Effects of Lower Symmetry and Dimensionality on Raman Spectra in 2D WSe2

TL;DR

This study investigates how reduced symmetry and dimensionality in 2D WSe2 alter Raman spectra, revealing new peaks and mode activations explained by group theory and density functional calculations, with experimental validation.

Contribution

It predicts and confirms that certain Raman inactive modes in bulk WSe2 become active in thin films due to symmetry reduction, advancing understanding of vibrational properties in 2D materials.

Findings

New Raman peaks at ~310 cm-1 and 176 cm-1 observed in thin films.

Raman inactive modes in bulk become active in 2D due to symmetry changes.

Mode frequencies shift with thickness, influenced by surface effects.

Abstract

We report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe2), induced by the reduction of symmetry going from 3D to 2D. In general, Raman frequencies in 2D materials follow quite closely the frequencies of corresponding eigenmodes in the bulk. However, while the modes that are Raman active in the bulk are also Raman active in the thin films, the reverse is not always true due to the reduced symmetry in thin films. Here, we predict from group theory and density functional calculations that two intra-layer vibrational modes which are Raman inactive in bulk WSe2 in our experimental configuration become Raman active in thin film WSe2, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at ~310 cm-1 and 176 cm-1 in thin film WSe2. Interestingly, the bulk mode at ~310 cm-1 that is Raman inactive can in fact be detected in Raman measurements under specific wavelengths of irradiation, suggesting that in this case, crystal symmetry selection rules may be broken due to resonant scattering. Both theory and experiment indicate that the and modes blue-shift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/IR activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D.