Photophysics in Zero‐Dimensional Potassium‐Doped Cesium Copper Chloride Cs₃Cu₂Cl₅ Nanosheets and Its Application for High‐Performance Flexible X‐Ray Detection

Abstract Zero‐dimensional Cs₃Cu₂Cl₅ exhibits intriguing optical properties, which can meet the basic requirements of ideal scintillator application. Here, green emitter Cs₃Cu₂Cl₅ nanosheets are successfully synthesized, and by doping with 2% potassium (K⁺), their photoluminescence quantum yield (70.23% to 81.39%), radioluminescence intensity, and stability are improved. Further experimental and theoretical studies point out that: (1) K⁺ brings the neighboring [Cu₂Cl₅]³⁻ dimers groups closer, leading to lattice shrinkage and lower lattice constants; (2) such compact crystal structure results in stronger exciton–photon coupling and reduced phonon–electron coupling strength, which is beneficial to form self‐trapped excitons and enhanced luminescence; (3) lower lattice constants also induce decreased bandgap (2.58 to 2.51 eV) and increased Stokes shift (217 to 223 nm), for less self‐absorption and higher quantum efficiency. Finally, area‐controllable uniform flexible Cs₃Cu₂Cl₅:2%K⁺–polystyrene film is successfully achieved with excellent X‐ray sensibility. Such flexible film shows low cost (2.8223 $/g), broadband response (20–160 keV), and high spatial resolution (5 lp mm⁻¹) that is comparable to commercial CsI:Tl wafer. Moreover, it fits well with nonplanar surfaces for high‐quality medical and industrial flexible images applications. It is believed that this work can provide a viable tactic to enhance X‐ray detection in metal halide scintillators.