Exciton regulation and carrier dynamics in WS₂ coupled with gap-adjustable plasmonic nanocavity

Layered transition metal dichalcogenides (TMDCs) exhibit exceptional physical properties and unique optical features. Plasmonic nanocavity provides an efficient and practical solution for fruitful exciton regulation related emission properties by manipulating light-matter interactions, which is not normally available. For practical applications, an ideal scenario is to enhance the exciton emission and to realize active regulation simultaneously. Here, we designed and fabricated an anisotropic nanocavity using monolayer biphenyl-4-thiol (BPT) and WS₂ separated Ag nanowire and Au film. For the 1L WS₂, emission intensity was enhanced by ~ 631-fold with a dichroic ratio of 2.3. For few-layer WS₂ (2L WS₂ as an example), the resonant wavelength of plasmonic nanocavity matches well with the energy of indirect exciton. Consequently, the enhancement effect of indirect exciton (~ 521 folds) is significantly greater than that of direct exciton (~ 316 folds). The effective modulation of the spectral emission dominated by indirect exciton or direct exciton can be achieved by varying excitation power. Specifically, plasmonic nanocavity can induce fruitful exciton emission properties in 2L WS₂ at low temperature, including direct exciton, interlayer exciton and different types of indirect exciton emissions, which are usually not observed. Transient absorption spectroscopy further revealed that non-radiative and radiative recombination process of exciton and trion in few-layer WS₂ were accelerated in the nanocavity. Our findings provide a prototypical plasmonic hybrid system for anisotropic enhancement of photoluminescence at the nanoscale to achieve active modulation, offering a new opportunity to build high-efficiency and high-quality photonic devices with multi-functionalities.