H. Kataura

Optical properties of single-wall carbon nanotubes

Four kinds of single-wall carbon nanotubes (SWNTs) with different diameter distribution have been synthesized and optical absorption spectra have been measured. Three large absorption bands due to the optical transitions between spike-like density of states, characteristics of SWNTs, were observed from infrared to visible region. Comparing with the calculated energy band, it has been concluded that the first and the second lowest absorption bands are due to the optical transitions between spikes in semiconductor phases and the third one is due to that in metallic phases. Absorption Peaks sensitively shifted to higher energy side with decreasing tube diameters as the band calculation predicted. Resonance Raman spectra were also measured using various laser lines. When the excitation is in an energy region corresponding to the absorption band of metallic phase, spectra have shown Breit-Wigner-Fano line shape, which is a sign of metallic phase. Using these results, we can easily characterize SWNTs from the optical absorption spectra without Raman measurements and transmission electron microscope observations.

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.

Direct observation of Tomonaga-Luttinger-liquid state in carbon nanotubes at low temperatures.

The electronic transport properties of conventional three-dimensional metals are successfully described by Fermi-liquid theory. But when the dimensionality of such a system is reduced to one, the Fermi-liquid state becomes unstable to Coulomb interactions, and the conduction electrons should instead behave according to Tomonaga–Luttinger-liquid (TLL) theory. Such a state reveals itself through interaction-dependent anomalous exponents in the correlation functions, density of states and momentum distribution of the electrons. Metallic single-walled carbon nanotubes (SWNTs) are considered to be ideal one-dimensional systems for realizing TLL states. Indeed, the results of transport measurements on metal–SWNT and SWNT–SWNT junctions have been attributed to the effects of tunnelling into or between TLLs, although there remains some ambiguity in these interpretations. Direct observations of the electronic states in SWNTs are therefore needed to resolve these uncertainties. Here we report angle-integrated photoemission measurements of SWNTs. Our results reveal an oscillation in the π-electron density of states owing to one-dimensional van Hove singularities, confirming the one-dimensional nature of the valence band. The spectral function and intensities at the Fermi level both exhibit power-law behaviour (with almost identical exponents) in good agreement with theoretical predictions for the TLL state in SWNTs.

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.

Phase Transition in Confined Water Inside Carbon Nanotubes.

In materials confined within nanometer channels in single-walled carbon nanotube (SWNT) bundles, interesting properties which are not observed in bulk materials are expected. In the present paper, we report an X-ray diffraction (XRD) study on water adsorption in SWNT bundles. It was found that a substantial amount of water is absorbed inside SWNTs at room temperature (RT). The desorption-adsorption of water molecules occurred reversibly above RT. We found that the liquid-like water is transformed into a new solid form, i.e., ice nanotubes, at 235 K.

Electrochemical tuning of electronic states in single-wall carbon nanotubes studied by in situ absorption spectroscopy and ac resistance

Electrochemical doping of single-wall carbon nanotube (SWNT) films and concomitant changes in their electronic states were investigated by in situ measurements of optical absorption spectra as well as of ac resistance using a nonaqueous electrolyte solution. A systematic, consistent, and reversible variation of these properties induced by the shift in the electrode potential demonstrated the practicability of fine and continuous tuning of their electronic states. Analysis of the potential dependence of the absorbance at 0.68 eV enabled the estimation of average values of the electron affinity (4.8 eV) and the first ionization potential (5.4 eV) of semiconducting SWNTs.

Photoconductivity in Semiconducting Single-Walled Carbon Nanotubes

We have observed the photoconductive response of film samples of single-walled carbon nanotubes for the first time. Two peaks in the photoconductivity excitation spectra around 0.7 and 1.2 eV are observed at room temperature, which can be interpreted as a photocurrent in semiconducting nanotubes. At a low temperature, we found a marked change in the intensity of the spectrum. In this paper, we discuss this temperature dependence and the mechanism of photoconductivity.

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Abstract Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube–wall corrugation was caused by heat treatment (HT) at 1000 to 1700xa0°C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900xa0°C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000xa0°C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400xa0°C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube–tube interconnections created by HT.

Hydrogen storage in single-walled carbon nanotube bundles and peapods

Abstract Desorption of molecular hydrogen from single-walled carbon nanotube (SWNT) bundles and peapods (C 60 encapsulated SWNTs) have been observed at about 350 K by the temperature-programmed-desorption (TPD) method. A treatment with NaOH after purification is found to be crucial for obtaining clean SWNTs and for desorbing hydrogen at this moderate temperature. A clear difference observed in gas evolution from SWNTs and peapods shows that the storage site for the hydrogen molecule is an inter-tube space and that `sub-nanometer sized spaces are indispensable for storing hydrogen molecules in this system.