TY  - JOUR
TI  - Sensors based on piezoelectric resonators
AU  - Benes, E.
AU  - Gröschl, M.
AU  - Burger, W.
AU  - Schmid, M.
T2  - Sensors and Actuators A: Physical
AB  - A review of sensors based on piezoelectric crystal resonators is presented. The survey focuses on the fundamental resonator modes rather than on the variety of surrounding support configurations in special sensor applications. First, the general properties of vibrating crystal sensors and their inherent superiority are described. The sensor concepts utilizing either homogeneous resonators with temperature and pressure (stress) as primary measurants or composite resonators with areal mass density and viscoelastic properties of the 'foreign' layer as primary measurands are discriminated. A comparison between bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators with respect to their primary sensitivity functions and principal capabilities for sensor applications is given and the importance of recent investigations on Lamb wave and horizontal polarized shear wave (HPLW) interdigital transducer (IDT) resonators is acknowledged. The importance of mode purity for high dynamic range sensors based on resonators and some aspects of the demand on specialized electronics are emphasized. The present state of established sensors based on primary sensitivities, e.g., quartz-crystal thermometers, pressure transducers, thin-film thickness and deposition-rate monitors, viscoelastic layer analysers (crystal/liquid composite resonators) is reviewed. A selection of the most promising recently investigated vibrating crystal sensors utilizing indirect sensitivities is described, including the wide field of analyte-selective coatings and resonator-based immunosensors or immunoassays. Finally, the potential of alternative piezoelectric materials for future sensor developments is briefly discussed.
DA  - 1995/05/01/
PY  - 1995
DO  - 10.1016/0924-4247(95)00846-2
VL  - 48
IS  - 1
SP  - 1
EP  - 21
J2  - Sens. Actuators A
SN  - 0924-4247
ER  - 

TY  - JOUR
TI  - Surface reduction state determines stabilization and incorporation of Rh on α-Fe2O3(11̅02)
AU  - Kraushofer, Florian
AU  - Resch, Nikolaus
AU  - Eder, Moritz
AU  - Rafsanjani-Abbasi, Ali
AU  - Tobisch, Sarah
AU  - Jakub, Zdenek
AU  - Franceschi, Giada
AU  - Riva, Michele
AU  - Meier, Matthias
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Advanced Materials Interfaces
AB  - Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as-synthesized catalyst prior to use. Here, Rh adsorption and incorporation on the (11̅02) surface of hematite (α-Fe2O3) are studied, which switches from a stoichiometric (1 × 1) termination to a reduced (2 × 1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favored that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) data, as well as density functional theory (DFT), it is concluded that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer.
DA  - 2021///
PY  - 2021
DO  - 10.1002/admi.202001908
VL  - 8
IS  - 8
SP  - 2001908
J2  - Adv. Mater. Interfaces
LA  - en
SN  - 2196-7350
ER  - 

TY  - JOUR
TI  - Quest for a pristine unreconstructed SrTiO3(001) surface: An atomically resolved study via noncontact atomic force microscopy
AU  - Sokolović, Igor
AU  - Franceschi, Giada
AU  - Wang, Zhichang
AU  - Xu, Jian
AU  - Pavelec, Jiří
AU  - Riva, Michele
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Setvín, Martin
T2  - Physical Review B
AB  - The surfaces of perovskite oxides affect their functional properties, and while a bulk-truncated (1×1) termination is generally assumed, its existence and stability is controversial. Here, such a surface is created by cleaving the prototypical SrTiO3(001) in ultrahigh vacuum, and its response to thermal annealing is observed. Atomically resolved noncontact atomic force microscopy (nc-AFM) shows that intrinsic point defects on the as-cleaved surface migrate at temperatures above 200∘C. At 400∘C–500∘C, a disordered surface layer forms, albeit still with a (1×1) pattern in low-energy electron diffraction (LEED). Purely TiO2-terminated surfaces, prepared by wet-chemical treatment, are also disordered despite their (1×1) periodicity in LEED.
DA  - 2021/06/10/
PY  - 2021
DO  - 10.1103/PhysRevB.103.L241406
VL  - 103
IS  - 24
SP  - L241406
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Ni-modified Fe3O4(001) surface as a simple model system for understanding the oxygen evolution reaction
AU  - Mirabella, Francesca
AU  - Müllner, Matthias
AU  - Touzalin, Thomas
AU  - Riva, Michele
AU  - Jakub, Zdenek
AU  - Kraushofer, Florian
AU  - Schmid, Michael
AU  - Koper, Marc T. M.
AU  - Parkinson, Gareth S.
AU  - Diebold, Ulrike
T2  - Electrochimica Acta
AB  - Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.
DA  - 2021/09/01/
PY  - 2021
DO  - 10.1016/j.electacta.2021.138638
VL  - 389
SP  - 138638
J2  - Electrochim. Acta
LA  - en
SN  - 0013-4686
ER  - 

TY  - JOUR
TI  - Direct assessment of the acidity of individual surface hydroxyls
AU  - Wagner, Margareta
AU  - Meyer, Bernd
AU  - Setvin, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Nature
AB  - The state of deprotonation/protonation of surfaces has far-ranging implications in chemistry, from acid–base catalysis and the electrocatalytic and photocatalytic splitting of water, to the behaviour of minerals and biochemistry. An entity’s acidity is described by its proton affinity and its acid dissociation constant pKa (the negative logarithm of the equilibrium constant of the proton transfer reaction in solution). The acidity of individual sites is difficult to assess for solids, compared with molecules. For mineral surfaces, the acidity is estimated by semi-empirical concepts, such as bond-order valence sums, and increasingly modelled with first-principles molecular dynamics simulations. At present, such predictions cannot be tested—experimental measures, such as the point of zero charge, integrate over the whole surface or, in some cases, individual crystal facets. Here we assess the acidity of individual hydroxyl groups on In2O3(111)—a model oxide with four different types of surface oxygen atom. We probe the strength of their hydrogen bonds with the tip of a non-contact atomic force microscope and find quantitative agreement with density functional theory calculations. By relating the results to known proton affinities of gas-phase molecules, we determine the proton affinity of the different surface sites of In2O3 with atomic precision. Measurements on hydroxylated titanium dioxide and zirconium oxide extend our method to other oxides.
DA  - 2021/04//
PY  - 2021
DO  - 10.1038/s41586-021-03432-3
VL  - 592
IS  - 7856
SP  - 722
EP  - 725
J2  - Nature
LA  - en
SN  - 1476-4687
ER  - 

TY  - JOUR
TI  - Resolving the structure of a well-ordered hydroxyl overlayer on In2O3(111): Nanomanipulation and theory
AU  - Wagner, Margareta
AU  - Lackner, Peter
AU  - Seiler, Steffen
AU  - Brunsch, Achim
AU  - Bliem, Roland
AU  - Gerhold, Stefan
AU  - Wang, Zhiming
AU  - Osiecki, Jacek
AU  - Schulte, Karina
AU  - Boatner, Lynn A.
AU  - Schmid, Michael
AU  - Meyer, Bernd
AU  - Diebold, Ulrike
T2  - ACS Nano
AB  - Changes in chemical and physical properties resulting from water adsorption play an important role in the characterization and performance of device-relevant materials. Studies of model oxides with well-characterized surfaces can provide detailed information that is vital for a general understanding of water–oxide interactions. In this work, we study single crystals of indium oxide, the prototypical transparent contact material that is heavily used in a wide range of applications and most prominently in optoelectronic technologies. Water adsorbs dissociatively already at temperatures as low as 100 K, as confirmed by scanning tunneling microscopy (STM), photoelectron spectroscopy, and density functional theory. This dissociation takes place on lattice sites of the defect-free surface. While the In2O3(111)-(1 × 1) surface offers four types of surface oxygen atoms (12 atoms per unit cell in total), water dissociation happens exclusively at one of them together with a neighboring pair of 5-fold coordinated In atoms. These O–In groups are symmetrically arranged around the 6-fold coordinated In atoms at the surface. At room temperature, the In2O3(111) surface thus saturates at three dissociated water molecules per unit cell, leading to a well-ordered hydroxylated surface with (1 × 1) symmetry, where the three water OWH groups plus the surface OSH groups are imaged together as one bright triangle in STM. Manipulations with the STM tip by means of voltage pulses preferentially remove the H atom of one surface OSH group per triangle. The change in contrast due to strong local band bending provides insights into the internal structure of these bright triangles. The experimental results are further confirmed by quantitative simulations of the STM image corrugation.
DA  - 2017/11/28/
PY  - 2017
DO  - 10.1021/acsnano.7b06387
VL  - 11
IS  - 11
SP  - 11531
EP  - 11541
J2  - ACS Nano
SN  - 1936-0851
ER  - 

TY  - JOUR
TI  - Interplay between adsorbates and polarons: CO on rutile TiO2(110)
AU  - Reticcioli, Michele
AU  - Sokolović, Igor
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Setvin, Martin
AU  - Franchini, Cesare
T2  - Physical Review Letters
AB  - Polaron formation plays a major role in determining the structural, electrical, and chemical properties of ionic crystals. Using a combination of first-principles calculations, scanning tunneling microscopy, and atomic force microscopy, we analyze the interaction of polarons with CO molecules adsorbed on the reduced rutile TiO2(110) surface. Adsorbed CO shows attractive coupling with polarons in the surface layer, and repulsive interaction with polarons in the subsurface layer. As a result, CO adsorption depends on the reduction state of the sample. For slightly reduced surfaces, many adsorption configurations with comparable adsorption energies exist and polarons reside in the subsurface layer. At strongly reduced surfaces, two adsorption configurations dominate: either inside an oxygen vacancy, or at surface Ti5c sites, coupled with a surface polaron. Similar conclusions are predicted for TiO2(110) surfaces containing near-surface Ti interstitials. These results show that polarons are of primary importance for understanding the performance of polar semiconductors and transition metal oxides in catalysis and energy-related applications.
DA  - 2019/01/09/
PY  - 2019
DO  - 10.1103/PhysRevLett.122.016805
VL  - 122
IS  - 1
SP  - 016805
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Sexiphenyl on Cu(100): nc-AFM tip functionalization and identification
AU  - Wagner, Margareta
AU  - Setvín, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Surface Science
T3  - Surface Structure and Dynamics – in Honor of Karl-Heinz Rieder
AB  - The contrast obtained in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images is determined by the tip termination and symmetry. Functionalizing the tip with a single metal atom, CO molecule or organic species has been shown to provide high spatial resolution and insights into tip-surface interactions. A topic where this concept is utilized is the adsorption of organic molecules at surfaces. With this work we aim to contribute to the growing database of organic molecules that allow assignment by intra-molecular imaging. We investigated the organic molecule para-sexiphenyl (C36H26, 6P) on Cu(100) using low-temperature STM and non-contact AFM with intra-molecular resolution. In the sub-monolayer regime we find a planar and flat adsorption with the 6P molecules rotated 10° off the 〈010〉 directions. In this configuration, four of six phenyl rings occupy almost equivalent sites on the surface. The 6P molecules are further investigated with CO-functionalized tips, in comparison to a single-atom metal and 6P-terminated tip. We also show that the procedure of using adsorbed CO to characterize tips introduced by Hofmann et al., Phys. Rev. B 112 (2014) 066101 is useful when the tip is terminated with an organic molecule.
DA  - 2018/12/01/
PY  - 2018
DO  - 10.1016/j.susc.2018.03.004
VL  - 678
SP  - 124
EP  - 127
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The growth of metastable fcc Fe78Ni22 thin films on H-Si(100) substrates suitable for focused ion beam direct magnetic patterning
AU  - Gloss, Jonas
AU  - Horký, Michal
AU  - Křižáková, Viola
AU  - Flajšman, Lukáš
AU  - Schmid, Michael
AU  - Urbánek, Michal
AU  - Varga, Peter
T2  - Applied Surface Science
AB  - We have studied the growth of metastable face-centered-cubic, non-magnetic Fe78Ni22 thin films on silicon substrates. These films undergo a magnetic (paramagnetic to ferromagnetic) and structural (fcc to bcc) phase transformation upon ion beam irradiation and thus can serve as a material for direct writing of magnetic nanostructures by the focused ion beam. So far, these films were prepared only on single-crystal Cu(100) substrates. We show that transformable Fe78Ni22 thin films can also be prepared on a hydrogen-terminated Si(100) with a 130-nm-thick Cu(100) buffer layer. The H-Si(100) substrates can be prepared by hydrofluoric acid etching or by annealing at 1200 °C followed by adsorption of atomic hydrogen. The Cu(100) buffer layer and Fe78Ni22 fcc metastable thin film were deposited by thermal evaporation in ultra-high vacuum. The films were consequently transformed in-situ by 4 keV Ar+ ion irradiation and ex-situ by a 30 keV Ga+ focused ion beam, and their magnetic properties were studied by magneto-optical Kerr effect magnetometry. The substitution of expensive copper single crystal substrate by standard silicon wafers dramatically expands application possibilities of metastable paramagnetic thin films for focused-ion-beam direct magnetic patterning.
DA  - 2019/03/01/
PY  - 2019
DO  - 10.1016/j.apsusc.2018.10.263
VL  - 469
SP  - 747
EP  - 752
J2  - Appl. Surf. Sci.
SN  - 0169-4332
ER  - 

TY  - JOUR
TI  - Surface structures of ZrO2 films on Rh(111): From two layers to bulk termination
AU  - Lackner, Peter
AU  - Zou, Zhiyu
AU  - Mayr, Sabrina
AU  - Choi, Joong-Il Jake
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Surface Science
AB  - We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2–10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2–4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Above this thickness, films show a (2 × 1) or a distorted (2 × 2) surface structure with respect to cubic ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 °C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 °C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2, as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO2/Pt(111) system.
DA  - 2019/01/01/
PY  - 2019
DO  - 10.1016/j.susc.2018.09.004
VL  - 679
SP  - 180
EP  - 187
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Adsorption of CO on the Ca3Ru2O7(001) surface
AU  - Mayr-Schmölzer, Wernfried
AU  - Halwidl, Daniel
AU  - Mittendorfer, Florian
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Redinger, Josef
T2  - Surface Science
AB  - The adsorption of CO molecules at the Ca3Ru2O7(001) surface was studied using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Ca3Ru2O7 can be easily cleaved along the (001) plane, yielding a smooth, CaO-terminated surface. The STM shows a characteristic pattern with alternating dark and bright stripes, resulting from the tilting of the RuO6 octahedra. At 78 K, CO adsorbs at an apical surface O at the channel edge with a predicted binding energy of Eads=−0.85 eV. After annealing at room temperature, the CO forms a strong bond (Eads=−2.04 eV) with the apical O and the resulting carboxylate takes the place of the former surface O. This carboxylate can be decomposed by scanning the surface with a high sample bias voltage of +2.7 V, restoring the original surface.
DA  - 2019/02/01/
PY  - 2019
DO  - 10.1016/j.susc.2018.10.004
VL  - 680
SP  - 18
EP  - 23
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Surface oxides on Pd(111): STM and density functional calculations
AU  - Klikovits, J.
AU  - Napetschnig, E.
AU  - Schmid, M.
AU  - Seriani, N.
AU  - Dubay, O.
AU  - Kresse, G.
AU  - Varga, P.
T2  - Physical Review B
AB  - The formation of one-layer surface oxides on Pd(111) has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Besides the Pd5O4 structure determined previously, structural details of six different surface oxides on Pd(111) will be presented. These oxides are observed for preparation in oxygen-rich conditions, approaching the thermodynamic stability limit of the PdO bulk oxide at an oxygen chemical potential of -0.95 to -1.02 eV (570-605 K, 5x10(-4) mbar O-2). Sorted by increasing oxygen fraction in the primitive unit cell, the stoichiometry of the surface oxides is Pd5O4, Pd9O8, Pd20O18, Pd23O21, Pd19O18, Pd8O8, and Pd32O32. All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd(111) substrate. DFT calculations were used to simulate STM images as well as to determine the stability of the surface oxide structures. Simulated and measured STM images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than Pd5O4, we conclude that Pd5O4 is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures.
DA  - 2007/07//
PY  - 2007
DO  - 10.1103/PhysRevB.76.045405
VL  - 76
IS  - 4
SP  - 045405
J2  - Phys. Rev. B
SN  - 1098-0121
ER  - 

TY  - JOUR
TI  - Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(110): Contact angle, surface structure and composition
AU  - Napetschnig, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - We have investigated the structure and morphology of Co and Pd clusters grown at room temperature on an alumina film on NiAl(110) by scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. We have also studied the clusters after annealing to 300 degrees C and Pd clusters deposited at 300 degrees C. Mixed Co-Pd clusters obtained by sequential deposition at room temperature were also studied. Pure Co deposited at room temperature forms a single type of clusters, most or all of them with close-packed planes parallel to the oxide surface. Their shape can be approximated by truncated spheres with a high contact angle of 115-125 degrees. These clusters are stable upon annealing up to 300 degrees C. Pd clusters deposited at room temperature grow in two different modes. At the reflection domain boundaries the clusters grow in their thermodynamically favorable shape. The clusters do not have a single crystallographic orientation and their shape can be approximated by a truncated sphere with a high contact angle of about 110 degrees, especially at very low coverages (below 0.05 ML). At the antiphase domain boundaries. the Pd clusters grow in (1 11) orientation and on some of them small (111) facets appear at their tops already at low coveraizes. For higher coverages of Pd, the majority of Pd clusters are rather flat with a large Pd(111) facet on top. The clusters' shape at the antiphase domain boundaries differs from the thermodynamically favorable one, due to kinetic limitations, especially at higher coverages. Annealing the Pd clusters to 300 degrees C leads to re-structuring of these Pd clusters. They transform into higher and more rounded clusters and a thin disordered alumina film is formed on top of the clusters. When Pd is deposited at 300 degrees C, about 16% of the Pd clusters have a steep slope and rounded tops. The rest of the Pd forms lower clusters, goes subsurface and is covered by a disordered alumina film. When Co and Pd are deposited sequentially, Pd covers the Co clusters forming a shell. The resulting mixed clusters are still truncated spheres with a lowered contact angle. For deposition in the reverse order (first Pd and then Co) we found that Co forms an alloy with Pd already at room temperature.
DA  - 2007/08/01/
PY  - 2007
DO  - 10.1016/j.susc.2007.05.047
VL  - 601
IS  - 15
SP  - 3233
EP  - 3245
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Nanotemplate with holes: Ultrathin alumina on Ni3Al(111)
AU  - Schmid, M.
AU  - Kresse, G.
AU  - Buchsbaum, A.
AU  - Napetschnig, E.
AU  - Gritschneder, S.
AU  - Reichling, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - We have determined the structure of the ultrathin (root 67 x root 67)R12.2 degrees aluminum oxide on Ni3Al(111) by a combination of scanning tunneling microscopy and density functional theory. In addition to other local defects, the main structural feature of the unit cell is a 0.4-nm-diameter hole reaching down to the metal substrate. Understanding the structure and metal growth on this oxide allows us to use it as a template for growing highly regular arrays of nanoparticles.
DA  - 2007/11/09/
PY  - 2007
DO  - 10.1103/PhysRevLett.99.196104
VL  - 99
IS  - 19
SP  - 196104
J2  - Phys. Rev. Lett.
SN  - 0031-9007
ER  - 

TY  - JOUR
TI  - Ultrathin alumina film on Cu-9at%Al(111)
AU  - Napetschnig, E.
AU  - M. Schmid
AU  - Varga, P.
T2  - Surface Science
AB  - We have investigated the structure of the clean and the oxidized (111) surface of a Cu-Al alloy with 9 at% Al by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), low energy ion scattering (LEIS) and low energy electron diffraction (LEED). Annealing of the clean crystal at 680 degrees C leads to segregation of Al to the surface. The Al concentration at the annealed surface is 23 +/- 2% and domains with a (root 3 x root 3)R30 degrees superstructure are visible, as well as small Cu(111) areas and disordered patches. Oxidation at 680 degrees C leads to the formation of a well-ordered flat alumina film with two very similar oxide structures. One oxide structure has a nearly commensurate rectangular cell rotated by 30 degrees with respect to a close-packed row of the substrate and grows in three different domains. The second structure has a commensurate cell consisting of four equivalent building blocks and has a rectangular centered symmetry. This structure is rotated by 18 degrees with respect to a close-packed row of the substrate and grows in six different domains. The rectangular building blocks of these two oxide structures have a similar thickness, the same surface termination and the same number and arrangement of the atoms as the oxide film on NiAl(110) [G. Kresse, M. Schmid, E. Napetschnig, M. Shishkin, L. Kohler, P. Varga, Science 308 (2005) 1448]. In contrast to the oxide on NiAl(110), alumina on the Cu-Al alloy crystal does not show stress-induced domain boundaries and grows in large defect-free domains. Thus, I'd deposited on this oxide nucleates not only on domain boundaries and steps but also on the unperturbed oxide, forming (111)-oriented clusters.
DA  - 2008/05/15/
PY  - 2008
DO  - 10.1016/j.susc.2008.02.040
VL  - 602
IS  - 10
SP  - 1750
EP  - 1756
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Ion-beam-induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100)
AU  - Shah Zaman, Sameena
AU  - Oßmer, Hinnerk
AU  - Jonner, Jakub
AU  - Novotný, Zbyněk
AU  - Buchsbaum, Andreas
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Physical Review B
AB  - We have grown 22-ML-thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) γ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) α phase. A structural transformation of these films from fcc to bcc can be induced by Ar+ ion irradiation. Scanning-tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing Ar+ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low-energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions.
DA  - 2010/12/01/
PY  - 2010
DO  - 10.1103/PhysRevB.82.235401
VL  - 82
IS  - 23
SP  - 235401
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Antiphase domain boundaries at the Fe3O4(001) surface
AU  - Parkinson, Gareth S.
AU  - Manz, Thomas A.
AU  - Novotný, Zbyněk
AU  - Sprunger, Phillip T.
AU  - Kurtz, Richard L.
AU  - Schmid, Michael
AU  - Sholl, David S.
AU  - Diebold, Ulrike
T2  - Physical Review B
AB  - Antiphase domain boundaries (APDBs) in the (√2×√2)R45∘ reconstruction of the Fe3O4(001) surface were investigated using scanning tunneling microscopy (STM) and density functional theory [(DFT) + U] calculations. The equilibrium structure of the APDBs is interpreted in terms of the distorted B-layer model for the (√2×√2)R45∘ reconstruction in which a lattice distortion couples to charge order in the subsurface layers. The APDBs are observed after prolonged annealing at 700 °C, indicating that they are extremely stable. DFT + U calculations reveal that the APDB structure is linked to a disruption in the subsurface charge-order pattern, leading to an enrichment of Fe2+ cations at the APDB. Simulated STM images reproduce the appearance of the APDBs in the experimental data and reveal that they are preferential adsorption sites for hydrogen atoms.
DA  - 2012/05/24/
PY  - 2012
DO  - 10.1103/PhysRevB.85.195450
VL  - 85
IS  - 19
SP  - 195450
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Surface preparation of TiO2 anatase (101): Pitfalls and how to avoid them
AU  - Setvín, Martin
AU  - Daniel, Benjamin
AU  - Mansfeldova, Vera
AU  - Kavan, Ladislav
AU  - Scheiber, Philipp
AU  - Fidler, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Surface Science
AB  - TiO2 anatase is a material of high technological importance, yet studies on well-defined model surfaces are scarce. The main impediment to fundamental research of this material is the lack of high-purity single crystals of a sufficient size. Natural anatase crystals always contain impurities, while synthetic crystals are pure but usually very small. We discuss optimum surface preparation procedures that result in clean surfaces under UHV conditions and the best ways to mount small crystals on sample plates. The influence of bulk impurities on surface preparation is discussed. The most troublesome impurity in natural TiO2 crystals is Fe. Upon annealing the crystal in partial O2 pressure Fe segregates, resulting in overgrowth of iron oxide on the surface. Based on the temperature-dependence of O-induced Fe segregation, optimum sample treatment procedures are proposed. Finally, we show that the surface roughness on the anatase (101) surface increases with the number of sputter-anneal cycles. Possible reasons and ways to revert this process are described.
DA  - 2014/08//
PY  - 2014
DO  - 10.1016/j.susc.2014.04.001
VL  - 626
SP  - 61
EP  - 67
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Segregated carbon on Pt10Ni90(100) studied by scanning tunneling microscopy
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Varga, P.
T2  - Surface Science
AB  - Scanning tunneling microscopy is used to study the arrangement of segregated carbon atoms with atomic resolution. Individual carbon atoms are visible only under special tip conditions, while they normally do not directly appear on STM topographs. Under all tip conditions, carbon atoms affect the corrugation of their metal neighbours, reducing the apparent height by 20 to 40 pm in the p(2 × 2) and 40 to 70 pm in the c(2 × 2) superstructure. Therefore the existence, structure and amount of carbon can be also derived from images without directly visible carbon atoms. A substrate lattice distortion in regions of the carbon c(2 × 2) superstructure was observed, exhibiting areas of the p4g structure known from earlier LEED studies of Ni(100).
DA  - 1993/09/10/
PY  - 1993
DO  - 10.1016/0039-6028(93)90103-Q
VL  - 294
IS  - 3
SP  - L952
EP  - L958
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Subsurface islands and superstructures of Cu on Pb(111)
AU  - Nagl, C.
AU  - Platzgummer, E.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Speller, S.
AU  - Heiland, W.
T2  - Surface Science
AB  - A new growth mode, namely subsurface island growth, has been identified by means of scanning tunneling microscopy. When Cu is deposited on Pb(111), 3D-Cu islands with a thickness between 1 and 11 layers are immersed several layers deep into the Pb substrate. The Cu islands, which can be classified in four different types, are furthermore covered by a monatomic Pb film. On flat Cu islands, this Pb film shows a corrugation that is strongly dependent on the number of Cu layers beneath. Single Cu layers were found to form a new type of Cu-Pb superstructure islands.
DA  - 1996/05/15/
PY  - 1996
DO  - 10.1016/0039-6028(95)01200-1
VL  - 352-354
SP  - 540
EP  - 545
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Segregation of impurities on Cr(100) studied by AES and STM
AU  - Schmid, M.
AU  - Pinczolits, M.
AU  - Hebenstreit, W.
AU  - Varga, P.
T2  - Surface Science
AB  - With increasing annealing temperature, Auger electron spectroscopy (AES) of a Cr(100) single crystal shows segregation of C, N and O as the dominating segregating species, indicating competitive segregation of these elements. An STM study of N structures shows a c(2 × 2) superstructure at N coverages up to 1/2. The local N coverage can be increased by insertion of N-rich domain boundaries up to 2/3, where a c(3√2 × √2)R ± 45° structure forms, followed by a first-order phase transformation to a p(1 × 1) structure. The existence of patches of the N-rich p(1 × 1) structure at coverages below 2/3 is probably due to additional carbon impurities stabilizing this structure. The possibility of inverse corrugation on the pure Cr(100) surface is discussed.
DA  - 1997/04/20/
PY  - 1997
DO  - 10.1016/S0039-6028(96)01539-7
VL  - 377-379
SP  - 1023
EP  - 1027
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - CHAP
TI  - Ionization of LiF by hyperthermal multiply charged ions
AU  - Neidhart, T.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Ionization of Solids by Heavy Particles
A2  - Baragiola, Raul A.
T3  - NATO ASI Series
AB  - Sputtering of a LiF thin film (0.25 µm) surface by singly- (He+, Ne+ and Ar+) and doubly-charged (Ne++ and Ar++) ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F- ions is only slightly higher for doubly-charged ions compared to singly-charged projectiles at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model based on calculations by Walkup and Avouris. A comparison is given with former measurements on a LiF single crystal.
CY  - New York
DA  - 1993/08/31/
PY  - 1993
ET  - 1
VL  - B 306
SP  - 447
EP  - 453
PB  - Plenum
SN  - 0-306-44489-5
ER  - 

TY  - CHAP
TI  - Desorption from LiF(100) by singly- and doubly-charged hyperthermal He ions
AU  - Neidhart, T.
AU  - Schmid, M
AU  - Varga, P.
T2  - Desorption Induced by Electronic Transitions DIET V
A2  - Burns, A. R.
A2  - Stechel, E. B.
A2  - Jennison, D. R.
T3  - Springer Series in Surface Sciences
AB  - Sputtering of a LiF(100) surface by singly- and doubly-charged He ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F- ions is only slightly higher for doubly-charged ions compared to singly-charged at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model based on calculations by Walkup and Avouris.
CY  - Berlin
DA  - 1993///
PY  - 1993
VL  - 31
SP  - 129
EP  - 132
PB  - Springer
SN  - 978-3-540-56473-7
ER  - 

TY  - JOUR
TI  - Sputtering of LiF(100) with low energetic Ne+ and Ne2+ ions
AU  - Wutte, D.
AU  - Diebold, U.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - Sputtering of a LiF(100) surface by singly and doubly charged Ne ions with impact energies between 10 and 500 eV has been performed. The emission of Li+, Li0, F+ F0 and LiF+ was measured by means of a quadrupole mass spectrometer. The yield of sputtered Li+ ions and Li0 atoms decreases with decreasing impact energy and is slightly higher for Ne2+ compared to Ne+ at impact energies below 100 eV. The situation changes very drastically regarding the emitted F particles. In the whole energy range investigated, the F+ and F0 yield was about one order of magnitude larger if Ne2+ projectiles were used instead of Ne+. According to the experimental data we propose the following model for the emission of F+ [1,2]: (1) The F ion of the solid surface is changed to F+ by Auger neutralization of the Ne2+ into Ne+ or by double resonance transition into doubly excited states of Ne0 with subsequent autoionization into Ne+. These processes take place several Ångströms in front of the surface. (2) The initial repulsive forces between the F+ ion and the surface are changed into attractive ones within a very short time by rearrangement of the alkali halide lattice [2]. (3) The neutralized projectile sputters the F+ as in a normal sputtering event on a surface with low surface binding energy, and F+ can he emitted as long as the kinetic energy of the projectile is sufficiently high.
DA  - 1992/03/01/
PY  - 1992
DO  - 10.1016/0168-583X(92)95029-Q
VL  - 65
IS  - 1-4
SP  - 167
EP  - 172
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - A computer-controlled system for the measurement of complete admittance spectra of piezoelectric resonators
AU  - Schmid, M
AU  - Benes, E.
AU  - Sedlaczek, R.
T2  - Measurement Science and Technology
AB  - A measurement system for the characterisation of the electrical properties of piezoelectric resonators is presented. Admittance of the resonator is measured at successive frequencies controlled by an algorithm which uses small frequency steps when a resonance is found, and large steps between the resonances. During the measurement, characteristic frequencies are marked for each resonance. Further evaluation of the data is done by a least-squares fit, which yields the parameters of the equivalent circuit for each resonance in closed form.
DA  - 1990///
PY  - 1990
DO  - 10.1088/0957-0233/1/9/021
VL  - 1
IS  - 9
SP  - 970
EP  - 975
J2  - Meas. Sci. Technol.
SN  - 0957-0233
ER  - 

TY  - JOUR
TI  - Ion-induced erosion of tungsten surfaces studied by a sensitive quartz-crystal-microbalance technique
AU  - Golczewski, A.
AU  - Kuzucan, A.
AU  - Schmid, K.
AU  - Roth, J.
AU  - Schmid, M.
AU  - Aumayr, F.
T2  - Journal of Nuclear Materials
AB  - A highly sensitive quartz-crystal-microbalance (QCM) technique was used to study erosion of polycrystalline tungsten films due to impact of deuterium, carbon and argon ions, as well as retention of deuterium in these films. Polycrystalline tungsten films coated onto a SC-cut quartz crystal were bombarded by ions with impact energies from 100 eV up to a few keV and the frequency change due to mass loss (sputtering, desorption) or mass gain (implantation, adsorption) during bombardment was determined. Our setup was capable of detecting mass-changes as small as 10^-5 µg/s, which corresponds to a removal (or deposition) of only 10^-4 W monolayers/s. While our total sputtering yields for deuterium and argon projectiles compare well with the results of previous work, we derive new data on sputtering of tungsten by carbon ions. In addition we demonstrate that our setup is well suited for determining deuterium retention rates in tungsten.
DA  - 2009/06/15/
PY  - 2009
DO  - 10.1016/j.jnucmat.2009.01.279
VL  - 390-391
SP  - 1102
EP  - 1105
J2  - J. Nucl. Mater.
SN  - 0022-3115
ER  - 

TY  - JOUR
TI  - Total sputter yield of LiF induced by hyperthermal ions measured by a quartz microbalance
AU  - Neidhart, T.
AU  - Toth, Z.
AU  - Hochhold, M.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - The total sputter yield of LiF induced by singly charged hyperthermal He+, Ne+ and Ar+ ions with a kinetic energy of 5-500 eV is presented. The measurements have been performed with a highly sensitive quartz crystal microbalance. Results of the total sputter yield for Au show excellent agreement with the literature. The results on LiF show a large and slowly decreasing total sputter yield at low energies. This fact is interpreted as evidence for electronic processes in the sputtering of LiF.
DA  - 1994/05/02/
PY  - 1994
DO  - 10.1016/0168-583X(94)95601-4
VL  - 90
IS  - 1-4
SP  - 496
EP  - 500
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - Secondary ion emission from lithium fluoride under impact of slow multicharged ions
AU  - Neidhart, T.
AU  - Pichler, F.
AU  - Aumayr, F.
AU  - Winter, H. P.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged Ar ions (charge state q ≤ 9, impact energy E_k ≤ 500 eV). The F- ions originate from collisional energy transfer, almost independent of the primary ion charge, whereas the F+ yield strongly increases with q. The F+ ions are produced via interatomic Auger transitions from the F- 2p valence band into projectile states, and their desorption from LiF is controlled by Coulomb interaction of F+ with Li+ and F- surface ions, and LiF lattice relaxation. At high impact energy, emission of Li+ is also mainly due to collisional energy transfer, but toward lower Ek the primary ion charge plays an increasingly important role. The present measurements demonstrate that secondary ions account for less than 0.1% of our earlier measured total sputter yields from LiF.
DA  - 1995/05/03/
PY  - 1995
DO  - 10.1016/0168-583X(95)00169-7
VL  - 98
IS  - 1-4
SP  - 465
EP  - 468
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - The accuracy of quantitative LEED in determining chemical composition profiles of substitutionally disordered alloys: a case study
AU  - Sporn, M.
AU  - Platzgummer, E.
AU  - Forsthuber, S.
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
T2  - Surface Science
AB  - We explore the accuracy of chemical composition profiles of substitutionally disordered alloys determined experimentally by LEED (low-energy electron diffraction) I(E) analysis. We analyse experimental I(E) spectra of pure Rh(111) for its known chemical composition by comparing them to calculations assuming a substitutionally disordered PtxRh1-x alloy surface. The layer concentrations known to be 100% Rh are reproduced with a maximum error of 8% when the Pendry R-factor (RP) is employed. This error is considerably smaller than estimated by error bars derived from the variance of RP. We argue that the same accuracy can be expected for compositional depth profiles to be determined for alloys exhibiting weak chemical order and negligible lattice distortions such as PtxRh1-x.
DA  - 1998/10/30/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00596-2
VL  - 416
IS  - 3
SP  - 423
EP  - 429
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Trajectory-dependent neutralization of 1 keV He+ ions scattered from Pb(111) and Pb films on Cu(100)
AU  - Platzgummer, E.
AU  - Borrell, M.
AU  - Nagl, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - We observe trajectory-dependent neutralization of 1 keV He+ ions scattered from the Pb(111) surface. Varying the azimuthal angle of the ion trajectories we observe neutralization by charge densities of the next and second next neighbors of the target atom occurring at polar angles between 40 and 15°. This corresponds to ion-neighbor distances between 2.3 and 1 Å. We adjust a three-parameter neutralization model for Pb to fit the trajectory dependence of the survival probability for the well known Pb(111) surface geometry. Good agreement with experimental results is found only if a shell-like neutralization region is centered at a radius of 1.46 Å. In regard to the spatial distribution of the neutralization rate we consider the Pb 6sp electrons to be the source of the long distance neutralization that leads to the trajectory dependence. The same neutralization model is then applied for a structural analysis of ultrathin Pb films evaporated on Cu(100), where we confirm the existence of a Pb-Cu surface alloy on the c(4×4) Pb/Cu(100) and the existence of a Pb overlayer on the c(2×2) Pb/Cu(100). It turns out that the neutralization model developed for Pb atoms embedded in the (111) surface holds also for adsorbed Pb atoms.
DA  - 1998/09/03/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00388-4
VL  - 412-413
SP  - 202
EP  - 212
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Threshold for Potential Sputtering of LiF
AU  - Hayderer, G.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Winter, H P.
AU  - Aumayr, F.
AU  - Wirtz, L.
AU  - Lemell, C.
AU  - Burgdörfer, J.
AU  - Hägg, L.
AU  - Reinhold, C. O.
T2  - Physical Review Letters
AB  - We have measured total sputtering yields for impact of slow ( ≤100 eV) singly and doubly charged ions on LiF. The minimum potential energy necessary to induce potential sputtering (PS) from LiF was determined to be about 10 eV. This threshold coincides with the energy necessary to produce a cold hole in the valence band of LiF by resonant neutralization. This allows the first unambiguous identification of PS induced by cold holes. Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides.
DA  - 1999/11/08/
PY  - 1999
DO  - 10.1103/PhysRevLett.83.3948
VL  - 83
IS  - 19
SP  - 3948
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Observation of a threshold in potential sputtering of LiF surfaces
AU  - Hayderer, G.
AU  - Lemell, C.
AU  - Wirtz, L.
AU  - Schmid, M.
AU  - Burgdörfer, J.
AU  - Varga, P.
AU  - Winter, HP.
AU  - Aumayr, F.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - A quartz-crystal microbalance technique is used for measuring total sputtering yields for LiF under impact of slow (20 eV, 100 eV, 500 eV and 1000 eV kinetic energy) singly and doubly charged ions. At low kinetic energies ([less-than-or-equals, slant]100 eV) potential sputtering (PS) (i.e., sputtering due to the projectiles potential energy) clearly dominates over kinetically induced sputtering. New insight into the mechanisms for PS is gained by determining the minimum potential energy necessary to induce PS. The measured potential energy threshold at around 10 eV provides evidence that PS can already be induced by the production of cold holes in the valence band of LiF via resonant neutralisation.
DA  - 2000/04//
PY  - 2000
DO  - 10.1016/S0168-583X(99)01070-8
VL  - 164-165
SP  - 517
EP  - 521
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - Vibration modes of mass-loaded planoconvex quartz crystal resonators
AU  - Benes, Ewald
AU  - Schmid, Michael
AU  - Kravchenko, Victor
T2  - Journal of the Acoustical Society of America
AB  - Layered piezoelectric resonators exhibit a complex spectrum of eigenfrequencies, which can be understood better if the associated vibration patterns are examined. For a closer insight into such a system, planoconvex AT-cut quartz crystals with a thick layer of copper on one side are investigated experimentally. Since presently no comprehensive three-dimensional theory of layered resonators is available, the experimental results can be compared only with one-dimensional descriptions of layered resonators or with theoretical models for unloaded contoured quartz crystals. The anharmonic spectrum  of the crystals is used to find a formula by which the eigenfrequency of the corresponding infinite plate can be determined from measured frequencies of planoconvex crystals. If this correction  is applied to experimental frequency spectra of composite quartz-film resonators, excellent agreement with the one-dimensional model is found. As theoretically predicted, also even harmonic overtones of heavily loaded crystals can be excited electrically. The vibration patterns are examined with an ultrasensitive laser-speckle technique. The measurements of the odd harmonic overtones  agree well with the theory of energy trapping in planoconvex  quartz crystals. An increase of the crystal's active area with  increasing mass load is observed. The even overtones frequently exhibit no such energy trapping; in these cases, unusual vibration patterns occur.
DA  - 1991///
PY  - 1991
DO  - 10.1121/1.401940
VL  - 90
IS  - 2
SP  - 700
EP  - 706
J2  - J. Acoust. Soc. Am.
ER  - 

TY  - JOUR
TI  - Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors
AU  - Ostermaier, C.
AU  - Pozzovivo, G.
AU  - Basnar, B.
AU  - Schrenk, W.
AU  - Schmid, M.
AU  - Tóth, L.
AU  - Pécz, B.
AU  - Carlin, J.-F.
AU  - Gonschorek, M.
AU  - Grandjean, N.
AU  - Strasser, G.
AU  - Pogany, D.
AU  - Kuzmik, J.
T2  - Applied Physics Letters
AB  - We report on an annealing-induced “gate sinking” effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 °C  the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 °C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm
DA  - 2010///
PY  - 2010
DO  - 10.1063/1.3458700
VL  - 96
IS  - 26
SP  - 263515
J2  - Appl. Phys. Lett.
SN  - 00036951
ER  - 

TY  - JOUR
TI  - Determination of electron-induced total sputter yield of LiF
AU  - Neidhart, T.
AU  - Sporn, M.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - We present measurements of the total sputter yield for LiF at 150°C induced by electrons with an impact energy between 10 and 500 eV. A nearly linear increase with kinetic energy has been measured. The decrease of the yield with increasing electron dose for energies below 100 eV is also shown. For very high electron dose the yield even becomes zero and simultaneously a red coloration of the surface is observed. Low energy ion scattering (LEIS) measurements at such a surface showed a Li enrichment to more than 90%. To reach such a composition with electrons between 40 and 80 eV kinetic energy sputtering of about 20 monolayers LiF is necessary.
DA  - 1995/06/03/
PY  - 1995
DO  - 10.1016/0168-583X(95)00062-3
VL  - 101
IS  - 1-2
SP  - 127
EP  - 130
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - A LEED study of NO superstructures on the Pd(111) surface
AU  - Kostelník, Petr
AU  - Šikola, Tomas
AU  - Varga, Peter
AU  - Schmid, Michael
T2  - Journal of Physics: Condensed Matter
AB  - We have examined two adsorption structures of NO on the Pd(111) surface and the transformation between them. Low-energy electron diffraction (LEED) I(V) curves of the Pd(111)-p(2 x 2)-NO and Pd(111)-c(4 x 2)-NO surface structures were acquired and analyzed using tensor LEED. Our structural models confirm a previous study by scanning tunneling microscopy and DFT (Hansen et al 2002 Surf. Sci. 496 1). In the c(4 x 2)-NO structure, which forms at an NO coverage of 0.5 monolayers (ML), the NO molecules occupy fcc and hcp hollow sites and are almost upright with only slight tilting, possibly related to NO-NO repulsion. In the p(2 x 2)-NO structure (0.75 ML), with two NO molecules in hollow sites and one in an on-top site, we find strong tilting of the on-top molecule. Upon heating, thermal desorption of NO leads to a transition from the p(2 x 2) to the c(4 x 2) structure, which leads to splitting of the diffraction spots and/or streaky spots. The transition is discussed in terms of domain walls.
DA  - 2009///
PY  - 2009
DO  - 10.1088/0953-8984/21/13/134005
VL  - 21
IS  - 13
SP  - 134005
J2  - J. Phys.: Condens. Matter
SN  - 0953-8984
ER  - 

