Molecular Dynamics Simulations of Metal Surfaces: Surface Melting and Non-Melting, and Tip-Surface Interactions

Abstract

Molecular dynamics simulations have been used in order to gain some insight into two problems of interest to this workshop.The first concerns surface melting, well known to occur on several open fcc metal surfaces such as Pb(110) and A1(110), but not on the well-packed (111) surfaces which remain crystalline up to melting. This (111) stability is not observed on rare-gas crystals, where all orientations appear to melt, and must therefore be related to the nature of atomic interactions at a metal surface. Non-melting of (111) faces is at the origin of many interesting phenomena observed on small metallic particles, such as overheating when only (111) faces are present and presence of sharp edges on crystal shapes, leading to collapse of isolated monoatomic steps on a (111) surface close to melting.The second problem concerns tip-surface interactions in STM/AFM experiments. While a large amount of experimental data has been collected, the microscopic aspects involved when a tip approaches a substrate and comes possibly into contact with it, are still largely unexplored. We have used a model system consisting of a gold tip on a Pb(110) surface to study tip-induced deformations, contact formation with local heating, adhesion, and total force acting on the tip, as a function of distance and of temperature up to the surface melting regime.