Enhanced magnetization of nanoscale colloidal cobalt particles

Seven-Nanometer Hexagonal Close Packed Cobalt Nanocrystals for High-Temperature Magnetic Applications through a Novel Annealing Process

Seven-nanometer cobalt nanocrystals are synthesized by colloidal chemistry. Gentle annealing induces a direct structural transition from a low crystalline state to the hexagonal close packed (hcp) phase without changing the size, size distribution, and the lauric acid passivating layer. The hcp structured nanocrystals can be easily redispersed in solvent for further application and processing. We found that the magnetization at saturation and the magnetic anisotropy are strongly modified through the annealing process. Monolayer self-assembly of the hcp cobalt nanocrystals is obtained, and due to the dipolar interaction, ferromagnetic behavior close to room temperature has been observed. This work demonstrates a novel approach for obtaining small size hcp structured cobalt magnetic nanocrystals for many technological applications.

The Effect of Membrane Charge on Gold Nanoparticle Synthesis via Surfactant Membranes

The effect of vesicle membrane structure and charge on the synthesis of gold nanoparticles was investigated. The vesicle membranes were comprised of either negatively charged soy lipids or mixtures of charge neutral and negatively charged soy lipids. Palladium ions bound to the membranes served as the catalyst for metal particle synthesis using an electroless metallization procedure. The size range of particles synthesized using membranes comprised of only negatively charged lipids (5-15 nm) was significantly smaller than those synthesized using mixtures of negatively charged and charge-neutral lipids (2-180 nm). X-ray diffraction revealed that the average crystallite size decreased with increasing palladium ion content of the membranes. It also showed that the average crystallite size was smaller for particles synthesized using vesicles comprised of only soy phoshohydroxyethanol lipids than for particles synthesized using vesicles comprised of only soy phosphatidic acid lipids. Particles synthesized with membranes comprised of only negatively charged lipids were encapsulated within the resulting lipid membrane matrix. FT-IR of the lipid matrix indicated that the matrix was formed as the result of ionic bridging of the lipid phosphate headgroups with gold ions. Copyright 1999 Academic Press.

Preparation and Properties of Monodisperse Magnetic Cobalt Colloids Grafted with Polyisobutene

A single-step method to synthesize monodisperse metallic cobalt particles of diameter around 8 nm is described. The particles are sterically stabilized by modified polyisobutene and form stable ferrofluids in toluene. The core-shell cobalt spheres have a narrow size distribution and are nonaggregated. Various techniques, such as infrared spectroscopy, X-ray diffraction, and elemental analysis establishes the presence of grafted modified polyisobutene on the surface of cobalt particles. The hydrodynamic thickness of the grafted polyisobutene calculated from sedimentation and viscosity measurements and independently from the dynamic light scattering is ca. 10 nm. The dependence of magnetization has been measured for ferrofluid, showing superparamagnetic behavior above 247 K. Due to anisotropy, superposition of magnetic curves is not observed below this temperature. The concentration dependence of the sedimentation coefficient agrees qualitatively with the theory for dipolar spheres. SAXS data on concentrated ferrofluid dispersion indicate interpenetration of polyisobutene chains grafted to the cobalt particles. Copyright 1998 Academic Press.

Size Dependence of Structural Metastability in Semiconductor Nanocrystals

The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases.