Self-Trapped Excitons in 3R ZnIn₂S₄ with Broken Inversion Symmetry for High-Performance Photodetection

Exploring novel materials with intrinsic self-trapped excitons (STEs) is crucial for advancing optoelectronic technologies. In this study, 2D 3R-phase ZnIn₂S₄, featuring broken inversion symmetry_, is introduced to investigate intrinsic STEs. This material exhibits a broadband photoluminescence (PL) emission with a full width at half maximum of 164 nm and a large Stokes shift of ≈0.6 eV, which arises from the distortion of [ZnS₄]^6- tetrahedral unit induced by the symmetry breaking and strong electron-phonon coupling. The photophysical properties of the STEs exhibit a high Huang-Rhys factor (15.0), rapid STEs formation time (166 fs), and extended STEs lifetime (1039 ps), as demonstrated by experimental evidence from temperature-dependent PL, Raman spectroscopy, and ultrafast absorption spectroscopy. Additionally, STE-induced photoconductive effect is elucidated, indicating that intrinsic STEs in 3R-ZnIn₂S₄ can provide a synergistic effect that enhances absorption capacity, localization, and lifetime by capturing the self-trapped hole state. Consequently, the 2D 3R-ZnIn₂S₄ photodetector exhibits remarkable broad-spectrum photosensitivity, including a photo-switching ratio of 11286, response times of less than 0.6 ms, responsivity of 15.2 A W^-1, detectivity of 1.02 × 10¹¹ Jones, and external quantum efficiency of 5032% under 375 nm light. These findings provide new ideas for exploring materials with intrinsic STEs to achieve novel high-performance photodetector applications.