Anomalous Raman spectra and thickness-dependent electronic properties of WSe2

Typical Raman spectra of transition-metal dichalcogenides (TMDs) display two prominent peaks, ${E}_{2g}$ and ${A}_{1g}$, that are well separated from each other. We find that these modes are degenerate in bulk WSe${}_{2}$ yielding one single Raman peak in contrast to other TMDs. As the dimensionality is lowered, the observed peak splits in two. In contrast, our ab initio calculations predict that the degeneracy is retained even for WSe${}_{2}$ monolayers. Interestingly, for minuscule biaxial strain, the degeneracy is preserved, but once the crystal symmetry is broken by a small uniaxial strain, the degeneracy is lifted. Our calculated phonon dispersion for uniaxially strained WSe${}_{2}$ shows a good match to the measured Raman spectrum, which suggests that uniaxial strain exists in WSe${}_{2}$ flakes, possibly induced during the sample preparation and/or as a result of the interaction between WSe${}_{2}$ and the substrate. Furthermore, we find that WSe${}_{2}$ undergoes an indirect-to-direct band-gap transition from bulk to monolayers, which is ubiquitous for semiconducting TMDs. These results not only allow us to understand the vibrational and electronic properties of WSe${}_{2}$, but also point to effects of the interaction between the monolayer TMDs and the substrate on the vibrational and electronic properties.