Ligand-Directed Stabilization of Ternary Phases: Synthetic Control of Structural Dimensionality in Solution-Grown Cesium Lead Bromide Nanocrystals

Lead halide perovskites are a versatile class of semiconductors that provide considerable opportunities for tunability of absorption maxima, exciton binding energies, and band gaps as a function of composition and dimensional confinement. Considerable attention has focused on the design and synthesis of related frameworks with reduced structural dimensionality that provide an expanded palette of optical transitions with varying degrees of exciton localization. In this work, we demonstrate the ligand-mediated navigation of the cesium—lead—bromine ternary phase diagram demarcating distinctive regimes wherein 3D CsPbBr3 and 0D Cs4PbBr6 nanocrystals can be stabilized. The denticity, steric bulk, and concentration of aliphatic amine ligands strongly modifies the supersaturation of lead monomers, scaling proportionately to their complexation coefficients and ability to form ordered passivating ligand shells. The added ligands strongly alter the trajectory of nucleation and growth processes, stabilizing either Pb-rich or Pb-deficient compositions across the ternary phase diagram. These parameters furthermore exert considerable influence on the physical dimensions of the obtained nanocrystals. By altering the monomer supersaturation and dynamics of crystal growth, the molecular amines thus provide a means of controlling both structural dimensionality and nanocrystal size. The reversible interconversion of CsPbBr3 and Cs4PbBr6 is furthermore illustrated upon the ligand-mediated addition/leaching of PbBr2. A dissolution—reprecipitation process with signatures of the inverse Kirkendall effect is observed to bring about the transformation of ordered stacks of CsPbBr3 nanoplatelets to single-crystalline hexagonal Cs4PbBr6 nanoplatelets with well-defined facets.