Pt-induced structures on Si(1 1 0) studied by STM

The structures induced by platinum (Pt) adsorption on Si(1 1 0) surface have been studied by scanning tunneling microscopy (STM) for coverage up to 2 monolayers (ML). Three surface phases have been found to form: “5×4”, “13×2” and “6×5” for Pt coverages 0.3, 0.5 and 1 ML respectively. All structures are formed by one-dimensional rows aligned along the direction. At the coverage >1 ML islands of, probably, Pt silicide start to form in form of 1D nanowires.     

Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.     

Electronic structure of PdAg(1 0 0) ordered surface alloys using synchrotron radiation

PdAg(1 0 0) ordered surface alloys have been prepared by e-beam evaporation technique in situ in UHV and were investigated using high energy photoemission and normal incidence X-ray standing wave (NIXSW) techniques with wiggler radiation. The line shapes of 3d core levels of Ag and Pd exhibited interesting changes with the increase of Pd concentration in the alloys. The Ag 3d core level exhibited a disorder induced broadening that gradually increased with Pd concentration. On the other hand Pd 3d core level exhibited an asymmetry on the high binding energy side of the peak which gradually increased with Pd concentration. Interestingly, Pd core level did not exhibit a broadening with increase of Pd content in the alloy.     

Preparation and characterisation of Au(1 1 0) and Cu(1 1 0) surfaces for applications in ambient environments

The preparation of metal surfaces that in ambient conditions are flat and smooth over micron length scales is desirable for a wide range of applications. Scanning probe microscopy (SPM) studies of biomolecular adsorption and cell attachment require such well-prepared substrates. Standard polishing finishes are often found to exhibit considerable roughness and damage including scratches when investigated by SPM. We have prepared by means of UHV technology Au(1 1 0) and Cu(1 1 0) surfaces that when in ambient air exhibit a more homogeneous morphology and are considerably smoother than conventional polished surfaces. SPM techniques and the optical technique of reflection anisotropy spectroscopy (RAS) are used to characterise the morphological and electronic properties of these surfaces, respectively. The RA response of both Au(1 1 0) and Cu(1 1 0) surfaces in ambient conditions can be interpreted in terms of optical transitions between surface-modified bulk bands.     

Structural fluctuation of dimers on Ge(0 0 1) surface near transition temperature

The structural fluctuation of the orientational arrangement of buckled dimers on a Ge(0 0 1) surface near the transition temperature of the order-disorder phase transition is investigated by time-resolving dynamical Monte Carlo simulations. STM images averaged in a finite period are derived from the simulation. The coexistence of the c(4 × 2) and the apparent (2 × 1) domains in the STM images observed by experiments is reproduced in the simulated STM images. We show that the coexistence on the Ge(0 0 1) surface can be attributed to the critical slowing down near the transition temperature.     

Surface diffusion of Pb clusters on Cu(1 1 1) influenced by size dependent cluster-substrate misfit: Monte Carlo tight binding simulation model

The migration of two-dimensional Pb clusters on Cu(1 1 1) surface is studied in the framework of three-dimensional (3D) continuum space tight binding (TB) computational model. Monte Carlo simulations based on many-body TB interactions reveal significant influence of cluster-substrate misfit on diffusion behavior of two-dimensional (2D) clusters. The analysis of pair distribution function (PDF) demonstrates that cluster lattice constant depends on the number of atoms N for 1 < N < 10. The observed effect is more pronounced for heteroepitaxial than homoepitaxial systems. It can be explained in the framework of model accounting for enhanced charge transfer in heteroepitaxy and strong influence of surface potential on cluster atomic arrangement. The variation of lattice constant leads to variable misfit which affects the island migration behavior. The results are discussed in a physical model that implies cluster diffusion with size dependent cluster-substrate misfit.     

Adsorption of Nd on the Mo(1 1 0) surface

The adsorption of Nd on the Mo(1 1 0) surface has been studied by low energy electron diffraction and Auger electron spectroscopy. It has been found that at low coverages Nd adatoms form a rich amount of dilute (n × 2) commensurate structures, which can be explained as forming zigzag chains oriented along the [1 1 0] direction. Monte Carlo simulations indicate that the formation of the zigzag chain structures is initiated by the indirect lateral interaction between Nd adatoms.     

