Core and valence band photoemission Spectroscopy of well-ordered ultrathin TiOx films on Pt(111)

Well-ordered ultrathin TiOx layers on Pt(111) surface, prepared by reactive evaporation of Ti in oxygen, were characterized by means of Ti 2p and O 1s core level and by valence band photoelectron spectroscopy. Depending on the details of the preparation procedures, a total of six well-ordered structures, each of them characterized by a well-defined low energy electron diffraction pattern, were obtained. The core level data show that this wide range of structures can be rationalized in two main groups, i.e., a group of three stoichiometric (labeled as k', rect, and rect') and a group of a three substoichiometric (labeled as z, z', and w) ordered films. The valence band data are rather consistent with this basic distinction. In fact, valence band spectra relative to stoichiometric or substoichiometric films share common features and are quite different from spectra relative to the other group. On the other hand, the valence band data appear to be more sensitive to the details of the film structure by also displaying electronic features that are particular to each individual film. The valence band data are discussed with the aid of theoretical and experimental results for bulk surfaces and compounds available in the literature. It turns out that mixing with Pt states plays a major role in determining the electronic structure of the reduced substoichiometric films, whose spectral data are also consistent with a stoichiometry close to TiO and with the presence of a Ti-Pt interface. This finding is in agreement with previously reported photoelectron diffraction data. The stoichiometric films show a valence band structure that is strongly reminiscent of the one measured on the stoichiometric bulk TiO2 surface. Deviations from the bulk band structure appear in the form of a narrowing of the band and in a shift toward lower binding energy. The band narrowing effect is attributed to the spatial confinement of the TiO2-like films, while the shift is attributed to mixing of film and Pt substrate derived states. Finally, the rect structure shows a (film thickness dependent) anomalous spectral shape that is tentatively attributed to its peculiar geometric structure.