Growth and structure of ultrathin vanadium oxide layers on Pd(111)

The growth of thin vanadium oxide films on Pd(111) prepared by reactive evaporation of vanadium in an oxygen atmosphere has been studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and ab initio density-functional-theory (DFT) calculations. Two-dimensional (2D) oxide growth is observed at coverages below one-half of a monolayer (ML), displaying both random island and step-flow growth modes. Above the critical coverage of 0.5 ML, three-dimensional oxide island growth is initiated. The morphology of the low-coverage 2D oxide phase depends strongly on the oxide preparation conditions, as a result of the varying balance of the mobilities of adspecies on the substrate terraces and at the edges of the growing oxide islands. Under typical V oxide evaporation conditions of $p({\mathrm{O}}_{2})=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\mathrm{mbar},$ $T(\mathrm{substrate})=523\mathrm{K},$ the 2D oxide film exhibits a porous fractal-type network structure with atomic-scale ordered branches, showing a $p(2\ifmmode\times\else\texttimes\fi{}2)$ honeycomb structure. Ab initio DFT total-energy calculations reveal that a surface oxide model with a formal ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ stoichiometry is energetically the most stable configuration. The simulated STM images show a $(2\ifmmode\times\else\texttimes\fi{}2)$ honeycomb structure in agreement with experimental observation. This ${\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\ensuremath{-}\mathrm{V}}_{2}{\mathrm{O}}_{3}$ layer is very different from bulk ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and represents an interface stabilized oxide structure. The V oxide layers decompose on annealing above 673 K and 2D island structures of V/Pd surface alloy and metallic V are then formed on the Pd(111) surface.