Growth and Characterization of Ultrathin V₂O_y (y ≈ 5) Films on Au(111)

The growth of vanadium oxide ultrathin films on Au(111) using physical vapor deposition of V and oxidation under 50 mbar of oxygen has been investigated with scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure analysis. Depending on the coverage, two different well-ordered coincidence monolayer structures with a V ∼5+ oxidation state are observed. These arrangements do not correspond to any known bulk structures, and they are determined by the interaction between the oxide layer and the Au(111) surface, a finding that highlights the ability of vanadium oxides to adopt different structures. After completion of the first layer, a two-layer thick film which wets the substrate and has a surface structure similar to that of V 6 O 13 (001) is formed. This film does not exhibit a well-defined azimuthal orientation, which leads to ringlike structures in the low-energy electron diffraction pattern. The formation of the V 6 O 13 (001)-like film is shown to precede the nucleation and growth of V 2 O 5 (001) crystallites at higher thicknesses. This finding is in contrast with the common assumption that V 2 O 5 islands start to grow as soon as the monolayer is completed.