Effect of S contamination on properties of Fe(1 0 0) surfaces

The effect of S contamination on the properties of Fe(1 0 0) is examined using density functional theory (DFT) calculations. S is adsorbed at 1/2 monolayer coverage in atop, bridge and hollow sites in a c(2 × 2) arrangement. The effect of S on the clean surface properties is first examined for the three adsorption sites and compared with experimental and other theoretical data. S is found to adsorb preferentially in hollow sites on the isolated surface in agreement with experiment. The adhesion energy at different interfacial separations is then calculated and the effect of S on the interfacial properties of Fe(1 0 0) is characterised quantitatively and qualitatively. S is found to enhance adhesion at larger separations though at the equilibrium interfacial separation the maximum interfacial strength is reduced.     

Hyperthermal scattering of Cs from Pt(1 1 1): the effect of image charge on the ion-surface energy transfer

We have studied the energy exchange between hyperthermal (5-100 eV) Cs⁺ projectiles and a Pt(1 1 1) surface by measuring the kinetic energy of the scattered ions. The scattering geometry was chosen to be in-plane with specular scattering angles, and the energy of the scattered ions was analyzed as functions of incidence energy and angle. For low incidence energy (<40 eV), the energy transfer to the Pt surface is substantially enhanced due to the attractive image charge force between Cs⁺ and the surface. The image charge effects are highlighted by the different energy transfer on Pt(1 1 1) and Si(1 1 1) surfaces. Analysis of the experimental results using two- and three-dimensional theoretical models revealed a well depth of 1 eV for the image charge potential. Hyperthermal Cs⁺ ions scatter from Pt(1 1 1) predominantly via double collisions with Pt atoms, though the scattering phenomena are insensitive to the impact site at the surface.     

Digital image correlation approach to cracking and decohesion in a brittle coating/ductile substrate system

By using a digital image correlation technique, the full/local field strain in a brittle coating/ductile substrate system during tension has been successfully monitored. One of the most important experimental results indicates that the distribution of interfacial shear stress in the segmented coating is antisymmetric about the center, which clarifies several controversial assumptions introduced in theoretical models. Two key mechanical properties of thermal barrier coatings, fracture strength in coating and interfacial adhesion strength, were determined as 35.0 ± 4.6 and 14.1 ± 3.2 MPa, respectively, which are consistent with available experimental data.     

Electron dynamics in H/Na/Cu(1 1 1) collisions

Surface adsorbates induce strong local perturbations in the electronic structure and potentials in their surroundings. Consequently, charge transfer processes between projectiles and adsorbate-covered surfaces are strongly affected. The theoretical calculations and experiment measurements reported herein are focused on the H⁻/Na/Cu(1 1 1) system. The electron dynamics at the Na/Cu(1 1 1) surface and the influence of Na adsorbates on the H⁻-Cu(1 1 1) charge transfer are treated and discussed in detail. The ion fractions are mainly influenced by the ion exit trajectories. At low Na coverage, they exhibit a maximum near the 60° exit angle from surface. The calculations and experimental data are in good agreement.     

Why the case for clean surfaces does not hold water: Structure and morphology of hydroxylated nickel oxide (1 1 1)

We report an experimental and theoretical analysis of the √3 × √3-R30° and 2 × 2 reconstructions on the NiO (1 1 1) surface combining transmission electron microscopy, X-ray photoelectron spectroscopy, and reasonably accurate density functional calculations using the meta-GGA hybrid functional TPSSh. While the main focus here is on the surface structure, we also observe an unusual step morphology with terraces containing only even numbers of unit cells during annealing of the surfaces. The experimental data clearly shows that the surfaces contain significant coverage of hydroxyl terminations, and the surface structures are essentially the same as those reported on the MgO (1 1 1) surface implying an identical kinetically-limited water-driven structural transition pathway. The octapole structure can therefore be all but ruled out for single crystals of NiO annealed in or transported through humid air. The theoretical analysis indicates, as expected, that simple density functional theory methods for such strongly-correlated oxide surfaces are marginal, while better consideration of the metal d-electrons has a large effect although it is still not perfect.     

