Three-dimensional structure of the calcite-water interface by surface X-ray scattering

The three-dimensional structure of the calcite (104)-water interface has been determined with surface X-ray scattering. Nine crystal truncation rods (including specular and non-specular rods) were measured providing both vertical and lateral sensitivity to the interfacial structure. The results reveal that calcite is nearly ideally terminated with a single surface hydration layer that includes two inequivalent water molecules having distinct heights of 2.3 ± 0.1 and 3.5 ± 0.2 Å, each with a well-defined lateral registry with respect to the calcite surface. No additional layering of water is observed beyond this surface hydration layer. Small displacements in the outer two calcium carbonate layers were also observed. These results are compared with previous experimental and computational results.     

Structure analysis of stoichiometric LiNbO3(0 0 0 1) surfaces using low-energy neutral scattering spectroscopy

The atomic structure of LiNbO₃(0 0 0 1) surface was investigated by low-energy neutral scattering spectroscopy (LENS). Poled stoichiometric LiNbO₃ (SLN) samples were prepared for the measurements. The LENS was developed for surface structure and composition analysis particularly of highly insulating materials and was successfully applied to the structure analysis of the SLN(0 0 0 1) surface. The polar angle dependences of intensity of scattered He⁰ from the poled SLN surfaces indicate obvious differences between the negatively and the positively charged surfaces. It is suggested that O atoms cover the surfaces, and the first metal layers underneath the O layer consist of Li and Nb for negatively and positively charged surfaces, respectively, parallel to the applied electric field.     

Overpotential deposition of Ag monolayer and bilayer on Au(1 1 1) mediated by Pb adlayer underpotential deposition/stripping cycles

Ultra-thin Ag films on the Au(1 1 1) surface were prepared via overpotential deposition (OPD) in the presence of Pb²⁺ ions. By carrying out repetitive Pb adlayer underpotential deposition (UPD) and stripping cycles during Ag bulk deposition, the two-dimensional growth of Ag films was significantly enhanced in high OPD. The Ag monolayer sample was made by comparing the voltammetry curves, in which the signatures for Pb adlayer UPD on Au(1 1 1) changed to that on Ag(1 1 1). As demonstrated by the X-ray specular reflectivity measurements, nearly complete monolayer and bilayer films can be made with optimized deposition procedures. On subatomic scale, however, we found that these films have significant higher root-mean-square displacement amplitudes than those underpotentially deposited Ag monolayer and bilayer on either Au(1 1 1) or Pt(1 1 1).     

Surface structure analysis based on the exclusive use of the specular LEED spot – a theoretical study

The exclusive use of the specularly reflected beam (the (0,0) spot) may be a more practical way of collecting data for a LEED I–V structure analysis under certain experimental conditions. In this paper we discuss the special properties of the (0,0) spot intensity and test its sensitivity towards structural changes for the model system CO/Ni(1 1 1) within the framework of a R factor analysis. It is found that the (0,0) spot can, indeed, be used for a reliable structure determination if the energy range is increased by collecting data at different polar and azimuthal angles of incidence. The R factor contrast is, however, reduced with respect to a conventional LEED I–V analysis.     

Surface X-ray scattering studies of the growth of Pd thin films on the Pt(0 0 1) electrode surface and the effects of the adsorption of CO

The morphology of electrochemically deposited Pd films on the Pt(0 0 1) electrode surface has been examined through the combination of cyclic voltammetry (CV) and in situ surface X-ray scattering (SXS). Analysis of SXS measurements has indicated that the Pd grows via pseudomorphic island formation, with the partial occupation of successive layers occurring at a first layer occupation of 0.8 ML. Further Pd deposition sees the formation of larger islands built onto the now complete monolayer, characteristic of pseudomorphic Stranski–Krastanov (SK) growth. In the H_UPD potential region the effect of CO on the surface expansion of the multilayer Pd film is negligible. In the hydrogen evolution region, however, the effect of the adsorption of CO has been shown to produce surface normal expansion and in-plane disorder of the Pd film. It is suggested that hydrogen permeation into the Pd film is enhanced on the CO-poisoned surface.     

