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Typical Raman spectra of transition metal dichalcogenides (TMDs) display twoprominent peaks, E2g and A1g, that are well separated from each other. We findthat these modes are degenerate in bulk WSe2 yielding one single Raman peak. Asthe dimensionality is lowered, the observed peak splits in two as a result ofbroken degeneracy. In contrast to our experimental findings, our phonondispersion calculations reveal that these modes remain degenerate independentof the number of layers. Interestingly, for minuscule biaxial strain thedegeneracy is preserved but once the crystal symmetry is broken by uniaxialstrain, the degeneracy is lifted. Our calculated phonon dispersion foruniaxially strained WSe2 shows a perfect match to the measured Raman spectrumwhich suggests that uniaxial strain exists in WSe2 flakes possibly inducedduring the sample preparation and/or as a result of interaction between WSe2and the substrate. Furthermore, we find that WSe2 undergoes an indirect todirect bandgap transition from bulk to monolayers which is ubiquitous forsemiconducting TMDs. These results not only allow us to understand thevibrational properties of WSe2 but also provides detailed insight to theirphysical properties.

Anomalous Raman Spectra and Thickness Dependent Electronic properties of WSe2