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二维过渡金属硫族化合物（TMDs）中层内振动模的Davydov组分与其层间耦合密切相关。尽管带边共振拉曼光谱能极大地增强TMDs拉曼峰的强度，但Davydov组分的拉曼峰极易被带边光致发光信号所压制，因此所有组分的拉曼峰在室温下难以同时被实验观测。本文通过构建少层TMDs与石墨烯薄片的范德华异质结，利用超低波数拉曼光谱证实了其良好的界面耦合质量并精准测定了其中TMDs和石墨烯薄片成分的层数。利用带边共振拉曼光谱技术，同时观测到了异质结中MoS2、MoSe2和WS2成分A模各Davydov组分的拉曼峰。研究表明，上述现象起源于三种机制的共同作用： 1）二维过渡金属硫族化合物成分的对称性降低，可以激活A模Davydov劈裂红外禁戒模； 2）界面电荷转移可有效抑制荧光背景； 3）异质结中光激发载流子的非辐射弛豫有效抑制了TMDs成分的能带填充效应。进一步研究发现，界面耦合对异质结中TMDs成分层内振动模的微扰导致其A模频率整体蓝移。本研究为二维材料范德华异质结的界面耦合与声子调控提供了研究范例，并揭示了异质结成分层数、对称性破缺及界面耦合对异质结成分声子行为的协同调控机制。
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关键词:
- Davydov 组分 /
- 异质结 /
- 过渡金属硫族化合物 /
- 层间耦合 /
- 拉曼光谱
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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