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Optimizing Control of Fe Catalysts for Carbon Nanotube Growth

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One must control the size distribution of catalyst Fe nano-particles (NPs) very carefully if one is to have any chance of growing “super-aligned” carbon nanotube (CNT) forests which can be spun directly into yarns and pulled directly into long sheets. Control of the Fe Nps size is important during all phases, including: the catalyst deposition, annealing and forest growth. As a result, it is important to understand how NPs are affected by various experimental factors as well as how those catalyst NPs then cause the growth of the forests. This paper focuses on two key experimental factors: The as-deposited thickness of the Fe catalyst film and the use of hydrogen gas (H2) during anneal and growth. We found that the sheet resistance (R s) of as-deposited Fe films is directly related to the average film thickness and can be used to estimate whether the films can catalyze the growth of super-aligned forests. The height of the CNT forests decrease with decreasing R s, but only slowly. More importantly, CNTs grown on the largest and the smallest R s films are less aligned. Instead, they are more curled and wavy due to the Fe NP dynamics. The use of Hydrogen (H2) affects the formation of Fe NPs from the as-deposited film as well as their composition during the forest growth. We find that the addition of H2 to a CNT forest growth process at 680 °C (C2H2/He [30/600 sccm]) increases the CNT alignment substantially. H2 can also reduce iron-oxides which otherwise would impede the formation of NPs. As a result, H2 has multiple roles: besides its chemical reactivity, H2 is important for catalyst reconstruction into NPs having a proper size distribution as well as surface density.
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Document Type: Research Article

Publication date: 2012-07-01

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  • Journal for Nanoscience and Nanotechnology (JNN) is an international and multidisciplinary peer-reviewed journal with a wide-ranging coverage, consolidating research activities in all areas of nanoscience and nanotechnology into a single and unique reference source. JNN is the first cross-disciplinary journal to publish original full research articles, rapid communications of important new scientific and technological findings, timely state-of-the-art reviews with author's photo and short biography, and current research news encompassing the fundamental and applied research in all disciplines of science, engineering and medicine.
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