TY  - JOUR
TI  - Reduced thickness of contamination layers determined from C 1s- and CKVV-lines
AU  - Ebel, Maria F.
AU  - Schmid, M.
AU  - Ebel, H.
AU  - Vogel, A.
T2  - Journal of Electron Spectroscopy and Related Phenomena
AB  - In the electron spectra of hydrocarbon contamination layers, carbon is the dominating chemical element. Its two characteristic lines (C 1s and CKVV) can be measured easily and evaluated for the reduced thickness, x, of the overlayer. For calibration an “infinite” thick layer, either graphite after removal of its own contamination layer by argon-ion etching, or thin films of diffusion-pump oil, can be used, since the content of carbon in these reference samples does not influence the reference value according to x = ∞. The standard deviation of x as obtained by this method is within ± 40%.
DA  - 1984///
PY  - 1984
DO  - 10.1016/0368-2048(84)80076-6
VL  - 34
IS  - 3
SP  - 313
EP  - 316
J2  - J. Electron Spectrosc. Relat. Phenom.
SN  - 0368-2048
ER  - 

TY  - JOUR
TI  - A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces
AU  - Golczewski, A.
AU  - Dobes, K.
AU  - Wachter, G.
AU  - Schmid, M.
AU  - Aumayr, F.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - We describe a highly sensitive quartz-crystal-microbalance technique capable of determining erosion as well as implantation and retention rates for fusion relevant surfaces under ion bombardment. Total sputtering yields obtained with this technique for Ar ion impact on polycrystalline gold and tungsten surfaces are presented. The results compare well with existing experimental data as well as theoretical predictions and thus demonstrate the feasibility of the developed technique. Our setup is capable of detecting mass-changes as small as 10-5 [mu]g/s, which corresponds to a removal of only 10-4 W monolayers/s.
DA  - 2009/02//
PY  - 2009
DO  - 10.1016/j.nimb.2008.10.088
VL  - 267
IS  - 4
SP  - 695
EP  - 699
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - Vanadium surface oxides on Pd(111): A structural analysis
AU  - Klein, C.
AU  - Kresse, G.
AU  - Surnev, S.
AU  - Netzer, F. P.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - Scanning tunneling microscopy studies of vanadium oxides grown on Pd(111) show interesting structures especially in the low-coverage region. Evaporation of V in an oxygen background at elevated sample temperature (250 °C) results in the formation of a nonperiodic honeycomb-like structure growing from the steps, which starts to transform into an ordered phase at a vanadium coverage of ≈0.2 ML (monolayer). At 0.31 ML the entire surface is covered by this well-ordered open (4×4) structure. Annealing this structure in H2 atmosphere transforms the phase into a V2O3 surface oxide with (2×2) periodicity, whose optimal coverage is reached at 0.5 ML vanadium. Models for both ordered structures have been suggested before on the basis of ab initio density-functional theory (DFT) calculations and molecular-dynamics simulations and these models are now unambiguously confirmed by quantitative low-energy electron-diffraction (LEED) analyses. In the (4×4) phase, the V atoms are surrounded by four oxygen atoms in an unusual tetrahedral coordination leading to a V5O14 stoichiometry. This tetrahedral coordination allows the oxide to adopt open loosely packed two-dimensional (2D) and 1D structures, which are stabilized by the surface-oxide interface energy. Furthermore, it is shown that state of the art DFT calculations can indeed predict complex structures exactly as well as that modern quantitative LEED is capable of dealing with very large unit cells.
DA  - 2003/12/30/
PY  - 2003
DO  - 10.1103/PhysRevB.68.235416
VL  - 68
IS  - 23
SP  - 235416
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Coexistence of fcc- and bcc-like crystal structures in ultrathin Fe films grown on Cu(111)
AU  - Biedermann, A.
AU  - Rupp, W.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - We report on bcc-like phases in ultrathin Fe films grown by thermal deposition on Cu(111) previously thought to consist exclusively of fcc phases distinguished only by their magnetic order. Our scanning tunneling microscopy and spectroscopy data together with published x-ray photoelectron diffraction results [M. T. Kief and W. F. Egelhoff, Jr., Phys. Rev. B 47, 10785 (1993)] provide us with sufficient detail to deduce the film structure. Two growth regimes are considered: (1) films with 1–2 monolayer average thickness grown near 200 K, which nucleate as bcc-like bilayer islands: Larger islands show bcc-like fringes coexisiting with an fcc center domain; i.e., the bcc-like phase is stable only within a certain distance to a step edge. The presence of a bcc-like bilayer phase provides a straightforward explanation for the ferromagnetism previously observed in these films. In addition we find that the bcc-like phase can be promoted by H adsorption at 80  K. The bcc domains form "displacement vortex" structures to simultaneously minimize film stress and interface energy. (2) In films grown at room temperature, between pseudomorphic fcc areas, we observe a more ideal but still strained bcc phase in regions with a local thickness of at least 4 monolayers. Also in this growth regime, the fcc-bcc transformation is facilitated by step edges, which are abundant due to the imperfect layer-by-layer growth.
DA  - 2006/04/15/
PY  - 2006
DO  - 10.1103/PhysRevB.73.165418
VL  - 73
IS  - 16
SP  - 165418
EP  - 16
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Ion-beam induced fcc-bcc transition in ultrathin Fe films for ferromagnetic patterning
AU  - Rupp, W.
AU  - Biedermann, A.
AU  - Kamenik, B.
AU  - Ritter, R.
AU  - Klein, Ch.
AU  - Platzgummer, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Applied Physics Letters
AB  - Ar+ ion irradiation is used to induce a structural change from fcc to bcc in a 1.5  nm thick Fe film epitaxially grown on a Cu(100) crystal. Scanning tunneling microscopy and low-energy electron diffraction show the nucleation of bcc nanocrystals, which grow with increasing ion dose. As a consequence of the structural change, the irradiated iron film becomes strongly ferromagnetic at room temperature. We present a model for the process of the transformation and demonstrate writing a magnetic pattern at the 100  nm scale by ion-beam projection lithography.
DA  - 2008///
PY  - 2008
DO  - 10.1063/1.2969795
VL  - 93
IS  - 6
SP  - 063102
EP  - 3
J2  - Appl. Phys. Lett.
ER  - 

TY  - JOUR
TI  - Two-dimensional oxide on Pd(111)
AU  - Lundgren, E.
AU  - Kresse, G.
AU  - Klein, C.
AU  - Borg, M.
AU  - Andersen, J. N.
AU  - De Santis, M.
AU  - Gauthier, Y.
AU  - Konvicka, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.
DA  - 2002/06/03/
PY  - 2002
DO  - 10.1103/PhysRevLett.88.246103
VL  - 88
IS  - 24
SP  - 246103
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Surface structure and composition of Pt50Rh50(110): room temperature analysis of the (1 × 3) missing-row reconstruction
AU  - Koller, R.
AU  - Gauthier, Y.
AU  - Klein, C.
AU  - De Santis, M.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The room temperature structure and composition of the clean Pt50Rh50(110) surface is investigated by low energy ion scattering (LEIS), scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and grazing incidence X-ray diffraction (GIXRD). While Pt25Rh75(1 1 0) reconstructs with a (1 × 2) missing-row structure, Pt50Rh50(110) exhibits a (1 × 3) structure in analogy with Pt80Fe20(110) and Pt90Co10(110). Three missing rows lead to the formation of (111) facets in which all atomic sites are enriched with platinum. Similarly all sites directly underneath are enriched with Rh leading to oscillation of the Pt concentrations similar to that of (111) surfaces. These composition changes are accompanied by a marked inwards relaxation of the top row (-10%) and large buckling in layers 3-5. Additionally the atomic positions of the facets are shifted laterally towards the valleys formed by the missing rows. Consistent pictures are derived from LEED, GIXRD, STM and LEIS concerning the composition while some discrepancy--similar to that recorded for pure Pt--are found concerning the interlayer distances between LEED and X-rays.
DA  - 2003/05/01/
PY  - 2003
DO  - 10.1016/S0039-6028(03)00381-9
VL  - 530
IS  - 3
SP  - 121
EP  - 135
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Structure of Ag(111)-p(4 x 4)-O: No Silver Oxide
AU  - Schmid, M.
AU  - Reicho, A.
AU  - Stierle, A.
AU  - Costina, I.
AU  - Klikovits, J.
AU  - Kostelník, P.
AU  - Dubay, O.
AU  - Kresse, G.
AU  - Gustafson, J.
AU  - Lundgren, E.
AU  - Andersen, J. N.
AU  - Dosch, H.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - The structure of the oxygen-induced p(4×4) reconstruction of Ag(111) is determined by a combination of scanning tunneling microscopy, surface x-ray diffraction, core level spectroscopy, and density functional theory. We demonstrate that all previous models of this surface structure are incorrect and propose a new model which is able to explain all our experimental findings but has no resemblance to bulk silver oxide. We also shed some light on the limitations of current density functional theories and the potential role of van der Waals interactions in the stabilization of oxygen-induced surface reconstructions of noble metals.
DA  - 2006/04/14/
PY  - 2006
DO  - 10.1103/PhysRevLett.96.146102
VL  - 96
IS  - 14
SP  - 146102
EP  - 4
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Strain-induced local surface chemical ordering observed by STM
AU  - Ritz, G.
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Varga, P.
T2  - Physical Review B
AB  - PtNi alloys are known to exhibit a tendency towards chemical ordering, which also effects surface segregation. Scanning tunneling microscopy results obtained in the strain field of dislocations on PtxNi1-x(110) surfaces show a (2×1) superstructure of alternating Pt and Ni atoms in some regions close to the dislocation core. In other regions, the apparent height of all surface atoms is equal, in agreement with low energy ion scattering results yielding a surface concentration of almost 100% Ni. This indicates that the strain present in the vicinity of dislocations influences both the surface composition and chemical order. The experimental results are compared to simulation calculations of chemical ordering and segregation, using embedded atom method potentials and linear elasticity theory. The simulations indicate that the (2×1) superstructure is due to an L10 ordered phase in regions where the tetragonal distortion of the L10 phase with respect to the cubic substrate can alleviate stress. It is argued that this dislocation-induced ordering can immobilize dislocations.
DA  - 1996/06//
PY  - 1996
DO  - 10.1103/PhysRevB.53.16019
VL  - 53
IS  - 23
SP  - 16019
EP  - 16026
J2  - Phys. Rev. B
SN  - 0163-1829
ER  - 

TY  - JOUR
TI  - Surface effects on Pt-Ni single crystals calculated with the embedded-atom method
AU  - Stadler, H.
AU  - Hofer, W.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - Recent scanning-tunneling-microscopy (STM) experiments on Pt25Ni75(111) and Pt10Ni90(110) surfaces showing lattice mismatch dislocations near the surface and surface ordering phenomena have been verified in simulation calculations using the embedded-atom method (EAM). It was found that the EAM can be successfully applied for the calculation of ordering effects and crystal defects such as dislocations using energy minimization routines and Monte Carlo calculations. By comparing experimental data and EAM results, it was possible to determine the depth of the dislocations and therefore the number of layers with Pt enrichment on the surface. For the (111) face exhibiting surface ordering phenomena, the calculated short-range order is in agreement with STM data where the two species could be clearly distinguished.
DA  - 1993/10/15/
PY  - 1993
DO  - 10.1103/PhysRevB.48.11352
VL  - 48
IS  - 15
SP  - 11352
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Direct observation of surface chemical order by scanning tunneling microscopy
AU  - Schmid, M.
AU  - Stadler, H.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - We present the first scanning tunneling microscopy (STM) study which allows clear discrimination of two chemical species in a metal alloy. Special tunneling conditions, which we attribute to an adsorbate at the STM tip, cause a difference in corrugation between Pt and Ni atoms of 0.3 Å. The STM data reveal chemical short-range order at the surface, which is in agreement with embedded atom simulations and can be understood as small domains of an L10 ordered phase.
DA  - 1993/03/08/
PY  - 1993
DO  - 10.1103/PhysRevLett.70.1441
VL  - 70
IS  - 10
SP  - 1441
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Inverse corrugation and corrugation enhancement of Pb superstructures on Cu(111) and (110)
AU  - Nagl, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - Contrary to intuitive expectation, a monatomic hexagonal close-packed Pb film on Cu(111) shows an "inverse" corrugation, i.e. the Pb atoms in on-top adsorption sites appear lowest in scanning tunneling microscopy (STM) images, as well as in effective medium theory (EMT) simulation. On small subsurface Cu islands on a Pb(111) substrate, which are also covered by a monatomic Pb film, the corrugation of this film was found to be strongly dependent on the thickness of the Cu island (the thinner the island, the larger the corrugation). EMT simulations reproduce this enhanced corrugation qualitatively and thus further confirm the formation of these subsurface Cu islands. On a p(8 × 1) superstructure of Pb/Cu(110), similarly, the lowest coordinated Pb atoms also show the lowest apparent height in STM images.
DA  - 1996/12/20/
PY  - 1996
DO  - 10.1016/S0039-6028(96)00907-7
VL  - 369
IS  - 1-3
SP  - 159
EP  - 168
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Pt(100) quasihexagonal reconstruction: A comparison between scanning tunneling microscopy data and effective medium theory simulation calculations
AU  - Ritz, G.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Borg, A.
AU  - Rønning, M.
T2  - Physical Review B
AB  - The interpretation of scanning tunneling microscopy (STM) data is usually limited to first-layer effects, but with increasing resolution of the STM images deeper-layer effects may also become visible in the top-layer corrugations. We have investigated the clean Pt(100) surface, which is known to be pseudohexagonally reconstructed and for which there is some evidence for a second-layer reconstruction. The big unit cell makes it difficult to investigate the deeper layers by traditional methods like low-energy-electron diffraction (LEED). We have, therefore, used a “fingerprint” technique to compare highly resolved STM data of the clean Pt(100) surface to effective-medium-theory simulation calculations in order to determine the geometric structure of the second atomic layer. We were able to show that STM can be sensitive to deeper layer effects and that excellent agreement could only be achieved for an unreconstructed second layer. The simulation results also agree well with the corrugations determined by LEED whereas the maximum corrugation amplitude is higher than previously derived from helium-diffraction measurements.
DA  - 1997/10/15/
PY  - 1997
DO  - 10.1103/PhysRevB.56.10518
VL  - 56
IS  - 16
SP  - 10518
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Adatom interlayer diffusion on Pt(111): an embedded atom method study
AU  - Leonardelli, G.
AU  - Lundgren, E.
AU  - Schmid, M.
T2  - Surface Science
AB  - We use embedded atom method (EAM) potentials to calculate the Schwoebel barriers for a large number of hopping and exchange processes of Pt and Ni adatoms descending steps of the Pt(111) surface. The barriers we find for hopping processes are too high to play any role in homo- and heteroepitaxy, but we find very low and even negative Schwoebel barriers for exchange processes at concave corners and kinks. On straight steps we find the process taking place on B-steps rather than on A-steps, with very similar Schwoebel barriers for Ni and Pt adatoms. We also find a strong dependence of the Schwoebel barrier on the lateral relaxation of step edges as caused by surface stress. For vicinal surfaces with high step density this effect causes an increase of the Schwoebel barrier if the width of the (111)-terraces is reduced.
DA  - 2001/09/01/
PY  - 2001
DO  - 10.1016/S0039-6028(01)01212-2
VL  - 490
IS  - 1-2
SP  - 29
EP  - 42
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - A misfit structure in the Co/Pt(111) system studied by scanning tunnelling microscopy and embedded atom method calculations
AU  - Lundgren, E.
AU  - Leonardelli, G.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - We have performed scanning tunneling microscopy (STM) measurements of thin films of Co and CoPt alloys on Pt(111). In contrast to the interpretation of an earlier low energy electron diffraction investigation by Tsay and Shern [J.S. Tsay, C.S. Shern, Surf. Sci. 396 (1998) 313] we find that the structure observed upon annealing a Pt(111) sample with a Co film of 2 monolayer thickness does not consist of a rotated Co film but rather of a CoPt alloy film with hexagonal areas of fcc stacking, single and double tacking faults, delimited by misfit dislocations (Shockley partial dislocations). Comparison of STM images with embedded atom method (EAM) calculations confirms our model. The structure at the Pt-PtCo interface in our model is similar to that proposed by Henzler [M. Henzler, Surf. Sci. 419 (1999) 321].
DA  - 2002/02/10/
PY  - 2002
DO  - 10.1016/S0039-6028(01)01754-X
VL  - 498
IS  - 3
SP  - 257
EP  - 265
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Growth and decay of the Pd(111)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects
AU  - Gabasch, Harald
AU  - Unterberger, Werner
AU  - Hayek, Konrad
AU  - Klötzer, Bernhard
AU  - Kresse, Georg
AU  - Klein, Christof
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Surface Science
AB  - Growth and decomposition of the Pd<sub>5</sub>O<sub>4</sub> surface oxide on Pd(111) were studied at sample temperatures between 573 and 683 K and O2 gas pressures between 10<sup>-7</sup> and 6 × 10<sup>-5</sup> mbar, by means of an effusive O2 beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd5O4 surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10<sup>-6</sup> mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O2 pressure pulse and then following further growth kinetics in the lower (10<sup>-6</sup> mbar) pressure range. Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd<sub>5</sub>O<sub>4</sub> layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a (√67 x √67)R12.1° superstructure. Isothermal decay of the Pd<sub>5</sub>O<sub>4</sub> oxide layer at 693 K involves at first a rearrangement into the  (√67 x √67)R12.1° structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.
DA  - 2006/01/01/
PY  - 2006
DO  - 10.1016/j.susc.2005.09.052
VL  - 600
IS  - 1
SP  - 205
EP  - 218
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying?
AU  - Hofer, W. A.
AU  - Ritz, G.
AU  - Hebenstreit, W.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Redinger, J.
AU  - Podloucky, R.
T2  - Surface Science
AB  - Recent STM studies achieved chemical resolution on PtRh and PtNi alloy surfaces. By a first-principles method employing the Tersoff-Hamann model, we have simulated STM scans on PtRh and PtNi(100) surfaces by calculating the apparent heights of individual surface atoms. The difference in apparent heights between Pt and Rh atoms is caused by changes in the density of states due to alloying. The simulations for the PtNi(100) surface, however, yield apparent heights of Pt and Ni atoms below atomic resolution, indicating that in the experiment, tip-sample interactions are responsible for chemical and atomic resolution.
DA  - 1998/05/15/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00140-X
VL  - 405
IS  - 2-3
SP  - L514
EP  - L519
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Oxygen-induced vacancy formation on a metal surface
AU  - Schmid, M.
AU  - Leonardelli, G.
AU  - Sporn, M.
AU  - Platzgummer, E.
AU  - Hebenstreit, W.
AU  - Pinczolits, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - Using scanning tunneling microscopy, low-energy ion scattering, and quantitative low-energy electron diffraction, we find about 17% metal vacancies on the oxygen-covered Cr(100) surface. The oxygen atoms occupy all the hollow sites of the first layer, including those neighboring a Cr vacancy. We argue that the vacancy formation is energetically favored and not caused by stress but by electronic effects.
DA  - 1999/01/11/
PY  - 1999
DO  - 10.1103/PhysRevLett.82.355
VL  - 82
IS  - 2
SP  - 355
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations
AU  - Hebenstreit, W.
AU  - Redinger, J.
AU  - Horozova, Z.
AU  - Schmid, M.
AU  - Podloucky, R.
AU  - Varga, P.
T2  - Surface Science
AB  - Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab-initio calculations of the local density of states (LDOS) by means of the full-potential linearized augmented plane wave (FLAPW) method, the protrusions could be attributed to the anion Cl-. The calculations show that a higher density of occupied states at the Cl sites than for the Na sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the bandgap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers.
DA  - 1999/04/01/
PY  - 1999
DO  - 10.1016/S0039-6028(99)00095-3
VL  - 424
IS  - 2-3
SP  - L321
EP  - L328
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Surface and subsurface alloy formation of vanadium on Pd(111)
AU  - Konvicka, Ch.
AU  - Jeanvoine, Y.
AU  - Lundgren, E.
AU  - Kresse, G.
AU  - Schmid, M.
AU  - Hafner, J.
AU  - Varga, P.
T2  - Surface Science
AB  - We have studied the submonolayer growth of vanadium on the Pd(111) surface at different substrate temperatures. By using LEIS (Low energy ion spectroscopy), AES (Auger electron spectroscopy), STM (Scanning tunneling microscopy), XPD (X-ray photoelectron diffraction) and ab-initio local-density-functional calculations we find that V atoms deposited at room temperature substitute surface Pd atoms. In addition, islands are formed on the surface, which consist mostly of the substituted Pd atoms. At higher temperatures, V diffuses into subsurface layers and at a temperature of 300 °C only a small amount of V is observed in the top-most layer. By using STM a (√3 x √3)R30° superstructure is observed to be formed and XPD measurements demonstrate that this structure is due to V atoms incorporated in the second layer. This finding is confirmed by ab-initio calculations. Further, a model for the (√3 x √3)R30° structure based on the experiments and the ab-initio calculations is given.
DA  - 2000/09/10/
PY  - 2000
DO  - 10.1016/S0039-6028(00)00643-9
VL  - 463
IS  - 3
SP  - 199
EP  - 210
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Direct imaging of catalytically important processes in the oxidation of CO over RuO2(110)
AU  - Over, H.
AU  - Seitsonen, A. P.
AU  - Lundgren, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Journal of the American Chemical Society
AB  - Ruthenium dioxide (RuO2) reveals unique and promising redox properties, making RuO2 a potential candidate for a versatile oxidation catalyst. Recently Zhang and Kisch reported, for instance, that RuO2 is a robust and efficient catalyst for room temperature (RT) oxidation of CO by humid air; recall that typical metal oxides do not tolerate humidity. In this contribution we present scanning tunneling microscopy (STM) data which directly image the catalytically important processes occurring on the RuO2(110) surface after exposing the pristine surface to CO and O2. These data are complemented by density functional theory (DFT) calculations.

The following processes are governing the catalytic activity of RuO2 on atomic scale. The reactants from the gas phase encounter strongly binding adsorption sites on the RuO2(110) surface in the form of the under-coordinated Ru atoms. For instance, CO adsorbs on the stoichiometric RuO2(110) surface by 1.2 eV (over the 1f-cus-Ru), while on the reduced RuO2(110) surface the CO binding energies are 1.61 eV and 1.85 eV for adsorption over 1f-cus-Ru and 2f-cus-Ru atoms, respectively. The RuO2 surface provides an active oxygen species to react with CO, i.e. the under-coordinated (bridging) lattice oxygen atoms Obr. The recombination of adsorbed CO with Obr creates vacancies, which are identified with STM. At RT, oxygen molecules from the gas phase can efficiently dissociate on RuO2(110) via the molecular precursor state. This leads to weakly held O atoms, which are grouped in pairs as imaged with STM. It is argued that the weakly held oxygen plays an important role in replenishing the consumed (bridging) lattice oxygen atoms on the (partially) reduced RuO2(110) surface.
DA  - 2001/11/01/
PY  - 2001
DO  - 10.1021/ja016408t
VL  - 123
IS  - 47
SP  - 11807
EP  - 11808
J2  - J. Am. Chem. Soc.
ER  - 

TY  - JOUR
TI  - Oxygen adsorption on Al(111): low transient mobility
AU  - Schmid, M.
AU  - Leonardelli, G.
AU  - Tscheließnig, R.
AU  - Biedermann, A.
AU  - Varga, P.
T2  - Surface Science
AB  - Adsorption of oxygen on Al(111) is studied by scanning tunneling microscopy at 80 and 300 K. After adsorption at 130-195 K, STM images taken at 80 K show pairs of oxygen adatoms with interatomic distances mainly between one and three Al interatomic spacings. This clearly shows that dissociation of the oxygen molecules results in a rather low transient mobility of the two oxygen atoms, a fact which is in contrast to previous work [Phys. Rev. Lett. 68 (1992) 624]. We also find evidence for oxygen atoms in a second metastable adsorption site at these temperatures. At room temperature, we find groups of two or more oxygen atoms in adjacent fcc hollow sites, but no single oxygen atoms. We therefore explain the room-temperature results by part of the oxygen pairs remaining or becoming nearest neighbors, whereas others separate by diffusion and their oxygen atoms attach to other pairs or groups, forming the larger groups found. The pairs and larger groups are stable due to an attractive interaction of oxygen atoms in adjacent fcc hollow sites.
DA  - 2001/05/10/
PY  - 2001
DO  - 10.1016/S0039-6028(01)00967-0
VL  - 478
IS  - 3
SP  - L355
EP  - L362
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The structure of the oxygen induced (1×5) reconstruction of V(100)
AU  - Koller, R.
AU  - Bergermayer, W.
AU  - Kresse, G.
AU  - Hebenstreit, E. L. D.
AU  - Konvicka, C.
AU  - Schmid, M.
AU  - Podloucky, R.
AU  - Varga, P.
T2  - Surface Science
AB  - The adsorption of 1 Langmuir oxygen on the V(100) surface at 200 °C leads to a (1x5) reconstruction. The structure of this surface was determined by scanning tunneling microscopy (STM), low energy electron diffraction (quantitative LEED) and density functional theory calculations. The STM images show dark lines every fifth vanadium lattice constant. Between these (1x5) lines, oxygen resides in four-fold coordinated hollow sites with a coverage of ~70%. From the LEED analysis (Pendry R-factor = 0.17) it was found that in the dark lines oxygen adsorbs in bridge sites, in agreement with the ab initio calculations. The driving force behind this reconstruction is surface stress induced by the vanadium-oxygen bonds.
DA  - 2001/05/30/
PY  - 2001
DO  - 10.1016/S0039-6028(01)00978-5
VL  - 480
IS  - 1-2
SP  - 11
EP  - 24
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Experimental and simulated STM images of stoichiometric and partially reduced RuO2(110) surfaces including adsorbates
AU  - Over, H.
AU  - Seitsonen, A. P.
AU  - Lundgren, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - We present experimental and DFT-simulated STM images of ultrathin RuO2(110) films on Ru(0001), including adsorbates such as oxygen and CO. We are able to identify the under-coordinated O atoms on the RuO2(110) surface with STM, i.e. the bridging O atoms and the on-top O atoms. The partial reduction of the RuO2(110) surface by CO exposure at room temperature leads to a surface where part of the bridging O atoms have been removed and some of the vacancies are occupied by bridging CO. When dosing 10 L of CO at room temperature, all the RuO2(110) surface becomes mildly reduced in that all bridging O atoms are replaced by bridging CO molecules. Annealing the surface to 600 K produces holes on the terraces of such a mildly reduced RuO2(110) surface. These pits are not generated by the recombination of lattice O with CO, but rather these pits are assigned to a complex temperature-induced rearrangement of surface atoms in the topmost RuO2 double layer of RuO2(110). With this process the bridging O atoms are again populated and surplus Ru atoms agglomerate in small islands at the rims of the holes.
DA  - 2002/08//
PY  - 2002
DO  - 10.1016/S0039-6028(02)01853-8
VL  - 515
IS  - 1
SP  - 143
EP  - 156
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The structure of the oxygen-induced c(6×2) reconstruction of V(110)
AU  - Koller, R.
AU  - Bergermayer, W.
AU  - Kresse, G.
AU  - Konvicka, C.
AU  - Schmid, M.
AU  - Redinger, J.
AU  - Podloucky, R.
AU  - Varga, P.
T2  - Surface Science
AB  - The adsorption of 2.4 Langmuir oxygen on V(110) induces a c(6×2) reconstruction with an oxygen coverage of 0.5 ML. Its structure was determined using STM, quantitative LEED and ab initio density functional calculations in combination with molecular dynamics. Driven by the strong vanadium-oxygen bonding, the vanadium atoms at the surface are significantly rearranged compared to their bulk positions. The reconstructed geometry offers threefold and fourfold coordinated hollow sites, which are partially occupied by oxygen. The large set of structural data derived from LEED I-V analysis (R_Pe=0.11) and ab initio calculations, as well as the experimental and simulated STM images agree well. Additionally the structure of clean V(110) was determined.
DA  - 2002/06/20/
PY  - 2002
DO  - 10.1016/S0039-6028(02)01722-3
VL  - 512
IS  - 1-2
SP  - 16
EP  - 28
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Interaction of oxygen with palladium deposited on a thin alumina film
AU  - Shaikhutdinov, Sh.
AU  - Heemeier, M.
AU  - Hoffmann, J.
AU  - Meusel, I.
AU  - Richter, B.
AU  - Bäumer, M.
AU  - Kuhlenbeck, H.
AU  - Libuda, J.
AU  - Freund, H. -J.
AU  - Oldman, R.
AU  - Jackson, S. D.
AU  - Konvicka, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(110) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400-500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300-500 K. For large and crystalline Pd particles, the system exhibits adsorption-desorption properties which are very similar to those of the Pd(111) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ca. 5-7 kJmol^-1 for 1 and 3-5 nm Pd particles studied.
DA  - 2002/04/01/
PY  - 2002
DO  - 10.1016/S0039-6028(01)01850-7
VL  - 501
IS  - 3
SP  - 270
EP  - 281
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Superstructures of carbon on V(100)
AU  - Bergermayer, W.
AU  - Koller, R.
AU  - Konvicka, C.
AU  - Schmid, M.
AU  - Kresse, G.
AU  - Redinger, J.
AU  - Varga, P.
AU  - Podloucky, R.
T2  - Surface Science
AB  - Carbon adsorption on V(100) was studied by both experimental methods and density functional theory. At low carbon coverages of theta_C = 0.18 ML and oxygen below the experimental detection limit, measured scanning tunneling microscopy (STM) images show both areas of local c(2x2) structure and <010> oriented rows of C atoms. At higher coverages of ThetaC = 0.41 ML, mainly <010> oriented C rows with some local p(1x2) patterns are formed. The observed c(2x2) pattern is attributed to the presence of oxygen, since a mixture of carbon and oxygen favours the c(2x2) superstructure according to both the STM and the ab initio results. The calculations show that for theta_C = 0.50 ML the p(1x2) structure is more stable than c(2x2) by 0.13 eV per adsorbed atom. From the ab initio results it is predicted that p(1x2) changes into c(2x2) at a mixed coverage of about theta_C ≈ 0.37 ML and theta_O ≈ 0.13 ML. The geometry of the c(2x2) structure was determined using quantitative LEED showing good agreement with the ab initio data. Also the simulated STM images agree well with the experimental STM data.
DA  - 2002/01/20/
PY  - 2002
DO  - 10.1016/S0039-6028(01)01659-4
VL  - 497
IS  - 1-3
SP  - 294
EP  - 304
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Stabilizing single metal adatoms at room temperature: Pd on C- and O-covered V(100)
AU  - Konvicka, C.
AU  - Hammerschmid, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The initial stages of Pd thin film growth on clean and C- and O-precovered V(100) surfaces at room temperature (RT) and 200 °C have been studied by means of scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). In the presence of C and O, the adsorption of Pd atoms in the clean four-fold hollow sites of the V surface is strongly preferred. Upon deposition of Pd in the submonolayer range, it was possible to stabilize single Pd adatoms and small clusters at RT. Annealing such a surface at 200 °C leads to the formation of rectangular Pd islands (size ≈40×40 Å2) and to the compression of the initial C and O adlayer in the areas between the Pd. For higher Pd coverages we found that oxygen acts as an anti-surfactant, shifting the onset of second layer growth dramatically. The reason for the change of the growth mode of Pd on V(100) from Stranski–Krastanov to a Volmer–Weber-type growth in the presence of oxygen can be found in a higher free energy of the film/substrate interface compared to the clean surface.
DA  - 2002/01/10/
PY  - 2002
DO  - 10.1016/S0039-6028(01)01605-3
VL  - 496
IS  - 3
SP  - 209
EP  - 220
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Complex surface reconstructions solved by ab initio molecular dynamics
AU  - Kresse, G.
AU  - Bergermayer, W.
AU  - Podloucky, R.
AU  - Lundgren, E.
AU  - Koller, R.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Applied Physics A: Materials Science & Processing
AB  - Abstract.   Complex surface reconstructions and surface oxides, in particular, often exhibit complicated atomic arrangements, which are difficult to resolve with traditional experimental methods, such as low energy electron diffraction (LEED), surface X-ray diffraction (SXRD) or scanning tunnelling microscopy (STM) alone. Therefore, ab initio density functional calculations are used as a supplement to the experimental techniques, but even then the structural determination usually relies on a simple trial and error procedure, in which conceivable models are first constructed and then tested for their stability in ab initio calculations. An exhaustive search of the configuration space is usually difficult and requires a significant human effort. Solutions to this problem, such as simulated annealing, have long been known, but are usually considered to be too time-consuming in combination with first principles methods. In this work, we show that ab initio density functional codes are now sufficiently fast to perform extensive finite temperature molecular dynamics. The merits of this approach are exemplified for two cases, for a complex two-dimensional surface oxide on Pd(111), and for the oxygen induced c(6×2) reconstruction of V(110).
DA  - 2003/03/15/
PY  - 2003
DO  - 10.1007/s00339-002-2007-2
VL  - 76
IS  - 5
SP  - 701
EP  - 710
J2  - Appl. Phys. A
ER  - 

TY  - JOUR
TI  - Structure of the cobalt-filled missing-row reconstruction of Pt(110)
AU  - Klein, C.
AU  - Koller, R.
AU  - Lundgren, E.
AU  - Máca, F.
AU  - Redinger, J.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - The atomic structure of 0.5 monolayer (ML) Co deposited on Pt(110) was investigated by quantitative low-energy electron diffraction and ab initio density functional theory calculations, showing a pronounced inward relaxation and a filling of the missing-row sites of the Pt(110) substrate by Co atoms. Up to this Co coverage no significant intermixing of Pt atoms with Co atoms was observed by scanning tunneling microscopy, resulting in an alternating arrangement of pure Co and Pt rows.
DA  - 2004/10/18/
PY  - 2004
DO  - 10.1103/PhysRevB.70.153403
VL  - 70
IS  - 15
SP  - 153403
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Structure of a thin oxide film on Rh(100)
AU  - Gustafson, J.
AU  - Mikkelsen, A
AU  - Borg, M.
AU  - Andersen, J. N.
AU  - Lundgren, E.
AU  - Klein, C.
AU  - Hofer, W.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Köhler, L.
AU  - Kresse, G.
AU  - Kasper, N.
AU  - Stierle, A.
AU  - Dosch, H.
T2  - Physical Review B
AB  - The initial oxidation of Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction, scanning tunneling microscopy, and density functional theory. We report a structural study of an oxygen induced structure displaying a c(8×2) periodicity at an oxygen pressure above 10−5 mbar and using a sample temperature of 700 K . Our experimental and theoretical data demonstrate that this structure is due to the formation of a thin surface oxide with a hexagonal trilayer O-Rh-O structure.
DA  - 2005/03/31/
PY  - 2005
DO  - 10.1103/PhysRevB.71.115442
VL  - 71
IS  - 11
SP  - 115442
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Chemical ordering and composition fluctuations at the (001) surface of the Fe64Ni36 Invar alloy
AU  - Ondráček, M.
AU  - Máca, F.
AU  - Kudrnovský, J.
AU  - Redinger, J.
AU  - Biedermann, A.
AU  - Fritscher, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which combines first-principles calculations and low-temperature scanning tunneling microscopy (STM) experiments. Density functional theory calculations show that Fe-Ni alloy surfaces are buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the STM image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c(2×2) chemical order (50% Fe) with frequent Fe-rich defects on the Fe64Ni36(001) surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The STM images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition.
DA  - 2006/12/15/
PY  - 2006
DO  - 10.1103/PhysRevB.74.235437
VL  - 74
IS  - 23
SP  - 235437
EP  - 7
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Oxygen-deficient line defects in an ultrathin aluminum oxide film
AU  - Schmid, M.
AU  - Shishkin, M.
AU  - Kresse, G.
AU  - Napetschnig, E.
AU  - Varga, P.
AU  - Kulawik, M.
AU  - Nilius, N.
AU  - Rust, H.-P.
AU  - Freund, H.-J.
T2  - Physical Review Letters
AB  - A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements.
DA  - 2006/07/28/
PY  - 2006
DO  - 10.1103/PhysRevLett.97.046101
VL  - 97
IS  - 4
SP  - 046101
EP  - 4
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Oxygen-induced step bunching and faceting of Rh(553): Experiment and ab initio calculations
AU  - Gustafson, J.
AU  - Resta, A.
AU  - Mikkelsen, A.
AU  - Westerström, R.
AU  - Andersen, J. N.
AU  - Lundgren, E.
AU  - Weissenrieder, J.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Kasper, N.
AU  - Torrelles, X.
AU  - Ferrer, S.
AU  - Mittendorfer, F.
AU  - Kresse, G.
T2  - Physical Review B
AB  - Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh(553) surface, a surface vicinal to (111), undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about 10–6  mbar and a temperature of 380  °C this leads to (331) facets with one-dimensional oxide chains along the steps, coexisting with (111) facets. Further increase of the pressure and temperature results in (111) facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.
DA  - 2006/07/15/
PY  - 2006
DO  - 10.1103/PhysRevB.74.035401
VL  - 74
IS  - 3
SP  - 035401
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Combined STM, LEED and DFT study of Ag(100) exposed to oxygen near atmospheric pressures
AU  - Costina, I.
AU  - Schmid, M.
AU  - Schiechl, H.
AU  - Gajdos, M.
AU  - Stierle, A.
AU  - Kumaragurubaran, S.
AU  - Hafner, J.
AU  - Dosch, H.
AU  - Varga, P.
T2  - Surface Science
AB  - We have investigated the interaction of molecular oxygen with the Ag(100) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O2, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O2 at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at [approximate]470 K and 10 mbar O2 pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.
DA  - 2006/02/01/
PY  - 2006
DO  - 10.1016/j.susc.2005.11.020
VL  - 600
IS  - 3
SP  - 617
EP  - 624
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Structure and catalytic reactivity of Rh oxides
AU  - Gustafson, J.
AU  - Westerström, R.
AU  - Resta, A.
AU  - Mikkelsen, A.
AU  - Andersen, J.N.
AU  - Balmes, O.
AU  - Torrelles, X.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Hammer, B.
AU  - Kresse, G.
AU  - Baddeley, C.J.
AU  - Lundgren, E.
T2  - Catalysis Today
AB  - Using a combination of experimental and theoretical techniques, we show that a thin RhO2 surface oxide film forms prior to the bulk Rh2O3 corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO2 surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh2O3 corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(111), Rh(100) and Pt25Rh75(100) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO2 production coincides with the formation of the thin RhO2 surface oxide film. In the case of Pt25Rh75(100), our measurements demonstrate that the formation of bulk Rh2O3 corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase.
DA  - 2009/07/30/
PY  - 2009
DO  - 10.1016/j.cattod.2008.11.011
VL  - 145
IS  - 3-4
SP  - 227
EP  - 235
J2  - Catal. Today
SN  - 0920-5861
ER  - 

TY  - JOUR
TI  - Analysis of vibration-isolating systems for scanning tunneling microscopes
AU  - Schmid, M.
AU  - Varga, P.
T2  - Ultramicroscopy
AB  - For high-resolution and nanotechnology devices, such as the STM, the effective isolation of environmental vibrations plays a key role. Different types of vibration-reducing systems are analyzed by means of mechanical four-pole theory.
A comparison of one- and two-stage spring-suspended systems with magnetic eddy-current damping shows that one-stage systems suffer from poor isolation at high frequencies, which can be improved by additional elastomer elements. Contrary to common belief, two-stage spring systems with eddy-current damping for the upper stage only are superior to similar systems with eddy-current damping of both stages.
Plate stacks with elastomer elements yield good amplitude reduction at high frequencies, while vibrations in the 10-100 Hz range may be even enhanced. Several stacks with different arrangements of masses and spring constants are compared, and it is shown that stacks should have only few (2 or 3) plates. For optimum performance it is necessary to combine the plate stack inside the vacuum chamber with soft (pneumatic or spring) suspension of the whole chamber.
DA  - 1992/07//
PY  - 1992
DO  - 10.1016/0304-3991(92)90493-4
VL  - 42-44
IS  - Part 2
SP  - 1610
EP  - 1615
J2  - Ultramicroscopy
SN  - 0304-3991
ER  - 

