Newly observed first-order resonant Raman modes in few-layer MoS$_2$

TL;DR

This paper reports the discovery of two new first-order Raman modes in few-layer MoS$_2$, which serve as a reliable marker for layer number and are linked to symmetry-related phonons not observed in monolayers.

Contribution

It provides a theoretical and experimental analysis of nearly degenerate phonons in few-layer 2D materials, explaining the emergence of new Raman modes due to symmetry considerations.

Findings

New Raman modes at 286 cm$^{-1}$ and 471 cm$^{-1}$ identified in few-layer MoS$_2$

These modes are absent in monolayer MoS$_2$ and indicate layer number

Nearly degenerate phonons of different symmetry cause pseudo-activation effects

Abstract

We report two new first-order Raman modes in the spectra of few-layer MoS$_2$ at 286~cm$^{-1}$ and 471~cm$^{-1}$ for excitation energies above 2.4~eV. These modes appear only in few-layer MoS$_2$; therefore their absence provides an easy and accurate method to identify single-layer MoS$_2$. We show that these modes are related to phonons that are not observed in the single layer due to their symmetry. Each of these phonons leads to several nearly degenerate phonons in few-layer samples. The nearly degenerate phonons in few-layer materials belong to two different symmetry representations, showing opposite behavior under inversion or horizontal reflection. As a result, Raman active phonons exist in few-layer materials that have nearly the same frequency as the symmetry forbidden phonon of the single layer. We provide here a general treatment of this effect for all few-layer two-dimensional crystal structures with an inversion center or a mirror plane parallel to the layers. We show that always nearly degenerate phonon modes of different symmetry must occur and, as a result, similar pseudo-activation effects can be excepted.