The effect of arc parameters on the growth of carbon nanotubes
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The effect of arc parameters on the growth of carbon nanotubes J. M. Lauerhaas, J. Y. Dai, A. A. Setlur, and R. P. H. Chang Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 10 October 1996; accepted 16 January 1997)
The influence of starting carbon material on the generation of carbon nanotubes is investigated. Comparisons are made between oriented graphite, randomly oriented graphite, carbon black, and a polycyclic aromatic hydrocarbon as carbon sources in helium and hydrogen arcs. Transmission electron microscopy investigation of the redeposited rod formed on the cathode and the soot from the chamber walls provides evidence for the building blocks that lead to the nanostructures formed. It is postulated that polycyclic aromatic hydrocarbons are precursors for carbon nanotube growth in a hydrogen arc. While, in the case of helium, low molecular weight carbon ions and molecules have been previously hypothesized by others to be the building blocks for nanotube growth.
I. INTRODUCTION
Gram quantities of fullerenes and nanotubes are now routinely synthesized.1,2 Fullerenes and nanotubes have been used as starting materials in a variety of experiments.3–9 A large volume of published work has been focused on examining the carbon species that are formed as a result of operating a dc carbon arc.10–16 In addition, experiments have been performed to fill the nanotubes with metallic or carbide species.17–22 These nanostructured materials are of particular interest for the study of quantized effects. Furthermore, these materials may be used in quantum electronic and optical applications.23–27 Recently, Chang and co-workers have studied the effect of gas in the dc arc on the carbon species produced.28 For example, when helium was replaced by hydrogen the arc still produced nanotubes in the redeposited rod on the cathode, but a large reduction of carbon “soot” deposited on the chamber walls was observed. Very little amorphous carbon was observed either inside or outside the nanotubes. Open nanotubes with clean termination of the graphitic sheets and empty interiors were also observed. The quality and the percent yield of openended nanotubes depended sensitively on the hydrogen gas pressure and other operating parameters of the arc discharge. Time of flight mass spectrometry indicated that there was a disproportionately large reduction of C60 in the “soot”, but an increased presence of polycyclic aromatic hydrocarbons (PAH’s). This led us to postulate a new mechanism for the formation of nanotubes in a hydrogen arc. This hypothesis was checked by considering how copper- and germanium-filled carbon nanotubes are formed in a hydrogen arc and not in a helium arc.29,30 1536
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 6, Jun 1997
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There are several major differences between helium and hydrogen: (i) the thermal conductivity of hydrogen is 20% higher than helium at room temperature, (ii)
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