K. Meinel

Quasicrystalline structure formation in a classical crystalline thin-film system

The discovery of quasicrystals—crystalline structures that show order while lacking periodicity—forced a paradigm shift in crystallography. Initially limited to intermetallic systems, the observation of quasicrystalline structures has recently expanded to include ‘soft’ quasicrystals in the fields of colloidal and supermolecular chemistry. Here we report an aperiodic oxide that grows as a two-dimensional quasicrystal on a periodic single-element substrate. On a Pt(111) substrate with 3-fold symmetry, the perovskite barium titanate BaTiO3 forms a high-temperature interface-driven structure with 12-fold symmetry. The building blocks of this dodecagonal structure assemble with the theoretically predicted Stampfli–Gähler tiling having a fundamental length-scale of 0.69 nm. This example of interface-driven formation of ultrathin quasicrystals from a typical periodic perovskite oxide potentially extends the quasicrystal concept to a broader range of materials. In addition, it demonstrates that frustration at the interface between two periodic materials can drive a thin film into an aperiodic quasicrystalline phase, as proposed previously. Such structures might also find use as ultrathin buffer layers for the accommodation of large lattice mismatches in conventional epitaxy.

Scanning tunnelling microscopy on the growth and structure of NiO(100) and CoO(100) thin films

We have prepared ordered thin films of NiO and CoO in (100) orientation by evaporating Ni (Co) in an O2 atmosphere onto Ag(100). The films have been analysed by scanning tunnelling microscopy and low-energy electron diffraction. In the initial stage (coverage up to a few monolayers), growth and structure of the grown films drastically depend on the preparation conditions (in particular, on the temperature of the substrate during deposition and post-annealing). In this case we also observe strong interactions with the substrate. Ag atoms are partially removed from the substrate terraces and form islands or migrate to step edges. No indications for incorporation in the oxide thin films are seen. The oxidic features grow on top of the substrate or in the vacancy islands within the first layer of the substrate left behind by the removed Ag atoms. At low substrate temperatures (near room temperature) an essential part of the oxidic features corresponds to a precursor state rather than to the fully developed (100) oxide film which only develops after post-annealing to higher temperatures (typically around 500 K). I/U characteristics and the sample bias dependency of the contrast of the islands grown have been utilised for identification of whether an oxide reaction had taken place or not. The surfaces of the oxide precursor show a typical defect structure similar to those found on cleaved NiO(100) (M. R. Castell etal., Phys. Rev. B:Condens. Matter, 1997, 55, 7859). This feature shows ‘random walk’ at room temperature.

Scanning tunneling microscopy and spectroscopy investigation of the atomic and electronic structure of CoO islands on Ag(0 0 1)

Ultrathin CoO-films were deposited on Ag(0 0 1) by evaporation of Co in O2-atmosphere. During subsequent heating (400–500 K) well-ordered island with (0 0 1) and (1 1 1) orientation are formed. Scanning tunneling microscopy (STM) images of CoO(0 0 1) islands show a corrugation with the atomic periodicity of the Ag(0 0 1) substrate for gap voltages in the range of −1.5 to +1.5 V. (1 1 1) oriented Co oxide islands reveal a moire-like modulation with hexagonal structure. The heights of the different islands, as measured by STM, depend in a characteristic way on the bias voltage. Maps of the tunneling current as a function of bias voltage have been recorded using scanning tunneling spectroscopy (STS) at a temperature of 100 K. The evaluation of the multidimensional data set allows the characterization of the CoO(0 0 1) and (1 1 1) oriented Co oxide islands as well as the Ag(0 0 1) substrate, with respect to the electronic density of states and the influence on the imaging contrast in STM.

High-resolution LEED analysis of strained Cu layers on Ru(0001)

Scanning tunnelling microscopic observations have revealed that the relaxation of the lattice strain in the Cu film growing on Ru(0001) occurs in four different stages that are connected with different superstructures depending on the film thickness. Using high-resolution low-energy electron diffraction (HRLEED) the satellite spots of the different superstructures of Cu films with a thickness up to 7 ML (monolayers) grown at 520 K could be identified and quantitatively analysed. However, for Cu films thicker than 2 ML the diffraction patterns are very complex because satellite spots of several superstructures are incoherently superposed. Surprisingly, the structural data derived in a local scale by scanning tunnelling microscopy (STM) are highly representative for the entire surface, analysing by low-energy electron diffraction (LEED). This demonstrates the stability of the relaxation process. Corrugated Cu(111) layers formed after a deposition of 4 ML are rotated with respect to the Ru lattice within a small range of angles of only ±0.7°.

Activation and isomerization of n-butane on sulfated zirconia model systems—an integrated study across the materials and pressure gaps

Butane activation has been studied using three types of sulfated zirconia materials, single crystalline epitaxial films, nanocrystalline films, and powders. A surface phase diagram of zirconia in interaction with SO(3) and water was established by DFT calculations, which was verified by LEED investigations on single-crystalline films and by IR spectroscopy on powders. At high sulfate surface densities a pyrosulfate species is the prevailing structure in the dehydrated state; if such species are absent, the materials are inactive. Theory and experiment show that the pyrosulfate can react with butane to give butene, H(2)O and SO(2), hence butane can be activated via oxidative dehydrogenation. This reaction occurred on all investigated materials; however, isomerization could only be proven for powders. Transient and equilibrium adsorption measurements in a wide pressure and temperature range (isobars measured via UPS on nanocrystalline films, microcalorimetry and temporal analysis of products measurements on powders) show weak and reversible interaction of butane with a majority of sites but reactive interaction with <5 micromol g(-1) sites. Consistently, the catalysts could be poisoned by adding sodium to the surface in a ratio S/Na = 35. Future research will have to clarify what distinguishes these few sites.