The ZnO non-polar (10 0) surface: an X-ray structural investigation

We performed a structural analysis of the non-polar ZnO (10 0) surface by means of grazing incidence X-ray diffraction. The analysis was conducted on ten rods, smooth surface domains, though of small coherent width, having been obtained after several Ar⁺ sputtering–annealing cycles. The surface derived from the bulk structure exposes one ZnO dimer per unit cell, parallel to the [001] axis. All the existing models, derived from ab initio calculations or low-energy electron diffraction (LEED) analyses, consist in a surface dimer whose O and Zn atoms are shifted inwards, with the O pointing outwards with respect to Zn. Whereas the LEED studies conclude on a dimer distance greater than in the bulk, the theoretical studies agree on a dimer contracted by amounts ranging from 5.5 to 7.5%. This contraction is interpreted as a result of the strong ionicity of ZnO, and is associated with a moderate dimer rotation. The latter, however, is found between 2.3 and 11.4°. Despite the discrepancies between the models, the Zn atom is always found shifted downwards by more than 0.25 Å. This is unambiguously rejected by our data, which show that the Zn atom keeps very close to its bulk position. It is displaced downwards by ΔZ_Zn=−0.06±0.02 Å, and it moves along [001] towards O by ΔX_Zn=0.05±0.02 Å. We denote a trend for the O atom to be displaced downwards too, with a concomitant displacement towards Zn. The faint X-ray scattering of O prevents us from assessing its position with accuracy. Depending on the choice of position for Zn in the error bar range, the buckling is evaluated as between −6.5 and 3°, or between −4 and 0.5°. The dimer distance is evaluated equal to 1.90 Å, with a deviation equal either to 0.06 or 0.11 Å.     

Surface alloys, overlayer and incommensurate structures of Bi on Cu(1 1 1)

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(1 1 1). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (√3 × √3)R30° unit cell. For increasing coverage, de-alloying occurs, leading to an overlayer structure at a coverage of 0.5 ML in which the Bi atoms form zigzag chains. The surface contains three domains of this phase. Finally, at a slightly higher coverage of 0.53 ML, the unit cell is compressed in one direction, leading to a uniaxial-incommensurate phase with three rotational domains.The structure determination includes relaxations in the topmost layers and therefore allows a detailed comparison of the most important bond distances. This shows that an increased charge density of the Cu(1 1 1) surface is the main driving force for the different phases.     

New method to characterize mesoscopic range and very small strain with using multi-wave X-ray diffraction

A new technique to observe mesoscopic-range strain fields (up to several hundreds of nm) is proposed, using modulation of the crystal-truncation-rod (CTR) scattering caused by Bragg reflection. This technique is particularly sensitive to small, long-range strain fields near crystal surfaces and interfaces, which are usually difficult to be discriminated by using Bragg reflection. A simple interpretation can be made for the modulation profile: the technique is physically simple with a few parameters fitted to the data and, independently of any model, is able to determine the total displacement due to mesoscopic strain field for depths up to several hundreds of nanometers. We applied this method to a Si(0 0 1) wafer whose surface is covered with a thermal oxide layer 3 nm thick. On the basis of the expressions we obtained for the modulation profile a least-squares fitting was carried out to give a result that under the oxide layer there exists a total displacement of −0.16 Å. It was also revealed from the visibility of the modulation profile that the total displacement has a static fluctuation of at least ±0.13 Å in the lateral direction. The new method can be used for the correction of the errors of the X-ray standing wave (XSW) method produced by strained layers near crystal surfaces.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Modification of UO2 crystal morphologies through hydroxylation

Atomic scale computer simulation is used to predict the surface energies of UO₂, subject to different hydroxide coverages. It was found that the {1 1 1} surface dominates dry UO₂, resulting in an octahedral morphology. However, the {1 0 0} surfaces were strongly stabilized by hydroxylation relative to the {1 1 1} surfaces. Consequently, even a modest hydroxylation of 30% substantially truncates the octahedron crystal morphology, and a fully cubic morphology is predicted at 80% hydroxide coverage.     

