Kenji Sumiyama

Synthesis of Ag–In binary sulfide nanoparticles—structural tuning and their photoluminescence properties

In this report, we demonstrate the synthesis of binary sulfide Ag–In–S NPs using a Ag–In thiolate complex. Thermal decomposition of the thiolate complex provides Ag/AgInS2 heterostructured nanoparticles (NPs). A metathesis reaction between the thiolate complex and sulfur source leads to the formation of nearly monodispersed AgInS2 NPs with a chalcopyrite-like or orthorhombic structure. AgInS2 NPs with a chalcopyrite-like structure exhibited room temperature photoluminescence (PL). Spectral shift of the PL band depending on the excitation laser intensity and characteristic behavior of the PL decay time varying over a wide energy range within the PL band were observed. These results indicate that the PL of the AgInS2 NPs may be attributed to the donor–acceptor (D–A) pair recombination.

Magnetic carriers of iron nanoparticles coated with a functional polymer for high throughput bioscreening

We synthesized iron nanoparticles, 15–20 nm in size, having saturation magnetization, Ms, of 170emu∕g. They were embedded in copolymer beads of styrene (St) and glycidyl methacrylate (GMA), which were coated with poly-GMA by seed polymerization. The resultant Fe∕St-GMA/GMA beads had diameters of 100–200 nm and Ms of 60emu∕g. By coating with poly-GMA, the zeta potential of the beads changed from −93.7 to −54.8mV, as measured by an electrophoresis method. This facilitated (as revealed by gel electrophoresis method) nonspecific protein adsorption suppression, a requisite for nanoparticles to be applied to carriers for bioscreening.

Phase control and its mechanism of CuInS2 nanoparticles.

CuInS(2) nanoparticles (NPs) usually take chalcopyrite-(CP) structure. Recently, CuInS(2) NPs with pseudo-wurtzite (WZ) structure, which is thermodynamically less favored, have been synthesized. However, the formation mechanism of this metastable-phase has not been understood yet. In this report, the key issue of phase selectivity of CuInS(2) (CIS) NPs has been investigated using various metal sources and ligands. Experimental results suggested that the crystalline structure and morphology of CIS NPs were decided by the stability of indium ligand complex; the active ligand reduces the precipitation rate of In(2)S(3), resulting in pre-generation of Cu(2)S seed NPs. Crystallographic analogy and superionic conductivity of Cu(2)S remind us that the formation of WZ CIS NPs is attributed to the pre-generation of Cu(2)S seed NPs and the following cation exchange reaction. In order to confirm this hypothesis, Cu(2-)(x)S seed NPs with various structures have been annealed in indium-ligand solution. This experiment revealed that the crystalline structure of CIS NP was determined by that of pre-generation Cu(2-)(x)S NPs. Our results provide the important information for the phase control and synthesis of ternary chalcogenide NPs with a novel crystalline structure.

Effect of Sulfur Concentration on the Morphology of Carbon Nanofibers Produced from a Botanical Hydrocarbon

Carbon nanofibers (CNF) with diameters of 20–130 nm with different morphologies were obtained from a botanical hydrocarbon: Turpentine oil, using ferrocene as catalyst source and sulfur as a promoter by simple spray pyrolysis method at 1,000 °C. The influence of sulfur concentration on the morphology of the carbon nanofibers was investigated. SEM, TEM, Raman, TGA/DTA, and BET surface area were employed to characterize the as-prepared samples. TEM analysis confirms that as-prepared CNFs have a very sharp tip, bamboo shape, open end, hemispherical cap, pipe like morphology, and metal particle trapped inside the wide hollow core. It is observed that sulfur plays an important role to promote or inhibit the CNF growth. Addition of sulfur to the solution of ferrocene and turpentine oil mixture was found to be very effective in promoting the growth of CNF. Without addition of sulfur, carbonaceous product was very less and mainly soot was formed. At high concentration of sulfur inhibit the growth of CNFs. Hence the yield of CNFs was optimized for a given sulfur concentration.

Formation of ordered CoAl alloy clusters by the plasma-gas condensation technique

CoxAl1−x alloy clusters were synthesized from a mixture of Co and Al metal vapors generated by the sputtering of pure metal targets. We observed that the produced alloy clusters were uniform in size, ranging from approximately 20 nm for Al-rich clusters to 10 nm for Co-rich clusters. For a wide average composition range (x≈0.4–0.7), the alloy clusters have the ordered B2 (CsCl-type) structure. In the Co-rich cluster aggregates (x=0.76), the clusters are composed of face-centered-cubic (fcc) Co and minor CoAl(B2) clusters. In the Al-rich aggregates (x=0.23), the clusters are mainly composed of the fcc-Al phase, although clusters occasionally possess a “core-shell structure” with the CoAl(B2) phase surrounded by an Al-rich amorphous phase. These observations are in general agreement with our prediction based on the equilibrium phase diagram. We also noticed that the average composition depends not only on the relative amount of Co and Al vapors, but also on their absolute amount, and even on the Ar gas flow...

