Strain-dependent electronic structure and optical properties of monolayer indium selenide: A density functional

In this work we systematically investigate the impact of lateral strain on the electronic structure and optical properties of monolayer indium selenide (InSe) and its atomic origin with density functional theory, tightbinding models and many-particle perturbation theory. Free-standing InSe has an indirect band gap due to its special bonding nature. Compressive strain enhances the coupling between Se 4p(x)/4p(y) states more significantly than Se 4p(z) states, eventually reordering the valence bands and converting the band gap from indirect to direct. The selection rules in single-particle regime are still valid when electron-hole interaction is included. The lowest exciton is active under vertically polarized light for free-standing InSe with a low transition matrix element. Under compressive strain the lowest exciton becomes active under lateral polarization and its transition matrix element increases significantly, indicating improvement of the photoluminescence performance. This study provides thorough understanding into the impact of strain as well as guides to tailor the electronic structure and optical properties of monolayer InSe.