  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.
