Adatom binding at the surface ledges of a jellium metal

The nature of a surface ledge on a jellium metal has been simulated by sharply cutting the uniform positive charge into the prescribed ledge and allowing the negative electronic charge to show a compensating modulation, falling off tangentially to the surface. The size of the associated fall-off parameter was determined by energy minimization and turned out to be quite independent of step height and comparable to that for the charge fall-off perpendicular to the surface. To this ledge a model adatom was brought, consisting of a Topp-Hopfield ion core and a single valence electron distribution. When approaching the ledge from the lower terrace, the adatom binding increased by about 40% more than its flat surface value. Conversely, the adatom approaching the ledge across the upper terrace experienced a decrease in binding (balustrade effect) also of about 40% of the flat surface binding. A particular application of the results to condensation from the vapor phase is the prediction that the spacing between a series of ledges will tend to become more uniform as the condensation proceeds.     

Adatom mobility for the solid-on-solid model

The relaxation of a crystal surface through surface diffusion is studied within the solid-on-solid model. Two types of (conserved) dynamics are considered. ForArrhenius dynamics we show that the relevant transport coefficient, the adatom mobility, has a simple analytic form: It is independent of orientation, and depends exponentially on the inverse temperature, for any surface dimensionalityd. Together with the expression for the orientation-dependent stiffness this completely determines the macroscopic evolution equation for the surface. The predictions of the macroscopic theory are checked against simulations of profile evolution and roughening ind=1. For one-dimensionalMetropolis dynamics we provide an upper bound on the adatom mobility and obtain numerical estimates of its actual value, which indicate a nontrivial orientation dependence in this case. An alternative derivation of the macroscopic dynamics directly from the master equation is presented and discussed in relation to previous approximate work.Dedicated to Professor H. Wagner on the occasion of his 60th birthday     

The surface susceptibility of a simple metal

The magnetic surface response of a simple metal to a uniform magnetic field is calculated for a jellium model of a metal surface. We find a positive surface contribution to the measured susceptibility of finite samples.     

Binding of an adatom to a simple metal surface

The density functional formalism of Hohenberg and Kohn is used to investigate the energies, charge densities and forces which hold an adatom on the surface of a simple metal. The valence wavefunction of the adatom is fitted to the Herman-Skillman solutions at large distance and is simplified somewhat in the core region. The field of the ion is represented by the Ashcroft pseudopotential. For the metal the jellium model is used. Detailed calculations are carried out for a sodium adatom on a sodium surface. Simply juxtaposing adatom and surface gives a binding energy of about $\text{1}\text{3}\text{eV}$. This value is approximately twice the surface energy per atom in the close-packed plane. Charge redistributions as determined variationally increase the binding energy by about 10%. The redistribution is primarily a dipole induced on the adatom at close distances, but at somewhat larger distances a prolate quadrupole also appears on the atom. A small amount of charge is also drawn from the metal toward the atom. The equilibrium distance for the adatom turns out to be 1.66 Å from the surface, as compared with 1.52 Å, the observed value for one-half the distance between the close-packed planes. Contour plots of the piling-up of electronic charge between the adatom and the metal are presented.     

Adatom kinetics on and below the surface: the existence of a new diffusion channel

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.     

Adatom self-organization induced by quantum confinement of surface electrons

We present a novel mechanism of nanostructure growth based on quantum confinement of surface-state electrons. Ab initio calculations and the kinetic Monte Carlo simulations reveal the phenomenon of confinement-induced adatom self-organization in quantum corrals. Our studies indicate that new atomic-scale nanostructures can be engineered exploiting the quantum confinement of surface electrons.     

Adatom transport on strained Cu(001): surface crowdions

Surface strain is often suggested as a means to control the self-assembled growth of nanostructures. Strain affects both the kinetics of nucleation and the free energies of formation of the desired nanostructure. It is demonstrated here that diffusion on some strained surfaces may be mediated by newly identified adatom transport mechanism: the formation and motion of a surface crowdion.     

Adatom ascending at step edges and faceting on fcc metal (110) surfaces

Using first-principles total-energy calculations, we show that an adatom can easily climb up at monatomic-layer-high steps on several representative fcc metal (110) surfaces via a place exchange mechanism. Inclusion of such novel adatom ascending processes in kinetic Monte Carlo simulations of Al(110) homoepitaxy as a prototypical model system can lead to the existence of an intriguing faceting instability, whose dynamical evolution and kinetic nature are explored in comparison with experimental observations.     

Drift mobility correlation in a simple cubic lattice

A Monte Carlo method is described for the calculation of the drift mobility of interacting ions on a simple cubic lattice in an electric field. This formalism provides a useful operational model for electromigration in interstitial solid solutions. The results, which show a correlation in the drift of the ions, can be interpreted either in terms of a deviation from the Nernst-Einstein relation in the sense of vacancy wind effects or as an intrinsic correlation effect in the diffusion coefficient of the charge carriers. It is also shown that the usual tracer correlation factor can be calculated upon subtracting the square of the average drifts from the average squared displacements.     

Low-energy electron diffraction on the planes non parallel with the surface

Some problems of the interpretation of the geometry of low-energy electron diffraction patterns obtained on metal samples with a general crystallographic orientation near that of some densely occupied atomic plane are discussed. It appears that for the interpretation of these diffraction patterns the commonly used classical model of the diffraction on the surface plane cannot be used. The new geometrical model — model of the diffraction on densely occupied planes non parallel with the surface — in constructed. Using this model the calculation of the diffraciton patterns is made for a surface with an orientation close to the densely occupied (100) plane. The calculated diffraction patterns are compared with those obtained on monocrystalline niobium with known surface orientation near that of the (100) plane. Experimental results confirm the mentioned model to be rightful. This diffraction model can be used for determination of the polycrystalline grain orientation by means of the selected area low-energy electron diffraction. The experiments were made in the emission electron microscope adapted for low-energy electron diffraction.     

Comments on chemisorption at a metal surface

The effect of the gradient correction to the exchange and correlation energies on chemisorption properties is examined using the evaluation of B_χc arrived at by Rasolt and Geldart. Quantities common to the study of hydrogen and alkali chemisorption on a high density metal are calculated for various forms of the energy functional within the linear response approach. It is found that the gradient correction is a quantitatively significant term, lowering binding energies by ≈ 10% and increasing the dipole sizably.     

Influence of surface roughness on adsorbate mobility

Field-emission fluctuations of the coadsorption system W(112)Ni-K were investigated by the cross-correlation technique in two perpendicular crystallographic directions at various temperatures. In- and cross-channel movements show a considerable reduction of _max which is the time delay corresponding to the cross-correlation function maximum with increasing Ni coverage at a constant potassium probe concentration. _max, its activation energy and the signal velocity are discussed in terms of the well-known W(112)Ni surface structure.     

On the structure of a water molecule chemisorbed on a metal surface

A model using the electrostatic image force and the harmonic restoring force has been adapted to find the equilibrium structure of a chemisorbed water molecule on a metal surface. An estimate of the maximum possible increase in the HOH interbond angle upon chemisorption due to image forces is given.