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.

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.

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The formation of novel vanadium oxide cluster molecules by oxidative two-dimensional evaporation from vanadium oxide nanostructures is reported on a Rh(111) metal surface. The structure and stability of the planar V6O12 clusters and the physical origin of their 2D evaporation process have been elucidated by high-resolution scanning tunneling microscopy (STM) and ab initio density functional theory calculations. The surface diffusion of the clusters has been followed in elevated-temperature STM experiments, and the diffusion parameters have been extracted, indicating diffusion by hopping of the entire surface stabilized cluster units.

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Oxide structures with nanometric dimensions exhibit novel physical and chemical properties, with respect to bulk oxide materials, due to the spatial confinement and the proximity of the substrate. They derive their atomic structure and morphology, on the one hand, from the interactions at the interface between the oxide overlayer and the substrate and, on the other hand, from kinetic constraints during the growth process. Here we describe the formation of vanadium oxide nanostructures on a single-crystal metal surface and their characterization by scanning tunnelling microscopy (STM) and ab initio density functional theory (DFT) calculations. We show that vanadium oxide nanostructures can be formed on Rh(111) with morphologies ranging from quasi-zero- to three-dimensional and that the oxide growth can be tuned into a particular dimensionality by careful adjustment of experimental parameters. These ‘artificial oxide phases’ display new physical and chemical properties, which make them potentially interesting materials for nanotechnology applications.

A surface stabilized monolayer phase of nickel oxide, c(4 x 2)-Ni(3)O(4), has been found to grow epitaxially under reactive deposition conditions on Pd(100), in the presence of other adsorbed phases and in competition with them. High-quality scanning tunneling microscopy data are reported and discussed, including a detailed analysis of the defects and of the border morphology of this new phase. The data are discussed in the light of ab initio simulations of the electronic, energetic, and geometric properties of such a phase. A hybrid-exchange density functional theory approach has been used, and a slab model is adopted where palladium is simulated by a thin film covered on both sides by regular epilayers. A growth model has been developed that explains both the unusual stoichiometry of the phase and the observed defects.