Atomic Scale Insights into Reversible Oxygen Storage in Vanadium Oxide Thin Films.

Monolayer vanadium oxide films grown on Pt(111) can be reversibly switched between an oxygen-poor and an oxygen-rich composition, equivalent to VO and VO, respectively. While the overall oxygen storage capacity of the film is quantified by X-ray photoelectron spectroscopy, the atomic binding sites of the extra O species are determined by low-temperature scanning tunneling microscopy and electron diffraction. In the O-poor phase, the oxide takes the form of a honeycomb lattice that gets partially covered with vanadyl (V=O) groups at higher O exposure.

Light Emission from Single Oxygen Vacancies in CuO Films Probed with Scanning Tunneling Microscopy.

Global photoluminescence (PL) and spatially resolved scanning tunneling microscopy (STM) luminescence are compared for thick CuO films grown on Au(111). While the PL data reveal two peaks at 750 and 850 nm, assigned to radiative electron decays via localized gap states induced by O vacancies, a wide-band emission between 700 and 950 nm is observed in STM luminescence. The latter is compatible with cavity plasmons stimulated by inelastic electron tunneling and contains no spectral signature of the CuO defects.

Termination-dependent electronic structure and atomic-scale screening behavior of the CuO(111) surface.

By combining differential conductance (d/d) spectroscopy with a scanning tunneling microscope and hybrid density functional theory simulations we explore the electronic characteristics of the (1 × 1) and (√3 × √3)30° terminations of the CuO(111) surface close to thermodynamic equilibrium. Although frequently observed experimentally, the composition and atomic structure of these two terminations remain controversial. Our results show that their measured electronic signatures, such as the conduction band onset deduced from d/dmeasurements, the bias-dependent appearance of surface topographic features, as well as the work function retrieved from field emission resonances unambiguously confirm their recent assignment to a (1 × 1) Cu-deficient (CuD) and a (√3 × √3)30° nano-pyramidal reconstruction.

Electron stimulated desorption of vanadyl-groups from vanadium oxide thin films on Ru(0001) probed with STM.

Low-temperature scanning tunnelling microscopy (STM) is employed to study electron-stimulated desorption of vanadyl groups from an ultrathin vanadium oxide film. The vanadia patches are prepared by reactive vapour deposition of V onto a Ru(0001) surface and comprise a highly ordered network of six and twelve membered V-O rings, some of them terminated by upright V[double bond, length as m-dash]O groups. The vanadyl units can be desorbed via electron injection from the STM tip in a reliable fashion.

A fiber scanning tunneling microscope for optical analysis at the nanoscale.

A hybrid scanning tunneling/optical near-field microscope is presented, in which an optical fiber tip coated with 100 nm thick Ag/Cr films scans the surface. The tip metallization enables operating the instrument via a current-based distance control and guarantees sub-nanometer spatial resolution in the topographic channel. The fiber tip simultaneously serves as nanoscale light source, given the optical transparency of the metal coating.

Growth and characterization of Ca-Mo mixed oxide films on Mo(001).

Calcium-molybdate ultrathin films were prepared on a Mo(001) crystal and characterized by means of scanning tunneling microscopy (STM), electron diffraction, photoelectron spectroscopy, and density functional theory (DFT). The films were grown via reactive Ca deposition, followed by a vacuum annealing step to trigger Mo diffusion from the support into the Ca-O ad-layer. A series of crystalline oxide configurations was revealed that evolves from a (3 × 3) to a (4 × 4) and (6 × 6) superstructure with increasing annealing temperature and finally decays to a binary MoO phase.

Tungsten deposits facilitate oxidation of the NiAl(110) surface.

The alumina film formed by oxidation of NiAl(110) has gained enormous attention as a surface-science compatible model system for a crystalline and atomically flat oxide surface. A main disadvantage is its small thickness of only 0.5 nm that limits possible uses in catalytic studies at elevated temperature and pressure.

