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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) efficient 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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图 1 本征MoS2中A模的Davydov组分 (a) 1—4L TMDs中A模所有Davydov组分原子位移示意图; (b) 本征1—4LM的差分反射谱; (c) 本征1—4LM的低频模; (d) 本征1—4LM中A模的Davydov组分缺失
Figure 1. Davydov components of A-mode in intrinsic MoS2: (a) Schematic diagram of atomic displacements for all the Davydov components of A-mode in 1–4L TMDs; (b) Differential reflectance spectra of intrinsic 1–4LM; (c) Low-frequency modes of intrinsic 1–4LM; (d) Failure to observe all the Davydov components of A-mode in intrinsic 1–4LM.
图 2 MoS2/石墨烯异质结的层间耦合与拉曼增强机制 (a) 低频模: nLM/4LG vs 本征nLM; (b) 高频模: nLM/4LG vs 本征nLM; (c) PL谱: nLM/4LG vs 本征nLM; 一阶拉曼增强机制: (d1—d2) 1LM/4LG vs 本征1LM; (e1—e2) nLM/4LG vs 本征nLM, $ n \geqslant 2 $.
Figure 2. Interlayer coupling and Raman enhancement mechanism in MoS2/Gr heterostructures: (a) Low-frequency modes: nLM/4 LG vs. intrinsic nLM; (b) High-frequency modes: nLM/4 LG vs. intrinsic nLM; (c) PL spectra: nLM/4 LG vs. intrinsic nLM; Mechanism of first-order Raman enhancement: (d1–d2) 1 LM/4 LG vs. intrinsic 1 LM; (e1–e2) nLM/4 LG vs. intrinsic nLM, $ n \geqslant 2 $.
图 3 MoS2/石墨烯异质结中A模的Davydov组分 (a—d) 1—4LM/4LG的Davydov组分(室温); (e—h) 各组分峰强随激发光能量的变化(1—4LM/4LG)
Figure 3. Davydov components of A-mode in MoS2/Gr heterostructures: (a–d) Davydov components in 1–4LM/4LG (room temperature); (e–h) Excitation energy dependence of component peak intensities (1–4LM/4LG).
图 4 异质结策略的跨材料普适性 (a—d) MoSe2体系与(e—h) WS2体系中的Davydov组分观测
Figure 4. Universality of heterostructure strategies. Observation of Davydov components in (a–d) MoSe2 system and (e–h) WS2 system
图 5 A(1)模频率的层数依赖关系 (a—c) 本征nL MoS2、nL MoSe2、nL WS2样品中A(1)模频率随层数的变化(蓝色菱形); nL MoS2/4LG、nL MoSe2/4LG、nL WS2/4LG异质结样品中A(1)模频率随层数的变化(红色圆形)
Figure 5. Dependence of the Raman peak of A(1) mode on the number of layers (a–c) in intrinsic nL MoS2, nL MoSe2, nL WS2 (blue diamond) and in nL MoS2/4LG, nL MoSe2/4LG, nL WS2/4LG heterstructures(red dot).
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