Excitation energy dependence of Raman spectra of few-layer WS2

초록

Raman spectra of few-layer WS<inf>2</inf> have been measured with up to seven excitation energies, and peculiar resonance effects are observed. The two-phonon acoustic phonon scattering signal close to the main E<inf>2</inf><inf>g</inf>1 peak is stronger than the main peaks for excitations near the A or B exciton states. The low-frequency Raman spectra show a series of shear and layer-breathing modes that are useful for determining the number of layers. In addition, hitherto unidentified peaks (X<inf>1</inf> and X<inf>2</inf>), which do not seem to depend on the layer thickness, are observed near resonances with exciton states. The polarization dependences of the two peaks are different: X<inf>1</inf> vanishes in cross polarization, but X<inf>2</inf> does not. At the resonance with the A exciton state, the Raman-forbidden, lowest-frequency shear mode for odd number of layers appears as strong as that for the allowed case of even number of layers. This mode also exhibits a strong Breit-Wigner-Fano line shape and an anomalous polarization behavior at this resonance. © 2017 Elsevier B.V.

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