Surface Review and Letters

In this investigation, we have studied the kinetics and mechanism of desorption of CO from the Cu(110) surface using a new Monte Carlo simulation and putting emphasis on high order lateral interaction. According to our simulated TPD spectra, for β=10 K/s the maximum desorption rate occurs at T_m=218.6 K. Furthermore, analysis of simulated TPD spectra of CO desorption shows that it is strongly lateral-interactive and results an activation energy of CO desorption from Cu(110) that is E_d=66.6 Kj/mol. These simulated results are compared with other reported results and show excellent agreement. After that we have investigated the kinetics and mechanism of desorption of CO₂ from the Cu(100) surface using a Monte Carlo simulation. According to our simulated TPD spectra, for β=0.5 K/s the maximum desorption rate occurs at T_m=89.7 K. Analysis of simulated TPD spectra of CO₂ desorption shows that it is not strongly lateral-interactive and results in an activation energy of CO desorption from Cu(100) that is E_d=25.2 Kj/mol. Finally, the CO/Cu(110) system is compared with the CO₂/Cu(100) system.

The electronic structure of a grain boundary (GB) in α-Fe with segregated S and C impurity atoms was calculated by the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. An Fe₁₇₆ cluster model was used to simulate the local environment of the Σ5 (013), 36.9° [100] tilt boundary.

The results show a site competition on the GB between S and C. The C drives the S away from the boundaries. The C segregation is 4.77 eV more favorable than the S segregation.

Both interstitials weaken Fe–Fe bonds perpendicular to the GB plane and simultaneously form new interstitial-Fe bonds, although C presents the possibility of connecting the two halves of the crystal with a bridge Fe–C–Fe bond. These results should be important for understanding the complex mechanism for decohesion α-Fe.

We have carried out the calculations of vibrational dynamics in the low-dimensional structure with dimensions on the nanometer scale by using the matching formalism. The nanostructure model consists in the surface step produced at a perpendicular interface between two truncated thin films with different thickness. The theoretical approach determines the vibrational field in the direction where the translation symmetry is broken. The calculation concerns in particular the phonon dispersion curves localized on the interface and step edge, and employs the matching procedure in the harmonic approximation. The nearest and next nearest neighbors elastic force constants between the mass sites in the model, as well as a modification of the elastic strain field induced by the presence of the step, are considered. Analytic expressions are obtained for the phonon dispersion relations of the localized surface and edge elastic waves and the bulk phonons near a step. The breakdown of translational symmetry perpendicular to the step edge gives rise to several Raleigh-like branches localized in the neighborhood of step and interface. The effects of varying the elastic force constants at an interface and the strain field parameter near the step are studied. These factors influence the number of localized modes as well as their frequency, intensity and attenuation.

The positron annihilation parameter (mean lifetime) increases with increasing Mg contents for annealed and quenched samples (50_XX). Positron parameter τ, s (Doppler broadening) increase with increasing Q.T for the same (50_XX). The preferred orientation is one of the most important parameters that can be observed by XRD for materials, for such measurements in our observation: the dependence of the preferred orientation on the Mg content, the dependence of the preferred orientation on the Q.T, and the dependence of the preferred orientation on the degree of deformation.

The Kelvin probe is a sophisticated instrument which is very sensitive to changes in surface conditions, such as deformation, texture, phase transformation and contamination. Efforts have been made to use this technique to diagnose wear. In this study, the effect of the grain boundary (GB) on the electron work function (EWF) was examined with the aim of investigating the contribution of changes in grain size to total changes in the EWF during wear. Copper and aluminum were studied as examples. It was demonstrated that the EWF dropped in the vicinity of GB's and the mean EWF decreased as the grain size decreased. The mechanism responsible for the changes in the EWF with respect to the GB is discussed.

Amorphous silicon oxide nanowires (a-SiONW's) were synthesized on a NiO-catalyzed silicon substrate through a simple thermal annealing at 1000–1100°C without any additional silicon source materials. The synthesized nanowires were highly pure (no metal catalyst contamination) and very long (hundreds of micrometers). Most nanowires had a diameter in the range of 50–100 nm. The nickel oxide nanoparticles play a peculiar role in the synthesis of a-SiONW's. The growth of a-SiONW's is most likely controlled by the solid–liquid–solid (SLS) mechanism. IR absorption characteristics clearly showed bending (796 cm^-1) and asymmetric stretching (1075 cm^-1) modes of Si–O–Si bonds in a-SiONW's. An extraordinary UV emission at 370 nm was observed from a-SiONW's, which could be attributed to defect centers of oxygen excess in the nanowires.

We have modeled the dissociative chemisorption of water on the Si(100)-(2×1) surface using a generalized gradient approximation of density functional theory and a periodic slab model of the surface. For the energetically favorable structures, scanning tunneling microscope topographs of the filled states are simulated. These exhibit distinctively dark characteristics where water "islands" are formed, in agreement with experimental findings. In addition, they indicate that the hydrogen-atom and hydroxyl-radical adducts display somewhat different contrasts. Furthermore, in the case of a partial saturation of a Si dimer a prominent brightness is predicted for the unsaturated Si atoms if their dimer-forming counterparts are saturated by hydroxyl species, while in the case of hydrogen saturation the contrast is rather dim.

