Unveiling the complex phonon nature and phonon cascades in 1L to 5L WSe₂ using multiwavelength excitation Raman...

Tungsten diselenide (WSe₂) is a 2D semiconducting material, promising for novel optoelectronic and phononic applications. WSe₂ 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 WSe₂ 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-Γ 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 WSe₂ flake thicknesses. Finally, we report the observation of phonon-cascades for all WSe₂ flake thicknesses, indicating strong phonon-electron interactions during early carrier relaxation processes in WSe₂. This research provides a solid foundation and reference for future investigations of the vibrational properties of WSe₂, paving the way for further development of this material towards applications.