K. Sumiyama

Magic numbers in transition metal (Fe, Ti, Zr, Nb, and Ta) clusters observed by time-of-flight mass spectrometry

We have measured time-of-flight (TOF) mass spectra of transition metal free clusters, TMn (TM=Fe, Ti, Zr, Nb, and Ta and n is the number of atoms per cluster), produced by a laser vaporization source. The size resolved TOF intensities at n=7, 13, 15 are much higher than those at the neighboring n values for all TMn. Such specific n values are assigned to the magic numbers of these transition metal clusters and can be related to pentagonal bipyramid, icosahedron, and bcc structure units. The other magic numbers are observed for larger TMn: n=19 and 23 for Fe, n=19 and 25 for Ti, being attributable to the polyicosahedron. The TOF spectra of Nb and Ta clusters are similar to each other and display the common magic number of n=22.

Formation and size control of a Ni cluster by plasma gas condensation

We have constructed a plasma-gas-condensation type cluster deposition apparatus and tried to find the optimum operation conditions for controlling the cluster size. Transmission electron microscope (TEM) observation has been done to evaluate sizes of Ni clusters produced when varying the volume of a cluster growth region, sputtering power, and inert gas pressure. The mean cluster size decreases by decreasing the volume of growth region and the sputtering power. The smallest cluster obtained in this work is about 2.3 nm in diameter. We have considered the following two models for the cluster growth: (1) a cluster–cluster collision growth and (2) an atomic vapor condensation growth. The cluster growth speed estimated from the former is too slow, while that from the latter is reasonable in comparison with the present experiments. When stable embryos are made from atom collisions, they grow up faster and the final cluster sizes estimated from the latter model are consistent with those observed by TEM.

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...

Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies

We systematically studied structural and magnetic characteristics of size- monodispersed Fe and oxide-coated Fe cluster assemblies with the mean cluster sizes of 7–16 nm. Transmission electron microscopy and scanning electron microscopy (SEM) observations show that the Fe clusters in the assemblies maintain their original size at room temperature. In the SEM images, a random stacking of the Fe clusters and a porous structure with a low cluster packing fraction of about 25% are observed. For the Fe cluster assemblies, magnetic coercivity (Hc) at room temperature increases from 4×101 to 4×102 Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at T⩾100 K and the fitting values of blocking temperature TB from Hc=Hc0[1−(T/TB)1/2], we estimated the values of magnetic anisotropy constant K of the order of 106 erg/cm3 from TB=KV/25kB, which is larger by an order of magnitude than the bulk Fe value (5×105 erg/cm3). Such a large effective anisotropy at T⩾100 ...

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...

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.

Magnetic properties and magnetoresistance in small iron oxide cluster assemblies

We report the magnetic properties and magnetoresistance (MR) in small iron oxide (Fe3−xO4 and Fe3O4) cluster assemblies. Half-metallic Fe3O4 cluster assembly with grain size of 10–15 nm is shown to exhibit a MR value of about 8% at T=30 K and a peak around the Verwey transition temperature Tv=115 K which is a little lower than the Tv value (∼120 K) of single crystal specimens. Even at T=5 K, the magnetization is not saturated in fields up to 50 kOe. The MR behaviors of a Fe3−xO4-coated iron cluster assembly and a sample which was prepared by embedding the Fe3−xO4-coated iron clusters into a MgO matrix are also studied for comparison. The MR value of the latter is over one time larger than that of the former and is also larger than those of the Fe3O4 cluster assembly at various temperatures. It suggests that the barrier layer is important for enhancing the MR effect at high temperatures.

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.

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.

Structure and magnetic properties of Co/CoO and Co/Si core–shell cluster assemblies prepared via gas-phase

Abstract This review article summarizes briefly some important achievements of our recent reserach on anatase and/or rutile TiO2 thin films, fabricated by helicon RF magnetron sputtering, with good crystal quality and high density, and gives the-state-of-the-art of the knowledge on systematic interrelationship for fabrication conditions, crystal structure, composition, optical properties, and bactericidal abilities, and on the effective surface treatment to improve the optical reactivity of the obtained films.