Response of silicane and germanane to uni-axial compression: superstructures, polymorph nano-ribbons, and extreme...

Germanane (GeH) and silicane (SiH) are the fully hydrogenated forms of germanene and silicene, the Ge- and Si-analogues of graphene. Here we use density-functional theory calculations to probe the properties of GeH and SiH sheets and their dependence on applied uni-axial compression. We find that GeH polymorphs with distinct hydrogen arrangements have markedly different energy band gaps. We also show that, when compressed, GeH forms superstructures with parts in low- and wide-gap geometries, enabling the creation of alternating polymorph nano-ribbons. An alternative to superstructure formation is the adoption by GeH of a corrugated form with extreme bending. Silicane shows similar behavior under compression, with either high corrugation, or successive parts with different geometries. Finally, we demonstrate that interaction with a substrate can influence the relative stability of GeH overlayer polymorphs. Overall, the results reveal ways to enhance the functionalities of these two-dimensional materials through the formation of superstructures with sizeable quantum well effects or outstanding mechanical response.