Probing the uniaxial strains in MoS2 using polarized Raman spectroscopy: A first-principles study

Characterization of strain in two-dimensional (2D) crystals is important for understanding their properties and performance. Using first-principles calculations, we study the effects of uniaxial strain on the Raman-active modes in monolayer MoS2. We show that the in-plane E' mode at 384 cm(-1) and the out-of-plane A(1)' mode at 403 cm(-1) can serve as fingerprints for the uniaxial strain in this 2D material. Specifically, under a uniaxial strain, the doubly degenerate E' mode splits into two nondegenerate modes: one is the E-parallel to' mode in which atoms vibrate in parallel to the strain direction, and the other is the E-perpendicular to' mode in which atoms vibrate perpendicular to the strain direction. The frequency of the E-parallel to' mode blueshifts for a compressive strain, but redshifts for a tensile strain. In addition, due to the strain-induced anisotropy in the MoS2 lattice, the polarized Raman spectra of the E-parallel to' and E-perpendicular to' modes exhibit distinct angular dependence for specific laser polarization setups, allowing for a precise determination of the direction of the uniaxial strain with respect to the crystallographic orientation. Furthermore, we find that the polarized Raman intensity of the A(1)' mode also shows evident dependence on the applied strain, providing additional effective clues for determining the direction of the strain even without knowledge of the crystallographic orientation. Thus, polarized Raman spectroscopy offers an efficient nondestructive way to characterize the uniaxial strains in monolayer MoS2.