Anomalous excitonic resonance Raman effects in few-layered MoS2.

The resonance effects on the Raman spectra from 5 to 900 cm(-1) of few-layered MoS2 thin films up to 14-layers were investigated by using six excitation energies. For the main first-order Raman peaks, the intensity maximum occurs at ∼2.8 eV for single layered and at ∼2.5 eV for few-layered MoS2, which correspond to the band-gap energy. At the excitation energy of 1.96 eV, several anomalous behaviors are observed. Many second-order peaks are anomalously enhanced even though the main first-order peaks are not enhanced. In the low-frequency region (<100 cm(-1)), a broad peak centered at ∼38 cm(-1) and its second order peak at 76 cm(-1) appear for the excitation energy of 1.96 eV. These anomalous resonance effects are interpreted as occuring due to the strong resonance with excitons or exciton-polaritons.