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We report the observation and interpretation of new Raman peaks in few-layertungsten diselenide (WSe2), induced by the reduction of symmetry going from 3Dto 2D. In general, Raman frequencies in 2D materials follow quite closely thefrequencies of corresponding eigenmodes in the bulk. However, while the modesthat are Raman active in the bulk are also Raman active in the thin films, thereverse is not always true due to the reduced symmetry in thin films. Here, wepredict from group theory and density functional calculations that twointra-layer vibrational modes which are Raman inactive in bulk WSe2 in ourexperimental configuration become Raman active in thin film WSe2, due toreduced symmetry in thin films. This phenomenon explains the Raman peaks weobserve 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 bedetected in Raman measurements under specific wavelengths of irradiation,suggesting that in this case, crystal symmetry selection rules may be brokendue to resonant scattering. Both theory and experiment indicate that the andmodes blue-shift with decreasing thickness, which we attribute to surfaceeffects. Our results shed light on a general understanding of the Raman/IRactivities of the phonon modes in layered transition metal dichalcogenidematerials and their evolution behavior from 3D to 2D.

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