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

Abstract

The temperature evolution of the resonant Raman scattering from high-quality bilayer 2H-MoS$$_{2}$$ 2 encapsulated in hexagonal BN flakes is presented. The observed resonant Raman scattering spectrum as initiated by the laser energy of 1.96 eV, close to the A excitonic resonance, shows rich and distinct vibrational features that are otherwise not observed in non-resonant scattering. The appearance of 1st and 2nd order phonon modes is unambiguously observed in a broad range of temperatures from 5 to 320 K. The spectrum includes the Raman-active modes, i.e. E$$_{\text {1g}}^{2}$$ 1g 2 ($$\Gamma$$ Γ ) and A$$_{\text {1g}}$$ 1g ($$\Gamma$$ Γ ) along with their Davydov-split counterparts, i.e. E$$_{\text {1u}}$$ 1u ($$\Gamma$$ Γ ) and B$$_{\text {1u}}$$ 1u ($$\Gamma$$ Γ ). The temperature evolution of the Raman scattering spectrum brings forward key observations, as the integrated intensity profiles of different phonon modes show diverse trends. The Raman-active A$$_{\text {1g}}$$ 1g ($$\Gamma$$ Γ ) mode, which dominates the Raman scattering spectrum at T = 5 K quenches with increasing temperature. Surprisingly, at room temperature the B$$_{\text {1u}}$$ 1u ($$\Gamma$$ Γ ) mode, which is infrared-active in the bilayer, is substantially stronger than its nominally Raman-active A$$_{\text {1g}}$$ 1g ($$\Gamma$$ Γ ) counterpart.

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.…”

The effect of temperature and excitation energy on Raman scattering in bulk HfS2

“…The presence of IR-active modes in the Raman spectra can be related to strain, disorder, or Coulomb interactions [30]. In fact, the IR-active modes are apparent in the RS spectra in several TMDs due to the presence of symmetry breaking, disorder, or resonant conditions of RS excitation [12,22,25,[31][32][33]. However, the observation of both LO and TO modes of the A 2u mode can be related to the ionic bonding in HfS 2 .…”

“…Finally, let us address the effect of resonant excitation on the measured RS spectra, shown in figure 5. The resonant excitation may lead to a significant enhancement of the RS intensity in TMD as well as the activation of otherwise inactive modes [25,35]. This offers supplementary information on the coupling of particular phonons to electronic transitions of a specific symmetry [20].…”

“…More complicated is the description of the process in which the excited state coincides with a non-negligible density of electronic states in a process referred to as the resonant RS (RRS). The light-matter interaction in semiconductors measured by RRS can be modulated by the variation of the excitation energy and/or temperature [20][21][22][23][24][25]. The RS spectra excited with a series of lasers lines and measured at T = 300 K and T = 5 K are presented in figures 5(a) and (b), respectively.…”

Understanding the Heterointerfaces in Perovskite Solar Cells via Hole Selective Layer Surface Functionalization

“…This reduction in gap suggests the weaker interlayer interaction and vibrational motion of surface modified HTL layer, supporting relative displacement in higher wavelength zones. [30,31] Raman spectra have been taken at 300K using a 532 nm laser source at 2.33 eV excitation energy (Figure S10, Supporting Information). The first-order longitudinal (LO) mode and second-order longitudinal (2LO) of NiO x are represented by the stronger Raman peaks observed at ≈560 and 1095 cm −1 , respectively (Figure S10, Supporting Information).…”