Room-temperature Raman detection of all Davydov components of <inline-formula><tex-math id="Z-20250926165500">\begin{document}${\mathrm{A}}_1' $\end{document}</tex-math></inline-formula> mode in transition metal dichalcogenide

<sec>A comprehensive van der Waals heterostructure strategy has been implemented to be able to observe all Davydov components of the A-mode in few-layer transition-metal dichalcogenides (TMDs) at room temperature. In few-layer 2H-TMDs such as MoS<sub>2</sub>, MoSe<sub>2</sub>, and WS<sub>2</sub>, the A-mode phonon splits into <i>N</i> Davydov components that directly reflect the interlayer coupling strength and layer number. Under the resonance conditions near the band edge, however, strong photoluminescence (PL) and band filling effects severely obscure these Raman signals, particularly for infrared-active modes, rendering the observation of all the Davydov components at ambient temperature infeasible. In this work, few-layer (1–4 layers) TMD flakes are mechanically exfoliated and dry-transferred onto four-layer graphene, followed by high-vacuum annealing to improve the interfacial coupling quality. Ultralow-frequency Raman spectra of interlayer shear and breathing modes provide an unambiguous fingerprint for determining the layer numbers of both TMDs and graphene constituents, while differential reflectance spectra precisely determine the resonance energies of excitons.</sec><sec>Under resonance excitation with the A-exciton, the heterostructures exhibit a marked enhancement of A-mode Raman intensity accompanied by strong PL quenching. Raman peaks associated with all the Davydov components are simultaneously resolved for MoS<sub>2</sub>, MoSe<sub>2</sub>, and WS<sub>2</sub> 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 the forbidden components Raman-allowed; 2) interfacial charge transfer, which suppresses the PL background by depleting photoexcited carriers entering into graphene; and 3) efficient nonradiative relaxation pathways provided by graphene, which mitigates the band filling effect and restore resonant Raman scattering. Furthermore, the highest-frequency Davydov component A(1) exhibits an overall blue shift in the heterostructure relative to the intrinsic TMDs, with the magnitude of the shift decreasing as the layer number increases. This behavior can be explained by a diatomic linear-chain model in which interfacial van der Waals coupling enhances the force constants of intralayer vibrations.</sec><sec>This work thus establishes a general platform for Raman analysis of all the Davydov components of the A mode in two-dimensional (2D) TMDs at room temperature and elucidates how interface coupling, layer number, and symmetry breaking jointly govern phonon behavior. The approach offers valuable insights into phonon engineering and interface design in 2D heterostructures and may readily be extended to relevant systems such as WSe<sub>2</sub> and ReS<sub>2</sub>.</sec>