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.