Resonant-Raman study of Fröhlich exciton–phonon interaction in WS2 nanostructures

Raman spectroscopy has been widely used as an effective tool to characterize two-dimensional transition metal-dichalcogenides. Here we report a comparative study of the Fröhlich exciton–phonon interaction in monolayer (ML), bilayer (BL) and spiral (SPI) WS_2 nanostructures using angle-dependent polarized Raman (ADPR) and helicity-resolved Raman (HRR) measurements. HRR measurements show that the Raman modes maintain the helicity of the incident photons in all the three structures studied, which suggests that with resonance excitation, the Raman selection rules are seemingly violated due to the strong Fröhlich exciton–phonon interaction. Using ADPR measurements, we deduce the ratio of Raman tensor elements for the E_2g^1 mode for the different nanostructures.