Carbon nanotube growth on metal-catalyzed substrates in a laser oven apparatus
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Carbon nanotube growth on metal-catalyzed substrates in a laser oven apparatus Y. Suda, A. Tanaka, A. Okita, M.A. Bratescu, Y. Sakai, J. Nakamura1, G.Y. Xiong2, Z.F. Ren2 Graduate School of Information Science and Technology, Hokkaido University North 14, West 9, Sapporo 060-0814, Japan 1 Institute of Materials Science, University of Tsukuba Tsukuba, Ibaraki, 305-8573, Japan 2 Department of Physics, Boston College 140 Commonwealth Ave, Chestnut Hill, Massachusetts 02467, USA ABSTRACT Carbon nanotubes (CNTs) were grown on Ni- and Fe-coated SiO2/Si substrates in a laser oven apparatus. The grown CNTs were analyzed by scanning electron microscopy. It is speculated that the CNTs grow out from the metal nanoparticle after laser-ablated carbon clusters have been dissolved in it. In a range of oven temperatures between 800 and 1100oC, growth of CNTs was achieved at a temperature ≥ 1000oC. The thickness of the Ni film controlled the CNTs diameter. INTRODUCTION Since carbon nanotubes (CNTs) have excellent physical and electronic properties, such as field emission property, high electrical conductivity and high tensile strength, they have been widely studied for various applications to field electron emitters, scanning probes and electronic devices [1]. Several techniques, laser ablation [2], arc discharge [3] and chemical vapor deposition (CVD) [4], have been developed for the production of single-walled CNTs (SWNTs). In the laser ablation technique, the process of CNT growth is thought to be as follows: (1) carbon and metal catalyst (Ni/Co) clusters are vaporized at a temperature of ~4000ºC by laser ablation (LA) of the graphite target containing the catalyst metals, (2) these clusters are gradually cooled by collision with ambient gas (Ar) atoms and condense to liquid-phase nanoparticles, and (3) while the temperature of nanoparticles drops to the oven temperature (~1200ºC), which is below the carbon-metal eutectic temperature, Teu (e.g., Teu for C-Ni = 1326.5ºC), carbon atoms are separated from the nanoparticles and form CNTs in a gas phase [5]. The optimal oven temperature for this process has been determined to be ~1200oC [6]. On the other hand, the CVD technique using CH4 gas enables SWNTs to form on metal-catalyzed substrates at a lower temperature (~900oC) than that in the laser ablation method [7]. Since it is well known that catalyst metals, especially Ni, are efficient catalysts for dissociation of CH4 (CH4 → C + H2) [8], carbon atoms can be generated from this dissociation process on the catalyst surface even at a low temperature of ~500oC and dissolved in it. The dissociated H2 molecules also play an important role in reducing the catalyst. We are interested in the difference of the optimal temperatures for the SWNT growth by the laser ablation and CVD. To investigate this difference, we have studied the laser-thermal chemical vapor deposition (LTCVD) technique [9]. LTCVD is a combination of LA and CVD.
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The carbon source is supplied from an LA plume of graphite and reaches metal-catalyzed Si
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