Adatom-substrate Interaction Research Articles

Samarium chemisorption on group-IV semiconductors

Samarium is an excellent test system with which to study the interaction between chemisorbed metallic overlayers and different kinds of substrates because the $4f$ configuration is sensitive to the Sm valence state. We used this unique feature to study the chemisorption of Sm on cleaved Ge(111) and Si(111) surfaces. In both cases, synchrotron radiation photoemission spectra exhibited only divalent Sm features at coverages below 2-3 \AA{} and a mixture of trivalent and divalent Sm features at higher coverages. This transition was accompanied by strong adatom-substrate chemical interactions as revealed by large core-level shifts.

Critical temperature of two-dimensional condensation of atoms adsorbed in registry with solid surfaces: I. Registered cell model in the mean field approximation

A statistical mechanical theory based on a cell model is presented for the critical temperature T c of two-dimensional condensation of adatoms on solid surfaces. The cell for an adatom is to be inherently defined by the corrugation of the adatom-substrate potential and the array of the cells is to be in registry with the substrate structure. Adatom-adatom and adatom-substrate interaction potentials are assumed to be of Lennard-Jones type, and interactions between adatoms are treated in the mean field approximation. Calculated T c is examined as a function of potential parameters including the lattice incompatibility between the substrate and the adlayer. Especially, the dependence of T c on the lattice incompatibility shows a good qualitative agreement with experimental observations. These results are discussed in terms of thermal motion of adatoms in the cells.

Quantum model for kinetics of helium atoms near surfaces of free-electron metals: II. Overlap-induced dissipation

A quantum model developed formerly to describe inelastic collisions of low-energy helium atoms with surfaces of free-electron metals is extended to include the electronically induced dissipative scattering arising from the overlap-induced electron-helium atom pseudopotential. It is shown that the repulsive adatom-substrate Hartree-Fock interaction, known from the work of Zaremba and Kohn to be proportional to the overlap S between the adatom and substrate electronic densities, is succeeded by a series of nonadiabatic higher-order terms which also give rise to dissipative processes in adatom-surf ace collisions. The leading contribution to this series which contains the information on the decay rate of the adparticle initial state is calculated up to the order S 2 using the formalism of the surface response function developed earlier. Such a formulation does not require the use of any adjustable parameters and the results obtained indicate the importance of this scattering mechanism with respect to the polarization-induced dissipation discussed in Part I [Surface Sci. 117 (1982) 116].

Adatom coalescence and cluster motion on amorphous surfaces via a molecular dynamics computer simulation

A molecular dynamics computer simulation technique of model systems was used to (a) determine the extent of coalescence of a dispersion of adatoms on the amorphous surface as a function of the adatom-substrate interaction energy, and (b) evaluate the effect of the adatom-substrate interaction energy on the mobility of a 10 atom cluster on amorphous and crystalline surfaces. Results show that at low temperatures the diffusion of adatoms on the amorphous surface is limited by the high density of sites from which adatoms require a significant amount of thermal energy in order to escape. At higher temperatures, the amorphous surface can relax and accommodate adatoms and clusters more readily than a low index crystalline surface, again inhibiting diffusion. Such results help explain the experimentally observed high sticking coefficients and higher densities of small clusters experimentally observed in depositions on amorphous surfaces as compared to crystalline surfaces.

Adatom coalescence and cluster motion on amorphous surfaces via a molecular dynamics computer simulation

Abstract A molecular dynamics computer simulation technique of model systems was used to (a) determine the extent of coalescence of a dispersion of adatoms on the amorphous surface as a function of the adatom-substrate interaction energy, and (b) evaluate the effect of the adatom-substrate interaction energy on the mobility of a 10 atom cluster on amorphous and crystalline surfaces. Results show that at low temperatures the diffusion of adatoms on the amorphous surface is limited by the high density of sites from which adatoms require a significant amount of thermal energy in order to escape. At higher temperatures, the amorphous surface can relax and accommodate adatoms and clusters more readily than a low index crystalline surface, again inhibiting diffusion. Such results help explain the experimentally observed high sticking coefficients and higher densities of small clusters experimentally observed in depositions on amorphous surfaces as compared to crystalline surfaces.

Model hamiltonian for adatom-substrate polarization interaction

Using the properties of the surface response function we set up a model hamiltonian which describes the adatom-substrate polarization interactions encountered in adsorption kinetics and similar phenomena. The Van der Waals interaction emerges from this model as the second order long-wavelength correction to the ground state energy.

Cluster adsorption on amorphous and crystalline surfaces: A molecular dynamics study of model Pt on Cu and model Pd on Pt

Molecular dynamics was used to study the structure, dispersion and short-time behavior of ten-atom clusters adsorbed onto amorphous and crystalline substrates, in which the cluster atoms differed from the substrate atoms. Two adatom–substrate model systems were chosen; one, in which the interaction energy between adatom pairs was greater than that between substrate pairs, and the other, in which the reverse was true. At relatively low temperature ranges, increased dispersion of cluster atoms occurred: (a) on the amorphous substrate as compared to the FCC(100) surface, (b) with increasing reduced temperature, and (c) with adatom–substrate interaction energy stronger than adatom–adatom interaction. Two dimensional clusters (rafts) on the FCC(100) surface displayed migration of edge atoms only, indicating a mechanism for the cluster rotation and shape changes found in experimental studies.

Single atom chemisorption on a bcc metal

The effect of chemisorption of a single atom on the (001) surface of a bcc metal is investigated via the Green's function and the phase shift techniques using the LCAO method and the tight-binding approximation. In particular, we obtain the change in the electronic density of states Δ n due to two different binding sites, the on-site and the centered four-fold-site. For each site, the adatom energy level E a is placed both inside and outside the band, and several adatom-substrate interaction strengths σ are considered. By varying these parameters we obtain a qualitative understanding of the effects on Δ n due to either the adsorption of different atoms or a readjustment of E a arising from a flow of charge onto or away from the adsorbed atom. We compare our results to previous studies of chemisorption on the (001) surface of an s-band simple cubic crystal and find that, although some details distinctive of the bcc metal arise, the overall qualitative features are the same.

Superfluid helium in a restricted geometry

A set of boson models, incorporating an excitation spectrum which produces a linear relation between the temperature and the superfluid density in narrow channels at low temperatures as seen by pobell et al. (see abstr. A25131 of 1972), are presented. These energy states are interpreted as arising from adatom-substrate interactions resulting from the heterogeneity of the surface.