Tungsten and molybdenum oxide nanostructures: two-dimensional layers and nanoclusters.

W- and Mo-oxides form an interesting class of materials, featuring structural complexities, stoichiometric flexibility, and versatile physical and chemical properties that render them attractive for many applications in diverse fields of nanotechnologies. In nanostructured form, novel properties and functionalities emerge as a result of quantum size and confinement effects. In this topical review, W- and Mo-oxide nanosystems are examined with particular emphasis on two-dimensional (2D) layers and small molecular-type clusters.

Decomposition of Methanol on Mixed CuO-CuWO Surfaces.

Mixed CuO(2 × 1)-CuWO layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2 × 1) surface oxide with adsorbed (WO) clusters. The adsorption and decomposition of methanol on these well-defined CuO-CuWO surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200-300 K or decompose into CH and possibly CO.

Phase behaviour of 2D MnWO and FeWO ternary oxide layers on Pd(1 0 0).

The structure and properties of ternary oxide materials at the nanoscale are poorly explored both on experimental and theoretical levels. With this work we demonstrate the successful on-surface synthesis of two-dimensional (2D) ternary oxide, MnWO and FeWO , nanolayers on a Pd(1 0 0) surface and the understanding of their new structure and phase behaviour with the help of state-of-art surface structure and spectroscopy techniques. We find that the 2D MnWO and FeWO phases, prepared under identical thermodynamic conditions, exhibit similar structural properties, reflecting the similarity of the bulk MnWO and FeWO phases with the wolframite structure.

Two-Dimensional Iron Tungstate: A Ternary Oxide Layer With Honeycomb Geometry.

The exceptional physical properties of graphene have sparked tremendous interests toward two-dimensional (2D) materials with honeycomb structure. We report here the successful fabrication of 2D iron tungstate (FeWO ) layers with honeycomb geometry on a Pt(111) surface, using the solid-state reaction of (WO) clusters with a FeO(111) monolayer on Pt(111). The formation process and the atomic structure of two commensurate FeWO phases, with (2 × 2) and (6 × 6) periodicities, have been characterized experimentally by combination of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) and understood theoretically by density functional theory (DFT) modeling.

Lattice Strain Defects in a Ceria Nanolayer.

An ultrathin two-dimensional CeO2 (ceria) phase on a Cu(110) surface has been fabricated and fully characterized by high-resolution scanning tunneling microscopy, photoelectron spectroscopy, and density functional theory. The atomic lattice structure of the ceria/Cu(110) system is revealed as a hexagonal CeO2(111)-type monolayer separated from the Cu(110) surface by a partly disordered Cu-O intercalated buffer layer. The epitaxial coupling of the two-dimensional ceria overlayer to the Cu(110)-O surface leads to a nanoscopic stripe pattern, which creates defect regions of quasi-periodic lattice distortions.

Growth of Ceria Nano-Islands on a Stepped Au(788) Surface.

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm²) has been generated on the Au(788) surface by the postoxidation method.

Redox processes at a nanostructured interface under strong electric fields.

Manipulation of chemistry and film growth via external electric fields is a longstanding goal in surface science. Numerous systems have been predicted to show such effects but experimental evidence is sparse. Here we demonstrate in a custom-designed UHV apparatus that the application of spatially extended, homogeneous, very high (>1 V nm(-1)) DC-fields not only changes the system energetics but triggers dynamic processes which become important much before static contributions appreciably modify the potential energy landscape.

Metal tungstates at the ultimate two-dimensional limit: fabrication of a CuWO₄ nanophase.

Metal tungstates (with general formula MWO4) are functional materials with a high potential for a diverse set of applications ranging from low-dimensional magnetism to chemical sensing and photoelectrocatalytic water oxidation. For high level applications, nanoscale control of film growth is necessary, as well as a deeper understanding and characterization of materials properties at reduced dimensionality. We succeeded in fabricating and characterizing a two-dimensional (2-D) copper tungstate (CuWO4).

Nanostripe pattern of NaCl layers on Cu(110).

A sodium chloride monolayer on a Cu(110) surface gives rise to a highly corrugated periodic nanostripe pattern of the (100) lattice as observed by scanning tunneling microscopy and low-energy electron diffraction. As revealed by density-functional calculations, this pattern is a consequence of the frustration of the overlayer-substrate chemical bonding produced by epitaxial mismatch. The coexistence of regions of strong Cu-Cl covalent and weak nonbonding interactions leads to a chemically induced topographic modulation here realized in a two-dimensional dielectric.

Ordered arrays of size-selected oxide nanoparticles.

A bottom-up approach to produce a long-range ordered superlattice of monodisperse and isomorphic metal-oxide nanoparticles (NP) supported onto an oxide substrate is demonstrated. The synthetic strategy consists of self-assembling metallic NP on an ultrathin nanopatterned aluminum oxide template followed by a morphology-conserving oxidation process, and is exemplified in the case of Ni, but is generally applicable to a wide range of metallic systems. Both fully oxidized and core-shell metal-metal-oxide particles are synthesized, up to 3-4 nm in diameter, and characterized via spectroscopic and theoretical tools.

Tailor-made ultrathin manganese oxide nanostripes: 'magic widths' on Pd(1 1 N) terraces.

The growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of 'magic width' stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries.

The two-dimensional cobalt oxide (9 × 2) phase on Pd(100).

