Understanding the Energy Barriers of the Reversible Ion Exchange Process in CsPbBr_1.5Cl_1.5@Y₂O₃:Eu³⁺ Macroporous Composites and Their Application in Anti-Counterfeiting Codes

The photoinduced reversible ion exchanges in mixed halide perovskites and the resulting luminescent variations make them promising for constructing anti-counterfeiting patterns; however, its understanding in an interfacial view is lacking. In this work, nominal CsPbBr_1.5Cl_1.5 (CPBC) nanocrystals (NCs) were introduced into macroporous Y₂O₃:Eu³⁺ (MYE) to realize emission color variations from red emission of MYE to green emission of halide NCs. The large surface area of MYE helps the formation of Y-Cl/Br bonds which induces fluctuation in the halide composition, while water and intrinsic halogen defects have also been proved to be essential in the reversible ion segregation process. The PL variations of several samples with different pore sizes were investigated upon irradiation of light with different photon energies and excitation power at certain temperatures. According to combined results of density functional theory calculation, the research reveals the presence of two energy barriers that would be overcome correspondingly by the excitation photon and the concentration difference in the ion exchange and recovery process. A photochromic anti-counterfeiting quick response (QR) code was constructed facilely with the perovskite composites. This work provides a deeper understanding from the interfacial aspect and also proposes a feasible strategy to realize reversible PL variation for anti-counterfeiting applications.