Metamorphoses of Cesium Lead Halide Nanocrystals

Following the impressive development of bulk lead-based perovskite photovoltaics, the "perovskite fever" did not spare nanochemistry. In just a few years, colloidal cesium lead halide perovskite nanocrystals have conquered researchers worldwide with their easy synthesis and color-pure photoluminescence. These nanomaterials promise cheap solution-processed lasers, scintillators, and light-emitting diodes of record brightness and efficiency. However, that promise is threatened by poor stability and unwanted reactivity issues, throwing down the gauntlet to chemists.More generally, Cs-Pb-X nanocrystals have opened an exciting chapter in the chemistry of colloidal nanocrystals, because their ionic nature and broad diversity have challenged many paradigms established by nanocrystals of long-studied metal chalcogenides, pnictides, and oxides. The chemistry of colloidal Cs-Pb-X nanocrystals is synonymous with change: these materials demonstrate an intricate pattern of shapes and compositions and readily transform under physical stimuli or the action of chemical agents. In this Account, we walk through four types of Cs-Pb-X nanocrystal metamorphoses: change of structure, color, shape, and surface. These transformations are often interconnected; for example, a change in shape may also entail a change of color.The ionic bonding, high anion mobility due to vacancies, and preservation of cationic substructure in the Cs-Pb-X compounds enable fast anion exchange reactions, allowing the precise control of the halide composition of nanocrystals of perovskites and related compounds (e.g., CsPbCl₃ ⇄ CsPbBr₃ ⇄ CsPbI₃ and Cs₄PbCl₆ ⇄ Cs₄PbBr₆ ⇄ Cs₄PbI₆) and tuning of their absorption edge and bright photoluminescence across the visible spectrum. Ion exchanges, however, are just one aspect of a richer chemistry.Cs-Pb-X nanocrystals are able to capture or release (in short, trade) ions or even neutral species from or to the surrounding environment, causing major changes to their structure and properties. The trade of neutral PbX₂ units allows Cs-Pb-X nanocrystals to cross the boundaries among four different types of compounds: 4CsX + PbX₂ ⇄ Cs₄PbBr₆ + 3PbX₂ ⇄ 4CsPbBr₃ + PbX₂ ⇄ 4CsPb₂X₅. These reactions do not occur at random, because the reactant and product nanocrystals are connected by the Cs⁺ cation substructure preservation principle, stating that ion trade reactions can transform one compound into another by means of distorting, expanding, or contracting their shared Cs⁺ cation substructure.The nanocrystal surface is a boundary between the core and the surrounding environment of Cs-Pb-X nanocrystals. The surface influences nanocrystal stability, optical properties, and shape. For these reasons, the dynamic surface of Cs-Pb-X nanocrystals has been studied in detail, especially in CsPbX₃ perovskites. Two takeaways have emerged from these studies. First, the competition between primary alkylammonium and cesium cations for the surface sites during the CsPbX₃ nanocrystal nucleation and growth governs the cube/plate shape equilibrium. Short-chain acids and branched amines influence that equilibrium and enable shape-shifting synthesis of pure CsPbX₃ cubes, nanoplatelets, nanosheets, or nanowires. Second, quaternary ammonium halides are emerging as superior ligands that extend the shelf life of Cs-Pb-X colloidal nanomaterials, boost their photoluminescence quantum yield, and prevent foreign ions from escaping the nanocrystals. That is accomplished by combining reduced ligand solubility, due to the branched organic ammonium cation, with the surface-healing capabilities of the halide counterions, which are small Lewis bases.