Atomically thin MoS$_{ 2}$ crystals have been recognized as a quasi-2D semiconductor with remarkable physics properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS$_{ 2}$, especially in the low-frequency range ($<$50 cm$^{ -1}$). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS$_{ 2}$ 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 $E_{ 2g}^{ 2}$ 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 MoS$_{ 2}$ 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.
Categories: cond-mat.mes-hall cond-mat.mtrl-sci