TY  - JOUR
TI  - p(n x 1) superstructures of Pb on Cu(110)
AU  - Nagl, C.
AU  - Pinczolits, M.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Robinson, I. K.
T2  - Physical Review B
AB  - The structures of the p(n×1) superstructures (n=4, 5, and 9) of Pb on Cu(110) in the coverage range between FTHETA=0.75 and 0.8 are revealed by atomically resolved scanning tunneling microscopy. All three superstructures are formed by substitution of every nth row of Cu atoms (n=4, 5, and 9) in the [001] direction by Pb atoms. The Pb atoms in between are lined up in the [1¯10] direction. The p(4×1) structure appears in two different modifications: one with substitutional rows of Pb atoms and one with a simple overlayer structure without substituted rows of Cu atoms. Alternating succession of these two modifications results in p(12×1) domains. It is further shown that the p(9×1) structure is not a succession of p(4×1) and p(5×1) but a superstructure on its own. The p(5×1) structure proposed here agrees with previous x-ray-diffraction data at least as well as the quasihexagonal model proposed earlier. We have, in addition, identified the nature of the phase that has been described incommensurate obtained by desorption of Pb upon annealing above 600 K.
DA  - 1995/12/15/
PY  - 1995
DO  - 10.1103/PhysRevB.52.16796
VL  - 52
IS  - 23
SP  - 16796
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Direct Observation of a New Growth Mode: Subsurface Island Growth of Cu on Pb(111)
AU  - Nagl, C.
AU  - Platzgummer, E.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Speller, S.
AU  - Heiland, W.
T2  - Physical Review Letters
AB  - Atomically resolved scanning tunneling microscopy on Cu/Pb(111) reveals a new growth mode, contrary to the Volmer-Weber mode expected from the significantly lower surface energy of Pb. (111)-oriented Cu islands with a thickness of 3–11 layers are immersed in the Pb substrate and covered by a single close-packed Pb layer. This subsurface growth mode occurring at room temperature can be explained by simple thermodynamic considerations.
DA  - 1995/10/16/
PY  - 1995
DO  - 10.1103/PhysRevLett.75.2976
VL  - 75
IS  - 16
SP  - 2976
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - STM study of the (111) and (100) surfaces of PdAg
AU  - Wouda, P. T.
AU  - Schmid, M.
AU  - Nieuwenhuys, B. E.
AU  - Varga, P.
T2  - Surface Science
AB  - The (111) and (100) surfaces of the Pd67Ag33 alloy have been imaged with atomic resolution by scanning tunneling microscopy. On the (111) surface, it was found that Pd atoms appear in the images about 25 pm higher than the Ag atoms. The surface concentration of palladium was determined as a function of annealing temperature (720-920 K) and was found to vary between 5 and 11%. Analysis of the relative positions of the Pd atoms showed a tendency towards formation of isolated palladium sites. On the (100) surface, the palladium concentration in the first layer is extremely low and the system has to be forced into a non-equilibrium state to find palladium atoms in the first monolayer. Chemical contrast here amounts to a 60 pm apparent height difference.
DA  - 1998/11/20/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00673-6
VL  - 417
IS  - 2-3
SP  - 292
EP  - 300
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Unreconstructed Au(100) monolayers on a Au3Pd(100) single-crystal surface
AU  - Aschoff, M.
AU  - Speller, S.
AU  - Kuntze, J.
AU  - Heiland, W.
AU  - Platzgummer, E.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Baretzky, B.
T2  - Surface Science
AB  - The Au3Pd(100) single-crystal surface was studied with ion scattering methods, low-energy electron diffraction (LEED) and scanning tunneling microscopy. The crystal is covered at room temperature with a pure, (100)-ordered gold layer. Palladium is found in the second layer only. The lattice constant of the gold surface as evaluated by ion scattering and a tensor low-energy electron diffraction (TLEED) analysis is equal to the bulk lattice constant of 4.017 Å as evaluated by X-ray analysis. The surface lattice constant of the gold layer on the alloy surface is 0.08 Å smaller than that of bulk gold.
DA  - 1998/10/09/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00564-0
VL  - 415
IS  - 3
SP  - L1051
EP  - L1054
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - High temperature growth of Pt on the Rh(111) surface
AU  - Duisberg, M.
AU  - Dräger, M.
AU  - Wandelt, K.
AU  - Gruber, E. L. D.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The epitaxial growth of Pt on the Rh(111) surface at 700 K was studied with AES, UPS, ISS and STM. From AES and ISS measurements a 2D growth mode is concluded at this substrate temperature. The morphology of the surface is studied by photoemission spectra of adsorbed Xe (PAX) and STM. A disperse distribution of the Pt atoms is suggested by PAX and is consistent with an incorporation of these atoms into the first substrate layer. Atomically and chemically resolved STM measurements confirm these conclusions. The interaction of CO with the surface alloy is investigated by UPS. The CO-induced features in UP spectra show significant differences in the peak positions and shape between the clean substrate and the surface precovered with different amounts of Pt. The CO induced emissions are, thus, used for a quantitative titration of Pt on the Rh surface.
DA  - 1999/08/02/
PY  - 1999
DO  - 10.1016/S0039-6028(99)00037-0
VL  - 433-435
SP  - 554
EP  - 558
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Pt25Rh75(111), (110), and (100) studied by scanning tunnelling microscopy with chemical contrast
AU  - Hebenstreit, E. L. D.
AU  - Hebenstreit, W.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - Scanning tunneling microscopy images with chemical contrast allowed the direct determination of the composition and short-range order behaviour of the clean Pt25Rh75(111), (110), and (100) alloy surfaces. All measurements were performed at room temperature and showed a strong platinum enrichment depending on the preparation temperature. In the top layers of both Pt25Rh75(111) and Pt25Rh75(110)-(1x2) we find a preference for unlike nearest neighbors. Pt25Rh75(100) exhibits after a preparation temperature of 900°C a preference for clustering whereas after a preparation temperature of 600°C, comparable to Pt25Rh75(111) and (110) ordering tendencies appear. Additional investigations of the chemical identity of atoms surrounding hollow positions show mostly small deviations from a random distribution. However, the number of hollow sites surrounded by Rh atoms only can be significantly affected by the short range order.
Pt25Rh75(110) exhibits a (1x2) missing-row reconstruction after annealing above 700°C. After the first annealing of the sputtered surface it is accompanied by mesoscopic long range "waves" with a height of approximately 2 nm and a wavelength up to 200 nm depending on the preparation temperature.
DA  - 1999/11/01/
PY  - 1999
DO  - 10.1016/S0039-6028(99)00880-8
VL  - 441
IS  - 2-3
SP  - 441
EP  - 453
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Chemical discrimination on atomic level by STM
AU  - Varga, P.
AU  - Schmid, M.
T2  - Applied Surface Science
AB  - Chemical information with spatial atomic resolution on multicomponent surfaces (especially alloys) can be achieved by STM (scanning tunnelling microscopy) with constant current imaging. Therefore STM can not only be used for determination of the crystallographic structure of single crystal surfaces but also for finding the chemical composition of bimetallic surfaces. This possibility makes the STM a unique instrument to find out the local chemical structure of multicomponent surfaces on an atomic scale. This ability can be used for studying in great detail segregation processes on metal surfaces. Examples of chemical discrimination between different metals on low index single crystal surfaces of bulk alloys we have seen so far are PtNi, PtRh, PtCo, PtAu and AgPd. For surfaces where the identification of the alloy constituents in the STM images is ambiguous (e.g., because of unknown or equal concentrations), it will be shown how ab initio calculations of the electron density of states using the FLAPW (full potential linearized augmented plane waves) method can help to interpret constant current STM topographs just by following the simple theory of Tersoff-Hamann. On the other hand, tip changes (e.g., adsorbates) can strongly influence the chemical contrast on constant current STM images.
DA  - 1999/03//
PY  - 1999
DO  - 10.1016/S0169-4332(98)00514-5
VL  - 141
IS  - 3-4
SP  - 287
EP  - 293
J2  - Appl. Surf. Sci.
SN  - 0169-4332
ER  - 

TY  - JOUR
TI  - Temperature-dependent segregation on Pt25Rh75(111) and (100)
AU  - Platzgummer, E.
AU  - Sporn, M.
AU  - Koller, R.
AU  - Forsthuber, S.
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
T2  - Surface Science
AB  - Surface segregation is studied on Pt25Rh75(111) and Pt25Rh75(100) by LEED intensity analysis and LEIS. Although both equilibrated surfaces are strongly Pt-enriched (up to 80 at.%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The Pt concentration grows continuously on Pt25Rh75(111) until 1000°C, whereas it reaches a maximum enrichment around 500°C on Pt25Rh75(100) and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the Pt25Rh75(100) surface by the interchange of Pt and Rh atoms within the near-surface layers, and on the Pt25Rh75(111) surface by a diffusion of Pt atoms from bulk to the near-surface region. The latter leads to an overall Pt enrichment of several layers, and is only observed after annealing at 1100°C. The presence of carbon contamination on the Pt25Rh75(100) surface causes a significant reduction of the Pt segregation. There is excellent agreement between the top-layer concentrations derived by LEIS and quantitative LEED.
DA  - 1999/01/04/
PY  - 1999
DO  - 10.1016/S0039-6028(98)00800-0
VL  - 419
IS  - 2-3
SP  - 236
EP  - 248
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Temperature-dependent segregation and (1×2) missing-row reconstruction of Pt25Rh75(110)
AU  - Platzgummer, E.
AU  - Sporn, M.
AU  - Koller, R.
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
T2  - Surface Science
AB  - The surface structure and composition of the clean Pt25Rh75(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). For the equilibrated Pt25Rh75(110) surface we observe a (1x2) missing-row reconstruction in analogy to the pure Pt(110) surface, and a significant Pt enrichment of the topmost atomic layer (up to 80 at.% Pt). As the same strong surface enrichment in Pt was found in a previous study on the (100) and (111) surface of the same bulk composition, this means that in contrast to Pt-Ni and Pt-Co alloys, for Pt25Rh75 alloys the segregation behavior is not influenced extensively by the surface orientation. In addition to the structure analysis by LEED we performed LEIS experiments to determine the temperature induced changes of the surface composition and structure. Since the Pt segregation is less pronounced at elevated temperature, the surface reveals a temperature induced deconstruction of the (1x2) structure around 750°C, resulting in an fcc(110) (1x1) surface at high temperature. Temperature dependent measurements further show a hysteresis-like behavior of the top-layer composition, which is attributed to an enhanced Pt segregation on the (1x2) reconstructed surface.
DA  - 1999/03/01/
PY  - 1999
DO  - 10.1016/S0039-6028(98)00924-8
VL  - 423
IS  - 1
SP  - 134
EP  - 143
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Adsorbate migration on PdAg(111)
AU  - Wouda, P. T.
AU  - Schmid, M.
AU  - Nieuwenhuys, B. E.
AU  - Varga, P.
T2  - Surface Science
AB  - Scanning Tunneling Microscopy was used to study the adsorption of oxygen on the PdAg(111) surface. Oxygen atoms appear as dark holes of about 40 pm depth, whereas palladium atoms appear as bright spots. By comparing consecutive STM images, oxygen was found to occupy only palladium sites: the oxygen was found to travel exclusively to and from the palladium atoms. The surface concentration of oxygen was low under all the experimental conditions used, as indicated by STM as well as Auger electron spectroscopy.
DA  - 1999/03/01/
PY  - 1999
DO  - 10.1016/S0039-6028(98)00937-6
VL  - 423
IS  - 1
SP  - L229
EP  - L235
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - An atomic-scale study of the Co induced dendrite formation on Pt(111)
AU  - Lundgren, E.
AU  - Stanka, B.
AU  - Koprolin, W.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - We have studied the initial growth of Co on the Pt(111) surface in a temperature range of 300-400 K with scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. In agreement with previous work by Grütter and Dürig, when depositing 0.1 ML Co on Pt(111) at 400 K, the formation of large dendrites is observed. These dendrites are formed in conjunction with a Co induced local reconstruction of the Pt(111) surface, resulting in dislocations. The dendrites, however, show no evidence for any dislocations below their surface, the local reconstruction is observed to be lifted by the formation of the dendrites. LEIS data suggest that the dendrites consist mainly of Pt, implying, together with the STM data, that Co is incorporated underneath the Pt. A model for this process is proposed.
DA  - 1999/03/10/
PY  - 1999
DO  - 10.1016/S0039-6028(98)00931-5
VL  - 423
IS  - 2-3
SP  - 357
EP  - 363
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Segregation and chemical ordering in the surface layers of Pt25Co75(111): a LEED/STM study
AU  - Gauthier, Y.
AU  - Baudoing-Savois, R.
AU  - Bugnard, J. M.
AU  - Hebenstreit, W.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - Segregation and chemical ordering on Pt25Co75(111) are studied by quantitative low energy electron diffraction (LEED) analysis and scanning tunnelling microscopy (STM). Although LEED patterns do not show any sign of superstructure, LEED calculations undoubtedly point to a surface which contains about the same amount of both species and reveal significant short range chemical ordering (down to the third layer). Pt and Co surface sites are locally arranged with a (1x2) unit cell, in the manner of the ordered tetragonal L10 phase. More direct evidence is given by STM images which exhibit parallel Pt and Co monoatomic chains a few lattice constants long and an apparent height difference of about 0.2 Å for Pt and Co sites. LEED shows that the Pt sublattice in the top layer actually resides 0.1 Å above the Co one. Otherwise the surface is bulklike, with weak relaxations of interlayer distances. The use of a chemically ordered model for the LEED analysis, in which sublattice occupancies are optimised, results in a remarkable improvement of the fit with experiment as compared to a totally random distribution; however, most interestingly, it does not modify the average layer concentration profile versus depth (55, 5 and 35 at% Pt respectively for the three outermost layers). The distortions needed for the tetragonal L1_0 phase with respect to the fcc L1_2 phase explains why chemical order does not extend over larger domains. Finally, both techniques yield complementary pictures and quite consistent results as to the top layer content and chemical order.
DA  - 2000/11/01/
PY  - 2000
DO  - 10.1016/S0039-6028(00)00751-2
VL  - 466
IS  - 1-3
SP  - 155
EP  - 166
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Temperature-dependent segregation reversal and (1×3) missing-row structure of Pt90Co10(110)
AU  - Platzgummer, E.
AU  - Sporn, M.
AU  - Koller, R.
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
T2  - Surface Science
AB  - The surface structure and composition of the clean Pt90Co10(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). Through LEED I-V analysis we find a (1x3) missing-row reconstruction on the equilibrated Pt90Co10(110) surface - comparable with the pure Pt(110) (1x3) surface - in which all atomic positions in the topmost layer and in the (111) oriented micro facets are Pt-enriched. Due to the fact that the unreconstructed Pt25Co75(110) surface is known to exhibit an almost pure Co top layer the Pt segregation reported in this study is undoubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by LEIS experiments performed on the hot Pt90Co10(110) surface, in which simultaneously temperature induced changes of the surface composition and qualitative changes in the surface structure are monitored. The measured low energy ion spectra do not only reproduce the calculated first-layer composition of the LEED analysis, they also show a less pronounced Pt segregation at temperatures around 750°C, and eventually a reversed Pt segregation above 750°C, i.e. Co enrichment of the Pt90Co10(110) surface with respect to the bulk concentration. We find a clear correlation between the thermal deconstruction and the surface composition. The striking segregation reversal during temperature variation is attributed to the high excess value of the mixing enthalpy, which implies a structure-dominated segregation behavior.
DA  - 2000/05/10/
PY  - 2000
DO  - 10.1016/S0039-6028(00)00351-4
VL  - 453
IS  - 1-3
SP  - 214
EP  - 224
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - STM and STS of bulk electron scattering by subsurface objects
AU  - Schmid, M.
AU  - Crampin, S.
AU  - Varga, P.
T2  - Journal of Electron Spectroscopy and Related Phenomena
AB  - Results obtained on aluminium and copper surfaces are used to demonstrate the ability of scanning tunnelling microscopy (STM) and spectroscopy (STS) to detect subsurface structures through their influence on the electronic structure. Subsurface Ar bubbles in Al lead to a quantum well bounded by the outer surface and the top of the bubbles. Using z(V) spectroscopy, where the STM feedback loop keeps the current constant while ramping the voltage, it is possible to detect the energy steps between the quantum well states; combined with a one-dimensional model employing a realistic potential for the electrons, this allows an estimate of the thickness of the quantum well, i.e., the depth of the bubbles. Simulated STM images calculated with a three-dimensional scattering theory reproduce many details of the interference pattern, and confirm the size and geometry of the bubbles. Interference patterns attributed to subsurface scatterers have been also detected on Cu(111) and Cu(100). We propose that the patterns observed on Cu(111) are due to focusing of electron waves in certain crystallographic directions, whereas those on Cu(100) are unexplained up to now.
DA  - 2000/08//
PY  - 2000
DO  - 10.1016/S0368-2048(00)00108-0
VL  - 109
IS  - 1-2
SP  - 71
EP  - 84
J2  - J. Electron Spectrosc. Relat. Phenom.
SN  - 0368-2048
ER  - 

TY  - JOUR
TI  - Segregation and ordering at Fe1-xAlx(100) surfaces - a model case for binary alloys
AU  - Blum, V.
AU  - Hammer, L.
AU  - Meier, W.
AU  - Heinz, K.
AU  - Schmid, M.
AU  - Lundgren, E.
AU  - Varga, P.
T2  - Surface Science
AB  - The geometrical structure, chemical order and composition of (100) oriented surfaces of the binary alloy system Fe_1-xAl_x were investigated in the Fe-rich regime (x = 0.03, 0.15, and 0.30) using quantitative low-energy electron diffraction. Low-energy He+ ion scattering and scanning tunneling microscopy were additionally employed to characterize the x = 0.15 sample. The equilibrium structures developing with increasing bulk Al content can be consistently explained by the interplay between Al surface segregation and ordering processes which are controlled by atomic interactions similar to those in the bulk. These interactions divide the process of Al segregation to the very surface into two steps whereby Al atoms occupy sites of two different sublattices of c(2x2) periodicity with different probability. While one sublattice is already completely filled at low bulk Al concentration, the other sublattice fills only gradually with increasing bulk Al content. The local order in deeper layers is consistent with the bulk phase diagram.
DA  - 2001/03/01/
PY  - 2001
DO  - 10.1016/S0039-6028(00)00987-0
VL  - 474
IS  - 1-3
SP  - 81
EP  - 97
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Exchange processes in interlayer diffusion – kinks, corners and the growth mode
AU  - Schmid, M.
AU  - Lundgren, E.
AU  - Leonardelli, G.
AU  - Hammerschmid, A.
AU  - Stanka, B.
AU  - Varga, P.
T2  - Applied Physics A: Materials Science & Processing
AB  - Abstract.   Interlayer diffusion, i.e. mass transport between different terraces, is known to be an essential process for obtaining layer-by-layer growth, avoiding formation of three-dimensional (3D) islands when growing thin films. We present experimental results for the growth of cobalt on Pt(111), which demonstrate the importance of kinks and corners for interlayer diffusion. We show that Co grows two-dimensionally as long as strain caused by the Pt-Co interface keeps the step edges rough, with a high kink density, and then transforms to 3D growth with straight steps. The results for growth with adsorbed carbon monoxide show that CO acts as a surfactant, causing two-dimensional growth unless heterogeneous nucleation occurs. Again, this process is related to roughening of the steps, being a new mechanism for the action of a surfactant. A scanning tunneling microscopy study at the atomic scale confirms the fact that step descent happens only at kinks and (concave) corners, and in conjunction with simulations allows us to identify some of the relevant atomic-exchange processes. We finally argue that the dependence of the growth mode on the step morphology, together with straightening of the steps by step–step interaction, can lead to an instability of the growth mode.
DA  - 2001/04/15/
PY  - 2001
DO  - 10.1007/s003390100753
VL  - 72
IS  - 4
SP  - 405
EP  - 412
J2  - Appl. Phys. A
ER  - 

TY  - JOUR
TI  - Nucleation of bcc Iron in Ultrathin fcc Films
AU  - Biedermann, Albert
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Physical Review Letters
AB  - Needle-shaped bcc nucleation centers in fcc films of Fe on Cu(100) are observed by scanning tunneling microscopy. They form virtually without mass transfer and nearly under conservation of volume, which causes a large strain within the nascent bcc grain. The corresponding strain energy almost equals the gain in structural energy, rendering the bcc nucleation very sensitive to any effect influencing this subtle balance. We suggest that modifying the film by straining, alloying, or surface adsorption may inhibit the bcc nucleation and lead to thick metastable fcc films.
DA  - 2001/01/15/
PY  - 2001
DO  - 10.1103/PhysRevLett.86.464
VL  - 86
IS  - 3
SP  - 464
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Scanning tunneling spectroscopy on clean and contaminated V(001)
AU  - Bischoff, M. M. J.
AU  - Konvicka, C.
AU  - Quinn, A. J.
AU  - Schmid, M.
AU  - Redinger, J.
AU  - Podloucky, R.
AU  - Varga, P.
AU  - van Kempen, H.
T2  - Surface Science
AB  - Scanning tunneling spectroscopy (STS) measurements on clean V(001), carbon-covered V(001) and the oxygen-induced V(001) (1x5) reconstruction are reported. The clean V(001) surface shows a strong surface state 0.03 eV below the Fermi level. Isolated impurities shift the surface state 0.05 eV upwards in energy and broaden the peak observed in dI/dV. No significant influence of monoatomic steps on the surface state could be observed. For tunneling resistances down to about 1 MOhm the surface state is unaffected by the tip of the STM. A surface state is detected around +0.75 eV in small c(2x2) patches which are observed at higher carbon (and oxygen) coverages. The oxygen induced (1x5) reconstruction of V(001) shows a peak at similar energy (+0.63 eV) in the areas with O and C atoms in fourfold hollow sites and a peak around +0.91 eV above the rows of bridge-site oxygen. Ab-initio band structure calculations confirm the existence of a surface state of dz2 symmetry with an energy close to that observed experimentally on clean V(001). This agreement provides strong evidence that the V(001) surface is not magnetic (at least at room temperature) as predicted by the calculations. We also compare the experimentally observed peak shifts on the carbon and oxygen covered surfaces with calculational results for carbon-covered geometries.
DA  - 2002/07//
PY  - 2002
DO  - 10.1016/S0039-6028(02)01783-1
VL  - 513
IS  - 1
SP  - 9
EP  - 25
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Geometry of the valence transition induced surface reconstruction of Sm(0001)
AU  - Lundgren, E.
AU  - Andersen, J. N.
AU  - Nyholm, R.
AU  - Torrelles, X.
AU  - Rius, J.
AU  - Delin, A.
AU  - Grechnev, A.
AU  - Eriksson, O.
AU  - Konvicka, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.
DA  - 2002/03/18/
PY  - 2002
DO  - 10.1103/PhysRevLett.88.136102
VL  - 88
IS  - 13
SP  - 136102
J2  - Phys. Rev. Lett.
ER  - 

TY  - CHAP
TI  - Segregation and surface chemical ordering-an experimental view on the atomic scale
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Alloys and Alloy Surfaces
A2  - Woodruff, D.P.
T3  - The Chemical Physics of Solid Surfaces
AB  - Scanning tunneling microscopy with atomic resolution and chemical contrast offers unique possibilities in studying segregation and chemical ordering of alloy surfaces. Chemical contrast in STM can have three different reasons, (a) true topographic effects, (b) different density of states of the alloy constituents, and (c) tip-sample interaction depending on the chemical identity of the atom imaged.
The composition and chemical order on surfaces is determined by an interplay of ordering and segregation. If the chemical ordering is weak, segregation is mainly determined by the differences in surface energies. On the surfaces of alloys with a strong tendency towards ordering, segregation and ordering can either compete, e.g., in cases where preserving the bulk chemical order requires the surface to assume the bulk composition, or ordering can enhance segregation, e.g. in cases where bulk ordering allows a pure-metal termination. In cases where only short-range chemical ordering occurs at the surface, the trends in surface chemical ordering were found to correspond well to the ordering tendency observed in the bulk.
We also show that the study of surface composition and chemical order is essential for understanding adsorption on alloy surface. Even weak ordering can lead to significant changes in the availability of some adsorption sites. We could also obtain STM images of an alloy surface with chemical contrast and images of adsorbates in the same surface area, revealing the chemical structure of adsorption sites. We could thereby demonstrate the ligand effect, i.e., the dependence of adsorption strength on the atoms neighbouring an adsorption site.
DA  - 2002///
PY  - 2002
VL  - Volume 10
SP  - 118
EP  - 151
PB  - Elsevier
SN  - 1571-0785
ER  - 

TY  - JOUR
TI  - Two-dimensional alloy of immiscible metals: Single and binary monolayer films of Pb and Sn on Rh(111)
AU  - Yuhara, J.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - The single and binary metal films of Pb and Sn on Rh(111) have been studied at room temperature by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. Both Pb and Sn are mobile at low coverage and form commensurate overlayers of (4×4)-Pb and c(2×4)-Sn, respectively. From atomically resolved STM images, the atomic arrangements of (4×4)-Pb and c(2×4)-Sn have been identified to be hexagonal and rectangular structures, respectively. With increasing coverage, both commensurate phases change into incommensurate phases followed by island formation (Stranski-Krastanov growth). This shows that Rh(111) is static and inert enough to support two-dimensional (2D) phases of Pb and/or Sn without alloying at room temperature. The Pb-Sn bimetallic film on Rh(111) forms an ordered 2D alloy of PbSn3 with an incommensurate structure close to (sqrt[7]×sqrt[7]), contrary to the immiscibility of Pb and Sn in the bulk. From atomically resolved STM images, the atomic arrangement of this (sqrt[7]×sqrt[7])-(Pb,Sn) structure has been determined.
DA  - 2003/05/12/
PY  - 2003
DO  - 10.1103/PhysRevB.67.195407
VL  - 67
IS  - 19
SP  - 195407
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Reconstruction of the clean and H covered "magnetic live surface layer" of Fe films grown on Cu(100)
AU  - Biedermann, Albert
AU  - Tscheliessnig, Rupert
AU  - Klein, Christof
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Surface Science
AB  - The surface of 6-7 monolayer thick fcc Fe films grown at room temperature on a Cu(100) substrate is characterized by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The STM images show a p4g(2 × 2) structure at 5 and 80 K, but not at 300 K. LEED, however, indicates an expansion of the interlayer distance and lateral distortions of similar magnitude both at 150 K and at 300 K. No evidence for a significant change of the surface structure is detected by a LEED spot profile analysis between 150 and 300 K. We attribute the apparent absence of the reconstruction in the STM images at 300 K to surface dynamics caused by domain boundary motion. The particular surface structure with bond angles and distances similar to bcc Fe suggests a driving force of the reconstruction which is similar to that operative in the fcc-to-bcc transition of bulk Fe. Dosing less than 5 L H2 decorates the p4g(2 × 2) surface reconstruction, while higher hydrogen doses transform the surface reconstruction to p(2 × 1).
DA  - 2004/08/10/
PY  - 2004
DO  - 10.1016/j.susc.2004.06.150
VL  - 563
IS  - 1-3
SP  - 110
EP  - 126
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Growth of Ce on Rh(111)
AU  - Napetschnig, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - We have studied the growth of cerium films on Rh(111) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh3. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (111)-plane of CeRh3, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh3(100). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.
DA  - 2004/05/01/
PY  - 2004
DO  - 10.1016/j.susc.2004.03.006
VL  - 556
IS  - 1
SP  - 1
EP  - 10
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Local atomic structure of ultra-thin Fe films grown on Cu(100)
AU  - Biedermann, A.
AU  - Tscheliessnig, R.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Applied Physics A: Materials Science & Processing
AB  - Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering.
DA  - 2004/03/01/
PY  - 2004
DO  - 10.1007/s00339-003-2435-7
VL  - 78
IS  - 6
SP  - 807
EP  - 816
J2  - Appl. Phys. A
ER  - 

TY  - JOUR
TI  - Growth of ultrathin Fe films on Cu(111) by pulsed laser deposition
AU  - Schiechl, H.
AU  - Rauchbauer, G.
AU  - Biedermann, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - Ultrathin Fe films have been grown on Cu(111) by pulsed laser deposition (PLD) and thermal deposition (TD) and analyzed by scanning tunneling microscopy (STM) and low-energy ion scattering (LEIS). PLD was performed using nanosecond pulses of a Nd:YAG laser providing three different wavelengths. Compared to the widely investigated Fe films grown by thermal deposition on Cu(111), which exhibit bilayer and multi-layer (3D) island growth at low coverage, PLD-grown films show enhanced layer-by-layer growth when using sufficiently high laser fluence. However, by increasing the laser spot size on the Fe target and adjusting the laser power to achieve an unchanged deposition rate, resulting in a lower laser fluence, we observe bilayer growth reminiscent of the TD films. Using STM and LEIS, we observe an increasing number of Fe atoms implanted into the Cu substrate with increasing laser fluence and consequently mixing of Fe and Cu in the layer-by-layer films even at a preparation temperature of 200 K. We therefore suggest that the reason for layer-by-layer growth in this system is not the high instantaneous deposition rate of PLD, but the implantation of Fe atoms due to their higher kinetic energy at higher fluences.
DA  - 2005/12/01/
PY  - 2005
DO  - 10.1016/j.susc.2005.07.016
VL  - 594
IS  - 1-3
SP  - 120
EP  - 131
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Thin films of Co on Pt(111): Strain relaxation and growth
AU  - Lundgren, E.
AU  - Stanka, B.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - The growth, structure and morphology of thin Co layers with a thickness ranging from 1 to 15 monolayers deposited at room temperature on Pt(111) have been studied by the use of scanning tunneling microscopy. We demonstrate that the first Co layer grows preferably in the Pt fcc lattice sites, with a high density of defects due to the lattice mismatch. The second Co layer is found to exhibit a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The growth of thin Co films is observed to be mostly in terms of flat layers (two dimensional) up to a Co coverage of about 3.5 ML. At higher coverages, we find that the Co grows in (three dimensional) islands and we show that the growth is characterized by a mainly twinned fcc-like stacking. We argue that the reason for the two dimensional growth mode at lower Co coverages is due to the strained interface between the Co overlayers and the Pt(111) surface, resulting in a large number of kinks and corners which facilitate interlayer diffusion. For higher coverage such sites become less common, due to the decreasing influence of the strained interface, resulting in no interlayer diffusion leading to a three dimensional growth mode. The implications by these observations on the magnetic properties of the Co/Pt(111) interface system are discussed.
DA  - 2000/07/15/
PY  - 2000
DO  - 10.1103/PhysRevB.62.2843
VL  - 62
IS  - 4
SP  - 2843
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions
AU  - Buchsbaum, A.
AU  - Rauchbauer, G.
AU  - Varga, P.
AU  - Schmid, M.
T2  - Review of Scientific Instruments
AB  - The growth of ultrathin films, deposited by laser ablation, crucially depends on the energy of the ablated species. Therefore, a time-of-flight (TOF) spectrometer has been constructed and measurements have been carried out in order to determine the energy distribution of laser-ablated Fe and Pt atoms and ions in the plasma created by nanosecond pulses of a frequency-doubled neodymium doped yttrium aluminum garnet laser. The experiments have been performed in ultrahigh vacuum at relatively low laser power. For measuring the spectra of the neutrals, a cross-beam electron source for postionization and electric as well as magnetic fields for repelling the ions are employed. Nevertheless, measurements of neutral particles are restricted to low plasma densities due to electrostatic shielding within the plasma, leading to an inefficient deflection of charged particles by electrostatic and magnetic fields. Test measurements have been performed by utilizing the TOF spectrometer as a pressure gauge and also by chopping the electron beam, running the TOF spectrometer as a residual gas mass spectrometer. The spectra of the laser-ablated plasmas have shown plasma conditions with a Debye length of approximately 10^−4  m, densities of 10^15–10^16  m^−3 and ion energies up to 150  eV. Neutral spectra have shown an unexpectedly low fraction of neutrals (10^−3–10^−4) and hyperthermal energies up to several 10 eV, possibly contributed by recombination of ions and electrons in the plasma. Even though gas spectra had demonstrated the expected sensitivity of the TOF spectrometer for low-energy neutrals, no thermally evaporated neutral atoms could be found.
DA  - 2008/04/04/
PY  - 2008
DO  - 10.1063/1.2901607
VL  - 79
IS  - 4
SP  - 043301
J2  - Rev. Sci. Instrum.
ER  - 

TY  - JOUR
TI  - Sputtering of Au and Al2O3 surfaces by slow highly charged ions
AU  - Hayderer, G.
AU  - Cernusca, S.
AU  - Hoffmann, V.
AU  - Niemann, D.
AU  - Stolterfoht, N.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Winter, HP.
AU  - Aumayr, F.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - A quartz crystal microbalance technique was used for measuring total sputtering yields for polycrystalline Au and Al2O3 under impact of slow (100-1500 eV) multiply charged Ar and Xe ions. Up to the highest charge states investigated (Xe25+), the sputter yield for the Au target remains independent on the projectile charge state and can be well described by kinetic sputtering only. For Al2O3, on the contrary, a dramatic increase in total sputtering yield with increasing projectile charge state was found, showing that in this case potential sputtering (PS), i.e., sputtering due to the potential energy of the projectile, clearly dominates over kinetically induced sputtering. Results can be explained within the defect-mediated desorption model of PS.
DA  - 2001/08//
PY  - 2001
DO  - 10.1016/S0168-583X(01)00668-1
VL  - 182
IS  - 1-4
SP  - 143
EP  - 147
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - Sputter yields of insulators bombarded with hyperthermal multiply charged ions
AU  - Varga, P.
AU  - Neidhart, T.
AU  - Sporn, M.
AU  - Libiseller, G.
AU  - Schmid, M.
AU  - Aumayr, F.
AU  - Winter, H. P.
T2  - Physica Scripta
AB  - The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bombarded with hyperthermal highly charged Ar^q+ ions (q = 1-9) has been measured. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 potential sputtering (enhancement of the sputter yield with increasing charge state of the primary ion) has been observed. All other targets showed normal collision induced sputtering. From that result it is obvious that the mechanisms for sputtering can not be explained by the Coulomb explosion model, because in this model insulators like MgO and semiconductors like Si and GaAs should also show charge state dependence of the sputtering yield. Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excitations in the valence band are localized by formation of self trapped excitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and below the surface causes the formation of STE and/or STH. Therefore the potential sputtering can be explained as a defect mediated sputtering process which is well known in electron stimulated desorption (ESD) where the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neutral cations are either evaporated (as it is the case for the alkali halides) or have to be removed by momentum transfer by the impinging projectiles. Therefore it is very likely that in the case of SiO2 for very low impact energy mainly only oxygen is enhanced sputtered, the surface is enriched in Si and the potential sputtering effect decreases with increasing ion dose.
DA  - 1997///
PY  - 1997
DO  - 10.1088/0031-8949/1997/T73/100
VL  - T73
SP  - 307
EP  - 310
J2  - Phys. Scripta
SN  - 1402-4896
ER  - 

TY  - JOUR
TI  - Metastable surface oxide on CoGa(100): Structure and stability
AU  - Vlad, A.
AU  - Stierle, A.
AU  - Marsman, M.
AU  - Kresse, G.
AU  - Costina, I.
AU  - Dosch, H.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - We investigated the structure and formation of a surface oxide and bulk β-Ga2O3 on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a p2mm symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential ∼1.6 eV above the stability limit for bulk β-Ga2O3. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.
DA  - 2010/03/02/
PY  - 2010
DO  - 10.1103/PhysRevB.81.115402
VL  - 81
IS  - 11
SP  - 115402
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Highly ordered Pd, Fe, and Co clusters on alumina on Ni3Al(111)
AU  - Buchsbaum, Andreas
AU  - De Santis, Maurizio
AU  - Tolentino, Helio C. N.
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Physical Review B
AB  - Template-mediated growth of metals has attracted much interest due to the remarkable magnetic and catalytic properties of clusters in the nanometer range and provides the opportunity to grow clusters with narrow size distributions. We have grown well-ordered Fe and Co clusters on the ultrathin aluminum oxide on Ni3Al(111), a template with a 4.1 nm lattice. The structure of the ≈0.5-nm-thick oxide film exhibits holes reaching down to the metal substrate at the corners of the (√67×√67)R12.2° unit cell. Pd atoms trapped in these corner holes create metallic nucleation sites where Fe as well as Co clusters can nucleate and form a well-ordered hexagonal arrangement on the oxide nanomesh. We have studied these Fe and Co clusters and applied different methods such as scanning tunneling microscopy and surface x-ray diffraction to determine the morphology and crystallography of the clusters. For Fe, we found cluster growth in either bcc[110] or bcc[100] direction, depending on the deposition temperature and for Co we found close-packed planes on top of the clusters and random stacking of close-packed planes. Pd clusters grow with fcc(111) orientation.
DA  - 2010/03/11/
PY  - 2010
DO  - 10.1103/PhysRevB.81.115420
VL  - 81
IS  - 11
SP  - 115420
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Mismatch dislocations caused by preferential sputtering of a platinum-nickel alloy surface
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Stadler, H.
AU  - Slama, C.
AU  - Varga, P.
T2  - Applied Physics A: Materials Science & Processing
AB  - Preferential sputtering and recoil mixing of a Pt25Ni75(111) single crystal surface leads to platinum enrichment in the upper monolayers, thereby increasing the lattice constant in these layers. This results in subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. While the subsurface dislocations are only visible as shallow ditches in STM topographs, the Burgers vectors of the dislocation system can be determined by means of atomically resolved images of dislocations reaching the surface. A comparison with simulations of lattice relaxation using embedded-atom potentials shows good agreement with STM data and further allows the determination of the thickness of the Pt enrichment. We have estimated the Pt concentration in these layers from the dislocation density and studied the annealing behaviour of the surface.
DA  - 1992/11/01/
PY  - 1992
DO  - 10.1007/BF00348334
VL  - 55
IS  - 5
SP  - 468
EP  - 475
J2  - Appl. Phys. A
ER  - 

TY  - JOUR
TI  - Embedded-atom method calculations applied to surface segregation of Pt-Ni single crystals
AU  - Stadler, H.
AU  - Hofer, W.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The embedded-atom method, a model for the calculation of various crystal and alloy properties, has also been applied to the study of surface segregation phenomena. We employ this formalism for calculating surface segregation on PtxNi1-x single-crystal low-index faces which are known to show an orientation-dependent segregation behaviour. Pt enrichment in the topmost layer and alternating segregation profiles for the (100) and (111) surfaces are found to be in accordance with experimental data. The results for the (110) surface, showing a different behaviour, are discussed. In addition, surface relaxation is calculated and found to correspond with experimental results.
DA  - 1993/05/10/
PY  - 1993
DO  - 10.1016/0039-6028(93)90804-S
VL  - 287-288
IS  - Part 1
SP  - 366
EP  - 370
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Intrinsic defects on a TiO2(110)(1×1) surface and their reaction with oxygen: a scanning tunneling microscopy study
AU  - Diebold, Ulrike
AU  - Lehman, Jeremiah
AU  - Mahmoud, Talib
AU  - Kuhn, Markus
AU  - Leonardelli, Georg
AU  - Hebenstreit, Wilhelm
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Surface Science
AB  - We report a scanning tunneling microscopy (STM) study of the rutile TiO2(110) surface. The surface was prepared by sputtering and annealing in an ultrahigh vacuum (UHV). After annealing to 1100 K in UHV, a (1×1) surface with a terrace width of ~100 Å is obtained. The terraces are separated by monoatomic step edges running predominantly parallel to <001> and type directions. Approximately half of the <001>-type steps have a kinked appearance that is attributed to a (4×1)-reconstructed step edge. Atomic models for autocompensated step edges are presented. Oxygen vacancies (point defects) in the bridging oxygen rows are created by the high-temperature anneal in UHV. In STM images, these oxygen vacancies appear as bright features centered on dark rows. Their density is 7±3% per surface unit cell and is reduced upon exposure to molecular oxygen at room temperature. Dark features on bright rows are also seen; these are not affected by molecular oxygen and are tentatively assigned to subsurface defects. Hydroxyl groups from spurious water in the oxygen gas stream are observed to adsorb dissociatively at step edges and on the in-plane Ti rows on the terraces. The appearance of the surface oxygen vacancies depends on the state of the STM tip; asymmetric tips skew the appearance of the point defects and may even render images where they are invisible. Tip changes occur frequently, especially when the surface has been exposed to oxygen, and may lead to images that are hard to interpret. The "normal" tip state where the vacancies appear as bright spots connecting bright rows can be regained reproducibly by scanning with a high (up to +10 V) tip voltage; the tip is then possibly covered with substrate material. The oxygen vacancies show strong interactions with the STM tip, i.e. tip-induced oxidation and mobility. These interactions depend strongly on the state of the tip, and are enhanced by the presence of oxygen in the ambient. A model for the tip-induced oxidation is presented where oxygen atoms hop between the tip and sample to explain these effects.
DA  - 1998/08/11/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00356-2
VL  - 411
IS  - 1-2
SP  - 137
EP  - 153
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Hochauflösende Rastertunnelmikroskopie unterscheidet Atome
AU  - Varga, Peter
AU  - Schmid, Michael
AU  - Redinger, Josef
T2  - Physik in unserer Zeit
AB  - Rastertunnelmikroskopie mit atomarer Auflösung ermöglicht die chemische Analyse an Oberflächen von metallischen Legierungen und ultradünnen Filmen auf atomarer Skala und trägt damit zum besseren Verständnis von katalytischen Prozessen bei.
DA  - 2000///
PY  - 2000
DO  - 10.1002/1521-3943(200009)31:5<215::AID-PIUZ215>3.0.CO;2-0
VL  - 31
IS  - 5
SP  - 215
EP  - 221
J2  - Physik in unserer Zeit
LA  - German
ER  - 

