The chemical potential in surface segregation calculations: AgPd alloys

We put forward a technique for calculating the surface segregation profile in substitutional disordered alloys. The surface internal energy and the effective bulk and surface chemical potentials are calculated using the full charge density exact muffin-tin orbitals method, combined with the coherent potential approximation. The application of our approach is demonstrated to the close-packed surface of Ag_cPd_1−c random alloys with 0 < c < 1. The surface concentration profile, surface energy and segregation energy are investigated as functions of bulk composition. The present results are compared with former theoretical and experimental data. It is found that at low temperature, Ag segregates to the surface layer for the entire bulk composition range. At 0 K, the subsurface layer contains 100% Pd for c ? 0.4, and somewhat more than (2c − 1) Ag in alloys with c > 0.5. The temperature dependence of the segregation profile is significant for Pd rich alloys and for alloys with intermediate concentrations. At temperatures ?600 K, the subsurface layer is obtained to be almost bulk like.     

Model calculation of the surface tension of liquid Ga-Bi alloy

A convenient model, based on some assumptions, for calculating the composition and temperature dependence of the surface tension of binary liquid alloys is reported. The theoretical calculations of the surface tension of gallium-rich-bismuth alloys are presented. The calculated results are compared with the reported experimental data. A relatively good agreement with experimental behavior of the composition dependence of the surface tension was found, but a disagreement was observed with experimental temperature behavior of the surface tension of these alloys. The calculations were conducted in the temperature range from almost 320 K to about 800 K. The surface tension was calculated from eutectic composition (x_Bi = 0.0022) to x_Bi = 0.1, and worked out by linear equations. The model calculation and analysis indicate a first order surface phase transition in this system, which is in accord with experimental findings. For this system, γ decreases linearly with increasing temperature at fixed Bi mole fraction x_Bi, and thus, suggesting a positive surface excess entropy. It is also found that the surface tension isotherms show the linear dependence on the concentration, in the logarithm scale of x_Bi, in the very narrow concentration range.     

Surface properties of liquid In-Zn alloys

The measurements of surface tension and density of zinc, indium and liquid In-Zn alloys containing 0.9, 0.85, 0.75, 0.70, 0.60, 0.40, 0.25 and 0.10 mole fraction of In were carried out using the method of maximum pressure in gaseous bubbles (MBP) as well as dilatometric technique. The technique of sessile drop was additionally applied in the measurements of surface tension for pure indium and zinc. The measurements were performed at temperature range 474-1151 K. The isotherms of surface tension calculated based on Butler's equation at 700 and 1100 K corresponded well with the experimental values for zinc content lower than 0.6 mole fraction. The surface tension calculated for alloys of higher zinc concentrations (0.6 < X_Zn < 0.95) had a positive value of the surface tension temperature coefficient (dσ/dT), which did not coincide with the experimental results. The density as well as molar volume of liquid In-Zn alloys showed almost identical behaviour like the ideal solutions. The observed little deviations were contained within assessed experimental errors.     

Nitrogen adsorption and thin surface nitrides on Cu(1 1 1) from first-principles

Experimental studies of nitrogen adsorbed on a Cu(1 1 1) surface show that the surface layer undergoes a reconstruction to form a pseudo-(1 0 0) structure. We use ab initio techniques to demonstrate the theoretical stability of this reconstructed surface phase over a range of conditions. We systematically investigate the chemisorption of N on the Cu(1 1 1) surface, from 0.06 to 1 ML coverage. A peculiar atomic relaxation of N atoms for 0.75 ML is identified, which results in the formation of a (metastable) “N-trimer cluster” on the surface. We have also investigated surface nitride formation, as suggested from experiments. A surface nitride-like structure similar to the reported pseudo-(1 0 0) reconstruction is found to be highly energetically favored. Using concepts from “ab initio atomistic thermodynamics”, we predict that this surface nitride exists for a narrow range of nitrogen chemical potential before the formation of bulk Cu₃N.     

