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Among the most common few-layers transition metal dichalcogenides (TMDs),WSe2 is the most challenging material from the lattice dynamics point of view.Indeed, for a long time the main two phonon modes (A1g and E12g) have beenwrongly assigned. In the last few years, these two modes have been properlyinterpreted, and their quasi-degeneracy in the monolayer has been used for itsidentification. In this work, we show that this approach has a limited validityand we propose an alternative, more general approach, based on multi-phononbands. Moreover, we show and interpret all the peaks (about 40) appearing inthe Raman spectra of monolayers, bilayers, and trilayers of WSe2 by combiningexperimental wavelength- and polarization-dependent Raman studies withdensity-functional theory calculations providing the phonon dispersions, thepolarization-resolved first-order Raman spectra, and the one- and two-phonondensity of states. This complete study not only offers a method to distinguishbetween monolayers, bilayers, and trilayers with no need of optical images andatomic force microscopy, but it also sheds light on the interpretation ofsingle and multi-phonon bands appearing in the inelastic light scatteringexperiments of layered WSe2; some of these bands were never observed before,and some were observed and uncertainly assigned. We promote the fullunderstanding of the lattice dynamics of this material that is crucial for therealization of optoelectronics devices and of novel phononic metamaterials,such as TMDs superlattices.

New insights in the lattice dynamics of monolayers, bilayers, and trilayers of WSe2 and unambiguous determination of few-layer-flakes' thickness