Effects of hot phonons and thermal stress in micro-Raman spectra of molybdenum disulphide

TL;DR

This study distinguishes the effects of hot phonons and thermal stress in micro-Raman spectra of MoS2, revealing that nonequilibrium phonon populations are smaller than previously thought, which advances understanding of energy relaxation in 2D materials.

Contribution

It provides a detailed analysis separating phonon and stress effects in Raman spectra of MoS2, supported by first-principles predictions, enhancing insight into thermal and electronic properties of 2D materials.

Findings

Hot optical phonons cause Raman shifts in MoS2.

Thermally-induced stress results in Raman blue shifts.

Nonequilibrium phonon populations are smaller than in suspended graphene.

Abstract

Micro-Raman spectroscopy has become an important tool in probing thermophysical behavior in emerging functional materials such as two-dimensional (2D) layered structures. Localized heating by the focused Raman excitation laser beam is expected to produce both stress and nonequilibrium temperature distributions in the material. Here we investigate the effects of hot optical phonons in the Raman spectra of molybdenum disulphide (MoS2) and distinguish them from those caused by thermally-induced compressive stress, which causes a Raman frequency blue shift in the focused Raman laser spot. With the stress effect accounted for in micro-Raman measurements, the degree of nonequilibrium between the hot optical phonons and the acoustic phonon bath is found to be much smaller than those observed in prior micro-Raman measurements of suspended graphene. The observation agrees with a first-principles based theoretical prediction of overpopulated zone-center optical phonons compared to other optical phonons in the Brillouin zone and acoustic phonons. The findings provide detailed insight into the energy relaxation processes in this emerging 2D transition metal dichalcogenide (TMD) electronic and optoelectronic material and clarify an important question in micro-Raman measurements of thermal transport in this and other 2D materials.