Surface oxides of the oxygen-copper system: Precursors to the bulk oxide phase?

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we performed density-functional theory calculations, investigating the interaction of oxygen and copper, focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system. By employing the technique of “ab initio atomistic thermodynamics”, we show that surface oxides are only metastable at relevant pressures and temperatures of technical catalysis, with no stable chemisorption phase observed even at very low coverage. Although exhibiting only metastability, these surface oxides resemble the bulk oxide material both geometrically and electronically, and may serve as a precursor phase before onset of the bulk oxide phase. Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we show that a Cu₂O(1 1 1) surface with Cu vacancies has a lower free energy than the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant.     

Adsorption, ordering, and chemistry of nitrobenzene on Si(1 0 0)-2 × 1

Surface chemistry of nitrobenzene on Si(1 0 0)-2 × 1 has been investigated using multiple internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR), Auger electron spectroscopy (AES) and thermal desorption mass spectrometry. Molecular adsorption of nitrobenzene at submonolayer coverages is dominating at cryogenic temperatures (100 K). As the surface temperature is increased to 160 K, chemical reaction involving nitro group occurs, while the phenyl entity remains intact. Thus, a barrier of approximately 40.8 kJ/mol is established for the interaction of the nitro group of nitrobenzene with the Si(1 0 0)-2 × 1 surface. Further annealing of the silicon surface leads to the decomposition of nitrobenzene. The concentration of nitrogen and oxygen remains constant on a surface within the temperature interval studied here. AES studies also suggest that the majority of carbon-containing products remain bound to the surface at temperatures as high as 1000 K. The only chemical reaction leading to the release of the gaseous products is benzene formation around 670 K. The amount of benzene accounts only for a few percent of the surface species, while the rest of the phenyl groups connected to the silicon surface via a nitrogen linker remain stable even at elevated temperatures, opening an opportunity for stable surface coatings.     

The water-silicas interfacial interaction energies

The water-silicas interfacial interaction energies were calculated for samples of quartz, silicas and silicas outgassed at high temperatures using own and published data of the spreading pressure of water, its surface tension, its contact angle and using formulas obtained by the combination of the Young equation with a general equation of pair interaction. The values obtained for 18 different samples were in the range 7.80-6.92 kJ mol⁻¹. Lower values of energies are for samples that contain relatively less amounts of water at P/P₀ = 0.25 and are characterized also by relatively low values of surface pressures.     

Bulk and surface properties of liquid Al-Mg, Au-Sn, and Mg-Tl compound forming alloys

We have used phenomenological models based on statistical mechanics to study the bulk and surface properties of Au-Sn, Al-Mg, and Mg-Tl binary liquid alloys. Our study reveals that the three alloys are weakly ordered systems with the most stable intermetallic complexes at temperatures of 823 K, 1073 K and 923 K been AuSn, Al₃Mg₂ and Mg₄Tl, respectively. An analysis of Warren-Cowley short-range order parameter indicates that the weakest intermetallic compound is Al-Mg while Au-Sn is observed to be more chemically ordered than Mg-Tl. Furthermore, our surface properties calculations shows that Mg-atom and Sn-atom segregate to the surface over the whole concentration range of Al-Mg and Au-Sn alloys, respectively. In Mg-Tl, there is a competing effect between Tl-atom being drawn into the bulk and at the same time Tl-atom wanting to segregate to the surface over the whole concentration range.     

Bulk and surface properties of liquid X-Zr (X=Ag, Cu) compound forming alloys

The phase diagrams of the Ag-Zr and Cu-Zr systems exhibit the existence of different intermetallic compounds in the solid state, and since the structure of a liquid alloy is in some respects similar to that of a crystal, the compound formation phenomenon in these liquid alloy systems has been analysed through the study of surface properties (surface tension and surface composition), dynamic properties (chemical diffusion and viscosity) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of the compound formation model (CFM). Moreover, the associative tendency between unlike constituent elements qualitatively expressed by the microscopic functions indicates the glass-forming ability of both systems at higher Zr-concentrations. These results are in agreement with reported experimental data and confirm the applicability of a statistical mechanical theory in conjunction with the CFM to describe the mixing behaviour of compound forming alloys.     

