Excited Excitonic States in 1L, 2L, 3L, and Bulk WSe₂ Observed by Resonant Raman Spectroscopy

Abstract

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.

Effect of disorder on Raman scattering of single-layerMoS2

“…These modes involve phonons at the zone edge of the Brillouin zone, which may be activated by the momentum contribution of a defect, allowing the Raman selection rule to be satisfied. Similar peaks have been previously reported in other transition metal dichalcogenides (TMDs) under electronic resonance condition, and ascribed to disorder-induced modes originated at the zone edge [32]. Some of these modes, while still clearly distinguishable, overlap with the E and A 1 peaks, affecting the uncertainty in the fitting process.…”

Symmetry-Dependent Exciton-Phonon Coupling in 2D and BulkMoS2Observed by Resonance Raman Scattering

“…It has been observed that the Raman spectrum is significantly dependent on the laser excitation energy [8][9][10][11][12][13]. Scheuschner et al [14] measured the REP of the A ]g phonon mode of 1L and 2L MoS2 in the range of laser energies of the A and B excitons (1.75-2.05 eV) [14], and observed that the amplitude of the REP associated with the B transition is larger than the A REP amplitude.…”

Field-effect transistors based on wafer-scale, highly uniform few-layer p-type WSe2

“…45,46,48,49 Finally, few-layer WSe 2 samples show a set of three second-order resonance modes at 360 cm -1 , 375 cm -1 , and 395 cm -1 . 44 In Ref. 43, these peaks are assigned to combinations of the main vibrational modes with acoustical phonon modes to form secondary Raman peaks.…”