Adsorption of squaraine molecules to Au(111) and Ag(001) surfaces.

The adsorption of anilino squaraines, an important chromophore for the use in organic solar cells, to Ag(001) and Au(111) has been studied with scanning tunneling microscopy. Self-assembly into square building blocks with eight molecules per unit cell is revealed on the Ag surface, while no ordering effects occur on gold. The squaraine-silver interaction is mediated by the carbonyl and hydroxyl oxygens located in the center of the molecule.

Temperature-dependent phase evolution of copper-oxide thin-films on Au(111).

The formation of ultrathin copper oxide layers on an Au(111) surface is explored with scanning tunneling microscopy and density functional theory. Depending on the thermal treatment of as-grown Cu-O samples, a variety of thin-film morphologies is observed. Whereas 1D oxide stripes with Au[112[combining macron]] and Au[11[combining macron]0] orientation emerge at 450 and 550 K annealing, respectively, a planar (2 × 2) Cu-O network with specific domain structure develops at higher temperature.

Incorrect DFT-GGA predictions of the stability of non-stoichiometric/polar dielectric surfaces: the case of Cu2O(111).

Scanning tunneling microscopy (STM) and hybrid density functional theory (DFT) have been used to study the stability and electronic characteristics of the Cu2O(111) surface. We challenge previous interpretations of its structure and composition and show that only appropriate (hybrid) calculations can correctly account for the relative thermodynamic stability of stoichiometric versus Cu-deficient terminations. Our theoretical finding of the stoichiometric surface to be most stable at oxygen-lean conditions is confirmed by an excellent matching between STM spectroscopy data and the calculated surface electronic structure.

Erratum: Titration of Ce^{3+} Ions in the CeO_{2}(111) Surface by Au Adatoms [Phys. Rev. Lett. 111, 206101 (2013)].

This corrects the article DOI: 10.1103/PhysRevLett.111.

Formation of Water Chains on CaO(001): What Drives the 1D Growth?

Formation of partly dissociated water chains is observed on CaO(001) films upon water exposure at 300 K. While morphology and orientation of the 1D assemblies are revealed from scanning tunneling microscopy, their atomic structure is identified with infrared absorption spectroscopy combined with density functional theory calculations. The latter exploit an ab initio genetic algorithm linked to atomistic thermodynamics to determine low-energy H2O configurations on the oxide surface.

Molecular Adsorption Changes the Quantum Structure of Oxide-Supported Gold Nanoparticles: Chemisorption versus Physisorption.

STM conductance spectroscopy and mapping has been used to analyze the impact of molecular adsorption on the quantized electronic structure of individual metal nanoparticles. For this purpose, isophorone and CO2, as prototype molecules for physisorptive and chemisorptive binding, were dosed onto monolayer Au islands grown on MgO thin films. The molecules attach exclusively to the metal-oxide boundary, while the interior of the islands remains pristine.

Exploring routes to tailor the physical and chemical properties of oxides via doping: an STM study.

Doping opens fascinating possibilities for tailoring the electronic, optical, magnetic, and chemical properties of oxides. The dopants perturb the intrinsic behavior of the material by generating charge centers for electron transfer into adsorbates, by inducing new energy levels for electronic and optical excitations, and by altering the surface morphology and hence the adsorption and reactivity pattern. Despite a vivid scientific interest, knowledge on doped oxides is limited when compared to semiconductors, which reflects the higher complexity and the insulating nature of many oxides.

Activating Nonreducible Oxides via Doping.

Nonreducible oxides are characterized by large band gaps and are therefore unable to exchange electrons or to form bonds with surface species, explaining their chemical inertness. The insertion of aliovalent dopants alters this situation, as new electronic states become available in the gap that may be involved in charge-transfer processes. Consequently, the adsorption and reactivity pattern of doped oxides changes with respect to their nondoped counterparts.

Phonon-mediated electron transport through CaO thin films.

