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

Abstract

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 MoS2/WS2 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 MoS2/WS2 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 MoS2/WS2 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.

Nonlinear Optical Properties from Engineered 2D Materials

“…After that, one photon with fixed energy is emitted, and the semiconductor decays back to the ground state, as shown in Figure 3b. Due to almost the same photoluminescent process, the emission spectra of 2PPL should generally possess a similar shape, peak position, and lifetime compared to the PL spectra [54][55][56][57][58]. 2PPL is induced with the two-photon absorption (TPA), an NLO absorption process involving simultaneous absorption of two photons and the transition from the ground state to the excited state [53].…”

“…After that, one photon with fixed energy is emitted, and the semiconductor decays back to the ground state, as shown in Figure 3b. Due to almost the same photoluminescent process, the emission spectra of 2PPL should generally possess a similar shape, peak position, and lifetime compared to the PL spectra [54][55][56][57][58].…”

Recent progress of MXene as an energy storage material

“…2,3 Notable examples of emerging 2D materials include graphene, 4,5 black phosphorus, 6,7 borophene 8 and transition metal dichalcogenides (TMD). 9,10 Graphene has been the most representative and promising material in the field of 2D materials research over the past few years. Its simple structure composed of carbon atoms gives it exceptional properties such as conductivity, thermal conductivity, and mechanical performance.…”