Multiple excitations and temperature study of the disorder-induced Raman bands in MoS2

Raman spectroscopy has been extensively used to probe disorder in graphene and other carbon-related materials, and disorder-induced (DI) Raman bands are prominent even for low defect densities. The DI bands in MoS2 have been studied in the last years, but a multiple excitation study using laser excitation energies near the excitonic energies was still lacking. In this work, we investigate the low-frequency defect-induced Raman bands in MoS2 coming from the acoustic phonon branches near the Brillouin zone edge using samples produced by mechanical exfoliation and chemical vapor deposition, recorded with different laser excitation energies close to the resonance with the excitonic transitions, and measured at different temperatures, from 100 K to 400 K. Our results show that the defect-induced Raman processes are affected by both excitation energy and temperature. We find that the temperature of measurement affects the linear dependence between the intensities of the DI peaks and the defect concentration. In particular, we observed that the ratio of intensities of the DI longitudinal acoustic (LA) and transversal acoustic (TA) modes with respect to the first-order E ' mode is about the same for the two different samples when results are corrected by the defect density. We show in this work that the largest intensity of the DI peaks occurs for laser energies in the resonance with the excitonic transitions. Finally, we introduce a general expression that provides the parameters for the quantification of defects in MoS2 samples based on the intensity of the DI Raman bands, measured at different laser energies across the excitonic transitions.

Automated summary

In this study, Raman spectroscopy was used to investigate low-frequency defect-induced Raman bands in MoS2, revealing the influence of excitation energy and temperature on defect-induced Raman processes. Additionally, it was observed that the ratio of intensities of the DI longitudinal and transversal acoustic modes with respect to the first-order E' mode was similar for two different samples when corrected by defect density. The largest intensity of the DI peaks was found to occur for laser energies in resonance with excitonic transitions, highlighting the importance of laser excitation energy in studying defects in MoS2 samples.