On-Surface Reactions in the Growth of High-Quality CdSe Nanocrystals in Nonpolar Solutions.

On-surface reaction mechanisms during the growth of high-quality CdSe nanocrystals are studied quantitatively and systematically by introducing a cyclic growth scheme. Prior to the repeating growth cycles, presynthesized CdSe QD seeds from a conventional scheme are reacted with an activated Se precursor, which is found to include three elementary steps and generate Se-terminated CdSe QDs. The cyclic growth in amine-octadecene solution includes two repeating half-reactions. The first half-reaction is between cadmium carboxylates in the bulk solution and the Se-terminated QDs, and the other is between the Se precursor in the bulk solution and the Cd-terminated QDs generated by the first half-reaction. While two elementary steps in the Se-surface half-reaction can be quantitatively treated as parallel kinetics, two elementary steps for the Cd-surface half-reaction must be treated as consecutive steps. These elementary steps are found to possess substantially different reaction rates as well as activation energies. Results indicate that, in the growth of compound semiconductor nanocrystals with metal carboxylates as cationic precursor (or ligands), the elementary step between activated anionic precursors in the bulk solution and the cationic sites on the surface of nanocrystals would be the rate-limiting step. This rate-limiting step should be the one that causes nucleation (or formation of small clusters by solution reactions) to be substantially faster than the corresponding growth through on-surface reactions.