Lattice steps and adatom binding on W(211)

The role of a lattice step in affecting the binding of adatoms in its vicinity has been explored for tungsten as well as rhenium adatoms on the (211) plane of tungsten. On this plane adatoms are confined to sites in a one-dimensional channel along [1̄11]. From the equilibrium distribution of an adatom over all sites in a channel it is possible to derive the relative free energy of binding at the different sites. Such measurements have been made using a field ion microscope supported by an automated data handling system. Contrary to expectations based on potential calculations using standard pair interactions, atom binding is found to be stronger close to the plane edges rather than in the center, where the number of distant neighbors around the adatom is larger. These edge effects have a surprisingly long range, extending at least over three sites, and are strongly dependent upon the chemical identity of the adatom.     

A study of local density of states of atom clusters on the W(110) surface

The kinetic energy distributions of field desorbed He ions from tungsten clusters of one to five atoms on a W(110) surface are measured using a high resolution pulsed-laser time-of-flight atom probe. The He field ion energy distribution from the single W adatom shows an extra peak-like feature centered at 2.7 eV above the Fermi level. It has a full width at half maximum (fwhm) of 2.3 eV. The data from two tungsten adatoms separated by two lattice constants have nearly the same feature with the extra peak located at 2.5 eV above the Fermi level. These peaks arise from resonance tunneling with the adatom local density of states (LDOS). The He ion energy distribution of a tungsten dimer has an extra peak centered at 1.5 eV above the Fermi level. The fwhm is about 4 eV. The spectra from four and five tungsten adatom clusters show only one peak each, similar to that from a flat plane.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

Chemisorption of a monolayer of atoms on a bcc metal surface

The change in the density of states due to the adsorption of a monolayer of atoms on the (001) surface of a bcc metal is presented. The substrate is described by the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight-Binding (TB) approximation, and both the Green's function formalism and the phase shift technique are employed. Each adatom is represented by a single nondegenerate energy level. Two binding sites for the commensurate monolayer are considered: the on-site and the centered fourfold-site. By assuming that screening of the charges on the adatoms is complete within the surface layer of atoms, the selfconsistency condition of satisfying Friedel's sum rule can be met by varying the orbital energies of the adatoms and the surface plane of atoms of the substrate. The changes in the density of states show strongly skewed bonding and antibonding resonances which occur at different energies for the two binding sites even though equal binding strengths are assumed. A comparison with previous single adatom results shows that the shape and position of the bonding resonance are dependent upon adatom coverage.     

Diffusion of single adatoms of platinum,iridium and gold on platinum surfaces

Diffusion of single adatoms of platinum iridium and gold on platinum surfaces has been studied experimentally using field ion microscopy and theoretically using Morse potentials to describe the adatom bonding. The order of increasing activation energies for the surfaces studied was (111), (113), (011), (133), (001) and calculated activation energies agreed satisfactorily with the experimental values. On (113) and (011) platinum the activation energies for the adsorbates increased in the order Au, Pt, Ir. The calculations show that this variation arises largely from differences in the adsorbate binding energy, with the differences in adatom size having little effect. Diffusion of Au on (011) platinum occurred only along the direction of the surface channels, as is to be expected from the surface structure. In contrast, Pt and Ir adatoms were found to diffuse in two dimensions on (011). Since direct inter-channel jumps by adatoms should require large activation energies, it is proposed that the observed inter-channel adatom transfers proceed by a surface vacancy mechanism.     

Surface diffusion of tungsten adatoms induced by low-energy bombardment with He atoms

Using low-temperature field ion microscope techniques, we studied at the atomic level the elementary events of radiation-induced surface diffusion of tungsten adatoms on the ideally pristine surface. The experiments on surface-damage formation and adatom displacements have been performed in situ with a source of neutral helium atoms with an energy of 5?keV. It was demonstrated that the low-energy He atomic bombardment at grazing incidence was able to induce a substantial surface mobility of adatoms. Most of the radiation-induced adatom displacements were oriented along the direction of irradiation. The long impact-induced jumps of adatoms, spanning more than a nearest-neighbor distance, were revealed. Performed molecular dynamics simulations reproduce the general experimental trends and conclusions.     

