Deep-ultraviolet Raman scattering spectroscopy of monolayer WS 2

Raman scattering measurements of monolayer WS 2 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 E_2g^1 to A 1 g 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 E_2g^1 peak with the 1.58 eV laser light comes from the fact that valley polarization of the laser light surpasses the E_2g^1 mode since the E_2g^1 mode is the helicity-exchange Raman mode. On the other hand, the disappearance of the A 1 g peak with the 2.33 eV laser light might be due to the strain effect on the electron-phonon matrix element.