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.