Interfacial Structural Transformation for the Synthesis of Lead‐Free Double Perovskite Nanocrystals

Abstract Lead‐free halide double perovskite nanocrystals have emerged as one of the most promising alternatives to lead halide perovskite nanocrystals due to their non‐toxicity, high stability, and outstanding optoelectronic properties. However, conventional synthesis methods often result in impurities due to increased constituent elements. In this study, an efficient water‐oil biphasic interface‐driven approach is introduced for synthesizing lead‐free double perovskite nanocrystals, enabling controlled structural transformations from 0D to 2D and 3D structures. Starting from 0D Cs 3 BiBr 6 , a gradual cation exchange is achieved, forming 3D Cs 2 AgBiBr 6 . Real‐time monitoring reveals the slow insertion of Ag + ions as the key to the structural transformation. The resulting Cs 2 AgBiBr 6 nanocrystals exhibit exceptional stability, maintaining their integrity for over 120 days under ambient conditions without significant degradation, showing no considerable material decomposition. Additionally, this method allows for the successful synthesis of 2D layered double perovskite Cs 4 ZnBi 2 Br 12 , which has not previously been reported in experimental studies. This biphasic synthesis strategy provides a universal and reliable method for producing high‐quality double perovskite nanocrystals while offering valuable insights into their structural dynamics and properties.