Low-Frequency Raman Modes and Electronic Excitations In Atomically Thin MoS2 Crystals

TL;DR

This study investigates low-frequency Raman modes and electronic excitations in atomically thin MoS2 crystals, revealing thickness-dependent vibrational modes and a resonance-enhanced electronic feature linked to spin-orbit coupling.

Contribution

It provides new insights into vibrational and electronic properties of monolayer and multilayer MoS2 using low-frequency Raman spectroscopy and first-principles calculations.

Findings

Identified two contrasting low-frequency Raman modes with thickness dependence.

Assigned modes as shear and compression vibrations through calculations.

Observed a resonance-enhanced electronic Raman feature related to spin-orbit splitting.

Abstract

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.