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Growth of Vertically Aligned Carbon Nanotube Arrays on Al Substrates through Controlled Diffusion of Catalyst Zhaoli Gao, Xinfeng Zhang, Kai Zhang, and Matthew M F Yuen J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Jun 2015 Downloaded from http://pubs.acs.org on June 4, 2015
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Growth of Vertically Aligned Carbon Nanotube Arrays on Al Substrates through Controlled Diffusion of Catalyst Zhaoli Gao†,‡, Xinfeng Zhang†, Kai Zhang†, Matthew M. F. Yuen†*
† Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China ‡ Present address: Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
ABSTRACT Atomic diffusion induced catalyst evolution affects the iron catalyst lifetime and the collective termination of VACNT growth on a metal substrate. We demonstrated that controlled catalyst diffusion in the pretreatment stage is capable of extending the iron catalyst lifetime on an aluminum thin film substrate to allow enhanced vertically aligned carbon nanotube (VACNT) growth by a conventional low pressure thermal chemical vapor deposition. In adopting a fast-heating pretreatment to control the catalyst loss from diffusion, catalyst lifetime was extended, leading to the VACNT growth with heights up to 65% taller than those without the diffusion control strategy. Atomic diffusion of iron catalyst along the grain boundaries of aluminum thin films was found to play a critical role in the catalyst loss
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mechanism. This work provides the basic understanding of catalyst diffusion control to facilitate the growth of VACNT arrays on metallic substrates for various applications such as through silicon via interconnects micro-electrodes, field emission devices, etc.
1. Introduction CNT is a material with extraordinary electrical 1-2 and thermal 3-4 properties. Integration of vertically aligned carbon nanotube (VACNT) arrays onto a metal substrate is a necessary step in bringing these arrays into practical applications such as through-silicon via (TSV) interconnects 5-7
, micro-electrodes 8-9 and field emission devices 10-11. Conventionally, VACNT arrays are
grown on alumina (Al2O3) substrates, then transferred onto the conductive metal substrates through the wet chemical 12-13 or metallic coating 14-15 approaches which inevitably involve complex processes and introduce additional interfacial resistances between the CNTs and the metal substrates. Directly growing VACNT arrays on the metal substrates, providing robust mechanical, thermal and electrical contacts between the CNTs and metal substrates, has attracted intense interest furthering development of CNT devices 16-17. Progress has been reported in growing long VACNT arrays (>100 µm) using solution catalysts, e.g., Fe(NO3)3 solutions 18-20, and vapor catalysts e.g., ferrocene 17, 21, however it remains difficult to achieve specific CNT patterns for applications like TSV interconnects and micro-electrodes. In order to solve this problem, a physical vapor deposited thin film catalyst, taking advantage of the photolithography developed patterns, has to be implemented. Hiraoka et al. 22 used water-assisted CVD and demonstrated the growth of single- and double-walled CNT on Ni-based alloy foils with the catalytic thin films of Al2O3 (30 nm)/Fe (1 nm). Park et al. 8 succeeded in fabricating VACNT micro-electrodes on conductive substrates (Pt and glassy carbon) using a photolithography patterned catalyst. The lengths of the VACNT arrays achieved
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were 2 µm and 10 µm at a synthesis temperature of 600 °C and 700 °C, respectively. Nessim et al. 23 grew 2 µm VACNT arrays with an Fe catalyst on a Ta underlayer by preheating the inlet gas to 770 °C. Burt et al.24 varied the substrate temperature to adjust the grain sizes of the sputtered Al thin film underlayer, and showed growth of VACNT arrays with a maximum length of 18 µm. Cola et al. 25 reported the growth of 50 µm long VACNT arrays on Cu using a plasma enhanced chemical vapor deposition (PECVD) process. Esconjauregui et al. 26 report the growth of carbon nanotube forests on conductive substrates, i.e., W, Ti, and TiN, with an ultra-thin Al2O3 layer of 0.5 nm thick. Recently, Zhong et al. 16 in the same group of Esconjauregui reported a catalyst diffusion control scheme for enhanced VACNT growth on metallic Ti substrates by applying an Fe/Ti/Fe catalyst structure, enabling the fabrication of high areal density (~1012 cm2) VACNTs with lengths of around 20 µm. Rao et al. 27 demonstrated a promising methodology to grow CNTs on a variety of substrates, e.g. Cu, Pt and diamond in adoption of graphene on top of the substrates. More recently, Rao et al. 28 showed that graphenecovered silicon substrates enable CNT forest growth with heights up to twice as compared against bare silicon substrates along with an increase in tube density up to 30%, and suggested that graphene enhances the catalytic activity of Fe nanoparticles. Although encouraging progress on VACNT growth on metallic thin films has been achieved, the majority of the work focused on the modification of the CNT synthesis conditions, often involving complex processes, e.g. PECVD, carbon precursor preheating, additional buffer layer, and growth control of VACNTs on metal substrates remains a challenge. Effects such as catalyst pretreatment/dewetting process are still not well understood, e.g., effects of catalyst loss during the pretreatment process on catalyst lifetime and the mechanism of catalyst loss. The Fe thin film catalyst undergoes a dewetting process at the prescribed pretreatment temperature to form
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catalyst nanoparticles on the substrate surface. Alongside with the dewetting process, catalysts lose from the substrate surface due to diffusion29-32. Bedewy et al. 33 claimed that catalyst loss through diffusion causes the collective deactivation of the catalyst leading to termination of VACNT growth. Kim et al. 31, using an in-situ transmission electron microscopy, also showed that termination of VACNT array growth is intrinsically linked to the catalyst loss from diffusion. In addition, Ostwald ripening34-36 is another process causing the loss of active catalysts, and the concurrent presence of catalyst diffusion and Ostwald ripening was revealed by Sakura et al.37, and the competition between these two processes is affected by the catalystunderlayer interactions and diverse synthesis systems used. Given the considerably higher atomic diffusion coefficients for the Fe in metals than those in oxides, e.g., ~2.89 × 10-9 cm2/s 38 for Fe in Al at a temperature of 927 K, which is four orders of magnitude higher than that in Al2O3 (
