Structure of Stoichiometric and Oxygen-Rich Ultrathin\nFeO(111) Films Grown on Pd(111)

Abstract

Monolayer thin FeO(111)\nfilms were grown on Pd(111) and oxidized\nby atomic oxygen (O). The stoichiometric and oxidized films were studied\nin detail by scanning tunneling microscopy (STM) and density functional\ntheory (DFT) calculations. Compared to the previously studied FeO(111)/Pt(111)\nsystem, small structural differences were observed for stoichiometric\nFeO monolayer films. Upon O exposure, the stoichiometric FeO film\nreconstructs, leading to the formation of new O-rich structures incorporating\nincreasing amounts of additional O atoms. At low O exposures, the\nSTM images exhibit bright features of regularly sized triangular structures\nassigned to O-adatom dislocation loops. A model of this O-rich structure\ncomposed of four-fold O-coordinated Fe atoms is proposed and confirmed\nby DFT calculations. Furthermore, these O dislocation loops induce\nthe inversion of the FeO film and enclose portions of the film in\nwhich the order of the high-symmetry domains is inverted. For higher\nO exposures, the formation of FeO_2–<i>x</i> islands coexisting with O-adatom dislocations and stoichiometric\nFeO patches was observed. These FeO_2–<i>x</i> islands are reminiscent of the O-rich structures previously\nreported for FeO supported on Pt(111) and are catalytically active\ntoward CO oxidation.