Methanol adsorption on silicon (0 0 1)

Using a first-principles pseudopotential technique, we have investigated the adsorption of CH₃OH on the Si(0 0 1) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (0 0 1) is as follows: the gas phase CH₃OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si-OCH₃ and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5 ML, i.e., one molecule per Si-Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.     

Cu(1 1 1) and Cu(0 0 1) surface electronic states. Comparison between theory and experiment

Recent advances in both the experimental resolution and in the computational capabilities motivate new studies of surface properties. In particular, a detailed comparison between theoretical and experimental data is expected to provide a better insight into surface and image states. In this work we present a joint effort analyzing such features of the Cu(1 1 1) and Cu(0 0 1) surfaces. The experiments are performed by using both linear and non-linear angle-resolved photoemission. From the theoretical point of view, we make use of the Green function embedding technique within density functional theory. We are able to account for the image states by suitably modifying the effective potential in the Kohn-Sham equation and the generalized boundary conditions on the Green function. Comprehensive theoretical and experimental results on the effective mass and the binding energy of the Shockley state and the first image states are reported.     

Modeling the influence of the porosity of laser-ablated silicon films on their photoluminescence properties

Nanostructured porous Si-based films produced by pulsed laser ablation (PLA) from a silicon target in residual helium gas can exhibit both size-dependent (1.6-3.2 eV) and fixed photoluminescent (PL) bands (1.6 and 2.2 eV) with their relative contributions depending on the film porosity. We study the influence of prolonged oxidation in ambient air on properties of the fixed PL bands, associated with oxidation phenomena, and their correlation with structural properties of the films. In addition, we propose a model describing the appearance of surface radiation states for oxidized films of various porosities. Our experiments and numerical simulations led to a conclusion that the 1.6 eV PL is due to a mechanism involving a recombination through the interfacial layer between Si core and an upper oxide of nanocrystals. This mechanism gives the optimal porosity of 73% for the most efficient production of 1.6 eV PL centers that is in excellent agreement with our experimental results.     

The effect of Me-Ti inter-atomic interactions on wetting in CaF2/(Me-Ti) systems: Ab-initio considerations

CaF₂ is a thermodynamically stable, non-reactive compound, displaying a relatively high contact angle with pure liquid metals. A remarkable decrease of this contact angle takes place when small amounts of Ti are added to liquid In, while a relatively small change of the contact angle is observed when it was added to liquid Sn. In order to understand the reason for this different behavior, ab-initio calculations were carried out in the framework of Density Functional Theory. The effect of the In-Ti and Sn-Ti inter-atomic interactions in the vicinity of CaF₂(1 1 1) slab is discussed using the results of modest calculations for several Me-Ti configurations on CaF₂(1 1 1) slabs. The results of the calculations indicate that the level of the interaction between the Me and the Ti atom affects the (Me-Ti)/CaF₂ interface composition, the interfacial energy and the wetting behavior.For the system with stronger inter-atomic attraction in the melt (Sn-Ti alloys), Ti atoms prefer to be surrounded by Sn atoms, and only weakly affect the metal/substrate interfacial energy and, thus the observed contact angle. However, for weak inter-atomic attraction (In-Ti alloys) an enhanced Ti adsorption at the metal/substrate interface takes place and leads to decrease the interfacial energy and improved wetting. The differences in the wetting behavior for these systems are discussed in terms of the total energy of each system, the electron charge re-distribution and the electron Density of States.     

The formation of sharp NiO(1 0 0)-cobalt interfaces

An atomically sharp interface between an antiferromagnetic oxide and a ferromagnetic metal may be obtained by the deposition of an epitaxial oxide buffer nanolayer in between. The buffer layer consists of the oxide of the ferromagnetic metal. The concept has been demonstrated on the NiO(1 0 0)-Co system, where the inclusion of a 1-2 ML CoO(1 0 0) interlayer inhibits the interfacial redox reaction which takes place between NiO and Co metal in the absence of the buffer layer.     

