Enhanced structural and magnetic properties of fcc colloidal crystals of cobalt nanoparticles.

We report the elaboration of supercrystals made up of dodecanoic acid-coated 8.1 nm-Co nanocrystals with controlled supercrystallinity, morphology and magnetic properties. Supercrystal growth is controlled using a solvent-mediated ligand-ligand interaction strategy. Either face-centered cubic supercrystalline films or single colloidal crystals composed of cobalt nanocrystals are obtained. The change in supercrystal morphology is explained by Flory-type solvation theory using Hansen solubility colloidal parameters. The use of the same batch of Co nanocrystals for the fabrication of supercrystalline films and colloidal crystals enables accurate comparative structural and magnetic studies using (high-resolution) transmission electron microscopy, field emission gun scanning electron microscopy, grazing incidence small-angle X-ray scattering and vibrating sample magnetometry. The nearest neighbor distance between nanoparticles is interpreted using theoretical models proposed in the literature. We evidence the increase in both geometric anisotropy and magnetic dipolar interactions for colloidal crystals compared to supercrystalline films.

Simpler and Cleaner Synthesis of Variously Capped Cobalt Nanocrystals Applied in the Semihydrogenation of Alkynes.

Unlike the classical organometallic approach, we report here a synthetic pathway requiring no reducing sources or heating to produce homogeneous hexagonal-close-packed cobalt nanocrystals (Co NCs). Involving a disproportionation process, this simple and fast (6 min) synthesis is performed at room temperature in the presence of ecofriendly fatty alcohols to passivate Co NCs. Through a recycling step, the yield of Co NCs is improved and the waste generation is limited, making this synthetic route cleaner. After an easy exchange of the capping ligands, we applied them as unsupported catalysts in the stereoselective semihydrogenation of alkynes.

Hcp cobalt nanocrystals with high magnetic anisotropy prepared by easy one-pot synthesis.

Herein we describe the first synthesis of pure mono-disperse spherical hcp-nanocrystals ferromagnetic at room temperature. Our strategy, based on the simple combination of oleylamine and ClCo(PPh3)3, allows the one-pot synthesis of size-controlled hcp-nanocrystals. The size and shape of the nanocrystals can be tuned by varying the reaction time or the concentration.

Mesoscopic void structures in cobalt nanocrystal films formed from drying concentrated colloidal solutions.

Here we report the formation of void (hole) structures when concentrated colloidal solutions of magnetic nanocrystals are subjected to a magnetic field during slow evaporation. This presents a new type of solid mesostructure obtained by self-assembly of nanocrystals. The voids are characterized by a cylindrical shape with either circular or elliptical base. We show that the morphology of these patterns is essentially controlled by the fraction of the volume occupied by the magnetic phase to the total volume of the film. Monte Carlo simulations carried out using a Stockmayer fluid model agree remarkably well with the experiments for the formation of void structures in the range of considered volume fractions.

Tuning of copper nanocrystals optical properties with their shapes.

Copper nanocrystals are obtained by chemical reduction of copper ions in mixed reverse micelles. A large excess of reducing agent favors producing a new generation of shaped copper nanocrystals as nanodisks, elongated nanocrystals, and cubes. By using UV-Visible spectroscopy and numerical optical simulations we demonstrate that the optical properties are tuned by the relative proportions of spheres and nanodisks.

Anisotropic copper nanocrystals synthesized in a supersaturated medium: nanocrystal growth.

In the chemical reduction of copper ions in mixed reverse micelles it is found that a large excess of reducing agent favors the production of a new generation of copper nanocrystals. At low reducing agent concentration, nanocrystals are mostly spherical, while in the supersaturated regime, they have various shapes such as pentagons, squares, triangles, and elongated forms. The nanocrystal structures, characterized by high-resolution transmission electron microscopy, are based on the face-centered cubic structure. A tentative explanation for the growth mechanism of copper nanocrystals having various shapes is proposed.