The structure of ultrathin H-passivated [112] silicon nanowires

We report on the atomic structure and energetics of H-passivated silicon nanowires that are oriented along the [112] crystallographic direction and have effective diameters of approximately 1 nm and below. Using a genetic algorithm structural optimization followed by ab initio density functional theory calculations, we find that at certain values of the hydrogen chemical potential the nanowires can take relatively stable (magic) structures with rectangular cross sections bounded by monohydride {110} and {111} facets with dihydride wire edges. Variations in the chemical potential of hydrogen alter the wire structures retrieved by the optimization, the most prominent example of this being that the {111} nanofacets acquire trihydride terminations instead of monohydride ones when the H chemical potential is raised. While the trihydride-passivated wires have already been experimentally observed, the magic-number monohydride-facetted wires found here may serve as useful predictions to be tested in the future.