TY  - JOUR
TI  - On the role of kinks and strain in heteroepitaxial growth: An STM study
AU  - Lundgren, E.
AU  - Schmid, M.
AU  - Leonardelli, G.
AU  - Hammerschmid, A.
AU  - Stanka, B.
AU  - Varga, P.
T2  - Surface Review and Letters
AB  - Interlayer diffusion of Co over steps of vacancy islands on the Pt(111) surface as studied by scanning tunneling microscopy is presented. It is demonstrated that Co atoms descend Pt steps by an exchange diffusion process at the step edge with the Pt atoms. Further, the exchange diffusion process is observed to occur at the corners (kinks) of the vacancy islands. The importance of kinks concerning whether the growth mode of a heteropitaxial film is two-dimensional or three-dimensional is demonstrated for the case of thin Co films on Pt(111). We argue that the strain in the Co film is to a large extent responsible for the kink formation.
DA  - 2000///
PY  - 2000
DO  - 10.1142/S0218625X00000750
VL  - 7
IS  - 5-6
SP  - 673
EP  - 677
J2  - Surf. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Segregated Si on Fe96.5Si3.5(110): Domain-wall structures in a two-dimensional alloy
AU  - Biedermann, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - The surface structures formed by segregated silicon on a bcc Fe96.5Si3.5(110) surface have been studied by scanning tunneling microscopy (STM). Additionally Auger-electron spectroscopy and low-energy ion-scattering spectroscopy have been used to obtain the chemical and structural information necessary to decide between possible configurations suggested by the STM images. Near the expected saturation coverage a substitutional c(1×3)Si superstructure weakly disordered by 〈110〉 aligned domain boundaries was observed. Their arrangement is equivalent to the incommensurate domain-wall structures of chemisorption systems like H/Fe(110), which allow a continuous increase of the surface coverage by successive insertion of equidistantly arranged heavy domain walls. Our observations suggest a possible increase of the silicon coverage above the 33.3% of the perfect c(1×3) structure at least up to 40% corresponding to a c(1×5)Si commensurate domain-wall structure. Correlation analysis of the disordered phase at 15% coverage, obtained by short annealing, allows a qualitative determination of the interaction forces among individual silicon atoms on the surface, yielding attraction of third-nearest neighbors and repulsion of nearest neighbors.
DA  - 1994/12/15/
PY  - 1994
DO  - 10.1103/PhysRevB.50.17518
VL  - 50
IS  - 23
SP  - 17518
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - An STM study of the step structure of Pb(110) and Pb(111)
AU  - Speller, S.
AU  - Heiland, W.
AU  - Biedermann, A.
AU  - Platzgummer, E.
AU  - Nagl, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The (110) and (111) surfaces of lead are investigated by scanning tunneling microscopy. They are atomically resolved with corrugations of 0.1 Å and 0.6 Å respectively. The STM images allow conclusions about the motion of the atoms at step edges and step step interaction. At room temperature the steps on both surfaces are below the roughening transition. The influence of impurities and tip surface interaction on the step fluctuation is discussed.
DA  - 1995/07/01/
PY  - 1995
DO  - 10.1016/0039-6028(95)00257-X
VL  - 331-333
IS  - Part 2
SP  - 1056
EP  - 1061
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Chemically resolved STM on a PtRh(100) surface
AU  - Wouda, P. T.
AU  - Nieuwenhuys, B. E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - Scanning tunnelling microscopy on PtRh(100) (molar bulk composition 1 : 1) has revealed the possibility of direct determination of the surface of this system. During measurements at low tunnelling resistance (<500 kΩ), the Pt and Rh atoms appear with a clearly observable apparent height difference of more than 20 pm. No long range ordering has been found. Variation of the sample preparation method and comparison of STM and Auger electron spectroscopy measurements led to the conclusions that there is a preferential surface segregation of platinum, that rhodium atoms are the ones with the highest apparent height, and that there is limited tendency of clustering on the surface. Furthermore, it was found that platinum atoms preferentially populate the step edges on this crystal surface.
DA  - 1996/07//
PY  - 1996
DO  - 10.1016/0039-6028(96)00022-2
VL  - 359
IS  - 1-3
SP  - 17
EP  - 22
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Scanning tunneling microscopy studies of niobium carbide (100) and (110) surfaces
AU  - Tsong, R. M.
AU  - Schmid, M.
AU  - Nagl, C.
AU  - Varga, P.
AU  - Davis, R. F.
AU  - Tsong, I. S. T.
T2  - Surface Science
AB  - Scanning tunneling microscopy (STM) studies were conducted on the surfaces of NbC0.75(100) and NbC0.865(110) single crystals after in situ cleaning treatments of sputtering and annealing cycles. STM images show atom-resolved structures of both surfaces. On NbC0.75(100), localized areas of a square (1 × 1) structure were observed, together with regions of hexagonal structure, indicative of a coexisting surface phase, possibly that of Nb4C3-x. On the NbC0.865(110) surface, a ridge-and-valley grating structure consisting of both (4 × 1) and (5 × 1) geometries was observed over large areas. The nanometer-scale faceting phenomenon may be common to the (110) surfaces of most transition-metal carbides.
DA  - 1996/10/10/
PY  - 1996
DO  - 10.1016/0039-6028(96)00804-7
VL  - 366
IS  - 1
SP  - 85
EP  - 92
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Surface structure of the missing-row reconstruction of VC0.8(110): a scanning tunneling microscopy analysis
AU  - Gauthier, Y.
AU  - Schmid, M.
AU  - Hebenstreit, W.
AU  - Varga, P.
T2  - Surface Science
AB  - Scanning tunnelling microscopy (STM) was used to study the (110) surface of a VC0.8 sample. The surface shows a missing-row reconstruction, i.e., a grating structure with ridges and valleys oriented along the [001] direction and (100) and (010) facets. We did not find unreconstructed (110) terraces. The regular spacing of the ridges corresponds to a periodicity of (3 × 1) or (4 × 1), depending on preparation, presumably related to different concentrations of carbon vacancies. In the STM images, we can also observe apparent pairing of atoms in the rows, leading to the larger c(6 × 2) and (4 × 2) superstructure cells, which also show up in LEED. We attribute these additional periodicities to ordering of carbon vacancies in the surface rows.
DA  - 2003/12/20/
PY  - 2003
DO  - 10.1016/j.susc.2003.10.016
VL  - 547
IS  - 3
SP  - 394
EP  - 402
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Atomic structure of an Al-Co-Ni decagonal quasicrystalline surface
AU  - Yuhara, J.
AU  - Klikovits, J.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Yokoyama, Y.
AU  - Shishido, T.
AU  - Soda, K.
T2  - Physical Review B
AB  - We have analyzed the structure and composition of the first layer of an Al72Co16Ni12 tenfold surface by means of scanning tunneling microscopy (STM), ion scattering spectroscopy (ISS), and Auger electron spectroscopy (AES). High-resolution STM images reveal local structures that have decagonal symmetry in addition to the usual pentagonal symmetry of the surface. This quasicrystal surface resembles a random tiling instead of an ideal quasiperiodic tiling. After annealing at 1100 K , the total surface atomic density found by ISS is (9±1)×10^14 cm−2 . The surface densities of Al and TM (transition metal, i.e., Co and Ni) are determined as (8±1)×10^14 cm−2 and (1.0±0.2)×10^14 cm−2 , respectively from ISS, indicating a similar density of Al and much lower density of the TM atoms in the surface layer than in a truncated bulk. The Al surface atomic density agrees well with the number of corrugation maxima in the STM images. A model of the arrangement of the Al atoms in the top layer is presented. Scanning tunneling spectroscopy (STS) is performed to study the local electronic structure. The STS spectrum at the corrugation maxima is similar to that at the corrugation minima. A few ≈0.12 nm high protrusions in the STM images are attributed to local oxide clusters due to their STS spectra different from the corrugation maxima and through in situ STM observations during exposure to O2 gas at 2×10^−6 Pa at RT.
DA  - 2004/07/07/
PY  - 2004
DO  - 10.1103/PhysRevB.70.024203
VL  - 70
IS  - 2
SP  - 024203
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Lattice mismatch dislocations in a preferentially sputtered alloy studied by scanning tunneling microscopy
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Stadler, H.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - Scanning tunneling microscopy (STM) on a sputtered and annealed Pt25Ni75(111) single crystal reveals a network of subsurface lattice mismatch dislocations caused by platinum enrichment due to preferential sputtering and recoil mixing. Atomically resolved STM topographs are compared with simulations of these dislocations using embedded atom potentials. This allows one to estimate the depth of the dislocations, and thus the thickness of Pt enrichment, which is three monolayers on the 500 eV Xe+ sputtered and five monolayers on the Ar+ sputtered surface, compatible with the depth of radiation damage.
DA  - 1992///
PY  - 1992
DO  - 10.1103/PhysRevLett.69.925
VL  - 69
IS  - 6
SP  - 925
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Surface stress, surface elasticity, and the size effect in surface segregation
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
AU  - Stoltze, P.
AU  - Jacobsen, K. W.
AU  - Nørskov, J. K.
T2  - Physical Review B
AB  - Surface stress and surface elasticity of low-index fcc surfaces have been studied using effective-medium theory potentials. In addition to total-energy calculations giving stress components and elastic data for the surface as a whole, the use of artificial atoms with modified size allows us to probe the stress and elasticity of individual layers. This method of artificial atoms provides a direct way to study the contribution of atomic size to segregation in alloys as well as the driving force of reconstructions driven by surface stress. As an example, we give a qualitative explanation of the face-dependent segregation of Pt-Ni alloys. We also compare results of these atomic-scale calculations with continuum elasticity.
DA  - 1995/04/15/
PY  - 1995
DO  - 10.1103/PhysRevB.51.10937
VL  - 51
IS  - 16
SP  - 10937
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Deconvolution of STM images using entropy as a regularization functional
AU  - Böhmig, S.
AU  - Schmid, M.
AU  - Störi, H.
T2  - Fresenius' Journal of Analytical Chemistry
AB  - The Maximum Entropy approach is applied to restore and sharpen scanning tunneling microscopy (STM) images with atomic resolution. Based on the STM theory of Tersoff and Hamann the process of data acquisition can be approximated by the convolution of a localised atomic density of states (i.e. narrow spots in the reconstruction) of the sample and a Gaussian resolution function which limits the resolution. In STM practice a good and robust estimation of the atomic core positions is necessary for different reasons, such as to be able to calculate the characteristics of the atomic lattice or to study non-periodicities.
DA  - 1995/10/25/
PY  - 1995
DO  - 10.1007/s0021653530439
VL  - 353
IS  - 3
SP  - 439
EP  - 442
J2  - Fresenius J. Anal. Chem.
ER  - 

TY  - JOUR
TI  - Surface composition of Pt25Ni75(111) investigated by ISS and STM
AU  - Weigand, P.
AU  - Nagl, C.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Fresenius' Journal of Analytical Chemistry
AB  - Ion scattering spectroscopy results show Pt enrichment in the topmost atomic layers of a Pt25Ni75(111) single crystal due to preferential sputtering. The increased lattice constant caused thereby leads to subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. Agreement is found for the Pt concentration estimated from the density of dislocations and the results obtained by ISS. Thermal annealing induces further segregation of Pt in the topmost atomic layer. The composition of the subsurface layers has been studied and a strong dependence on the annealing temperature has been found. The observed Pt enrichment in the first monolayer for the thermodynamic equilibrium state agrees well with a thermodynamic theory.
DA  - 1993/01/01/
PY  - 1993
DO  - 10.1007/BF00321431
VL  - 346
IS  - 1
SP  - 281
EP  - 283
J2  - Fresenius J. Anal. Chem.
ER  - 

TY  - JOUR
TI  - Preferential sputtering of Pt-Ni alloy single crystals studied by scanning tunneling microscopy
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Slama, C.
AU  - Stadier, H.
AU  - Weigand, P.
AU  - Varga, P.
T2  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
AB  - Due to its composition, the altered layer of preferentially sputtered alloys has a lattice constant different from that of the bulk. This lattice mismatch can lead to the formation of dislocations or reconstructions, which have been studied on different crystallographic faces. While a subsurface dislocation network exists on the (111) plane, parallel dislocations are found below the (110) surface. The (100) surface exhibits a shifted-row reconstruction, which is tentatively attributed to the stress induced by lattice mismatch between the bulk and the Pt-enriched surface. The annealing process of the sputtered Pt25Ni75(111) surface is studied in detail by evaluation of the mismatch dislocations and low energy ion scattering data.
DA  - 1993/07/02/
PY  - 1993
DO  - 10.1016/0168-583X(93)96028-B
VL  - 82
IS  - 2
SP  - 259
EP  - 268
J2  - Nucl. Instrum. Methods B
SN  - 0168-583X
ER  - 

TY  - JOUR
TI  - Chemical analysis of PtxNi1−x alloy single crystal surfaces by scanning tunnelling microscopy
AU  - Biedermann, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Fresenius' Journal of Analytical Chemistry
AB  - Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt25Ni75(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt10Ni90(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.
DA  - 1994/01/01/
PY  - 1994
DO  - 10.1007/BF00323272
VL  - 349
IS  - 1
SP  - 201
EP  - 203
J2  - Fresenius J. Anal. Chem.
ER  - 

TY  - JOUR
TI  - Domain wall structures in an ordered Si/Fe(110) surface alloy
AU  - Biedermann, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - During the annealing process of an Fe96.5Si3.5(100)/(110) bicrystal, silicon and impurity carbon segregate to the surface. The structures formed by the segregands on the (110) surface have been studied by STM (geometry) and AES (chemical information). Silicon substitutes iron surface atoms and forms a two-dimensional alloy, whereas carbon occupies hollow sites in the first monolayer, leading to a distortion of the substrate lattice. The structures are based on a c(1 × 3)Si [theta] = 1/3 ordered surface alloy. Additional silicon as well as the co-segregating impurity carbon are inserted into this structure by formation of domain walls. If the density of these nearly straight and parallel domain walls becomes high enough, commensurate domain wall structures with c(1 × n) supercells can be observed.
DA  - 1995/07/01/
PY  - 1995
DO  - 10.1016/0039-6028(95)00339-8
VL  - 331-333
IS  - Part 1
SP  - 787
EP  - 793
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Competitive segregation of Si and P on Fe96.5Si3.5 (100) and (110)
AU  - Biedermann, A.
AU  - Schmid, M.
AU  - Reichl, B.
AU  - Varga, P.
T2  - Fresenius' Journal of Analytical Chemistry
AB  - Annealing an Fe96.5Si3.5 (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 × 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 × 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface.
DA  - 1995/10/25/
PY  - 1995
DO  - 10.1007/s0021653530259
VL  - 353
IS  - 3
SP  - 259
EP  - 262
J2  - Fresenius J. Anal. Chem.
ER  - 

TY  - JOUR
TI  - Probing the surface phase diagram of Fe3O4(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface
AU  - Novotny, Zbynek
AU  - Mulakaluri, Narasimham
AU  - Edes, Zoltan
AU  - Schmid, Michael
AU  - Pentcheva, Rossitza
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Physical Review B
AB  - Reduced terminations of the Fe3O4(001) surface were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Fe atoms, deposited onto the thermodynamically stable, distorted B-layer termination at room temperature (RT), occupy one of two available tetrahedrally coordinated sites per (√2×√2)R45° unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a “Fe-dimer” termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+U results indicate a progressive reduction of surface iron from Fe3+ to Fe2+ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe3O4(001) suggests possible exchange bias in this system.
DA  - 2013/05/08/
PY  - 2013
DO  - 10.1103/PhysRevB.87.195410
VL  - 87
IS  - 19
SP  - 195410
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Interface-confined mixing and buried partial dislocations for Ag bilayer on Pt(111)
AU  - Aït-Mansour, Kamel
AU  - Brune, Harald
AU  - Passerone, Daniele
AU  - Schmid, Michael
AU  - Xiao, Wende
AU  - Ruffieux, Pascal
AU  - Buchsbaum, Andreas
AU  - Varga, Peter
AU  - Fasel, Roman
AU  - Gröning, Oliver
T2  - Physical Review B
AB  - The trigonal strain-relief pattern formed by an Ag bilayer on Pt(111) is a prominent example for dislocation networks and their use as nanotemplates. However, its atomic structure has not been solved. Combining scanning tunneling microscopy, low-energy ion scattering, and x-ray photoelectron diffraction, we demonstrate that, unexpectedly, about 22% of the atoms exchange across the Ag/Pt interface, and that the partial dislocations defining the trigonal network are buried in the Pt interface layer. We present an embedded-atom-method simulation identifying the lowest energy structure compatible with all experimental findings.
DA  - 2012/08/03/
PY  - 2012
DO  - 10.1103/PhysRevB.86.085404
VL  - 86
IS  - 8
SP  - 085404
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Bulk and surface characterization of In2O3(001) single crystals
AU  - Hagleitner, Daniel R.
AU  - Menhart, Manfred
AU  - Jacobson, Peter
AU  - Blomberg, Sara
AU  - Schulte, Karina
AU  - Lundgren, Edvin
AU  - Kubicek, Markus
AU  - Fleig, Jürgen
AU  - Kubel, Frank
AU  - Puls, Christoph
AU  - Limbeck, Andreas
AU  - Hutter, Herbert
AU  - Boatner, Lynn A.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Physical Review B
AB  - A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 ± 0.5 × 10^5 Ω cm, which translates into a carrier concentration of ≈10^12 cm−3. The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) Å from the previously accepted value of 10.117 Å. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 °C) surface show a step height of 5 Å, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 Å height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (EF). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.
DA  - 2012/03/28/
PY  - 2012
DO  - 10.1103/PhysRevB.85.115441
VL  - 85
IS  - 11
SP  - 115441
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Segregation and reconstructions of PtxNi1-x(100)
AU  - Hebenstreit, W.
AU  - Ritz, G.
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Varga, P.
T2  - Surface Science
AB  - It is known that on (100) surfaces of PtxNi1 - x single crystals Pt segregates. With increasing Pt concentration in the surface the transition from unreconstructed Ni(100) to the pseudo hexagonal Pt(100) reconstruction occurs via a shifted row reconstruction and several pseudo hexagonal (n × 1) superstructures (n = 7, 12 and 19) consisting of similar (7 × 1) and (5 × 1) subcells. This was revealed by atomically resolved scanning tunnelling microscopy (STM). From low energy ion scattering measurements it becomes clear that the formation of the pseudo hexagonal structure leads to strong amplification of Pt segregation. Chemically resolved STM on the atomic scale shows that Pt prefers the highly coordinated four-fold hollow sites in the pseudo hexagonal structures and Ni is pushed into nearly on-top or bridge sites. Therefore the strong tendency of Pt to increase its coordination is proposed as the driving force of the reconstructions. Corrugations and chemical ordering measured by STM within the pseudo hexagonal reconstructions are confirmed by simulations based on embedded atom method potentials.
DA  - 1997/10/23/
PY  - 1997
DO  - 10.1016/S0039-6028(97)00392-0
VL  - 388
IS  - 1-3
SP  - 150
EP  - 161
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The Rh(100)-(3 × 1)-2O structure
AU  - Gustafson, J.
AU  - Lundgren, E.
AU  - Mikkelsen, A.
AU  - Borg, M.
AU  - Klikovits, J.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Andersen, J. N.
T2  - Journal of Physics: Condensed Matter
AB  - The O adsorption on Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. In addition to the well known (2 × 2), (2 × 2)-pg and c(8 × 2) structures at coverages of 0.25, 0.5 and 1.75 ML respectively, an intermediate (3 × 1) structure with a coverage of 2/3 ML is identified.
DA  - 2012/06/06/
PY  - 2012
DO  - 10.1088/0953-8984/24/22/225006
VL  - 24
IS  - 22
SP  - 225006
J2  - J. Phys.: Condens. Matter
SN  - 0953-8984, 1361-648X
ER  - 

TY  - JOUR
TI  - Interaction of oxygen with PtRh(100) studied with STM
AU  - Wouda, P. T.
AU  - Schmid, M.
AU  - Hebenstreit, W.
AU  - Varga, P.
T2  - Surface Science
AB  - The adsorption of oxygen at 500°C on a Pt50Rh50(100) single crystal surface was studied using UHV-STM and Auger electron spectroscopy. Images were taken of the p(3 × 1) phase; of a mixed phase with p(2 × 2), c(2 × 2) and (3 × 3) units; and of rhodium oxide patches. Possible models for these structures involving surface reconstruction are presented. Exposure of the p(3 × 1)O/PtRh(100) to H2 at room temperature led to the conversion to the p(1 × 1) substrate structure. The ordering and composition of this substrate structure after reduction is discussed.
DA  - 1997/10/23/
PY  - 1997
DO  - 10.1016/S0039-6028(97)00375-0
VL  - 388
IS  - 1-3
SP  - 63
EP  - 70
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Surface alloying and superstructures of Pb on Cu(100)
AU  - Nagl, C.
AU  - Platzgummer, E.
AU  - Haller, O.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The growth of submonolayer and monolayer Pb films on Cu(100) has been investigated by STM. In the submonolayer region a disordered surface alloy is found in spite of the immiscibility of bulk Cu and Pb. At a coverage of theta = 3/8 a c(4 × 4) superstructure is observed. The atomic arrangement of the c(4 × 4) superstructure unit cell could be revealed; it is formed by linear chains of Pb atoms in two rotational domains. Increasing the coverage to theta = 0.5, a c(2 × 2) structure can be observed. The transition to the c(5√2 × √2)R45° superstructure of the dense overlayer proceeds via the insertion of antiphase domain boundary. The model for the c(5√2 × √2)R45° superstructure presented in literature is confirmed.
DA  - 1995/07/01/
PY  - 1995
DO  - 10.1016/0039-6028(95)00387-8
VL  - 331-333
IS  - Part 1
SP  - 831
EP  - 837
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The nitrogen-induced herringbone reconstruction of Cr(110)
AU  - Schmid, M.
AU  - Pinczolits, M.
AU  - Hebenstreit, W.
AU  - Varga, P.
T2  - Surface Science
AB  - Segregation of nitrogen causes herringbone-like (3 × n) surface reconstructions, which were studied by atomically resolved scanning tunneling microscopy (STM). The main building blocks of the reconstruction are domains of (√6 × √6)R ± 35° cells, separated by domain boundaries running in the [110] direction. Since the density of Cr atoms in the surface is reduced by the reconstruction, the driving force of the reconstruction is believed to be compressive stress caused by the incorporation of N atoms into the surface.
DA  - 1997/11/06/
PY  - 1997
DO  - 10.1016/S0039-6028(97)00474-3
VL  - 389
IS  - 1-3
SP  - L1140
EP  - L1146
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study
AU  - Gauthier, Y.
AU  - Dolle, P.
AU  - Baudoing-Savois, R.
AU  - Hebenstreit, W.
AU  - Platzgummer, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1-0.4 Å above that of Co, whereas LEED shows that Pt atoms are geometrically ~0.04 Å higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (<c1> = 62.6% Pt, <c3> = 53.5%) and Pt-depleted layers (<c2> = 6.9%, <c4> = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L1_2 PtCo3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.
DA  - 1998/01/20/
PY  - 1998
DO  - 10.1016/S0039-6028(97)00665-1
VL  - 396
IS  - 1-3
SP  - 137
EP  - 155
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Anti-corrugation and nitrogen c(2 × 2) on Cr(100): STM on atomic scale and quantitative LEED
AU  - Sporn, M.
AU  - Platzgummer, E.
AU  - Pinczolits, M.
AU  - Hebenstreit, W.
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
T2  - Surface Science
AB  - We present a LEED I-V analysis of c(2 × 2)-N/Cr(100). We found nitrogen residing in fourfold hollow sites and exclude adsorption models in which nitrogne adsorbs on a metal site (on-top, substitutional or second-layer interstitial). We achieved a Pendry R-factor of 0.16 for the best-fit structure. Nitrogen resides at a vertical distance of 0.36 Å above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 Å (7.5% with respect to the bulk value of 1.44 Å). The interlayer spacing between the second and the third layer is contracted to 1.41 Å. The second chromium layer is buckled (0.13 Å). The second-layer chromium atom beneath a nitrogen atom resides deeper in the bulk. The nitrogen bond length to the four first-layer chromium atoms amounts to 2.07 Å, the bond length to the second-layer chromium atom amounts to 1.97 Å. The nitrogen position in c(2 × 2)-N determined by LEED is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 × 1)-Cr(100) are imaged as hillocks and chromium atoms as depressions. This is anti-corrugation of clean Cr(100). Anti-corrugation seems to be related to a surface state of clean Cr(100) and is lifted in p(1 × 1)-N/Cr(100) at a (local) nitrogen coverage of 1 monolayer.
DA  - 1998/01/20/
PY  - 1998
DO  - 10.1016/S0039-6028(97)00660-2
VL  - 396
IS  - 1-3
SP  - 78
EP  - 86
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Submonolayer growth of Pb on Cu(111): surface alloying and de-alloying
AU  - Nagl, C.
AU  - Haller, O.
AU  - Platzgummer, E.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Surface Science
AB  - In spite of the immiscibility of Pb in bulk Cu, atomically resolved scanning tunneling microscopy reveals surface alloy formation of Pb deposited on Cu(111), even at 300 K. Due to kinetic limitations at room temperature, the incorporation of Pb is restricted to advance from step edges, while after annealing to 470 K or higher, embedded Pb atoms are found to be randomly distributed over terraces. At low tunneling voltages, standing waves of surface-state electrons scattered by embedded Pb atoms could be observed. The maximum packing density of the surface alloy is about 40% (= 0.4 ML) of a close-packed Pb overlayer. Thus, deposition above 0.4 ML and subsequent annealing results in hexagonal close-packed Pb regions, whereas on the non-annealed surface hexagonal close-packed Pb islands are already found at 0.2 ML. Eventually, at 1 ML the surface alloy is entirely replaced by a Pb overlayer.
DA  - 1994/12/20/
PY  - 1994
DO  - 10.1016/0039-6028(94)90189-9
VL  - 321
IS  - 3
SP  - 237
EP  - 248
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Growth of ultrathin cobalt oxide films on Pt(111)
AU  - De Santis, Maurizio
AU  - Buchsbaum, Andreas
AU  - Varga, Peter
AU  - Schmid, Michael
T2  - Physical Review B
AB  - Cobalt surface oxides were grown on Pt(111) by depositing Co and dosing with molecular oxygen at temperatures ranging between 300 and 740 K. Oxidation of 1 monolayer (ML) Co results in a two-dimensional (2D) moiré structure, observed using both low-energy electron diffraction and scanning tunneling microscopy and interpreted as a polar (oxygen terminated) CoO(111) atomic bilayer. It is expanded by 2.7 ± 0.6% in the surface plane with respect to bulk CoO. An almost-flawless moiré pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moiré pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupy half of the moiré cell. Low-temperature annealing (450 K) can be used to create a zigzag phase, which is mainly observed in 1-ML-thick areas after several cycles of Co deposition (1 ML each) and oxidation at 10−7 mbar. The CoO films obtained by deposition/oxidation cycles exhibit Stranski-Krastanov growth; the structure of the 2D layer between the islands depends on the thermal treatment. It exhibits the moiré pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moiré pattern different from that of the first layer, presumably a wurtzite-like structure. Above the third layer, we observe only small three-dimensional islands, which exhibit a bandgap. We have also studied oxidation of surface alloys obtained by depositing Co and annealing. On these surfaces, we found a quasi-(3 × 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moirélike structures might be useful as templates for metal cluster growth.
DA  - 2011///
PY  - 2011
DO  - 10.1103/PhysRevB.84.125430
VL  - 84
IS  - 12
SP  - 125430
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - In-situ magnetic nano-patterning of Fe films grown on Cu(100)
AU  - Shah Zaman, Sameena
AU  - Dvořák, Petr
AU  - Ritter, Robert
AU  - Buchsbaum, Andreas
AU  - Stickler, Daniel
AU  - Oepen, Hans Peter
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Journal of Applied Physics
AB  - Metastable paramagnetic face-centered cubic (fcc) Fe films grown on a Cu(100) single crystal at room temperature can be transformed to the ferromagnetic body-centered cubic (bcc) structure by ion irradiation. We have employed this technique to write small ferromagnetic patches by Ar+ irradiation through a gold coated SiN mask with regularly arranged 80-nm diameter holes, which was placed on top of the as-prepared fcc Fe films. Nanopatterning was performed on both 8-monolayer (ML) Fe films grown in ultrahigh vacuum as well as 22-ML films stabilized by dosing carbon monoxide during growth. The structural transformation of these nano-patterned films was investigated using scanning tunneling microscopy. In both 8 and 22-ML fcc Fe films, the bcc needles are found to protrude laterally out of the irradiated part of the sample, limiting the resolution of the technique to a few 10 nm. The magnetic transformation was confirmed by magnetic force microscopy.
DA  - 2011/07/27/
PY  - 2011
DO  - 10.1063/1.3609078
VL  - 110
IS  - 2
SP  - 024309
J2  - J. Appl. Phys.
SN  - 10897550
ER  - 

TY  - JOUR
TI  - Anisotropic two-dimensional electron gas at SrTiO3(110)
AU  - Wang, Zhiming
AU  - Zhong, Zhicheng
AU  - Hao, Xianfeng
AU  - Gerhold, Stefan
AU  - Stöger, Bernhard
AU  - Schmid, Michael
AU  - Sánchez-Barriga, Jaime
AU  - Varykhalov, Andrei
AU  - Franchini, Cesare
AU  - Held, Karsten
AU  - Diebold, Ulrike
T2  - Proceedings of the National Academy of Sciences
AB  - Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG––the SrTiO3(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising “semiheavy” band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.
DA  - 2014/03/03/
PY  - 2014
DO  - 10.1073/pnas.1318304111
VL  - 111
IS  - 11
SP  - 3933
EP  - 3937
J2  - Proc. Natl. Acad. Sci. USA
LA  - en
SN  - 0027-8424, 1091-6490
ER  - 

TY  - JOUR
TI  - Real-space imaging of the Verwey transition at the (100) surface of magnetite
AU  - de la Figuera, Juan
AU  - Novotny, Zbynek
AU  - Setvin, Martin
AU  - Liu, Tijiang
AU  - Mao, Zhiqiang
AU  - Chen, Gong
AU  - N’Diaye, Alpha T.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Schmid, Andreas K.
AU  - Parkinson, Gareth S.
T2  - Physical Review B
AB  - Effects of the Verwey transition on the (100) surface of magnetite were studied using scanning tunneling microscopy and spin polarized low-energy electron microscopy. On cooling through the transition temperature TV, the initially flat surface undergoes a rooflike distortion with a periodicity of ∼0.5 μm due to ferroelastic twinning within monoclinic domains of the low-temperature monoclinic structure. The monoclinic c axis orients in the surface plane, along the [001]c directions. At the atomic scale, the charge-ordered (√2×√2)R45∘ reconstruction of the (100) surface is unperturbed by the bulk transition, and is continuous over the twin boundaries. Time resolved low-energy electron microscopy movies reveal the structural transition to be first order at the surface, indicating that the bulk transition is not an extension of the Verwey-like (√2×√2)R45∘ reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-TV bulk of magnetite are unrelated phenomena.
DA  - 2013/10/29/
PY  - 2013
DO  - 10.1103/PhysRevB.88.161410
VL  - 88
IS  - 16
SP  - 161410(R)
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Stoichiometry-driven switching between surface reconstructions on SrTiO3(001)
AU  - Gerhold, Stefan
AU  - Wang, Zhiming
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Surface Science
AB  - Controlling the surface structure on the atomic scale is a major difficulty for most transition metal oxides; this is especially true for the ternary perovskites. The influence of surface stoichiometry on the atomic structure of the SrTiO3(001) surface was examined with scanning tunneling microscopy, low-energy electron diffraction, low-energy He+ ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 0.8 monolayer (ML) strontium and 0.3 ML titanium, with subsequent annealing to 850 °C in 4 × 10^-6 mbar O2, reversibly switches the surface between c(4 × 2) and (2 × 2) reconstructions, respectively. The combination of LEIS and XPS shows a different stoichiometry that is confined to the top layer. Geometric models for these reconstructions need to take into account these different surface compositions.
DA  - 2014/03//
PY  - 2014
DO  - 10.1016/j.susc.2013.10.015
VL  - 621
SP  - L1
EP  - L4
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Enhancement of STM images and estimation of atomic positions based on maximum entropy deconvolution
AU  - Böhmig, S. D.
AU  - Schmid, M.
AU  - Störi, H.
T2  - Surface Science
AB  - A new method for restoration and sharpening of scanning tunneling microscopy (STM) data is presented. According to the STM theory of Tersoff and Hamann it is assumed that the response of the STM can be approximated by the convolution of a localized atomic density of states of the sample and a Gaussian, which limits the resolution. Therefore, one must find the solution of an inverse problem, which is done by minimizing the mean square deviation between the measured and the reconstructed image using entropy as a regularization functional. This nonlinear method is shown to be superior to linear filters such as the Wiener filter in that the solution carries as minimal information as is necessary to fit the data and is not constrained to low frequencies. On metals, where atomically resolved STM images show mainly geometrical information, the centers of mass of the resulting peaks are taken as the atomic positions which are compared to those estimated visually from the STM images. The method has been applied to both the periodic Cu(111) surface and to the nonperiodic shifted row reconstruction of Pt10Ni90(100).
DA  - 1994/06/20/
PY  - 1994
DO  - 10.1016/0039-6028(94)91152-5
VL  - 313
IS  - 1-2
SP  - 6
EP  - 16
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The shifted-row reconstruction of PtxNi1-x(100)
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Böhmig, S. D.
AU  - Weigand, P.
AU  - Varga, P.
T2  - Surface Science
AB  - Scanning tunneling microscopy on Pt10Ni90(100) and Pt25Ni75(100) single crystals reveals close-packed rows of atoms, which are shifted by <110> along the direction of the rows into a bridge position and slightly outward of the surface. Maximum entropy deconvolution of atomically resolved STM data shows that all atoms between the shifted rows are close to the unreconstructed positions. The density of the shifted rows increases with increasing Pt surface concentration up to a maximum value of each 5th row shifted. The reconstruction shows little dependence on the carbon contamination of the surface, but it is lifted by a full c(2 × 2) coverage of carbon monoxide, which can be imaged simultaneously with the substrate, indicating an on-top position of CO. The driving force of the shifted-row reconstruction is discussed.
DA  - 1994/10/20/
PY  - 1994
DO  - 10.1016/0039-6028(94)90103-1
VL  - 318
IS  - 3
SP  - 289
EP  - 298
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - An STM study of growth and alloying of Cr on Ru(0001) and CO adsorption on the alloy
AU  - Engelhardt, M. P.
AU  - Schmid, M.
AU  - Biedermann, A.
AU  - Denecke, R.
AU  - Steinrück, H.-P.
AU  - Varga, P.
T2  - Surface Science
AB  - We have investigated the growth of Cr on Ru(0001) using scanning tunneling microscopy (STM). In the submonolayer regime (theta_Cr = 0.25 ML) monatomically high islands are formed at 300 K, accompanied by step decoration. The island density decreases upon heating to 700 K. Small amounts of Cr are incorporated in the Ru substrate at this temperature. For higher Cr coverages (2 ML), three-dimensional growth is observed at 300 K. Annealing this Cr layer to 500 and 700 K leads to the formation of Cr islands (4-5 layers high) with an elongated shape and bcc(110) structure with a pseudomorphic Cr monolayer covering the remaining substrate. As shown by atomically resolved images, the Cr bcc(110) islands have Kurdjumov-Sachs orientation with respect to the Ru(0001) substrate. After annealing at 1000 K, the formation of a hexagonal, chemically disordered CrRu alloy is observed, in agreement with a previous study [Engelhardt et al., Surf. Sci. 512 (2002) 107]. The room-temperature STM investigation of CO adsorption on the CrRu alloy with approx. 34% Cr in the first layer shows that adsorbed molecular CO resides only on Ru atoms, not on Cr, mostly in an on-top geometry. The CO occupation of Ru sites increases slightly with the number of neighbouring Cr atoms, indicating a weak ligand effect. We also find indications that the few threefold hollow sites surrounded by three Cr atoms become occupied by atomic C or O, due to dissociation of a small fraction of the adsorbed CO molecules; thus, we cannot unambiguously determine whether molecular CO can bind to these threefold sites at room temperature.
DA  - 2005/03/10/
PY  - 2005
DO  - 10.1016/j.susc.2005.01.022
VL  - 578
IS  - 1-3
SP  - 124
EP  - 135
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - The surface oxide as a source of oxygen on Rh(111)
AU  - Lundgren, E.
AU  - Gustafson, J.
AU  - Resta, A.
AU  - Weissenrieder, J.
AU  - Mikkelsen, A.
AU  - Andersen, J. N.
AU  - Köhler, L.
AU  - Kresse, G.
AU  - Klikovits, J.
AU  - Biederman, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Journal of Electron Spectroscopy and Related Phenomena
AB  - The reduction of a thin surface oxide on the Rh(111) surface by CO is studied in situ by photoemission spectroscopy, scanning tunneling microscopy, and density functional theory. CO molecules are found not to adsorb on the surface oxide at a sample temperature of 100[thin space]K, in contrast to on the clean and chemisorbed oxygen covered surface. Despite this behavior, the surface oxide may still be reduced by CO, albeit in a significantly different fashion as compared to the reduction of a phase containing only chemisorbed on surface oxygen. The experimental observations combined with theoretical considerations concerning the stability of the surface oxide, result in a model of the reduction process at these pressures suggesting that the surface oxide behaves as a source of oxygen for the CO-oxidation reaction.
DA  - 2005/06//
PY  - 2005
DO  - 10.1016/j.elspec.2005.01.004
VL  - 144-147
SP  - 367
EP  - 372
J2  - J Electron Spectrosc. Relat. Phenom.
SN  - 0368-2048
ER  - 

TY  - JOUR
TI  - Stressing Pd atoms: Initial oxidation of the Pd(110) surface
AU  - Westerström, R.
AU  - Weststrate, C. J.
AU  - Resta, A.
AU  - Mikkelsen, A.
AU  - Schnadt, J.
AU  - Andersen, J. N.
AU  - Lundgren, E.
AU  - Schmid, M.
AU  - Seriani, N.
AU  - Harl, J.
AU  - Mittendorfer, F.
AU  - Kresse, G.
T2  - Surface Science
AB  - We have investigated the oxygen induced structures of the Pd(110) surface in the pressure range of 10<sup>-5</sup>-10<sup>-3</sup> mbar of oxygen, at a sample temperature of around 300 °C. These structures, denoted as "(7x√3)" and "(9x√3)", are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [1 -1 0] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation.
DA  - 2008///
PY  - 2008
DO  - 10.1016/j.susc.2008.05.033
VL  - 602
IS  - 14
SP  - 2440
EP  - 2447
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Ultra-thin Fe films grown on Cu by pulsed laser deposition: Intermixing and bcc-like structures
AU  - Rauchbauer, G.
AU  - Buchsbaum, A.
AU  - Schiechl, H.
AU  - Varga, P.
AU  - Schmid, M.
AU  - Biedermann, A.
T2  - Surface Science
AB  - Pulsed laser deposition (PLD) with nanosecond pulses has been used for growing ultrathin Fe films on Cu(100) and Cu(111) single crystal surfaces. We have studied the morphology as well as the crystallographic structure of these films by scanning tunneling microscopy, and we compare the films with thermally deposited (TD) films. For Fe/Cu(100), bcc-like (nanomartensitic) structures are found in roughly the same thickness range for PLD and TD films but occupy a lower fraction of the films when deposited by PLD. The situation is different for thin Fe/Cu(111) films, where PLD films exhibit a higher bcc-like fraction, especially in islands of two monolayers thickness. Similar to TD films, we also observe surface reconstructions with bcc-like bond angles for the otherwise fee Fe/Cu(100) films in the thickness range above 5 ML. For both Fe/Cu(111) and Fe/Cu(100), we find a stronger intermixing between substrate and film compared to films grown by thermal deposition. Even in the seventh monolayer of Fe/Cu(100), approximately 10% Cu have been measured. We argue that the compositional heterogeneity is the reason for the absence of long-range order in the bcc-like phases, hiding them from diffraction techniques. We also discuss the results in the context of the magnetic properties of these films described in the literature. (c) 2008 Elsevier B.V. All rights reserved.
DA  - 2008/04/15/
PY  - 2008
DO  - 10.1016/j.susc.2008.02.024
VL  - 602
IS  - 8
SP  - 1589
EP  - 1598
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Growth and structure of an ultrathin tin oxide film on Rh(111)
AU  - Yuhara, J.
AU  - Tajima, D.
AU  - Matsui, T.
AU  - Tatsumi, K.
AU  - Muto, S.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Journal of Applied Physics
AB  - The oxidation of submonolayer tin films on a Rh(111) surface by O2 gas was studied using low energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy(XPS), and scanning tunneling microscopy. A uniform tin oxide monolayer film formed at oxidation temperatures around 500 °C and a partial pressure of 2 × 10 ^-7 mbar O2 . The tin oxide film had (2 × 2) periodicity on the Rh(111) surface, and the resulting tin coverage was determined to be 0.5 ML. Using XPS, the compositional ratio O/Sn was determined to be 3/2. XPS spectra showed a single component for the Sn and O peaks, indicating a uniform bonding environment. Finally, ab initio density-functional theory total energy calculations and molecular dynamics simulations were performed using the projector augmented wave method to determine the detailed structure of the tin oxide thin film.
DA  - 2011/01/20/
PY  - 2011
DO  - 10.1063/1.3537871
VL  - 109
IS  - 2
SP  - 024903
J2  - J. Appl. Phys.
SN  - 0021-8979, 1089-7550
ER  - 

TY  - JOUR
TI  - Segregation phenomena on Pt_x Ni_1−x low-index single crystal surfaces studied by STM
AU  - Varga, P.
AU  - Schmid, M.
AU  - Hofer, W.
T2  - Surface Review and Letters
AB  - Surface segregation changes the composition of alloy surfaces. It influences both the geometrical and the chemical structure of the surface. In this paper segregation phenomena are shown for low index single crystal surfaces of different PtNi alloys which can be seen only by scanning tunneling microscopy (STM).
STM experiments performed with atomic resolution revealed the existence of subsurface dislocation networks. A closer study of the conditions of their existence allowed us to understand the effects of preferential sputtering and annealing on the segregation behavior (i.e. building up a rather stable altered layer and its disappearance only at elevated temperatures). In addition, local chemical ordering in small domains and shifted row reconstructions with a large and varying periodicity (i.e. phenomena that are hardly seen by other methods like e.g. LEED) have been observed.
DA  - 1996/10//
PY  - 1996
DO  - 10.1142/S0218625X9600276X
VL  - 03
IS  - 05-06
SP  - 1831
EP  - 1845
J2  - Surf. Rev. Lett.
SN  - 0218-625X, 1793-6667
ER  - 

TY  - JOUR
TI  - Atomic-scale structure and catalytic reactivity of the RuO2(110) surface
AU  - Over, H.
AU  - Kim, Y. D.
AU  - Seitsonen, A. P.
AU  - Wendt, S.
AU  - Lundgren, E.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Morgante, A.
AU  - Ertl, G.
T2  - Science
AB  - Exposure of a Ru(0001) surface to large doses of O2 at elevated temperatures leads to the growth of an epitaxial layer of RuO2 with (110) surface orientation whose structure was analyzed quantitatively by combination of low energy electron diffraction, scanning tunneling microscopy and density functional calculations. The surface exposes essentially bridging O atoms and Ru atoms not capped by oxygen. The latter play the role of coordinatively unsaturated sites (cus) - a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces - onto which CO may become chemisorbed and from where it may react with neighboring lattice-O to CO2. The distortion of the surface lattice thereby caused is restored by uptake of oxygen from the gas phase, i.e., the oxide surface itself is actively participating in the catalytic reaction. In this way a general mechanism originally proposed by Mars and van Krevelen could be verified.
DA  - 2000/02/25/
PY  - 2000
DO  - 10.1126/science.287.5457.1474
VL  - 287
IS  - 5457
SP  - 1474
EP  - 1476
J2  - Science
ER  - 

TY  - JOUR
TI  - Carbon monoxide-induced adatom sintering in a Pd–Fe3O4 model catalyst
AU  - Parkinson, Gareth S.
AU  - Novotny, Zbynek
AU  - Argentero, Giacomo
AU  - Schmid, Michael
AU  - Pavelec, Jiří
AU  - Kosak, Rukan
AU  - Blaha, Peter
AU  - Diebold, Ulrike
T2  - Nature Materials
AB  - The coarsening of catalytically active metal clusters is often accelerated by the presence of gases, but the role played by gas molecules is difficult to ascertain and varies from system to system. We use scanning tunnelling microscopy to follow the CO-induced coalescence of Pd adatoms supported on the Fe3O4(001) surface at room temperature, and find Pd-carbonyl species to be responsible for mobility in this system. Once these reach a critical density, clusters nucleate; subsequent coarsening occurs through cluster diffusion and coalescence. Whereas CO induces the mobility in the Pd/Fe3O4 system, surface hydroxyls have the opposite effect. Pd atoms transported to surface OH groups are no longer susceptible to carbonyl formation and remain isolated. Following the evolution from well-dispersed metal adatoms into clusters, atom-by-atom, allows identification of the key processes that underlie gas-induced mass transport.
DA  - 2013/08//
PY  - 2013
DO  - 10.1038/nmat3667
VL  - 12
IS  - 8
SP  - 724
EP  - 728
J2  - Nat. Mater.
LA  - en
SN  - 1476-1122
ER  - 