The two-dimensional (2D) Co oxide monolayer phase with (9 × 2) structure on Pd(100) has been investigated experimentally by scanning tunneling microscopy (STM) and theoretically by density functional theory (DFT). The high-resolution STM images reveal a complex pattern which on the basis of DFT calculations is interpreted in terms of a coincidence lattice, consisting of a CoO(111)-type bilayer with significant symmetry relaxation and height modulations to reduce the polarity in the overlayer. The most stable structure displays an unusual zig-zag type of antiferromagnetic ordering.

Kondo versus magnetic coupling of cobalt dimers in a Cu-O (2 × 1) reconstruction.

Co atoms and dimers embedded in a Cu(110)(2 × 1)-O surface oxide are investigated via low-temperature STM/STS experiments and first-principles simulations. It is found that Co dimers incorporated into adjacent rows of the Cu(110)(2 × 1)-O reconstruction show Kondo resonances decoupled from each other, whereas they are antiferromagnetically coupled (and do not exhibit a Kondo effect) when they are aligned on the same row. This shows that it is possible to decouple single carriers of the Kondo effect via a proper choice of the adsorption and host geometry.

Oxide-metal nanowires by oxidation of a one-dimensional Mn-Pd alloy: stability and reactivity.

Distinct one-dimensional (1D) oxide nanowires decorating the step edges of a stepped Pd(1 1 9) surface are formed by partial and complete oxidation of a 1D Mn-Pd alloy. Under full postoxidation treatment at 470-570 K, 1D MnO(2) nanowires coupled pseudomorphically to the Pd steps are obtained. Oxidized nanowires, which maintain the basic structural pattern of the 1D Mn-Pd alloy, are instead prepared by exposure of the Mn-Pd alloy to O(2) at 90 K and subsequent short heating to 400 K.

Interplay between magnetic, electronic, and vibrational effects in monolayer Mn3O4 grown on Pd(100).

The surface stabilized MnO(100)-like monolayer, characterized by a regular c(4 x 2) distribution of Mn vacancies, is studied by hybrid functionals and discussed in the light of available scanning tunneling microscopy and high-resolution electron energy loss spectroscopy data. We show that the use of hybrid functionals is crucial to account for the intermingled nature of magnetic interactions, electron localization, structural distortions, and surface phonons. The proposed Pd(100) supported Mn(3)O(4) structure is excellently compatible with the experiments previously reported in literature.

Two-dimensional manganese oxide nanolayers on Pd(100): the surface phase diagram.

Two-dimensional manganese oxide layers have been grown on Pd(100) and have been characterized by means of scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy (XPS). The complex surface phase diagram of MnO(x) on Pd(100) is reported, where nine different novel Mn oxide phases have been detected as a function of the chemical potential of oxygen μ(O). Three regions of the chemical potential of oxygen can be identified, in which structurally related oxide phases are formed, often in coexistence at the surface.

Chemical reactivity of Ni-Rh nanowires.

The properties of bimetallic Ni-Rh nanowires, fabricated by decorating the steps of vicinal Rh(111) surfaces by stripes of self-assembled Ni adatoms, have been probed by STM, photoemission, and ab initio density functional theory calculations. These Ni-Rh nanowires have specific electronic properties that lead to a significantly enhanced chemical reactivity towards oxygen. As a result, the Ni-Rh nanowires can be oxidized exclusively, generating novel quasi-one-dimensional oxide structures.

Model reaction studies on vanadium oxide nanostructures on Pd(111).

Deuterium desorption and reaction between deuterium and oxygen to water has been studied on ultrathin vanadium oxide structures prepared on Pd(111). The palladium sample was part of a permeation source, thus enabling the supply of atomic deuterium to the sample surface via the bulk. Different vanadium oxide films have been prepared by e-beam evaporation in UHV under oxygen atmosphere.

Experimental and theoretical study of a surface stabilized monolayer phase of nickel oxide on Pd(100).

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.

Reactivity of V2O3(0001) surfaces: molecular vs dissociative adsorption of water.

The adsorption of water on V2O3(0001) surfaces has been investigated by thermal desorption spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy with use of synchrotron radiation. The V2O3(0001) surfaces have been generated in epitaxial thin film form on a Rh(111) substrate with three different surface terminations according to the particular preparation conditions. The stable surface in thermodynamic equilibrium with the bulk is formed by a vanadyl (VO) (1x1) surface layer, but an oxygen-rich (radical3xradical3)R30 degrees reconstruction can be prepared under a higher chemical potential of oxygen (microO), whereas a V-terminated surface consisting of a vanadium surface layer requires a low microO, which can be achieved experimentally by the deposition of V atoms onto the (1x1) VO surface.

Planar vanadium oxide clusters: two-dimensional evaporation and diffusion on Rh(111).

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.

Reversible dynamic behavior in catalyst systems: oscillations of structure and morphology.

Exposing vanadium oxide nanoparticles on a Pd(111) surface to reducing conditions is shown to cause a spreading of the oxide over the metal until a reduced oxide phase covers the entire surface. Reoxidation reverses this process and oxide island structures and bare metal patches are reestablished. The physical origin of this wetting-dewetting process is revealed at the atomic level by in situ variable-temperature scanning tunneling microscopy and in terms of a surface oxide phase stability diagram, as calculated by density functional theory as a function of the chemical potential of oxygen and the vanadium concentration.