Layer‐dependent resonant Raman scattering of a few layer MoS2

Abstract

We report resonant Raman scattering of MoS2 layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS2 in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single‐ and few‐layer MoS2 samples which are absent in the bulk. The Raman mode at ~230 cm−1 appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at ~179 cm−1 shows asymmetric character for a few‐layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the Γ‐M direction. The most intense spectral region near 455 cm−1 shows a layer‐dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm−1 is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS2 layers down to a few atoms thick. Copyright © 2012 John Wiley & Sons, Ltd.