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.

O-mediated layer growth of Cu on Ru(0001)

The film growth of Cu on clean and O-precovered Ru(0001) at different growth temperatures form 300 K to 450 K was investigated by scanning tunnelling microscopy (STM). Cu films on clean Ru(0001) grow in a multilayer mode at these temperatures. By using an O precoverage in the range of 0.1 monolayers (ML) up to a saturation coverage of 0.5 ML on clean Ru(0001), at 400 K different growth regimes are obtained. For ML a multilayer mode is preserved which changes into an O-induced two-dimensional (2D) growth for higher (0.2-0.5 ML). STM reveals the formation of an O/Cu surfactant structure on the surface due to migration of O initially located at the Ru surface. Its surface coverage rises linearly with O precoverage up to ML where it covers the surface completely. By increasing up to 0.5 ML, a drastic change in the morphology and density of the 2D islands occurs, which is accompanied by a change of the O/Cu surfactant structure. The O/Cu surfactant structure displays some order on a local scale for low , which changes into a disordered structure for ML. Structural similarities to oxidized surfaces of Cu(111) and the structures induced by postadsorption on Cu/Ru(0001) are discussed. Different models of surfactant mechanisms are presented to explain the observations. The locally ordered O/Cu surfactant structure (for ML) together with specific Cu film defects induce a heterogeneous nucleation of Cu with a high island density. Different mobilities of migrating Cu adatoms are established on top of the small islands and on the O/Cu structure resulting in enhanced interlayer diffusion explaining the observed 2D growth. The average island density only slightly changes within the temperatures investigated. In contrast, the saturated and disordered O/Cu surfactant structure (for = 0.4 - 0.5 ML) causes homogeneous nucleation. For this structure, the island density strongly depends on temperature and gives rise to an Arrhenius-like behaviour. The observed 2D growth is attributed to a reduction of the interlayer diffusion barrier. Cu growth on a formerly annealed Cu/O/Ru(0001) film system yields an almost perfect layer-by-layer growth caused by heterogeneous nucleation at periodically arranged Cu film defect sites. The relationship of the O/Cu surfactant structures to the ordered O/Cu bilayer on Ru(0001) - interpreted as a disrupted -like oxidized surface - was revealed.

The multilayer growth mode in the epitaxy of Ag on Ag(111) analysed by SPALEED

Abstract The morphology of atomically stepped hillocks formed during homoepitaxial growth on Ag(111) is analysed by SPALEED in the deposition range of 0–20 monolaycrs (ML). At 220 K the inclination of the facets increases linearly with coverage, with the terrace widths being correspondingly reduced. Facets with (100)-like steps are preferred over those containing (111)-like steps resulting in a more triangular shape of the hillocks. At room temperature the hexagonally shaped hillocks are larger owing to enhanced terrace widths. Moreover, the beginning interlayer diffusion limits the height of the hillocks to ~ 10 layers effecting a much slower reduction of the terrace widths after deposition of 3.7 ML.

Oxygen-induced restructuring of strained Cu layers on Ru(0001)

Abstract The interaction of O with strained Cu layers on Ru(0001) has been studied by means of high-resolution low-energy electron diffraction and scanning tunneling microscopy. An O-induced (3 × 2√3) structure with glide-plane symmetry is observed after exposure in the range 50–200 Langmuir at temperatures between 500 and 700 K on thin Cu films on Ru(0001). At Cu coverages > 2 monolayers the structure coexists with different heteroepitaxial Cu Ru (0001) strain structures. Since O adsorption is accompanied by a drastic mass transport of Cu atoms, we suggest that a two-dimensional O Cu surface structure is formed containing Cu in more than one layer.

Effects and structures of the O/Cu surfactant layer in O-mediated film growth of Cu on Ru(0 0 0 1)

The O-mediated Cu-film growth on O-precovered Ru(0 0 0 1) is investigated by means of scanning tunneling microscopy for growth temperatures between 300 and 600 K. Cu-films on clean Ru(0 0 0 1) grow in a multilayer mode. For O precoverages (Θ) between 0.2 ML (monolayer) and the saturation coverage (Θ=0.5ML), a layer-by-layer growth is observed at growth temperatures between 350 and 450 K. On Cu-islands, an O/Cu surfactant layer is formed, which floats on-top of the growing film and induces the layerwise Cu-film growth. The surface coverage of the O/Cu surfactant layer linearly rises with the O precoverage up to Θ≈0.4ML, where it completely covers the surface. Two different types of the surfactant layer are identified, inducing different surfactant mechanisms. For Θ=0.1–0.4ML, the O/Cu surfactant structure (A-type) displays some local order and induces inhomogeneous nucleation at the misfit-induced relaxation structure of the Cu-film. The layer-wise growth is explained by the concept of two mobilities, implying a large attempt frequency for adatom jumps over the interlayer diffusion barrier at the steps. For Θ = 0.4–0.5 ML, a disordered O/Cu surfactant layer is established (B-type), inducing homogeneous nucleation. The layer-wise Cu-film growth is attributed to a reduction of the effective interlayer diffusion barrier. Cu-film growth at 400 K on the ordered (3×2√3)O/Cu structure formed at temperatures around 520 K yields the conclusion that the O/Cu surfactant structures are composed of randomly arranged O–Cu–O strings and disrupted “Cu2O(1 1 1)” fragments.