Study of the structural and atomic diffusive properties of the ordered Cu3Au (110) surface

The structural properties, the formation and migration energies of a single vacancy migrating intralayer and interlayer in the CuAu‐terminated (110) surface of Cu₃Au ordered alloy have been calculated and discussed by using the modified analytical embedded‐atom method (MAEAM) and molecular dynamics (MD) methods. The surface layer exhibits rippling that the Au atoms are raised above Cu atoms about 0.117 Å in the topmost layer. The displacements of the topmost two layers are comparatively larger, while the third layer relaxes slightly and there are no changes in the nether layers. From energy minimization, the vacancy is most likely to be formed in the first layer (1L), especially on the Au site. The surface vacancy shows the smallest formation energy compared to the interlayer and bulk vacancies, while the corresponding value converges after the fifth layer (5L). For Cu vacancy originally sited in the second layer (2L) and migrated intralayer and interlayer, the diffusion without causing the local disorder is the most favorable, and the vacancy tends to migrate to the topmost layer. In the topmost layer of the CuAu‐terminated (110) surface, the circularity path is preferred over the beeline path. Copyright © 2011 John Wiley & Sons, Ltd.     

Atomic and electronic structure of unreduced and reduced CeO2 surfaces: a first-principles study

The atomic and electronic structure of (111), (110), and (100) surfaces of ceria (CeO2) were studied using density-functional theory within the generalized gradient approximation. Both stoichiometric surfaces and surfaces with oxygen vacancies (unreduced and reduced surfaces, respectively) have been examined. It is found that the (111) surface is the most stable among the considered surfaces, followed by (110) and (100) surfaces, in agreement with experimental observations and previous theoretical results. Different features of relaxation are found for the three surfaces. While the (111) surface undergoes very small relaxation, considerably larger relaxations are found for the (110) and (100) surfaces. The formation of an oxygen vacancy is closely related to the surface structure and occurs more easily for the (110) surface than for (111). The preferred vacancy location is in the surface layer for CeO2(110) and in the subsurface layer (the second O-atomic layer) for CeO2(111). For both surfaces, the O vacancy forms more readily than in the bulk. An interesting oscillatory behavior is found for the vacancy formation energy in the upper three layers of CeO2(111). Analysis of the reduced surfaces suggests that the additional charge resulting from the formation of the oxygen vacancies is localized in the first three layers of the surface. Furthermore, they are not only trapped in the 4f states of cerium.     

The negative adsorption of chromium atoms on alloy-oxide film boundaries during oxidation in air and anodic passivation of Fe-Cr and Ni-Cr alloys

Equilibrium of Cr atoms between the surface layer and bulk of a binary alloy was analyzed. The Gibbs adsorption equation was used to obtain the dependence of the adsorption activity of atoms in the surface layer on their activity in the bulk. An approximate thermodynamic method was used to calculate the adsorption of Fe (Ni) and Cr atoms in the surface layers of Fe-Cr and Ni-Cr alloys. According to calculations, there was negative adsorption, X _Cr ≪ 1, in the surface layer of the alloys caused by a large difference between the Gibbs surface energies of Cr and Fe (or Ni). The negative adsorption of Cr shifted chemical reaction equilibria on the alloy-oxide film boundary both in oxidation in air and in anodic passivation, 3FeO (NiO) + 2Cr = Cr₂O₃ + 3Fe(Ni), toward oxide film enrichment in the FeO (or NiO) oxide. A unified method for calculating the composition of oxide films on alloys was used for both processes. The method was based on the use of the initial data on the Gibbs surface energy of metals constituting alloys. The calculated oxide film compositions were close to the experimental X-ray photoelectron spectroscopy data.     

Confinement effects for ionic carriers in SrTiO3 ultrathin films: first-principles calculations of oxygen vacancies

One-dimensional confinement effects are modelled within the hybrid HF-DFT LCAO approach considering neutral and single-charged oxygen vacancies in SrTiO(3) ultrathin films. The calculations reveal that confinement effects are surprisingly short-range in this partly covalent perovskite; already for film thickness of 2-3 nm (and we believe, similar size nanoparticles) only the surface-plane defect properties differ from those in the bulk. This includes a pronounced decrease of the defect formation energy (by ～1 eV), a much deeper defect band level and a noticeable change in the electronic density redistribution at the near-surface vacancy site with respect to that in the bulk. The results also show that the size effect pertains to the interactions between the oxygen vacancy and two neighboring titanium atoms and orientation (parallel or perpendicular to the surface) of the Ti-V(O)-Ti complex. In particular, we predict considerable oxygen vacancy segregation towards the surface.     