Tungsten cluster migration on nanoparticles: minimum energy pathway and migration mechanism

Saddle point searches have been employed to investigate the migration mechanisms of W clusters on W nanoparticles, and to determine the corresponding migration energies for the possible migration paths of these clusters. The tungsten clusters containing up to four adatoms are found to prefer 2D-compact structures with relatively low binding energies. The effect of interface and vertex regions on the migration behavior of the clusters is significantly strong, as compared to that of nanoparticle size. The migration mechanisms are quite different when the clusters are located at the center of the nanoparticle and near the interface or vertex areas. Near the interfaces and vertex areas, the substrate atoms tend to participate in the migration processes of the clusters, and can join the adatoms to form a larger cluster or lead to the dissociation of a cluster via the exchange mechanism, which results in the adatom crossing the facets. The lowest energy paths are used to be determined the energy barriers for W cluster migrations (from 1- to 4-atoms) on the facets, edges and vertex regions. The calculated energy barriers for the trimers suggest that the concerted migration is more probable than the successive jumping of a single adatom in the clusters. In addition, it of interest to note that the dimer shearing is a dominant migration mechanism for the tetramer, but needs to overcome a relatively higher migration energy than other clusters.     

Photoemission studies of Ar,Kr and Xe adsorbed on Al(111): Dipole moments,polarizabilities and spatial distributions

Submonolayers of rare gases adsorbed on Al(111) have been studied using photoemission techniques. Coverage-dependent core level binding energy shifts and work function changes have been measured; polarizabilities and dipole moments are deduced. Results indicate that adatom spatial distributions for Xe submonolayers adsorbed on Al(111) at 40 K are best described by a random 2-dimensional distribution, and there is negligible charge transfer from the Al substrate onto the Xe adatoms in the photoionization process.     

Hydrogen chemisorption on metal surfaces

The adsorption of H atoms on metal (jellium) surfaces has been investigated using linear response theory within the density functional formalism. The adsorbate is represented initially by a 1S orbital on the H atom, which perturbs the jellium surface and indirectly the adsorbate itself. The interaction energy curves, atomic binding energies, induced dipole moments, chemical shifts associated with the adsorbate, and vibrational excitation energies at the equilibrium internuclear separation have been calculated for a single H atom chemisorbed on metal surfaces. The sum of the atomic binding energy and the ionization potential of the H 1S level may be regarded as the initial state energy in the case of photoemission from the chemisorbed H. The rather satisfactory overall agreement between the theory and the experimental results for binding energies, vibrational excitation energies, and dipole moments suggests that this simple formalism could also have useful applications in more complicated chemisorbed systems.     

On the description of the coadsorption of cesium and selenium atoms on the silicon surface

Adsorption of cesium and selenium atoms on silicon surface has been considered in terms of the Anderson-Newns-Muskat model with allowance for the dipole-dipole repulsion. The results obtained have been applied to the problem of coadsorption of Cs and Se atoms. It is shown that even a small addition of cesium adatoms to a submonolayer selenium adatom film on silicon strongly increases the charge of selenium adatoms. This effect is initiated by the mutual enhancement of the dipole fields of the Cs and Se adatom sublattices. All the results obtained agree well with experimental observations.     

Direct observation of adatom-substitutional atom interaction on dilute metal alloy surfaces

The field ion microscope is used to study interactions of a migrating tungsten adatom with substitutional rhenium atoms in the (110) plane of a W-3%Re alloy. By observing about 300 migration periods on a single (110) surface plane of a small field ion emittier, each position of the adatom with respect to deduced Re atom locations can be identified. The interaction is found to be attractive with a strength of 90 ± 7 meV at the closest equilibrium separation, and repulsive with a strength of at least 80 meV at the second closest separation. No interaction could be observed for larger separations indicating a strength of less than 10 meV. Results of a control experiment, diffusion of single W adatoms on the (110) plane of pure W, are also presented for comparison.     

Atom binding at overlayers: Re on W(211)

Observations with the field-ion microscope have been made of the formation and structure of rhenium overlayers on W(211), as well as of the behavior of Re adatoms bound on top of the layer and at the layer edge. A well-ordered pseudomorphic rhenium film is formed by deposition and annealing at T > 500 K, and its structure has been established using a migrating Re adatom as a probe. Relative binding energies at a close-packed edge of the layer have been deduced from the frequency with which different sites are occupied by a rhenium adatom. The relative strength of binding varies markedly with the number of sites available at the edge. For a layer with 5 sites, binding is found to deviate dramatically from the predictions of the usual pairwise bonding schemes, in that interactions at the central site, where the coordination is highest, are weaker than at adjacent sites.     

Direct identification of atomic binding sites on a crystal

The atomic resolution of the field ion microscope, in conjunction with its ability to remove and identify individual atomic layers, allowed us to map unambiguously the unit cell of the (111) plane of tungsten and to determine directly the location of single tungsten atoms adsorbed on this plane. Adatoms have been observed to occupy two binding sites only. The predominant site corresponds to a normal lattice position. The second site is of similar symmetry, in that the adatom sits between three first layer atoms; however, at this position the adatom is located above an atom in the second rather than the third lattice layer. The former site is favored energetically, but only by ≈ 0.5 eV. All observations have been made at high fields, however, it is shown from studies of migration and other effects that the binding sites identified in the field ion microscope are typical of a normal, field free environment.     

