Layer‐dependent resonant Raman scattering of a few layer MoS 2

We report resonant Raman scattering of MoS 2 layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS 2 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 MoS 2 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 MoS 2 layers down to a few atoms thick. Copyright © 2012 John Wiley & Sons, Ltd.