Tungsten selenide is a semiconductor from the transition metal dichalcogenide series. It has four active Raman modes, as well as other hexagonal 2D materials, denominated E1g (175 cm-1), E2g 1 (248 cm-1), A1g (251 cm-1), and A2u (308 cm-1). WSe2 consists of slabs of trilayers with ionic-covalent metal-chalcogenide bonds within each trilayer. In general, the most predominant bands in the Raman spectra are related to the E2g 1 and A1g modes, which in the few-layer limit present a small difference of less than 5 cm-1. Therefore, in most of the literature only one strong and broad vibrational mode around 250 cm-1 is observed, and this peak normally presents a blue shift with decreasing the thickness of the WSe2 samples. Moreover, by analysing the phonon dispersion and electronic structure of this material, these two modes are degenerated in energy.
Nevertheless, these peaks can be resolved by using polarized Raman scattering, where one peak is observable in one polarization configuration and the other in the opposite one. The latter, due to the nature of each mode, where the E2g 1 mode is associated with in-plane opposing motions of Se and W atoms while the A1g mode originates from out-of-plane relative motions of Se atoms. In polarized Raman spectra, these peaks show opposite shifts with the increase of layer thickness: the in-plane mode exhibits red shifts, while the out-of-plane mode exhibits blue shifts. Regarding the A2u2 vibration, this mode should be Raman inactive but due to a break of symmetry in the few-layers limit it can be observed in the spectra.
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