Excitation energy dependence of Raman spectra of few-layer WS2

TL;DR

This study investigates how excitation energy influences the Raman spectra of few-layer WS2, revealing resonance effects, layer-dependent modes, and unidentified peaks with distinct polarization behaviors.

Contribution

It provides new insights into resonance effects and layer-specific Raman modes in WS2, including the discovery of unidentified peaks and their polarization properties.

Findings

Resonance effects enhance certain Raman signals near exciton energies.

Unidentified peaks X1 and X2 do not depend on layer thickness.

Layer-dependent shear and breathing modes are observed.

Abstract

Raman spectra of few-layer WS2 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 E2g1 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 (X1 and X2), 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: X1 vanishes in cross polarization, but X2 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.