Showing papers by "Harutoshi Takeo published in 1999"

Stable icosahedral nanoparticles in an as-grown Au–Fe alloy

Abstract: The morphologies of Au—Fe alloy nanoparticles with an 11% Fe mean atomic concentration were investigated by means of high-resolution electron microscopy. The icosahedral morphology was very frequently found in the particles that were generated by the vapor condensation method with helium and deposited on an amorphous carbon film. A high fraction of the icosahedral morphology still remained even after the supported particles were annealed at 723 K for 1 h in a vacuum; this suggests the minimum-energy structure for the alloy particles is icosahedral. As for the low production rate of the icosahedral morphology in the pure Au particles produced under the same conditions, the present results indicate that the icosahedral form is stabilized by the addition of Fe atoms into the Au particles.

Energy-resolved collision-induced dissociation of Aln+ clusters (n=2–11) in the center of mass energy range from few hundred meV to 10 eV

Abstract: Energy-resolved collision-induced dissociation (CID) of Aln+ (n=2–11) in collision with argon is presented for the energy ranges from few hundred meV to 10 eV in the center of mass frame. The experiments were carried out with a recently constructed secondary ion tandem mass spectrometer, that is described in detail. The collision energy dependence is measured for the total and the partial dissociation cross sections, and the dissociation thresholds for the individual processes are estimated. The release of Al+ is found to be the dominating channel for n 8, the cross section for the release of Al+ and Al are comparable. The release of more than one neutral atom from the larger clusters (n>6) is found to be in good agreement with sequential atom loss. In the case of the smaller clusters, on the other hand, fission is the energetically favorable process. The closed shell cluster, Al7+ (20 valence electrons), is found to be exceptionally stable and the adiabatic ionization potential of Al7 is found to be lower than that of the monomer. The stability of Al7+ is further reflected in the dissociation dynamics of the next neighbor, Al8+. The high stability of Al7+ as well as the dissociation dynamics of Al8+ are treated in the simple frame of the electronic shell model. Unlike Al7+, Al3+ (with 8 valence electrons) shows no sign of increased stability, and the dissociation dynamics seems to be controlled by the spin selection rules, rather than the energetics. In the present work, general trends and the dissociation dynamics of individual clusters are discussed. Qualitative information on the development of the geometric and electronic structure, with increasing cluster size, is deduced and discussed in terms of a transition from a covalent to a metallic character. Finally, this work is compared to earlier theoretical and experimental approaches to Aln+ clusters.

Optical and X-ray diffraction study of AgI clusters incorporated into zeolite LTA

Abstract: AgI clusters are incorporated into cages of zeolite LTA. The loading density of AgI is systematically changed from 0.2 to 4.0 molecules per α-cage. The electronic states and structures of the AgI clusters, with regard to their optical absorption spectra and X-ray powder diffraction patterns, are discussed. At a dilute loading, a notable absorption band, assigned to the AgI molecule, appears at 5.6 eV. With increasing loading densities, new absorption bands appear at 4.7, 4.2, 4.1, and 3.6 eV. These bands show that (AgI) n (n = 2 − 4) clusters with larger sizes are formed with increasing loading density. These clusters seem to exist in α-cages. The X-ray diffraction (XRD) patterns exhibit superlattice reflections at high loading densities. The appearance of the extra reflections has a correlation with the formation of (AgI)3 and (AgI)4 clusters. The Ag and I atoms of the cluster seem to be situated at the position of tetrahedral-based symmetry in the α-cage.