Low-frequency Raman modes and electronic excitations in atomically thin MoS2films

Atomically thin MoS${}_{2}$ crystals have been recognized as quasi-two-dimensional semiconductors with remarkable physical properties. We report our Raman scattering measurements on multilayer and monolayer MoS${}_{2}$, especially in the low-frequency range ($<$50 cm${}^{\ensuremath{-}1}$). We find two low-frequency Raman modes with a contrasting thickness dependence. When increasing the number of MoS${}_{2}$ layers, one mode shows a significant increase in frequency while the other decreases following a $1/N$ ($N$ denotes the number of unit layers) trend. With the aid of first-principles calculations we assign the former as the shear mode ${E}_{2g}^{2}$. The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates vibrational modes in an atomically thin crystal as well as a more precise way to characterize the thickness of atomically thin MoS${}_{2}$ films. In addition, we observe a broad feature around 38 cm${}^{\ensuremath{-}1}$(5 meV) which is visible only under near-resonance excitation and pinned at a fixed energy, independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in a conduction band at $K$ points in their Brillouin zone.