Observation of intravalley phonon scattering of 2s excitons in MoSe₂ and WSe₂ monolayers

Abstract

We present a high-resolution resonance Raman study of hBN encapsulated MoSe2 and WSe2 monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe2 A2s and MoSe2 A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates for k = 0 optical phonons are comparable for both 1s and 2s states despite differences in the envelope functions. We also observe multiple new dispersive Raman peaks including a peak at the WSe2 A2s resonance that demonstrates non-linear dispersion and peak-splitting behavior that suggests the dispersion relations for dark excitonic states at energies near the 2s state are extremely complex.

Exciton spectroscopy and diffusion in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

“…WSe 2 phonons are spectrally different compared to MoSe 2 . The degenerate A 1 (Γ)/E (Γ) phonons are located at 250 cm −1 and, similar to MoSe 2 , a strong and recently discovered peak at 495 cm −1 is also observed here and attributed to multi-phonon processes at K and M points or combination of Γ point phonons [58]. The Raman analysis and the assignment of the Raman peaks further confirms high quality CVD-grown MLs.…”

“…The main Raman peaks of MoSe 2 and WSe 2 can be identified in the contour plot (detailed Raman spectra can be found in the Supplementary Material, section B). For ML MoSe 2 we observe the A 1 (Γ) phonon at 241 cm −1 and the E (Γ) at 291 cm −1 [57], while a strong peak at 458 cm −1 has been associated to other peaks to form triplets [58]. Interestingly, we also observe a strong peak at 531 cm −1 , recently assigned to multi-phonon processes associated with either both K and M point phonons or a combination of Γ point phonons [58].…”

“…The spectral shape and amplitude of the differential reflectivity signal also depends on thin-film interference effects related to the thickness of top, bottom hBN and SiO 2 layers [69] The main Raman peaks of MoSe 2 (Fig. 6a) include the A 1 (Γ) phonon at 241 cm −1 and the E (Γ) at 291 cm −1 [57], while a strong peak at 458 cm −1 has been associated two other peaks to form triplets, see supplementary material of [58]. A strong peak at 531 cm −1 was recently assigned to multi-phonon processes either associated with both K and M point phonons or combinations of Γ point phonons [58].…”

Excited Rydberg states in MoSe2/WSe2 heterostructures

“…The constant 520 cm -1 peak from the Si substrate was used as an internal reference which allowed us to calibrate the Raman shift of the TMD peaks to within 0.5 cm -1 . The intensities of the Raman peaks were also converted to absolute scattering probability using the Si peak intensity as a reference 28 with corrections for Fabry-Perot effects modelled using the reflectivity spectra as were performed in our previous paper 8 . The laser power incident on the sample was kept below 100 µW to avoid photo-doping and laser heating effects 45,46 .…”

“…Despite the Raman scattering at the A1s and B1s excitonic resonances being approximately two orders of magnitude stronger than at the A2s and B2s, key phonons which are only observed at these 2s resonances 8 allow them to be clearly identified. In the case of MoSe2 the 2s specific peaks are at 480, 530 and 580 cm -1 (see Figs 1 and 2 and reference 8 ). As shown in the colour plots in Figures 1 and 2, the peaks have a clear outgoing resonance with the A2s state and incoming and outgoing resonances with the B2s states.…”