Systematic fractional change in the yield of various isomers of fullerenes was revealed to strongly depend on temperature of a buffer gas. A new kinetic consideration is proposed for understanding the observed temperature- and pressure-dependence of yield of fullerenes. The model consists of three competitive reactions in consideration of plausible behaviors of a precursor, (1) decomposition into smaller fragments, (2) isomerization leading to formation of a stable fullerene cage, and (3) growth into a large carbon cluster. Arrhenius activation energy of formation of stable fullerenes was determined to be 0.8eV for both C60 and C70, while a higher energy of 2.0-3.3eV for seven different isomers of higher fullerenes raging from C76 to C84. Correlation in the activation energy is noted for a series of higher fullerenes with different sizes, suggesting the existence of a specific precursors in their formation process.
Photoionization/fragmentation of endohedralfullerenes was investigated by use of laser-desorption time-of flight (LD-TOF) mass spectroscopy. The variety distribution of the parent ion (LaC82+) was found to be bimodal, as has previously shown for laser desorbed C60+. The fragment ions have velocity distribution corresponding predominantly to the fast parent ion distribution. The LD-TOF mass spectra taken with a relatively low laser fluence were independent of the decay time of the extraction pulses, showing only a monotonically decreasing patterns of LaC2n+ (as n decreased). However, with higher laser fluence, it was shown that the mass distributions drastically changed from the monotonically decreasing pattern to that of C2n+, and LaC2n+ with magic numbers. Based on these findings, a plausible photoionization/fragmentation mechanism is presented and discussed.
Infrared pulse laser ablation of graphite followed by tightly focused ultraviolet pulse laser irradiation yield C10- predominantly when the delay time between the two laser pulses is set at about 50-200 microseconds. The mechanism of the formation of C10- is deduced to be the attachment of slow photoelectrons to neutral C10 produced by fragmentation of hot neutral clusters of larger sizes. There is an indication that C10 and its anion have cyclic structure in contrast to the other chainlike clusters.
Magnetic susceptibility, electron spin resonance and 13C-NMR data on NH3K3C60 reveal that NH3K3C60 is a narrow-band metal with a transition to an insulating ground state at 40K. This transition suppresses the superconductivity that is expected to occur around 30K according to a simple empirical relation between Tc and the cell volume in alkali-metal-doped C60 superconductors.This is a first indication of the proximity of superconductivity and insulator in fullerene materials.
A new ternary superconducting fulleride was synthesized by intercalation of potassium into Ba3C60. Bulk superconductivity with the onset temperature was 5.6K was observed in K3Ba3C60, which forms a body-centered-cubic (bcc) lattice parameter of a=11.24Å. A nave electron counting predicts that the Fermi level of K3Ba3C60 is at the center of the t1g-derived band, in sharp contrast with also superconducting Ba6C60. This result indicates that the superconductivity of fullerides occurs at the half-filled states both of the t1u- and t1g-derived bands.
Formation of C60 aggregates has been found in toluene solution by using photoluminescence spectroscopy. From the detailed investigation of the temperature dependence of luminescence, it has been confirmed that the formation of C60 aggregates occurs simultaneously with the liquid-solid transition of the solution and that the aggregates remain in liquid-solution at temperatures (170-210K) slightly higher than the melting point of the solution in the warming process. the 0-0 transition energy in their luminescence spectrum has been found to be blue-shifted in comparison with that of the C60 crystal.
We found evidence for polymerization of C70 induced by high pressure. X-ray diffraction, infrared spectra, ultraviolet-visible absorption spectra unambiguously confirmed the occurrence of cross linking of C70 molecules. As was observed in C60 polymers, C70 polymer is not soluble in toluene, and revert to conventional monomer crystals by heating to 300°C at ambient pressure.
Electric conductance of a multi-wall carbon nanotube shows a decreasing temperature dependence with decreasing temperature, but with a kink about 15 K. Temperature dependence of conductance both above and below 15 K which can be explained in terms of two-dimensional weak localization with different parameters in each region. Magnetoconductance reveals oscillation with magnetic field at low temperatures, which can be explained by universal conductance fluctuations.
We have succeeded to align multi-wall carbon nanotubes under magnetic field. Carbon nanotubes align along magnetic field owing to anisotropy in magnetic susceptibility. With use of aligned carbon nanotubes the diamagnetic susceptibility for magnetic field parallel and perpendicular to the tube axis are determined to be -8.0 and -9.5 emu/g at 300 K, respectively.
back to Joint Studies Programs