Excited excitonic states in 1L, 2L, 3L, and bulk WSe2 observed by resonant Raman spectroscopy.

Resonant Raman spectroscopy (RRS) is a very useful tool to study physical properties of materials since it provides information about excitons and their coupling with phonons. We present in this work a RRS study of samples of WSe2 with one, two, and three layers (1L, 2L, and 3L), as well as bulk 2H-WSe2, using up to 20 different laser lines covering the visible range. The first- and second-order Raman features exhibit different resonant behavior, in agreement with the double (and triple) resonance mechanism(s). From the laser energy dependence of the Raman intensities (Raman excitation profile, or REP), we obtained the energies of the excited excitonic states and their dependence with the number of atomic layers. Our results show that Raman enhancement is much stronger for the excited A' and B' states, and this result is ascribed to the different exciton-phonon coupling with fundamental and excited excitonic states.