Balance of graphite deposition and multishell carbon nanotube growth in the carbon arc discharge
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Balance of graphite deposition and multishell carbon nanotube growth in the carbon arc discharge P. M. Ajayan, Ph. Redlich, and M. R¨uhle Institut f ur ¨ Werkstoffwissenschaft, Max-Planck-Institut f¨ur Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany (Received 22 February 1996; accepted 24 June 1996)
Except for atomistic models of individual carbon nanotube growth, there is still no precise understanding of the large scale deposition of carbon during the arc discharge. We study the microstructure of cathode deposits in detail using scanning electron microscopy, focusing on two distinct regimes found in the deposits having different large scale morphology. The shell grown circumferentially consists of extended graphite layers with preferred orientation, and the structure reveals close similarity to pyrolitic graphite. The core region is a porous assembly of nanotubes and nanoparticles. We conclude that closed nanostructures self-assemble from a dense carbon vapor, whereas pyrographitic shell grows by continuous deposition on exposed substrates.
I. BACKGROUND Growth of multishell nanometer size tubes of carbon by dc electric arc discharges has caught the fancy of scientists due to low dimensionality of the material,1 some of the predicted structure sensitive properties, such as electrical conductivity,2,3 and possible applications, for example, as field-emitting sources.4 Carbon nanotubes were first observed by Iijima5 as condensed needles on the cathode surface after a brief electric arc discharge had taken place between two pure graphite electrodes in an inert gas (He) atmosphere. Later, conditions such as the inert gas pressure, voltage, and current were modified to obtain a substantial yield of nanotubes during the arc discharge.6 Under the best conditions the anode evaporates continuously, depositing on the facing cathode surface into a cigar-shaped deposit which contains 75% of the evaporated anode mass. The deposit formed consists of a grey, hard outer region (shell) and a soft, black interior (core). The core that can be easily removed from the shell is loosely packed with closed multilayer carbon structures with aspect ratios (length-to-diameter) ranging from one (onion-like) to several thousands (tubular). Various research groups have since tried to understand the growth mechanism of nanotubes by arc-discharge deposition,7–17 but a clear picture has still not evolved. The majority of the growth models proposed concentrated on atomistic aspects of the buildup of single tubes, while a few studies, trying to explain the growth pattern inside the deposits discussing the microstructure of the core where the nanotubes are present, have neglected the shell region that grows simultaneously with the nanotubes. For the formation of nanotubes in the arc discharge, several mechanisms have been considered to explain why tubes remain open, allowing extended length growth.17 As possible external factors, either the influence of an electric 244
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