Theoretical STM images of alkaline-earth metal adsorbed Si(1 1 1)3 × 2 surfaces

We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction.     

Structural and electronic properties of V, Nb and Ta nanoclusters by tight-binding molecular dynamics simulations

We present tight-binding molecular dynamics (TBMD) calculations on V, Nb and Ta nanoclusters with N = 15, 65, 175 and 369 atoms. We found that rhombic dodecahedra are energetically favoured over rhombic hexahedra and icosahedra, with V forming the most compact cluster with the gyration radius increasing with the cluster size. In addition, from the calculated electronic density of states we found that the cluster size plays an important role in the HOMO-LUMO gap and that an increase in cluster size results in enhancement of the electronic density around the Fermi level. Furthermore, we found that the small clusters (N = 15, 65) exhibit energy gap that persists even at 900 K. These findings originate from charge transfer occurring between the inner and outer cluster atoms; interestingly, we found that in the small N = 15, 65 clusters electronic charge accumulates at each surface site at the expense of the inner cluster atoms, while in the larger clusters, N = 175 and 369, charge is gathered on the central particles of the cluster, suggesting different sub-surface character of the clusters. These findings are in agreement with available experimental and theoretical data and promise to offer important information for creating nanostructured materials with improved properties suitable for multiple technological applications.     

A comparison between optical properties of TiO2 nanowires obtained by EMA method and experiment

Optical properties of TiO₂ nanowires, synthesized by two-step thermal evaporation process, have been studied experimentally and theoretically. Based on the theoretical method optical constants of nanowires have been calculated with the use of the effective medium approximation (EMA). As evidenced by X-ray diffraction patterns our synthesized nanowires, whose diameters and lengths were within the ranges of 50-90 nm and 500-1500 nm, respectively, were found to be crystalline rutile TiO₂ with the major refraction being along the (1 1 0) direction. The experimental data of the reflectance of TiO₂ nanowires has been obtained using spectrometer in wavelength 250-800 nm, and then, compared with the spectrum of reflectance predicted by the EMA theoretical model. Our measured experimental optical data has been found to be in good accord with our predicted results spectrum with the use of the EMA modeling; this agreement indicates that our estimation of the volume fraction from atomic force microscopy (AFM) data was accurate.     

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.     

The specific oxidation mechanism in the initial stages of O2 adsorption on submonolayer Ba covered W(1 1 0) surface

The initial oxidation of a polarized 0.35-ML Ba-covered W(1 1 0) surface has been investigated in detail by high-resolution photoemission spectroscopy using synchrotron radiation. Upon small exposures (?0.15 L) to O₂, both interfacial W 4f (Ba-coordinated W) and Ba 4d core levels shift simultaneously toward smaller binding energies and then fix during 0.15-0.25 L, finally disappear at 0.6 L. The concurrent negative shifts of both interfacial W 4f and Ba 4d peaks can be attributed to be a consequence of the increased occupancy of 5d states. This explanation is supported by the variations of 5d stats in valence band spectra upon the initial oxidation. Our results clearly demonstrate that charge rearrangement is the dominant effect in the surface reactivity of our oxidized surface. We also found that this charge redistribution is associated with a structural change during oxidation, which is in contrary to previous studies of alkali-metal promotion. That is, oxygen is first chemisorbed atop the polarized Ba adlayer, then incorporates beneath Ba adatoms to form a covalent Ba-O-W complex upon further dosage, and reacts strongly with the Ba overlayer to become an ionic Ba⁺²O⁻² surface dipole layer at 0.6 L.     

