Role of Gas-phase Reactions and Thermal Gradient Control in Carbon Nanotube Synthesis
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Role of Gas-phase Reactions and Thermal Gradient Control in Carbon Nanotube Synthesis Seul Ki Youn,1 Baskar Pagadala Gopi,2 Kenneth B. K. Teo2 and Hyung Gyu Park1,* 1 Department of Mechanical and Process Engineering, ETH Zürich, Zürich, Switzerland 2 AIXTRON, Anderson Road, Swavesey, Cambridge CB24 4FQ, United Kingdom * To whom correspondence should be addressed: [email protected] ABSTRACT We investigate the role of precursor thermal rearrangement and surface catalytic reactions in the synthesis of vertically aligned carbon nanotubes (VA-CNTs) by acetylene-based, chemical vapor deposition (CVD) and demonstrate a millimeter-long growth of single-walled CNT (SWNT) without water assistance. A substrate heater was used to create an ascending temperature gradient from gas injection to catalyst substrate. Whereas temperature of catalyst substrates primarily determines their catalytic activity, it is a thermal condition of a gaseous mixture in the CVD chamber that also influence growth yield and structural features of as-grown CNTs. Employing Egloff’s characterization, [1] we discuss the importance of various gas thermal zones in producing high-quality nanotubes with augmented growth efficiency. We continue to report production of millimeter-long, VA-SWNT having a mean diameter of 1.7 ± 0.7 nm, catalyzed by iron on an alumina support. Important finding is that a million of aspect ratio of SWNT arrays can be produced, without water assistance, via combined action of an ascending temperature gradient toward catalyst substrate and low partial pressures of acetylene carbon feedstock. Our results do not only emphasize the role of precursor thermal rearrangement in CNT synthesis, but also offer a practical route to the modulation of such complex phenomena for an ultrahigh-yield growth of narrow VA-SWNT. INTRODUCTION VA-CNTs find use in many applications including MEMS/NEMS, nanofluidic membranes, electrical and gecko brush contact, field emitters and fuel cell electrodes. Each application has a requirement such as height of a CNT forest, uniformity in height, and diameter and wall number of average nanotube comprising the CNT forest. Despite the importance, exact control of VA-CNT structure and quality is yet far-fetching. For the controlled synthesis of VACNT, therefore, a detailed understanding of the growth process is crucial. Studies about VA-CNT growth have mostly focused on action and control of catalyst. Recently, however, there is growing interest in the research community about the role of thermal rearrangement of carbon precursors in the mechanism of nanotube growth. CVD conditions of VA-CNT synthesis usually incorporate high temperatures, sub-atmospheric to atmospheric pressures, carbon source and reducing gases (hydrocarbons and hydrogen), inert carrier gas (argon, helium, nitrogen, etc.) and catalyst. Reactions in the gas phase, or pyrolysis, have been assumed a complete decomposition that yields atomic carbon as a reactant for heterogeneous catalysis for the CNT synthesis. An imbalance between carbon flux and catalytic
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