Structural anisotropy results in strain-tunable electronic and optical properties in monolayer GeX and SnX (X = S, Se, Te).

Using first-principles calculations, the structural and electronic properties of group-IV monochalcogenide monolayers are investigated. It is demonstrated that all the monolayers employed here possess moderate indirect bandgaps. In-plane elastic stiffness calculation demonstrates the structural anisotropy in these materials, further resulting in anisotropic response to in-plane strains in their electronic properties and anisotropic optical properties. The bandgaps of GeX and SnX monolayers can be linearly reduced by applied in-plane compressive strains and the semiconductor-to-metal transition can be realized under large compressive strains; while tensile strains exert less influence on the electronic properties in comparison to compressive strains. Some monolayers will experience indirect-to-direct bandgap transition when subjected to proper strains. A further insight into the variation of bandgaps of these monolayers can be obtained from the changing band edges.