TY  - JOUR
TI  - Pt3Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation
AU  - Antlanger, Moritz
AU  - Mayr-Schmölzer, Wernfried
AU  - Pavelec, Jiří
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Varga, Peter
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Physical Review B
AB  - We have studied the surface of pure and oxidized Pt3Zr(0001) by scanning tunneling microscopy (STM), Auger electron microscopy, and density functional theory (DFT). The well-annealed alloy surface shows perfect long-range chemical order. Occasional domain boundaries are probably caused by nonstoichiometry. Pt3Zr exhibits ABAC stacking along [0001]; only the A-terminated surfaces are seen by STM, in agreement with DFT results showing a lower surface energy for the A termination. DFT further predicts a stronger inward relaxation of the surface Zr than for Pt, in spite of the larger atomic size of Zr. A closed ZrO2 film is obtained by oxidation in 10^-7 mbar O2 at 400 °C and post-annealing at ≈800 ∘C. The oxide consists of an O-Zr-O trilayer, equivalent to a (111) trilayer of the fluorite structure of cubic ZrO2, but contracted laterally. The oxide forms a (√19×√19)R23∘ superstructure. The first monolayer of the substrate consists of Pt and contracts, similar to the metastable reconstruction of pure Pt(111). DFT calculations show that the oxide trilayer binds rather weakly to the substrate. In spite of the O-terminated oxide, bonding to the substrate mainly occurs via the Zr atoms in the oxide, which strongly buckle down toward the Pt substrate atoms if near a Pt position. According to DFT, the oxide has a band gap; STM indicates that the conduction band minimum lies ≈2.3 eV above E_F.
DA  - 2012/07/30/
PY  - 2012
DO  - 10.1103/PhysRevB.86.035451
VL  - 86
IS  - 3
SP  - 035451
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - A metastable Fe(A) termination at the Fe3O4(001) surface
AU  - Parkinson, Gareth S.
AU  - Novotný, Zbynek
AU  - Jacobson, Peter
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Surface Science
AB  - A metastable Fe(A) terminated Fe3O4(001) surface was prepared by tailoring the surface preparation conditions. STM, LEIS and LEED are utilized to demonstrate that annealing the Ar+ sputtered surface to 350 °C produces an Fe(A) terminated surface with a (√2 × √2)R45° superstructure. Within the superstructure both single Fe atoms and Fe dimer species are observed. The surface is reoxidized upon annealing to higher temperatures, eventually leading to the recovery of the energetically favorable Jahn-Teller distorted surface at 700 °C. The ability to reproducibly prepare the Fe(A) termination in this simple manner will allow investigations into the structure-function relationship for this important technological material.
DA  - 2011/08//
PY  - 2011
DO  - 10.1016/j.susc.2011.05.018
VL  - 605
IS  - 15-16
SP  - L42
EP  - L45
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Subsurface cation vacancy stabilization of the magnetite (001) surface
AU  - Bliem, R.
AU  - McDermott, E.
AU  - Ferstl, P.
AU  - Setvin, M.
AU  - Gamba, O.
AU  - Pavelec, J.
AU  - Schneider, M. A.
AU  - Schmid, M.
AU  - Diebold, U.
AU  - Blaha, P.
AU  - Hammer, L.
AU  - Parkinson, G. S.
T2  - Science
AB  - Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (√2 x √2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace.
DA  - 2014/12/05/
PY  - 2014
DO  - 10.1126/science.1260556
VL  - 346
IS  - 6214
SP  - 1215
EP  - 1218
J2  - Science
LA  - en
SN  - 0036-8075, 1095-9203
ER  - 

TY  - JOUR
TI  - Point defects at cleaved Srn+1RunO3n+1(001) surfaces
AU  - Stöger, Bernhard
AU  - Hieckel, Marcel
AU  - Mittendorfer, Florian
AU  - Wang, Zhiming
AU  - Schmid, Michael
AU  - Parkinson, Gareth S.
AU  - Fobes, David
AU  - Peng, Jin
AU  - Ortmann, John E.
AU  - Limbeck, Andreas
AU  - Mao, Zhiqiang
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - Physical Review B
AB  - The (001) surfaces of cleaved Sr3Ru2O7 and Sr2RuO4 samples were investigated using low-temperature scanning tunneling microscopy and density functional theory calculations. Intrinsic defects are not created during cleaving. This experimental observation is consistent with calculations, where the formation energy for a Sr and O vacancy, 4.19 eV and 3.81 eV, respectively, is significantly larger than that required to cleave the crystal, 1.11 eV/(1 × 1) unit cell. Surface oxygen vacancies can be created through electron bombardment, however, and their appearance is shown to vary strongly with the imaging conditions. Point defects observed on as-cleaved surfaces result from bulk impurities and adsorption from the residual gas.
DA  - 2014/10/30/
PY  - 2014
DO  - 10.1103/PhysRevB.90.165438
VL  - 90
IS  - 16
SP  - 165438
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Progress in monitoring thin film thickness by use of quartz crystals
AU  - Benes, E.
AU  - Schmid, M.
AU  - Thorn, G.
T2  - Thin Solid Films
AB  - The sensor function of high-accuracy quartz crystal thin film thickness monitors is determined by the properties of the unloaded crystal and the acoustic impedance zF of the film material. This material constant zF is often not precisely known, but it can be determined by the introduced two-frequency or Auto-Z-Match® technique which uses the frequencies of two different quasiharmonic modes of the crystal. In comparison with standard crystal thickness measurement techniques as well as with atomic absorption spectrometry and the conventional microbalance, the superiority of the Auto-Z-Match® technique is demonstrated for the evaporation materials copper, titanium, aluminium and lead. As the two-frequency technique allows the determination of the effective acoustic impedance of the film and thus thickness, determinations with higher accuracy are possible in the case of poorly known or unknown z-values. Other contributions to the practical performance and accuracy of quartz crystal thin film thickness monitors, such as the influence of crystal temperature, thin films stresses and the relaxation phenomenon, as well as the occurrence of frequency jumps, are discussed on the basis of the most recent research results.
DA  - 1989/07//
PY  - 1989
DO  - 10.1016/0040-6090(89)90907-3
VL  - 174
IS  - Part 1
SP  - 307
EP  - 314
J2  - Thin Solid Films
SN  - 0040-6090
ER  - 

TY  - JOUR
TI  - Identification of adsorbed molecules via STM tip manipulation: CO, H2O, and O2 on TiO2 anatase (101)
AU  - Setvin, Martin
AU  - Daniel, Benjamin
AU  - Aschauer, Ulrich
AU  - Hou, Weiyi
AU  - Li, Ye-Fei
AU  - Schmid, Michael
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - Physical Chemistry Chemical Physics
AB  - While Scanning Tunneling Microscopy (STM) has evolved as an ideal tool to study surface chemistry at the atomic scale, the identification of adsorbed species is often not straightforward. This paper describes a way to reliably identify H2O, CO and O2 on the TiO2 anatase (101) surface with STM. These molecules are of a key importance in the surface chemistry of this and many other (photo-) catalytic materials. They exhibit a wide variety of contrasts in STM images, depending on the tip condition. With clean, metallic tips the molecules appear very similar, i.e., as bright, dimer-like features located in the proximity of surface Ti5c atoms. However, each species exhibits a specific response to the electric field applied by the STM tip. It is shown that this tip–adsorbate interaction can be used to reliably ascertain the identity of such species. The tip–adsorbate interactions, together with comparison of experimental and calculated STM images, are used to analyse and revisit the assignments of molecular adsorbed species reported in recent studies.
DA  - 2014/09/17/
PY  - 2014
DO  - 10.1039/C4CP03212H
VL  - 16
IS  - 39
SP  - 21524
EP  - 21530
J2  - Phys. Chem. Chem. Phys.
LA  - en
SN  - 1463-9084
ER  - 

TY  - JOUR
TI  - Vacancy clusters at domain boundaries and band bending at the SrTiO3(110) surface
AU  - Wang, Zhiming
AU  - Hao, Xianfeng
AU  - Gerhold, Stefan
AU  - Schmid, Michael
AU  - Franchini, Cesare
AU  - Diebold, Ulrike
T2  - Physical Review B
AB  - Antiphase domain boundaries (APDBs) in the (n×1)(n=4,5) reconstructions of the SrTiO3(110) surface were studied with scanning tunneling microscopy, x-ray photoemission spectroscopy, and density functional theory (DFT) calculations. Two types of APDBs form on each reconstruction; they consist of TixOy vacancy clusters with a specific stoichiometry. The presence of these clusters is controlled by the oxygen pressure during annealing. The structural models of the vacancy clusters are resolved with DFT, which also shows that their relative stability depends on the chemical potential of oxygen. The surface band bending can be tuned by controlling the vacancy clusters at the domain boundaries.
DA  - 2014/07/23/
PY  - 2014
DO  - 10.1103/PhysRevB.90.035436
VL  - 90
IS  - 3
SP  - 035436
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Adjusting island density and morphology of the SrTiO3(110)-(4 × 1) surface: Pulsed laser deposition combined with scanning tunneling microscopy
AU  - Gerhold, Stefan
AU  - Riva, Michele
AU  - Yildiz, Bilge
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Surface Science
AB  - The first stages of homoepitaxial growth of the (4 × 1) reconstructed surface of SrTiO3(110) are probed by a combination of pulsed laser deposition (PLD) with in-situ reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Considerations of interfacing high-pressure PLD growth with ultra-high-vacuum surface characterization methods are discussed, and the experimental setup and procedures are described in detail. The relation between RHEED intensity oscillations and ideal layer-by-layer growth is confirmed by analysis of STM images acquired after deposition of sub-monolayer amounts of SrTiO3. For a quantitative agreement between RHEED and STM results one has to take into account two interfaces: the steps at the circumference of islands, as well as the borders between two different reconstruction phases on the islands themselves. Analysis of STM images acquired after one single laser shot reveals an exponential decrease of the island density with increasing substrate temperature. This behavior is also directly visible from the temperature dependence of the relaxation times of the RHEED intensity. Moreover, the aspect ratio of islands changes considerably with temperature. The growth mode depends on the laser pulse repetition rate, and can be tuned from predominantly layer-by-layer to the step-flow growth regime.
DA  - 2016/09//
PY  - 2016
DO  - 10.1016/j.susc.2016.03.010
VL  - 651
SP  - 76
EP  - 83
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Fe3O4(110)–(1 × 3) revisited: Periodic (111) nanofacets
AU  - Parkinson, Gareth S.
AU  - Lackner, Peter
AU  - Gamba, Oscar
AU  - Maaß, Sebastian
AU  - Gerhold, Stefan
AU  - Riva, Michele
AU  - Bliem, Roland
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Surface Science
AB  - The structure of the Fe3O4(110)–(1 × 3) surface was studied with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflection high-energy electron diffraction (RHEED). The so-called one-dimensional reconstruction is characterized by bright rows that extend hundreds of nanometers in the [ 1 ¯ 10] direction and have a periodicity of 2.52 nm in [001] in STM. It is concluded that this reconstruction is the result of a periodic faceting to expose {111}-type planes with a lower surface energy.
DA  - 2016/07//
PY  - 2016
DO  - 10.1016/j.susc.2016.02.020
VL  - 649
SP  - 120
EP  - 123
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Adsorption of water at the SrO surface of ruthenates
AU  - Halwidl, Daniel
AU  - Stöger, Bernhard
AU  - Mayr-Schmölzer, Wernfried
AU  - Pavelec, Jiri
AU  - Fobes, David
AU  - Peng, Jin
AU  - Mao, Zhiqiang
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - Nature Materials
AB  - Although perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Srn+1RunO3n+1 (n = 1, 2) using low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighbouring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr–Sr bridge positions, circling the surface OH with a measured activation energy of 187 ± 10 meV. At higher coverage, dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites.
DA  - 2016///
PY  - 2016
DO  - 10.1038/nmat4512
VL  - 15
SP  - 450
EP  - 455
J2  - Nat. Mater.
LA  - en
SN  - 1476-1122
ER  - 

TY  - JOUR
TI  - A multitechnique study of CO adsorption on the TiO2 anatase (101) surface
AU  - Setvin, Martin
AU  - Buchholz, Maria
AU  - Hou, Weiyi
AU  - Zhang, Cui
AU  - Stöger, Bernhard
AU  - Hulva, Jan
AU  - Simschitz, Thomas
AU  - Shi, Xiao
AU  - Pavelec, Jiri
AU  - Parkinson, Gareth S.
AU  - Xu, Mingchun
AU  - Wang, Yuemin
AU  - Schmid, Michael
AU  - Wöll, Christof
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - The adsorption of carbon monoxide on the anatase TiO2 (101) surface was studied with infrared reflection absorption spectroscopy (IRRAS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT). The IRRAS data reveal only one CO band at ≈2181 cm^-1 for both stoichiometric and reduced TiO2(101) surfaces. From TPD, an adsorption energy of 0.37 ± 0.03 eV is estimated for the isolated molecule, which shifts to slightly smaller values at higher coverages. Combining STM imaging and controlled annealing of the sample confirms the adsorption energies estimated from TPD and the slight repulsive intermolecular interaction. CO molecules desorb from electron-rich, extrinsic donor defect sites at somewhat higher temperatures. Confronting the experimental results with DFT calculations indicates that the anatase (101) surface does not contain any significant concentration of subsurface oxygen vacancies in the near-surface region. Comparison with CO adsorption on the rutile TiO2(110) surface shows that the tendency for excess electron localization in anatase is much weaker than in rutile.
DA  - 2015/09/10/
PY  - 2015
DO  - 10.1021/acs.jpcc.5b07999
VL  - 119
IS  - 36
SP  - 21044
EP  - 21052
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - An atomic-scale view of CO and H2 oxidation on a Pt/Fe3O4 model catalyst
AU  - Bliem, Roland
AU  - van der Hoeven, Jessi
AU  - Zavodny, Adam
AU  - Gamba, Oscar
AU  - Pavelec, Jiri
AU  - de Jongh, Petra E.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Angewandte Chemie International Edition
AB  - Metal–support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal oxide supports, yet the atomic-scale mechanisms are rarely known. In this report, scanning tunneling microscopy was used to study a Pt1-6/Fe3O4 model catalyst exposed to CO, H2, O2, and mixtures thereof at 550 K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface. H2 and O2 dissociate on the metal clusters and spill over onto the support. The former creates surface hydroxy groups, which react with the support, ultimately leading to the desorption of water, while oxygen atoms react with Fe from the bulk to create new Fe3O4(001) islands. The presence of the Pt is crucial because it catalyzes reactions that already occur on the bare iron oxide surface, but only at higher temperatures.
DA  - 2015/11/16/
PY  - 2015
DO  - 10.1002/anie.201507368
VL  - 54
IS  - 47
SP  - 13999
EP  - 14002
J2  - Angew. Chem. Int. Ed.
LA  - en
SN  - 1521-3773
ER  - 

TY  - JOUR
TI  - Adsorption of formic acid on the Fe3O4(001) surface
AU  - Gamba, Oscar
AU  - Noei, Heshmat
AU  - Pavelec, Jiří
AU  - Bliem, Roland
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Stierle, Andreas
AU  - Parkinson, Gareth S.
T2  - The Journal of Physical Chemistry C
AB  - The adsorption of formic acid (HCOOH) on the Fe3O4(001) surface was studied using X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy (IRRAS), low-energy electron diffraction, and scanning tunneling microscopy (STM). At room temperature, HCOOH dissociates to form formate (HCOO–) and hydroxyl groups, facilitated by the close proximity of undercoordinated Fe3+/O2– cation/anion pairs at the Fe3O4(001) surface. Bidentate formate species are observed in IRRAS, and their position on Fe–Fe bridge sites can be inferred from STM data. At 70 K, HCOOH is adsorbed both dissociatively and molecularly, suggesting two active sites for dissociation. Our study also demonstrates that IRRAS is possible on Fe3O4 single crystals with good sensitivity but that unusual peak shapes occur because the substrate is midway between a perfect conductor and a perfect dielectric.
DA  - 2015/09/03/
PY  - 2015
DO  - 10.1021/acs.jpcc.5b05560
VL  - 119
IS  - 35
SP  - 20459
EP  - 20465
J2  - J. Phys. Chem. C
LA  - en
SN  - 1932-7447, 1932-7455
ER  - 

TY  - JOUR
TI  - Nickel-oxide-modified SrTiO3 (110)-(4 × 1) surfaces and their interaction with water
AU  - Gerhold, Stefan
AU  - Riva, Michele
AU  - Wang, Zhiming
AU  - Bliem, Roland
AU  - Wagner, Margareta
AU  - Osiecki, Jacek
AU  - Schulte, Karina
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - Nickel oxide (NiO), deposited onto the strontium titanate (SrTiO3) (110)-(4 × 1) surface, was studied using photoemission spectroscopy (PES), X-ray absorption near edge structure (XANES), and low-energy He+ ion scattering (LEIS), as well as scanning tunneling microscopy (STM). The main motivation for studying this system comes from the prominent role it plays in photocatalysis. The (4 × 1) reconstructed SrTiO3(110) surface was previously found to be remarkably inert toward water adsorption under ultrahigh-vacuum conditions. Nickel oxide grows on this surface as patches without any apparent ordered structure. PES and LEIS reveal an upward band bending, a reduction of the band gap, and reactivity toward water adsorption upon deposition of NiO. Spectroscopic results are discussed with respect to the enhanced reactivity toward water of the NiO-loaded surface.
DA  - 2015/09/03/
PY  - 2015
DO  - 10.1021/acs.jpcc.5b06144
VL  - 119
IS  - 35
SP  - 20481
EP  - 20487
J2  - J. Phys. Chem. C
LA  - en
SN  - 1932-7447, 1932-7455
ER  - 

TY  - JOUR
TI  - Growth of an ultrathin zirconia film on Pt3Zr examined by high-resolution x-ray photoelectron spectroscopy, temperature-programmed desorption, scanning tunneling microscopy, and density functional theory
AU  - Li, Hao
AU  - Choi, Joong-Il Jake
AU  - Mayr-Schmölzer, Wernfried
AU  - Weilach, Christian
AU  - Rameshan, Christoph
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Schmid, Michael
AU  - Rupprechter, Günther
T2  - The Journal of Physical Chemistry C
AB  - Ultrathin (~3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 x 10^-7 mbar of O2 at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO2 films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO2 films were determined by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) (including depth profiling), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Oxidation mainly leads to ultrathin trilayer (O-Zr-O) films on the alloy; only a small area fraction (10-15%) is covered by ZrO2 clusters (thickness ~0.5-10 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the Pt3Zr substrate by ZrO2, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO2 films are between those of metallic Zr and thick (bulklike) ZrO2. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrOx is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO2 films or metal/ZrO2 interfaces. Furthermore, our results indicate that the common approach of calculating core level shifts by DFT including final-state effects should be taken with care for thicker insulating films, clusters, and bulk insulators.
DA  - 2015///
PY  - 2015
DO  - 10.1021/jp5100846
VL  - 119
IS  - 5
SP  - 2462
EP  - 2470
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Molecular ordering at the interface between liquid water and rutile TiO2(110)
AU  - Serrano, Giulia
AU  - Bonanni, Beatrice
AU  - Di Giovannantonio, Marco
AU  - Kosmala, Tomasz
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Di Carlo, Aldo
AU  - Cheng, Jun
AU  - VandeVondele, Joost
AU  - Wandelt, Klaus
AU  - Goletti, Claudio
T2  - Advanced Materials Interfaces
AB  - The pivotal importance of TiO2 as a technological material involves most applications in an aqueous environment, but the single-crystal TiO2/bulk-water interfaces are almost completely unexplored, since up to date solid/liquid interfaces are more difficult to access than surfaces in ultrahigh vacuum (UHV). Only a few techniques (as scanning probe microscopy) offer the opportunity to explore these systems under realistic conditions. The rutile TiO2(110) surface immersed in high-purity water is studied by in situ scanning tunneling microscopy. The large-scale surface morphology as obtained after preparation under UHV conditions remains unchanged upon prolonged exposure to bulk water. Moreover, in contrast to UHV, atomically resolved images show a twofold periodicity along the [001] direction, indicative of an ordered structure resulting from the hydration layer. This is consistent with density-functional theory based molecular dynamics simulations where neighboring interfacial molecules of the first water layer in contact with the bulk liquid form dimers. By contrast, this dimerization is not observed for a single adsorbed water monolayer, i.e., in the absence of bulk water.
DA  - 2015/11/01/
PY  - 2015
DO  - 10.1002/admi.201500246
VL  - 2
IS  - 17
SP  - 1500246
J2  - Adv. Mater. Interfaces
LA  - en
SN  - 2196-7350
ER  - 

TY  - JOUR
TI  - Dual role of CO in the stability of subnano Pt clusters at the Fe3O4(001) surface
AU  - Bliem, Roland
AU  - Hoeven, Jessi E. S. van der
AU  - Hulva, Jan
AU  - Pavelec, Jiri
AU  - Gamba, Oscar
AU  - Jongh, Petra E. de
AU  - Schmid, Michael
AU  - Blaha, Peter
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Proceedings of the National Academy of Sciences
AB  - Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1–CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase.
DA  - 2016/08/09/
PY  - 2016
DO  - 10.1073/pnas.1605649113
VL  - 113
IS  - 32
SP  - 8921
EP  - 8926
J2  - Proc. Natl. Acad. Sci. USA
LA  - en
SN  - 0027-8424, 1091-6490
ER  - 

TY  - JOUR
TI  - Composition and local atomic arrangement of decagonal Al-Co-Cu quasicrystal surfaces
AU  - Zenkyu, R.
AU  - Yuhara, J.
AU  - Matsui, T.
AU  - Shah Zaman, S.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review B
AB  - We investigated the composition of decagonal Al-Co-Cu surface by Auger electron spectroscopy (AES) and low-energy ion scattering (LEIS). The surface composition after annealing was Al richer and Co poorer compared to that after sputtering or bulk composition. Two types of the characteristic clusters were observed by scanning tunneling microscopy (STM) and no bias voltage dependence of the image was observed. On the other hand, scanning tunneling spectroscopy revealed a subtle difference of local density of states in unoccupied states between different sites. Structural optimization using ab initio calculations based on density functional theory (DFT) was performed on several compositional models, which are based on the W-(AlCoNi) bulk model. The surface structures of two types of the characteristic clusters were determined by comparison of the STM image and the simulated image of the structures obtained by DFT. The topmost layer was composed of Al and Cu atoms, and the compositional ratio was consistent with the AES and LEIS results.
DA  - 2012/09/17/
PY  - 2012
DO  - 10.1103/PhysRevB.86.115422
VL  - 86
IS  - 11
SP  - 115422
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Ion-beam-induced magnetic and structural phase transformation of Ni-stabilized face-centered-cubic Fe films on Cu(100)
AU  - Gloss, Jonas
AU  - Shah Zaman, Sameena
AU  - Jonner, Jakub
AU  - Novotny, Zbynek
AU  - Schmid, Michael
AU  - Varga, Peter
AU  - Urbánek, Michal
T2  - Applied Physics Letters
AB  - Metastable face-centered cubic (fcc) Fe/Cu(100) thin films are good candidates for ion-beam magnetic patterning due to their magnetic transformation upon ion-beam irradiation. However, pure fcc Fe films undergo spontaneous transformation when their thickness exceeds 10 ML. This limit can be extended to approximately 22 ML by deposition of Fe at increased CO background pressures. We show that much thicker films can be grown by alloying with Ni for stabilizing the fcc γ phase. The amount of Ni necessary to stabilize nonmagnetic, transformable fcc Fe films in dependence on the residual background pressure during the deposition is determined and a phase diagram revealing the transformable region is presented.
DA  - 2013/12/26/
PY  - 2013
DO  - 10.1063/1.4856775
VL  - 103
IS  - 26
SP  - 262405
J2  - Appl. Phys. Lett.
SN  - 0003-6951, 1077-3118
ER  - 

TY  - JOUR
TI  - Following the reduction of oxygen on TiO2 anatase (101) step by step
AU  - Setvin, Martin
AU  - Aschauer, Ulrich
AU  - Hulva, Jan
AU  - Simschitz, Thomas
AU  - Daniel, Benjamin
AU  - Schmid, Michael
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - Journal of the American Chemical Society
AB  - We have investigated the reaction between O2 and H2O, coadsorbed on the (101) surface of a reduced TiO2 anatase single crystal by scanning tunneling microscopy, density functional theory, temperature-programmed desorption, and X-ray photoelectron spectroscopy. While water adsorbs molecularly on the anatase (101) surface, the reaction with O2 results in water dissociation and formation of terminal OH groups. We show that these terminal OHs are the final and stable reaction product on reduced anatase. We identify OOH as a metastable intermediate in the reaction. The water dissociation reaction runs as long as the surface can transfer enough electrons to the adsorbed species; the energy balance and activation barriers for the individual reaction steps are discussed, depending on the number of electrons available. Our results indicate that the presence of donor dopants can significantly reduce activation barriers for oxygen reduction on anatase.
DA  - 2016/08/03/
PY  - 2016
DO  - 10.1021/jacs.6b04004
VL  - 138
IS  - 30
SP  - 9565
EP  - 9571
J2  - J. Am. Chem. Soc.
SN  - 0002-7863
ER  - 

TY  - JOUR
TI  - Magnetism of FePt surface alloys
AU  - Honolka, J.
AU  - Lee, T. Y.
AU  - Kuhnke, K.
AU  - Enders, A.
AU  - Skomski, R.
AU  - Bornemann, S.
AU  - Mankovsky, S.
AU  - Minar, J.
AU  - Staunton, J.
AU  - Ebert, H.
AU  - Hessler, M.
AU  - Fauth, K.
AU  - Schutz, G.
AU  - Buchsbaum, A.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Kern, K.
T2  - Physical Review Letters
AB  - The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, X-ray magnetic circular dichroism and ab-initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex non-collinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk L1_0 magnets is not essential to achieve high anisotropy values.
DA  - 2009/02/13/
PY  - 2009
DO  - 10.1103/PhysRevLett.102.067207
VL  - 102
IS  - 6
SP  - 067207
EP  - 4
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - High island densities in pulsed laser deposition: Causes and implications
AU  - Schmid, M.
AU  - Lenauer, C.
AU  - Buchsbaum, A.
AU  - Wimmer, F.
AU  - Rauchbauer, G.
AU  - Scheiber, P.
AU  - Betz, G.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - By studying metal growth on Pt(111), we determine the reasons for the high island densities observed in pulsed laser deposition (PLD) compared to conventional thermal deposition. For homoepitaxy by PLD with moderate energies (≲100 eV) of the deposited ions, high island densities are caused by the high instantaneous flux of arriving particles. Additional nuclei are formed at high ion energies (≳200 eV) by adatoms created by the impinging ions. For heteroepitaxy, the island density is also increased by intermixing (deposited material implanted in the surface), creating an inhomogeneous potential energy surface for diffusing atoms. We discuss implications for layer-by-layer growth and sputter deposition.
DA  - 2009/08/10/
PY  - 2009
DO  - 10.1103/PhysRevLett.103.076101
VL  - 103
IS  - 7
SP  - 076101
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Observation and destruction of an elusive adsorbate with STM: O2/TiO2(110)
AU  - Scheiber, Philipp
AU  - Riss, Alexander
AU  - Schmid, Michael
AU  - Varga, Peter
AU  - Diebold, Ulrike
T2  - Physical Review Letters
AB  - When a slightly defective rutile TiO2(110) surface is exposed to O2 at elevated temperatures, the molecule dissociates at defects, filling O vacancies (VO) and creating O adatoms (Oad) on Ti5c rows. The adsorption of molecular O2 at low temperatures has remained controversial. Low-temperature scanning tunneling microscopy of O2, dosed on TiO2(110) at a sample temperature of ≈100 K and imaged at 17 K, shows a molecular precursor at VO as a faint change in contrast. The adsorbed O2 easily dissociates during the STM measurements, and the formation of Oad’s at both sides of the original VO is observed.
DA  - 2010/11/15/
PY  - 2010
DO  - 10.1103/PhysRevLett.105.216101
VL  - 105
IS  - 21
SP  - 216101
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Room temperature water splitting at the surface of magnetite
AU  - Parkinson, Gareth S.
AU  - Novotný, Zbyněk
AU  - Jacobson, Peter
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Journal of the American Chemical Society
AB  - An array of surface science measurements has revealed novel water adsorption behavior at the Fe3O4(001) surface. Following room temperature exposure to water, a low coverage of hydrogen atoms is observed, with no associated water hydroxyl group. Mild annealing of the hydrogenated surface leads to desorption of water via abstraction of surface oxygen atoms, leading to a reduction of the surface. These results point to an irreversible splitting of the water molecule. The observed phenomena are discussed in the context of recent DFT calculations ( Mulakaluri, N. ; Pentcheva, R. ; Scheffler, M. J. Phys. Chem. C 2010, 114, 11148 ), which show that the Jahn–Teller distorted surface isolates adsorbed H in a geometry that could kinetically hinder recombinative desorption. In contrast, the adsorption geometry facilitates interaction between water hydroxyl species, which are concluded to leave the surface following a reactive desorption process, possibly via the creation of O2.
DA  - 2011///
PY  - 2011
DO  - 10.1021/ja203432e
VL  - 133
IS  - 32
SP  - 12650
EP  - 12655
J2  - J. Am. Chem. Soc.
ER  - 

TY  - JOUR
TI  - Unusual cluster shapes and directional bonding of an fcc metal: Pt/Pt(111)
AU  - Schmid, Michael
AU  - Garhofer, Andreas
AU  - Redinger, Josef
AU  - Wimmer, Florian
AU  - Scheiber, Philipp
AU  - Varga, Peter
T2  - Physical Review Letters
AB  - Small clusters of Pt adatoms grown on Pt(111) exhibit a preference for the formation of linear chains, which cannot be explained by simple diffusion-limited aggregation. Density functional theory calculations show that short chains are energetically favorable to more compact configurations due to strong directional bonding by dz2-like orbitals, explaining the stability of the chains. The formation of the chains is governed by substrate distortions, leading to funneling towards the chain ends.
DA  - 2011/06/29/
PY  - 2011
DO  - 10.1103/PhysRevLett.107.016102
VL  - 107
IS  - 1
SP  - 016102
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Nickel carbide as a source of grain rotation in epitaxial graphene
AU  - Jacobson, Peter
AU  - Stöger, Bernhard
AU  - Garhofer, Andreas
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Caudillo, Roman
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - ACS Nano
AB  - Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 × 1 configurations. We have investigated graphene grown by chemical vapor deposition (CVD) on the nickel carbide (Ni2C) reconstruction of Ni(111) with scanning tunneling microscopy (STM). The presence of excess carbon, in the form of Ni2C, prevents graphene from adopting the preferred 1 × 1 configuration and leads to grain rotation. STM measurements show that residual Ni2C domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate Ni2C dissolution after graphene growth. Density functional theory (DFT) calculations predict a very weak (van der Waals type) interaction of graphene with the underlying Ni2C, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as Ni2C can play a major role in the quality of epitaxial graphene.
DA  - 2012/04/24/
PY  - 2012
DO  - 10.1021/nn300625y
VL  - 6
IS  - 4
SP  - 3564
EP  - 3572
J2  - ACS Nano
SN  - 1936-0851, 1936-086X
ER  - 

TY  - JOUR
TI  - Ordered array of single adatoms with remarkable thermal stability: Au/Fe3O4(001)
AU  - Novotný, Zbyněk
AU  - Argentero, Giacomo
AU  - Wang, Zhiming
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Physical Review Letters
AB  - Gold deposited on the Fe3O4(001) surface at room temperature was studied using scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). This surface forms a (√2×√2)R45° reconstruction, where pairs of Fe and neighboring O ions are slightly displaced laterally producing undulating rows with “narrow” and “wide” hollow sites. At low coverages, single Au adatoms adsorb exclusively at the narrow sites, with no significant sintering up to annealing temperatures of 400 °C. We propose the strong site preference to be related to charge and orbital ordering within the first subsurface layer of Fe3O4(001)-(√2×√2)R45°. Because of its high thermal stability, this could prove an ideal model system for probing the chemical reactivity of single atomic species.
DA  - 2012/05/24/
PY  - 2012
DO  - 10.1103/PhysRevLett.108.216103
VL  - 108
IS  - 21
SP  - 216103
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - (Sub)surface mobility of oxygen vacancies at the TiO2 anatase (101) surface
AU  - Scheiber, Philipp
AU  - Fidler, Martin
AU  - Dulub, Olga
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Hou, Weiyi
AU  - Aschauer, Ulrich
AU  - Selloni, Annabella
T2  - Physical Review Letters
AB  - Anatase is a metastable polymorph of TiO2. In contrast to the more widely studied TiO2 rutile, O vacancies (VO’s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface VO’s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures ≥200 K. After an initial decrease of the VO density, a temperature-dependent dynamic equilibrium is established where VO’s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, density functional theory calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously distributed subsurface defects in the reduced sample.
DA  - 2012/09/28/
PY  - 2012
DO  - 10.1103/PhysRevLett.109.136103
VL  - 109
IS  - 13
SP  - 136103
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Reaction of O2 with subsurface oxygen vacancies on TiO2 anatase (101)
AU  - Setvín, Martin
AU  - Aschauer, Ulrich
AU  - Scheiber, Philipp
AU  - Li, Ye-Fei
AU  - Hou, Weiyi
AU  - Schmid, Michael
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - Science
AB  - Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2–) at fivefold-coordinated Ti sites was transformed to peroxo (O22–) and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.
DA  - 2013/08/30/
PY  - 2013
DO  - 10.1126/science.1239879
VL  - 341
IS  - 6149
SP  - 988
EP  - 991
J2  - Science
LA  - en
SN  - 0036-8075, 1095-9203
ER  - 

TY  - JOUR
TI  - The role of defects in the local reaction kinetics of CO oxidation on low-index Pd surfaces
AU  - Vogel, D.
AU  - Spiel, C.
AU  - Schmid, M.
AU  - Stöger-Pollach, M.
AU  - Schlögl, R.
AU  - Suchorski, Y.
AU  - Rupprechter, G.
T2  - The Journal of Physical Chemistry C
AB  - The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented ?m-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10?5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.
DA  - 2013/06/13/
PY  - 2013
DO  - 10.1021/jp312510d
VL  - 117
IS  - 23
SP  - 12054
EP  - 12060
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Water adsorption at the tetrahedral titania surface layer of SrTiO3(110)-(4 × 1)
AU  - Wang, Zhiming
AU  - Hao, Xianfeng
AU  - Gerhold, Stefan
AU  - Novotny, Zbynek
AU  - Franchini, Cesare
AU  - McDermott, Eamon
AU  - Schulte, Karina
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - The interaction of water with oxide surfaces is of great interest for both fundamental science and applications. We present a combined theoretical (density functional theory (DFT)) and experimental (scanning tunneling microscopy (STM) and photoemission spectroscopy (PES)) study of water interaction with the two-dimensional titania overlayer that terminates the SrTiO3(110)-(4 x 1) surface and consists of TiO4 tetrahedra. STM and core-level and valence band PES show that H2O neither adsorbs nor dissociates on the stoichiometric surface at room temperature, whereas it does dissociate at oxygen vacancies. This is in agreement with DFT calculations, which show that the energy barriers for water dissociation on the stoichiometric and reduced surfaces are 1.7 and 0.9 eV, respectively. We propose that water weakly adsorbs on two-dimensional, tetrahedrally coordinated overlayers.
DA  - 2013/12/12/
PY  - 2013
DO  - 10.1021/jp407889h
VL  - 117
IS  - 49
SP  - 26060
EP  - 26069
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Metal adatoms and clusters on ultrathin zirconia films
AU  - Choi, Joong Il Jake
AU  - Mayr-Schmölzer, Wernfried
AU  - Valenti, Ilaria
AU  - Luches, Paola
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - The Journal of Physical Chemistry C
AB  - Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO2 films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO2 films as well as the (1̅11) surface of monoclinic ZrO2. STM shows decreasing cluster size, indicative of increasing metal–oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO2/Pt3Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal–oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO2, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO2 film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between −1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO2, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO2{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces.
DA  - 2016/05/12/
PY  - 2016
DO  - 10.1021/acs.jpcc.6b03061
VL  - 120
IS  - 18
SP  - 9920
EP  - 9932
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Direct view at excess electrons in TiO2 rutile and anatase
AU  - Setvin, Martin
AU  - Franchini, Cesare
AU  - Hao, Xianfeng
AU  - Schmid, Michael
AU  - Janotti, Anderson
AU  - Kaltak, Merzuk
AU  - Van de Walle, Chris G.
AU  - Kresse, Georg
AU  - Diebold, Ulrike
T2  - Physical Review Letters
AB  - A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in TiO2 rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.
DA  - 2014/08/18/
PY  - 2014
DO  - 10.1103/PhysRevLett.113.086402
VL  - 113
IS  - 8
SP  - 086402
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Charge trapping at the step edges of TiO2 anatase (101)
AU  - Setvin, Martin
AU  - Hao, Xianfeng
AU  - Daniel, Benjamin
AU  - Pavelec, Jiri
AU  - Novotny, Zbynek
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Kresse, Georg
AU  - Franchini, Cesare
AU  - Diebold, Ulrike
T2  - Angewandte Chemie International Edition
AB  - A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO2 anatase (101) surface are trapped at step edges. Consequently, steps act as preferred adsorption sites for O2. In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide-ranging applications of titania.
DA  - 2014///
PY  - 2014
DO  - 10.1002/anie.201309796
VL  - 53
IS  - 18
SP  - 4714
EP  - 4716
J2  - Angew. Chem. Int. Ed.
LA  - en
SN  - 1521-3773
ER  - 

TY  - JOUR
TI  - High chemical activity of a perovskite surface: reaction of CO with Sr3Ru2O7
AU  - Stöger, Bernhard
AU  - Hieckel, Marcel
AU  - Mittendorfer, Florian
AU  - Wang, Zhiming
AU  - Fobes, David
AU  - Peng, Jin
AU  - Mao, Zhiqiang
AU  - Schmid, Michael
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - Physical Review Letters
AB  - Adsorption of CO at the Sr3Ru2O7(001) surface was studied with low-temperature scanning tunneling microscopy (STM) and density functional theory. In situ cleaved single crystals terminate in an almost perfect SrO surface. At 78 K, CO first populates impurities and then adsorbs above the apical surface O with a binding energy Eads=−0.7 eV. Above 100 K, this physisorbed CO replaces the surface O, forming a bent CO2 with the C end bound to the Ru underneath. The resulting metal carboxylate (Ru-COO) can be desorbed by STM manipulation. A low activation (0.2 eV) and high binding (−2.2 eV) energy confirm a strong reaction between CO and regular surface sites of Sr3Ru2O7; likely, this reaction causes the “UHV aging effect” reported for this and other perovskite oxides.
DA  - 2014/09/10/
PY  - 2014
DO  - 10.1103/PhysRevLett.113.116101
VL  - 113
IS  - 11
SP  - 116101
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Stabilizing single Ni adatoms on a two-dimensional porous titania overlayer at the SrTiO3(110) surface
AU  - Wang, Zhiming
AU  - Hao, Xianfeng
AU  - Gerhold, Stefan
AU  - Mares, Petr
AU  - Wagner, Margareta
AU  - Bliem, Roland
AU  - Schulte, Karina
AU  - Schmid, Michael
AU  - Franchini, Cesare
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - Nickel vapor-deposited on the SrTiO3(110) surface was studied using scanning tunneling microscopy, photoemission spectroscopy (PES), and density functional theory calculations. This surface forms a (4 ? 1) reconstruction, composed of a 2-D titania structure with periodic six- and ten-membered nanopores. Anchored at these nanopores, Ni single adatoms are stabilized at room temperature. PES measurements show that the Ni adatoms create an in-gap state located at 1.9 eV below the conduction band minimum and induce an upward band bending. Both experimental and theoretical results suggest that Ni adatoms are positively charged. Our study produces well-dispersed single-adatom arrays on a well-characterized oxide support, providing a model system to investigate single-adatom catalytic and magnetic properties.
DA  - 2014/08/28/
PY  - 2014
DO  - 10.1021/jp506234r
VL  - 118
IS  - 34
SP  - 19904
EP  - 19909
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Reducing the In2O3(111) surface results in ordered indium adatoms
AU  - Wagner, Margareta
AU  - Seiler, Steffen
AU  - Meyer, Bernd
AU  - Boatner, Lynn A.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Advanced Materials Interfaces
AB  - When the In2O3 surface is subjected to reducing conditions it responds with the formation of an ordered array of isolated In adatoms. The facile back-and-forth between the oxidized and reduced surface should allow tuning surface properties in a judicious and clear-cut manner. On a more fundamental level, the ordered adatom structure is an entirely novel way of any oxide surface to respond to reducing conditions.
DA  - 2014/11/01/
PY  - 2014
DO  - 10.1002/admi.201400289
VL  - 1
IS  - 8
SP  - 1400289
J2  - Adv. Mater. Interfaces
LA  - en
SN  - 2196-7350
ER  - 

TY  - JOUR
TI  - Interplay between steps and oxygen vacancies on curved TiO2(110)
AU  - Miccio, Luis A.
AU  - Setvin, Martin
AU  - Müller, Moritz
AU  - Abadía, Mikel
AU  - Piquero, Ignacio
AU  - Lobo-Checa, Jorge
AU  - Schiller, Frederik
AU  - Rogero, Celia
AU  - Schmid, Michael
AU  - Sánchez-Portal, Daniel
AU  - Diebold, Ulrike
AU  - Ortega, J. Enrique
T2  - Nano Letters
AB  - A vicinal rutile TiO2(110) crystal with a smooth variation of atomic steps parallel to the [1?10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1?11] zigzag faceting to straight [1?10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (Obr vacs) vanish. The [1?10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (St) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of Obr vacs plus St protrusions at steps, suggesting that both Obr vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti3+ polaron near steps and at terraces.
DA  - 2016/03/09/
PY  - 2016
DO  - 10.1021/acs.nanolett.5b05286
VL  - 16
IS  - 3
SP  - 2017
EP  - 2022
J2  - Nano Lett.
SN  - 1530-6984
ER  - 

