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The growth and geometric structure of ultrathin zinc oxide films on Pd(111) has been studied by scanning tunneling microscopy, low-energy electron diffraction, and density functional theory calculations. For sub-monolayer coverages, depending on the oxygen pressure, two well-ordered zinc oxide phases with (4 × 4) and (6 × 6) coincidence structures form, which are attributed to H-terminated Zn6O5 and graphite-like Zn6O6 layers, respectively. The (6 × 6) phase exhibits a pronounced oxygen pressure dependence: at low p(O2) a well-ordered (6 × 6) two-dimensional array of O vacancies develops, yielding a layer with a formal Zn25O24 stoichiometry, while at high p(O2) the Zn6O6 monolayer transforms into bilayer islands. For oxide coverages up to 4 monolayers the graphite-like Zn6O6 structure is thermodynamically the most stable phase over a large range of oxygen chemical potentials, before it converges to the bulk-type wurtzite structure. Under oxygen-poor conditions a compressed overlayer of Zn adatoms can be stabilized on top of the Zn6O6 structure.

The reduction of vanadium oxide monolayer structures on Rh͑111͒ has been investigated by variabletemperature scanning tunneling microscopy, low energy electron diffraction, photoelectron spectroscopy of the core levels, and the valence band, and by probing the phonon spectra of the oxide structures in high-resolution electron energy loss spectroscopy. A sequence of oxide phases has been observed following the reduction from the highly oxidized ͑ ͱ 7 ϫ ͱ 7͒R19.1°V-oxide monolayer: ͑5 ϫ 5͒, ͑5 ϫ 3 ͱ 3͒rect, ͑9 ϫ 9͒, and "wagon-wheel" oxide structures are formed with decreasing chemical potential of oxygen O. The structures have been simulated by ab initio density functional theory, and structure models are presented. The various V-oxide structures are interrelated by common V u O coordination units, and the reduction progresses mainly via the removal of V v O vanadyl groups. All oxide structures are stable at the appropriate O only in the twodimensional V-oxide/Rh͑111͒ phase diagram and are thus stabilized by the metal-oxide interface. The results demonstrate that oxides in ultrathin layer form display modified physical and chemical properties as compared to the bulk oxides.

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The growth and structure of ultrathin vanadium oxide films on Rh͑111͒ has been studied by scanning tunneling microscopy, low-energy electron diffraction, high-resolution x-ray photoelectron spectroscopy, highresolution electron energy-loss spectroscopy, and ab initio density-functional-theory calculations. For submonolayer coverages ͓⌰Ͻ0.6 MLE ͑monolayer equivalents͔͒, depending on the oxide preparation route ͑reactive evaporation vs postoxidation͒, two well-ordered V-oxide phases with (ͱ7ϫͱ7)R19.1°and (ͱ13 ϫͱ13)R13.8°structures and similar electronic and vibrational signatures have been observed. The ͱ7 and ͱ13 phases are interface stabilized and exhibit high formal oxidation states (ϳ5 ϩ). In the oxide coverage range 0.6Ͻ⌰Ͻ1.2 MLE, i.e., after the completion of the first oxide layer, the ͱ7 and ͱ13 structures are replaced by several coexisting V-oxide phases, where the oxidation state of the V atoms progressively decreases from 4 ϩ to 2 ϩ with increasing oxide coverage. For coverages exceeding 2 MLE a bulk-type V 2 O 3 phase with corundum structure grows epitaxially on the Rh͑111͒ surface. The observed growth mode is examined by assessing kinetic and energetic effects in the ultrathin oxide film growth. The importance of the oxide-free areas of the metal support for the formation of highly oxidized V-oxide layers at the initial stages of growth is discussed.

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.