Quasi-Hexagonal to Lepidocrocite-like Transition in TiO2 Ultrathin Films on Cu(001)

The structure and growth mechanism of TiO2 ultrathin films on Cu(001) were investigated by experimental and theoretical studies. Multiple experimental techniques, including photoemission spectroscopies and scanning probe microscopy, were used to carefully characterize the chemical, electronic, and morphological structure of the metal oxide layer. A complete and flat TiO2 bilayer with a lepidocrocite-like structure is observed at the earliest stages of its growth on the Cu(001) surface under controlled experimental conditions. The employed multitechnique approach reveals that a transition between two different structures of the TiO2 film, that is, the quasi-hexagonal and lepidocrocite-like structure, occurs as a function of the coverage passing from one to two monolayers. Ab initio calculations indicate that the lepidocrocite-like structure is thermodynamically more stable on Cu(001) than the quasi-hexagonal one, thus supporting the observed transition.