Soft magnetic property and magnetic exchange correlation in high-density Fe-Co alloy cluster assemblies

Magnetically soft Fe-Co alloy cluster-assembled films were produced at room temperature by an energetic cluster deposition. Size-monodispersed Fe-Co alloy clusters whose average sizes ranged between 8 and 14 nm were obtained using a plasma-gas-condensation-type cluster deposition apparatus. Positively charged clusters in a cluster beam were accelerated electrically and deposited onto a negatively biased substrate together with neutral clusters from the same cluster source, leading to formation of high-density Fe-Co alloy cluster-assembled films. Magnetic properties were observed for these films prepared by changing the bias voltage. Based on the temperature dependence of magnetic coercivity, we estimated the effective magnetic anisotropy constant, Keff, and magnetic exchange length Lex (or Dex) as a function of the packing fraction in the Fe-Co cluster-assembled films: Keff decreases and Lex (or Dex) increases dramatically with the packing fraction.

Shape-induced simple cubic arrangement in three-dimensional nanocube self-assemblies

The simple cubic packing rarely appears in nature primarily due to its low packing density. Here, we present that the cubic particle shape induces a preferred formation of three-dimensional arrays of simple cubic arrangement, confirmed by transmission electron microscopy and small-angle x-ray scattering. The calculation of van der Waals interaction energy between the particle cores showed that the highly coordinated, simple cubic packing is energetically the most stable, being consistent with the present experimental result.

Random dipolar ferromagnetism in Co∕CoO core-shell cluster assemblies observed by electron holography

The formation of large-scale ferromagnetic domains has been detected in quasi-three-dimensional random assemblies of monodisperse 6nm Co∕CoO core-shell clusters by electron holography. This is primarily attributed to the magnetic dipole interactions between these clusters, because the exchange coupling between neighboring Co cores is effectively interrupted by antiferromagnetic CoO surface layers. The magnetic field-cooled effect has been also detected by in-situ electron microscope experiment. The field-induced alignment of magnetic moments during the cooling experiment is ascribable to the exchange interactions between Co cores and CoO shells.

Growth of Vertically Aligned Carbon Nanotubes from Highly Active Fe–Ti–O Nanoparticles Prepared by Liquid-Phase Synthesis

Monodispersed Fe–Ti–O nanoparticles and Fe–O nanoparticles were prepared by liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs were obtained in the case of Fe–Ti–O nanoparticles, whereas few CNTs were grown when Fe–O particles were used. CNTs with a diameter of 4.3±0.7 nm were grown from Fe–10 at. % Ti nanoparticles with a diameter of 3.8±0.6 nm. These CNTs mainly consist of double- and triple-walled CNTs. The catalytic activity of nanoparticles peaks at around 20–25 at. % Ti. The diameter controllability of Fe–20 at. % Ti nanoparticles was lower than that of Fe–10 at. % Ti nanoparticles, although a 200-µm-thick CNT forest was grown within 10 min. The energy-dispersive X-ray spectroscopy (EDS) analysis of the Fe–Ti–O nanoparticles before and after CNT growth revealed the depletion of Ti in the nanoparticles during the CNT growth, indicating the extraction of Ti from the nanoparticles.

Composite state control and magnetic properties of Co and Si cluster assemblies prepared with double-glow-discharge sources

Using a double-glow-discharge-cluster-source system, in which one glow discharge was a dc mode and the other an rf discharge mode, Co and Si clusters were independently produced and simultaneously deposited on a substrate. When a separation plate was inserted between two glow-discharge chambers, a mixture of Co and Si clusters was obtained: small Co clusters were distributed at random, while the Si clusters were aggregated to form large secondary particles. Without inserting the separation plate, on the other hand, core-shell clusters were obtained: a Co core was surrounded by small Si crystallites. The magnetization measurement indicated that the magnetic coercive force of Co∕Si core-shell cluster assemblies was much smaller than that of Co cluster assemblies in which Co clusters were covered with antiferromagnetic CoO shells, indicating that the Si shell prevented Co cluster surfaces from their oxidation. Therefore, the present double-cluster-source system is useful in fabricating various sorts of clust...