Understanding mixing of Ni and Pt in the Ni/Pt(111) bimetallic catalyst via molecular simulation and experiments

Molecular dynamics (MD) simulations employing embedded atom method potentials and ultrahigh vacuum (UHV) experiments were carried out to study the mixing process between the Ni and Pt atoms in the Ni/Pt(111) bimetallic system. The barrier for a Ni atom to diffuse from the top surface to the subsurface layer is rather high (around 1.7 eV) as calculated using the nudged elastic band (NEB) method. Analysis of the relaxation dynamics of the Ni atoms showed that they undergo diffusive motion through a mechanism of correlated hops. At 600 K, all Ni atoms remain trapped on the top surface due to large diffusion barriers. At 900 K, the majority of Ni atoms diffuse to the second layer and at 1200 K diffusion to the bulk is observed. We also find that smaller Ni coverages and the presence of Pt steps facilitate the Ni-Pt mixing. By simulated annealing simulations, we found that in the mixed state, the Ni fraction oscillates between layers, with the second layer being Ni-richer at equilibrium. The simulation results at multiple time scales are consistent with the experimental data.     

Chain structures of surface hydroxyl groups formed via line oxygen vacancies on TiO2(110) surfaces studied using noncontact atomic force microscopy

Structures of surface hydroxyl groups arranged on a reduced TiO2(110) surface that had line oxygen vacancies were studied using noncontact atomic force microscopy (NC-AFM). NC-AFM results revealed that by increasing the density of oxygen vacancies on the TiO2(110) surface, line oxygen vacancies were formed by removal of oxygen atoms in a bridge oxygen row on the TiO2(110) surface. After the TiO2(110) surface with the line oxygen vacancies was exposed to water, the surface showed hydroxyl chain structures that were composed of hydroxyl groups linearly arranged in a form of two rows on the line oxygen vacancies and on a neighboring bridge oxygen row. In-situ NC-AFM measurements of these surfaces exposed to water at room temperature revealed that hydroxyl chain structures were formed at the line oxygen vacancy. Annealing above 500 K was sufficient to remove the hydroxyl chain structures on the TiO2(110) surface and allowed line oxygen vacancies to reappear on the surface. The line oxygen vacancies are active sites for water dissociation. In conclusion, the formation of the hydroxyl chain structure suggests that the surface hydroxyl groups on a TiO2(110) surface can be controlled by preparing oxygen vacancy structures on the surface.     

SnO2(110)弛豫表面构型与电子结构的第一性原理研究

采用基于第一性原理的密度泛函方法对SnO₂(110)表面的构型和电子结构进行了系统研究. 结果表明, 与理想表面相比, 表面弛豫导致表层五配位Sn原子向体相方向位移, 六配位Sn原子以及表面氧原子往真空方向移动, 而桥氧原子位置基本保持不变. 当表面厚度小于3 nm时, 表面能和表层原子的弛豫大小随着层数的增加出现振荡现象. 由能带计算结果得知, 以桥氧的2p_y/2i>p_z轨道为主要成分的能带出现在体相的带隙中. 进一步考察了弛豫对表面电子结构的影响.     

Importance of bulk properties in the structure and evolution of cleavage surfaces of quasicrystals

The present work reviews the recent achievements in probing bulk properties of quasicrystals by using cleavage surfaces and surface sensitive techniques. In particular, it is shown that the cluster–subcluster-based structure of the cleavage surface of icosahedral Al–Pd–Mn quasicrystals can be related to the presence of stable atom clusters in the bulk, which force the crack front to circumvent them. Furthermore, by subjecting cleavage surfaces of differently pre-annealed Al–Pd–Mn quasicrystals to a post-cleavage heat treatment, we demonstrate that bulk vacancies migrate toward the surface, where they initiate structure and composition changes. These studies allow us to characterize Al–Pd–Mn quasicrystals with respect to their bulk vacancy concentration. As-grown Al–Pd–Mn quasicrystals are found to contain a supersaturation of all chemical species of vacancies in near stoichiometric composition, whereas long term pre-annealed material has a much lower, and predominantly Al, vacancy concentration. Analogous experiments for decagonal Al–Ni–Co quasicrystals show that as-grown Al–Ni–Co has a lower vacancy concentration than as-grown Al–Pd–Mn.     

