Strain induced new phase and indirect-direct band gap transition of monolayer InSe

The effect of in-plane strain on monolayer InSe has been systematically investigated by using first-principles calculations. It is found that monolayer InSe exhibits superior mechanical flexibility, which can sustain a tensile strain up to 27% in the armchair direction. More importantly, a new phase with inversion symmetry denoted as phase-II is obtained when the tensile strain increases over 25% along the zigzag direction, which is predicted to be metallic and thermodynamically stable at room temperature. And the phase-II InSe could show an out-of-plane negative Poisson's ratio after the uniaxial tensile strain is larger than 5%. Moreover, both uniaxial and biaxial compressive strains can trigger the indirect-to-direct band gap transition in the pristine monolayer InSe and its band gap decreases monotonously with the applied tensile strain, which offers an effective method to tune the electronic properties of monolayer InSe for its promising application in electronics and optoelectronics.