Symmetry-dependent phonon renormalization in monolayer MoS2 transistor

TL;DR

This study demonstrates how electron doping in monolayer MoS2 affects phonon frequencies, especially the A₁g mode, using Raman spectroscopy and first-principles calculations, enabling nondestructive doping level assessment.

Contribution

It reveals the symmetry-dependent phonon renormalization in monolayer MoS2 and explains the selective sensitivity of the A₁g mode to electron doping.

Findings

A₁g phonon softens and broadens with doping

E₂g¹ mode remains inert to doping

Theoretical analysis explains symmetry-dependent effects

Abstract

Strong electron-phonon interaction which limits electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in-situ Raman scattering from single layer MoS$_2$ electrochemically top-gated field effect transistor (FET), we show softening and broadening of A$_{1g}$ phonon with electron doping whereas the other Raman active E$_{2g}^{1}$ mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why A$_{1g}$ mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single layer MoS$_2$-based FETs, which have a high on-off ratio and are of enormous technological significance.