Initial formation of contact layers on Ni/SiC samples studied by XPS

This study deals with the quantitative assessment of the coverage and thickness of Ni silicide films formed during annealing of SiC substrates with sputtered thin films of Ni. The analytical approach involves the use of XPS and depth profiling by means of successive ion etchings and XPS analyses. For either 3 or 6 nm initial Ni film thickness, a 10 nm Ni₂Si product is formed. On top of this product, the C released is accumulated in a very thin (1–2 nm) film. In neither case, the Ni₂Si covers the whole surface, although the coverage is almost complete (～90%) in the latter case. For the greater initial Ni‐film thickness of 17 nm, the thickness of the Ni₂Si product corresponds well to the value of 25 nm expected from the Ni/Ni₂Si stoichiometric relationship. This thickness is significantly greater than a critical level and the film covers the whole surface. Carbon is similarly accumulated in a very thin layer on the top surface, although the major part of C (～70%) is found inside the main reaction product layer. Copyright © 2008 John Wiley & Sons, Ltd.     

Nitrogen/gold codoping of the TiO2(101) anatase surface. A theoretical study based on DFT calculations

The interaction between implanted nitrogen atoms, adsorbed gold atoms, and oxygen vacancies at the anatase TiO(2)(101) surface is investigated by means of periodic density functional theory calculations. Substitutional and interstitial configurations for the N-doping have been considered, as well as several adsorption sites for Au adatoms and different types of vacancies. Our total energy calculations suggest that a synergetic effect takes place between the nitrogen doping on one hand and the adsorption of gold and vacancy formation on the other hand. Thus, while pre-implanted nitrogen increases the adsorption energy for gold and decreases the energy required for the formation of an oxygen vacancy, pre-adsorbed gold or the presence of oxygen vacancies favors the nitrogen doping of anatase. The analysis of the electronic structure and electron densities shows that a charge transfer takes place between implanted-N, adsorbed Au and oxygen vacancies. Moreover, it is predicted that the creation of vacancies on the anatase surface modified with both implanted nitrogen and supported gold atoms produces migration of substitutional N impurities from bulk to surface sites. In any case, the most stable configurations are those where N, Au and vacancies are close to each other.     

Molecular dynamics investigation of oxygen vacancy diffusion in rutile

Oxygen vacancy diffusion in rutile was studied by Born-Oppenheimer molecular dynamics techniques in the framework of the semiempirical molecular orbital method MSINDO. Migration of an oxygen vacancy from the rutile (110) surface towards the bulk was simulated. The metadynamics technique was employed to accelerate the diffusion processes. In this way, transition state structures and activation energies for the diffusion processes were obtained. Rate constants and the time scale of diffusion processes were estimated for different temperatures using the calculated activation energy. It was found that the vacancies in the bulk are less stable than on the surface. The feasibility of oxygen vacancy diffusion under experimental conditions is discussed.     

Low-temperature diffusion of oxygen through ordered carbon vacancies in Zr2C(x): the formation of ordered Zr2C(x)O(y)

Investigations are performed on low-temperature oxygen diffusion in the carbon vacancy ordered ZrC(0.6)and thus induced formation of the oxygen atom ordered ZrC(0.6)O(0.4). Theoretically, a superstructure of Zr(2)CO can be constructed via the complete substitution of carbon vacancies with O atoms in the Zr(2)C model. In the ordered ZrC(0.6), the consecutive arrangement of vacancies forms the vacancy channels along some zone axes in the C sublattice. Through these vacancy channels, the thermally activated oxygen diffusion is significantly facilitated. The oxygen atoms diffuse directly into and occupy the vacancies, producing the ordered ZrC(0.6)O(0.4). Relative to the ordered ZrC(0.6), the Zr positions are finely tuned in the ordered ZrC(0.6)O(0.4) because of the ionic Zr-O bonds. Because of this fine adjustment of Zr positions and the presence of oxygen atoms, the superstructural reflections are always observable in a selected area electron diffraction (SAED) pattern, despite the invisibility of superstructural reflections in ZrC(0.6) along some special zone axes. Similar to the vacancies in ordered ZrC(0.6), the ordering arrangement of O atoms in the ordered ZrC(0.6)O(0.4) is in nanoscale length, thus forming the nano superstructural domains with irregular shapes.     

Oxygen vacancies on TiO2 (110) from first principles calculations

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.     

Structure and Thermodynamic Characteristics of Impurity Centers in Lithium-Doped Cadmium Oxide: an <Emphasis Type="Italic">Ab Initio</Emphasis> Paw-Study

The ab initio projector augmented wave (PAW) method is used to calculate the electronic structure of Li-doped cadmium oxide with NaCl structure. The preference energy for Li atoms in interstitial sites and the energy of impurity oxidation are calculated. Interstitial positions for Li atoms are shown to be stable under thermodynamic equilibrium, but Li atoms can substitute Cd atoms in presence of vacancies in the oxygen sublattice. We consider the following complexes: one Li atom in the interstitial site and the other Li atom in Cd position; one Li atom in Cd position and one oxygen vacancy; a pair of oxygen vacancies; and show that these complexes are formed to have the shortest possible distance between their components. The band gap substantially decreases when Li atoms occupy interstitial sites to explain considerable increase of experimental conductivity.     