A theoretical study of structural and electronic properties of a missing dimer defect on Si- and C-terminated SiC(0 0 1)

We present a theoretical study of the geometrical and electronic properties of the C- and Si-terminated -SiC(0 0 1) surfaces in the vicinity of the missing dimer defect. The experimental results suggest that the atomic structures of these two surfaces may be considerably modified by external stress. In our present study we have considered the possible influence of this factor on the surface geometry of both systems. We have shown that the structural differences between the C- and Si-terminated surfaces lead to their different behaviour in the presence of a missing dimer and applied stress. In the case of the C-terminated c(2×2) surface, the missing dimer defect causes the buckling of the adjacent carbon dimers lying in the line of the defect (dimer atoms adjacent to the defect have vertical positions lower by 0.18 Å). This effect becomes more pronounced in the presence of compressive stress — the stress of 8% leads to the buckling of these two dimers of around 0.5 Å. The vertical positions of silicon atoms located directly below the defect were increased by 0.2 Å. We have also found that the missing dimer influences the structure of the carbon dimers on the neighbouring lines of dimers. Contrary to the C-terminated surface, the missing dimer defect on the Si-terminated SiC(0 0 1)-p(2×1) surface remains neutral for silicon dimers located in the line of defect, i.e. the dimers do not change their geometrical properties in unstrained structure nor in the presence of a tensile stress. On the other hand, this defect modifies considerably the geometry of the dimers from the two neighbouring lines of dimers by reducing their bond lengths and vertical positions. Changes in the geometrical properties of the second neighbour dimers (with respect to the defect) in these two lines are also noticeable. Moreover, we have found that the presence of a missing dimer modifies significantly the positions of the adjacent subsurface carbon atoms.     

Electron density profile at the interface of SiO2/Si(0 0 1)

In this report we present grazing incidence X-ray reflectivity (GIXR) study of SiO₂/Si(0 0 1) system. We have analysed the X-ray reflectivity data using recursive formalism based on matrix method and distorted wave Born approximation (DWBA). From the analysis of the reflectivity data we could obtain the electron density profile (EDP) at the interface of the dielectric SiO₂ film and the Si(0 0 1) substrate. The EDP obtained from the matrix method follows the DWBA scheme only when two transition layers are considered at the interface of SiO₂/Si. The layer which is in proximity with the Si substrate has a higher electron density value than the Si and SiO₂ values and it appears as a maximum in the EDP. The layer which is in proximity with the dielectric SiO₂ layer has an electron density value lower than the SiO₂ value and it appears as a minimum in the EDP. When the thickness of the SiO₂ layer is increased the lower density layer diminishes and the higher density layer persists.     

Simulation of the interface of (1 0 0) rutile with aqueous ionic solution

The interface of the rutile (1 0 0) surface with NaCl solutions has been simulated by classical molecular dynamics. In contrast to earlier simulations the protonation and hydroxylation equilibriums have been adjusted for different pH values (4, 7.4, and 9). The short range order close to the surface is described by two water layers with some orientational order and intermediate layers of positive or negative ions depending on the surface charge. A Stern model is confirmed with a dense layer of counterions on the charged TiO₂ surface and a diffuse layer, which only consists of few ions in our system. The increase of orientational order of the water molecules close to the surface is described by an exponential function with a decay parameter of 1.9 Å, superposed by a damped oscillation which is independent of the pH value. The diffusion is significantly slower than in the bulk within a range of 13 Å from the surface. We propose a common approach for describing the different z-dependences of orientational order and of the diffusion coefficients.     

Selforganized nanopatterning of Si(0 0 1)

The (2 × n) superstructure of Si(0 0 1) consists of elongated (2 × 1) reconstructed stripes separated by a dimer-vacancy line every few nanometers, thus offering a means to obtain a nanopattered Si(0 0 1) surface. Scanning tunneling microscopy (STM) investigations of Si(0 0 1) substrates that were deoxidized at 880-920 °C reveal that the formation of the (2 × n) depends strongly on the Si coverage of the topmost surface layer. It forms only in a narrow coverage window ranging from 0.6 to 0.8 ML. Systematic Monte Carlo simulations by an algorithm that combines the diffusion of monomers and dimers with the simultaneous deposition of Si onto the Si(0 0 1) surface, corroborate the STM results and suggest Si deposition as a viable alternative for introducing dimer vacancies in a well-defined manner.     