Measurement of the dipole moments of single 5-d transition metal adatoms on the tungsten (110) plane

The surface-induced dipole moments of single 5-d transition metal atoms adsorbed on the W(110) plane have been experimentally determined from field emission work function changes. The resulting values are found to be small (less than 0.5 Debye) and exhibit a monotonically decreasing behavior across the series from Ta to Pt. This same type of d-electron dependence is also found for the relative sizes of the field emission current jumps associated with adsorption of the various atoms on the W(110) plane. The data are interpreted using the concept of charge transfer, and a qualitative discussion of the adatom polarizabilities and desorption fields in relation to this model is given.     

碱金属原子与W,Pt表面的电荷交换

本工作利用热能碱金属原子与炽热W,Pt表面的相互作用,研究了碱金属原子与金属表面的电荷交换,以及金属表面吸附原子后表面功函数的变化。 关键词：     

Diffusion mechanisms for iron on tungsten

Diffusion of iron atoms on clean W(1 0 0) and W(1 1 0) as well as on Fe/W(1 0 0) and Fe/W(1 1 0) surfaces was investigated by means of exhaustive first-principle calculations. Comparison of the activation energy barriers obtained for hopping and exchange migration processes shows that the surface diffusion proceeds via jumps to the nearest sites. The activation energies are higher for Fe adatom on clean tungsten than those for Fe adatoms moving on iron-covered tungsten. The magnetism of the underlying Fe/W(1 0 0) films has a pronounced influence on the diffusion, as evidenced not only by a reduced activation energy barrier but also by a change of the stable adsorption place. Fe atoms reaching step edges are trapped there and eventually diffuse along the steps more slowly than the adatoms on the terraces. The rate of diffusion increases upon depositing a row of Fe atoms along steps.     

表面Cu原子间相互作用对Cu(001)表面跳跃扩散行为的影响

采用嵌入原子方法的原子间相互作用势，利用准静态分子动力学模拟研究了Cu原子在Cu(001)表面吸附所导致的基体晶格畸变以及对其附近的另一个吸附原子自扩散行为的影响.研究结果表明，吸附原子的存在可以导致多达10层的Cu基体晶格产生畸变.两个吸附原子所产生的晶格畸变应力场之间的相互作用，可以导致吸附原子运动活性的增加.通过比较同一路径上往返跳跃扩散势垒的差异发现，在原子间相互作用势的有效距离之外，两个吸附原子的扩散行为可以认为是存在晶格畸变应力场相互作用的两个独立吸附原子的扩散；在原子间相互作用势的有效距离之 关键词： 表面吸附原子 晶格畸变 表面二聚体 扩散     

Adatom mobility on the surface planes of a simple metal

An approach using the Density Functional Approximation, which had in an earlier paper been applied to the binding of an adatom to a jellium metal (r_s = 4a₀), has been extended to a partially structured metal surface by introducing a layer of surface atoms to replace an equivalent layer of jellium. The model has been used to estimate the adatom motion energies over several quite close packed planes for a simple surface. An important preliminary step was to determine the general electron density contours for each surface by a simple variational method. The ensuing shielding charge distributions have an important bearing on the adatom motion energy. Adatom energies were calculated at three positions: (1) A above a surface atom, (2) B above a bridge position between two surface atoms and (3) C above a central position between three or four neighboring atoms. The motion energy was taken to be E_B ? E_C. As might have been expected this quantity was larger for the less closely packed planes, although it was always quite small due to the nature of the metal — large r_S, small ion core and typical s-p binding. To a rather surprising degree the strength of the shielding charge, the energies and the positions of the adatoms proved to be quite smooth functions of a parameter chosen to measure the close packing of the surface, namely the square of the interplanar distance divided by the surface area per atom.     

Non—conservation of energy arising from atomic dipole interactions and its effects on light field and coupled atoms

The interactions between coupled atoms and a single mode of a quantized electromagnetic field, which involve the terms originating from the dipole interactions, are discussed. In the usual Jaynes－Cummings model for coupled atoms, the terms of non-conservation of energy originating from dipole interactions are neglected, however, we take them into consideration in this paper. The effects of these terms on the evolutions of quantum statistic properties and squeezing of the field, the squeezing of atomic dipole moments and atomic population inversion are investigated. It has been shown that the coupling between atoms modulates these evolutions of fields and atoms. The terms of non-conservation of energy affect these evolutions of fields and atoms slightly. They also have effects on the squeezing of the field, the squeezing of atomic dipole and atomic population inversions. The initial states of atoms also affect these properties.