Shinzou Suzuki

Diameter control of single-walled carbon nanotubes

Laser furnace technique has been used for diameter selective formation of single-walled carbon nanotubes (SWNTs) using NiCo, RhPt and RhPd catalysts. Particularly RhPd catalyst can produce very thin SWNTs in high yield. In this work, we investigated the furnace temperature dependence for each catalyst to control the diameter of SWNTs more strictly. Further, we investigated the effect of the gas flow velocity to see the growth time of SWNTs. When the flow velocity was changed from 1.2 to 12 mm/s, the diameter distributions of the SWNTs obtained were changed significantly. This result suggests that the nucleation and the growth of SWNTs are much slower than the fullerene formation. Using the slow growth nature of SWNTs, there is a possibility of further diameter control of SWNTs.

Gas adsorption in the inside and outside of single-walled carbon nanotubes

Abstract Adsorption properties of nitrogen and oxygen gases in single-walled carbon nanotube (SWNT) bundles were investigated by the isotherm and X-ray diffraction (XRD) studies. In the as-grown (AG) nanotubes with close-ended caps, both the gases are adsorbed only in the interstitial channels between triangular packed nanotubes. In the heat-treated (HT) nanotubes with open ends, the gases are adsorbed first in the inside of tubes, and next in the interstitial channels. In each site, gases can be adsorbed with the stoichiometory of C 20 N 2 or C 20 O 2 as a monolayer. These results indicate that the inside of nanotube has strong affinity for gas adsorption than the interstitial channels of bundles.

High-yield fullerene encapsulation in single-wall carbon nanotubes

We have successfully synthesized single-wall carbon nanotubes encapsulating specified fullerenes (peapods) in high yield using a sublimation method. Side and section images by HRTEM indicate that almost all nanotubes are filled with high-density fullerene chains. We measured Raman spectra to estimate macroscopic yield. The observed Raman intensity of C 70 molecules in the C 70 -peapods is 1/10 of that in C 70 film, which indicates a filling rate of C 70 to be higher than 26 %. In the case of C 60 -peapods, Raman spectrum has changed rapidly by laser irradiation. Since the final spectrum at room temperature is similar to that of the orthorhombic polymer phase, a formation of one-dimensional photopolymers inside SWNTs is suggested.

Anomaly of X-ray Diffraction Profile in Single-Walled Carbon Nanotubes

X-ray diffraction (XRD) studies on single-walled carbon nanotube (SWNT) samples prepared by the arc-discharge method were reported. The XRD profile was basically explained to be a result of triangular packing of SWNTs with a lattice constant of 17.1 A and an average nanotube radius of 7.1 A. We found an anomalous change in XRD profiles before and after heat-treatment of the SWNT samples in air at ~350°C. Combined with gravimetric measurements and resistivity measurements, a detailed simulation of the XRD profiles showed that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs.

Time period for the growth of single-wall carbon nanotubes in the laser ablation process: evidence from gas dynamic studies and time resolved imaging

Abstract Single-wall carbon nanotubes (SWNTs) were synthesized by laser ablation of Ni–Co-graphite composite targets at 1200°C under argon gas. The effects of the temperature gradient near the target and the gas flow rate on the diameter distribution of SWNTs were studied in order to understand their growth dynamics. Raman spectroscopy was used to analyze the diameter distribution of SWNTs. The flow rate was found to affect the relative yields of SWNTs having different diameters when the temperature gradient around the target was large. Scattering images from the ablated species at different flow rates, recorded by a high-speed video camera, indicated that 10 ms after the ablation the velocities of backward moving species increased with increasing flow rate. These findings are used to estimate the time required for determining the diameter distribution and the growth of SWNTs.

Growth of single-walled carbon nanotubes from the condensed phase

Abstract Single-walled carbon nanotubes (SWNTs) were grown from the condensed phase by thermally post-annealing of a soot-like material, possibly containing precursors for SWNT growth. The soot-like material was obtained by laser ablation of Ni–Co–graphite composite targets at 550–700 °C. This initial material did not contain any SWNTs but post-annealing treatment inside an electric furnace at 1200 °C under argon flow resulted in SWNTs. The soot-like material prepared at lower temperatures (room temperature to 400 °C did not yield SWNTs even after post-annealing. These results indicate that the precursors for the growth of SWNTs can form only above a certain threshold temperature, about 550 °C. Once these precursors are present, SWNTs can grow by annealing of the metal–carbon mixture. A growth model is proposed based on these results.

Gas Storage in Single-Walled Carbon Nanotubes

Abstract X-ray diffraction (XRD) and electrical resistance measurement on single-walled carbon nan-otube (SWNT) samples prepared by the arc-discharge method are reported. The XRD profile of heat-treated sample indicated that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs. We also found that the electrical resistance of SWNT soot is significantly affected by exposing to the oxygen gas and humid air.

Photoconductivity of single-walled carbon nanotubes

We have investigated photoconducting properties of mat samples of single-walled carbon nanotubes (SWNTs) in order to clarify the mechanism of photoconductivity. Two peaks are observed in photoconductivity spectra around 0.7 and 1.2 eV for both samples of SWNTs synthesized by arc discharge and laser ablation methods, which can be interpreted as the photocurrent in the semiconductor phase of nanotubes. No threshold in applied voltage is observed for the occurrence of photoconductivity. Results show that the photoconductivity is an intrinsic feature of SWNTs and that junction areas do not play an important role on photoconductivity.

Nuclear magnetic resonance (NMR) and electron spin resonance (ESR) characterization of sublimed C60 solid

Two kinds of electron spin resonance (ESR) signals were found in sublimed carbon-sixty ( C60) solid, one of which is extremely sensitive to air while the other is relatively stable. Although the origin of these signals could not be clarified, it was found that successive sublimation leads to a C60 solid which does not show any detectable ESR signal. 13C-nuclear magnetic resonance (NMR) study showed that the sublimed solid is much more homogeneous than the solution-grown solids, and that C60 orientational ordering transition around 262 K is a first-order phase transition.

Superconductivity and site-selective intercalation in KRbCsC60

Ternary-alkali-metal-intercalated fullerides, KRbCsC 60 , was prepared by method employing alkali metal azides reported by Bensebaa et al . (F. Bensebaa, B. Xiang and L. Kevan: J. Phys. Chem. 96 (1992) 6118). We found that KRbCsC 60 is a superconductor with the transitipn temperature of 28(bulk)∼29 K (onset). It was also found, from nuclear magnetic resonance (NMR) study of 87 Rb and 133 Cs in this system, that the alkali atoms are site-selectively intercalated into C 60 face-centered-cubic (fcc) lattice. The largest alkali-metal ion, Cs, occupies octahedral sites of the C 60 fcc lattice, and the K and Rb atoms occupy mainly tetrahedral sites.