Structure of Reduced Ultrathin TiOx Polar Films on Pt(111)

The structure of well-ordered reduced TiOx nanolayers (NLs) on Pt(111), prepared by reactive evaporation of Ti in an oxygen background and postannealed in different conditions, is discussed on the basis of experimental and theoretical data. All of the observed NLs are formed by a wetting 2D Ti-O bilayer where the Ti atoms are at the interface and the O atoms form the topmost layer. Different structures and stoichiometries depend on the actual Ti coverage and on the conditions of the postannealing. They represent the final products of a self-assembling process where the Ti atoms tend to organize in pseudoepitaxial regions, whereas the O atoms in the topmost layer (more abundant due to stoichiometry constraints) solve the crowding problem by creating pseudoepitaxial regions that exhibit dislocation lines and alternate with regions where Ti vacancies (defects or holes) occur. Given the inherent similarity among such reduced phases, as confirmed by an analysis of their electronic structure, it turns out that subtle effects are at the origin of the large variety of possible structures and that kinetic effects during their synthesis can play a role in driving toward a specific phase.