Enabling\nMulticolor Information Encryption: Oleylammonium-Halide-Assisted\nReversible Phase Conversion between Cs<sub>4</sub>PbX<sub>6</sub> and\nCsPbX<sub>3</sub> Nanocrystals

Recently,\nhalide perovskites have been recognized for their thermochromic\ncharacteristics, showing significant potential in information encryption\napplications. However, the limited luminescence color gamut hinders\nthe encryption of complex multicolor information. Herein, for the\nfirst time, multicolor thermochromic perovskites with luminescence\ncovering the entire visible spectrum have been designed. Oleylammonium\nhalide salts facilitate a reversible phase transformation between\nnonluminescent Cs<sub>4</sub>PbX<sub>6</sub> nanocrystals (NCs) and\nluminescent CsPbX<sub>3</sub> NCs upon heating or cooling. This process\noccurs without the need for external addition or removal of ligands\nor metal salts, enabling efficient and intelligent information encryption.\nA proof-of-concept demonstration successfully encrypts and decrypts\nmulticolor digital information. This work not only advances the understanding\nof phase transformations in perovskites but also highlights their\nsignificant potential for information encryption applications.