Trion-to-exciton emission modulation in monolayer WS₂via plasmonic nanocavities

Plasmonic coupling provides a promising strategy for tailoring the emission properties of atomically thin semiconductors, while the regulation of exciton and trion populations in plasmonic-TMD hybrids remains a topic of ongoing investigation. In this work, we construct monolayer WS₂ integrated with size-controlled silver nanocube-on-mirror (NCoM) nanocavities and explore the influence of cavity resonance on its optical emission characteristics. By adjusting the nanocube dimensions, the plasmonic modes are engineered to simultaneously couple with the excitation field and WS₂ emission, leading to strongly enhanced photoluminescence. Considering the effective excitation area of individual nanocavities, a local enhancement factor of approximately 10³ is obtained under optimized resonance conditions. Furthermore, excitation-power-dependent measurements reveal a pronounced redistribution between exciton and trion contributions. Unlike the WS₂/Ag planar structure, where trion emission remains dominant, the optimally coupled NCoM structure exhibits a gradual evolution toward exciton-dominated emission with increasing excitation power. This transition originates from the interplay among plasmonic resonance matching, localized electromagnetic confinement, and excitation-induced carrier and thermal effects at the metal-semiconductor interface. These findings highlight plasmonic nanocavities as an effective platform for manipulating excitonic emission processes in two-dimensional semiconductor systems.