Growth and Self-Assembly of CsPbBr Nanocrystals in the TOPO/PbBr Synthesis as Seen with X-ray Scattering.

Despite broad interest in colloidal lead halide perovskite nanocrystals (LHP NCs), their intrinsic fast growth has prevented controlled synthesis of small, monodisperse crystals and insights into the reaction mechanism. Recently, a much slower synthesis of LHP NCs with extreme size control has been reported, based on diluted TOPO/PbBr precursors and a diisooctylphosphinate capping ligand. We report new insights into the nucleation, growth, and self-assembly in this reaction, obtained by synchrotron-based small-angle X-ray scattering and optical absorption spectroscopy. We show that dispersed 3 nm Cs[PbBr] agglomerates are the key intermediate species: first, they slowly nucleate into crystals, and then they release Cs[PbBr] monomers for further growth of the crystals. We show the merits of a low Cs[PbBr] monomer concentration for the reaction based on oleate ligands. We also examine the spontaneous superlattice formation mechanism occurring when the growing nanocrystals in the solvent reach a critical size of 11.6 nm.