TY  - JOUR
TI  - Adsorption and incorporation of transition metals at the magnetite Fe3O4(001) surface
AU  - Bliem, Roland
AU  - Pavelec, Jiri
AU  - Gamba, Oscar
AU  - McDermott, Eamon
AU  - Wang, Zhiming
AU  - Gerhold, Stefan
AU  - Wagner, Margareta
AU  - Osiecki, Jacek
AU  - Schulte, Karina
AU  - Schmid, Michael
AU  - Blaha, Peter
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Physical Review B
AB  - The adsorption of Ni, Co, Mn, Ti, and Zr at the (√2×√2)R45°-reconstructed Fe3O4(001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the (√2×√2)R45° reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (√2×√2)R45° reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1×1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe3O4.
DA  - 2015/08/26/
PY  - 2015
DO  - 10.1103/PhysRevB.92.075440
VL  - 92
IS  - 7
SP  - 075440
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Aggregation and electronically induced migration of oxygen vacancies in TiO2 anatase
AU  - Setvin, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Physical Review B
AB  - The influence of the electric field and electric current on the behavior of oxygen vacancies (VOs) in TiO2 anatase was investigated with scanning tunneling microscopy (STM). At the anatase (101) surface VOs are not stable; they migrate into the bulk at temperatures above 200 K. Scanning a clean anatase (101) surface at a sample bias greater than ≈+4.3 V results in surface VOs in the scanned area, suggesting that subsurface VOs migrate back to the surface. To test this hypothesis, surface VOs were first created through bombardment with energetic electrons. The sample was then mildly annealed, which caused the VOs to move to the subsurface region, where they formed vacancy clusters. These VO clusters have various, distinct shapes. Scanning VO clusters with a high STM bias reproducibly converts them back into groupings of surface VO, with a configuration that is characteristic for each type of cluster. The dependence of the subsurface-to-surface VO migration on the applied STM bias voltage, tunneling current, and sample temperature was investigated systematically. The results point towards a key role of energetic, “hot” electrons in this process. The findings are closely related to the memristive behavior of oxides and oxygen diffusion in solid-oxide membranes.
DA  - 2015/05/06/
PY  - 2015
DO  - 10.1103/PhysRevB.91.195403
VL  - 91
IS  - 19
SP  - 195403
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Coexistence of trapped and free excess electrons in SrTiO3
AU  - Hao, Xianfeng
AU  - Wang, Zhiming
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
T2  - Physical Review B
AB  - The question whether excess electrons in SrTiO3 form free or trapped carriers is a crucial aspect for the electronic properties of this important material. This fundamental ambiguity prevents a consistent interpretation of the puzzling experimental situation, where results support one or the other scenario depending on the type of experiment that is conducted. Using density functional theory with an on-site Coulomb interaction U, we show that excess electrons form small polarons if the density of electronic carriers is higher than ≈10^20 cm^-3. Below this value, the electrons stay delocalized or become large polarons. For oxygen-deficient SrTiO3, small polarons confined to Ti3+ sites are immobile at low temperature but can be thermally activated into a conductive state, which explains the metal-insulator transition observed experimentally.
DA  - 2015/02/12/
PY  - 2015
DO  - 10.1103/PhysRevB.91.085204
VL  - 91
IS  - 8
SP  - 085204
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Electron transfer between anatase TiO2 and an O2 molecule directly observed by atomic force microscopy
AU  - Setvin, Martin
AU  - Hulva, Jan
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Proceedings of the National Academy of Sciences
AB  - Activation of molecular oxygen is a key step in converting fuels into energy, but there is precious little experimental insight into how the process proceeds at the atomic scale. Here, we show that a combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) experiment can both distinguish neutral O2 molecules in the triplet state from negatively charged (O2)− radicals and charge and discharge the molecules at will. By measuring the chemical forces above the different species adsorbed on an anatase TiO2 surface, we show that the tip-generated (O2)− radicals are identical to those created when (i) an O2 molecule accepts an electron from a near-surface dopant or (ii) when a photo-generated electron is transferred following irradiation of the anatase sample with UV light. Kelvin probe spectroscopy measurements indicate that electron transfer between the TiO2 and the adsorbed molecules is governed by competition between electron affinity of the physisorbed (triplet) O2 and band bending induced by the (O2)− radicals. Temperature–programmed desorption and X-ray photoelectron spectroscopy data provide information about thermal stability of the species, and confirm the chemical identification inferred from AFM/STM.
DA  - 2017/03/28/
PY  - 2017
DO  - 10.1073/pnas.1618723114
VL  - 114
IS  - 13
SP  - E2556
EP  - E2562
J2  - Proc. Natl. Acad. Sci. USA
LA  - en
SN  - 0027-8424, 1091-6490
ER  - 

TY  - JOUR
TI  - Physical-chemical stability of fluorinated III-N surfaces: Towards the understanding of the (0001) AlxGa1-xN surface donor modification by fluorination
AU  - Reiner, Maria
AU  - Schellander, Josef
AU  - Denifl, Günter
AU  - Stadtmueller, Michael
AU  - Schmid, Michael
AU  - Frischmuth, Tobias
AU  - Schmid, Ulrich
AU  - Pietschnig, Rudolf
AU  - Ostermaier, Clemens
T2  - Journal of Applied Physics
AB  - Gallium nitride based high electron mobility transistors are widely known for their operational instabilities regarding interface defects to the dielectric. In this paper, we discuss a III-N surface treatment that results in an electrically more defined interface and hence a narrower distribution of electrically present interface states compared to the original, untreated interface. This surface modification is caused by a remote plasma fluorination of the III-N surface. We show that it is a very distinctive surface processing which cannot be reproduced by other plasma techniques or ion implantation. Applying physical and chemical analyses, the fluorination is found to have a remarkable stability towards temperatures up to 700 °C and is also stable in air for up to 180 h. However, an aqueous clean allows the surface to return to its original state. Even though the exact physical origin of the responsible surface donor cannot be inferred, we suggest that fluorine itself might not directly represent the new surface donor but that it rather activates the III-N surface prior to the dielectric deposition or even substitutes and hence reduces the concentration of surface hydroxides.
DA  - 2017/06/09/
PY  - 2017
DO  - 10.1063/1.4985345
VL  - 121
IS  - 22
SP  - 225704
J2  - J. Appl. Phys.
SN  - 0021-8979
ER  - 

TY  - JOUR
TI  - Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy
AU  - Setvín, Martin
AU  - Wagner, Margareta
AU  - Schmid, Michael
AU  - Parkinson, Gareth S.
AU  - Diebold, Ulrike
T2  - Chemical Society Reviews
AB  - Metal oxides are abundant in nature and they are some of the most versatile materials for applications ranging from catalysis to novel electronics. The physical and chemical properties of metal oxides are dramatically influenced, and can be judiciously tailored, by defects. Small changes in stoichiometry introduce so-called intrinsic defects, e.g., atomic vacancies and/or interstitials. This review gives an overview of using Scanning Probe Microscopy (SPM), in particular Scanning Tunneling Microscopy (STM), to study the changes in the local geometric and electronic structure related to these intrinsic point defects at the surfaces of metal oxides. Three prototypical systems are discussed: titanium dioxide (TiO2), iron oxides (Fe3O4), and, as an example for a post-transition-metal oxide, indium oxide (In2O3). Each of these three materials prefers a different type of surface point defect: oxygen vacancies, cation vacancies, and cation adatoms, respectively. The different modes of STM imaging and the promising capabilities of non-contact Atomic Force Microscopy (nc-AFM) techniques are discussed, as well as the capability of STM to manipulate single point defects.
DA  - 2017/04/03/
PY  - 2017
DO  - 10.1039/C7CS00076F
VL  - 46
IS  - 7
SP  - 1772
EP  - 1784
J2  - Chem. Soc. Rev.
LA  - en
SN  - 1460-4744
ER  - 

TY  - JOUR
TI  - Well-ordered In adatoms at the In2O3(111) surface created by Fe deposition
AU  - Wagner, Margareta
AU  - Lackner, Peter
AU  - Seiler, Steffen
AU  - Gerhold, Stefan
AU  - Osiecki, Jacek
AU  - Schulte, Karina
AU  - Boatner, Lynn A.
AU  - Schmid, Michael
AU  - Meyer, Bernd
AU  - Diebold, Ulrike
T2  - Physical Review Letters
AB  - Metal deposition on oxide surfaces usually results in adatoms, clusters, or islands of the deposited material, where defects in the surface often act as nucleation centers. Here an alternate configuration is reported. After the vapor deposition of Fe on the In2O3(111) surface at room temperature, ordered adatoms are observed with scanning tunneling microscopy. These are identical to the In adatoms that form when the sample is reduced by heating in ultrahigh vacuum. Density functional theory calculations confirm that Fe interchanges with In in the topmost layer, pushing the excess In atoms to the surface where they arrange as a well-ordered adatom array.
DA  - 2016/11/11/
PY  - 2016
DO  - 10.1103/PhysRevLett.117.206101
VL  - 117
IS  - 20
SP  - 206101
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Oxygen-stabilized Rh adatoms: 0D oxides on a vicinal surface
AU  - Mittendorfer, Florian
AU  - Franz, Thomas
AU  - Klikovits, Jan
AU  - Schmid, Michael
AU  - Merte, Lindsay R.
AU  - Shah Zaman, Sameena
AU  - Varga, Peter
AU  - Westerström, Rasmus
AU  - Resta, Andrea
AU  - Andersen, Jesper N.
AU  - Gustafson, Johan
AU  - Lundgren, Edvin
T2  - The Journal of Physical Chemistry Letters
AB  - We have investigated the initial oxidation of the Rh(113) and Rh(223) vicinal surfaces by STM and ab initio simulations. Upon adsorption of small amounts of oxygen, the surface morphology is completely altered. Surprisingly, oxygen-stabilized Rh adatoms can be observed on the (113) facets, with oxide-like electronic properties. We present models of these "0D oxide" phases and discuss reasons for their stability.
We have investigated the initial oxidation of the Rh(113) and Rh(223) vicinal surfaces by STM and ab initio simulations. Upon adsorption of small amounts of oxygen, the surface morphology is completely altered. Surprisingly, oxygen-stabilized Rh adatoms can be observed on the (113) facets, with oxide-like electronic properties. We present models of these "0D oxide" phases and discuss reasons for their stability.
DA  - 2011/10/19/
PY  - 2011
DO  - 10.1021/jz2011308
VL  - 2
IS  - 21
SP  - 2747
EP  - 2751
J2  - J. Phys. Chem. Lett.
SN  - 1948-7185
ER  - 

TY  - JOUR
TI  - Cluster nucleation and growth from a highly supersaturated adatom phase: silver on magnetite
AU  - Bliem, Roland
AU  - Kosak, Rukan
AU  - Perneczky, Lukas
AU  - Novotny, Zbynek
AU  - Gamba, Oscar
AU  - Fobes, David
AU  - Mao, Zhiqiang
AU  - Schmid, Michael
AU  - Blaha, Peter
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - ACS Nano
AB  - The atomic-scale mechanisms underlying the growth of Ag on the (√2x√2)R45°-Fe3O4(001) surface were studied using scanning tunneling microscopy and density functional theory based calculations. For coverages up to 0.5 ML, Ag adatoms populate the surface exclusively; agglomeration into nanoparticles occurs only with the lifting of the reconstruction at 720 K. Above 0.5 ML, Ag clusters nucleate spontaneously and grow at the expense of the surrounding material with mild annealing. This unusual behavior results from a kinetic barrier associated with the (√2x√2)R45° reconstruction, which prevents adatoms from transitioning to the thermodynamically favorable 3D phase. The barrier is identified as the large separation between stable adsorption sites, which prevents homogeneous cluster nucleation and the instability of the Ag dimer against decay to two adatoms. Since the system is dominated by kinetics as long as the (√2x√2)R45° reconstruction exists, the growth is not well described by the traditional growth modes. It can be understood, however, as the result of supersaturation within an adsorption template system.
DA  - 2014/07/22/
PY  - 2014
DO  - 10.1021/nn502895s
VL  - 8
IS  - 7
SP  - 7531
EP  - 7537
J2  - ACS Nano
SN  - 1936-0851
ER  - 

TY  - JOUR
TI  - Construction and evaluation of an ultrahigh-vacuum-compatible sputter deposition source
AU  - Lackner, Peter
AU  - Choi, Joong Il Jake
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Review of Scientific Instruments
AB  - A sputter deposition source for the use in ultrahigh vacuum (UHV) is described, and some properties of the source are analyzed. The operating principle is based on the design developed by Mayr et al. [Rev. Sci. Instrum. 84, 094103 (2013)], where electrons emitted from a filament ionize argon gas and the Ar+ ions are accelerated to the target. In contrast to the original design, two grids are used to direct a large fraction of the Ar+ ions to the target, and the source has a housing cooled by liquid nitrogen to reduce contaminations. The source has been used for the deposition of zirconium, a material that is difficult to evaporate in standard UHV evaporators. At an Ar pressure of 9×10^−6 mbar in the UHV chamber and moderate emission current, a highly reproducible deposition rate of ≈1 ML in 250 s was achieved at the substrate (at a distance of ≈50 mm from the target). Higher deposition rates are easily possible. X-ray photoelectron spectroscopy shows a high purity of the deposited films. Depending on the grid voltages, the substrate gets mildly sputtered by Ar+ ions; in addition, the substrate is also reached by electrons from the negatively biased sputter target.
DA  - 2017/10/01/
PY  - 2017
DO  - 10.1063/1.4998700
VL  - 88
IS  - 10
SP  - 103904
J2  - Rev. Sci. Instrum.
SN  - 0034-6748
ER  - 

TY  - JOUR
TI  - Ordered hydroxyls on Ca3Ru2O7(001)
AU  - Halwidl, Daniel
AU  - Mayr-Schmölzer, Wernfried
AU  - Fobes, David
AU  - Peng, Jin
AU  - Mao, Zhiqiang
AU  - Schmid, Michael
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - Nature Communications
AB  - As complex ternary perovskite-type oxides are increasingly used in solid oxide fuel cells, electrolysis and catalysis, it is desirable to obtain a better understanding of their surface chemical properties. Here we report a pronounced ordering of hydroxyls on the cleaved (001) surface of the Ruddlesden-Popper perovskite Ca3Ru2O7 upon water adsorption at 105 K and subsequent annealing to room temperature. Density functional theory calculations predict the dissociative adsorption of a single water molecule (E
                        ads = 1.64 eV), forming an (OH)ads group adsorbed in a Ca-Ca bridge site, with an H transferred to a neighboring surface oxygen atom, Osurf. Scanning tunneling microscopy images show a pronounced ordering of the hydroxyls with (2 × 1), c(2 × 6), (1 × 3), and (1 × 1) periodicity. The present work demonstrates the importance of octahedral rotation and tilt in perovskites, for influencing surface reactivity, which here induces the ordering of the observed OH overlayers. As ternary perovskite-type oxides are increasingly used in fuel cells and catalysis, greater understanding of their surface chemical properties is required. Here the authors report a pronounced ordering of hydroxyls on the cleaved (001) surface of Ca3Ru2O7 perovskite induced by O-octahedral rotation and tilt.
DA  - 2017/06/20/
PY  - 2017
DO  - 10.1038/s41467-017-00066-w
VL  - 8
IS  - 1
SP  - 23
J2  - Nat. Commun.
LA  - En
SN  - 2041-1723
ER  - 

TY  - JOUR
TI  - Disorder and defect healing in graphene on Ni(111)
AU  - Jacobson, Peter
AU  - Stöger, Bernhard
AU  - Garhofer, Andreas
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Caudillo, Roman
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry Letters
AB  - The structural evolution of graphene on Ni(111) is investigated as a function of growth temperature by scanning tunneling microscopy (STM). Low temperature (400–500 °C) growth results in a continuous but highly defective film with small ordered graphene domains and disordered domains composed of Stone-Wales (SW)-like defects. As the growth temperature is increased, the disordered domains shrink leaving small clusters of defects alongside epitaxially matched graphene. Density functional theory (DFT) calculations indicate the crucial role of the metallic support for the healing of SW defects, as the interaction with the substrate leads to a stabilization of the reaction intermediate. This work highlights the effect of the graphene-substrate interaction on the temperature dependence of the defect concentration in epitaxial graphene on Ni(111).
DA  - 2012///
PY  - 2012
DO  - 10.1021/jz2015007
VL  - 3
SP  - 136
EP  - 139
J2  - J. Phys. Chem. Lett.
SN  - 1948-7185
ER  - 

TY  - JOUR
TI  - Water adsorption at zirconia: from the ZrO2(111)/Pt3Zr(0001) model system to powder samples
AU  - Lackner, Peter
AU  - Hulva, Jan
AU  - Köck, Eva-Maria
AU  - Mayr-Schmölzer, Wernfried
AU  - Choi, Joong Il J.
AU  - Penner, Simon
AU  - Diebold, Ulrike
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Klötzer, Bernhard
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
T2  - Journal of Materials Chemistry A
AB  - We present a comprehensive study of water adsorption and desorption on an ultrathin trilayer zirconia film using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), as well as scanning tunneling microscopy (STM) at different temperatures. The saturation coverage is one H2O per surface Zr atom, with about 12% dissociation. The monolayer TPD peak (180 K, desorption barrier 0.57 ± 0.04 eV) has a tail towards higher temperatures, caused by recombinative desorption from defect sites with dissociated water. STM shows that the defects with the strongest H2O adsorption are found above subsurface dislocations. Additional defect sites are created by multiple water adsorption/desorption cycles; these water-induced changes were also probed by CO2 TPD. Nevertheless, the defect density is much smaller than in previous studies of H2O/ZrO2. To validate our model system, transmission Fourier-transform infrared absorption spectroscopy (FTIR) studies at near-ambient pressures were carried out on monoclinic zirconia powder, showing comparable adsorption energies as TPD on the ultrathin film. The results are also compared with density functional theory (DFT) calculations, which suggest that sites with strong H2O adsorption contain twofold-coordinated oxygen.
DA  - 2018/09/18/
PY  - 2018
DO  - 10.1039/C8TA04137G
VL  - 6
IS  - 36
SP  - 17587
EP  - 17601
J2  - J. Mater. Chem. A
LA  - en
SN  - 2050-7496
ER  - 

TY  - JOUR
TI  - Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide
AU  - Riva, Michele
AU  - Kubicek, Markus
AU  - Hao, Xianfeng
AU  - Franceschi, Giada
AU  - Gerhold, Stefan
AU  - Schmid, Michael
AU  - Hutter, Herbert
AU  - Fleig, Juergen
AU  - Franchini, Cesare
AU  - Yildiz, Bilge
AU  - Diebold, Ulrike
T2  - Nature Communications
AB  - Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules.
DA  - 2018/09/13/
PY  - 2018
DO  - 10.1038/s41467-018-05685-5
VL  - 9
IS  - 1
SP  - 3710
J2  - Nat. Commun.
LA  - en
SN  - 2041-1723
ER  - 

TY  - JOUR
TI  - High-affinity adsorption leads to molecularly ordered interfaces on TiO2 in air and solution
AU  - Balajka, Jan
AU  - Hines, Melissa A.
AU  - DeBenedetti, William J. I.
AU  - Komora, Mojmir
AU  - Pavelec, Jiri
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Science
AB  - Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO2) photocatalysts exposed to air and solution. Using a combination of atomic-scale microscopy and spectroscopy, we show that TiO2 selectively adsorbs atmospheric carboxylic acids that are typically present in parts-per-billion concentrations while effectively repelling other adsorbates, such as alcohols, that are present in much higher concentrations. The high affinity of the surface for carboxylic acids is attributed to their bidentate binding. These self-assembled monolayers have the unusual property of being both hydrophobic and highly water-soluble, which may contribute to the self-cleaning properties of TiO2. This finding is relevant to TiO2 photocatalysis, because the self-assembled carboxylate monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis.
Self-assembled monolayers that form on titanium dioxide surfaces in air are attributed to atmospheric organic acids.
DA  - 2018/08/24/
PY  - 2018
DO  - 10.1126/science.aat6752
VL  - 361
IS  - 6404
SP  - 786
EP  - 789
J2  - Science
LA  - en
SN  - 0036-8075, 1095-9203
ER  - 

TY  - JOUR
TI  - Apparatus for dosing liquid water in ultrahigh vacuum
AU  - Balajka, Jan
AU  - Pavelec, Jiri
AU  - Komora, Mojmir
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Review of Scientific Instruments
AB  - The structure of the solid-liquid interface often defines the function and performance of materials in applications. To study this interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows bringing a surface in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice. The ice is at cryogenic temperature, which reduces its vapor pressure to the UHV range. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. In test experiments, a rutile TiO2(110) single crystal exposed to liquid water showed unprecedented surface purity, as established by X-ray photoelectron spectroscopy and scanning tunneling microscopy. These results enabled us to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed.
DA  - 2018/08/01/
PY  - 2018
DO  - 10.1063/1.5046846
VL  - 89
IS  - 8
SP  - 083906
J2  - Rev. Sci. Instrum.
SN  - 0034-6748
ER  - 

TY  - JOUR
TI  - Prototypical organic–oxide interface: Intramolecular resolution of sexiphenyl on In2O3(111)
AU  - Wagner, Margareta
AU  - Hofinger, Jakob
AU  - Setvín, Martin
AU  - Boatner, Lynn A.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - ACS Applied Materials & Interfaces
AB  - The performance of an organic semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. Although an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material are essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In2O3(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide, is based on. The onset of nucleation and formation of the first monolayer are followed with atomically resolved scanning tunneling microscopy and noncontact atomic force microscopy (nc-AFM). Annealing to 200 °C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests an essentially planar adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with a poor long-range order. Eventually, the molecules reorient into an ordered monolayer. This first monolayer has a densely packed, well-ordered (2 × 1) structure with one 6P per In2O3(111) substrate unit cell, that is, a molecular density of 5.64 × 1013 cm–2.
DA  - 2018/03/28/
PY  - 2018
DO  - 10.1021/acsami.8b02177
VL  - 10
IS  - 16
SP  - 14175
EP  - 14182
J2  - ACS Appl. Mater. Interfaces
LA  - en
AN  - world
ER  - 

TY  - JOUR
TI  - Surface oxides on close-packed surfaces of late transition metals
AU  - Lundgren, Edvin
AU  - Mikkelsen, Anders
AU  - Andersen, Jesper N.
AU  - Kresse, Georg
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Journal of Physics: Condensed Matter
AB  - In recent years, the formation of thin, well-ordered but complex surface oxides on late transition metals has been discovered. The driving force for this line of research has been the strong incentive to increase the partial pressure of oxygen from ultra-high vacuum to conditions more relevant for heterogeneous catalysis. Here we review the present status of the research field. Compared to oxygen adatom superstructures, the structure of the surface oxides has proven to be extremely complex, and the investigations have therefore relied on a combination of several experimental and theoretical techniques. The approach to solving the structures formed on close-packed surfaces of Pd and Rh is presented in some detail. Focusing on the structures found, we show that the surface oxides share some general properties with the corresponding bulk oxides. Nevertheless, of all surface oxide structures known today, only the two-dimensional surface oxides on Pd(100) and Pt(110) have the same lattice as the bulk oxides (PdO and PtO, respectively). In addition to two-dimensional oxides, including the O-Rh-O trilayers found on Rh, one-dimensional oxides were observed at ridges or steps of open surfaces such as (110) or vicinal surfaces. Finally, we briefly report on a few studies of the reactivity of surface oxides with well-known structure.
DA  - 2006///
PY  - 2006
VL  - 18
IS  - 30
SP  - R481
EP  - R499
J2  - J. Phys.: Condens. Matter
SN  - 0953-8984
ER  - 

TY  - JOUR
TI  - The Pd(100)-(√5 x √5)R27°-O surface oxide: A LEED, DFT and STM study
AU  - Kostelník, Petr
AU  - Seriani, Nicola
AU  - Kresse, Georg
AU  - Mikkelsen, Anders
AU  - Lundgren, Edvin
AU  - Blum, Volker
AU  - Šikola, Tomás
AU  - Varga, Peter
AU  - Schmid, Michael
T2  - Surface Science
AB  - Using low energy electron diffraction (LEED), density functional theory (DFT) and scanning tunneling microscopy (STM), we have re-analyzed the Pd(100)-(√5 x √5)R27°-O surface oxide structure consisting, in the most recent model, of a strained PdO(101) layer on top of the Pd(100) surface [M. Todorova et al., Surf. Sci. 541 (2003) 101]. Both, DFT simulations using the Vienna Ab initio Simulation Package (VASP) and tensor LEED I(V) analysis of newly acquired LEED experimental data, show that the PdO(101) model is essentially correct. However, compared to the previous study, there is a horizontal shift of the PdO(101) layer with respect to the Pd(100) substrate. The atomic coordinates derived by DFT and LEED (RP = 0.162) are in excellent agreement with each other. We also present STM images with atomic resolution showing domain boundaries on the surface oxide and discuss the bonding geometry between the surface oxide and the substrate.
DA  - 2007/03/15/
PY  - 2007
DO  - 10.1016/j.susc.2007.01.026
VL  - 601
IS  - 6
SP  - 1574
EP  - 1581
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Oxidation of Pd(553): From ultrahigh vacuum to atmospheric pressure
AU  - Westerström, R.
AU  - Gustafson, J.
AU  - Resta, A.
AU  - Mikkelsen, A.
AU  - Andersen, J. N.
AU  - Lundgren, E.
AU  - Seriani, N.
AU  - Mittendorfer, F.
AU  - Schmid, M.
AU  - Klikovits, J.
AU  - Varga, P.
AU  - Ackermann, M. D.
AU  - Frenken, J. W. M.
AU  - Kasper, N.
AU  - Stierle, A.
T2  - Physical Review B
AB  - The oxidation of a vicinal Pd(553) surface has been studied from ultrahigh vacuum (UHV) to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below 10(-6) mbar and that the (553) surface is preserved. In the pressure range between 10(-6) and 1 mbar and at a sample temperature of 300-400 degrees C, a surface oxide forms and rearranges the (553) surface facets and forming (332) facets. Most of the surface oxide can be described as a PdO(101) plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the (√6 x √6) Pd5O4 surface oxide can be observed on (111) terraces larger than those of the (332) terraces. Increasing the O pressure above 1 mbar results in the disappearance of the (332) facets and the formation of PdO bulk oxide.
DA  - 2007/10/15/
PY  - 2007
DO  - 10.1103/PhysRevB.76.155410
VL  - 76
IS  - 15
SP  - 155410
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Water agglomerates on Fe3O4(001)
AU  - Meier, Matthias
AU  - Hulva, Jan
AU  - Jakub, Zdeněk
AU  - Pavelec, Jiří
AU  - Setvin, Martin
AU  - Bliem, Roland
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
AU  - Parkinson, Gareth S.
T2  - Proceedings of the National Academy of Sciences
AB  - Determining the structure of water adsorbed on solid surfaces is a notoriously difficult task and pushes the limits of experimental and theoretical techniques. Here, we follow the evolution of water agglomerates on Fe3O4(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH–H2O bonds drive the formation of partially dissociated water dimers at low coverage, but a surface reconstruction restricts the density of such species to one per unit cell. The dimers act as an anchor for further water molecules as the coverage increases, leading first to partially dissociated water trimers, and then to a ring-like, hydrogen-bonded network that covers the entire surface. Unraveling this complexity requires the concerted application of several state-of-the-art methods. Quantitative temperature-programmed desorption (TPD) reveals the coverage of stable structures, monochromatic X-ray photoelectron spectroscopy (XPS) shows the extent of partial dissociation, and noncontact atomic force microscopy (AFM) using a CO-functionalized tip provides a direct view of the agglomerate structure. Together, these data provide a stringent test of the minimum-energy configurations determined via a van der Waals density functional theory (DFT)-based genetic search.
DA  - 2018/06/19/
PY  - 2018
DO  - 10.1073/pnas.1801661115
VL  - 115
IS  - 25
SP  - E5642
EP  - E5650
J2  - Proc. Natl. Acad. Sci. USA
LA  - en
SN  - 0027-8424, 1091-6490
ER  - 

TY  - JOUR
TI  - Formation and dynamics of small polarons on the rutile TiO2(110) surface
AU  - Reticcioli, Michele
AU  - Setvin, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
T2  - Physical Review B
AB  - Charge trapping and the formation of polarons is a pervasive phenomenon in transition-metal oxide compounds, in particular at the surface, affecting fundamental physical properties and functionalities of the hosting materials. Here we investigate via first-principles techniques the formation and dynamics of small polarons on the reduced surface of titanium dioxide, an archetypal system for polarons, for a wide range of oxygen vacancy concentrations. We report how the essential features of polarons can be adequately accounted for in terms of a few quantities: the local structural and chemical environment, the attractive interaction between negatively charged polarons and positively charged oxygen vacancies, and the spin-dependent polaron-polaron Coulomb repulsion. We combined molecular-dynamics simulations on realistic samples derived from experimental observations with simplified static models, aiming to disentangle the various variables at play. We find that depending on the specific trapping site, different types of polarons can be formed, with distinct orbital symmetries and a different degree of localization and structural distortion. The energetically most stable polaron site is the subsurface Ti atom below the undercoordinated surface Ti atom, due to a small energy cost to distort the lattice and a suitable electrostatic potential. Polaron-polaron repulsion and polaron-vacancy attraction determine the spatial distribution of polarons as well as the energy of the polaronic in-gap state. In the range of experimentally reachable oxygen vacancy concentrations, the calculated data are in excellent agreement with observations, thus validating the overall interpretation.
DA  - 2018/07/20/
PY  - 2018
DO  - 10.1103/PhysRevB.98.045306
VL  - 98
IS  - 4
SP  - 045306
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Pushing the detection of cation nonstoichiometry to the limit
AU  - Riva, Michele
AU  - Franceschi, Giada
AU  - Lu, Qiyang
AU  - Schmid, Michael
AU  - Yildiz, Bilge
AU  - Diebold, Ulrike
T2  - Physical Review Materials
AB  - Nanoscale complex-oxide thin films prepared by well-established growth techniques, such as pulsed-laser deposition or molecular-beam epitaxy, often exhibit compositions that deviate from the ideal stoichiometry. Even small variations in composition can lead to substantial changes in the technologically relevant electronic, magnetic, and optical properties of these materials. To assess the reasons behind this variability, and ultimately to allow tuning the properties of oxide films with precise control of the deposition parameters, high-resolution detection of the nonstoichiometry introduced during growth is needed. The resolution of current techniques, such as x-ray diffraction, fluorescence, or spectroscopy, is limited to estimating composition differences in the percent level, which is often insufficient for electronic-device quality. We develop an unconventional approach based on scanning tunneling microscopy for enabling the determination of cation imbalance introduced in thin films with exceptionally small detection limit. We take advantage of the well-controlled surface reconstructions on SrTiO3(110), and use the established relation between those reconstructions and the surface composition to assess the cation excess deposited in pulsed-laser grown SrTiO3(110) films. We demonstrate that a <0.1% change in cation nonstoichiometry is detectable by our approach. Furthermore, we show that, for thin films that accommodate all the nonstoichiometry at the surface, this method has no fundamental detection limit.
DA  - 2019/04/25/
PY  - 2019
DO  - 10.1103/PhysRevMaterials.3.043802
VL  - 3
IS  - 4
SP  - 043802
J2  - Phys. Rev. Materials
ER  - 

TY  - JOUR
TI  - A multi-technique study of CO2 adsorption on Fe3O4 magnetite
AU  - Pavelec, Jiri
AU  - Hulva, Jan
AU  - Halwidl, Daniel
AU  - Bliem, Roland
AU  - Gamba, Oscar
AU  - Jakub, Zdenek
AU  - Brunbauer, Florian
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - The Journal of Chemical Physics
AB  - The adsorption of CO2 on the Fe3O4(001)-(√2 ×√2)R45° surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO2 binds most strongly at defects related to Fe2+, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages,CO2 adsorbs at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (√2 × √2)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO2 multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements.
DA  - 2017/01/03/
PY  - 2017
DO  - 10.1063/1.4973241
VL  - 146
IS  - 1
SP  - 014701
J2  - J. Chem. Phys.
SN  - 0021-9606
ER  - 

TY  - JOUR
TI  - Research update: Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties
AU  - Urbánek, Michal
AU  - Flajšman, Lukáš
AU  - Křižáková, Viola
AU  - Gloss, Jonáš
AU  - Horký, Michal
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - APL Materials
AB  - Focused ion beam irradiation of metastable Fe78Ni22 thin films grown on Cu(100) substrates is used to create ferromagnetic, body-centered cubic patterns embedded into paramagnetic, face-centered-cubic surrounding. The structural and magnetic phase transformation can be controlled by varying parameters of the transforming gallium ion beam. The focused ion beam parameters such as the ion dose, number of scans, and scanning direction can be used not only to control a degree of transformation but also to change the otherwise four-fold in-plane magnetic anisotropy into the uniaxial anisotropy along a specific crystallographic direction. This change is associated with a preferred growth of specific crystallographic domains. The possibility to create magnetic patterns with continuous magnetization transitions and at the same time to create patterns with periodical changes in magnetic anisotropy makes this system an ideal candidate for rapid prototyping of a large variety of nanostructured samples. Namely, spin-wave waveguides and magnonic crystals can be easily combined into complex devices in a single fabrication step.
DA  - 2018/06/01/
PY  - 2018
DO  - 10.1063/1.5029367
VL  - 6
IS  - 6
SP  - 060701
J2  - APL Mater.
ER  - 

TY  - JOUR
TI  - Incipient ferroelectricity: A route towards bulk-terminated SrTiO3
AU  - Sokolović, Igor
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Setvin, Martin
T2  - Physical Review Materials
AB  - An investigation of bulk-terminated (001) surfaces of SrTiO3, a prototypical cubic perovskite, was made possible with a novel cleaving procedure. Controlled application of strain on a SrTiO3 single-crystal results in a flat cleavage with μm-size domains of SrO and TiO2. Distribution of these two terminations is dictated by ferroelectric domains induced by strain during the cleavage process. Atomically resolved scanning tunneling microscopy/atomic force microscopy measurements reveal the presence of point defects in a well-defined concentration of (14±2)%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the TiO2 termination. These intrinsic defects stabilize the surface by balancing the interplay between ferroelectricity, surface polarity, and surface charge.
DA  - 2019/03/14/
PY  - 2019
DO  - 10.1103/PhysRevMaterials.3.034407
VL  - 3
IS  - 3
SP  - 034407
J2  - Phys. Rev. Materials
ER  - 

TY  - JOUR
TI  - Structure of the ultrathin aluminum oxide film on NiAl(110)
AU  - Kresse, Georg
AU  - Schmid, Michael
AU  - Napetschnig, Evelyn
AU  - Shishkin, Maxim
AU  - Köhler, Lukas
AU  - Varga, Peter
T2  - Science
AB  - The well-ordered aluminum oxide film formed by oxidation of the NiAl(110) surface is the most intensely studied metal surface oxide, but its structure was previously unknown. We determined the structure by extensive ab initio modeling and scanning tunneling microscopy experiments. Because the topmost aluminum atoms are pyramidally and tetrahedrally coordinated, the surface is different from all Al2O3 bulk phases. The film is a wide-gap insulator, although the overall stoichiometry of the film is not Al2O3 but Al10O13. We propose that the same building blocks can be found on the surfaces of bulk oxides, such as the reduced corundum (0001) surface.
DA  - 2005/06/03/
PY  - 2005
DO  - 10.1126/science.1107783
VL  - 308
IS  - 5727
SP  - 1440
EP  - 1442
J2  - Science
ER  - 

TY  - JOUR
TI  - One-dimensional PtO2 at Pt steps: Formation and reaction with CO
AU  - Wang, J. G.
AU  - Li, W. X.
AU  - Borg, M.
AU  - Gustafson, J.
AU  - Mikkelsen, A.
AU  - Pedersen, T. M.
AU  - Lundgren, E.
AU  - Weissenrieder, J.
AU  - Klikovits, J.
AU  - Schmid, M.
AU  - Hammer, B.
AU  - Andersen, J. N.
T2  - Physical Review Letters
AB  - Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO2 oxide structure forms at the steps of the Pt(332) surface after O2 exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.
DA  - 2005/12/15/
PY  - 2005
DO  - 10.1103/PhysRevLett.95.256102
VL  - 95
IS  - 25
SP  - 256102
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Kinetics of the reduction of the Rh(111) surface oxide: Linking spectroscopy and atomic-scale information
AU  - Klikovits, J.
AU  - Schmid, M.
AU  - Gustafson, J.
AU  - Mikkelsen, A.
AU  - Resta, A.
AU  - Lundgren, E.
AU  - Andersen, J. N.
AU  - Varga, P.
T2  - The Journal of Physical Chemistry B
AB  - The reduction of the surface oxide on Rh(111) by H2 was observed in situ by scanning tunneling microscopy (STM) and high-resolution core level spectroscopy (HRCLS). At room temperature, H2 does not adsorb on the oxide, only in reduced areas. Reduction starts in very few sites, almost exclusively in stepped areas. One can also initiate the reduction process by deliberately creating defects with the STM tip allowing us to examine the reduction kinetics in detail. Depending on the size of the reduced area and the hydrogen pressure, two growth regimes were found. At low H2 pressures or small reduced areas, the reduction rate is limited by hydrogen adsorption on the reduced area. For large reduced areas, the reduction rate is limited by the processes at the border of the reduced area. Since a near-random distribution of the reduction nuclei was found and the reduction process at defects starts at a random time, one can use JohnsonMehlAvramiKolmogoroff (JMAK) theory to describe the process of reduction. The microscopic data from STM agree well with spatially averaged data from HRCLS measurements.
DA  - 2006/05/01/
PY  - 2006
DO  - 10.1021/jp0611875
VL  - 110
IS  - 20
SP  - 9966
EP  - 9975
J2  - J. Phys. Chem. B
ER  - 

TY  - JOUR
TI  - Unusual process of water formation on RuO2(110) by hydrogen exposure at room temperature
AU  - Knapp, M.
AU  - Crihan, D.
AU  - Seitsonen, A. P.
AU  - Resta, A.
AU  - Lundgren, E.
AU  - Andersen, J. N.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Over, H.
T2  - The Journal of Physical Chemistry B
AB  - The reduction mechanism of the RuO2(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO2(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (ObrH) species and subsequently part of these ObrH groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.
DA  - 2006/07/01/
PY  - 2006
DO  - 10.1021/jp0626622
VL  - 110
IS  - 29
SP  - 14007
EP  - 14010
J2  - J. Phys. Chem. B
ER  - 

TY  - JOUR
TI  - Fabrication of a well-ordered nanohole array stable at room temperature
AU  - Aït-Mansour, K.
AU  - Buchsbaum, A.
AU  - Ruffieux, P.
AU  - Schmid, M.
AU  - Gröning, P.
AU  - Varga, P.
AU  - Fasel, R.
AU  - Gröning, O.
T2  - Nano Letters
AB  - We report on the fabrication of a new type of nanotemplate surface consisting of a hexagonally well-ordered array of one monolayer deep holes with a tunable size of about 4 nm2 and a fixed spacing of 7 nm. The nanohole array fabrication is based on the strain-relief trigonal network formed in the 2 monolayer Ag on Pt(111) system. Removing about 0.1 ML of the Ag top layer of this surface structure, for example, by He- or Ar-ion sputtering, leads to the formation of nanoholes at specific domains of the trigonal network, which are stable at room temperature.
DA  - 2008/07/01/
PY  - 2008
DO  - 10.1021/nl8013378
VL  - 8
IS  - 7
SP  - 2035
EP  - 2040
J2  - Nano Lett.
ER  - 

TY  - JOUR
TI  - Step-orientation-dependent oxidation: From 1D to 2D oxides
AU  - Klikovits, J.
AU  - Schmid, M.
AU  - Merte, LR.
AU  - Varga, P.
AU  - Westerström, R.
AU  - Resta, A
AU  - Andersen, JN
AU  - Gustafson, J
AU  - Mikkelsen, A
AU  - Lundgren, E
AU  - Mittendorfer, F
AU  - Kresse, G
T2  - Physical Review Letters
AB  - Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having {100} and {111} microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles.
DA  - 2008/12/31/
PY  - 2008
DO  - 10.1103/PhysRevLett.101.266104
VL  - 101
IS  - 26
SP  - 266104
J2  - Phys. Rev. Lett.
SN  - 0031-9007
ER  - 

TY  - JOUR
TI  - Zirconium-palladium interactions during dry reforming of methane
AU  - Köpfle, Norbert
AU  - Mayr, Lukas
AU  - Lackner, Peter
AU  - Schmid, Michael
AU  - Schmidmair, Daniela
AU  - Götsch, Thomas
AU  - Penner, Simon
AU  - Kloetzer, Bernhard
T2  - ECS Transactions
AB  - Catalytic investigations on chemical-vapor-deposition (CVD)-prepared Pd/Zr^0 /ZrO_x H_y inverse model catalysts and Pd/Zr intermetallic compound system in dry reforming of methane (DRM) are presented. DRM, which produces syngas, is an economically favourable way to operate an SOFC by reusing the already heated CO2 exhaust. The catalytic investigations of the Pd/Zr system yield important information for the design of novel electrode materials or external reforming catalysts. From a catalytic perspective, the initially bimetallic Pd-Zr pre-catalyst shows a distinct activity for dry reforming of methane. This activity can be ascribed to synergistic bifunctional cooperation of palladium and zirconium. Moreover, the investigations clearly demonstrate that metallic Zr is crucial to observe any activity. Therefore, different bulk and surface sensitive methods are used to follow the evolution of structural and redox changes of Zr. Studies of single-crystalline Pd(111) show that Zr^0 is located exclusively in subsurface layers after annealing in vacuum and prior to reaction.
DA  - 2017///
PY  - 2017
DO  - 10.1149/07801.2419ecst
VL  - 78
IS  - 1
SP  - 2419
EP  - 2430
J2  - ECS Trans.
LA  - en
SN  - 2151-2041, 2151-2043
ER  - 

TY  - JOUR
TI  - A full monolayer of superoxide: oxygen activation on the unmodified Ca3Ru2O7(001) surface
AU  - Halwidl, Daniel
AU  - Mayr-Schmölzer, Wernfried
AU  - Setvin, Martin
AU  - Fobes, David
AU  - Peng, Jin
AU  - Mao, Zhiqiang
AU  - Schmid, Michael
AU  - Mittendorfer, Florian
AU  - Redinger, Josef
AU  - Diebold, Ulrike
T2  - Journal of Materials Chemistry A
AB  - Activating the O2 molecule is at the heart of a variety of technological applications, most prominently in energy conversion schemes including solid oxide fuel cells, electrolysis, and catalysis. Perovskite oxides, both traditionally-used and novel formulations, are the prime candidates in established and emerging energy devices. This work shows that the as-cleaved and unmodified CaO-terminated (001) surface of Ca3Ru2O7, a Ruddlesden–Popper perovskite, supports a full monolayer of superoxide ions, O2−, when exposed to molecular O2. The electrons for activating the molecule are transferred from the subsurface RuO2 layer. Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA), predict the adsorption of O2− with Eads = 0.72 eV and provide a thorough analysis of the charge transfer. Non-contact atomic force microscopy (nc-AFM) and scanning tunnelling microscopy (STM) are used to resolve single molecules and confirm the predicted adsorption structures. Local contact potential difference (LCPD) and X-ray photoelectron spectroscopy (XPS) measurements on the full monolayer of O2− confirm the negative charge state of the molecules. The present study reports the rare case of an oxide surface without dopants, defects, or low-coordinated sites readily activating molecular O2.
DA  - 2018/04/03/
PY  - 2018
DO  - 10.1039/C8TA00265G
VL  - 6
IS  - 14
SP  - 5703
EP  - 5713
J2  - J. Mater. Chem. A
LA  - en
SN  - 2050-7496
ER  - 

TY  - JOUR
TI  - Oxide surface science
AU  - Diebold, Ulrike
AU  - Li, Shao-Chun
AU  - Schmid, Michael
T2  - Annual Review of Physical Chemistry
AB  - Most metals are oxidized under ambient conditions, and metal oxides show interesting and technologically promising properties. This has motivated much recent research on oxide surfaces. The combination of scanning tunneling microscopy with first-principles density functional theory–based computational techniques provides an atomic-scale view of the properties of metal-oxide materials. Surface polarity is a key concept for predicting the stability of oxide surfaces and is discussed using ZnO as an example. This review also highlights the role of surface defects for surface reactivity, and their interplay with defects in the bulk, for the case of TiO2. Ultrathin metal-oxide films, grown either through reactive evaporation on metal single crystals or through oxidation of metal alloys (such as Al2O3/NiAl), have gained popularity as supports for planar model catalysts. The surface oxides that form upon oxidation on Pt-group metals (e.g., Ru, Rh, Pd, and Pt) are considered as model systems for CO oxidation.
DA  - 2010/03//
PY  - 2010
DO  - 10.1146/annurev.physchem.012809.103254
VL  - 61
IS  - 1
SP  - 129
EP  - 148
J2  - Annu. Rev. Phys. Chem.
SN  - 0066-426X
ER  - 

