Chemical Instability of CsPbBr<sub>3</sub> Nanocrystals\nand the Reversible Transformation between CsPbBr<sub>3</sub> and Cs<sub>4</sub>PbBr<sub>6</sub> Nanocrystals as Driven by Synthetic Precursors

Perovskite nanocrystals attract growing\ninterest owing to their\nunique optoelectronic properties. However, their chemical stability\nis relatively poor due to their ionic nature. In this work, we found\nthat inorganic cations (e.g., Cs<sup>+</sup>) and ligands (e.g., didodecyldimethylammonium\nbromide, DDAB), used in the synthesis of CsPbBr<sub>3</sub> nanocrystals\n(NCs) at ambient temperature, quickly converted CsPbBr<sub>3</sub> NCs to Cs<sub>4</sub>PbBr<sub>6</sub> NCs during the synthesis or\npostsynthetic treatment. These cations (Cs<sup>+</sup>, DDA<sup>+</sup>) were involved in both the chemical composition and dissociation\nof CsPbBr<sub>3</sub> NCs. Nevertheless, DDA<sup>+</sup> induced the\ngeneration of an impurity in addition to Cs<sub>4</sub>PbBr<sub>6</sub> NCs due to its different nature from that of Cs<sup>+</sup>. The\ntransformation process was observed by optical spectroscopy and transmission\nelectron microscopy. The reverse transformation of Cs<sub>4</sub>PbBr<sub>6</sub> NCs to CsPbBr<sub>3</sub> NCs can be carried out completely\nby adding sufficient PbBr<sub>2</sub> into Cs<sub>4</sub>PbBr<sub>6</sub> NCs. Therefore, the forward and backward reactions were driven\nby Cs<sup>+</sup> and Pb<sup>2+</sup>. With additional DDAB for passivation\nduring the reverse transformation, the resulting product possessed\na better photoluminescence quantum yield (PLQY, ∼90%) compared\nwith that without the involvement of DDAB (∼69%). By considering\nthe Cs<sub>4</sub>PbBr<sub>6</sub> NCs as the Cs source, the reverse\ntransformation can be analogized to the synthesis of CsPbBr<sub>3</sub> NCs, providing a possible strategy for synthesizing luminescent\nperovskite NCs at room temperature with theoretical stoichiometry.