Thickness‐Dependent Optical Properties and Ultrafast Dynamics of Self‐Trapped Excitons in Symmetry‐Broken 3R‐ZnIn₂S₄

Abstract Benefiting from the broken symmetry, 2D 3R‐ZnIn₂S₄ exhibits unique optoelectronic properties and has gradually attracted attention. Herein, the optical properties and ultrafast exciton dynamics in 3R‐ZnIn₂S₄ nanoflakes are systematically investigated. The thickness‐dependent Raman, absorption, and photoluminescence (PL) spectra are measured to reveal the importance of thickness engineering in tuning the optical properties of 3R‐ZnIn₂S₄. Especially, 3R‐ZnIn₂S₄ exhibits characteristics of self‐trapped excitons (STEs), featuring a broad PL spectrum and a significant Stokes shift relative to absorption bandgap, which can be attributed to the broken inversion symmetry, as confirmed by second harmonic generation (SHG). The existence of STEs in 3R‐ZnIn₂S₄ endows it with unique optical properties and carrier dynamics. The thickness and wavelength‐dependent SHG demonstrate that the nonlinear optical response of 3R‐ZnIn₂S₄ can surpass the constraints of odd/even layer parity and achieve a broadband spectral response. Additionally, the ultrafast dynamics of STEs are revealed to consist of two components: a fast non‐radiative recombination process (≈200 ps) and a slower radiative recombination process (≈1 ns). Thickness‐dependent ultrafast absorption spectroscopy unveils that exciton dynamics remain stable across varying thicknesses, highlighting the consistency of exciton lifetimes. These findings reveal the potential of 3R‐ZnIn₂S₄ for optoelectronic applications, opening new pathways for its application in advanced technologies.