Relaxation of icosahedral-cage silicon clusters via tight-binding molecular dynamics

We have used a tight-binding total-energy expression for silicon to study the stability and structure of relatively large silicon clusters. Relaxation is performed using a fictitious Lagrangian and molecular dynamics. Specifically, we have looked at the relaxation of 50-, 60-, and 70-atom spheroidal hollow (icosahedral-cage-structure) clusters. We find that these clusters are not stable and relax into structures which look like puckered balls. Our results show that the unrelaxed clusters are metallic and the relaxed clusters are insulating with the Fermi level passing between localized states. We have also examined the relaxation of large clusters (size of about 100 atoms) extracted from pure diamond, fcc, and sc bulk silicon. We find that the cohesive energies of the relaxed icosahedral-cage clusters are greater than the relaxed diamond clusters and less than the relaxed fcc clusters. We also find that a single layer of flat silicon graphite is not stable and relaxes into a nonplanar layer with properties very similar to those of the relaxed icosahedral clusters.