Saeki Yamamuro

Gold-coated iron nanoparticles for biomedical applications

We describe the magnetic properties of gold-coated iron nanoparticles, and the effects of low pH and heat treatment. Acicular iron and spherical iron based nanoparticles were coated with a thin layer of gold. The morphology and magnetic properties of magnetic particles were examined using transmission electron microscopy and alternative gradient magnetometry. While the small spherical particles had relatively uniform layers coatings, the larger acicular particles had many gold clusters decorating the surface. The original acicular iron nanoparticles had a specific magnetic moment of 145 emu/g and a coercivity of 1664 Oe. Corrosion tests showed good corrosion resistance for gold-coated commercial iron particles even in a 1.0×10−3 M HCl solution at 80 °C for 12 h, compared with uncoated particles.

Iron particles nesting in carbon cages grown by arc discharge

Fine particles of iron and iron carbide wrapped in multilayered graphitic sheets, which were synthesized by arc discharge of carbon rods containing iron oxide (Fe2O3), were studied by transmission electron microscopy. The size of the wrapped particles was typically in the range 20–200 nm. Two kinds of nested materials were found; one was α-Fe and the other, Fe3C (cementite).

MONODISPERSED CR CLUSTER FORMATION BY PLASMA-GAS-CONDENSATION

Nanometer-sized Cr clusters in the size range of 7.6–13 nm have been produced by a plasma-gas-condensation-type cluster deposition apparatus, which combines a grow-discharge sputtering technique with an inert gas condensation technique. We have studied influences of the Ar gas pressure, PAr, and the Ar gas flow rate, VAr, on the size distribution of Cr clusters by transmission electron microscopy. Monodispersed Cr clusters are formed at both low PAr and low VAr. At low PAr, Cr clusters nucleate and grow only in the liquid-nitrogen-cooled growth region, and the deposition rate is rather low. At high PAr, on the other hand, a large amount of Cr clusters are formed even near the sputtering source, and the nucleation and growth occur over a wide region between the sputtering source and the growth region. Under this condition, the deposition rate is relatively high. Consequently, the formation mechanism of the monodispersed clusters is similar to that of monodispersed colloidal particles: The nucleation and gr...

Cobalt particles wrapped in graphitic carbon prepared by an arc discharge method

Fine particles of cobalt and cobalt carbide nesting in multilayered graphitic sheets, which were synthesized by an electric arc discharge of carbon rods containing cobalt oxide (CoO), were studied by transmission electron microscopy, including microdiffraction and energy dispersive x‐ray analysis. The size of the wrapped particles was typically in a range from 50 to 200 nm. Three phases of nested materials, hcp(α)‐Co, fcc(β)‐Co, and Co3C, were identified.

Morphological and magnetic characteristics of monodispersed Co-cluster assemblies

Co-cluster-assembled films have been prepared using a size-controllable cluster beam deposition system, by which monodispersed Co clusters with a mean diameter, d=6–13 nm are available. Their morphology and magnetic properties have been studied by scanning electron microscopy (SEM), small-angle x-ray scattering (SAXS) and magnetization measurements. The SEM images show that the film has a porous structure consisting of fine grains without a columnar texture and its density is about 25% of the bulk Co. The SAXS measurements indicate that monodispersivity of the incident clusters is maintained through their assembling process only for d=13 nm. All the specimens exhibit ferromagnetic behavior at room temperature and the magnetic coercive field Hc rapidly increases with decreasing temperature: Hc=168 kA/m (2.1 kOe) at 5 K. Such an enhancement in Hc is ascribed to the exchange anisotropy which arises from the antiferromagnetic Co–oxide layers covering the Co clusters, and to the assemblies of single-domain fer...

