Chemical Availability of Bromide Dictates CsPbBr₃ Nanocrystal Growth

Lack of detailed understanding of the growth mechanism of CsPbBr₃ nanocrystals has hindered sophisticated morphological and chemical control of this important emerging optoelectronic material. Here, we have elucidated the growth mechanism by slowing the reaction kinetics. When 1-bromohexane is used as an alternative halide source, bromide is slowly released into the reaction mixture, extending the reaction time from ∼3 s to greater than 20 min. This enables us to monitor the phase evolution of products over the course of reaction, revealing that CsBr is the initial species formed, followed by Cs₄PbBr₆, and finally CsPbBr₃. Furthermore, formation of monodisperse CsBr nanocrystals is demonstrated in a bromide-deficient and lead-abundant solution. The CsBr can be transformed only into CsPbBr₃ nanocubes if additional bromide is added. Our results indicate a fundamentally different growth mechanism for CsPbBr₃ in comparison with more established semiconductor nanocrystal systems and reveal the critical role of the chemical availability of bromide for the growth reactions.