Structure and anisotropic optical properties of γ-InSe under high pressure

Pressure is believed to be one effective method to modify the structure and properties of 2D materials. In the present work, the structural transition and anisotropic properties of gamma-InSe were in situ investigated under pressure using Raman, photoluminescence and infrared spectra, as well as the density functional theory. Results reveal that gamma-InSe would transform to one metallic phase of InSe-III above 11.4 GPa. Below this transition pressure, gamma-InSe completely transforms to the high temperature high pressure phase of InSe-II after three pressure up-down cycles. This is the first report on InSe-II obtained at ambient temperature. Besides phase transitions, one direct-to-indirect band transition is observed around 4 GPa. Angle-resolved polarized Raman spectra show all its three phonon modes exhibit 90 degrees periodic variations, yielding a 4-fold symmetry. Pressure would not change its 4fold symmetry, but its polarized angles vary with pressure. Therefore, pressure has important effect on gamma-InSe that induces its structure change and thus results in its optical anisotropy variation.