Investigation of electron–phonon interaction in bulk and nanoflakes of MoS2 using anomalous “b” mode in the resonant Raman spectra

Electron–phonon interaction in bulk and nanoflakes of MoS2 is investigated using Raman spectroscopy. Resonant Raman spectroscopic studies carried out on bulk and liquid exfoliated nanoflakes of MoS2 revealed a second order Raman mode (called the “b” mode), whose frequency in the case of nanoflakes was found to be largely different from that in bulk MoS2. Temperature dependent Raman spectra show larger variation in the frequency of the “b” mode in bulk MoS2 as compared to that in nanoflakes of MoS2. This anomalous behavior of the “b” mode could be attributed to the stronger electron–phonon coupling occurring in bulk MoS2, due to higher electron concentration in the same, as compared to that in nanoflakes of MoS2. A larger sulfur vacancy in bulk MoS2 as compared to that of nanoflakes was found to be responsible for higher electron concentrations. These findings are supported by energy dispersive x-ray analysis and x-ray photoelectron spectroscopic studies carried out on bulk and nanoflakes of MoS2. The present study suggests a more sensitive probe for the estimation of electron concentrations in the low limit range by following the “b” mode in resonance Raman spectra.