rXXXX American Chemical Society A dx.doi.org/10.1021/jp108624n | J. Phys. Chem. C XXXX, XXX, 000–000 ARTICLE pubs.acs.org/JPCC Nickel Catalyst-Assisted Vertical Growth of Dense Carbon Nanotube Forests on Bulk Copper Gowtam Atthipalli,* ,† Rigved Epur, † Prashant N. Kumta, †,‡,§ Mengjin Yang, † Jung-Kun Lee, † and Jennifer L. Gray † † Department of Mechanical Engineering and Materials Science, ‡ Department of Chemical and Petroleum Engineering, and § Department of Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ABSTRACT: Vertical growth of carbon nanotubes using thermal chemical vapor deposition (CVD) is demonstrated on bulk copper substrates by first sputtering a thin Ni film on the surface of copper. Vertical growth of carbon nanotubes occurred when the nickel film thickness was 20 nm and the carbon nanotubes were grown using a xylene source and additional ferrocene catalyst during CVD. These results show the effectiveness of this method in directly integrating carbon nanotubes with highly conductive substrates for applications where a conductive carbon nanotube network is desirable. ’ INTRODUCTION There has been ongoing interest in understanding and con- trolling the synthesis of carbon nanotubes (CNTs) ever since their initial discovery. 1 This is required in order to successfully integrate them into new devices and applications that exploit their excellent physical properties, including high mechanical strength, high aspect ratio, and conductivity. 2,3 There have been extensive publications in recent years to demonstrate the growth of nanotubes under different conditions. 4-7 These studies show that the resulting structure and the morphology of the nanotubes determine what types of applications they may be useful for. For example, randomly oriented, entangled CNTs have been sug- gested to be of use in capacitors because of the large surface area of the nanotubes, which is ideal for charge storage. 8 For use in applications such as electrodes, where the nanotubes might serve as an electrically conducting array or conducting support struc- ture, vertically aligned carbon nanotube growth directly on a common conducting substrate such as copper would be ideal. Copper is one of the most common conductive metals used in many applications because of its excellent thermal and electrical conductivity with respect to cost. The direct growth of nanotubes on Cu would provide good electrical contact to the nanotubes. These conductive, vertically aligned structures could be used as a conductive support for fabricating new structures for a variety of applications including energy storage, sensing, and nanoelectro- nic devices. 9 Typically, nanotubes are grown using a method such as chemical vapor deposition on metal catalyst particles or islands that are deposited on top of a semiconducting or insulating sub- strate such as SiO 2 . These nonconducting substrates facilitate the formation of small islands or nanoparticles of the catalyst metal on their surfaces, which are necessary for forming the CNTs. By comparison, fewer studies have been done on bulk metallic substrates. Although it has been demonstrated that Cu nano- particles may act as a catalyst for carbon nanotube growth, 10,11 it is difficult to grow CNTs directly on bulk Cu without the aid of an additional catalyst. Copper is a poor catalyst for CNT growth as carbon shows very little solubility in copper. 12,13 Deck and Vecchio 12 have reported that for transition metals to act as suc- cessful catalysts for CNT growth about 1 atom % carbon solu- bility in the solid solution is required. Among the transition metals, Fe, Co, Ni, and their alloys serve as the best catalysts for achieving directed growth of carbon nanotubes. 13-16 Growth of CNTs on copper has been reported using a multilayer structure consisting of titanium nitride as a barrier layer and a second layer of iron as the catalyst. 17 Recently, Li et al. 17 reported the growth of CNTs on copper using a thermal CVD technique and multi- layer structures consisting of a Ti buffer layer, an alumina layer, and finally a metal catalyst layer. In all of these cases, it is expected that the barrier layers may increase the electrical resistance between the copper and the nanotubes. However, if only a single thin film metallic catalyst layer is deposited on the copper, in this case, the increase in resistance may be limited to just the remaining native oxide on the copper surface and conductive catalyst par- ticles at the base of the nanotubes. Beyond simply finding a way to grow CNTs on copper, forcing them to grow in a primarily vertical direction is an additional challenge. Vertically aligned growth using thermal CVD is typically more difficult because of the absence of any electric fields or plasma. The plasma in a plasma-enhanced CVD (PECVD) system is Received: September 9, 2010 Revised: January 14, 2011