From Hot-Injection Synthesis to Heating-Up Synthesis of Cobalt Nanoparticles: Observation of Kinetically Controllable Nucleation

Just as nanoparticles display properties that differ from those of bulk samples of the same material, ensembles of nanoparticles can have collective properties that are different to those displayed by individual nanoparticles and bulk samples. Self-assembly has emerged as a powerful technique for controlling the structure and properties of ensembles of inorganic nanoparticles. Here we review different strategies for nanoparticle self-assembly, the properties of self-assembled structures of nanoparticles, and potential applications of such structures. Many of these properties and possible applications rely on our ability to control the interactions between the electronic, magnetic and optical properties of the individual nanoparticles.

Much progress has been made over the past ten years on the synthesis of monodisperse spherical nanocrystals. Mechanistic studies have shown that monodisperse nanocrystals are produced when the burst of nucleation that enables separation of the nucleation and growth processes is combined with the subsequent diffusion-controlled growth process through which the crystal size is determined. Several chemical methods have been used to synthesize uniform nanocrystals of metals, metal oxides, and metal chalcogenides. Monodisperse nanocrystals of CdSe, Co, and other materials have been generated in surfactant solution by nucleation induced at high temperature, and subsequent aging and size selection. Monodisperse nanocrystals of many metals and metal oxides, including magnetic ferrites, have been synthesized directly by thermal decomposition of metal-surfactant complexes prepared from the metal precursors and surfactants. Nonhydrolytic sol-gel reactions have been used to synthesize various transition-metal-oxide nanocrystals. Monodisperse gold nanocrystals have been obtained from polydisperse samples by digestive-ripening processes. Uniform-sized nanocrystals of gold, silver, platinum, and palladium have been synthesized by polyol processes in which metal salts are reduced by alcohols in the presence of appropriate surfactants.

We show that a relatively simple approach for controlling the colloidal synthesis of anisotropic cadmium selenide semiconductor nanorods can be extended to the size-controlled preparation of magnetic cobalt nanorods as well as spherically shaped nanocrystals. This approach helps define a minimum feature set needed to separately control the sizes and shapes of nanocrystals. The resulting cobalt nanocrystals produce interesting two- and three-dimensional superstructures, including ribbons of nanorods.