Alkylammonium Halides for Facet Reconstruction and Shape Modulation in Lead Halide Perovskite Nanocrystals

ConspectusThe interactions of halides and ammonium ions with lead halide perovskite nanocrystals have been extensively studied for improving their phase stability, controlling size, and enhancing their photoluminescence quantum yields. However, all these nanocrystals, which showed intense and color tunable emissions, mostly retained the six faceted cube or platelet shapes. Shape tuning needs the creation of new facets, and instead of composition variations by foreign ions interactions/substitutions, these require facet stabilizations with suitable ligands. Among most of the reported cases of lead halide perovskites, alkyl ammonium ions are used as a capping agent, which substituted in the surface Cs(I) sites of these nanocrystals. Hence, new surface ligands having a specific binding ability with different facets other than those in cube/platelet shapes are required for bringing stability to new facets and, hence, for tuning their shapes.In this Account, interactions of alkyl ammonium ions on the surface of perovskite nanocrystals and their impact on surface reconstructions are reviewed. Emphasizing the most widely studied CsPbBr₃ nanocrystals, the usefulness and impact of alkyl ammonium ions on the phase stability, high-temperature annealing, enhancement of the brightness and doping in these nanocrystals are first discussed. Then, nanocrystals formed under limited primary alkyl ammonium ions and also with specific tertiary ammonium ions having new facets are elaborated. Further, the treatment of excess alkyl ammonium halides to these newly formed multifaceted polyhedron nanocrystals under different conditions, which led to armed and step-armed structures, are discussed. The change in optical properties during these shape transformations is also presented. Finally, the shape-change mechanism with alkyl ammonium halide-induced dissolutions of {200} and {112} facets and formation of {110} and {002} facets are discussed. Further, in summary, future prospects of new ligand designing for stabilizing new facets of perovskite nanocrystals and obtaining new shapes and properties are proposed.