Surface energies and nanocrystal stability in the orthorhombic and π-phases of tin and germanium monochalcogenides

A new nanocrystalline cubic binary π-phase was recently discovered in the tin and germanium monosulfide and monoselenide systems. The structure and surface energies of the (100) and (010) surfaces of the orthorhombic phase and the (100) and (111) surfaces of the π phase were studied using density functional theory across several monochalcogenide systems. The thin film electronic band structures were calculated, demonstrating band gaps of ∼1 eV which are slightly lower than in the bulk. Surface states were found to form for the π phase but not for the orthorhombic phase surfaces. Additionally, the surface atomic structure and the chemical bonding in the vicinity of the surface were analyzed. It was found that the orthorhombic phase is stabilized as (010) platelets as observed experimentally, whereas the π phase cannot be stabilized without additional surface modifications.