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

Novel and covert fluorescence is quite desirable for fluorescent anticounterfeiting application. Here, Cs₂InCl₅·H₂O/Sb and Cs₂NaInCl₆/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 Cs₂InCl₅·H₂O/Sb to Cs₂NaInCl₆/Sb by an ion-exchange method. It was well done by just adding the NaCl precursor with different concentrations in the Cs₂InCl₅·H₂O/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 Cs₂InCl₅·H₂O/Sb, the white product, and Cs₂NaInCl₆/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 Cs₂InCl₅·H₂O/Sb to Cs₂NaInCl₆/Sb is highly promising in fluorescent anticounterfeiting application.