TY  - JOUR
TI  - Polarity compensation mechanisms on the perovskite surface KTaO3(001)
AU  - Setvin, Martin
AU  - Reticcioli, Michele
AU  - Poelzleitner, Flora
AU  - Hulva, Jan
AU  - Schmid, Michael
AU  - Boatner, Lynn A.
AU  - Franchini, Cesare
AU  - Diebold, Ulrike
T2  - Science
AB  - Compensating a polar surface
An ionic crystal surface can be electrostatically unstable, and the surface must reconstruct in some way to avoid this “polar catastrophe.” Setvin et al. used scanning probe microscopies and density functional theory to study the changes in the polar surface of the perovskite KTaO3. They observed several structural reconstructions as the surface cleaved in vacuum was heated to higher temperatures. These ranged from surface distortions to the formation of oxygen vacancies to the development of KO and TaO2 stripes. Hydroxylation after exposure to water vapor also stabilized the surface.
Science, this issue p. 572
The stacking of alternating charged planes in ionic crystals creates a diverging electrostatic energy—a “polar catastrophe”—that must be compensated at the surface. We used scanning probe microscopies and density functional theory to study compensation mechanisms at the perovskite potassium tantalate (KTaO3) (001) surface as increasing degrees of freedom were enabled. The as-cleaved surface in vacuum is frozen in place but immediately responds with an insulator-to-metal transition and possibly ferroelectric lattice distortions. Annealing in vacuum allows the formation of isolated oxygen vacancies, followed by a complete rearrangement of the top layers into an ordered pattern of KO and TaO2 stripes. The optimal solution is found after exposure to water vapor through the formation of a hydroxylated overlayer with ideal geometry and charge.
An ionic material can alleviate the energetic instability of its polar surface in several different ways.
An ionic material can alleviate the energetic instability of its polar surface in several different ways.
DA  - 2018/02/02/
PY  - 2018
DO  - 10.1126/science.aar2287
VL  - 359
IS  - 6375
SP  - 572
EP  - 575
J2  - Science
LA  - en
SN  - 0036-8075, 1095-9203
ER  - 

TY  - JOUR
TI  - Surface structure of TiO2 rutile (011) exposed to liquid water
AU  - Balajka, Jan
AU  - Aschauer, Ulrich
AU  - Mertens, Stijn F. L.
AU  - Selloni, Annabella
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - The rutile TiO2(011) surface exhibits a (2 × 1) reconstruction when prepared by standard techniques in ultrahigh vacuum (UHV). Here we report that a restructuring occurs upon exposing the surface to liquid water at room temperature. The experiment was performed in a dedicated UHV system, equipped for direct and clean transfer of samples between UHV and liquid environment. After exposure to liquid water, an overlayer with a (2 × 1) symmetry was observed containing two dissociated water molecules per unit cell. The two OH groups yield an apparent “c(2 × 1)” symmetry in scanning tunneling microscopy (STM) images. On the basis of STM analysis and density functional theory (DFT) calculations, this overlayer is attributed to dissociated water on top of the unreconstructed (1 × 1) surface. Investigation of possible adsorption structures and analysis of the domain boundaries in this structure provide strong evidence that the original (2 × 1) reconstruction is lifted. Unlike the (2 × 1) reconstruction, the (1 × 1) surface has an appropriate density and symmetry of adsorption sites. The possibility of contaminant-induced restructuring was excluded based on X-ray photoelectron spectroscopy (XPS) and low-energy He+ ion scattering (LEIS) measurements.
DA  - 2017/11/30/
PY  - 2017
DO  - 10.1021/acs.jpcc.7b09674
VL  - 121
IS  - 47
SP  - 26424
EP  - 26431
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - The role of surface defects in the adsorption of methanol on Fe3O4(001)
AU  - Gamba, Oscar
AU  - Hulva, Jan
AU  - Pavelec, Jiri
AU  - Bliem, Roland
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Topics in Catalysis
AB  - The adsorption of methanol (CH3OH) at the Fe3O4(001)−(√2 × √2)R45° surface was studied using X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed desorption (TPD). CH3OH adsorbs exclusively at surface defect sites at room temperature to form hydroxyl groups and methoxy (CH3O) species. Active sites are identified as step edges, iron adatoms, antiphase domain boundaries in the (√2 × √2)R45° reconstruction, and above Fe atoms incorporated in the subsurface. In TPD, recombinative desorption is observed around 300 K, and a disproportionation reaction to form methanol and formaldehyde occurs at 470 K.
DA  - 2017/05/01/
PY  - 2017
DO  - 10.1007/s11244-016-0713-9
VL  - 60
IS  - 6-7
SP  - 420
EP  - 430
J2  - Top Catal
LA  - en
SN  - 1022-5528, 1572-9028
ER  - 

TY  - JOUR
TI  - Self-limiting adsorption of WO3 oligomers on oxide substrates in solution
AU  - Müllner, Matthias
AU  - Balajka, Jan
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Mertens, Stijn F. L.
T2  - The Journal of Physical Chemistry C
AB  - Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO2(110) and magnetite Fe3O4(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO3 to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO3 spontaneously forms under all conditions of potential and pH at the tungsten–aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials.
DA  - 2017/09/14/
PY  - 2017
DO  - 10.1021/acs.jpcc.7b04076
VL  - 121
IS  - 36
SP  - 19743
EP  - 19750
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Formaldehyde adsorption on the anatase TiO2(101) surface: Experimental and theoretical investigation
AU  - Setvin, Martin
AU  - Hulva, Jan
AU  - Wang, Honghong
AU  - Simschitz, Thomas
AU  - Schmid, Michael
AU  - Parkinson, Gareth S.
AU  - Di Valentin, Cristiana
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - Formaldehyde (CH2O) adsorption on the anatase TiO2(101) surface was studied with a combination of experimental and theoretical methods. Scanning tunneling microscopy, noncontact atomic force microscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy were employed on the experimental side. Density functional theory was used to calculate formaldehyde adsorption configurations and energy barriers for transitions between them. At low coverages (<0.25 monolayer), CH2O binds via its oxygen atom to the surface 5-coordinated Ti atoms Ti5c (monodentate configuration). At higher coverages, many adsorption configurations with comparable adsorption energies coexist, including a bidentate configuration and paraformaldehyde chains. The adsorption energies of all possible adsorption configurations lie in the range from 0.6 to 0.8 eV. Upon annealing, all formaldehyde molecules desorb below room temperature; no other reaction products were detected.
DA  - 2017/04/27/
PY  - 2017
DO  - 10.1021/acs.jpcc.7b01434
VL  - 121
IS  - 16
SP  - 8914
EP  - 8922
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Methanol on anatase TiO2 (101): mechanistic insights into photocatalysis
AU  - Setvin, Martin
AU  - Shi, Xiao
AU  - Hulva, Jan
AU  - Simschitz, Thomas
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Di Valentin, Cristiana
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - ACS Catalysis
AB  - The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction.
DA  - 2017/10/06/
PY  - 2017
DO  - 10.1021/acscatal.7b02003
VL  - 7
IS  - 10
SP  - 7081
EP  - 7091
J2  - ACS Catal.
ER  - 

TY  - JOUR
TI  - Atomic-scale structure of the hematite α-Fe2O3(11̅02) “r-cut” surface
AU  - Kraushofer, Florian
AU  - Jakub, Zdenek
AU  - Bichler, Magdalena
AU  - Hulva, Jan
AU  - Drmota, Peter
AU  - Weinold, Michael
AU  - Schmid, Michael
AU  - Setvin, Martin
AU  - Diebold, Ulrike
AU  - Blaha, Peter
AU  - Parkinson, Gareth S.
T2  - The Journal of Physical Chemistry C
AB  - The α-Fe2O3(11̅02) surface (also known as the hematite r-cut or (012) surface) was studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and ab initio density functional theory (DFT)+U calculations. Two surface structures are stable under ultrahigh vacuum (UHV) conditions; a stoichiometric (1 × 1) surface can be prepared by annealing at 450 °C in ≈10–6 mbar O2, and a reduced (2 × 1) reconstruction is formed by UHV annealing at 540 °C. The (1 × 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by ≈0.2 eV with respect to the bulk. The work function is measured to be 5.7 ± 0.2 eV, and the VBM is located 1.5 ± 0.1 eV below EF. The images obtained from the (2 × 1) reconstruction cannot be reconciled with previously proposed models, and a new “alternating trench” structure is proposed based on an ordered removal of lattice oxygen atoms. DFT+U calculations show that this surface is favored in reducing conditions and that 4-fold-coordinated Fe2+ cations at the surface introduce gap states approximately 1 eV below EF. The work function on the (2 × 1) termination is 5.4 ± 0.2 eV.
DA  - 2018/01/25/
PY  - 2018
DO  - 10.1021/acs.jpcc.7b10515
VL  - 122
IS  - 3
SP  - 1657
EP  - 1669
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - Motional capacitance of layered piezoelectric thickness-mode resonators
AU  - Schmid, M.
AU  - Benes, E.
AU  - Burger, W.
AU  - Kravchenko, V.
T2  - IEEE Transactions on  Ultrasonics, Ferroelectrics and Frequency Control
AB  - The Butterworth-Van Dyke equivalent circuit for description of the electrical behavior of piezoelectric bulk resonators is considered. The motional capacitance, C1, in the circuit characterizes the strength of piezoelectric excitability of a vibration mode. For layered one-dimensional (1-D) structures this parameter can be calculated from the admittance given by the transfer matrix description of H. Nowotny and E. Benes (1987). Introducing the equivalent area of a vibration mode, the calculation is generalized for the three-dimensional (3-D) case of thickness-mode vibration amplitudes varying only slowly in the lateral directions. Detailed formulae are given for the case of singly rotated quartz crystals or ultrasonic transducers with additional layers on one or two sides. Good agreement of the calculated C1 with experimental data is shown for mass-loaded planoconvex AT-cut quartz crystals.
DA  - 1991/05//
PY  - 1991
DO  - 10.1109/58.79604
VL  - 38
IS  - 3
SP  - 199
EP  - 206
J2  - IEEE Trans. Ultrason. Ferroelectr. Freq. Control
SN  - 0885-3010
ER  - 

TY  - CONF
TI  - Investigation of quartz crystal thickness shear and twist modes using a new noninterferometric laser speckle measurement method
AU  - Hertl, S.
AU  - Benes, E.
AU  - Wimmer, L.
AU  - Schmid, M.
T2  - 39th Annual Symposium on Frequency Control. 1985
AB  - The measurement of the mechanical vibration amplitudes of quartz crystals has been carried out by a variety of different methods. Interferometric methods firstly were used to investigate out-of-plane vibrations. Enhanced interferometric setups also allow the measurement of in-plane vibrations. Holographic techniques present an instantaneous picture of the whole object, whereas the interferometric methods supply the vibration amplitude distribution point after point. 

A very powerful technique to study amplitude distributios of quartz crystals is the use of X-ray diffraction. Using this method, many quartz-crystal vibration modes have been investigated so far. The measurement setup, however, is complicated and the resolution of the graphic output is rather poor. 

Looking for a simple mechanical setup that is insensitive to environmental disturbances, we developed a new method which allows the measurement of in-plane vibration amplitudes of quartz crystals. The detection range is about one to some hundred nanometers. Due to the use of a microcomputer that controls the whole measurement the graphic output can easily be adapted to the user's needs.
C3  - 39th Annual Symposium on Frequency Control. 1985
DA  - 1985///
PY  - 1985
DO  - 10.1109/FREQ.1985.200895
SP  - 535
EP  - 543
ER  - 

TY  - JOUR
TI  - Layered piezoelectric resonators with an arbitrary number of electrodes (general one-dimensional treatment)
AU  - Nowotny, Helmut
AU  - Benes, Ewald
AU  - Schmid, Michael
T2  - Journal of the Acoustical Society of America
AB  - A general transfer matrix description for one-dimensional layered structures consisting of piezoelectric and nonpiezoelectric anisotropic layers of arbitrary number is used to calculate the electrical admittance matrix for such resonators with  N electrodes. The calculation is done in detail for linearly stacked resonators with two free surfaces as well as for ring resonators with a closed acoustical path. Experimental and theoretical results are given and compared for a ring resonator with two piezoelectric layers excited by four electrodes. Such a configuration can be used to generate unidirectional resonant waves.
DA  - 1991///
PY  - 1991
DO  - 10.1121/1.401915
VL  - 90
IS  - 3
SP  - 1238
EP  - 1245
J2  - J. Acoust. Soc. Am.
ER  - 

TY  - CONF
TI  - The role of localized rotational imbalance in drive level dependence phenomena
AU  - EerNisse, E.P.
AU  - Benes, E.
AU  - Schmid, M.
T2  - Frequency Control Symposium and PDA Exhibition, 2002. IEEE International
AB  - The motivation for this study comes from mode shape measurements made of a thickness shear quartz plate with a splatter accidentally attached during Cu deposition. The results show distinct flexural motions extending out to the blank edges in narrow bands. These flexural motions are not present for a clean blank. Finite element analysis calculations are performed for the motion of a contoured quartz thickness shear blank with a small amount of mass attached to the top surface. A 2-dimensional model of the blank is used to facilitate calculation. The added mass causes a localized perturbation of rotational motion that excites a small flexural component extending to the blank edge. This flexural component is similar to a nearby overtone flexural resonant mode. The magnitude of the induced flexural motion varies strongly as the location of the particle moves a few microns from a node to an antinode of the flexural component. This would explain why high level drive can cure a crystal exhibiting drive level dependence (DLD) without the particles being removed and cause others with particles originally not exhibiting DLD to become DLD. It also explains why some blanks with particle contamination function well without showing DLD.
C3  - Frequency Control Symposium and PDA Exhibition, 2002. IEEE International
DA  - 2002///
PY  - 2002
DO  - 10.1109/FREQ.2002.1075847
SP  - 2
EP  - 7
PB  - IEEE
ER  - 

TY  - JOUR
TI  - Bulk terminated NaCl(111) on aluminum: A polar surface of an ionic crystal?
AU  - Hebenstreit, W.
AU  - Schmid, M.
AU  - Redinger, J.
AU  - Podloucky, R.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - Atomically resolved scanning tunneling microscopy reveals the existence of triangular (111) bulk terminated NaCl islands. The islands can be grown by subsequent adsorption of Na and Cl2 on Al(111) and Al(100) or by conversion of stoichiometric NaCl(100) islands to NaCl(111) via additional Na adsorption. The NaCl(111) islands have Na-Cl-Na sandwich structure. Ab initio calculations of the electronic structure of these islands show that each of the Na atoms carries half a positive elementary charge, leaving the islands neutral and explaining the existence of an otherwise unstable surface.
DA  - 2000/12/18/
PY  - 2000
DO  - 10.1103/PhysRevLett.85.5376
VL  - 85
IS  - 25
SP  - 5376
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Interlayer diffusion of adatoms: A scanning-tunneling microscopy study
AU  - Lundgren, E.
AU  - Stanka, B.
AU  - Leonardelli, G.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - Low coverages of Co are deposited at room temperature on a Pt(111) surface with vacancy islands providing a high density of steps. Scanning-tunneling microscopy images with chemical contrast show that Co atoms do not descend Pt steps by diffusing over the step, but descend from the upper terrace to the lower by an exchange diffusion process with the Pt atoms at the step edge. The Co atoms descend a Pt step edge by this process neither at straight A nor at B steps, but rather at the corners or kinks of the vacancy islands.
DA  - 1999/06/21/
PY  - 1999
DO  - 10.1103/PhysRevLett.82.5068
VL  - 82
IS  - 25
SP  - 5068
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - High transient mobility of chlorine on TiO2(110): Evidence for "cannon-ball" trajectories of hot adsorbates
AU  - Diebold, Ulrike
AU  - Hebenstreit, Wilhelm
AU  - Leonardelli, Georg
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Physical Review Letters
AB  - Scanning tunneling microscopy was used to study the initial stages of Cl2 adsorption on TiO2(110). Cl atoms adsorb on the rows of fivefold coordinated surface Ti atoms, and mostly form well separated pairs (average distance 26 Å, atoms can be two or three rows apart). Abstractive adsorption results in 10% single Cl adatoms. We propose that Cl2 dissociates in an approximately upright position. The outer Cl atom is emitted along the bond axis and can surmount the substrate bridging oxygen rows in a “cannon-ball”-like trajectory. Channeling along the Ti rows leads to large average Cl-Cl distances.
DA  - 1998/07/13/
PY  - 1998
DO  - 10.1103/PhysRevLett.81.405
VL  - 81
IS  - 2
SP  - 405
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Potential sputtering of clean SiO2 by slow highly charged ions
AU  - Sporn, M.
AU  - Libiseller, G.
AU  - Neidhart, T.
AU  - Schmid, M.
AU  - Aumayr, F.
AU  - Winter, HP.
AU  - Varga, P.
AU  - Grether, M.
AU  - Niemann, D.
AU  - Stolterfoht, N.
T2  - Physical Review Letters
AB  - The recently discovered phenomenon of potential sputtering, i.e., the efficient removal of neutral and ionized target particles from certain insulator surfaces due to the potential rather than the kinetic energy of impinging slow highly charged ions, has now also been observed for stoichiometric SiO2 surfaces. Using a sensitive quartz crystal microbalance technique, total sputter yields induced by Ar^q+ (q≤14) and Xe^q+ (q≤27) ions have been determined for LiF and SiO2 surfaces. The primary mechanisms for potential sputtering (defect mediated sputtering) and its considerable practical relevance for highly charged ion-induced surface modification of insulators are discussed.
DA  - 1997///
PY  - 1997
DO  - 10.1103/PhysRevLett.79.945
VL  - 79
IS  - 5
SP  - 945
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Quantum wells and electron interference phenomena in Al due to subsurface noble gas bubbles
AU  - Schmid, M.
AU  - Hebenstreit, W.
AU  - Varga, P.
AU  - Crampin, S.
T2  - Physical Review Letters
AB  - Scanning tunneling microscopy on Ar ion bombarded and annealed aluminum surfaces shows electron interference between the surface and subsurface bubbles of implanted gas. The depth of the bubbles as determined from the energy dependence of the standing waves indicates a minimum around 6–7 layers on Al(111). The appearance and energy dependence of the interference pattern is in good agreement with scattering theory based on free electrons, and indicates the bubbles have a shape given by the Wulff construction.
DA  - 1996/03/25/
PY  - 1996
DO  - 10.1103/PhysRevLett.76.2298
VL  - 76
IS  - 13
SP  - 2298
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Scanning tunneling spectroscopy of one-dimensional surface states on a metal surface
AU  - Biedermann, A.
AU  - Genser, O.
AU  - Hebenstreit, W.
AU  - Schmid, M.
AU  - Redinger, J.
AU  - Podloucky, R.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - Scanning tunneling spectroscopy permits real-space observation of one-dimensional electronic states on a Fe(100) surface alloyed with Si. These states are localized along chains of Fe atoms in domain boundaries of the Fe(100) c(2×2)Si surface alloy, as confirmed by first-principles spin-polarized calculations. The calculated charge densities illustrate the d-like orbital character of the one-dimensional state and show its relationship to a two-dimensional state existing on the pure Fe(100) surface.
DA  - 1996/05/27/
PY  - 1996
DO  - 10.1103/PhysRevLett.76.4179
VL  - 76
IS  - 22
SP  - 4179
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Potential sputtering of lithium fluoride by slow multicharged ions
AU  - Neidhart, T.
AU  - Pichler, F.
AU  - Aumayr, F.
AU  - Winter, HP.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - Thin polycrystalline LiF films have been bombarded by slow (≤1 keV) multicharged Arq+ ions (q≤9), in order to study the resulting total sputter yields by means of a quartz crystal microbalance. More than 99% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective “potential sputtering” process already takes place far below 100 eV impact energy. It can be related to defect production in LiF following electron capture by the multicharged ions, and removes about one LiF molecule per 100 eV of projectile potential energy.
DA  - 1995/06/26/
PY  - 1995
DO  - 10.1103/PhysRevLett.74.5280
VL  - 74
IS  - 26
SP  - 5280
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Slowing down adatom diffusion by an adsorbate: Co on Pt(111) with and without preadsorbed CO
AU  - Kalousek, R.
AU  - Schmid, M.
AU  - Hammerschmid, A.
AU  - Lundgren, E.
AU  - Varga, P.
T2  - Physical Review B
AB  - Submonolayer deposition of cobalt on Pt(111) was studied by scanning tunneling microscopy at room temperature. It was found that deposition on a surface with adsorbed carbon monoxide (saturation coverage) leads to an island density almost ten times as high as that resulting from deposition on the clean platinum surface. Based on nucleation theory, we explain this fact as the result of a reduction of the diffusion coefficient of the Co adatoms in the presence of CO by more than two orders of magnitude. This effect is attributed to the displacement and/or rearrangement of the CO molecules necessary when Co adatoms diffuse over the “crowded” surface.
DA  - 2003/12/05/
PY  - 2003
DO  - 10.1103/PhysRevB.68.233401
VL  - 68
IS  - 23
SP  - 233401
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Using photoelectron spectroscopy to observe oxygen spillover to zirconia
AU  - Lackner, Peter
AU  - Zou, Zhiyu
AU  - Mayr, Sabrina
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Physical Chemistry Chemical Physics
AB  - X-ray photoelectron spectroscopy (XPS) of five-monolayer-thick ZrO2 films reveals a core level binding energy difference of up to 1.8 eV between the tetragonal and monoclinic phase. This difference is explained by positively charged oxygen vacancies in the tetragonal films, which are slightly reduced. Due to the large band gap of zirconia (≈5–6 eV), these charges shift the electron levels, leading to higher binding energies of reduced tetragonal films w.r.t. fully oxidized monoclinic films. These core level shifts have the opposite direction than what is usually encountered for reduced transition metal oxides. The vacancies can be filled via oxygen spillover from a catalyst that enables O2 dissociation. This can be either a metal deposited on the film, or, if the film has holes, the metallic (in our case, Rh) substrate. Our study also confirms that tetragonal ZrO2 is stabilized via oxygen vacancies and shows that the XPS binding energy difference between O 1s and Zr 3d solely depends on the crystallographic phase.
DA  - 2019/08/15/
PY  - 2019
DO  - 10.1039/C9CP03322J
VL  - 21
IS  - 32
SP  - 17613
EP  - 17620
J2  - Phys. Chem. Chem. Phys.
LA  - en
SN  - 1463-9084
ER  - 

TY  - JOUR
TI  - Crystallographic structure of ultrathin Fe films on Cu(100)
AU  - Biedermann, Albert
AU  - Tscheließnig, Rupert
AU  - Schmid, Michael
AU  - Varga, Peter
T2  - Physical Review Letters
AB  - We report the observation of bcc-like crystal structures in 2–4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.
DA  - 2001///
PY  - 2001
DO  - 10.1103/PhysRevLett.87.086103
VL  - 87
IS  - 8
SP  - 086103
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Influence of impurities on localized transition metal surface states: scanning tunneling spectroscopy on V(001)
AU  - Bischoff, M. M. J.
AU  - Konvicka, C.
AU  - Quinn, A. J.
AU  - Schmid, M.
AU  - Redinger, J.
AU  - Podloucky, R.
AU  - Varga, P.
AU  - van Kempen, H.
T2  - Physical Review Letters
AB  - The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to ∼0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.
DA  - 2001/03/12/
PY  - 2001
DO  - 10.1103/PhysRevLett.86.2396
VL  - 86
IS  - 11
SP  - 2396
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Adsorption sites and ligand effect for CO on an alloy surface: A direct view
AU  - Gauthier, Y.
AU  - Schmid, M.
AU  - Padovani, S.
AU  - Lundgren, E.
AU  - Buš, V.
AU  - Kresse, G.
AU  - Redinger, J.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - CO adsorption on a PtCo(111) surface was studied by scanning tunneling microscopy. Comparison of images with chemical contrast of Pt and Co and images showing the CO molecules indicates that CO resides exclusively on top of Pt sites and never on Co. CO bonding is highly sensitive to the chemical environment. The probability to find CO on a Pt atom increases drastically with the number of its Co nearest neighbors. Ab initio calculations show that this ligand effect is due to different positions of the center of the Pt d band.
DA  - 2001/06/28/
PY  - 2001
DO  - 10.1103/PhysRevLett.87.036103
VL  - 87
IS  - 3
SP  - 036103
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Kinetically assisted potential sputtering of insulators by highly charged ions
AU  - Hayderer, G.
AU  - Cernusca, S.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Winter, HP.
AU  - Aumayr, F.
AU  - Niemann, D.
AU  - Hoffmann, V.
AU  - Stolterfoht, N.
AU  - Lemell, C.
AU  - Wirtz, L.
AU  - Burgdörfer, J.
T2  - Physical Review Letters
AB  - A new form of potential sputtering has been found for impact of slow ( ≤1500 eV) multiply charged Xe ions (charge states up to q = 25) on MgOx. In contrast to alkali-halide or SiO2 surfaces this mechanism requires the simultaneous presence of electronic excitation of the target material and of a kinetically formed collision cascade within the target in order to initiate the sputtering process. This kinetically assisted potential sputtering mechanism has been identified to be present for other insulating surfaces as well.
DA  - 2001/04/16/
PY  - 2001
DO  - 10.1103/PhysRevLett.86.3530
VL  - 86
IS  - 16
SP  - 3530
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - STM studies of HCI-induced surface damage on highly oriented pyrolytic graphite
AU  - Hayderer, G.
AU  - Cernusca, S.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Winter, HP
AU  - Aumayr, F.
T2  - Physica Scripta
AB  - Scanning tunneling microscopy (STM) with atomic scale resolution has been applied to study surface defects in highly oriented pyrolytic graphite (HOPG) which have been produced by impact of 150 eV singly and multiply charged Ar ions (charge state up to 9+). The most prominent surface defects are protrusions. Their area density is in good agreement with the applied ion dose, implying that about every single ion impact causes one protrusion. A (√3 x √3) R 30° surface reconstruction, as characteristic for interstitial defects in HOPG, is observed in the vicinity of most defects. As the most remarkable result we find that the measured size of the hillocks (mean diameter and height) increases with projectile charge state.
DA  - 2001///
PY  - 2001
DO  - 10.1238/Physica.Topical.092a00156
VL  - T92
SP  - 156
EP  - 157
J2  - Phys. Scripta
SN  - 1402-4896
ER  - 

TY  - JOUR
TI  - Ultrathin films of Co on Pt(111): an STM view
AU  - Varga, P.
AU  - Lundgren, E.
AU  - Redinger, J.
AU  - Schmid, M.
T2  - physica status solidi (a)
AB  - The growth, structure and morphology of ultrathin Co layers with a thickness up to 15 layers deposited at room temperature on Pt(111) have been studied by using scanning tunnelling microscopy (STM) with atomic resolution and chemical discrimination between Co and Pt. This chemical contrast has been confirmed by simulations with an FLAPW (Full Potential Linearized Augmented Plane Waves) ab-initio computer code based on density functional theory. By the help of this contrast between Pt and Co atoms in STM constant current images it is shown that in the early stages of submonolayer growth Co is incorporated into the Pt surface, thereby forming dislocation lines. We were also able to demonstrate that Co atoms descend from the upper terrace to the lower one by an exchange diffusion process with the Pt atoms at the step edges. It is shown that this interlayer diffusion does not take place at straight steps, but rather at corners or kinks. The first completed Co monolayer (ML) is almost pseudomorphic (Co in the Pt fcc lattice sites) with a high density of defects due to the lattice mismatch. The second layer exhibits a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The step edges which are very rough at a coverage of two monolayers become smoother with increasing Co deposition. The growth mode is two-dimensional (layer-by-layer) around two to three monolayers and changes afterward into three-dimensional growth (island growth). We observe that the change of the step edge morphology is also correlated to this change from 2D to 3D growth mode. The reason for the 2D growth at the beginning is attributed to the strained interface between the Co overlayer and the Pt(111) surface which hinders the formation of straight steps. Therefore, many kinks and corners are formed, increasing the probability for interlayer diffusion by the above mentioned exchange process. With increasing number of layers the strain decreases, steps become smoother, interlayer diffusion decreases and therefore island growth develops. Up to the highest coverage (15 ML) studied the growth is characterised by a mainly twinned fcc-like stacking. Only a small amount of hcp stacking has been observed. Further experiments showed that preadsorption of carbon monoxide acts as a surfactant which extends the layer-by-layer growth up to higher Co coverages.
DA  - 2001///
PY  - 2001
DO  - 10.1002/1521-396X(200109)187:1<97::AID-PSSA97>3.0.CO;2-A
VL  - 187
IS  - 1
SP  - 97
EP  - 112
J2  - Physica Stat. Sol.
ER  - 

TY  - JOUR
TI  - When scanning tunneling microscopy gets the wrong adsorption site: H on Rh(100)
AU  - Klein, C.
AU  - Eichler, A.
AU  - Hebenstreit, E. L. D.
AU  - Pauer, G.
AU  - Koller, R.
AU  - Winkler, A.
AU  - Schmid, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.
DA  - 2003/04/29/
PY  - 2003
DO  - 10.1103/PhysRevLett.90.176101
VL  - 90
IS  - 17
SP  - 176101
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Self-limited growth of a thin oxide layer on Rh(111)
AU  - Gustafson, J.
AU  - Mikkelsen, A.
AU  - Borg, M.
AU  - Lundgren, E.
AU  - Köhler, L.
AU  - Kresse, G.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Yuhara, J.
AU  - Torrelles, X.
AU  - Quirós, C.
AU  - Andersen, J. N.
T2  - Physical Review Letters
AB  - The oxidation of the Rh(111) surface at oxygen pressures from 10-10 mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.
DA  - 2004/03/26/
PY  - 2004
DO  - 10.1103/PhysRevLett.92.126102
VL  - 92
IS  - 12
SP  - 126102
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Visualization of atomic processes on ruthenium dioxide using scanning tunneling microscopy
AU  - Over, H.
AU  - Knapp, M.
AU  - Lundgren, E.
AU  - Seitsonen, A. P.
AU  - Schmid, M.
AU  - Varga, P.
T2  - ChemPhysChem
AB  - The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2×2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2×2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.
DA  - 2004///
PY  - 2004
DO  - 10.1002/cphc.200300833
VL  - 5
IS  - 2
SP  - 167
EP  - 174
J2  - ChemPhysChem
ER  - 

TY  - JOUR
TI  - Understanding the structural deactivation of ruthenium catalysts on an atomic scale under both oxidizing and reducing conditions
AU  - Aßmann, Jens
AU  - Crihan, Daniela
AU  - Knapp, Marcus
AU  - Lundgren, Edvin
AU  - Löffler, Elke
AU  - Muhler, Martin
AU  - Narkhede, Vijay
AU  - Over, Herbert
AU  - Schmid, Michael
AU  - Seitsonen, Ari P.
AU  - Varga, Peter
T2  - Angewandte Chemie International Edition
AB  - No Abstract

The surface-science approach coupled with industrial catalyst research offers a synergistic strategy to improve the performance of industrial catalysts. The poorly understood microscopic processes that determine the structural deactivation of ruthenium-based catalysts during CO oxidation have been elucidated. Based on these results measures are proposed to improve the performance of ruthenium catalysts.
DA  - 2005///
PY  - 2005
DO  - 10.1002/anie.200461805
VL  - 44
IS  - 6
SP  - 917
EP  - 920
J2  - Angew. Chem. Int. Ed.
ER  - 

TY  - JOUR
TI  - Probing the geometry of copper and silver adatoms on magnetite: quantitative experiment versus theory
AU  - Meier, Matthias
AU  - Jakub, Zdeněk
AU  - Balajka, Jan
AU  - Hulva, Jan
AU  - Bliem, Roland
AU  - Thakur, Pardeep K.
AU  - Lee, Tien-Lin
AU  - Franchini, Cesare
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Allegretti, Francesco
AU  - Duncan, David A.
AU  - Parkinson, Gareth S.
T2  - Nanoscale
AB  - Accurately modelling the structure of a catalyst is a fundamental prerequisite for correctly predicting reaction pathways, but a lack of clear experimental benchmarks makes it difficult to determine the optimal theoretical approach. Here, we utilize the normal incidence X-ray standing wave (NIXSW) technique to precisely determine the three dimensional geometry of Ag1 and Cu1 adatoms on Fe3O4(001). Both adatoms occupy bulk-continuation cation sites, but with a markedly different height above the surface (0.43 ± 0.03 Å (Cu1) and 0.96 ± 0.03 Å (Ag1)). HSE-based calculations accurately predict the experimental geometry, but the more common PBE + U and PBEsol + U approaches perform poorly.
DA  - 2018///
PY  - 2018
DO  - 10.1039/C7NR07319D
VL  - 10
IS  - 5
SP  - 2226
EP  - 2230
J2  - Nanoscale
LA  - en
SN  - 2040-3364, 2040-3372
ER  - 

TY  - JOUR
TI  - Stability and catalytic performance of reconstructed Fe3O4(001) and Fe3O4(110) surfaces during oxygen evolution reaction
AU  - Müllner, Matthias
AU  - Riva, Michele
AU  - Kraushofer, Florian
AU  - Schmid, Michael
AU  - Parkinson, Gareth S.
AU  - Mertens, Stijn F. L.
AU  - Diebold, Ulrike
T2  - The Journal of Physical Chemistry C
AB  - Earth-abundant oxides are promising candidates as effective and low-cost catalysts for the oxygen evolution reaction (OER) in alkaline media, which remains one of the bottlenecks in electrolysis and artificial photosynthesis. A fundamental understanding of the atomic-scale reaction mechanism during OER could drive further progress, but a stable model system has yet to be provided. Here we show that Fe3O4 single crystal surfaces, prepared in ultrahigh vacuum (UHV) are stable in alkaline electrolytein the range pH 7–14 and under OER conditions in 1 M NaOH. Fe3O4(001) and Fe3O4(110) surfaces were studied with X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy in UHV, and atomic force microscopy in air. Fe3O4(110) is found to be more reactive for oxidative water splitting than (001)-oriented magnetite samples. Magnetite is electrically conductive, and the structure and properties of its major facets are well understood in UHV. With these newly obtained results, we propose magnetite (Fe3O4) as a promising model system for further mechanistic studies of electrochemical reactions in alkaline media and under highly oxidizing conditions.
DA  - 2019/04/04/
PY  - 2019
DO  - 10.1021/acs.jpcc.8b08733
VL  - 123
IS  - 13
SP  - 8304
EP  - 8311
J2  - J. Phys. Chem. C
SN  - 1932-7447
ER  - 

TY  - JOUR
TI  - High-coverage oxygen structures on Rh(111): adsorbate repulsion and site preference is not enough
AU  - Köhler, L.
AU  - Kresse, G.
AU  - Schmid, M.
AU  - Lundgren, E.
AU  - Gustafson, J.
AU  - Mikkelsen, A.
AU  - Borg, M.
AU  - Yuhara, J.
AU  - Andersen, J. N.
AU  - Marsman, M.
AU  - Varga, P.
T2  - Physical Review Letters
AB  - A new O induced structure on Rh(111) displaying a (2sqrt[3]×2sqrt[3])R30° periodicity with an oxygen coverage of 2/3 has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although O favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the (2sqrt[3]×2sqrt[3])R30° and (2×2)-3O structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images.
DA  - 2004/12/21/
PY  - 2004
DO  - 10.1103/PhysRevLett.93.266103
VL  - 93
IS  - 26
SP  - 266103
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Adsorption of CO on the Fe3O4(001) surface
AU  - Hulva, Jan
AU  - Jakub, Zdeněk
AU  - Novotny, Zbynek
AU  - Johansson, Niclas
AU  - Knudsen, Jan
AU  - Schnadt, Joachim
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - The Journal of Physical Chemistry B
AB  - The interaction of CO with the Fe3O4(001)-(√2 × √2)R45° surface was studied using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), the latter both under ultrahigh vacuum (UHV) conditions and in CO pressures up to 1 mbar. In general, the CO–Fe3O4 interaction is found to be weak. The strongest adsorption occurs at surface defects, leading to small TPD peaks at 115, 130, and 190 K. Desorption from the regular surface occurs in two distinct regimes. For coverages up to two CO molecules per (√2 × √2)R45° unit cell, the desorption maximum shows a large shift with increasing coverage, from initially 105 to 70 K. For coverages between 2 and 4 molecules per (√2 × √2)R45° unit cell, a much sharper desorption feature emerges at ∼65 K. Thermodynamic analysis of the TPD data suggests a phase transition from a dilute 2D gas into an ordered overlayer with CO molecules bound to surface Fe3+ sites. XPS data acquired at 45 K in UHV are consistent with physisorption. Some carbon-containing species are observed in the near-ambient-pressure XPS experiments at room temperature but are attributed to contamination and/or reaction with CO with water from the residual gas. No evidence was found for surface reduction or carburization by CO molecules.
DA  - 2018/01/18/
PY  - 2018
DO  - 10.1021/acs.jpcb.7b06349
VL  - 122
IS  - 2
SP  - 721
EP  - 729
J2  - J. Phys. Chem. B
SN  - 1520-6106
ER  - 

TY  - JOUR
TI  - A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion-surface collisions
AU  - Hayderer, G.
AU  - Schmid, M.
AU  - Varga, P.
AU  - Winter, HP.
AU  - Aumayr, F.
T2  - Review of Scientific Instruments
AB  - A quartz-crystal microbalance technique for measuring total sputter yields in ion-surface collisions is described. The electronic circuit to drive the quartz crystal ensures low noise and high frequency stability. By measuring the total sputter yields for impact of singly charged ions on LiF target films a sensitivity limit of 0.5% of a monolayer per minute could be achieved.
DA  - 1999///
PY  - 1999
DO  - 10.1063/1.1149979
VL  - 70
IS  - 9
SP  - 3696
EP  - 3700
J2  - Rev. Sci. Instrum.
ER  - 

TY  - JOUR
TI  - Substoichiometric ultrathin zirconia films cause strong metal–support interaction
AU  - Lackner, Peter
AU  - Choi, Joong Il Jake
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Journal of Materials Chemistry A
AB  - The strong metal–support interaction (SMSI) leads to substantial changes of the properties of an oxide-supported catalyst after annealing under reducing conditions. The common explanation is the formation of heavily reduced, ultrathin oxide films covering metal particles. This is typically encountered for reducible oxides such as TiO2 or Fe3O4. Zirconia (ZrO2) is a catalyst support that is difficult to reduce and therefore no obvious candidate for the SMSI effect. In this work, we use inverse model systems with Rh(111), Pt(111), and Ru(0001) as supports. Contrary to expectations, we show that SMSI is encountered for zirconia. Upon annealing in ultra-high vacuum, oxygen-deficient ultrathin zirconia films (≈ZrO1.5) form on all three substrates. However, Zr remains in its preferred charge state of 4+, as electrons are transferred to the underlying metal. At high temperatures, the stability of the ultrathin zirconia films depends on whether alloying of Zr and the substrate metal occurs. The SMSI effect is reversible; the ultrathin suboxide films can be removed by annealing in oxygen.
DA  - 2019/11/21/
PY  - 2019
DO  - 10.1039/C9TA08438J
VL  - 7
IS  - 43
SP  - 24837
EP  - 24846
J2  - J. Mater. Chem. A
LA  - en
SN  - 2050-7496
ER  - 

TY  - JOUR
TI  - Few-monolayer yttria-doped zirconia films: Segregation and phase stabilization
AU  - Lackner, Peter
AU  - Brandt, Amy J.
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - The Journal of Chemical Physics
AB  - For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to the stabilization of the tetragonal or cubic phase and increased oxygen ion conductivity. Most previous surface studies of yttria-doped zirconia were plagued by impurities, however. We have studied doping of pure, 5-monolayer ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). STM and LEED show that the tetragonal phase is stabilized by unexpectedly low dopant concentrations, 0.5 mol % Y2O3, even when the films are essentially fully oxidized (as evidenced by XPS core level shifts). XPS also shows Y segregation to the surface with an estimated segregation enthalpy of −23 ± 4 kJ/mol.
DA  - 2020/02/11/
PY  - 2020
DO  - 10.1063/1.5140266
VL  - 152
IS  - 6
SP  - 064709
J2  - J. Chem. Phys.
SN  - 0021-9606
ER  - 

TY  - JOUR
TI  - Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing
AU  - Flajšman, Lukáš
AU  - Wagner, Kai
AU  - Vaňatka, Marek
AU  - Gloss, Jonáš
AU  - Křižáková, Viola
AU  - Schmid, Michael
AU  - Schultheiss, Helmut
AU  - Urbánek, Michal
T2  - Physical Review B
AB  - Metastable face-centered-cubic Fe78Ni22 thin films are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxial magnetic in-plane anisotropy and saturation magnetization. Local magnetic anisotropy direction control eliminates the need for external magnetic fields, paving the way towards complex magnonic circuits with waveguides pointing in different directions. In the present study, we show that the magnetocrystalline anisotropy in transformed areas is strong enough to stabilize the magnetization in the direction perpendicular to the long axis of narrow waveguides. Therefore, it is possible to propagate spin waves in these waveguides in the favorable Damon-Eshbach geometry without the presence of any external magnetic field. Phase-resolved microfocused Brillouin light scattering yields the dispersion relation of these waveguides in zero as well as in nonzero external magnetic fields.
DA  - 2020/01/24/
PY  - 2020
DO  - 10.1103/PhysRevB.101.014436
VL  - 101
IS  - 1
SP  - 014436
J2  - Phys. Rev. B
ER  - 

TY  - JOUR
TI  - Epitaxial growth of complex oxide films: Role of surface reconstructions
AU  - Riva, Michele
AU  - Franceschi, Giada
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Physical Review Research
AB  - Achieving atomically flat and stoichiometric films of complex multicomponent oxides is crucial to integrating these materials in both established and emerging technologies. While pulsed laser deposition (PLD) can in principle produce these high-quality films, growth experiments often result in unsatisfactory morphologies with rough surfaces and nonstoichiometric compositions. To understand the cause, the growth needs to be followed at an atomic level from its early stages as a function of the growth conditions. By combining PLD with atomically resolved scanning tunneling microscopy, as well as surface spectroscopic and diffraction techniques, we address the origin of surface roughening in SrTiO3(110) homoepitaxial films and pinpoint optimal growth conditions. We highlight the importance of surface reconstructions at all stages of growth: The different sticking on coexisting surface structures is responsible for the roughening of SrTiO3(110) films and affects their stoichiometry.
DA  - 2019/10/29/
PY  - 2019
DO  - 10.1103/PhysRevResearch.1.033059
VL  - 1
IS  - 3
SP  - 033059
J2  - Phys. Rev. Research
ER  - 

