Multiwalled Carbon Nanotubes by Chemical Vapor Deposition Using Multilayered Metal Catalysts Lance Delzeit,* Cattien V. Nguyen, ² Bin Chen, ² Ramsey Stevens, ² Alan Cassell, ² Jie Han, ² and M. Meyyappan NASA Ames Research Center, Moffett Field, California 94035 ReceiVed: February 8, 2002; In Final Form: March 28, 2002 Multiwalled carbon nanotubes (MWCNTs) have been grown on various substrates by thermal chemical vapor deposition using multilayered metal catalysts. Ion beam sputtering is used to deposit various metal layers sequentially. Underlayers of Al (2-20 nm) are shown to influence the growth characteristics with Fe or Ni used as active catalysts. The as-sputtered catalyst surface, characterized using atomic force and scanning tunneling microscopies, consists of nanometer scale (<10 nm) catalyst particles. High-resolution transmission electron microscopy and Raman analysis are used to characterize the MWCNTs. Optimization of the two layers allows growth of MWCNT towers on patterned and unpatterned substrates. Introduction Carbon nanotubes (CNT) have been the subject of intensive research recently because of their unique electronic properties and extraordinary mechanical properties. While there is no doubt that the single-walled nanotube is the highly desirable material, multiwalled carbon nanotubes (MWCNTs) are also investigated widely because of their potential as field emitters and electrodes. Inexpensive and scalable growth techniques are critical to realize these applications, and chemical vapor deposition (CVD) with the aid of transition-metal catalysts has emerged as a viable technique since it is amenable to grow nanotubes on patterned substrates and in vertically aligned arrays. 1,2 Numerous re- searchers have reported MWCNT growth by CVD using hydrocarbon (CH 4 ,C 2 H 2 ,C 2 H 4 , etc.) or CO feedstock and Fe, Ni, or Co as catalysts supported on various substrates. 2-12 Continuous gas-phase CVD of MWCNTs aided by floating catalysts, generated in situ, has also been reported. 13,14 The choices for the catalyst, substrate, and the method to transfer the catalyst to the substrate are critical to the success of CVD of nanotubes. In many cases, the catalyst precursor and structure-directing agents or other additives, if any, are in a solution which is evaporated and calcined to prepare the catalyst formulation on the substrate. 1,4-8 One problem with solution-based catalyst preparation techniques is the difficulty in confining the catalyst within patterns, particularly in small- feature sizes needed for device development. Another problem is the long time required for and the cumbersome nature of catalyst preparation. A typical solution-based procedure involves many of the following steps: mixing, dissolution, refluxing, separation, cooling, gel formation, reduction, and drying/ annealing/calcination. Some of these steps take several hours to overnight processing. In contrast, physical deposition pro- cesses are quick, easy, and amenable to produce small patterns. For example, electron gun evaporation, 10 thermal evaporation, 11 pulsed laser deposition, 12 and magnetron sputtering 15 have been used in catalyst deposition. Nanochannel alumina templates made by anodization and followed by e-beam 2 or electrochemi- cal 9 deposition have been helpful in growing highly ordered array of MWCNTs. Ion beam sputtering is used in the present work to deposit an underlayer of Al followed by an active catalyst layer of either iron or nickel. The catalyst layer applied by physical processes in previous works 10-12,15 appears to be a thin film (under 100 nm) and smooth. However, apparently smooth films do not grow nanotubes, and catalyst nanoparticles are needed to enable nanotube growth. 16 Nanoparticle creation appears to be ac- complished by pretreatment of the catalyst film with am- monia, 11,12 ion bombardment in a plasma environment, 15 or any other approach to break a film into small particles prior to starting the CVD process. In ammonia pretreatment, the treatment time, temperature, NH 3 flow rate, and substrate seem to influence the resulting particle size. In general, thinner catalyst films yield smaller particle sizes with little agglomeration. 12 There is evidence 10 that nucleation and growth of nanotubes increase with a decrease in catalyst layer thickness at a fixed growth temperature. In plasmas, particle creation is enabled by a pretreatment in an inert gas plasma prior to the introduction of the hydrocarbon into the growth chamber. 15 The treatment time, substrate bias (and hence the ion energy), and the starting film thickness control the size distribution of resulting particles. We have previously found that introduction of a metal under- layer (such as Al or Ir) under the active catalyst layersinstead of any form of pretreatment of the catalyst layersappears to increase the surface roughness and to provide more active nucleation sites. 16 This approach is also the least time-consum- ing. Indeed in ref 16, we have successfully demonstrated growth of SWCNTs and shown that the density of the nanotubes can be varied by varying the thickness of the underlayer and the active catalyst layer. In this work, we extend this approach to grow MWCNT towers, and we discuss the results as a function of growth conditions and multilayer formulation. Experimental Work Ion beam sputtering is used for the deposition of an underlayer of Al followed by an active catalyst (Fe or Ni) to grow MWCNTs. The metals used in the experiments are 99.9+% * To whom correspondence should be addressed. MS 239-4. Phone: 650- 604-0236. Fax: 650-604-1088. E-mail: ldelzeit@mail.arc.nasa.gov. ² ELORET Corporation. 5629 J. Phys. Chem. B 2002, 106, 5629-5635 10.1021/jp0203898 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/14/2002