Effect of Temperature Gradient near the Target and Gas Flow Rate on the Diameter Distribution of Single-Walled Carbon Na
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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 Rahul Sen1, Hiromichi Kataura2, Yohsuke Ohtsuka1, Toshinobu Ishigaki1, Shinzo Suzuki1 and Yohji Achiba1 1 Department of Chemistry, Tokyo Metropolitan University, Tokyo 192-0397, JAPAN 2 Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, JAPAN ABSTRACT 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 oC under argon flow. 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. INTRODUCTION Electronic properties of single-walled carbon nanotubes (SWNTs) depend crucially on their structural parameters such as diameter and chirality [1]. It is important to understand the growth mechanism of SWNTs to achieve a control over their diameter distribution and to produce nanotubes of desired property. Laser ablation of metal-graphite composite targets in argon gas can produce high quality SWNTs [2], however the diameters and chiralities of the nanotubes vary considerably for a given set of growth conditions [3]. Bandow et al. [4] reported that the mean diameter of the nanotubes increased with increasing temperature of the growth environment in laser ablation. Diameters of SWNTs can also be changed by changing the catalyst system [5] or the catalyst ratio [6]. Despite these reports the process and mechanism of SWNT growth is not well understood. We consider that by changing various control parameters in the laser ablation process and by studying the dynamics of the ablated species, we can obtain an insight into the growth process of the SWNTs. We have studied the effect of the temperature gradient around the target and the gas flow rate on the diameter distribution of SWNTs. We also observed the scattering images of the laser-ablated species recorded by time-resolved photography. We estimated the velocities of the species inside the furnace and correlated them with the flow rates. Finally, the effect of the flow rate on the diameter distribution of SWNTs and the actual flow rate of the species are used to estimate the nucleation and growth times of SWNTs. EXPERIMENTAL SWNTs were grown by the laser ablation of metal-graphite composite targets using t
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