Strongly Tunable Raman Resonance in InSe under Pressure

The graphene-like layered semiconductor indium selenide has recently attracted widespread attention owing to its large tunability of the electronic states by varying layer thickness, chemical doping, or strain. However, the influence of the modulated inter- and intralayer chemical bonding upon lattice change on the optical and electrical properties is still in its infant stage. Here, we systematically investigate the high-pressure behaviors of the phonon modes and excitonic states in ϵ-InSe based on the measurements of Raman, absorption, and photoluminescence spectroscopy combined with theoretical calculations. We find drastically enhanced intensities ∼3 orders of magnitude for the polar and high-order Raman modes. Such intensity enhancements of the Raman modes are found to arise from the near resonance between the laser photon energy and the exciton B in InSe with increasing pressure. Further analysis indicates unexpected weakening of the intralayer In-In bonds and a decrease of polarity of the In-Se bonds upon compression, thus leading to an increase in the energy of exciton B. These results also explain the nonlinear pressure-dependent band gap transition. We demonstrate that InSe provides a versatile platform from which to explore the practical applications in flexible electronic and optoelectronic devices.