Preferential Growth of Carbon Nanotubes/Nanofibers Using Lithographically Patterned Catalysts
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Preferential Growth of Carbon Nanotubes/Nanofibers Using Lithographically Patterned Catalysts K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, and W. I. Milne Engineering Department, Cambridge University, Trumpington St, Cambridge CB2 1PZ, UK G. Pirio, P. Legagneux, F. Wycisk, and D. Pribat Thales Laboratoire Central de Recherches, Domaine de Corbeville, 91404 Orsay Cedex, France ABSTRACT In order to utilise the full potential of carbon nanotubes/nanofibers, it is necessary to be able to synthesize well aligned nanotubes/nanofibres at desired locations on a substrate. This paper examines the preferential growth of aligned carbon nanofibres by PECVD using lithographically patterned catalysts. In the PECVD deposition process, amorphous carbon is deposited together with the nanotubes due to the plasma decomposition of the carbon feed gas, in this case, acetylene. The challenge is to uniformly nucleate nanotubes and reduce the unwanted amorphous carbon on both the patterned and unpatterned areas. An etching gas (ammonia) is thus also incorporated into the PECVD process and by appropriately balancing the acetylene to ammonia ratio, conditions are obtained where no unwanted amorphous carbon is deposited. In this paper, we demonstrate high yield, uniform, ‘clean’ and preferential growth of vertically aligned nanotubes using PECVD. INTRODUCTION There has been considerable interest in the application of nanotubes in nanoelectronic devices [1-4], scanning probes [5-7], field emission sources [8-10] and supercapacitors [11]. The common techniques of depositing nanotubes include high pressure arcs, laser ablation and chemical vapour deposition (CVD) [12-18]. Unlike the former two deposition methods, CVD synthesis provides controlled, in-situ growth of nanotubes on substrates. The growth mechanism of carbon nanotubes by CVD is driven by the decomposition of a hydrocarbon gas using a suitable catalyst, the dissolution of carbon in the catalyst and the precipitation of graphitic, tubular carbon from the catalyst. Arrays of ‘spaghetti-like’ nanotubes and aligned carbon nanotubes have been deposited using various types of CVD [19-22]. It has also been reported recently that PECVD processes induce alignment of the nanotubes due to the electric field inherent in the plasma discharge [18,19,21,23]. Patterned growth of nanotubes is achieved by lithographically defining the catalyst areas before nanotube growth. The key requirements for the patterned growth of nanotubes are yield, uniformity and the elimination of unwanted carbonaceous by-products from the deposition process. These requirements are investigated in this paper and we demonstrate uniform preferential growth of aligned nanotubes using PECVD of acetylene and ammonia gases at 700°C.
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EXPERIMENTAL DETAILS Due to the catalytic growth nature of carbon nanotubes, it is possible to define the growth areas of carbon nanotubes on a substrate by pre-patterning the catalyst prior to nanotube deposition. The nickel catalyst used in the synthesis of carbon nanotubes was deposite
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