Takashi Yamasaki

Bottom-up synthesis and thread-in-bead structures of finite (n,0)-zigzag single-wall carbon nanotubes.

The last remaining synthetic target of finite single-wall carbon nanotube models, the zigzag nanotube, has been accomplished through bottom-up chemical synthesis. The zigzag nanotube was synthetically accessible without constructing long-sought yet elusive cyclacene structures but with a cycloarylene structure by devising its cutout positions. The persistent tubular shape was also perfected in this last model by cyclization of zigzag-shaped aromatic molecules with a synchronous topological arrangement. The crystal structure of this nanotube further revealed an entangled supramolecular assembly, which showed a novel way to align nanotube molecules by utilizing their open-end functional groups in a thread-in-bead molecular assembly.

Molecular bearings of finite carbon nanotubes and fullerenes in ensemble rolling motion

A carbonaceous bearing of a minimal form has been assembled using a finite carbon nanotube molecule and a functionalised fullerene molecule. With the van der Waals attraction, the bearing holds the fullerene journal tightly to prevent its run-out motion, and the journal with a shaft rolls anisotropically in the bearing despite the tight holding constraint. Ensemble motion has been revealed spectroscopically at the molar quantity level.

Solid-state structures of peapod bearings composed of finite single-wall carbon nanotube and fullerene molecules

Significance Carbonaceous entities possessing tubular and spherical shapes spontaneously form a host–guest complex. This supramolecular complex, so-called a peapod, is unique among host–guest pairs in that it is assembled solely by van der Waals interactions at the concave–convex interface of sp2-carbon networks. Recently, a molecular version of this supramolecular system revealed the presence of the extremely tight association concomitantly with the dynamic motions of the guest in apolar media. In this paper, an atomic-level structure of the molecular peapod is revealed by a crystallographic method to show the presence of an inflection-free surface inside the tubular molecule. Enjoying rotational freedom at this smooth surface, the guest fullerene molecule rolls dynamically even in the solid state. A supramolecular combination of carbon nanotube and fullerene, so-called a peapod, has attracted much interest, not solely because of its physical properties but also for its unique assembled structures of carbonaceous entities. However, the detailed structural information available was not sufficient for in-depth understanding of its structural chemistry or for exploratory research inspired by novel physical phenomena, mainly because of the severely inhomogeneous nature of currently available carbon nanotubes. We herein report solid-state structures of a molecular peapod. This structure, solved with a belt-persistent finite carbon nanotube molecule at the atomic level by synchrotron X-ray diffraction, revealed the presence of a smooth, inflection-free Hirshfeld surface inside the tube, and the smoothness permitted dynamic motion of the C60 guest molecule even in the solid state. This precise structural information may inspire the molecular design of carbonaceous machines assembled purely through van der Waals contacts between two neutral molecules.

Assessment of Fullerene Derivatives as Rolling Journals in a Finite Carbon Nanotube Bearing

Conformance assessment of rolling journals in a molecular bearing has been carried out with a combination of fullerenes and finite single-wall carbon nanotube molecules through quantitative analysis of the binding affinities. Endohedral fullerenes were applicable to three-body molecular bearings with slightly weaker binding affinities. Exohedral shaft moieties on C60 journals affected the binding affinities to reduce the binding constants to a considerable extent, and oval-spherical C70 journals were superior in tolerating bulky shaft attachments.

FABRICATION AND OPTICAL PROPERTIES OF TWO-DIMENSIONAL ORDERED ARRAYS OF SILICA MICROSPHERES

Two-dimensional ordered arrays of SiO2 (silica) microspheres, as a semi-two-dimensional photonic crystal, with large surface area were fabricated by using a newly developed self-organization method. We observed strong light scattering and hexagonal diffraction patterns for the hexagonally close-packed array of spheres with submicrometer size. Dips in the transmission spectra due to coupling between incident light and electromagnetic bound states of the photonic bands within the array were observed. From the transmission spectra, we determined the photonic band structure of the two-dimensional arrays of microspheres.

A Scaling Relation of Anomalous Hall Effect in Ferromagnetic Semiconductors and Metals

A scaling relation of the anomalous Hall effect recently found in a ferromagnetic semiconductor (Ti,Co)O2 is compared with those of various ferromagnetic semiconductors and metals. Many of these compounds with relatively low conductivity σxx≤104 Ω-1cm-1 are also found to exhibit similar relation: anomalous Hall conductivity σAH approximately scales as σAH∝σxx1.6, that is coincident with a recent theory. This relation is valid over five decades of σxx irrespective of metallic or hopping conduction.

Anomalous Hall effect in anatase Ti1−xCoxO2−δ above room temperature

Presence of spin polarized electrons was confirmed by observing anomalous Hall effect in a ferromagnetic semiconductor anatase Ti1−xCoxO2−δ up to 600K. The anomalous Hall resistivity exhibited apparent ferromagnetic hysteresis loop from 300to600K with insignificant change in the magnitude, indicating that the Curie temperature is higher than 600K. The measurements above 500K induced an annealing effect represented by the decrease in resistivity, whereas kept the anomalous Hall resistivity nearly constant.

Observation of anomalous Hall effect in EuO epitaxial thin films grown by a pulse laser deposition

We have found that there is a narrow but distinct window in oxygen pressure for growing phase-pure epitaxial EuO films by a pulsed laser deposition. With finely decreasing the oxygen pressure, the electrical property is varied from insulating to metallic with an enhancement in Curie temperature from 70 to 120 K. The anomalous Hall contribution was clearly observed in Hall resistance at 5 K in the highest electron density sample. The saturated anomalous Hall conductivity (0.2 S/cm) is rather high in comparison with those of the other ferromagnetic oxides, probably due to strong spin-orbit coupling in EuO.

Co-doped TiO2 films grown on glass: Room-temperature ferromagnetism accompanied with anomalous Hall effect and magneto-optical effect

Room-temperature ferromagnetic oxide semiconductor Co-doped TiO2 films are grown on glass substrates by sputtering method. Conducting films are ferromagnetic at room temperature that is consistent with the carrier-mediated nature of the ferromagnetism. Nearly full-polarized magnetization, large magneto-optical effect, and anomalous Hall effect are observed at room temperature. The magneto-optical effect shows nearly fourfold enhancement in a one-dimensional magnetophotonic crystal structure with a standard dielectric multilayer (SiO2/TiO2).

Ultrafast time-resolved faraday rotation in EuO thin films.

We have investigated the ultrafast spin dynamics in EuO thin films by time-resolved Faraday rotation spectroscopy. The photoinduced magnetization is found to be increased in a transient manner, accompanied with subsequent demagnetization. The dynamical magnetization enhancement showed a maximum slightly below the Curie temperature with prolonged tails toward both lower and higher temperatures and dominates the demagnetization counterpart at 55 K. The magnetization enhancement component decays in ~1 ns. The realization of the transient collective ordering is attributable to the enhancement of the f-d exchange interaction.