CNDO calculations of nickel atom clusters

CNDO molecular orbital calculations for nickel atom clusters containing from one to thirteen atoms in various geometric arrangements are presented. The parameters were selected so that an octahedral Ni₆ cluster had an equilibrium inter-nuclear distance, d band occupancy, binding energy, Fermi level, and d band width approximating those of bulk nickel. For clusters with a given number of atoms the stability always increased in the order linear < planar < three-dimensional cluster. The general assumption that the binding energy per atom in metal clusters is proportional to the number of nearest neighbours is supported by these CNDO calculations although this relation is certainly not exact for small clusters. Examination of the calculated orbitals does not indicate a separation of the d band into one part made up from atomic t_2g orbitals and another from e_g orbitals. Overall the CNDO method appears to present a reasonable approach to calculating properties of small metal clusters.     

Surface molecules and chemisorption: I. Adatom density of states

A useful picture of chemisorption on metal surfaces is one in which a localized molecule is formed between the adatom and its nearest neighbor substrate atoms. The interaction responsible for the molecule formation is treated as the coupling between the adsorbate state and a group orbital formed from a linear combination of atomic orbitals on the substrate atoms. Within the surface molecule picture, level width and level shift functions, given by Newns modification of the Anderson theory, have been calculated and the resulting adatom density of states function has been obtained. This has been done for model systems in which the substrate is either a free electron metal or a tightbinding p-band metal and the adsorbate is s or p like. The results show how it is possible to simultaneously have narrow virtual levels due to chemisorption (～ 1 eV) which previously implied weak interactions and also high binding energies (? 3 eV) as are observed experimentally.     

The theoretical and experimental study of the ionization processes during the low energy ion sputtering

The process by which atoms are ionized as they are sputtered from a metal surface has been analyzed both theoretically and experimentally. In the theoretical part the expressions for ionization coefficient R⁺ of atoms having the ionization energy much larger than the metal work function have been derived using a molecular orbital method. The effect of the level crossing was estimated in an approximate way. In the experimental part the SIMS experiments on clean Ni and Al surfaces and on Ni surface covered with a submonolayer of adsorbed K, Na and Al are reported. It has been found and it is for the first time reported that the energy distribution of ions sputtered from a submonolayer of adatoms is independent of energy (200–2500 eV) and mass (Ar⁺ Xe⁺ of incident ions and depends only upon the adsorption energy of the adatom. The energy distribution of ions sputtered from bulk samples has been found dependent on the primary ion energy. The measurement of the absolute value of R⁺ has shown that there is a strong correlation between the number of the adatom valence d-electrons and the value of R⁺, the value of R⁺ being smaller for atoms with more d-electrons. These experimental data have been compared with the theoretical expressions and the important role of the mechanism which takes into account the bending of the adatom energy level has been assessed.     

Density functional theory study of the adsorption of oxygen atoms on gold (111), (100) and (211) surfaces

The interaction of an oxygen atom with various gold surfaces was examined computationally using density functional theory. The relative binding energies for a range of possible adatom locations on each surface were obtained. The results demonstrated the relative importance of low-coordinated gold atoms to bind oxygen for a particular surface and a preference for three-fold adatom coordination over the two-fold and single-coordination sites. Pseudo-potential energy curves were obtained by following the calculated energy as a function of surface position. These general results provide a reference for the interaction of oxygen atoms with gold nanoparticles that may project faces similar to the surfaces studied here.     

Structural,electronic,and magnetic properties of vanadium atom-adsorbed MoSe_2 monolayer

Using the first-principles calculations, we study the structural, electronic, and magnetic properties of vanadium adsorbed MoSe_2 monolayer, and the magnetic couplings between the V adatoms at different adsorption concentrations. The calculations show that the V atom is chemically adsorbed on the MoSe_2 monolayer and prefers the location on the top of an Mo atom surrounded by three nearest-neighbor Se atoms. The interatomic electron transfer from the V to the nearestneighbor Se results in the polarized covalent bond with weak covalency, associated with the hybridizations of V with Se and Mo. The V adatom induces local impurity states in the middle of the band gap of pristine MoSe_2, and the peak of density of states right below the Fermi energy is associated with the V- dz~2 orbital. A single V adatom induces a magnetic moment of 5 μBthat mainly distributes on the V-3d and Mo-4d orbitals. The V adatom is in high-spin state, and its local magnetic moment is associated with the mid-gap impurity states that are mainly from the V-3d orbitals. In addition,the crystal field squashes a part of the V-4s electrons into the V-3d orbitals, which enhances the local magnetic moment.The magnetic ground states at different adsorption concentrations are calculated by generalized gradient approximations(GGA) and GGA+U with enhanced electron localization. In addition, the exchange integrals between the nearest-neighbor V adatoms at different adsorption concentrations are calculated by fitting the first-principle total energies of ferromagnetic(FM) and antiferromagnetic(AFM) states to the Heisenberg model. The calculations with GGA show that there is a transition from ferromagnetic to antiferromagnetic ground state with increasing the distance between the V adatoms. We propose an exchange mechanism based on the on-site exchange on Mo and the hybridization between Mo and V, to explain the strong ferromagnetic coupling at a short distance between the V adatoms. However, the ferromagnetic exchange mechanism is sensitive to both the increased inter-adatom distance at low concentration and the enhanced electron localization by GGA+U, which leads to antiferromagnetic ground state, where the antiferromagnetic superexchange is dominant.     

