Phosphine-Free Synthesis of High-Quality CdSe Nanocrystals in Noncoordination Solvents: “Activating Agent” and “Nucleating Agent” Controlled Nucleation and Growth

Abstract

CdSe nanocrystals (NCs) are prepared in noncoordination solvents (1-octadecene (ODE) and paraffin liquid) with long-chain primary alkylamine as the sole ligand, ODE-Se, and cadmium fatty acid salt as precursors. The obtained NCs meet the four fundamental parameters for high-quality NCs: high crystallinity, narrow size distribution, moderate photoluminescence quantum yield, and broad range size tunableness. Further, by simply regulating the relative molar ratio of alkylamine to cadmium precursor, the regular sized “nuclei” and final obtained NCs can be produced predictably within a certain size range. The size distribution of regular sized “nuclei” is very narrow (fwhm = 23 ± 1 nm), and the following focusing growth procedure vanishes. This indicates a different nucleation and growth kinetics from that of the well-established “focusing” and “defocusing” theory. By analyzing the conversion factor of precursors and the concentration of magic sized nanoclusters and regular sized “nuclei”, a subtle secondary nucleation mechanism, “quantized fusion”, was proposed. First, magic sized nanoclusters are formed as the critical nuclei; second, quantized critical nuclei will couple together and fuse into perfect regular sized “nuclei” by combining with stoichiometric monomers. The “quantized fusion” transformation could be regulated by the nature and ratios of the “activating agent” and the “nucleating agent”. These transition processes are very rapid and should be kinetically controlled. The molecular mechanism for monomer formation may be based on the traditional ester aminolysis reaction.