Influence of Defect Structures on Intervalley Scattering in Two-dimensional WSe₂ Revealed by Double-Resonance Raman Spectroscopy

Double-resonance Raman (DRR) scattering in two-dimensional (2D) materials describes the intravalley or intervalley scattering of an electron or a hole excited by incident photons. Although the presence of defects can provide additional momentum and influence the scattering process involving one or two phonons, only the idealized defects without any structural details are considered in traditional DRR theory. Here, the second-order DRR spectra of WSe₂ monolayer with different types of defects are calculated involving the combinations of acoustic and optical phonons in the vicinity of K (\bfK' ) and M points of the Brillouin zone. The electronic band structures are modified due to the presence of defects, and the band unfolding method is adopted to show the bending of valence and conduction bands for the defective WSe₂ monolayers. The associated phononic band structures also exhibit different changes in phonon dispersion curves, resulting in different DRR spectra corresponding to the different types of defects in the WSe₂ monolayers. For example, the existence of W vacancy in the WSe₂ monolayer would result in downshifts in vibrational frequencies and asymmetrical broadenings in linewidths for most combination modes due to the dramatic changes in contour shape of electronic valleys at K and \bfK' . Moreover, the scattering from K to Q is found to be forbidden for the two Se vacancies because of the elevation of conduction band at the Q point. Our work highlights the role of defect structures in the intervalley scattering and may provide better understanding in the underlying physics of DRR process in 2D materials.