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Few-layer tungsten diselenide (WSe2) is investigated using circularlypolarized Raman spectroscopy with up to eight excitation energies. The mainE2g1 and A1g modes near 250 cm-1 appear as a single peak in the Raman spectrumtaken without consideration of polarization but are resolved by usingcircularly polarized Raman scattering. The resonance behaviors of the E2g1 andA1g modes are examined. Firstly, both the E2g1 and A1g modes are enhanced nearresonances with the exciton states. The A1g mode exhibits Davydov splitting fortrilayers or thicker near some of the exciton resonances. The low-frequencyRaman spectra show shear and breathing modes involving rigid vibrations of thelayers and also exhibit strong dependence on the excitation energy. Anunidentified peak at ~19 cm-1 that does not depend on the number of layersappears near resonance with the B exciton state at 1.96 eV (632.8 nm). Thestrengths of the intra- and inter-layer interactions are estimated by comparingthe mode frequencies and Davydov splitting with the linear chain model, and thecontribution of the next-nearest-neighbor interaction to the inter-layerinteraction turns out to be about 34% of the nearest-neighbor interaction. Fanoresonance is observed for 1.58-eV excitation, and its origin is found to be theinterplay between two-phonon scattering and indirect band transition.

Excitonic Resonance Effects and Davydov Splitting in Circularly Polarized Raman Spectra of Few-Layer WSe2