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MWNTmetal2009 - IOP PUBLISHING NANOTECHNOLOGY...

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IOP P UBLISHING N ANOTECHNOLOGY Nanotechnology 20 (2009) 085302 (7pp) doi:10.1088/0957-4484/20/8/085302 Passivation oxide controlled selective carbon nanotube growth on metal substrates J B Bult 1 , W G Sawyer 2 , P M Ajayan 3 and L S Schadler 1 1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA 2 Department of Mechanical Engineering, University of Florida, Gainesville, FL 32611, USA 3 Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77251, USA E-mail: [email protected] Received 10 November 2008, in final form 18 December 2008 Published 2 February 2009 Online at stacks.iop.org/Nano/20/085302 Abstract Vertically aligned arrays of multi-wall carbon nanotubes (MWNT) are grown on Inconel 600, a nickel-based super-alloy. Using x-ray photoelectron spectroscopy (XPS) and chemical vapor deposition (CVD) growth of the MWNTs it is shown that a stable oxidation barrier is required for the stabilization of iron on the substrate and subsequent nanotube growth. This evidence for passivation oxide supported growth of MWNTs was then used to grow MWNTs on patterned oxidized substrates in a selective growth furnace. The unique advantage of this patterned growth on Inconel 600 is found to be the chromia passivation layer’s electrical conductivity (measured value of 1.08 μ m), creating the opportunity for low resistivity electrodes made from nanotubes. Inconel substrates with 100 μ m long aligned MWNTs are demonstrated to exhibit an average resistance value of 2 . 1. Introduction Vertically aligned carbon nanotube (CNT) arrays grown on conductive substrates [ 1–4 ] are of great interest for electronic applications such as field emitters [ 5 , 6 ], high surface area electrodes [ 7 ] and other applications requiring good electrical contact to the nanotube. Direct growth on metal substrates by thermal chemical vapor deposition (CVD) with direct in situ catalyst deposition is of particular interest in these applications due to the technique’s flexibility and ease of production. In the paper by Talapatra et al [ 8 ] it was shown that Inconel 600 (a nickel-based super-alloy) not only produces aligned CNTs, but also higher conductivity through the nanotube–substrate interface. However, these encouraging results have yet to be fully explained in the context of CNT–substrate interaction and its ability to support and control CNT growth. Furthermore, the significance of manipulating the oxide barrier on these conductive substrates has yet to be fully realized. It is well known that the substrate is critical to successful CVD growth of aligned CNTs [ 9 , 10 ]. It has also been generally observed [ 11 , 12 ] that CNTs can be grown on substrates that act as diffusion barriers. The postulated theory herein is that a stable diffusion barrier is required for CNT CVD growth and in the specific case of Inconel it is provided in the form of a conductive passivation oxide of chromium. Moreover, it is shown that, by controlling the oxide location by photolithography and etching, selective CNT
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