SEMIEMPIRICAL SIMULATIONS OF SOLID-SURFACES AND INTERFACES

Surfaces and interfaces in solids are of great practical interest because they affect several important properties of real materials. New semi-empirical methods of computing energies have made possible the simulation and modeling of real surfaces and interfaces. From simulations using the equivalent crystal theory (ECT), we show that the variation of the rigid adhesive energy with normal separation between two flat surfaces has a universal form independent of the material involved. The same universal behaviour is seen for an atomically sharp tip interacting with a flat surface. We also show that as two metal slabs approach each other, the surfaces can avalanche together when the rigid interfacial spacing falls below a critical distance. This is accompanied by a discontinuous decrease in the adhesive energy. Avalanche is inhibited when the two surfaces are out of registry and when only a few layers near the surface are allowed to relax. As the relaxing slabs get thicker a sharp avalanche reappears. Avalanche has serious consequences for the stability of the tip in the atomic force microscope (AFM). We have investigated, through simulations at room temperature, the Stability of two model AFM tip configurations for a number of fcc metals. Our results on a model single-atom tip are in excellent agreement with recent experiments on tunnelling through mechanically-controlled break junctions.