Local Atomic Off-Centering Mediated Efficient Self-Trapped Excitonic Emission in Cs₅Cu₃Cl₆I₂ Nanoplates

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, Cs₅Cu₃Cl₆I₂ 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 Cs₅Cu₃Cl₆I₂ 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.