A Monte Carlo simulation study of H2 layers on NaCl(0 0 1)

Monte Carlo simulations show that, at one monolayer coverage, H₂ molecules adsorbed on a NaCl(0 0 1) surface occupy all Na⁺ sites and form a commensurate c(2 × 2) structure. If the Cl⁻ sites are occupied as well, a bi-layer p(2 × 1) structure forms. An examination of the H₂ molecules’ rotational motion shows the molecular axes are azimuthally delocalized and so both of the structures acquire (1 × 1) symmetry in accord with experimental observations. These calculations also show that helicoptering o-H₂ (J = 1, m = ±1) prefer to sit on top of Na⁺ sites, while cartwheeling o-H₂ (J = 1, m = 0) prefers to locate over Cl⁻ sites, in agreement with other work.     

Stress modifications induced by dimer vacancies in the Si(0 0 1) surface: Monte Carlo simulations

By means of Monte Carlo simulations, we investigate the local stress modifications induced by dimer vacancies (DVs) in the Si(0 0 1) subsurface layers. In presence of n isolated compact DVs, the sites located below these defect rows are under clearly compressive stress in the third layer and under more and more tensile stress, as n increases, in the fourth layer. At higher DVs densities, analogous trends are observed, but the stress modifications are then slightly extended between the dimer rows. Applying our results to the Ge penetration in Si(0 0 1), we show how the knowledge of the local stress may allow predictions of a given impurity behaviour in the vicinity of the surface, provided that the impurity-defect and impurity-impurity interactions do not play a major role compared to the local stress modification induced by the presence of DVs.     

Strain-induced changes in electronic structures and work function for (0 0 1), (1 1 0) and (1 1 1) of AlCu3

Studies of strain-induced changes in surface properties of metal/alloy surface have long been concerned by lot of scientists. However, the strain effects on the work function (WF), and in particular, its physical mechanism have not been well understood. In this paper, we employed a first-principles method to study the effects of biaxial strain in WF on the (0 0 1), (1 1 0) and (1 1 1) surfaces of AlCu₃. The relationship among the WF change, atomic relaxation and charge transfer induced by strain was discussed. The calculated results showed that tensile strain decreased the WF, while the compressive strain increased the WF; a larger atomic relaxation often followed with a larger WF change. The sensitivity of the WF with respect to the strain was strongly dependent on the direction of the surface or the density of atom packed plane of the surface.     

Thionin adsorption on silicon (1 0 0): Structural analysis

In the search of new photoactive species that may be of use either in energy conversion or in photodynamic therapy, we studied the functionalization of silicon (1 0 0) with thionin using the glutaraldehyde coupling reaction. Surface properties were obtained by ellipsometric measurements, atomic force microscopy, and luminescence spectroscopy, which indicated the formation of a well organised monolayer in the silicon surface. In addition, the comparison of Raman spectroscopy data with first-principles pseudopotential calculations of the vibrational modes of the thionin molecule also indicate the formation of a monolayer of intact thionin molecules. Therefore, the simple functionalization process (involving only mild aqueous solution chemistry) described here works well and can be used to construct smooth monolayers of photoactive molecules with functional amine groups.     

D2 layers on MgO(0 0 1): Simulation study

In response to recent helium atom scattering (HAS) and neutron scattering results, Monte Carlo simulations and perturbation theory calculations have been performed for D₂ on MgO(0 0 1). Monte Carlo simulations predict that D₂ molecules form a series of interesting structures, p(2 × 2) → p(4 × 2) → p(6 × 2), with coverages Θ = 0.5, 0.75, 0.83 respectively, and followed by a formation of a top layer of p(6 × 2) unit cell symmetry. The three types of mono-layers are stable up to 13 K, whereas the top layer still exists up to 10 K. This is in partial agreement with the neutron scattering and HAS results that report c(2 × 2), c(4 × 2) and c(6 × 2); they agree in terms of coverage and stability, but disagree in terms of symmetry. A quantum mechanical examination of the D₂ molecules’ rotational motion shows the molecular axes are azimuthally delocalized and hence the simulated structures are c-type rather than p-type. These calculations also indicate that ortho-D₂ and helicoptering para-D₂ prefer cationic sites, while cartwheeling para-D₂ prefers anionic sites.     