Phase transformation and magnetic moment in FePt nanoparticles

The phase transformation from fcc to L1(0) in FePt nanoparticles was investigated in both thick film samples and self-assembled arrays as a function of the annealing temperature, using transmission electron microscopy, x-ray diffraction, differential scanning calorimetry, and magnetometry. A significant fraction of the surfactant decomposes into gaseous products below 500degreesC, removing the steric barrier between particle cores. This causes the particles to coalesce at the same annealing temperatures where the transformation to the high anisotropy phase occurs

Synthesis of copper and zinc sulfide nanocrystals via thermolysis of the polymetallic thiolate cage

Abstract In this paper report on the synthesis of copper and zinc sulfide nanocrystals (NCs) via the formation of polymetallic thiolate cages. Cu2S NCs derived from Cu–dodecanethiol complex formed well-defined spherers, which were sufficiently monodisperse (with a size distribution of ∼10% standard deviation of approximately 4.7 nm diameter on average) to generate ordered self-assemblies. An electron diffraction pattern and UV–vis spectrum of Cu2S NCs indicate that this process can provide pure b-chalcocite (Cu2S). Nearly monodisperse ZnS NCs with a size ranging from 3 to 7 nm were obtained by thermolysis of the S–Zn–dodecanethiol precursor. The electron diffraction pattern indicates that zinc sulfide NCs are either wurtzite or a mixture of wurtzite and zincblende. TEM observation and UV–vis spectra revealed that the growth rate of ZnS NCs depends strongly on the annealing temperature. UV–vis spectra of 3 nm ZnS NCs show sharp excitonic features and a large blue shift from the bulk material. The photoluminescence spectra exhibit a large red shift from the absorption band edges. These shifts could be attributed to recombination from the surface traps. The narrow size distribution of Cu2S and ZnS NCs led to the formation of ordered self-assemblies with various well-defined but nonclosed-packing.

Geometrical and electrical percolation in nanometre-sized Co-cluster assemblies

We deposited monodispersed Co-clusters in the size range of 6-13 nm on substrates using the plasma-gas-condensation cluster deposition system. The assembling process of the clusters from discontinuous to continuous networks was investigated by transmission electron microscopy (TEM) and in situ electrical conductivity measurement, and discussed in terms of the two-dimensional (2D) percolation concept. The electrical conductivity measurement indicates that the percolation process of Co clusters does not agree with a simple scaling-law: the critical conductivity exponent increases with increasing mean cluster diameter, d, although it is predicted to be independent of d in the ordinary 2D percolation theory. This anomaly is interpreted by the soft-percolation model, implying that there is distribution of electrical contacts between the clusters. The critical coverage of clusters (0.63) is much higher than the predicted one (0.45) irrespective of d, due mainly to the partial overlapping of deposited clusters, and partly to an attractive interaction between the clusters. Such cluster-overlapping also increases the critical thickness of electrical percolation with increasing d.

Direct visualization of dipolar ferromagnetic domain structures in Co nanoparticle monolayers by electron holography

Electron holography has revealed dipolar ferromagnetic domain structures in ordered monolayer arrays of Co nanoparticles. In zero-field-cooling experiments, we demonstrate the formation of micron-sized magnetically ordered regions with transverse domain walls, and a spatially varying moment order parameter. Truncated vortex structures that can be modified by an applied field are observed near the sample edges. The collectively magnetized state shows soft magnetic properties and long-range order that is stable over times over 1h at 108K.

Characteristic tunnel-type conductivity and magnetoresistance in a CoO-coated monodispersive Co cluster assembly

We have studied electrical conductivity, σ, and magnetoresistance in a CoO-coated monodispersive Co cluster assembly fabricated by a plasma–gas–aggregation-type cluster beam deposition technique. The temperature dependence of σ is described in the form of log σ vs 1/T for 7<T<80 K. The magnetoresistance ratio (ρ0−ρ3T)/ρ0 increases sharply with decreasing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase (by a factor of 6) is much larger than those observed for conventional metal–insulator granular systems. These results are ascribed to the Coulomb blockade effect in the monodispersed cluster assemblies.