Deep-ultraviolet Raman scattering spectroscopy of monolayer WS₂

Raman scattering measurements of monolayer WS₂ are reported as a function of the laser excitation energies from the near-infrared (1.58 eV) to the deep-ultraviolet (4.82 eV). In particular, we observed several strong Raman peaks in the range of 700∼850 cm⁻¹ with the deep-ultraviolet laser lights (4.66 eV and 4.82 eV). Using the first-principles calculations, these peaks and other weak peaks were appropriately assigned by the double resonance Raman scattering spectra of phonons around the M and K points in the hexagonal Brillouin zone. The relative intensity of the first-order E2g1 to A_1g peak changes dramatically with the 1.58 eV and 2.33 eV laser excitations, while the comparable relative intensity was observed for other laser energies. The disappearance of the E2g1 peak with the 1.58 eV laser light comes from the fact that valley polarization of the laser light surpasses the E2g1 mode since the E2g1 mode is the helicity-exchange Raman mode. On the other hand, the disappearance of the A_1g peak with the 2.33 eV laser light might be due to the strain effect on the electron-phonon matrix element.