Persistent nucleation and size dependent attachment kinetics produce monodisperse PbS nanocrystals

International audienceModern syntheses of colloidal nanocrystals yield extraordinarily narrow size distributions that are believed toresult from a rapid “burst of nucleation” (La Mer, JACS, 1950, 72(11), 4847–4854) followed by diffusionlimited growth and size distribution focusing (Reiss, J. Chem. Phys., 1951, 19, 482). Using a combinationof in situ X-ray scattering, optical absorption, and 13C nuclear magnetic resonance (NMR) spectroscopy,we monitor the kinetics of PbS solute generation, nucleation, and crystal growth from three thioureaprecursors whose conversion reactivity spans a 2-fold range. In all three cases, nucleation is found to beslow and continues during >50% of the precipitation. A population balance model based on a sizedependent growth law (1/r) fits the data with a single growth rate constant (kG) across all threeprecursors. However, the magnitude of the kG and the lack of solvent viscosity dependence indicatesthat the rate limiting step is not diffusion from solution to the nanoparticle surface. Several surfacereaction limited mechanisms and a ligand penetration model that fits data our experiments using a singlefit parameter are proposed to explain the result