Giant Enhancement of Defect-Bound Exciton Luminescence and Suppression of Band-Edge Luminescence in Monolayer WSe₂–Ag Plasmonic Hybrid Structures

We have investigated how the photoluminescence (PL) of WSe₂ is modified when coupled to Ag plasmonic structures at low temperature. Chemical vapor deposition (CVD) grown monolayer WSe₂ flakes were transferred onto a Ag film and a Ag nanotriangle array that had a 1.5 nm Al₂O₃ capping layer. Using low-temperature (7.5 K) micro-PL mapping, we simultaneously observed enhancement of the defect-bound exciton emission and quenching of the band edge exciton emission when the WSe₂ was on a plasmonic structure. The enhancement of the defect-bound exciton emission was significant with enhancement factors of up to ∼200 for WSe₂ on the nanotriangle array when compared to WSe₂ on a 1.5 nm Al₂O₃ capped Si substrate with a 300 nm SiO₂ layer. The giant enhancement of the luminescence from the defect-bound excitons is understood in terms of the Purcell effect and increased light absorption. In contrast, the surprising result of luminescence quenching of the bright exciton state on the same plasmonic nanostructure is due to a rather unique electronic structure of WSe₂: the existence of a dark state below the bright exciton state