Role of Magic-Sized Clusters in the Synthesis of CdSe Nanorods

The dynamics of the CdSe nanorod synthesis reaction have been studied, giving attention to the kinetics of magic-sized clusters (MSCs) that form as intermediates in the overall reaction. The MSCs have a distinct absorption peak, and the kinetics of this peak give insight into the overall reaction mechanism. In these studies, the reaction mixture consists primarily of Cd(phosphonate)(2) and trioctyl phosphine selenium in a solution of trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO). We find that the rate at which precursors react to form CdSe monomers and the rates at which monomers react to form nanoparticles can be varied by changing the chemistry of the reaction mixture. Decreasing the TOP concentration decreases the extent to which selenium is bound, both in the precursors and on the particles' surfaces, and thereby increases both the precursor to monomer and monomer to particle reaction rates. Decreasing the phosphonate concentration decreases the extent to which phosphonate binds cadmium in the precursors and on the surface of the nanoparticles, also increasing the rates of both reactions. This is also accomplished by the addition of inorganic acids which protonate the phosphonates. The presence of inorganic acids (impurities) is the primary reason that the overall synthesis reaction is faster in solutions made with technical grade rather than purified TOPO. The TOP and phosphonic acid concentrations are coupled because excess phosphonic acids react with TOP, forming TOPO and less strongly binding species, specifically phosphinic acids, phosphine oxides, and phosphines.