Ion-Mediated Ligand Exchange and Size Focusing of Semiconductor Nanocrystals in Ligand-Saturated Solutions

Controlling the morphology of colloidal semiconductor nanocrystals (NCs) remains to be a challenging task. Traditional growth strategies employ a high concentration of monomers to promote particle nucleation, which tends to oversaturate the solution with reactive species. This leads to secondary nucleation events and other dispersion-broadening processes. Here, we explore monomer-deprived synthetic conditions as a bilateral strategy for tuning both the shape and the surface-ligand chemistry of semiconductor colloids. Rather than controlling the nucleation phase, the present method employs a postsynthetic treatment based on low-temperature digestive ripening, where small particles grow at the expense of larger ones. The feasibility of the present approach was demonstrated by observing a 4-fold reduction in the CdSe nanoparticle size dispersion during the digestive ripening reaction, which was induced by high concentrations of L-type (amines) or X-type (oleic acid) ion-solubilizing ligands. In the latter case, a classically forbidden L → X ligand exchange was enabled by the concurrent process of the surface ion diffusion. The size-focusing capacity of the technique were subsequently demonstrated by ripening ZnSe NCs, which shape homogeneity is generally difficult to achieve.