Influence of substrate crystallography on the room temperature synthesis of AlN thin films by reactive sputtering

A pulsed DC reactive ion beam sputtering system has been used to synthesize aluminium nitride (AlN) thin films at room temperature by reactive sputtering. After systematic study of the processing variables, high-quality polycrystalline films with preferred c-axis orientation have been grown successfully on silicon and Au/Si substrates with an Al target under a N₂/(N₂ + Ar) gas flow ratio of 55%, 2 mTorr processing pressure and keeping the temperature of the substrate holder at room temperature. The crystalline quality of the AlN layer as well as the influence of the substrate crystallography on the AlN orientation has been characterized by high-resolution X-ray diffraction (HR-XRD). Best ω-FWHM (Full Width at Half Maximum) values of the (0 0 0 2) reflection rocking curve in the 1 μm thick AlN layers are 1.3°. Atomic Force Microscopy (AFM) measurements have been used to study the surface morphology of the AlN layer and Transmission Electron Microscopy (TEM) measurements to investigate the AlN/substrate interaction. AlN grew off-axis from the Si substrate but on-axis to the surface normal. When the AlN thin film is deposited on top of an Au layer, it grows along the [0 0 0 1] direction but showing a two-domain structure with two in-plane orientations rotated 30° between them.     

Surface core-level shift of Pd at the AgcPd1−c(1 1 1) surface: Nonlinear subsurface effects

The surface core-level binding-energy shift (SCLS) of Pd at the Ag_cPd_1−c(1 1 1) surface is calculated as a function of bulk concentration of the alloy. The equilibrium volume and the surface concentration profile used in the calculations refer to the 0 K case. The SCLSs are evaluated within the Z + 1 approximation. The results are analysed using the mixing enthalpy of the alloy and the bulk and surface chemical potentials. A relation of the SCLS to the bulk concentration is considered. This relation is shown to be mediated by the surface concentration profile which induces the observed nonlinear behaviour. The results are interpreted using a simple model for the alloy electronic structure.     

Combined STM, LEED and DFT study of Ag(1 0 0) exposed to oxygen near atmospheric pressures

We have investigated the interaction of molecular oxygen with the Ag(1 0 0) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O₂, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O₂ at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at ≈470 K and 10 mbar O₂ pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.     

Effects of thermo-hygro-mechanical densification on the surface characteristics of trembling aspen and hybrid poplar wood veneers

The effect of thermo-hygro-mechanical (THM) densification temperature on the surface color, roughness, wettability, and chemical composition of trembling aspen (Populus tremuloides) and hybrid poplar (Populus maximowiczii × P. balsamifera) veneers was investigated. Veneers were subjected to four THM densification temperatures (160 °C, 180 °C, 200 °C, and 220 °C). Veneer color darkened with increasing THM densification temperature. Surface roughness decreased between 160 °C and 200 °C. Wettability decreased after THM densification, but no significant difference was found between treated specimens. ATR-FTIR and XPS results confirmed that THM densification caused major chemical changes in veneer surfaces, and more pronounced at temperatures higher than 160 °C.     

Electrical and optical properties of Al-doped ZnO thin films by sol-gel process

Al-doped ZnO (AZO) thin films oriented along the (0 0 2) plane have been prepared by the sol-gel process and their electrical and optical properties with post-deposition heating temperature were investigated. The preferred c-axis orientation along the (0 0 2) plane was enhanced with increasing post-deposition heating temperature and the surface of the films showed a uniform and nano-sized microstructure. The electrical resistivity of the films decreased from 73 to 22 Ω cm as the post-deposition heating temperature increased from 500 to 650 °C; however, the film postheated at 700 °C increased greatly to 580 Ω cm. The optical transmittance of the films postheated below 650 °C was over 86%, but it decreased at 700 °C. The electrical and optical properties of the AZO films with post-deposition heating temperature are discussed.     

