Low Temperature Growth of Carbon Nanotubes and Nanofibres
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Low Temperature Growth of Carbon Nanotubes and Nanofibres J Robertson, S Hofmann, B Kleinsorge, *V Golokov, #C Ducati, *J Geng, M Cantoro, B Boskovic, *W Huck, *B F G Johnson, Engineering Dept, *Chemistry Dept, #Materials Science Dept, Cambridge University, Cambridge CB2 1PZ, UK Abstract This paper reviews work on low temperature growth of carbon nanotubes, on Si, on plastic, on carbon cloth, using sputtered and colloidal catalysts, and with nano-imprinted patterning. Carbon nanotubes are materials with a unique set of properties, such as a high aspect ratio, high stiffness, ballistic electron transport and high thermal conductivity. These lead to a desire to use them in various novel applications. Some of these applications such as interconnects in integrated circuits [1] or catalyst supports in polymer membrane fuel cells [2] call for growth at lower temperatures than are presently typical. For this reason, we have attempted here to lower the deposition temperature to see what was possible. Carbon nanotubes are usually grown by laser deposition, by the arc method or by chemical vapour deposition (CVD). The laser and arc methods involve local temperatures well over 1000C. CVD can be carried out on surface bound catalysts or unsupported catalysts and it typically involves process temperatures in the range 600-900C. A fourth method, a variant of CVD, is called plasma enhanced chemical vapour deposition (PECVD). The use of PECVD for growing carbon nanotubes was originated by Ren et al [3]. The process consists of depositing a thin layer of a catalyst such as Ni onto the substrate where nanotubes are desired to grow. The substrate is then heated under vacuum to restructure the catalyst into a nano-particulate form. A gas mixture of typically acetylene and ammonia is then introduced, and the plasma initiated. Merkulov et al [4] showed the importance of the restructuring of the catalyst and that the nanoparticle can be small enough, below 300nm, that only a single nanotube grows from each nanoparticle. In PECVD, the nanotubes grow vertically aligned to the substrate. Bower and Zhou [5] showed that the alignment arises from the electric field in the plasma sheath. Chhowalla [6] showed that the nanoparticle size varied with the thickness of the initial catalyst film. Teo et al [7] showed that the ammonia acts as an etchant to remove the competing amorphous carbon phase. They also showed that a Fig. 1. SEM image of single nanofibre grows if the nanoparticles are below 80 nm in carbon nanotubes grown size. at (a) 500C, (b) 270C and There was then a desire for the lower temperature (c) 120C
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deposition. Boskovic [8] showed the possibility of a nominally room thermal CVD (Ducati et al.) 100 100 temperature growth of carbon nanotubes PECVD (Ducati et al.) PECVD (this work) by PECVD. It that case, Ni powder was introduced into the plasma and non-aligned 10 10 CNTs were found to grow on the powder. This led Hofmann [9] to study the 1 1 low temperature process in more detail. activation 0.23 eV They de
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