Highly Modulated Dual Semimetal and Semiconducting Gamma-GeSe with Strain Engineering

Layered hexagonal Gamma--GeSe, a new polymorph of GeSe synthesized recently, shows strikingly high electronic conductivity in its bulk form (even higher than graphite) while semiconducting in the case of monolayer (1L). In this work, by using first-principles calculations, we demonstrate that, different from its orthorhombic phases of GeSe, the Gamma--GeSe shows a small spatial anisotropic dependence and a strikingly thickness-dependent behavior with transition from semimetal (bulk, 0.04 eV) to semiconductor (1L, 0.99 eV), and this dual conducting characteristic realized simply with thickness control in Gamma-GeSe has not been found in other 2D materials before. The lacking of d-orbital allows charge carrier with small effective mass (0.16 m0 for electron and 0.23 m0 for hole) which is comparable to phosphorene. Meanwhile, 1L Gamma--GeSe shows a superior flexibility with Young's modulus of 86.59 N/m, only one-quarter of that of graphene and three-quarters of that of MoS2, and Poisson's ratio of 0.26, suggesting a highly flexible lattice. Interestingly, 1L Gamma-GeSe shows an in-plane isotropic elastic modulus inherent with hexagonal symmetry while an anisotropic in-plane effective mass owing to shifted valleys around the band edges. We demonstrate the feasibility of strain engineering in inducing indirect-direct and semiconductor-metal transitions resulting from competing bands at the band edges. Our work shows that the free 1L Gamma-GeSe shows a strong light absorption (~106 cm-1) and an indirect bandgap with rich valleys at band edges, enabling high carrier concentration and a low rate of direct electron-hole recombination which would be promising for nanoelectronics and solar cell applications.