Drastic enhancement of the Raman intensity in few-layer InSe by uniaxial strain

The vibrational and electronic properties of two-dimensional (2D) materials can be efficiently tuned by external strain due to their good stretchability. Resonant Raman spectroscopy is a versatile tool to study the physics of phonons, electrons, and their interactions simultaneously, which is particularly useful for the investigation of strain effect on 2D materials. We report the resonant Raman study of strained few-layer InSe (gamma phase). Under similar to 1% of uniaxial tensile strain, one order of magnitude enhancement of Raman intensity for the longitudinal optical (LO) phonon is observed, while other modes exhibit only modest change. Further analysis demonstrates that it arises from the intraband electron-phonon scattering channel for a LO phonon in resonance. The large enhancement of Raman intensity provides us with a sensitive method to characterize the strain effect, and a mapping of the strain distribution in a wrinkled sample is demonstrated. In addition, we observe sizable redshifts of first-order optical phonon modes. The shift rate exhibits phonon-mode dependence, in excellent agreement with density functional perturbation theory calculations. Our study paves the way for sensitive strain quantification in few-layer InSe and its application in flexible electronic and optoelectronic devices.