Surface Review and Letters

The dielectric properties of the system Se₉₀Ge_10-xIn_x (x = 2, 4 and 6 at. %) were studied in the temperature range of 78–273 K and in the frequency range of 400 Hz–20 kHz. The density of localized states N(E) and the activation energy of hopping w_k for the samples were deduced at different temperatures. The exponents s, m₁ and m₂ of the equations σ_ac(ω) = Aω^s, ε′ = Bω^m₁ and ε′′ = Cω^m₂ have been deduced for the samples. The composition and temperature dependence of s, m₁, m₂, w_k and N(E) have been determined. The results are interpreted in terms of the correlated barrier hopping (CBH) model and the concept of localized states.

We investigate the dynamical properties of two isolated steps on the surface. We present the solution of the full dynamical problem arising from the absence of translation symmetry in two dimensions due to extended surface steps on the surface boundary of an insulating substrate. The calculations concern in particular the dynamics of localized modes of an atomic step on the surface of a cubic lattice. The theoretical approach determines the vibrational field in both steps. The matching method, which constitutes a powerful formalism for determining the vibrational properties of such disordered surfaces, is used. The model presented in this study consists of two monatomic steps as the interface between three coupled semi-infinite and single semi-infinite atomic layers. The dynamical properties of the perfect waveguides are presented and calculated numerically. The breakdown of translational symmetry perpendicular to the step edges gives rise to several Raleigh-like branches localized in the neighborhood of the steps. Typical dispersion curves for these modes along the steps are given with their polarization.

Yb:YAl₃(BO₃)₄ (YbYAB) is a self-frequency-doubling laser crystal. In this paper, we report a series of atomic force microscopy (AFM) observations on the growth and melting of the cleavage face of the crystal. Both growth and nucleation rates are very fast when the flux temperature is 0.5–1.5°C below the saturation point. These fast growth and nucleation rates are related to the nonsaturated bonds on the cleavage face formed during cleaving. The growth morphologies sensitively change to spiral melting holes when the flux temperature increases by only 0.5°C above the saturation point. When the temperature increases by another 1°C, the spiral melting mechanism transforms into a two-dimensional nucleation melting mechanism. The heights of the growth or melting steps are n × unit height of the structure layer in this direction. The melting step height increases much as the temperature increases by 1°C, but the growth step height increases only a little as the temperature decreases by 1°C. The melting steps are much wider than the growth steps, which reflects the fact that the melting centers (or melting nuclei), unlike the growth nuclei, are difficult to form. The growth morphology is not reverse-symmetric to the melting morphology as the temperature variation is reverse-symmetric on the saturation point.

The density functional theory method is used to explore the mechanism of dissociative adsorption of silane (SiH₄) on the S_A type stepped Si(100) surface. Two reaction paths are described that produce silyl (SiH₃) and hydrogen atom fragments adsorbed on the dimer bonds present on each terrace. It has been found that the initial stage of the dissociation of SiH₄ on the S_A type stepped Si(100) surface shows similarity to the dissociation of SiH₄ on the flat Si(100) surface; SiH₃ and hydrogen fragments bond to the Si dimer atoms by following the first reaction path.

We have synthesized poor quality diamond films, using high methane concentretion, by plasma-assisted chemical vapor deposition on silicon substrates. The roughness and dynamic critical exponents, α and β, of these films have been measured using an atomic force microscope. We show that the morphology, the roughness and the critical exponents of the diamond films are significantly different from those obtained with a low methane concentration. Our results are compared with Kardar–Parisi–Zhang theoretical predictions.

We report that the quality of atomic images reconstructed directly within the framework of holographic low energy electron diffraction can suffer from considerable atomic displacements. These are due to serious disturbances of the holographic object wave which exist when the object owns the same translational symmetry as the arrangement of beam splitters, for example when the latter induce a substrate reconstruction. We propose and test an iterative procedure for reducing these disturbances by a hybrid combination of the holographic reconstruction and some data fitting procedure. A reliable atomic image results after only a few iteration steps.

Two models of a V₂O₅ catalyst supported on TiO₂ are compared using DFT-GGA periodic calculations. In the first one, the energetically least favorable, a surface Ti atom of the TiO₂ anatase is substituted by a V atom, leading to a V⁺⁴ ion as active site. In the second model, the Ti atom is substituted by a VOH group; pentacoordinated V⁺⁵ ions are formed at the surface. This model leads to the most stable system with a gap between the occupied band mainly localized on the oxygen ions and the vacant band mainly localized on the metal ions. For the (100) face, it is concluded that the substitutions occur with the same energies at first neighbor and second neighbor metal positions. The adsorption energies are slightly larger for the (001) face, which is more stable.

