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A comprehensive van der Waals heterostructure strategy has been implemented to enable the observation of all Davydov components of the A-mode in few-layer transition-metal dichalcogenides (TMDs) at room temperature. In few-layer 2H-TMDs such as MoS2, MoSe2, and WS2, the A-mode phonon splits into N Davydov components that directly reflect interlayer coupling strength and layer number. Under the resonance conditions near band edge, however, strong photoluminescence (PL) and band filling effects severely obscure these Raman signals, particularly for infrared-active modes, rendering observation of all the Davydov components at ambient temperature infeasible. In this work, few-layer TMD flakes (1–4 layers) were mechanically exfoliated and dry-transferred onto four-layer graphene, followed by high-vacuum annealing to promote interfacial coupling quality. Ultralow-frequency Raman spectroscopy of interlayer shear and breathing modes provided an unambiguous fingerprint for determining the layer numbers of both TMDs and graphene constituents, while differential reflectance spectroscopy precisely located the exciton resonance energies.
Under resonance excitation with the A-exciton, the heterostructures exhibited a marked enhancement of A-mode Raman intensity accompanied by strong PL quenching. Raman peaks associated with all the Davydov components were simultaneously resolved for MoS2, MoSe2, and WS2 at room temperature. The activation of all the Davydov components arises from three synergistic mechanisms: (1) symmetry breaking at the TMDs/graphene interface, which renders forbidden components Raman-allowed; (2) interfacial charge transfer, which suppresses the PL background by depleting photoexcited carriers into graphene; and (3) effcient nonradiative relaxation pathways provided by graphene, which mitigate band filling effect and restore resonant Raman scattering. Furthermore, the highest-frequency Davydov component A(1) exhibited an overall blue shift in the heterostructure relative to the intrinsic TMDs, with the magnitude of the shift decreasing as layer number increased. This behavior is accounted for by a diatomic linear-chain model in which interfacial van der Waals coupling enhances the force constants of intralayer vibrations.
This work thus establishes a general platform for Raman analysis of all the Davydov components of the A mode in 2D TMDs at room temperature and elucidates how interface coupling, layer number, and symmetry breaking jointly govern phonon behavior. The approach offers valuable insights for phonon engineering and interface design in two-dimensional heterostructures and may readily be extended to related systems such as WSe2 and ReS2.-
Keywords:
- Davydov components /
- Heterostructures /
- Transition metal dichalcogenides /
- Interlayer coupling /
- Raman spectroscopy
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