Hideo Kohno

Atomic-Scale In-situ Observation of Carbon Nanotube Growth from Solid State Iron Carbide Nanoparticles

We have first observed the nucleation and growth process of carbon nanotubes (CNTs) from iron carbide (Fe 3C) nanoparticles in chemical vapor deposition with C 2H 2 by in situ environmental transmission electron microscopy. Graphitic networks are formed on the fluctuating iron carbide nanoparticles, and subsequently CNTs are expelled from them. Our atomic scale observations suggest that carbon atoms diffuse through the bulk of iron carbide nanoparticles during the growth of CNTs.

Self-organized chain of crystalline-silicon nanospheres

We have fabricated a self-organized chain of crystalline-silicon nanospheres via an extension of the vapor-liquid-solid mechanism. Transmission electron microscopy, electron energy-loss spectroscopy, and electron-induced x-ray fluorescence analyses have proved that the crystalline silicon nanospheres, of about 10 nm in diameter and at a nearly equal spacing, are supported in amorphous silica and carbon. The novel self-organized phenomenon is attributed to the periodic instability of catalysts and spontaneous oxidization during the growth of nanowhiskers.

Atomic-scale analysis on the role of molybdenum in iron-catalyzed carbon nanotube growth.

We have elucidated the synergetic role played by molybdenum in iron-catalyzed chemical vapor deposition growth of carbon nanotubes (CNTs) by in situ environmental transmission electron microscopy. Molybdenum can be well accommodated by Fe-based carbide nanoparticle catalysts of M(23)C(6)-type structure (M = Fe and Mo). We have also shown that molybdenum suppresses the nucleation of iron compounds that are known to exhibit no catalytic activity for the growth of CNTs.

Stepwise displacement of catalytically active gold nanoparticles on cerium oxide.

Aberration-corrected environmental transmission electron microscopy (ETEM) proved that catalytically active gold nanoparticles (AuNPs) move reversibly and stepwise by approximately 0.09 nm on a cerium oxide (CeO2) support surface at room temperature and in a reaction environment. The lateral displacements and rotations occur back and forth between equivalent sites, indicating that AuNPs are loosely bound to oxygen-terminated CeO2 and may migrate on the surface with low activation energy. The AuNPs are likely anchored to oxygen-deficient sites. Observations indicate that the most probable activation sites in gold nanoparticulate catalysts, which are the perimeter interfaces between an AuNP and a support, are not structurally rigid.

Temperature-Dependent Change in Shape of Platinum Nanoparticles Supported on CeO2 during Catalytic Reactions

The temperature dependence of the shape of Pt nanoparticles supported on CeO2 for CO oxidation was investigated using environmental transmission electron microscopy. Pt nanoparticles in CO/air become round at room temperature (when catalytic activity is low), while they become partially faceted at elevated temperature (when the catalytic activity is high). Based on a comparison between the shapes of the Pt nanoparticles in vacuum, N2, O2, CO, and CO/air at room temperature, 100, and 200 °C, we propose that the change in shape of the Pt nanoparticles is induced by the adsorption of CO molecules and O atoms.

Periodic instability in growth of chains of crystalline-silicon nanospheres

Abstract Chains of crystalline-silicon nanospheres were formed by a self-organized process via an extension of the vapor–liquid–solid mechanism using gold as catalyst. The diameter of nanospheres and the spacing between nanospheres were controlled by changing the growth conditions: the diameter and the spacing were small when the gold deposit was thin and the growth temperature was low. The spacing between the crystalline-silicon nanospheres was kept nearly proportional to the diameter of the chain. In order to reveal the growth mechanism of the chain, we simulated the periodic instability in chain growth, taking account of the supersaturation of silicon in vapor phase and molten catalyst, the kinetic roughening transition at the liquid–solid interface, and the interface tensions. Simulated instability was consistent with the experimental observations.

Origin and derived courses of eolian dust quartz deposited during marine isotope stage 2 in East Asia, suggested by ESR signal intensity

The ESR signal intensity of fine quartz (finer than 20 μm) during marine isotope stage (MIS) 2 in the Japanese Islands is spatially. Under the assumption that the values of ESR signal intensity of eolian fine quartz reflect those of the quartz at the in-source areas, the source area and trajectory of eolian dust are tentatively reconstructed. In northern Japan, where the ESR signal intensity is high (10–12), the eolian dust seems to have originated from Siberia and Mongolia where the Precambrian rocks with a high ESR signal intensity are widely exposed. In central and southern Japan, where the ESR signal intensity is medium (5.8–8.7), the eolian dust seems to be supplied from central Asia where Paleozoic–Mesozoic rocks with a medium ESR signal intensity are widely exposed. For the southernmost islands of Japan, where the ESR intensity signal increases again (9.7–13.4), seem to have an eolian dust supply from areas south of Himalayan Range where the Precambrian rocks are widely exposed. On the basis of ESR intensity signals, three major courses of eolian dust transport in MIS 2 are proposed: the winter monsoon in eastern and northern Japan, the summer subtropical jet in western and southern Japan, and the winter subtropical jet in the southernmost islands of Japan.

Silicon Nanoneedles Grown by a Simple Thermal Treatment Using Metal-Sulfur Catalysts

We found that dense silicon nanoneedles can be grown on silicon substrates epitaxially by heating with metal and sulfur via the vapor–liquid–solid process. Needles were examined by transmission electron microscopy and the growth direction was determined to be mostly . The results of our study revealed that sulfur can be used for fabricating self-organized silicon nanostructures.

One-phonon Raman scattering studies of chains of crystalline-Si nanospheres

Chains of crystalline-Si nanospheres were studied by means of Raman scattering spectroscopy. We found that the one-phonon Raman scattering peak from the chains was asymmetric and broader than that from bulk Si. This phenomenon can be attributed to a phonon confinement in the silicon nanospheres. The phonon confinement became more obvious by decreasing the size of the silicon nanospheres in the chains. We also found that the Si nanospheres in the chains were under compressive stress by the covering oxide layers through the analysis of the Raman shift.

Fabrication of iron silicide nanowires from nanowire templates

The authors demonstrate that Fe silicide nanowires can be fabricated from vapor-liquid-solid grown Si nanowire templates. By heating simple Si nanowires with Fe, Fe infuses into Si nanowires through a surface oxide layer and Si nanowires are converted to α-FeSi2 or e-FeSi partially. Furthermore, when Au catalyst forms Au∕Si heterostructured nanowires, heating with Fe results in the formation of β-FeSi2.