Transformation to soluble model for structural phase transition from (4 × 1) to (8 × “2”) of In-adsorbed Si(1 1 1) surface

The Ising model proposed previously for the structural phase transition from (4 × 1) to (8 × “2”) of In-adsorbed Si(1 1 1) surface, Hamiltonian of which is consisting of a two-spin interaction as well as a four-spin interaction is shown to be equivalent in thermodynamic properties to a soluble Ising model with two-spin interactions. Temperature dependence of the long range order and the transition temperature can now be determined from the exact formulae. Comparison between the simulation results and those from the exact formulae is made to see accuracy of the simulation.     

Simulation of adsorbate-induced faceting on curved surfaces

A simple solid-on-solid model, proposed earlier to describe overlayer-induced faceting of bcc(1 1 1) surface, is applied to faceting of curved surfaces covered by an adsorbate monolayer. Surfaces studied in this paper are formed by a part of sphere around the [1 1 1] pole. Results of Monte Carlo simulations show that the morphology of a faceted surface depends on the annealing temperature. At an initial stage the surface around the [1 1 1] pole consists of 3-sided pyramids and step-like facets, then step-like facets dominate and their number decreases with temperature, finally a single big pyramid is formed. It is shown that there is a reversible phase transition at which a faceted surface transforms to an almost spherical one. It is found that the temperature of this phase transition is an increasing function of the surface curvature. Simulation results show that measurements of high temperature properties performed directly and after fast cooling down to a low temperature lead to different results.     

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.     

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2)R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2)R45° phase by a first order transition, whereas the transition from c(9√2 × √2)R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step-step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step-step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu-Bi alloys is temperature dependent.     

The wettability of polytetrafluoroethylene and polymethylmethacrylate by aqueous solutions of Triton X-100 and propanol mixtures

Measurements of advancing contact angles (θ) were carried out for aqueous solutions of Triton X-100 (TX-100) and propanol mixtures at constant TX-100 concentration equal to 1 × 10⁻⁷, 1 × 10⁻⁶, 1 × 10⁻⁵, 1 × 10⁻⁴, 6 × 10⁻⁴ and 1 × 10⁻³ M, respectively, on polytetrafluoroethylene (PTFE) and polymethyhmethacrylate (PMMA). Using obtained results the changes of cosθ and adhesional tension against surface tension of all series of aqueous solutions of TX-100 and propanol mixtures (γ_LV) for PTFE and PMMA surfaces were shown. On the basis of these changes it was deduced that adsorption of TX-100 and propanol mixtures at PTFE-solution and solution-air interfaces is the same but the adsorption of TX-100 and propanol mixtures at solution-air interface is considerably higher than at PMMA-solution one. In the case of PTFE this conclusion was confirmed by relationship between cosθ and the reciprocal of the surface tension of solution. Extrapolation of the relationships between cosθ and/or adhesional tension and the surface tension of solutions to the points corresponding to the cosθ = 1 and adhsional tension equal to the surface tension of solution, the critical surface tension of PTFE and PMMA wetting was determined. The average values of critical surface tension of wetting determined from these relationships for PTFE are lying in the range of its surface tension values determined from contact angles of different kinds of liquids, which can be find in the literature, but for PMMA are considerably lower than the surface tension. The double value of the critical surface tension of PTFE wetting is equal to adhesion work of the solution to its surface and for PMMA there is not any correlation between these magnitudes.Using the measured values of the contact angles and Young equation the PTFE(PMMA)-aqueous solution interfacial tension was determined. The interfacial tension values of PTFE-aqueous solution were also calculated from the Fainerman and Miller equation in which the correcting parameter of nonideality of the surface monolayer was introduced and compared to those obtained from Young equation. From this comparison it results that the changes of PTFE-solution interface tension as a function of propanol concentration can be described by the Fainerman and Miller equation.     

Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3)R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450-525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S-Au(1 1 1) interaction.