Optimizing catalyst nanoparticle distribution to produce densely-packed carbon nanotube growth E. Terrado, I. Tacchini, A.M. Benito, W.K. Maser, M.T. Martı´nez * Instituto de Carboquı ´mica, CSIC, Miguel Luesma Casta ´ n 4, 50018 Zaragoza, Spain ARTICLE INFO Article history: Received 23 August 2008 Accepted 20 March 2009 Available online 27 March 2009 ABSTRACT The experimental parameters involved in the formation of the Ni catalytic nanoparticles on Si/SiO 2 substrates that seed carbon nanotube growth were investigated. It was found that after deposition of a nickel film on the substrate, the temperature and time of the thermal and reduction catalyst pre-treatment steps are crucial variables for optimized nanoparticle distribution with different average diameters, depending on the initial film thickness. Den- sely-packed carbon nanotube forests with interesting potential applications have been grown from this nanoparticle distribution. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) can become a key element in the nanotechnology development since their unique size and structure determine their electronic complexity and excellent physical and chemical properties [1,2] making them promis- ing candidates for interesting applications in such different areas as electronic, material science or medicine [3,4]. Nevertheless, the ability to grow CNTs with specific diam- eter and chirality directly on a substrate is the real challenge from a technological point of view. This control would allow the integration of the grown nanotubes into functional elec- tronic devices including field emission displays, gas sensors, resonators or supercapacitors. In the development of such applications, chemical vapour deposition (CVD) processes ap- pear to be the most suitable technique for direct CNT growth on substrates owing to their versatility in terms of the type of catalyst, carbon sources, environmental gases and process variables (pressure, temperature, reaction time) [5,6] that can be utilized. The CVD process for CNT growth on a supported catalyst is made up of several steps, one of the most important being the preparation of the metal nanoparticles on the substrate [7]. Once the hydrocarbon gas is let into the furnace, the carbon feedstock molecules decompose on the metal surface of the nanoparticle at a temperature ranging from 600 to 1100 °C. The role of such metal nanoparticles is very complex [8] and essential for CNT growth. In-depth knowledge of the function of these catalyst nanoparticles encompasses many factors such as their composition and morphology, size, density and distribution directly derived from their preparation meth- od, the catalyst–substrate interaction and the pre-treatment conditions. All of the mentioned parameters as a whole deter- mine the final catalytic properties of the metal nanoparticles. Since each nanoparticle acts as a seed for a nanotube, con- trolling catalyst nanoparticles is the shortest way of achieving the controlled growth of CNTs. The catalyst can be deposited onto the substrate by a chemical method [9,10] through the use of metal containing solutions or a physical method [7]. Physical methods include several techniques such as electron gun evaporation [11], thermal evaporation [12], ion-beam sputtering [13] and mag- netron sputtering [14] that are suitable to deposit thin catalyst films with a thickness under 20 nm. Subsequent heating and reduction steps allow small metal nanoparticles able to pro- mote CNT growth to be obtained quickly and easily [7]. These 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.03.045 * Corresponding author: Fax: +34 976733318. E-mail address: mtmartinez@icb.csic.es (M.T.Martı´nez). CARBON 47 (2009) 1989 – 2001 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon