Atomic-scale structure of the hematite α-Fe2O3(1 -1 0 2) "r-cut" surface

F. Kraushofer, Z. Jakub, M. Bichler, J. Hulva, P. Drmota, M. Weinold, M. Schmid, M. Setvin, U. Diebold, P. Blaha, and G. S. Parkinson

Institut für Angewandte Physik, TU Wien, 1040 Wien, Austria
Institute of Materials Chemistry, TU Wien, 1060 Wien, Austria

J. Phys. Chem. C 122 (2018) 1657-1669

The α-Fe2O3(1 -1 0 2) 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 tunnelling microscopy (STM), non-contact atomic force microscopy (nc-AFM), and ab initio density functional theory (DFT)+U calculations. Two surface structures are stable under ultra-high 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 favoured in reducing conditions, and that fourfold-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.

Corresponding author: Gareth S. Parkinson (parkinson at iap_tuwien_ac_at).

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