Atomically thin MoS2 crystals have been recognized as a quasi-2D semiconductor with remarkable physics properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS2, especially in the low-frequency range (<50 cm−1). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS2 layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes layer-number) trend. With the aid of first-principle calculations we assign the former as the shear mode E2g2 and the latter as the compression vibrational mode. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm−1 (~5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.
@article{arxiv.1209.1775,
title = {Low-Frequency Raman Modes and Electronic Excitations In Atomically Thin MoS2 Crystals},
author = {Hualing Zeng and Bairen Zhu and Kai Liu and Jiahe Fan and Xiaodong Cui and Q. M. Zhang},
journal= {arXiv preprint arXiv:1209.1775},
year = {2015}
}