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.

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.

2D self-organization of core/shell Co(hcp)/Co nanocrystals.

In this paper we report the preparation of ordered hexagonal 2D arrays of core/shell Cohcp/CoO nanocrystals. A full structural investigation has been carried out using high-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy.

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.

Self assemblies of nanocrystals: preparation, collective properties and uses.

Self-organization of nanocrystals depends on the type of nanomaterial and the coating. With silver nanocrystals, the self-organization is partially perturbed by the latter. With cobalt nanocrystals, the size distribution and thus the self-organization is controlled by the amount of reducing agent added during the chemical reaction and not by the micellar solution. It is possible to make "supra" crystals of cobalt nanoparticles in a face centered cubic (fcc) structure. By applying, during the evaporation process, an external magnetic field to ferrite nanocrystals dispersed in solution, nanocrystal organizations markedly change with their coating. Hence tubes are obtained with nanocrystals coated with citrate ions whereas thick films are produced when the coating is replaced by dodecanoic acid. Collective magnetic properties due to the organization in tubes are observed with a behavior similar to that observed with nanowires. The use of nanocrystals as a mask to produce various patterns on silicon is described.