Scanning tunneling microscopy has developed into a powerful tool for the characterization of conductive surfaces, for which the overlap of tip and sample wave functions determines the image contrast. On insulating layers, as the CaO thin film grown on Mo(001) investigated here, direct overlap between initial and final states is not enabled anymore and electrons are transported via hopping through the conduction-band states of the oxide. Carrier transport is accompanied by strong phonon excitations in this case, imprinting an oscillatory signature on the differential conductance spectra of the system.

Autocatalytic growth of ZnO nanorods from flat Au(111)-supported ZnO films.

Physical vapour deposition of ZnO on an Au(111) support has been investigated as a function of the oxygen chemical potential by means of scanning tunnelling microscopy and luminescence spectroscopy. Whereas a layer-by-layer growth of ZnO is revealed in oxygen excess, formation of oxide nanorods with large height-to-diameter ratio prevails at lower oxygen chemical potentials. We ascribe the formation of 3D nanostructures in the latter case to traces of Au atoms on the surface that promote trapping and dissociation of the incoming oxygen molecules.

Surface defects and their impact on the electronic structure of Mo-doped CaO films: an STM and DFT study.

The functionality of doped oxides sensitively depends on the spatial distribution of the impurity ions and their interplay with compensating defects in the lattice. In our combined scanning tunneling microscopy (STM) and density functional theory (DFT) study, we analyze defects occurring in Mo-doped CaO(001) films at the atomic scale. By means of topographic imaging, we identify common point and line defect in the doped oxide, in particular Mo donors and compensating Ca vacancies.

Titration of Ce3+ ions in the CeO2(111) surface by Au adatoms.

The role of surface and subsurface O vacancies for gold adsorption on crystalline CeO2(111) films has been investigated by scanning tunneling microscopy and density functional theory. Whereas surface vacancies serve as deep traps for the Au atoms, subsurface defects promote the formation of characteristic Au pairs with a mean atom distance of two ceria lattice constants (7.6 Å).

Adsorption, activation, and dissociation of oxygen on doped oxides.

Charge transfer in the presence of dopants is relevant for the adorption and activation of small molecules, such as O2 . Scanning tunneling microscopy and DFT calculations provide evidence for the formation of strongly bound superoxo species on chemically inert, Mo-doped CaO films. This oxygen surface species shows a high propensity to dissociate.

Charge competition with oxygen molecules determines the growth of gold particles on doped CaO films.

The influence of gas-phase oxygen on the growth of Au nanoparticles on Mo-doped CaO films has been investigated by means of low temperature scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Whereas at ideal vacuum conditions, only 2D Au islands develop on the oxide surface, the fraction of 3D deposits increases with increasing O2 pressure until they become the dominant species in 106 mbar oxygen. The morphology crossover arises from changes in the interfacial electron flow between Mo donors in the CaO lattice and different ad-species on the oxide surface.

Nanoparticles for heterogeneous catalysis: new mechanistic insights.

Metallic nanoparticles finely dispersed over oxide supports have found use as heterogeneous catalysts in many industries including chemical manufacturing, energy-related applications and environmental remediation. The compositional and structural complexity of such nanosized systems offers many degrees of freedom for tuning their catalytic properties. However, fully rational design of heterogeneous catalysts based on an atomic-level understanding of surface processes remains an unattained goal in catalysis research.

Probing the properties of metal-oxide interfaces: silica films on Mo and Ru supports.

The influence of metal-oxide interactions on the workfunction and band alignment in thin oxide films is investigated for silica mono- and bilayers grown on Mo(112) and Ru(0001) supports. By analyzing the position of field-emission resonances and the Kelvin-probe signal deduced from conductance and force spectroscopy, we have identified a substantial lowering of the workfunction in the monolayer films, with the oxide bands shifting accordingly. We explain this observation with a stronger coupling and a shorter binding length of the silica monolayer to the metal substrate, which removes the effect of electron spill-out, produces a positive interface dipole and reduces the workfunction of the system.