Enhancement of X-ray fluorescence intensity from an ultra-thin sandwiched layer at grazing-emission angles

Ni ultra-thin films sandwiched with carbon thin films of different thickness are measured by a laboratory grazing-emission X-ray fluorescence instrument. The Ni Kα intensity of the Ni ultra-thin film sandwiched with carbon layers is three times enhanced in comparison with the Ni ultra-thin film without carbon layers. In addition, oscillations caused by interference effects of directly observed X-ray beams and the reflected X-ray beams on the surface of the Pt substrate, are clearly observed. The periods of the oscillations depends on the thickness of the carbon layer, that is, the position of the Ni layer. Therefore, the thickness of the carbon layer can be estimated.     

Radical electrophilicities in solvent

A series of Ti-doped SnO₂(110) surfaces with different oxygen vacancies have been investigated by means of first principles DFT calculations combined with a slab model. Three kinds of defective SnO₂(110) surfaces are considered, including the formations of bridging oxygen (O _b ) vacancy, in-plane oxygen (O _i ) vacancy, and the coexistence of O _b and O _i vacancies. Our results indicate that Ti dopant prefers the fivefold-coordinated Sn site on the top layer for the surface with O _b or O _i vacancy, while the replacement of sublayer Sn atom becomes the most energetically favorable structure if the O _b and O _i vacancies are presented simultaneously. Based on analyzing the band structure of the most stable configuration, the presence of Ti leads to the variation of the band gap state, which is different for three defective SnO₂(110) surfaces. For the surface with O _b or O _i vacancy, the component of the defect state is modified, and the reaction activity of the corresponding surface is enhanced. Hence, the sensing performance of SnO₂ may be improved after introducing Ti dopant. However, for the third kind of reduced surface with the coexistence of O _b and O _i vacancies, the sublayer doping has little influence on the defect state, and only in this case, the Ti doping state partly appears in the band gap of SnO₂(110) surface.     

Effect of surface orientation on the segregation kinetics of Sb from a Cu single crystal

In a previous theoretical study it has been suggested that the bulk vacancy formation energy near a surface depends on the orientation of the surface. It has been suggested also that this dependency of the vacancy formation energy would influence the bulk diffusion coefficient near the surface. The experimental results presented in this paper support this hypothesis. The experimental results were obtained by measuring the bulk‐to‐surface segregation of Sb for a Cu(111) single crystal with 0.088 at.% Sb and for a Cu(110) single crystal with 0.082 at.% Sb. The experimental results were fitted with the vacancy‐modified Darken model and it was clear that the bulk diffusion coefficient beneath the (110) surface is higher than the bulk diffusion coefficient beneath the (111) surface. Copyright © 2003 John Wiley & Sons, Ltd.     

Wetting of mixed OHH(2)O layers on Pt(111)

We describe the effect of growth temperature and OHH(2)O composition on the wetting behavior of Pt(111). Changes to the desorption rate of ice films were measured and correlated to the film morphology using low energy electron diffraction and thermal desorption of chloroform to measure the area of multilayer ice and monolayer OHH(2)O exposed. Thin ice films roughen, forming bare (radical39 x radical39)R16 degrees water monolayer and ice clusters. The size of the clusters depends on growth temperature and determines their kinetic stability, with the desorption rate decreasing when larger clusters are formed by growth at high temperature. Continuous films of more than approximately 50 layers thick stabilize an ordered incommensurate ice film that does not dewet. OH coadsorption pins the first layer into registry with Pt, forming an ordered hexagonal (OH+H(2)O) structure with all the H atoms involved in hydrogen bonding. Although this layer has a similar honeycomb OH(x) skeleton to ice Ih, it is unable to reconstruct to match the bulk ice lattice parameter and does not form a stable wetting layer. Water aggregates to expose bare monolayer (OH+H(2)O), forming bulk ice crystallites whose size depend on preparation temperature. Increasing the proportion of water in the first layer provides free OH groups which stabilize the multilayer. The factors influencing multilayer wetting are discussed using density functional theory calculations to compare water adsorption on top of (OH+H(2)O) and on simple models for commensurate water structures. We show that both the (OH+H(2)O) structure and "H-down" water layers are poor proton acceptors, bonding to the first layer being enhanced by the presence of free OH groups. Formation of an ordered ice multilayer requires a water-metal interaction sufficient to wet the surface, but not so strong as to prevent the first layer relaxing to stabilize the interface between the metal and bulk ice.