Y. Ohtsuka

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.

Formation of Thin Single-Wall Carbon Nanotubes by Laser Vaporization of Rh/Pd-Graphite Composite Rod

Single-wall carbon nanotubes (SWNTs) were synthesized in high yield by laser vaporization of a Rh/Pd-graphite composite rod at 1200 °C. Lattice constants of the bundle were estimated from transmission-electron-microscope images and were found to be distributed between 1.0 and 1.5 nm. Nine Raman peaks originating from the breathing modes were observed at 204, 215, 229, 247, 262, 272, 287, 303 and 328 cm-1. These frequencies and lattice constants sharply indicate the presence of the SWNTs indexed from (5,5) to (8,8) which are thinner than the SWNTs obtained using Ni/Co catalyst.

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.

Resonance Raman Scattering of Br2 Doped Single-Walled Carbon Nanotube Bundles

Abstract We have measured Raman spectra of bromine doped single-walled carbon nanotubes (SWNTs) using various laser lines to clarify the electronic states of the doped SWNT. In the case of evacuated sample after full doping, two breathing mode peaks were observed simultaneously by visible laser excitations. We assigned the higher frequency peak to the doped SWNT bundles, and the other peak to the undoped portions in the sample. Intensity ratio between them decreased with decreasing excitation energy, and in the infrared region, the breathing mode band of the doped bundle was not observed. These results can be explained by a simple rigid band model.

Bundle Effects of Single-Wall Carbon Nanotubes

To see the bundle effects on the electronic and the vibrational properties of single-wall carbon nanotubes (SWNTs), we have measured the resonance Raman scattering of isolated SWNTs and thick bundles. For the measurements of the isolated SWNTs, we used an evacuated sample after bromine doping. A broad and asymmetric tangential mode band, which is a sign of the resonance of the metallic SWNTs and can be fitted by a Fano line shape, is not observed in the isolated SWNTs. This suggests the inter-tube interactions play an important role to the Fano interference. On the other hand, the purified sample shows very thick acquired bundles. We observed 4% higher breathing mode frequencies than in the pristine sample. Further, in the case of multi-wall nanotubes, we observed 5% higher breathing mode frequencies than the SWNTs. These results can be explained by the interlayer interactions.

Time and Space Evolution of Emitting Carbon Nanoparticles – Correlation with the Formation of Fullerenes and Carbon Nanotubes –

The temporal and spatial evolution of emitting carbon nanoparticles were investigated using a laser furnace apparatus combined with a high-speed video camera. An apparent increase in the blackbody emission intensity at Δt > 400 [.proportional]sec after laser vaporization of a graphite rod was clearly recognized. Also, it was found that this increasing tendency corresponds well to that of the fullerene yield, where fullerene species was obtained as sublimed carbon material using in situ sublimation method. These findings suggest that a certain exothermic process related to the formation of C 60 , other higher fullerenes, and carbon nanotubes should occur at Δt > 400 νsec inside the furnace.

Effect of Temperature Gradient near the Target and Gas Flow Rate on the Diameter Distribution of Single-Walled Carbon Nanotubes Grown by the Laser Ablation Technique

Gas dynamic and time resolved imaging studies have been performed on the growth of single-walled carbon nanotubes (SWNTs) in the laser ablation process. SWNTs were synthesized by laser ablation of Ni-Co catalyzed graphite targets at 1200 o C under argon gas. The effects of the temperature gradient near the target and the gas flow rate were studied in order to understand the effect of gas dynamics over the diameter distribution of SWNTs. The gas flow rate affects the diameter distribution of SWNTs especially when the growth species flow through a large temperature gradient. Scattering images from the growth species at different flow rates was recorded by high-speed video imaging. The results indicate that the velocities of these species are dependent on the gas flow rate but this dependence is evident 30 ms after the laser ablation. These findings are used to estimate the time period for the nucleation and the growth of SWNTs.

Optical absorption and resonance Raman scattering of carbon nanotubes

Four kinds of single-walled carbon nanotubes (SWNTs) with different diameter distributions were synthesized using NiY, NiCo, Ni and RhPd catalysts. Optical absorption and resonance Raman spectra were measured. For all the samples, three large absorption peaks were observed from the infrared to visible region. From the band calculation, it was found that they could be explained by the optical transitions between mirror image spikes in density of states caused by the one-dimensional van Hove singularities. Lower two peaks are originating from the semiconductor phases and the third one is from the metallic phases. In the Raman spectra, indeed, broad and asymmetric Fano line shape was observed when the excitation was at the optical transition of metallic SWNTs. Preliminary results about resonance Raman scattering of multiwall carbon nanotubes that have small core diameter and of Br2 doped SWNTs are shown.