Effect of hydrogen on ground-state structures of small silicon clusters

Abstract

We present results for ground-state structures of small ${\mathrm{Si}}_{n}\mathrm{H}$ $(2<~n<~10)$ clusters using the Car-Parrinello molecular dynamics. In particular, we focus on how the addition of a hydrogen atom affects the ground-state geometry, total energy, and the first excited electronic level gap of a ${\mathrm{Si}}_{n}$ cluster. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen bonds with two silicon atoms only in ${\mathrm{Si}}_{2}\mathrm{H},$ ${\mathrm{Si}}_{3}\mathrm{H}$ and ${\mathrm{Si}}_{5}\mathrm{H}$ clusters, while in other clusters (i.e., ${\mathrm{Si}}_{4}\mathrm{H},$ ${\mathrm{Si}}_{6}\mathrm{H},$ ${\mathrm{Si}}_{7}\mathrm{H},$ ${\mathrm{Si}}_{8}\mathrm{H},$ ${\mathrm{Si}}_{9}\mathrm{H},$ and ${\mathrm{Si}}_{10}\mathrm{H})$ hydrogen is bonded to only one silicon atom. Also in the case of a compact and closed silicon cluster hydrogen bonds to the cluster from outside. We find that the first excited electronic level gap of ${\mathrm{Si}}_{n}$ and ${\mathrm{Si}}_{n}\mathrm{H}$ fluctuates as a function of size and this may provide a first-principles basis for the short-range potential fluctuations in hydrogenated amorphous silicon. Our results show that the addition of a single hydrogen atom can cause large changes in the electronic structure of a silicon cluster, though the geometry is not much affected. Our calculation of the lowest-energy fragmentation products of ${\mathrm{Si}}_{n}\mathrm{H}$ clusters shows that hydrogen is easily removed from ${\mathrm{Si}}_{n}\mathrm{H}$ clusters.