Polarized Raman spectroscopy in monolayer ReSe2

Raman spectroscopy is a powerful tool to study two-dimensional compounds and has been widely used to obtain important information of their electronic and vibrational structures. In the case of graphene and MoS2-type compounds, the Raman spectrum is isotropic when the light polarization lies in the layer plane. However, for low-symmetry materials such as black phosphorus and triclinic transition metal dichalcogenides, the spectra are polarized dependent and polarized Raman spectroscopy should be used. By changing the angle between the light polarization and the crystallographic axes, the elements of the Raman tensors for the different phonon modes can be determined. Previous studies in black phosphorus showed that Raman tensor elements are complex numbers, but the physical origin of the phase differences are not yet well understood. In this work, polarized Raman spectroscopy is used to investigate the anisotropic behavior in monolayer ReSe2. The angular dependence of the polarized Raman spectra using different polarization configurations is obtained for the 18 Raman active modes as well as the Raman tensor elements for each mode. It was also observed that the principal axes of those Raman tensors are not along the crystallographic axes.