Atomic-Scale Structure of the Hematite alpha-Fe2O3(1(1)over-bar02) 'R-Cut' Surface

摘要

The alpha-Fe2O3(1 (1) over bar 02) 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 X 1) surface can be prepared by annealing at 450 degrees C in approximate to 10(-6) mbar O-2, and a reduced (2 X 1) reconstruction is formed by UHV annealing at 540 degrees C. The (1 X 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by approximate to 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 E-F. The images from the (2 X 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 E-F. The work function on the (2 X 1) termination is 5.4 +/- 0.2 eV.