Atomic-Scale Structure of the Hematite α-Fe₂O₃(1102) “R-Cut” Surface

Abstract

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 <i>ab initio</i> density functional theory (DFT)+<i>U</i> 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 <i>E</i>_F. 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+<i>U</i> 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 <i>E</i>_F. The work function on the (2 × 1) termination is 5.4 ± 0.2 eV.