Ion-Exchanging Lead-Free Perovskite with Tunable Emission Wavelengths for Chemical and Fluorescent Double-Modal Anticounterfeiting Application.

Novel and covert fluorescence is quite desirable for fluorescent anticounterfeiting application. Here, CsInCl·HO/Sb and CsNaInCl/Sb with high photoluminescence quantum yields (PLQYs) of 99.61 and 99.9%, respectively, were achieved. Considering the excellent optical performances together with the high similarity of the two crystal structures, we tried to realize the crystal structure transition from CsInCl·HO/Sb to CsNaInCl/Sb by an ion-exchange method. It was well done by just adding the NaCl precursor with different concentrations in the CsInCl·HO/Sb product. Interestingly, a gradual color change from yellow to orange, warm white, white, cool white, and blue was achieved in the process of crystal structure transition. The energy-transfer dynamic models of CsInCl·HO/Sb, the white product, and CsNaInCl/Sb were identified. The chemical reaction and UV fluorescence properties made it possible for application in chemical and fluorescent double-modal anticounterfeiting and highly decreased the possibility of being cracked and copied. Especially, when salt for daily cooking was used to replace NaCl, a similar phenomenon happened as that of the 99.9% NaCl precursor, which made it easy to be applicated. The combination of chemical and optical verifications provides two levels of security and unbreakable encryption. The results demonstrate that the transition from CsInCl·HO/Sb to CsNaInCl/Sb is highly promising in fluorescent anticounterfeiting application.