Growth rate of plasma-synthesized vertically aligned carbon nanofibers Vladimir I. Merkulov a, * , A.V. Melechko a,b , M.A. Guillorn a,c , D.H. Lowndes d , M.L. Simpson a,c a Molecular Scale Engineering and Nanoscale Technologies Research Group, Oak Ridge National Laboratory, P.O. Box 2008, MS 6006, Oak Ridge, TN 37831, USA b Center of Environmental Biotechnology, University of Tennessee, Knoxville, TN 37996, USA c Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA d Thin Film and Nanostructured Materials Physics Group, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 14 June 2002 Abstract Vertically aligned carbon nanofibers (VACNFs) were synthesized by direct-current plasma enhanced chemical vapor deposition using acetylene and ammonia as the gas source. The mechanisms responsible for changing the nanofiber growth rate were studied and phenomenological models are proposed. The feedstock for VACNF growth is suggested to consist mainly of radicals formed in the plasma and not the unexcited acetylene gas molecules. The growth rate is shown to increase dramatically by changing the radical transport mechanism from diffusive to forced flow, which was accomplished by increasing the gas flow in the direction perpendicular to the substrate. Ó 2002 Elsevier Science B.V. All rights reserved. Vertically aligned carbon nanofibers (VACNFs) prepared by plasma enhanced chemical vapor de- position (PECVD) [1–7] are an important material for various applications including electron field emitters [8,9], tips for scanning microscopy [10], and biological probes [11], among others. Despite the morphological similarity, the structure of VACNFs, which are composed of concentric graphene funnels and cones, is quite different from that of carbon nanotubes (CNTs), which consist of concentric graphene cylinders. To date, the crucial advantage of using PECVD-grown VACNFs is the ability to synthesize them deterministically [12], i.e., their location, height, tip and base diameters [12–14], and, to some extent, shape [15], orienta- tion [10], and chemical composition [2,16] can all be controlled, and mechanically and electrically reliable contact to the substrate can be established. It has been suggested [17] that in PECVD only VACNFs grown from the tip are aligned specifi- cally due to the presence of the plasma electric field in the growth process, whereas VACNFs grown 6 August 2002 Chemical Physics Letters 361 (2002) 492–498 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-865-576-2813. E-mail address: merkulovvi@ornl.gov (V.I. Merkulov). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)01016-3