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

Abstract

The growth of thin vanadium oxide films on Pd͑111͒ prepared by reactive evaporation of vanadium in an oxygen atmosphere has been studied by scanning tunneling microscopy ͑STM͒, low-energy electron diffraction ͑LEED͒, and ab initio density-functional-theory ͑DFT͒ calculations. Two-dimensional ͑2D͒ oxide growth is observed at coverages below one-half of a monolayer ͑ML͒, 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(O 2)ϭ2ϫ10 Ϫ7 mbar, T͑substrate͒ ϭ523 K, the 2D oxide film exhibits a porous fractal-type network structure with atomic-scale ordered branches, showing a p(2ϫ2) honeycomb structure. Ab initio DFT total-energy calculations reveal that a surface oxide model with a formal V 2 O 3 stoichiometry is energetically the most stable configuration. The simulated STM images show a (2ϫ2) honeycomb structure in agreement with experimental observation. This surface-V 2 O 3 layer is very different from bulk V 2 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 Pd͑111͒ surface.

Surface structure of nickel oxide layers on a Rh(111) surface

“…The Ni deposition rate has been controlled by a quartz microbalance. The STM experiments have been performed in a custom-designed variable-temperature STM system as described previously [9]. All STM images presented here have been recorded at room temperature and in constant-current mode.…”

The structure of the V2O3(0001) surface: A scanned-energy mode photoelectron diffraction study

“…The V 2 O 3 (0001) films were grown on a Pd(111) substrate using methods initially characterised by the group of Netzer and coworkers [6,7,9]. The Pd(111) substrate was first cleaned in situ by cycles of 1 keV Ar + ion bombardment and annealing to 600ºC to achieve a clean (1x1) ordered surface as indicated by SXPS and LEED.…”

“…Well-ordered V 2 O 3 (0001) films, as characterised by the low energy electron diffraction (LEED) pattern, have been grown on a range of substrates including W(110) [4], Cu 3 Au(100) [5], Au(111) [4], Pd(111) [6,7,8,9,10] and Rh(111) [11,12,13].…”

The local structure of OH species on the V2O3(0001) surface: A scanned-energy mode photoelectron diffraction study

“…The V 2 O 3 (0001) films were grown on a Pd(111) substrate using methods initially characterised by the group of Netzer and coworkers [6,7,9] A number of different methods were explored to produce a usable coverage of OH species on this surface. Initially, exposures to molecular water were conducted with the sample at room temperature.…”

“…This method of preparing the surface overcomes the complications of producing well-ordered stoichiometric surfaces on bulk oxide samples and is well-established for V 2 O 3 . In particular, well-ordered V 2 O 3 (0001) films, as characterised by their low energy electron diffraction (LEED) pattern, have been grown on a range of substrates including W(110) [4], Cu 3 Au(100) [5], Au(111) [4], Pd(111) [6,7,8,9,10] and Rh(111) [11,12,13], and here we followed the methodology established for growth on Pd(111). As a complement to this experimental structure determination we also present the results of ab initio density-functional theory (DFT) cluster calculations of the principle structural models discussed.…”