Y. Achiba

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.

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.

Detailed analysis of the mean diameter and diameter distribution of single-wall carbonnanotubes from their optical response

We report a detailed analysis of the optical properties of single-wall carbon nanotubes (SWCNTs) with different mean diameters as produced by laser ablation. From a combined study of optical absorption, high-resolution electron energy-loss spectroscopy in transmission, and tight-binding calculations we were able to accurately determine the mean diameter and diameter distribution in bulk SWCNT samples. In general, the absorption response can be well described assuming a Gaussian distribution of nanotube diameters and the predicted inverse proportionality between the nanotube diameter and the energy of the absorption features. A detailed simulation enabled not only a determination of the mean diameter of the nanotubes, but also gives insight into the chirality distribution of the nanotubes. The best agreement between the simulation and experiment is observed when only nanotubes within

A model for the C60 and C70 growth mechanism

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

of the armchair axis are considered. The mean diameters and diameter distributions from the optical simulations are in very good agreement with the values derived from other bulk sensitive methods such as electron diffraction, x-ray diffraction, and Raman scattering.

Encapsulated and hollow closed-cage structures of WS2 and MoS2 prepared by laser ablation at 450-1050°C

Abstract A new ring-stacking model is proposed for preferential generation of C60 with Ih symmetry and C70 with D5h symmetry in carbon soot. The model is constructed on the basis of the following two assumptions: (1) A carbon closed cage (fullerene) is formed by a sequential stacking with appropriate numbers and combinations of only even-numbered carbon rings. (2) The intermediates, as well as the final closed cages, consist of only hexagons and pentagons with no adjacent pentagons.

Evolution of superconductivity of KxC60 upon K-doping : microwave low-field signal and ESR study

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.

Magnetic properties of TDAE-C60 and TDAE-C70. A comparative study

Encapsulated and hollow closed-cage onion-like structures of WS2 and MoS2 were prepared by laser ablation of the corresponding layered structures in argon atmosphere at four varied temperatures. A detailed study for WS2 indicates that only metal-filled onion-like structures are produced at temperatures Tless-than-or-equals, slant650°C, whereas a mixture of metal-filled and hollow structures are produced at Tgreater-or-equal, slanted850°C. The encapsulated metal is identified to be predominantly the metastable β phase of tungsten. Very short tube-like or elongated polyhedral structures are also obtained at high temperatures.

HIGH-YIELD PRODUCTION OF SINGLE-WALL CARBON NANOTUBES IN NITROGEN GAS

Abstract By means of low magnetic-field micowave absorption (Low-Field Signal) and conventional ESR, we have studied the evolution of superconductivity in bulk KxC60 dependent on the variation of K- doping temperature and time. The first LFS as an occurrence of superconductivity is observed in a doping time as short as td = 0.5 h with a discrete Tc ∼ 19 K, and the volume of the superconducting phase increases on further dopings, while Tc changes slightly. The sample in a superconducting state exhibits strong ESR absorptions, which indicates the presence of a non-superconducting metallic phase coexisting with a superconducting phase in inhomogeneously doped KxC60. These phases are closely correlated and disappear at the same time by overdoping.

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 Magnetic properties of C60 and C70 complexes with tetrakis (dimethylamino) ethylene (TDAE) have been studied by Faraday-balance magnetization and ESR measurements. It has been confirmed that TDAE-C60 possesses about one radical spin per molecule and shows a ferromagnetic-type transition at Tc=16.7–17.5 K. The appearance of a new broad and intense ESR signal below Tc suggests that the radical spins in the magnetically ordered state localize on each C60 molecule and are ferromagnetically correlated. On the contrary, TDAE-C70 demonstrates the usual Curie-type narrow ESR peak at all temperatures suggesting the absence of a ferromagnetic transition at least down to 4.5 K.