Effect of hydrogen on ground state structures of small silicon clusters

We present results for ground state structures of small SinH (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 an Sin cluster. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen bonds with two silicon atoms only in Si2H, Si3H and Si5H clusters, while in other clusters (i.e. Si4H, Si6H, Si7H, Si8H, Si9H and Si10H) 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 Sin and SinH 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 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 SinH clusters shows that hydrogen is easily removed from SinH clusters. PACS numbers: 71.15.Pd, 73.22.-f, 61.46.+w