Emission Excitation Spectroscopy in WS₂ Monolayer Encapsulated in Hexagonal BN

Abstract

Resonant conditions of the Raman scattering in monolayer WS2 encapsulated in hexagonal BN are investigated using the Raman scattering excitation technique at low temperature (T = 5 K). The resonance of the detected signal with the neutral exciton leads to an extremely rich Raman scattering excitation spectrum, which displays also the Raman scattering features not reported so far. Moreover, the intensities of the observed phonon modes are strongly enhanced when the energy difference between the excitation laser and the neutral exciton emission (X 0) is equal to the corresponding phonon energy. The intensity profile of the out-of-plane A 1 phonon mode reflects the Lorentzian lineshape of the X 0 line with the linewidth of about 4 meV, which underlines the outgoing resonance conditions with the X 0 complex.

Layer-Dependent Valley Depolarization and Raman Phonon Softening of SnS2/MoS2 Vertical van der Waals Heterostructures

“…Strong resonant Raman scatterings are superimposed over the background of A exciton emission, which is not present for the non-resonant Raman spectra at 78 K (Figure S4c in the Supporting Information) and room-temperature helicity PL (Figure b). Similarly, with the previous reports on ML MoS 2 and WS 2 , − the enhanced Raman scattering is due to electron–phonon coupling in the vicinity of A excitonic resonances, which allows us to study the effect of EPC on the depolarization by the evolution of PL helicity with the resonant Raman modes. From the detailed analysis of the background exciton emission and the resonant Raman spectra after subtracting the backgrounds (Figure S4d–i in the Supporting Information), the Raman peaks at 456.0, 405.1, and 384.4 cm –1 can be attributed to B 2g 1 , A 1g , and E 2g 1 zone-center (Γ) vibrations of MoS 2 , respectively, while peaks at 260.6 and 229.6 cm –1 in the lower frequency range are assigned to B 2g 1 -ZA and LA phonons around the K and/or M point of the Brillouin zone, respectively.…”

Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS2

“…It has been well established in the literature that the RS in thin S-TMD layers can be significantly enhanced due to an electron–phonon coupling in the vicinity of excitonic resonances, particularly the so-called A exciton. 39 , 47 – 49 In order to study the effect of resonant conditions on the RS in the MoS BL, the temperature evolution of the PL spectra is measured in a broad range of temperature from T = 5 K –320 K. Note that the Raman measurements were carried out on the flat regions of the investigated sample (bubble-free and wrinkles free), see Fig. S1 in the Supplementary Information ( SI ) for optical and atomic force microscope (AFM) images.…”

“…This suggests that the effect of temperature on the resonant conditions of RS is more complex compared to the results of the RS excitation technique, where the laser energy is tuned in reference to the excitonic emission. 48 , 49 , 63 , 64…”

Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS$_{2}$

“…It has been well established in the literature that the RS in thin S-TMD layers can be significantly enhanced due to an electron-phonon coupling in the vicinity of excitonic resonances, particularly the so-called A exciton. 39,[45][46][47] In order to study the resonant conditions of the RS in the MoS 2 BL, the temperature evolution of the PL spectra is measured in a broad range of temperature from T =5 K to 320 K. The PL spectra measured at selected temperatures are shown in Fig. 1(a).…”

“…This suggests that the effect of temperature on the resonant conditions of RS is more complex compared to the results of the RS excitation technique, where the laser energy is tuned in reference to the excitonic emission. 46,47,61,62 Summarising, the observed temperature effect on the intensity of the phonon modes is complicated and cannot be understood in terms of simple incoming or outgoing resonance conditions of the RS. 63 In the presented experiment, the strength of the electron-phonon coupling is not only affected by the adjustment of the relative energy between the excitation and the emission (E L−X A ).…”