Atomic structure and electronic properties of Ta(1 1 2) and W(1 1 2) surfaces

We present results of theoretical and experimental studies of the structural and electronic properties of Ta(1 1 2) and W(1 1 2) surfaces. Atomic geometries of these surfaces and their electronic structures have been obtained from the first-principles pseudo-potential calculations based on the density functional theory and the use of the plane wave basis set. In the experimental part of our studies we have performed measurements of the photofield emission spectroscopy. The calculated density-of-states distributions are compared and discussed with energy distributions of electrons extracted from photofield emission characteristics. The photofield emission spectra measured for the surface regions of considered systems present very dense structures of features, while the corresponding dependences obtained for the bulk have much more smooth shape. Experimental results correspond very well with calculated density-of-states distributions.     

An example of chemistry-morphology interaction: making up for the geometric and energetic heterogeneities of the (1 0 0) surface of single crystalline silicon by high-temperature treatments in H2

A lot of work has been carried out to prepare chemically homogeneous (1 0 0) silicon surfaces. The hydrogen-terminated (1 0 0) silicon surfaces are the most promising ones, especially in view of their remarkable environmental stability. The simplest way to produce hydrogen-terminated surfaces (attack in water solution of HF of a sacrificial, thermally grown, oxide) results in strongly heterogeneous rough surfaces (although with prevailing dihydride terminations). These surfaces can, however, be flattened and homogenized by treating them in H₂ at high temperature (>850 C). The morphological and chemical changes undergone by the surface during the treatment are studied X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunnelling microscopy, infrared absorption spectroscopy in the attenuated total reflection mode, reflection high energy electron diffraction and thermal programmed desorption, and the mechanisms responsible for them are discussed.     

Shell model calculations of the adsorption and diffusion of K, Cl, and KCl on KCl(0 0 1)

Equilibrium adsorption positions and diffusion pathways of the ions K⁺ and Cl⁻ as well as of the molecule KCl on the terrace of the (0 0 1) surface of KCl were determined by shell model calculations allowing relaxations of the crystal lattice in the vicinity of the adsorbed species. For the ions each one adsorption position was found, in which the ions are located above the hollow site at the center of a slightly distorted square formed by two cations and two anions of the uppermost surface layer of the KCl crystal. Adsorption energies of −1.52 eV for K⁺ and −1.61 eV for Cl⁻ were calculated. Jumps of the ions occur from these positions to adjacent hollow positions in the ±[1 0 0] and ±[0 1 0] directions with a jump distance of a₀/2. The activation energies for the jumps result as 0.142 for K⁺ and 0.152 eV for Cl⁻ and the mean diffusion lengths as and . For the KCl molecule four distinct adsorption minima with energies between −0.932 and −0.825 eV were found. Because of the smaller lattice relaxation caused by the molecule the adsorption energies are considerably lower than for the single ions. In the position with the largest adsorption energy the ions of the admolecule are again placed above adjacent hollow sites. In two more adsorption positions only one ion is at the hollow site and the other one in a top position above an oppositely charged ion of the surface. In the fourth position with the smallest adsorption energy both ions are in top positions. Jumps between the different adsorption positions proceed by rotations of the molecule, in which one of its ions remains essentially attached to a local minimum position. The diffusion and desorption of a KCl molecule was studied by a Monte Carlo method, resulting in a mean diffusion length x_s (nm) = 0.39 exp[0.84 (eV)/2kT], which agrees rather well with an experimental value of . Values for the mean stay time as well as for the surface diffusion coefficient are derived.     

Anisotropy analysis of the surface energy of hcp (c/a < 1.633) metals

In this paper, anisotropy of the surface energy of 5 hcp metals Be, Hf, Ru, Ti and Y have been analyzed. The surface energies of three kinds of representative surfaces, (h 0 l), (h h l) and (h k 0) belong to [0 1 0], [] and [0 0 1] crystal band, respectively, have been calculated using the modified embedded atom method. For all 5 hcp metals, the (1 1 0) plane has the minimum surface energy in all 35 surfaces studied. Considering surface energy minimization solely, the (1 1 0) texture should be favorable in the hcp films. The fact that the short termination corresponds to much lower surface energy than long one implies the former is more stable for those surfaces having two possible terminations. Such as the prism plane (1 0 0), only the short termination was observed in experiment.