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.     

Superradiant decay of localized excitations in a linear atomic chain

The superradiant decay of localized excitations in a linear chain of two-level atoms has been investigated. It is shown that the excitation energy is not completely emitted in the cooperative stage of the process. The residual excitation forms waves propagating from the regions of initial localization to the chain edges.     

Influence of acceptor and donor adsorbates (CO,K, NH3) on Pt surface core-level shifts

4f core-level shifts have been measured for clean surfaces of Pt(111), Pt(331), and Pt(557). Surface peaks due to terrace sites are shifted toward lower binding energy (0.32 ± 0.05 eV) from the bulk peak, whereas peaks from step atoms are shifted by 0.58 ± 0.05 eV also to lower binding energy. The intensity ratios for the two sites differ considerably between the stepped Pt surfaces. Chemisorption of carbon monoxide on the Pt(331) surface is preferential to step sites, with a Pt 4f binding energy shift of ～ 1.29 eV toward higher binding energy. Chemisorption of potassium and ammonia also produces Pt 4f surface shifts which are at higher binding energy than the bulk peak. These experiments do not support the concept of electron donation by these adsorbates into metal d orbitals. The results are discussed in view of, and supported by, tight-binding LCAOMO calculations of potassium and ammonia interacting with a Pt(111) thin film.     

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.     

Mass-transport driven by surface instabilities in metals under reactive plasma/ion beam treatment at moderate temperature

This paper presents a generalized approach to the mechanisms of oxidation, hydrogenation and nitriding of metals under ion irradiation with reactive particles at elevated temperatures. Experimental results on the plasma oxidation of bilayered Y/Zr films, the plasma hydrogenation of Mg films and the ion beam (1.2 keV N ₂ ⁺ ) nitriding of stainless steel are presented and discussed. We make special emphasis on the analysis of surface effects and their role in the initiation of mixing of bilayered films, the ingress of reactive species in the bulk and the restructuring of the surface layers. It is suggested that primary processes driving reactive atoms from the surface into the bulk are surface instabilities induced by thermal and ballistic surface atom relocations under reactive adsorption and ion irradiation, respectively. The diffusion of adatoms and vacancies, at temperature when they become mobile, provide the means to relax the surface energy. It is recognized that the stabilizing effect of surface adatom diffusion is significant at temperatures below 300–350°C. As the temperature increases, the role of surface adatom diffusion decreases and processes in the bulk become dominant. The atoms of subsurface monolayers occupy energetically favorable sites on the surface, and result in reduced surface energy.     

An icosahedral quasicrystalline model for amorphous semiconductors

In disordered structures localized electronic states are expected to be found having a strong impact on the electronic properties. They are also expected to exist in an icosahedral quasicrystalline semiconductor which is studied therefore with respect to its electronic wave functions. The quasicrystalline model was introduced in part one of this series of articles, its electronic DOS in part two. Based upon the same LCAO tight binding scheme with five orbitals sp³s* per atom individual electronic wave functions of the quasicrystalline semiconductor are calculated using the Lanczos method on a cubic supercell of 20 000 atoms. The wave functions are analyzed regarding projections on the atomic orbitals, influence of the dangling bonds, spatial distribution, and — most interesting — localization measures like the participation ratio and the localization length which allow the definition of mobility edges.     

Classical theory of desorption rate velocity distribution of desorbed atoms; possibility of a compensation effect

A three-dimensional theory of atomic desorption is formulated using the thermal equilibrium hypothesis. The velocity distribution of desorbed atoms obtained is Maxwellian at the temperature of the crystal, whatever the interaction potential. As expected the desorption rate is equal to the product of a Boltzmann factor, exp(-U/kT), by a so-called preexponential factor. The case of an adatom linked by harmonic forces to four surface atoms of a unit cell is considered in particular. The result obtained, as well as those given by three-dimensional linear theory, show a compensation effect which seems to be too low if one compares calculated and experimental data. However, the calculated preexponential factor is mainly dependent on: (a) the adatom vibrational frequency; (b) the amount of potential energy which can be concentrated in the bond, characterized by its mean thermal value. It increases with respect to this quantity. This result suggest that high preexponential factor values are due to a long range collective interaction between the adatom and surface atoms, corresponding to the formation of a large “quasi molecule”.     

Surface-state localization at adatoms

Low-temperature scanning tunneling spectroscopy of magnetic and nonmagnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A line shape analysis of the bound state indicates that Ag and Cu adatoms on Ag(111) and Cu(111), respectively, decrease the surface-state lifetime, while a cobalt adatom causes no significant change.     

