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 ofdetectionsensitivity and spatial resolution is essential for the high-qualityspectrum of transition metal dichalcogenides (TMDs) and their heterostructures.Interfacial charge transfer (CT) causes a considerable reduction inthe photoluminescence of TMDs heterostructures compared to individualmonolayers. Here, we theoretically demonstrated that the fluorescenceenhancement of interfacial CT excitons in MoS2/WS2 heterostructures could be largely improved by the plexcitonic couplingbetween the metal plasmons and the CT excitons. High-order nonlinearityenables a huge two-photon-excited fluorescence (2PF) enhancement factoras high as 5 orders of magnitude, which is much higher than the one-photon-excitedfluorescence. Spectral dependence of the fluorescence lifetime ofthe MoS2/WS2 heterostructures coupled to thetip-substrate nanostructure is predicted by simulation. Theresonance wavelength matching between the minimum fluorescence lifetimeand the maximum electromagnetic field enhancement reveals a fasterinterfacial CT. Sub-5 nm spatial resolution could be obtained forthe optimal tip-enhanced 2PF configuration. The optimal 2PF collectionangles are & PLUSMN;32 & DEG; and & PLUSMN;28 & DEG; respectively for thevan der Waals (vdW) and lateral MoS2/WS2 heterostructures.Our findings are relevant for the comprehensive understanding of thehigh-resolution and high-sensitive 2PF of interfacial CT excitons,which is useful for the spectral measurement of 2D materials.