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Molecular Layer Deposition on Carbon Nanotubes Joseph J. Brown, Robert A. Hall, Paul E. Kladitis, Steven M. George, and Victor M. Bright ACS Nano, Just Accepted Manuscript • DOI: 10.1021/nn402733g • Publication Date (Web): 13 Aug 2013 Downloaded from http://pubs.acs.org on August 14, 2013
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ACS Nano
Molecular Layer Deposition on Carbon Nanotubes Joseph J. Brown,†,* Robert A. Hall,‡ Paul E. Kladitis,§ Steven M. George,‡,† Victor M. Bright† †
Department of Mechanical Engineering, University of Colorado, 427 UCB, Boulder, CO 803090427, United States. ‡Department of Chemistry and Biochemistry, University of Colorado, 215
UCB, Boulder, CO 80309-0215, United States. §Department of Electrical Engineering, Air Force Institute of Technology, Wright Patterson Air Force Base, OH 45433, United States. *Address correspondence to
[email protected].
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ABSTRACT
Molecular layer deposition (MLD) techniques were used to deposit conformal coatings on bulk quantities of carbon nanotubes (CNTs). Several metalcone MLD chemistries were employed, including alucone (trimethylaluminum/glycerol and trimethylaluminum/ethylene glycol), titanicone (TiCl4/glycerol), and zincone (diethyl zinc/glycerol). The metalcone MLD films grew directly on the CNTs and MLD initiation did not require atomic layer deposition (ALD) of an adhesion layer. Transmission electron microscopy revealed that MLD formed three-dimensional conformal deposits throughout a CNT scaffold. Mechanical testing was also performed on sheets of CNT networks coated by MLD. Young’s Modulus values improved from an initial value of 510 MPa for uncoated CNT sheet to values that range from 2.2 GPa, for 10 nm of glycerol alucone (AlGL), to 8.7 GPa for a composite 5 nm AlGL + 5 nm Al2O3 coating.
KEYWORDS: Carbon Nanotube, Molecular Layer Deposition, ALD, Mechanical Testing, Conformal, Composite, Scaffold
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ACS Nano
Translation of the interesting physical properties (high strength, high electrical conductivity, high current capacity, high thermal conductivity, low thermal expansion) of carbon nanotubes (CNTs) into useful macroscale applications has been a significant challenge since the inception of CNT research due to limitations on CNT availability and on the accessibility of technologies for measuring and utilizing the CNT properties in large-scale structures. Continual improvements in high volume carbon nanotube synthesis have resulted in current processes with sufficient yield to allow bulk testing and handling of carbon nanotubes. In bulk quantities, CNTs can provide a high surface area scaffolding material capable of supporting coatings that provide functionality for diverse new applications.1,2 These include structural composites, stabilization for material handling, strain or gas sensing, tissue scaffolds, separations, drug delivery, environmental remediation, catalysis, chromatography, and battery and supercapacitor electrodes.2,3 CNTs are most commonly available as powders that may be dispersed in suspension and then collected into different ensemble forms. In other cases, CNTs are now available as forests on substrates, or as yarns or sheets. Spinning of CNT sheets with incorporated atomized titania and boron particles has been identified as one mechanism of formation of CNT bulk composites with optimized properties.4 However, this mechanism is limited to only one structural modality, the formation of yarns or other linear structures. A more versatile approach is to add coating technologies as processing steps after the initial bulk CNT shape-forming processes that generate sheets, filaments, or other modalities of CNT ensembles. Liquid coatings have been used to provide customizable surface treatments (such as polymer-derived ceramic formation) on some CNT materials,1,5,6 but liquid-based processes can modify CNT forest shapes due to capillary forces.7–9 Some larger polymer molecules such as PDMS and PVA will readily infiltrate microporous CNT network structures,1 but these
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molecules may not reliably penetrate CNT bundles. Small molecules in liquid and gas phases may penetrate the tight, rope-like packing of single wall and double wall CNTs clumped into bundles in bulk ensembles.10–13 Gas phase deposition processes provide a complementary approach to conformal coating and bonding of CNTs. One such technology is atomic layer deposition (ALD), which deposits materials conformally based on the automated cycling of component gases.14,15 The growth of individual layers is a self-limiting reaction, resulting in linear growth of material, with precise thickness dependent upon the number of ALD cycles exposed to the substrate. Molecular layer deposition (MLD, Figure 1) uses a similar approach to generate controlled-thickness conformal organic or hybrid organic-inorganic films, and can be deposited in combination with inorganic ALD layers.16–18 The ALD and MLD processes are performed at relatively low temperatures, typically