TY  - JOUR
TI  - Growth of In2O3(111) thin films with optimized surfaces
AU  - Franceschi, Giada
AU  - Wagner, Margareta
AU  - Hofinger, Jakob
AU  - Krajňák, Tomáš
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Riva, Michele
T2  - Physical Review Materials
AB  - Indium oxide is widely employed in applications ranging from optoelectronics and gas sensing to catalysis, as well as in thin-film heterostructures. To probe the fundamentals of phenomena at the heart of In2O3-based devices that are tied to the intrinsic surface and interface properties of the material, well-defined single-crystalline In2O3 surfaces are needed. We report on how to grow atomically flat In2O3(111) thin films on yttria-stabilized zirconia substrates by pulsed laser deposition. The films are largely relaxed and reproduce the atomic-scale details of the surfaces of single crystals, except for line defects originating from the antiphase domain boundaries that form because of the one-on-four lattice match between the surface unit cells of In2O3(111) and of the substrate. While optimizing the growth conditions, we observe that the morphology of the films is ruled by the oxygen chemical potential, which determines the nature and diffusivity of adspecies during growth.
DA  - 2019/10/10/
PY  - 2019
DO  - 10.1103/PhysRevMaterials.3.103403
VL  - 3
IS  - 10
SP  - 103403
J2  - Phys. Rev. Materials
ER  - 

TY  - JOUR
TI  - Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water
AU  - Kraushofer, Florian
AU  - Mirabella, Francesca
AU  - Xu, Jian
AU  - Pavelec, Jiří
AU  - Balajka, Jan
AU  - Müllner, Matthias
AU  - Resch, Nikolaus
AU  - Jakub, Zdeněk
AU  - Hulva, Jan
AU  - Meier, Matthias
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - The Journal of Chemical Physics
AB  - Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe3O4(001)-(√2 × √2)R45° surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy and low-energy electron diffraction data show that extensive hydroxylation causes the surface to revert to a bulklike (1 × 1) termination. However, scanning tunneling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM studies leads us to believe that a similar structure exists at the solid-electrolyte interface during immersion at pH 7.
DA  - 2019/10/16/
PY  - 2019
DO  - 10.1063/1.5116652
VL  - 151
IS  - 15
SP  - 154702
J2  - J. Chem. Phys.
SN  - 0021-9606
ER  - 

TY  - JOUR
TI  - Polaron-driven surface reconstructions
AU  - Reticcioli, Michele
AU  - Setvin, Martin
AU  - Hao, Xianfeng
AU  - Flauger, Peter
AU  - Kresse, Georg
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
T2  - Physical Review X
AB  - Geometric and electronic surface reconstructions determine the physical and chemical properties of surfaces and, consequently, their functionality in applications. The reconstruction of a surface minimizes its surface free energy in otherwise thermodynamically unstable situations, typically caused by dangling bonds, lattice stress, or a divergent surface potential, and it is achieved by a cooperative modification of the atomic and electronic structure. Here, we combined first-principles calculations and surface techniques (scanning tunneling microscopy, non-contact atomic force microscopy, scanning tunneling spectroscopy) to report that the repulsion between negatively charged polaronic quasiparticles, formed by the interaction between excess electrons and the lattice phonon field, plays a key role in surface reconstructions. As a paradigmatic example, we explain the (1×1) to (1×2) transition in rutile TiO2(110).
DA  - 2017/09/25/
PY  - 2017
DO  - 10.1103/PhysRevX.7.031053
VL  - 7
IS  - 3
SP  - 031053
J2  - Phys. Rev. X
ER  - 

TY  - JOUR
TI  - Unraveling CO adsorption on model single-atom catalysts
AU  - Hulva, Jan
AU  - Meier, Matthias
AU  - Bliem, Roland
AU  - Jakub, Zdenek
AU  - Kraushofer, Florian
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
AU  - Parkinson, Gareth S.
T2  - Science
AB  - Understanding how the local environment of a “single-atom” catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper1, silver1, gold1, nickel1, palladium1, platinum1, rhodium1, and iridium1 species on Fe3O4(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal–CO bond. These effects could strengthen the bond (as for Ag1–CO) or weaken it (as for Ni1–CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.
DA  - 2021/01/22/
PY  - 2021
DO  - 10.1126/science.abe5757
VL  - 371
IS  - 6527
SP  - 375
EP  - 379
J2  - Science
LA  - en
SN  - 0036-8075, 1095-9203
ER  - 

TY  - JOUR
TI  - A high temperature dual-mode quartz crystal microbalance technique for erosion and thermal desorption spectroscopy measurements
AU  - Stadlmayr, Reinhard
AU  - Szabo, Paul Stefan
AU  - Biber, Herbert
AU  - Koslowski, Hans Rudolf
AU  - Kadletz, Elisabeth
AU  - Cupak, Christian
AU  - Wilhelm, Richard Arthur
AU  - Schmid, Michael
AU  - Linsmeier, Christian
AU  - Aumayr, Friedrich
T2  - Review of Scientific Instruments
AB  - An improved quartz crystal microbalance measurement method is described, which allows us to determine erosion, implantation, and release rates of thin films, during changing temperatures and up to 700 K. A quasi-simultaneous excitation of two eigenmodes of the quartz resonator is able to compensate for frequency drifts due to temperature changes. The necessary electronics, the controlling behavior, and the dual-mode temperature compensation are described. With this improved technique, quantitative in situ temperature-programmed desorption measurements are possible and the quartz crystal microbalance can be used for quantification of thermal desorption spectroscopy measurements with a quadrupole mass spectrometer. This is demonstrated by a study of the retention and release behavior of hydrogen isotopes in fusion-relevant materials. We find that more than 90% of the deuterium implanted into a thin film of beryllium is released during a subsequent temperature ramp up to 500 K.
DA  - 2020/12/01/
PY  - 2020
DO  - 10.1063/5.0012028
VL  - 91
IS  - 12
SP  - 125104
J2  - Rev. Sci. Instrum.
SN  - 0034-6748
ER  - 

TY  - JOUR
TI  - IrO2 surface complexions identified through machine learning and surface investigations
AU  - Timmermann, Jakob
AU  - Kraushofer, Florian
AU  - Resch, Nikolaus
AU  - Li, Peigang
AU  - Wang, Yu
AU  - Mao, Zhiqiang
AU  - Riva, Michele
AU  - Lee, Yonghyuk
AU  - Staacke, Carsten
AU  - Schmid, Michael
AU  - Scheurer, Christoph
AU  - Parkinson, Gareth S.
AU  - Diebold, Ulrike
AU  - Reuter, Karsten
T2  - Physical Review Letters
AB  - A Gaussian approximation potential was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO2 facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1×1) terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate and exhibit the theoretically predicted (1×1) periodicity and x-ray photoelectron spectroscopy core-level shifts. The obtained structures are analogous to the complexions discussed in the context of ceramic battery materials.
DA  - 2020/11/10/
PY  - 2020
DO  - 10.1103/PhysRevLett.125.206101
VL  - 125
IS  - 20
SP  - 206101
J2  - Phys. Rev. Lett.
ER  - 

TY  - JOUR
TI  - Atomically resolved surface phases of La0.8Sr0.2MnO3(110) thin films
AU  - Franceschi, Giada
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Riva, Michele
T2  - Journal of Materials Chemistry A
AB  - The atomic-scale properties of lanthanum–strontium manganite (La1−xSrxMnO3−δ, LSMO) surfaces are of high interest because of the roles of the material as a prototypical complex oxide, in the fabrication of spintronic devices and in catalytic applications. This work combines pulsed laser deposition (PLD) with atomically resolved scanning tunneling microscopy (STM) and surface analysis techniques (low-energy electron diffraction – LEED, X-ray photoelectron spectroscopy – XPS, and low-energy He+ ion scattering – LEIS) to assess the atomic properties of La0.8Sr0.2MnO3(110) surfaces and their dependence on the surface composition. Epitaxial films with 130 nm thickness were grown on Nb-doped SrTiO3(110) and their near-surface stoichiometry was adjusted by depositing La and Mn in sub-monolayer amounts, quantified with a movable quartz-crystal microbalance. The resulting surfaces were equilibrated at 700 °C under 0.2 mbar O2, i.e., under conditions that bridge the gap between ultra-high vacuum and the operating conditions of high-temperature solid-oxide fuel cells, where LSMO is used as the cathode. The atomic details of various composition-related surface phases have been unveiled. The phases are characterized by distinct structural and electronic properties and vary in their ability to accommodate deposited cations.
DA  - 2020/11/10/
PY  - 2020
DO  - 10.1039/D0TA07032G
VL  - 8
IS  - 43
SP  - 22947
EP  - 22961
J2  - J. Mater. Chem. A
LA  - en
SN  - 2050-7496
ER  - 

TY  - JOUR
TI  - Atomic-scale studies of Fe3O4(001) and TiO2(110) surfaces following immersion in CO2-acidified water
AU  - Mirabella, Francesca
AU  - Balajka, Jan
AU  - Pavelec, Jiri
AU  - Göbel, Markus
AU  - Kraushofer, Florian
AU  - Schmid, Michael
AU  - Parkinson, Gareth S.
AU  - Diebold, Ulrike
T2  - ChemPhysChem
AB  - Abstract Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water and studied its interaction with TiO2 and Fe3O4 surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO2 (p_CO2 ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)-(√2 × √2)R45° surface begins to dissolve at a pH 4.0–3.9 ( p_CO2 = 0.8–1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO3-) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates.
DA  - 2020/08/18/
PY  - 2020
DO  - 10.1002/cphc.202000471
VL  - 21
IS  - 16
SP  - 1788
EP  - 1796
J2  - ChemPhysChem
SN  - 1439-4235
ER  - 

TY  - JOUR
TI  - Carbide-modified Pd on ZrO2 as active phase for CO2-reforming of methane—A model phase boundary approach
AU  - Köpfle, Norbert
AU  - Ploner, Kevin
AU  - Lackner, Peter
AU  - Götsch, Thomas
AU  - Thurner, Christoph
AU  - Carbonio, Emilia
AU  - Hävecker, Michael
AU  - Knop-Gericke, Axel
AU  - Schlicker, Lukas
AU  - Doran, Andrew
AU  - Kober, Delf
AU  - Gurlo, Aleksander
AU  - Willinger, Marc
AU  - Penner, Simon
AU  - Schmid, Michael
AU  - Klötzer, Bernhard
T2  - Catalysts
AB  - Starting from subsurface Zr0-doped "inverse" Pd and bulk-intermetallic Pd0Zr0 model catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS), in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch reactor cell. Both intermetallic precursors develop a Pd0–ZrO2 phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO2 from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO2. This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO2 reference catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive carbidic/dissolved C-species inside Pd0 toward CO at the Pd/ZrO2 phase boundary, which serves the role of providing efficient CO2 activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO2 islands at the surface of a quasi-infinite Pd0 metal bulk. Strongly delayed Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much less active interfacial ZrO2–Pd0 state.
DA  - 2020/09//
PY  - 2020
DO  - 10.3390/catal10091000
VL  - 10
IS  - 9
SP  - 1000
J2  - Catalysts
LA  - en
ER  - 

TY  - JOUR
TI  - A quantitative LEED analysis of the oxygen-induced p(3×1) reconstruction of Pt25Rh75(100)
AU  - Sporn, M.
AU  - Platzgummer, E.
AU  - Gruber, E. L. D.
AU  - Schmid, M.
AU  - Hofer, W.
AU  - Varga, P.
T2  - Surface Science
AB  - Pt25Rh75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23 L O2, 600°C). A previous STM study on O/Pt50Rh50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant ("shifted rows"). We present a LEED I(E) analysis of Pt25Rh75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I(E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95 Å. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Å, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Å. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt25Rh75(100) at low oxygen coverage with STM. We resolve the shifted rows in real-space, and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt25Rh75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt25Rh75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt25Rh75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer.
DA  - 1998/10/30/
PY  - 1998
DO  - 10.1016/S0039-6028(98)00574-3
VL  - 416
IS  - 3
SP  - 384
EP  - 395
J2  - Surf. Sci.
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Propagation of spin waves through a Néel domain wall
AU  - Wojewoda, O.
AU  - Hula, T.
AU  - Flajšman, L.
AU  - Vaňatka, M.
AU  - Gloss, J.
AU  - Holobrádek, J.
AU  - Staňo, M.
AU  - Stienen, S.
AU  - Körber, L.
AU  - Schultheiss, K.
AU  - Schmid, M.
AU  - Schultheiss, H.
AU  - Urbánek, M.
T2  - Applied Physics Letters
AB  - Spin waves have the potential to be used as a next-generation platform for data transfer and processing as they can reach wavelengths in the nanometer range and frequencies in the terahertz range. To realize a spin-wave device, it is essential to be able to manipulate the amplitude as well as the phase of spin waves. Several theoretical and recent experimental works have also shown that the spin-wave phase can be manipulated by the transmission through a domain wall (DW). Here, we study propagation of spin waves through a DW by means of micro-focused Brillouin light scattering microscopy (μBLS). The 2D spin-wave intensity maps reveal that spin-wave transmission through a Néel DW is influenced by a topologically enforced circular Bloch line in the DW center and that the propagation regime depends on the spin-wave frequency. In the first regime, two spin-wave beams propagating around the circular Bloch line are formed, whereas in the second regime, spin waves propagate in a single central beam through the circular Bloch line. Phase-resolved μBLS measurements reveal a phase shift upon transmission through the domain wall for both regimes. Micromagnetic modeling of the transmitted spin waves unveils a distortion of their phase fronts, which needs to be taken into account when interpreting the measurements and designing potential devices. Moreover, we show that, by means of micromagnetic simulations, an external magnetic field can be used to move the circular Bloch line within the DW and to manipulate spin-wave propagation.
DA  - 2020/07/13/
PY  - 2020
DO  - 10.1063/5.0013692
VL  - 117
IS  - 2
SP  - 022405
J2  - Appl. Phys. Lett.
SN  - 0003-6951
ER  - 

TY  - JOUR
TI  - Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy
AU  - Sokolović, Igor
AU  - Reticcioli, Michele
AU  - Čalkovský, Martin
AU  - Wagner, Margareta
AU  - Schmid, Michael
AU  - Franchini, Cesare
AU  - Diebold, Ulrike
AU  - Setvín, Martin
T2  - Proceedings of the National Academy of Sciences
AB  - Interaction of molecular oxygen with semiconducting oxide surfaces plays a key role in many technologies. The topic is difficult to approach both by experiment and in theory, mainly due to multiple stable charge states, adsorption configurations, and reaction channels of adsorbed oxygen species. Here we use a combination of noncontact atomic force microscopy (AFM) and density functional theory (DFT) to resolve O2 adsorption on the rutile TiO2(110) surface, which presents a longstanding challenge in the surface chemistry of metal oxides. We show that chemically inert AFM tips terminated by an oxygen adatom provide excellent resolution of both the adsorbed species and the oxygen sublattice of the substrate. Adsorbed O2 molecules can accept either one or two electron polarons from the surface, forming superoxo or peroxo species. The peroxo state is energetically preferred under any conditions relevant for applications. The possibility of nonintrusive imaging allows us to explain behavior related to electron/hole injection from the tip, interaction with UV light, and the effect of thermal annealing.
DA  - 2020/06/30/
PY  - 2020
DO  - 10.1073/pnas.1922452117
VL  - 117
IS  - 26
SP  - 14827
EP  - 14837
J2  - Proc. Natl. Acad. Sci. USA
LA  - en
SN  - 0027-8424, 1091-6490
ER  - 

TY  - JOUR
TI  - Movable holder for a quartz crystal microbalance for exact growth rates in pulsed laser deposition
AU  - Franceschi, Giada
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Riva, Michele
T2  - Review of Scientific Instruments
AB  - Controlling the amount of material deposited by pulsed laser deposition (PLD) down to fractions of one atomic layer is crucial for nanoscale technologies based on thin-film heterostructures. Albeit unsurpassed for measuring growth rates with high accuracy, the quartz crystal microbalance (QCM) suffers from some limitations when applied to PLD. The strong directionality of the PLD plasma plume and its pronounced dependence on deposition parameters (e.g., background pressure and fluence) require that the QCM is placed at the same position as the substrate during growth. However, QCM sensors are commonly fixed off to one side of the substrate. This also entails fast degradation of the crystal, as it is constantly exposed to the ablated material. The design for a movable QCM holder discussed in this work overcomes these issues. The holder is compatible with standard transfer arms, enabling easy insertion and transfer between a PLD chamber and other adjoining vacuum chambers. The QCM can be placed at the same position as the substrate during PLD growth. Its resonance frequency is measured in vacuum at any location where it can be in contact with an electrical feedthrough, before and after deposition. We tested the design for the deposition of hematite (Fe2O3), comparing the rates derived from the QCM and from reflection high-energy electron diffraction oscillations during homoepitaxial growth.
DA  - 2020/06/01/
PY  - 2020
DO  - 10.1063/5.0007643
VL  - 91
IS  - 6
SP  - 065003
J2  - Rev. Sci. Instrum.
SN  - 0034-6748
ER  - 

TY  - JOUR
TI  - Fast low-noise transimpedance amplifier for scanning tunneling microscopy and beyond
AU  - Štubian, Martin
AU  - Bobek, Juraj
AU  - Setvin, Martin
AU  - Diebold, Ulrike
AU  - Schmid, Michael
T2  - Review of Scientific Instruments
AB  - A transimpedance amplifier has been designed for scanning tunneling microscopy (STM). The amplifier features low noise (limited by the Johnson noise of the 1 GΩ feedback resistor at low input current and low frequencies), sufficient bandwidth for most STM applications (50 kHz at 35 pF input capacitance), a large dynamic range (0.1 pA–50 nA without range switching), and a low input voltage offset. The amplifier is also suited for placing its first stage into the cryostat of a low-temperature STM, minimizing the input capacitance and reducing the Johnson noise of the feedback resistor. The amplifier may also find applications for specimen current imaging and electron-beam-induced current measurements in scanning electron microscopy and as a photodiode amplifier with a large dynamic range. This paper also discusses the sources of noise including the often neglected effect of non-balanced input impedance of operational amplifiers and describes how to accurately measure and adjust the frequency response of low-current transimpedance amplifiers.
DA  - 2020/07/01/
PY  - 2020
DO  - 10.1063/5.0011097
VL  - 91
IS  - 7
SP  - 074701
J2  - Rev. Sci. Instrum.
SN  - 0034-6748
ER  - 

TY  - JOUR
TI  - A model system for photocatalysis: Ti-doped α-Fe2O3(11̅02) single-crystalline films
AU  - Franceschi, Giada
AU  - Kraushofer, Florian
AU  - Meier, Matthias
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Riva, Michele
T2  - Chemistry of Materials
AB  - Hematite (α-Fe2O3) is one of the most investigated anode materials for photoelectrochemical water splitting. Its efficiency improves by doping with Ti, but the underlying mechanisms are not understood. One hurdle is separating the influence of doping on conductivity, surface states, and morphology, which all affect performance. To address this complexity, one needs well-defined model systems. We build such a model system by growing single-crystalline, atomically flat Ti-doped α-Fe2O3(11̅02) films by pulsed laser deposition (PLD). We characterize their surfaces, combining in situ scanning tunneling microscopy (STM) with density functional theory (DFT), and reveal how dilute Ti impurities modify the atomic-scale structure of the surface as a function of the oxygen chemical potential and Ti content. Ti preferentially substitutes subsurface Fe and causes a local restructuring of the topmost surface layers. Based on the experimental quantification of Ti-induced surface modifications and the structural model we have established, we propose a strategy that can be used to separate the effects of Ti-induced modifications to the surface atomic and electronic structures and bulk conductivity on the reactivity of Ti-doped hematite.
DA  - 2020/05/12/
PY  - 2020
DO  - 10.1021/acs.chemmater.9b04908
VL  - 32
IS  - 9
SP  - 3753
EP  - 3764
J2  - Chem. Mater.
SN  - 0897-4756
ER  - 

TY  - JOUR
TI  - Order–disorder phase transition of the subsurface cation vacancy reconstruction on Fe3O4(001)
AU  - Arndt, Björn
AU  - Lechner, Barbara A. J.
AU  - Bourgund, Alexander
AU  - Grånäs, Elin
AU  - Creutzburg, Marcus
AU  - Krausert, Konstantin
AU  - Hulva, Jan
AU  - Parkinson, Gareth S.
AU  - Schmid, Michael
AU  - Vonk, Vedran
AU  - Esch, Friedrich
AU  - Stierle, Andreas
T2  - Physical Chemistry Chemical Physics
AB  - We present surface X-ray diffraction and fast scanning tunneling microscopy results to elucidate the nature of the surface phase transition on magnetite (001) from a reconstructed to a non-reconstructed surface around 720 K. In situ surface X-ray diffraction at a temperature above the phase transition, at which long-range order is lost, gives evidence that the subsurface cation vacancy reconstruction still exists as a local structural motif, in line with the characteristics of a 2D second-order phase transition. Fast scanning tunneling microscopy results across the phase transition underpin the hypothesis that the reconstruction lifting is initiated by surplus Fe ions occupying subsurface octahedral vacancies. The reversible near-surface iron enrichment and reduction of the surface to stoichiometric composition is further confirmed by in situ low-energy ion scattering, as well as ultraviolet and X-ray photoemission results.
DA  - 2020/04/29/
PY  - 2020
DO  - 10.1039/D0CP00690D
VL  - 22
IS  - 16
SP  - 8336
EP  - 8343
J2  - Phys. Chem. Chem. Phys.
LA  - en
SN  - 1463-9084
ER  - 

TY  - JOUR
TI  - The growth of ultra-thin zirconia films on Pd3Zr(0001)
AU  - Choi, J. I. J.
AU  - Mayr-Schmölzer, W.
AU  - Mittendorfer, F.
AU  - Redinger, J.
AU  - Diebold, U.
AU  - Schmid, M.
T2  - Journal of Physics: Condensed Matter
AB  - Despite its importance in many areas of industry, such as catalysis, fuel cell technology and microelectronics, the surface structure and physical properties of ZrO2 are not well understood. Following the successful growth of ultra-thin zirconia on Pt3Zr(0001) (Antlanger et al 2012 Phys. Rev. B 86 035451), we report on recent progress into ZrO2 thin films, which were prepared by oxidation of a Pd3Zr(0001) crystal. Results from scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) as well as density-functional theory (DFT) are presented. Many sputter-annealing cycles are required for preparation of the clean Pd3Zr alloy surface, because oxygen easily dissolves in the bulk. By oxidation and post-annealing, a homogeneous ultra-thin ZrO2 film was obtained. This is an O-Zr-O trilayer based on cubic ZrO2(1 1 1). Using STM images corrected for distortion and creep of the piezo scanner the in-plane lattice parameter was determined as (351.2 ± 0.4) pm, slightly contracted with respect to the cubic ZrO2 bulk phase. The oxide forms an overlayer that is either incommensurate or has a very large superstructure cell (a = 8.3 nm); nevertheless its rotational orientation is always the same. In contrast to ultra-thin zirconia on Pt3Zr(0001), where the uppermost substrate layer is pure (but reconstructed) Pt, STM and XPS suggest a stoichiometric Pd3Zr below the oxide. The oxide film binds to the substrate mainly via bonds between oxygen and the Zr atoms in the substrate. The ultra-thin oxide shows large buckling in STM, confirmed by DFT calculations, where the buckling of the Zr layer can exceed 100 pm. Compared to the ZrO2 film on Pt3Zr(0001), the oxide on Pd3Zr(0001) has the advantage that the substrate below does not reconstruct, leading to a homogeneous oxide film.
DA  - 2014/06/04/
PY  - 2014
DO  - 10.1088/0953-8984/26/22/225003
VL  - 26
IS  - 22
SP  - 225003
J2  - J. Phys.: Condens. Matter
LA  - en
SN  - 0953-8984
ER  - 

TY  - JOUR
TI  - Local structure and coordination define adsorption in a model Ir1/Fe3O4 single-atom catalyst
AU  - Jakub, Zdenek
AU  - Hulva, Jan
AU  - Meier, Matthias
AU  - Bliem, Roland
AU  - Kraushofer, Florian
AU  - Setvin, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
AU  - Parkinson, Gareth S.
T2  - Angewandte Chemie International Edition
AB  - Single-atom catalysts (SACs) bridge homo- and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning-probe microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3O4(001) support. The two- and five-fold-coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square-planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism.
DA  - 2019///
PY  - 2019
DO  - 10.1002/anie.201907536
VL  - 58
IS  - 39
SP  - 13961
EP  - 13968
J2  - Angew. Chem. Int. Ed.
LA  - en
SN  - 1521-3773
ER  - 

TY  - JOUR
TI  - Partially dissociated water dimers at the water–hematite interface
AU  - Jakub, Zdenek
AU  - Kraushofer, Florian
AU  - Bichler, Magdalena
AU  - Balajka, Jan
AU  - Hulva, Jan
AU  - Pavelec, Jiri
AU  - Sokolović, Igor
AU  - Müllner, Matthias
AU  - Setvin, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Blaha, Peter
AU  - Parkinson, Gareth S.
T2  - ACS Energy Letters
AB  - The oxygen evolution reaction (OER) is thought to occur via a four-step mechanism with *O, *OH, and *OOH as adsorbed intermediates. Linear scaling of the *OH and **OOH adsorption energies is proposed to limit the oxides’ efficiency as OER catalysts, but the use of simple descriptors to screen candidate materials neglects potentially important water–water interactions. Here, we use a combination of temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), noncontact atomic force microscopy (nc-AFM), and density functional theory (DFT)-based computations to show that highly stable HO–H2O dimer species form at the (11̅02) facet of hematite; a promising anode material for photoelectrochemical water splitting. The UHV-based results are complemented by measurements following exposure to liquid water and are consistent with prior X-ray scattering results. The presence of strongly bound water agglomerates is generally not taken into account in OER reaction schemes but may play a role in determining the required OER overpotential on metal oxides.
DA  - 2019/02/08/
PY  - 2019
DO  - 10.1021/acsenergylett.8b02324
VL  - 4
IS  - 2
SP  - 390
EP  - 396
J2  - ACS Energy Lett.
ER  - 

TY  - JOUR
TI  - Adsorbate-induced structural evolution changes the mechanism of CO oxidation on a Rh/Fe3O4(001) model catalyst
AU  - Jakub, Zdenek
AU  - Hulva, Jan
AU  - Ryan, Paul T. P.
AU  - Duncan, David A.
AU  - Payne, David J.
AU  - Bliem, Roland
AU  - Ulreich, Manuel
AU  - Hofegger, Patrick
AU  - Kraushofer, Florian
AU  - Meier, Matthias
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Nanoscale
AB  - The structure of a catalyst often changes in reactive environments, and following the structural evolution is crucial for the identification of the catalyst's active phase and reaction mechanism. Here we present an atomic-scale study of CO oxidation on a model Rh/Fe3O4(001) “single-atom” catalyst, which has a very different evolution depending on which of the two reactants, O2 or CO, is adsorbed first. Using temperature-programmed desorption (TPD) combined with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we show that O2 destabilizes Rh atoms, leading to the formation of RhxOy clusters; these catalyze CO oxidation via a Langmuir–Hinshelwood mechanism at temperatures as low as 200 K. If CO adsorbs first, the system is poisoned for direct interaction with O2, and CO oxidation is dominated by a Mars-van-Krevelen pathway at 480 K.
DA  - 2020/03/12/
PY  - 2020
DO  - 10.1039/C9NR10087C
VL  - 12
IS  - 10
SP  - 5866
EP  - 5875
J2  - Nanoscale
LA  - en
SN  - 2040-3372
ER  - 

TY  - JOUR
TI  - Two-dimensional surface phase diagram of a multicomponent perovskite oxide: La0.8Sr0.2MnO3(110)
AU  - Franceschi, Giada
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Riva, Michele
T2  - Physical Review Materials
AB  - The many surface reconstructions of (110)-oriented lanthanum strontium manganite (La0.8Sr0.2MnO3, LSMO) were followed as a function of the oxygen chemical potential (μO) and the surface cation composition. Decreasing μO causes Mn to migrate across the surface, enforcing phase separation into A-site-rich areas and a variety of composition-related, structurally diverse B-site-rich reconstructions. The composition of these phase-separated structures was quantified with scanning tunneling microscopy, and these results were used to build a two-dimensional phase diagram of the LSMO(110) equilibrium surface structures.
DA  - 2021/09/24/
PY  - 2021
DO  - 10.1103/PhysRevMaterials.5.L092401
VL  - 5
IS  - 9
SP  - L092401
J2  - Phys. Rev. Materials
ER  - 

TY  - JOUR
TI  - Rapid oxygen exchange between hematite and water vapor
AU  - Jakub, Zdenek
AU  - Meier, Matthias
AU  - Kraushofer, Florian
AU  - Balajka, Jan
AU  - Pavelec, Jiri
AU  - Schmid, Michael
AU  - Franchini, Cesare
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - Nature Communications
AB  - Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline “r-cut” ($$1\bar{1}02$$) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.
DA  - 2021/11/10/
PY  - 2021
DO  - 10.1038/s41467-021-26601-4
VL  - 12
IS  - 1
SP  - 6488
J2  - Nat. Commun.
SN  - 2041-1723
ER  - 

TY  - JOUR
TI  - Oxygen-rich tetrahedral surface phase on high-temperature rutile VO2(110)_T single crystals
AU  - Wagner, Margareta
AU  - Planer, Jakub
AU  - Heller, Bettina S. J.
AU  - Langer, Jens
AU  - Limbeck, Andreas
AU  - Boatner, Lynn A.
AU  - Steinrück, Hans-Peter
AU  - Redinger, Josef
AU  - Maier, Florian
AU  - Mittendorfer, Florian
AU  - Schmid, Michael
AU  - Diebold, Ulrike
T2  - Physical Review Materials
AB  - Vanadium dioxide undergoes a metal-insulator transition from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g., as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many applications. In this paper, we present an atomic-scale investigation of the tetragonal rutile VO2(110)T single-crystal surface and report results obtained with scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy, supported by density-functional-theory-based calculations. The surface reconstructs into an oxygen-rich (2 × 2) superstructure that coexists with small patches of the underlying unreconstructed (110)−(1×1) surface when the crystal is annealed >600∘C. The best structural model for the (2 × 2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials, this reconstruction is more stable than the unreconstructed (110) surface and models proposed in the literature.
DA  - 2021/12/02/
PY  - 2021
DO  - 10.1103/PhysRevMaterials.5.125001
VL  - 5
IS  - 12
SP  - 125001
J2  - Phys. Rev. Materials
ER  - 

TY  - JOUR
TI  - Single Rh adatoms stabilized on α-Fe2O3(11̅02) by coadsorbed water
AU  - Kraushofer, Florian
AU  - Haager, Lena
AU  - Eder, Moritz
AU  - Rafsanjani-Abbasi, Ali
AU  - Jakub, Zdeněk
AU  - Franceschi, Giada
AU  - Riva, Michele
AU  - Meier, Matthias
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Parkinson, Gareth S.
T2  - ACS Energy Letters
AB  - Oxide-supported single-atom catalysts are commonly modeled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Herein, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and X-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on α-Fe2O3(11̅02) in the presence of 2 × 10–8 mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 °C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms and argue that their presence should be assumed when modeling single-atom catalysis systems.
DA  - 2022/01/14/
PY  - 2022
DO  - 10.1021/acsenergylett.1c02405
VL  - 7
IS  - 1
SP  - 375
EP  - 380
J2  - ACS Energy Lett.
ER  - 

TY  - JOUR
TI  - Reconstruction changes drive surface diffusion and determine the flatness of oxide surfaces
AU  - Franceschi, Giada
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Riva, Michele
T2  - Journal of Vacuum Science & Technology A
AB  - Surface diffusion on metal oxides is key in many areas of materials technology, yet it has been scarcely explored at the atomic scale. This work provides phenomenological insights from scanning tunneling microscopy on the link between surface diffusion, surface atomic structure, and oxygen chemical potential based on three model oxide surfaces: Fe2O3(1 -1 0 2), La1−xSrxMnO3(110), and In2O3(111). In all instances, changing the oxygen chemical potential used for annealing stabilizes reconstructions of different compositions while promoting the flattening of the surface morphology—a sign of enhanced surface diffusion. It is argued that thermodynamics, rather than kinetics, rules surface diffusion under these conditions: the composition change of the surface reconstructions formed at differently oxidizing conditions drives mass transport across the surface.
DA  - 2022/03//
PY  - 2022
DO  - 10.1116/6.0001704
VL  - 40
IS  - 2
SP  - 023206
J2  - J. Vac. Sci. Technol. A
SN  - 0734-2101
ER  - 

TY  - JOUR
TI  - CO oxidation by Pt2/Fe3O4: Metastable dimer and support configurations facilitate lattice oxygen extraction
AU  - Meier, Matthias
AU  - Hulva, Jan
AU  - Jakub, Zdenek
AU  - Kraushofer, Florian
AU  - Bobić, Mislav
AU  - Bliem, Roland
AU  - Setvin, Martin
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
AU  - Parkinson, Gareth S.
T2  - Science Advances
AB  - Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)–based calculations to study CO oxidation by a model Pt/Fe3O4(001) “single-atom” catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyze a reaction involving the oxide support to form CO2. Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. O_lattice extraction only becomes facile when both the Pt-dimer and the Fe3O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.
DA  - 2022///
PY  - 2022
DO  - 10.1126/sciadv.abn4580
VL  - 8
IS  - 13
SP  - eabn4580
J2  - Sci. Adv.
ER  - 

TY  - JOUR
TI  - Why and how Savitzky–Golay filters should be replaced
AU  - Schmid, Michael
AU  - Rath, David
AU  - Diebold, Ulrike
T2  - ACS Measurement Science Au
AB  - Savitzky–Golay (SG) filtering, based on local least-squares fitting of the data by polynomials, is a popular method for smoothing data and calculations of derivatives of noisy data. At frequencies above the cutoff, SG filters have poor noise suppression; this unnecessarily reduces the signal-to-noise ratio, especially when calculating derivatives of the data. In addition, SG filtering near the boundaries of the data range is prone to artifacts, which are especially strong when using SG filters for calculating derivatives of the data. We show how these disadvantages can be avoided while keeping the advantageous properties of SG filters. We present two classes of finite impulse response (FIR) filters with substantially improved frequency response: (i) SG filters with fitting weights in the shape of a window function and (ii) convolution kernels based on the sinc function with a Gaussian-like window function and additional corrections for improving the frequency response in the passband (modified sinc kernel). Compared with standard SG filters, the only price to pay for the improvement is a moderate increase in the kernel size. Smoothing at the boundaries of the data can be improved with a non-FIR method, the Whittaker–Henderson smoother, or by linear extrapolation of the data, followed by convolution with a modified sinc kernel, and we show that the latter is preferable in most cases. We provide computer programs and equations for the smoothing parameters of these smoothers when used as plug-in replacements for SG filters and describe how to choose smoothing parameters to preserve peak heights in spectra.
DA  - 2022/02/17/
PY  - 2022
DO  - 10.1021/acsmeasuresciau.1c00054
VL  - 2
IS  - 2
SP  - 185
EP  - 196
J2  - ACS Meas. Sci. Au
ER  - 

TY  - JOUR
TI  - Adsorption configurations of Co-phthalocyanine on In2O3(111)
AU  - Wagner, Margareta
AU  - Calcinelli, Fabio
AU  - Jeindl, Andreas
AU  - Schmid, Michael
AU  - Hofmann, Oliver T.
AU  - Diebold, Ulrike
T2  - Surface Science
AB  - Indium oxide offers optical transparency paired with electrical conductivity, a combination required in many optoelectronic applications. The most-stable In2O3(111) surface has a large unit cell (1.43 nm lattice constant). It contains a mixture of both bulk-like and undercoordinated O and In atoms and provides an ideal playground to explore the interaction of surfaces with organic molecules of similar size as the unit cell. Non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), and density functional theory (DFT) were used to study the adsorption of Co-phthalocyanine (CoPc) on In2O3(111). Isolated CoPc molecules adsorb at two adsorption sites in a ∼7:3 ratio. The Co atom sits either on top of a surface oxygen (‘F configuration’) or indium atom (‘S configuration’). This subtle change in adsorption site induces different bending of the molecules and differences in their electronic structure. According to DFT, the lowest unoccupied molecular orbital of the undistorted gas-phase CoPc remains mostly unaffected in the F configuration but is filled by one electron in S configuration. At coverages up to one CoPc molecule per substrate unit cell, a mixture of domains with molecules in F and S configuration are found. Molecules in F configuration first condense into a poorly ordered F-(2 × 2) structure and finally rearrange into an F-(1 × 1) structure in which the molecules partially overlap. The only ordered structure of molecules adsorbed in S configuration is an S-(1 × 1) superstructure at full monolayer coverage.
DA  - 2022/08/01/
PY  - 2022
DO  - 10.1016/j.susc.2022.122065
VL  - 722
SP  - 122065
J2  - Surf. Sci.
LA  - en
SN  - 0039-6028
ER  - 

TY  - JOUR
TI  - Structure of an ultrathin oxide on Pt3Sn(111) solved by machine learning enhanced global optimization
AU  - Merte, Lindsay R.
AU  - Bisbo, Malthe Kjær
AU  - Sokolović, Igor
AU  - Setvín, Martin
AU  - Hagman, Benjamin
AU  - Shipilin, Mikhail
AU  - Schmid, Michael
AU  - Diebold, Ulrike
AU  - Lundgren, Edvin
AU  - Hammer, Bjørk
T2  - Angewandte Chemie International Edition
AB  - Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure—the (4×4) surface oxide on Pt3Sn(111)—based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.
DA  - 2022///
PY  - 2022
DO  - 10.1002/anie.202204244
VL  - 61
IS  - 25
SP  - e202204244
J2  - Angew. Chem. Int. Ed.
LA  - en
SN  - 1521-3773
ER  - 

TY  - JOUR
TI  - Competing electronic states emerging on polar surfaces
AU  - Reticcioli, Michele
AU  - Wang, Zhichang
AU  - Schmid, Michael
AU  - Wrana, Dominik
AU  - Boatner, Lynn A.
AU  - Diebold, Ulrike
AU  - Setvin, Martin
AU  - Franchini, Cesare
T2  - Nature Communications
AB  - Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.
DA  - 2022/07/25/
PY  - 2022
DO  - 10.1038/s41467-022-31953-6
VL  - 13
IS  - 1
SP  - 4311
J2  - Nat. Commun.
LA  - en
SN  - 2041-1723
ER  - 

TY  - JOUR
TI  - Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001)
AU  - Wang, Zhichang
AU  - Reticcioli, Michele
AU  - Jakub, Zdenek
AU  - Sokolović, Igor
AU  - Meier, Matthias
AU  - Boatner, Lynn A.
AU  - Schmid, Michael
AU  - Parkinson, Gareth S.
AU  - Diebold, Ulrike
AU  - Franchini, Cesare
AU  - Setvin, Martin
T2  - Science Advances
AB  - Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO3. This intrinsically polar material cleaves along (001) into KO- and TaO2-terminated surface domains. At TaO2 terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO2 terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.
DA  - 2022/08/19/
PY  - 2022
DO  - 10.1126/sciadv.abq1433
VL  - 8
IS  - 33
SP  - eabq1433
J2  - Sci. Adv.
ER  - 

TY  - JOUR
TI  - Erratum for the Report “Reaction of O2 with subsurface oxygen vacancies on TiO2 anatase (101)” by M. Setvín, U. Aschauer, P. Scheiber, Y.-F. Li, W. Hou, M. Schmid, A. Selloni, U. Diebold
AU  - Setvín, Martin
AU  - Aschauer, Ulrich
AU  - Scheiber, Philipp
AU  - Li, Ye-Fei
AU  - Hou, W, Weiyi
AU  - Schmid, Michael
AU  - Selloni, Annabella
AU  - Diebold, Ulrike
T2  - Science
AB  - In the Report “Reaction of O2 with subsurface oxygen vacancies on TiO2 anatase (101),” new results indicate that the species referred as (O2–)ads is an adsorbed water molecule, and the species named (O22–)ads is a terminal OH group. Revisited identification of the species is described in Setvín et al., Phys. Chem. Chem. Phys. 16, 21524 (2014). The field-induced oxygen vacancy migration remains valid as described in the Report; the process is discussed in detail in Setvín et al., Phys. Rev. B 91, 195403 (2015). All theoretical results, notably the pathway in Fig. 1, remain valid as well.
DA  - 2015/07/17/
PY  - 2015
DO  - 10.1126/science.aac9659
VL  - 349
IS  - 6245
SP  - aac9659
EP  - aac9659
J2  - Science
ER  - 

TY  - JOUR
TI  - Water structures reveal local hydrophobicity on the In2O3(111) surface
AU  - Chen, Hao
AU  - Blatnik, Matthias A.
AU  - Ritterhoff, Christian L.
AU  - Sokolović, Igor
AU  - Mirabella, Francesca
AU  - Franceschi, Giada
AU  - Riva, Michele
AU  - Schmid, Michael
AU  - Čechal, Jan
AU  - Meyer, Bernd
AU  - Diebold, Ulrike
AU  - Wagner, Margareta
T2  - ACS Nano
AB  - Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In2O3(111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, XPS, nc-AFM) agree quantitatively with DFT calculations and show a series of distinct phases. The first three water molecules dissociate at one specific area of the unit cell and desorb above room temperature. The next three adsorb as molecules in the adjacent region. Three more water molecules rearrange this structure and an additional nine pile up above the OH groups. Despite offering undercoordinated In and O sites, the rest of the unit cell is unfavorable for adsorption and remains water-free. The first water layer thus shows ordering into nanoscopic 3D water clusters separated by hydrophobic pockets.
DA  - 2022/12/27/
PY  - 2022
DO  - 10.1021/acsnano.2c09115
VL  - 16
IS  - 12
SP  - 21163
EP  - 21173
J2  - ACS Nano
SN  - 1936-0851
ER  - 

TY  - JOUR
TI  - Resolving the intrinsic short-range ordering of K+ ions on cleaved muscovite mica
AU  - Franceschi, Giada
AU  - Kocán, Pavel
AU  - Conti, Andrea
AU  - Brandstetter, Sebastian
AU  - Balajka, Jan
AU  - Sokolović, Igor
AU  - Valtiner, Markus
AU  - Mittendorfer, Florian
AU  - Schmid, Michael
AU  - Setvín, Martin
AU  - Diebold, Ulrike
T2  - Nature Communications
AB  - Muscovite mica, KAl2(Si3Al)O10(OH)2, is a common layered phyllosilicate with perfect cleavage planes. The atomically flat surfaces obtained through cleaving lend themselves to scanning probe techniques with atomic resolution and are ideal to model minerals and clays. Despite the importance of the cleaved mica surfaces, several questions remain unresolved. It is established that K+ ions decorate the cleaved surface, but their intrinsic ordering – unaffected by the interaction with the environment – is not known. This work presents clear images of the K+ distribution of cleaved mica obtained with low-temperature non-contact atomic force microscopy (AFM) under ultra-high vacuum (UHV) conditions. The data unveil the presence of short-range ordering, contrasting previous assumptions of random or fully ordered distributions. Density functional theory (DFT) calculations and Monte Carlo simulations show that the substitutional subsurface Al3+ ions have an important role for the surface K+ ion arrangement.
DA  - 2023/01/13/
PY  - 2023
DO  - 10.1038/s41467-023-35872-y
VL  - 14
IS  - 1
SP  - 208
J2  - Nat Commun
LA  - en
SN  - 2041-1723
ER  -