Revealing defect-bound excitons in WS₂ monolayer at room temperature by exploiting the transverse electric polarized...

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS₂ monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si₃N₄/Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS₂ monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si₃N₄ layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS₂ monolayer can be modified by the Si₃N₄/Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS₂ monolayer attached on the Si₃N₄/Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers.