Humidity Assisted Selective Ion Exclusion in 0D Mixed Halide Perovskite Related Structures for Multimode Anti‐counterfeiting

Developing materials that are capable of switching the luminescence in response to a stimuli in solid‐state are highly desirable for advanced anticounterfieting applications. Herein, we report 0D mixed halide perovskite‐related structures (0D‐MHs) that undergo reversible phase transformation in the presence of humidity, which results in the rapid and reversible luminescence color switching from green to red. It is found that selective ion exclusion takes place in the presence of humidity, which results in the formation of either iodide‐rich 3D perovskite or bromide‐rich 3D perovskite. Iodide‐rich 3D perovskite is obtained under high humid conditions with surrounding humidity greater than 60%RH, and bromide‐rich 3D perovskite is obtained under low humid conditions with humidity less than 30%RH. This is attributed to the exclusion of MABr owing to its higher solubility in water at room temperature. As long as the reservoir of MAX is present, the decomposition of the 3D‐ perovskite does not occur, and the reversible transformation and multicolor tunable luminescence is observed. Furthermore, flexible, free‐standing films using a composite of PVDF and 0D MH is created which are stable in water and can be used for anti‐counterfeiting purposes. The free‐standing film remains stable even after a year under ambient room conditions, and the reversible phase transformation still takes place.