Electron-phonon processes in low symmetry 2Dmaterials investigated by polarized and resonance Raman spectroscopy

Raman spectroscopy is a powerful tool to study the behavior of phonons in optically anisotropic 2D materials since the intensities of the Raman peaks depend on the polarization of the light with respect to the crystallographic axes. I will present angle-resolved polarized Raman measurements in single-layer (1L) and bulk ReSe₂ and ReS₂ crystals that exhibit a triclinic symmetry, and using different laser excitation energies. Angle-resolved results allows the investigation of the effect of dimensionality on the Raman tensors, and the multiple excitation experiments exhibit peak enhancements by both single and the double-resonance processes. Phase differences between tensor elements are needed to describe the experimental results for low symmetry 2D crystals and results are explained considering the quantum model for the Raman intensities and the fact that Raman tensor elements are complex numbers near resonances. I will finally show that the wavevector dependence of the electron–phonon interaction is essential for explaining the distinct angle-resolved results for the different phonon modes of ReSe₂ and ReS₂.