Thermal Expansion Coefficient and Phonon Dynamics in Coexisting Allotropes of Monolayer WS2 Probed by Raman Scattering

TL;DR

This study investigates the temperature-dependent Raman spectra of monolayer WS2's different phases, revealing anomalous phonon behaviors, clarifying mode assignments, and estimating thermal expansion coefficients across phases from 4K to 330K.

Contribution

It provides a comprehensive analysis of phonon dynamics and thermal expansion in monolayer WS2 phases using Raman spectroscopy, clarifying mode assignments and substrate effects.

Findings

Anomalous temperature dependence of Raman modes observed.

Mode near 297 cm-1 is a forbidden first order Raman mode.

Thermal expansion coefficients estimated for different phases.

Abstract

We report a comprehensive temperature dependent Raman measurements on three different phases of monolayer WS2 from 4K to 330K in a wide spectral range. Our studies revels the anomalous nature of the first as well as the higher order combination modes reflected in the disappearance of the few modes and anomalous temperature evaluation of the phonon self-energy parameters attributed to the detuning of resonance condition and development of strain due to thermal expansion mismatch with the underlying substrate. Our detailed temperature dependence studies also decipher the ambiguity about assignment of the two modes in literature near ~ 297 cm-1 and 325 cm-1. Mode near 297 cm-1 is assigned as first order Raman mode, which is forbidden in the backscattering geometry and 325 cm-1 is assigned to the combination of and mode. We also estimated thermal expansion coefficient by systematically disentangling the substrate effect in the temperature range of 4K to 330K and probed its temperature dependence in 1H, 1T and 1T' phases.