An atomistic view of electrochemistry

One of the most important tasks of modern, physical electrochemistry is the development of an atomistic picture of the solid/liquid interface in order to provide the basis for a mechanistic understanding of electrochemical processes. Electrochemists seek answers to the same questions as their surface science colleagues (e.g., electronic and structure properties of surfaces and adlayers), but are faced with the fact that in electrochemistry the contact of the solid with a condensed phase, the electrolyte, makes life much more difficult. Nevertheless, electrochemists succeeded in the last 20 years to develop an electrochemical surface science by adopting experimental techniques and theoretical concepts from surface physicists.

This article describes the various routes electrochemists have used to obtain a detailed characterization of electrode surfaces in particular, and of the electrochemical interface in general. Success in physical electrochemistry is based on the development of non-traditional in situ methods to complement the classical, current- and voltage-based techniques. The former range from optical spectroscopies, linear and non-linear, to in situ X-ray diffraction and scanning tunneling microscopy. The current status of electrochemical surface science and its most important future goals are briefly addressed.     

Surface oxidation of liquid Sn

We report the results of an X-ray scattering study that reveals oxidation kinetics and formation of a previously unreported crystalline phase of SnO at the liquid-vapour interface of Sn. Our experiments reveal that the pure liquid Sn surface does not react with molecular oxygen below an activation pressure of ∼5.0 × 10⁻⁶ Torr. Above that pressure a rough solid Sn oxide grows over the liquid metal surface. Once the activation pressure has been exceeded the oxidation proceeds at pressures below the oxidation pressure threshold. The observed diffraction pattern associated with the surface oxidation does not match any of the known Sn oxide phases. The data have an explicit signature of the face-centred cubic structure, however it requires lattice parameters that are about 9% smaller than those reported for cubic structures of high-pressure phases of Sn oxides.     

Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer.     

Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2O3-mullite multi-coatings for carbon/carbon composites

Oxidation protective SiC-Al₂O₃-mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO₂/Al₂O₃ ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700-1800 °C. A denser coating surface was acquired with the decrease of SiO₂/Al₂O₃ ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.     

A phenomenological explanation for the buckling of surface alloys

A phenomenological model, in which the interactions between the nearest-neighbor (NN) atoms are described as bondings but not hard sphere contacts, is proposed to explain the unexpected reduced buckling in surface alloy systems. In the model, the forces acting on an adsorbate atom from its NN substrate atoms in different layers may be either repulsive or attractive, depending on whether the bond between the adsorbate atom and its NN substrate atoms is compressed or stretched. It is found that the forces on the adsorbate atom from its NN substrate atoms in the sub-surface layer play a more important role for the buckling of surface alloy than those from its NN substrate atoms in the surface layer do. The bucklings expected by the model are significantly smaller than those predicted by the simple hard sphere model and are in good agreement with the experiments when the equilibrium bond length of the NN adsorbate-substrate atom pairs is taken as the sum of the corresponding metal radii.     

Impact of atomic relaxation on the breaks of constant force surfaces in AFM

A calculation model for determination of the shapes of the constant force surfaces and profiles of lateral forces for the case of the AFM tip scanning the closely packed lattice in contact mode is proposed. Atomic relaxation is taken into account in this model. The existence of breaks on constant force surfaces, which was predicted earlier in an approximation of the fixed lattice, is confirmed. It is shown that due to non-zero atomic mobility, breaks appear for smaller scanning forces than assumed earlier. The shapes of the continuous constant force surfaces and profiles of lateral force components are computed. These results may be used for diagnostics of point defects on the surface.     

Macroscopic etch anisotropies and microscopic reaction mechanisms: a micromachined structure for the rapid assay of etchant anisotropy

A new technique for the rapid quantification of orientation-dependent etch rates, which uses micromachined test patterns and optical microscopy, has been developed. The etching of silicon in KOH etchants with and without isopropanol was studied. Etch rates measured with this technique are in good agreement with conventionally measured rates. In most cases, the etch rate anisotropies are well described by a simple model that is based on step-flow etching. Kinetic Monte Carlo simulations of etching were used to test the simple model and to generate approximate morphologies of the etched surfaces. Vicinal Si(110) surfaces display unusual, orientation-dependent etch rates in some etchants; the functional form of the etch rate anisotropy suggests that a morphological transition occurs on these highly reactive faces. In moderately concentrated KOH solutions where isopropanol is readily soluble, the measured etch rate anisotropies suggest that isopropanol stabilizes step-flow etching.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.