Stability and electronic properties of ultrathin films of silicon and germanium

Recent studies have examined the possibility of growing honeycomb silicene and germanene, the silicon and germanium analogues of graphene. Here we use first-principles calculations to examine the relative stability of a number of other single-layer structures that are derived from prominent surface reconstructions of group-IV semiconductors. We find that Si single-layers with the geometry of the root 3 x root 3 reconstruction are more stable than honeycomb silicene. Likewise, honeycomb germanene is less stable than germanium ultrathin films with a root 3 x root 3 or 7 x 7 arrangement. root 3 x root 3 Si and Ge single layers are metals with strong peaks at the Fermi level, and, in the case of Ge, they bear a magnetic moment. Overall, the results demonstrate that free-standing Si and Ge nano-sheets differ in key structural and electronic properties from graphene, and may thus provide systems with alternative functionalities.