Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides

TL;DR

This study explores low-temperature Raman excitation spectra of monolayer semiconducting transition metal dichalcogenides, revealing exciton-phonon interactions and higher-order phonon modes, demonstrating RSE's effectiveness in probing EPC.

Contribution

It provides detailed RSE spectra of four high-quality S-TMD monolayers, highlighting the role of excitonic states and demonstrating RSE as a powerful tool for studying exciton-phonon coupling.

Findings

Resonant enhancement of phonon modes in RSE spectra.

Correlation between exciton emissions and Raman-active modes.

Dependence of exciton-phonon coupling strength on excitonic ground state.

Abstract

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.