Growth and surface structure of vanadium oxide on anatase (0 0 1)

Abstract

Oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) of vanadium oxide on (1×4)-reconstructed anatase (001) thin films was studied using reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). XPS and UPS results showed that the vanadium was predominantly in the 5+ oxidation state after deposition of a monolayer at 525 K. After 1 ML of vanadia was deposited, the anatase (1×4)/(4×1) LEED and RHEED patterns were replaced by (1×1) patterns indicating that the vanadia lifts the reconstruction and suggesting that the monolayer is pseudomorphic. At 525 K, the V5+ oxidation state predominated in thicker films, however, no discernible LEED or RHEED patterns were seen after a few monolayers were deposited indicating that V2O5 epitaxy cannot be continued beyond 1 ML. When the growth temperature was increased to 750 K, RHEED patterns indicated no change in the surface structure after more than 20 ML of vanadia were deposited. Under these conditions, XPS peak positions were consistent with VO2. After growth at 775 K a c(2×2) LEED pattern attributed to half a monolayer of adsorbed oxygen on the VO2 surface was observed. The surface characterization data all pointed towards pseudomorphic growth of VO2 with a half monolayer of capping oxygen allowing the monolayer to achieve the V2O5 stoichiometry while maintaining the anatase structure. Bulk XRD data, however, were consistent with VO2(B), V6O13, and rutile VO2; none of which expose surfaces with the periodicity observed with RHEED and LEED. The reasons for the differences between the surface and bulk characterization are discussed.