Growth and the structure of epitaxial VO2 at the TiO2(110) surface

Epitaxial VO2 layers have been grown on the TiO2(110) rutile surface up to thicknesses of 5 ML. These ultrathin films have been characterized by means of x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS) measurements. LEED and XPD structural data demonstrate that the layer is both short- and long-range ordered, and that it has a rutile structure. The success in preparation of a single-crystalline epitaxial VO2 layer opens possibilities for studying the properties and the surface chemistry of this interesting oxide, so far complicated by the difficulties in growing macroscopic crystals. From the HeI spectra it turns out that the shape, position, and width of the 3d band closely resemble the UPS data of the bulk monoclinic semiconductive phase. The reported results could add new clues to a better understanding of the metal-to-semiconductor phase-transition phenomenon in VO2