Chemical Instability of CsPbBr₃ Nanocrystals and the Reversible Transformation between CsPbBr₃ and Cs₄PbBr₆ Nanocrystals as Driven by Synthetic Precursors

Perovskite nanocrystals attract growing interest owing to their unique optoelectronic properties. However, their chemical stability is relatively poor due to their ionic nature. In this work, we found that inorganic cations (e.g., Cs+) and ligands (e.g., didodecyldimethylammonium bromide, DDAB), used in the synthesis of CsPbBr3 nanocrystals (NCs) at ambient temperature, quickly converted CsPbBr3 NCs to Cs4PbBr6 NCs during the synthesis or postsynthetic treatment. These cations (Cs+, DDA+) were involved in both the chemical composition and dissociation of CsPbBr3 NCs. Nevertheless, DDA+ induced the generation of an impurity in addition to Cs4PbBr6 NCs due to its different nature from that of Cs+. The transformation process was observed by optical spectroscopy and transmission electron microscopy. The reverse transformation of Cs4PbBr6 NCs to CsPbBr3 NCs can be carried out completely by adding sufficient PbBr2 into Cs4PbBr6 NCs. Therefore, the forward and backward reactions were driven by Cs+ and Pb2+. With additional DDAB for passivation during the reverse transformation, the resulting product possessed a better photoluminescence quantum yield (PLQY, ∼90%) compared with that without the involvement of DDAB (∼69%). By considering the Cs4PbBr6 NCs as the Cs source, the reverse transformation can be analogized to the synthesis of CsPbBr3 NCs, providing a possible strategy for synthesizing luminescent perovskite NCs at room temperature with theoretical stoichiometry.