Theoretical considerations of energetics, dynamics, and structure at interfaces

Abstract

We review several of our recent studies investigating the structure and dynamics of two kinds of interfaces: solid–gas and solid–liquid types. For the former, we examine the energetics of some metal–vacuum interfaces and the dynamics of surface diffusion. Metal–vacuum interfaces are studied in the context of pseudopotential theory where the varied contributions to surface energy can be relatively easily categorized and interpreted while still providing quantitative accuracy. We examine the roles of the Madelung energy, surface dipole layer, Hartree energy, and electron response to the relaxation process. While qualitative behavior such as oscillatory multilayer relaxation can be seen even from the simplest ‘‘electrostatic’’ picture, quantitative predictions require both the complete theory and acknowledgment of the full three dimensionality of the system. Results are shown for the (111), (100), and (110) faces of aluminum. In exploring the dynamics of surface diffusion, we discuss a molecular dynamics (MD) simulation of model systems of lead monomers and dimers on a copper (110) surface (where there is a negligible size mismatch between dimer and substrate) and find that dimer diffusion rates can be two to three times those of the monomer. We show typical time evolutions of relevant dimer properties as well as vibrational densities of states and find that a librational or ‘‘wagging’’ mode of dimer motion acts as a ‘‘door-way state’’ for the diffusive jump and associate the elevation of the diffusion rate with its existence. We again employ MD to study solid–liquid interfaces both in equilibrium and under circumstances of rapid melting and recrystallization such as those found during laser annealing. We examine the (111), (100), and (110) faces of a fcc crystal and display profiles versus depth at various times of such properties as number density, temperature, potential energy, and diffusion constant, as well as growth rates. In particular, the close-packed (111) face enjoys a continuous growth process, uniform across the surface and mediated by an extensive interfacial region of liquid layering over the solid, while on the open, ‘‘rougher’’ (110) face the layering is suppressed and growth proceeds laterally across the exposed face in steps.