X-ray photoemission spectroscopy (XPS) measurements have been executed for TlTiOPO₄ to elucidate the general features in the electronic structure of the KTiOPO₄ family compounds. The peculiarities of the valence band structure have been discussed for the crystals. The persistence of core level binding energy differences O1s–P2p and O1s–Ti2p_3/2 has been detected in TlTiOPO₄ and KTiOPO₄, which relates well with the constancy of averaged P–O and Ti–O chemical bond lengths in this crystal family. The superstructure ordering of the TlTiOPO₄ surface subjected to polishing and annealing has been detected by reflectance high energy electron diffraction (RHEED). From comparison of surface crystallographic properties of TlTiOPO₄ and KTiOPO₄, the most typical superstructure indices have been revealed.

KHCO₃ compounds have been grown by the slow evaporation technique of an aqueous solution. The absorption process at the absorption edge of the compounds in the temperature range around the phase transition temperature (T_F≈322K) was investigated. The sample exhibited exponential behavior of the absorption edge following Urbach's rule. The Urbach parameters were determined and their temperature dependence was investigated, where drastic variations of these parameters at T_F were detected. The steepness parameter (s), the optical gap (E_g) and the disorder parameter (B) illustrate anomalous behavior with temperature. This behavior has been discussed in the light of the concept of order–disorder transition.

SiC short fibers, with an average diameter of 13 μm and length of 300–500 μm and chopped from SiC continuous fibers, were surface-modified by the semisolid mechanical stirring method to produce a discrete coating of aluminum particles. The SiC short fibers were introduced into semisolid state aluminum to prepare a slurry by the mechanical stirring method. As all of the short fibers were uniformly dispersed in the slurry, the temperature of the slurry was decreased as quickly as possible, but the slurry continued to be stirred until all of it was converted quickly into a powder-like composite. The surface morphology and axial section of the composite fiber and aluminum particles were examined using a scanning electron microscope (SEM) and an optical microscope; the element analysis of particles on the surface of short fibers in nanometer and submicron sizes was performed by Auger electron spectroscopy (AES). The results indicated that the powder-like composite was a mixture of pure aluminum powder and SiC short composite fiber, the surface of the composite fiber was coated by a discrete single layer of aluminum particles with nanometer, submicron and micron sizes, and the surface coverage of the aluminum particles on the surface of the short fibers ranged from 20 to 30%.

A thin-film bilayer structure consisting of polycrystalline Pb_0.85Sm_0.1TiO₃ and preferentially (111)-oriented Bi₂Ti₂O7 were prepared using the chemical solution deposition technique. Thin films were deposited by spin-coating. The structural properties of the films were examined by X-ray diffraction. The surface morphology and quality were studied by using an atomic force microscope. The films exhibit a good insulating property and resistance to breakdown. The clockwise hysteresis curve is referred to as polarization type switching, and the memory window is about 3.5 V. The accumulation capacitance and dielectric loss decrease with the increased annealing temperature. The (Pb, Sm)TiO₃/Bi₂Ti₂O₇ films in the "on" and "off" states are relatively stable.

Crystalline silicon kept at atmospheric pressure was irradiated with 775 nm multiple laser pulses of 150 fs duration at repetition rate 250 Hz. The laser pulses were circularly polarized, with a peak laser fluence of 0.03 J/cm². We observed surface damage at a much lower fluence and lower number of pulses compared to that reported in the literature. Surface damage, cracks and pits formation were observed. The evolution of the surface damage as a function of the number of laser pulses was recorded. The observations were in contrast to the findings in the literature that the silicon surface became structured when irradiated by multiple pulses.

We have measured, at room temperature, the resistivity, the surface roughness and the lateral surface correlation lengths of nanostructured platinum and gold thin films. The films' thickness d, deposited by vacuum arc plasma, is in the range 1.31≤d≤11.66 nm for platinum and 1.77≤d≤10.46 nm for gold. A theoretical estimate of our experimental data has been made.

Total energy calculations based on (1) density functional theory (DFT) in connection with ultrasoft pseudopotential and generalized gradient spin-polarized approximation (GGSA) and (2) the partial structural constraint path minimization (PSCPM) method have been used to investigate the energetically more favorable pathway for methylene (CH₂) insertion into the Ag–CF₃ bond followed by β-fluoride elimination to generate an isolated CH₂=CF_2(g) above the Ag(111) surface. The diffusion of the fcc-hollow site of CF_3(ads) toward the bridge site of CH_2(ads) is proposed as an energe*tically more favorable path for CH₂ insertion into the Ag–CF₃ bond to form the bridge site of CH₂CF_3(ads) on the Ag(111) surface. Then we proceed with β-fluoride elimination to form an isolated CH₂=CF_2(g) and the bridge site of F_(ads) on the Ag(111) surface. Our calculated energy barrier for β-fluoride elimination is 0.715 eV higher than that for CH₂ insertion on the Ag(111) surface. These calculated results imply that β-fluoride elimination rather than CH₂ insertion on the Ag(111) surface controls the CH₂=CF_2(g) formation rate as observed from temperature-programmed reaction (TPR) experimental data. Finally, we attribute these different energy barriers to the different transition state structures — largely distorted seven-centered versus less distorted four-centered — involved in these two different processes.

A modeling analysis of the growth mode of submonolayer amounts of Pt on Cu(111) and Cu on Pt(111) for different coverages and temperatures reproduces the known experimental behavior. An atom-by-atom analysis of the energetics using the BFS method for alloys provides a simple explanation of the underlying mechanisms leading to the observed behavior.