Strong influence of defects on the electronic structure of Pt adatoms and clusters on graphite

We study the specific impact of defects such as step edges at the graphite surface on the electronic configuration of adsorbed Pt atoms and Pt₈ clusters. Surface diffusion is strongly reduced by depositing Pt and Pt₈ into a thin rare gas layer. In this configuration a very narrow adatom Pt 4f spectrum is found at an exceptionally small binding energy, similar to Pt surfaces. Both, adatom and cluster spectra are strongly shifted towards higher binding energy when allowed to diffuse towards defects like step edges. The strong shifts are indicative of a chemical reaction at the step edges and are conjectured to be part of the particle size dependent binding energy shifts typically observed for transition metal clusters grown on the surface of graphite.     

Hierarchy of percolation thresholds and the mechanism for reduction of magnetic moments of transition metals intercalated into TiSe<Subscript>2</Subscript>

The concentration dependences of the effective magnetic moment of transition metal atoms intercalated into TiSe₂ are analyzed in the framework of the percolation theory. It is shown that, depending on the degree of localization of impurity states, the effective magnetic moment is determined by the overlap of 3d orbitals of transition metals or orbitals of titanium atoms coordinated by impurity atoms.     

Theory of surface chemical reactivity

Fundamental reactivity concepts of relevance to the reactivity of transition metal surfaces are reviewed using elementary quantum-chemical concepts. The Newns-Anderson tight binding model of chemisorption is presented and subsequently used to outline the electronic structure characteristics of weak versus strong chemisorption. Fundamental concepts such as electron localization and surface complex embedding energies are defined and used to help explain surface reactivity. The emphasis here is on establishing an understanding of the surface chemical bond as a function of adatom coordination number, degree of coordinative unsaturation of the surface atoms and electron occupation of the d-type valence electron band. We derive from formal chemisorption theory the important relationships that exist between measured chemisorption properties and the average position of the d-valence electron band. The Newns-Anderson model is also used to show the relationship that exists between the d-band center and the coordinative unsaturation of the metal surface atoms. The general conclusion is that for Group VIII metals the shift of the average energy of the surface local density of states correlates with the strength of the interaction of the surface atoms with the metal atoms next to the surface layer. The same model is then used to analyze the Shustorovich bond order conservation model. The BOC or its modern version UBI-QEP is found to be consistent with a surface interaction potential comprised of a two-body repulsive term along with a constant attractive interaction independent of the number of coordinating atoms. The concepts of weak and strong chemisorption provide a very good basis for the subsequent analysis of the Brønsted-Eyring-Polanyi (BEP) relation. The extreme values of the BEP proportionality constant are related to the concept of loose and tight transition states. This proportionality constant between transition state energy and reaction energy can be expressed in parameters from the Newns-Anderson model by identifying loose transition states with intermediates in which the bond to be activated has not yet been broken, whereas in tight transition states this bond can be considered to be broken. We conclude the paper with an analysis of surface reconstruction. The power of the surface-molecule complex view of chemisorption will be quite apparent. The paper has an extensive introductory section to relate the topics of the four sections that follow with important classical catalytic notions.     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

Characteristics of Li diffusion on silicene and zigzag nanoribbon

We perform a density functional study on the adsorption and diffusion of Li atoms on silicene sheet and zigzag nanoribbons. Our results show that the diffusion energy barrier of Li adatoms on silicene sheet is 0.25 eV, which is much lower than on graphene and Si bulk. The diffusion barriers along the axis of zigzag silicene nanoribbon range from 0.1 to0.25 eV due to an edge effect, while the diffusion energy barrier is about 0.5 eV for a Li adatom to enter into a silicene nanoribbon. Our calculations indicate that using silicene nanoribbons as anodes is favorable for a Li-ion battery.     

Adsorption spectroscopy by polariton-induced desorption (ASPID)

A new spectroscopic technique is proposed as a means of determining the energy and momentum of adsorbed atoms and molecules. The method, abbreviated to ASPID (adsorption spectroscopy by polariton-induced desorption), uses surface phonon polaritons (SPPs) to desorb atoms. Because one may create nearly-monoenergetic SPPs, the binding energy range is determined directly from the spectrum of final energies. A kinematical analysis may be used to analyze adatom momenta, and hence band structures. The angular distribution of the SPP-desorbed atoms may be used to monitor adsorbed-film temperature. We provide an estimate of the SPP desorption rate, and conclude that the proposed experiment is feasible.     

Bound states of an electron and a macroscopic cluster at a liquid helium surface

In a strong electric field, there are bound states of an electron at the surface of liquid helium, interacting with a large cluster of atoms in the bulk of liquid. This phenomenon is related to long-range interaction between the electron and the dipole moment of the cluster. The electron, holding the cluster under the liquid surface, is localized at this surface. One electron is capable of binding a cluster of up to 10⁶ atoms. The value of the binding energy may reach up to several kelvins.