Unveiling the complex phonon nature and phonon cascades in 1L to 5L WSe using multiwavelength excitation Raman scattering.

Tungsten diselenide (WSe) is a 2D semiconducting material, promising for novel optoelectronic and phononic applications. WSe has complex lattice dynamics and phonon structure. Numerous discrepancies in the literature exist regarding the interpretation and identification of phonon modes. This work presents a complete investigation of the vibrational properties of 1L to 5L flakes and bulk WSe using multi-wavelength Raman spectroscopy. We especially highlight measurements using 785 nm excitation, which have not been performed before. These allow us to solve inconsistences in the literature in terms of defect-activated non- point single phonon modes and Breit-Wigner-Fano type resonance. We identify 35 Raman peaks per flake thickness, which we attribute to either one-phonon or multi-phonon modes, including two-phonon scattering due to a van Hove singularity (vHs). The measurements are in excellent agreement with the theoretical predictions. Using photoluminescence measurements, we identify photon-exciton coupling leading to resonant Raman scattering, suggesting wavelength laser excitations best suited for further investigations of specific WSe flake thicknesses. Finally, we report the observation of phonon-cascades for all WSe flake thicknesses, indicating strong phonon-electron interactions during early carrier relaxation processes in WSe. This research provides a solid foundation and reference for future investigations of the vibrational properties of WSe, paving the way for further development of this material towards applications.