Effects of lower symmetry and dimensionality on Raman spectra in two-dimensional WSe2

We report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe${}_{2}$), induced by the reduction of symmetry going from three-dimensional (3D) to two-dimensional (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 intralayer vibrational modes, which are Raman inactive in bulk WSe${}_{2}$ in our experimental configuration become Raman active in thin film WSe${}_{2}$, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at \ensuremath{\sim}310 and 176 cm${}^{\ensuremath{-}1}$ in thin film WSe${}_{2}$. Interestingly, the bulk ${B}_{2g}^{1}$ mode at \ensuremath{\sim}310 cm${}^{\ensuremath{-}1}$ 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 ${E}_{2g}^{1}$ and ${B}_{2g}^{1}$ modes blueshift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/infrared activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D.