The surface structure obtained by deposition of a Ag monolayer on the ideal c(2 × 2) antiferromagnetic Mn monolayer on Ag(001) at 100 K and subsequent annealing at room temperature is determined by low energy electron diffraction. It is established that this system is actually a good realization of an inverted monolayer, i.e. a pseudomorphic Ag/Mn/Ag(001) structure that corresponds to a reversed composition of the two topmost layers with respect to the Mn overlayer. The Ag–Mn and Mn–Ag interlayer distances, d₁₂ = 1.97 ± 0.015 Å and d₂₃ = 1.97 ± 0.02 Å respectively, indicate only a fairly small contraction of ~ 3.5% (~ 1.5%) with respect to the ideal Ag bulk lattice (Mn monolayer on top) as compared to ~ 10% expected from atomic radii in bulk Mn and Ag. This clearly reveals a spectacular magnetovolume effect related to the high spin state of Mn in this two-dimensional structure.

The electronic properties of H impurity in an Fe Σ = 5, 53.1° [100] (012) symmetrical tilt grain boundary (GB) were studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. A large cluster containing 197 Fe atoms was used to simulate the local environment of the boundary. The most stable positions for one H atom and two H atoms at the GB core were determined. The total energy of the cluster decreases when the H atoms are at that location, making it a possible site for H accumulation. The binding energy found was less than that of a Σ = 5 [100] (013) GB. An analysis of the orbital interaction reveals that H–Fe bonding involves mainly the Fe 4s and H 1s orbitals. A higher contribution of d orbitals is present, which show a different behavior when compared with mixed dislocation and vacancy in the bulk Fe. The interatomic bonding along the Fe atom chains via H atoms is very inefficient, thus resulting in significant weakening of the interatomic bonding. H–H interaction was also analyzed.

In this paper we report the electrochemical behavior of heat-treated carbon blacks and Pt/C catalysts. Cyclic voltammetry indicates that the heat-treated carbon black as catalyst support does not improve the Pt/C catalyst's activity for methanol oxidation. An XPS study of a Pt-loaded carbon black indicates that the amounts of oxidized platinum and oxygen-functional groups on catalysts are decreased when the platinum particles are deposited on the heat-treated carbon surface. These changes in the surface and crystalline structural properties of carbon materials lead to the catalytic activity change in methanol electro-oxidation.

The AZS refractory materials with different composition to the commercial product were obtained, and the feasibility of formation of these products was studied taking like departure point the free energy of their formation. The AZS refractories were obtained by a traditional method of producing ceramic materials, the process of sintering.

Using STM, we have directly confirmed the incommensurate stacking of double atomic layers of graphene and monolayer h-BN on Ni(111). The formation of a graphene layer weakens the interfacial interaction between monolayer h-BN and Ni(111), resulting in insulating h-BN layers, while a pristine monolayer h-BN on Ni(111) is metallic. The STS spectra of the double atomic layers showed a tunneling character with a band gap of 0.5 eV.

The scattering and transmission properties of vibrational waves in a perturbed two-dimensional wave-guide in the harmonic approximation are presented. The perturbation is characterized by an isolated adatom defect boundary separating two planar waveguide crystalline lattices, illustrated by a linear chain of defect masses. The theoretical approach using the matching procedure for calculating phonon scattering at their domain boundaries is also presented. The reflection and transmission probabilities in the scattering region are calculated and illustrated for heavy and light defect masses. The conductance properties, in full accordance with the Landauer–Büttiker description of electron transport, are also calculated in these two cases. It is shown that the influence of defect masses can be explained by the strong decreases of conductance values caused by the asymmetric Fano resonance type. The realistic example presented here is instructive and may constitute a precursor model for analyzing the scattering of vibrational waves by an isolated defect in the two-dimensional crystalline lattice formed by atoms adsorbed on the surface of crystal substrates such as physisorbed noble gas films on close-packed atomic planes of the metal surface. A detailed discussion on the vibrational waves in the scattering region is given.

In this work we study the adsorption of Cs on S-covered Si(100)-(2 × 1) and Si(100)-(1 × 1) surfaces, as well as the adsorption of S on Cs-covered Si(100)-(2 × 1). The experiment was performed in an ultrahigh vacuum (UHV) chamber with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function (WF) measurements. Predeposited S increases the binding energy and the maximum amount of Cs that can be deposited on the surface. The presence of S inhibits the pattern of the characteristic WF curve of Cs on clean Si(100)-(2 × 1), i.e. an initial decrease to a minimum value, Φ_min, followed by an increase toward the value Φ_max of metallic Cs. The WF, instead, decreases to a value close to that of saturated Cs on clean Si(100)-(2 × 1), where it forms a plateau. This is characteristic of the covalent bonding of Cs with the semiconductor substrate. Independently of the sequence of Cs and S deposition, (a) the transition Si(100)-(2 × 1) → Si(100)-(1 × 1) occurs when Θ_S > 0.5 ML, and (b) the sites of Cs and S remain the same, with the Cs atoms residing between the S atoms. Heating of the S/Cs/Si composite surfaces to ~ 650 K causes a reorganization of the Cs and S adatoms in a tendency to form a Cs–S complex. The issue of site preference for Cs and S adatoms has been discussed in detail in the structural models provided.

