Atypical Exciton-Phonon Interactions in WS₂ and WSe₂ Monolayers Revealed by Resonance Raman Spectroscopy

Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS₂ and WSe₂ using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS₂ monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related X_A and X_B excitons), whereas Raman bands of WSe₂ are not enhanced at X_A/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS₂ and WSe₂ exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between X_C and X_A excitons with phonons explains the different Raman responses of WS₂ and WSe₂, and the absence of Raman enhancement for the WSe₂ modes at X_A/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).