Structure and reactivity of the hydrated hematite (0 0 0 1) surface

The structure of the hydroxylated hematite (0 0 0 1) surface was investigated using crystal truncation rod diffraction and density functional theory. The combined experimental and theoretical results suggest that the surface is dominated by two hydroxyl moieties—hydroxyls that are singly coordinated and doubly coordinated with Fe. The results are consistent with the formation of distinct domains of these surface species; one corresponding to the hydroxylation of the surface Fe-cation predicted to be most stable under UHV conditions, and the second a complete removal of this surface Fe species leaving the hydroxylated oxygen layer. Furthermore, our results indicate that the hydroxylated hematite surface structures are significantly more stable than their dehydroxylated counterparts at high water partial pressures, and this transition in stability occurs at water pressures orders of magnitude below the same transition for α-alumina. These results explain the observed differences in reactivity of hematite and alumina (0 0 0 1) surfaces with respect to water and binding of aqueous metal cations.     

On the geometrical and electronic structure of an ultra-thin crystalline silica film grown on Mo(1 1 2)

The atomic structure of a well-ordered silica film grown on a Mo(1 1 2) single crystal substrate is discussed in detail using the experimental and theoretical results available to date. New photoelectron spectroscopy results using synchrotron radiation and ultraviolet spectroscopy data are presented. The analysis unambiguously shows that the ultra-thin silica film consists of a two-dimensional network of corner-sharing [SiO₄] tetrahedra chemisorbed on the unreconstructed Mo(1 1 2) surface. The review also highlights the important role of theoretical calculations in the determination of the atomic structure of the silica films and in interpretation of experimental data.     

Diode laser spectroscopic measurements and theoretical calculations of line parameters of nitrogen-broadened water vapor overtone transitions in the 818-834 nm wavelength region

Nitrogen-broadened water vapor line parameters of the (2 1 1) ← (0 0 0) overtone band transitions in the 818-834 nm wavelength region are measured by using a tunable diode laser spectrometer. Water vapor is kept at its saturated vapor pressure at room temperature within a sample cell. Use of a balanced detector and a lock-in amplifier helps to increase the detection efficiency and the signal-to-noise ratio. The collisional broadening coefficients are extracted from the fitting of the experimental data by using a standard Voigt line profile. Collisional half-widths of water vapor lines perturbed by nitrogen are evaluated using the complex-valued implementation of the Robert-Bonamy (CRB) formalism. The rotational wave functions and the energy eigenvalues in the (2 1 1) vibrational state of water molecule are assumed on the basis of symmetry properties. Hence the outcome of this work also tests the validity of the theoretical approximations. The experimental data are compared with the corresponding theoretical values and the possible causes of deviation between the two have also been discussed.     

CF interaction with Si(1 0 0)-(2 × 1): Molecular dynamics simulation

In this study, the interaction of CF with the clean Si(1 0 0)-(2 × 1) surface at normal incidence and room temperature was investigated using molecular dynamics simulation. Incident energies of 2, 12 and 50 eV were simulated. C atoms, arising from dissociation, preferentially react with Si to form Si-C bonds. A Si_xC_yF_z interfacial layer is formed, but no etching is observed. The interfacial layer thickness increases with increasing incident energy, mainly through enhanced penetration of the silicon lattice. Silicon carbide and fluorosilyl species are formed at 50 eV, which is in good agreement with available experimental data. The level of agreement between the simulated and experimental results is discussed.     

Study of interface interactions in B4C/Ni-Cr-Si system

Boron carbide is a ceramic material possessing unique properties, and hence, high technological applications. The wetting behavior of ceramics by liquid alloys, which is of crucial importance in a variety of materials processing technologies, is determined by interfacial chemical interactions. In the present paper, the interfacial interactions of B₄C with a series of Ni-Cr-Si alloys with Ni/Cr = 3.4 have been studied. For this purpose, a thermodynamic method was developed to predict the interactions. In addition, the interfacial structure and chemical composition were examined using SEM-EDS technique. Good agreement was found between the theoretical calculations and experimental data. It was revealed that the amount of Si content in the alloys plays a major role on the interfacial reactions as well as the interface microstructure.