We have studied the spin-dependent quantum size effects in the reflection of 0–20 eV electrons from 2–10-ML-thick Fe microcrystals grown in situ on a W(110) surface in a spin-polarized low energy electron microscope. The goal is to obtain a better understanding of the magnetic ("asymmetry") image contrast and of the limits of spin polarization devices based on the quantum size contrast. The contrast is interpreted in terms of the band structure and of the spin-dependent inelastic mean free path. For the figure of merit of spin polarization devices, an upper limit of 5 × 10^-2 is obtained.

A photoemision study of the surface states on Be after Li adsorption at room temperature is reported. The surface band structure was mapped along four high symmetry directions of the SBZ. Fairly large shifts in the surface band locations were obtained but all surface states observed experimentally after Li adsorption were found to correspond to Be-derived states and no Li-derived surface states could be identified. The surface state bands located close to the Fermi level (E_F) were found to be affected the most and it is suggested that one surface state band which on the clean surface is located above E_F is pulled down below E_F after Li adsorption.

While the As-rich 2 × 4 reconstruction of GaAs(001) is well explained by the so-called β₂ structure, the atomic structure of the Ga-rich 4 × 2 phase has been discussed for a long time. In this review, the most important structural models for the GaAs(001) (4 × 2)/c(8 × 2) surface are compared from different theoretical and experimental points of view. The selected reconstructions include the recently proposed ζ model, a new mixed dimer model, and the well-known β, β₂, Cerdà and Skala models. The different structures are compared on the basis of total energy calculations, simulations of STM experimental images and interpretation of X-ray diffraction data. Only the ζ model satisfies all criteria, and provides therefore a satisfactory explanation of the atomic structure of GaAs(001)-(4 × 2).

By employing metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS, HeI) together with work function measurements and temperature-programmed desorption (TPD), the condensation of multilayer solvent systems, such as water and methanol on ultrathin Mo(100)-supported MgO films, has been investigated. In a first step, the techniques MIES/UPS and TPD are used to characterize the condensation of the pure solvent systems. Data collected in a coverage regime from submono- to multilayers at substrate temperatures between 100 and 500 K are surveyed in order to provide information about the buildup of the multilayer systems on an atomic level. Besides investigating the chemistry of pure phases, the present work provides insight into the chemistry of coadsorbed molecular species in a multilayer solvent environment. It is shown that thin films of amorphous water prepared at 100 K are a good candidate for studying aqueous multilayer chemistry under well-defined conditions, i.e. ultrahigh vacuum, by MIES. We are taking advantage of the unique properties of amorphous water films for investigating the interaction of water with coadsorbates, such as sodium and methanol. A deliberately chosen surface preparation technique in combination with the surface-sensitive electron-spectroscopic technique MIES enables the discrimination of various stages in the complex coadsorbate multilayer solvent reaction. Sodium has been chosen as a representative of a class of highly reactive and readily soluble species, and methanol as a surfactant.

The nature of oxide phases at metal–oxide interfaces, i.e. of oxide layers in the proximity of a metal surface, is assessed by critically examining the available data in the literature. The data reveal a trend towards the formation of reduced oxide phases with lower oxidation states in the vicinity of the interface with a metal. The physical origin of these interface-stabilized oxide layers is discussed and the possible causes include strong metal–metal bonding, high oxygen affinity of the substrate metal, reduction of the interfacial strain, and the stability of two-dimensional oxide phases.

Nucleation and growth are often discussed in terms of kinetics, i.e. the adsorption of atoms from the gas phase or a solution onto a surface, the diffusion of these atoms on that surface, and their attachment to a growing nucleus, island or layer. In recent years, scanning tunneling and atomic force microscopy studies have tremendously improved our understanding of such kinetic processes for a wide range of materials. At relatively low temperatures where diffusion is slow, and where typical deposition rates result in adatom concentrations that far exceed the equilibrium concentration of adatoms on the surface, growth is indeed controlled by irreversible atomic scale kinetics. But at higher temperatures this is not necessarily the case. Indeed, the equilibrium concentration of adatoms can be so high that it is only slightly increased by an external flux. Diffusion can be so fast that spatially separated regions on the surface interact on a time scale that is not slow relative to the growth process. In such cases reversible, collective phenomena are more important than individual atomic events, and thermodynamics is more important than kinetics. In this paper we examine a number of cases related to nucleation and growth on surfaces, where a deep and quantitative insight into the growth process can be obtained by detailed consideration of the thermodynamics involved. It is our hope that this paper will help to bring about a balanced understanding of these phenomena, where kinetics and thermodynamics are two poles on a continuum with an importance that depends on the particulars of each experiment.