Anomalous Raman Spectra and Thickness Dependent Electronic properties of WSe2

TL;DR

This study investigates the Raman spectral features and electronic properties of WSe2, revealing how layer thickness and strain influence vibrational modes and bandgap transitions, with implications for understanding TMD materials.

Contribution

The paper provides experimental and theoretical insights into the degeneracy lifting of Raman modes and strain effects in WSe2, highlighting the impact of uniaxial strain on phonon behavior.

Findings

Raman peak splitting occurs with decreasing layers due to broken degeneracy.

Uniaxial strain lifts phonon mode degeneracy, matching experimental spectra.

WSe2 transitions from indirect to direct bandgap from bulk to monolayer.

Abstract

Typical Raman spectra of transition metal dichalcogenides (TMDs) display two prominent peaks, E2g and A1g, that are well separated from each other. We find that these modes are degenerate in bulk WSe2 yielding one single Raman peak. As the dimensionality is lowered, the observed peak splits in two as a result of broken degeneracy. In contrast to our experimental findings, our phonon dispersion calculations reveal that these modes remain degenerate independent of the number of layers. Interestingly, for minuscule biaxial strain the degeneracy is preserved but once the crystal symmetry is broken by uniaxial strain, the degeneracy is lifted. Our calculated phonon dispersion for uniaxially strained WSe2 shows a perfect match to the measured Raman spectrum which suggests that uniaxial strain exists in WSe2 flakes possibly induced during the sample preparation and/or as a result of interaction between WSe2 and the substrate. Furthermore, we find that WSe2 undergoes an indirect to direct bandgap transition from bulk to monolayers which is ubiquitous for semiconducting TMDs. These results not only allow us to understand the vibrational properties of WSe2 but also provides detailed insight to their physical properties.