Tip-Enhanced Two-Photon-Excited Fluorescence of Interfacial Charge Transfer Excitons in MoS₂/WS₂ Heterostructures: Implications for Spectral Measurement of 2D Materials

Exploring tip-enhanced fluorescence (TEF) in terms of detection sensitivity and spatial resolution is essential for the high-quality spectrum of transition metal dichalcogenides (TMDs) and their heterostructures. Interfacial charge transfer (CT) causes a considerable reduction in the photoluminescence of TMDs heterostructures compared to individual monolayers. Here, we theoretically demonstrated that the fluorescence enhancement of interfacial CT excitons in MoS 2 /WS 2 heterostructures could be largely improved by the plexcitonic coupling between the metal plasmons and the CT excitons. High-order nonlinearity enables a huge two-photon-excited fluorescence (2PF) enhancement factor as high as 5 orders of magnitude, which is much higher than the one-photon-excited fluorescence. Spectral dependence of the fluorescence lifetime of the MoS 2 /WS 2 heterostructures coupled to the tip–substrate nanostructure is predicted by simulation. The resonance wavelength matching between the minimum fluorescence lifetime and the maximum electromagnetic field enhancement reveals a faster interfacial CT. Sub-5 nm spatial resolution could be obtained for the optimal tip-enhanced 2PF configuration. The optimal 2PF collection angles are ±32° and ±28° respectively for the van der Waals (vdW) and lateral MoS 2 /WS 2 heterostructures. Our findings are relevant for the comprehensive understanding of the high-resolution and high-sensitive 2PF of interfacial CT excitons, which is useful for the spectral measurement of 2D materials.