Colloidal nanocrystals: synthesis and shape-control, interparticle interactions & self-assembly
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Control over nanocrystal growth kinetics provides a powerful way of tailoring particle size and shape during synthesis. Investigations into the growth of gold nanocrystals demonstrated how reaction conditions can be adjusted to control the growth rate and produce monodisperse particles. Kinetic control during the synthesis of CdS, CdSe and CdTe nanoparticles allows the shape to be tuned, from rods to spheres, without modifying the reaction chemistry. The growth and optical properties of these shapeanisotropic semiconductor particles were studied, and these methods were extended to produce semiconductor heterostructure nanorods. Solvent-mediated interparticle interactions between nanocrystals dispersed in toluene and in supercritical carbon dioxide were also studied. Nanocrystal dispersions were characterized using small-angle X-ray scattering in order to obtain information about the pair interaction potential. In organic solvents, subtle differences in the concentration-dependent scattering from dispersions allowed second virial coefficients to be measured as a function of nanocrystal size. Interestingly, larger nanocrystals exhibited overall repulsive interactions, while smaller nanocrystals exhibited attractive interactions, which is likely due to differences in ligand coverage among the different sized particles. Nanocrystals coated with fluorinated ligands could be dispersed into supercritical carbon dioxide, and the relatively strong interparticle interactions were measured at different carbon dioxide densities. As expected, the interaction strength increased as the solvent density was lowered, due to a decreased ability of the solvent to solvate the capping ligands. The formation of metastable nanocrystal flocculates was also observed at all system conditions studied. The assembly of nanocrystals into ordered superlattices under equilibrium conditions is strongly influenced by nanocrystal interparticle interactions. The formation of binary superlattices was studied, and an ordered AB phase was observed from the coassembly of small gold and large iron nanocrystals. A non-equilibrium route, breath figure templating, was also used to produce nanocrystal films with hierarchal order and porous polymer films. Evaporation of a nanocrystal or polymer dispersion in a humid atmosphere causes water droplets to nucleate and grow at the solvent-air interface. The solute stabilizes the water droplets which assemble into ordered arrays to template the drying film. The design rules for producing macroporous nanocrystal and polymer films are discussed.