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 a 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 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 SiₙH clusters shows that hydrogen is easily removed from SiₙH clusters.