Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes:  Nucleation and Growth

The nucleation and growth of colloidal CdSe nanocrystals with a variety of elongated shapes were explored in detail. The critical size nuclei for the system were magic sized nanoclusters, which possessed a sharp and dominated absorption peak at 349 nm. The formation of the unique magic sized nuclei in a broad monomer concentration range was not expected by the classic nucleation theory. We propose that this was a result of the extremely high chemical potential environment, that is, very high monomer concentrations in the solution, required for the growth of those elongated nanocrystals. The shape, size, and size/shape distributions of the resulting nanocrystals were all determined by two related factors, the magic sized nuclei and the concentration of the remaining monomers after the initial nucleation stage. Without any size sorting, nearly monodisperse CdSe quantum structures with different shapes were reproducibly synthesized by using the alternative cadmium precursors, cadmium-phosphonic acid complexes. A reasonably large excess of the cadmium precursor, which is less reactive than the Se precursor, was found beneficial for the system to reach the desired balance between nucleation and growth. The shape evolution and growth kinetics of these elongated nanocrystals were consistent with the diffusion-controlled model proposed previously. The branched nanocrystals had to grow at very high monomer concentrations because the multiple growth centers at the end of each branch must be fed with a very high diffusion flux to keep all branches in the 1D-growth mode. The rice-shaped nanocrystals were found as special products of the 3D-growth stage. The growth of the nanocrystals in the 1D-growth stage was proven to be not unidirectional after the length of the nanocrystals reached a certain threshold. Experimental results indicate that coordinating solvents and two ligands with distinguishable coordinating abilities are both not intrinsic requirements for the growth of elongated CdSe nanocrystals.