The effect of different strain on the structural and optical properties of multilayer γ-InSe

Strain engineering is one promising approach to tune electronic and optical properties of 2D materials. But which type mechanical strain is most efficient is unclear. Here we systematically investigate and compare the tuning effect from four different type strains for multilayer γ-InSe, including in-plane uniaxial tensile strain, in-plane biaxial tensile strain, hydrostatic compressive strain and out-of-plane axial compressive strain. The evolution of its phonon modes and luminescent properties upon strain were investigated using in situ Raman and photoluminescence (PL) spectroscopy. All phonon modes exhibit blue shift upon compression strain, while red shift upon tensile strain. Band gap reduction was observed for both compression strain and tensile strain. Strain type has great effect on the shift rate of Raman mode and band gap. Among four types strain, biaxial tensile strain exhibits the largest shift rates, being 8.49, 9.24 and 8.65 cm-1/% for three phonon modes and 85 meV/% for band gap. It suggests that biaxial tensile strain is more efficient for γ-InSe. This result is helpful for practical strain-based manipulation for 2D materials.