Local Atomic Off‐Centering Mediated Efficient Self‐Trapped Excitonic Emission in Cs ₅ Cu ₃ Cl ₆ I ₂ Nanoplates

Abstract Cu(I)‐based low‐dimensional metal halides have received significant recognition attributable to their intriguing optoelectronic properties, which instigate the emergence of self‐trapped excitons (STEs) accompanied by intense broadband emissions. However, fundamental crystal structural origin of such STE is still elusive. Herein, we have synthesized Cu(I)‐based mixed halide, Cs 5 Cu 3 Cl 6 I 2 nanoplates (NPs) using room temperature ligand‐assisted reprecipitation method, which showed an intense blue emission with broad line‐width, large Stokes shift, long photoluminescence lifetime, high photoluminescence quantum yield (PLQY) of ∼75%. Temperature‐dependent PL intensity and line‐width analysis unfolded strong exciton‐phonon coupling in NP sample. Synchrotron X‐ray pair distribution function analysis determines the local Cu off‐centering, which provides the required lattice anharmonicity and softness for intense STE in Cs 5 Cu 3 Cl 6 I 2 NPs. The existence of such soft lattice structure associated with low‐energy phonons was verified by sound velocity, Raman spectroscopy and low‐temperature heat capacity measurements. The fluorescence microscopy and super‐resolution optical imaging were implemented at single‐particle level which exhibited minimal temporal PL intermittency with reasonable photostability under high‐intensity illumination. Accordingly, we hypothesize that the intense broadband emission of NPs are accompanied by the local atomic off‐centering‐driven lattice deformation during photo‐excitation process.