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

The growth of thin vanadium oxide films on Pd(lll) prepared by reactive eva poration of vanadium in an oxygen atmosphere has been studied by scanning t unneling microscopy (STM), low-energy electron diffraction (LEED), and ab i nitio density-functional-theory (DFT) calculations. Two-dimensional (2D) ox ide growth is observed at coverages below one-half of a monolayer (ML), dis playing both random island and step-flow growth modes. Above the critical c overage 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 mobiliti es of adspecies on the substrate terraces and at the edges of the growing o xide islands. Under typical V oxide evaporation conditions of p(O-2) = 2 x 10(-7) mbar, T(substrate) = 523 K, the 2D oxide film exhibits a porous frac tal-type network structure with atomic-scale ordered branches, showing a p( 2 x 2) honeycomb structure. Ab initio DFT total-energy calculations reveal that a surface oxide model with a formal V2O3 stoichiometry is energeticall y the most stable configuration. The simulated STM images show a (2 x 2) ho neycomb structure in agreement with experimental observation. This surface- V2O3 layer is very different from bulk V2O3 and represents an interface sta bilized oxide structure. The V oxide layers decompose on annealing above 67 3 K and 2D island structures of V/Pd surface alloy and metallic V are then formed on the Pd(111) surface.