Effect of Alignment on Transport Properties of Carbon Nanotube/Metallic Junctions
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Effect of Alignment on Transport Properties of Carbon Nanotube/Metallic Junctions Buzz Wincheski, Min Namkung, 1Jan Smits, 2Phillip Williams, and 3Robert Harvey NASA LaRC Hampton VA 23681, U.S.A. 1 Lockheed Martin Space Operation Hampton VA, 23681, U.S.A. 2 National Research Council Hampton VA, 23681, U.S.A. 3 Christopher Newport University Newport News VA, 23606, U.S.A. ABSTRACT Ballistic and spin coherent transport in single walled carbon nanotubes (SWCNT) are predicted to enable high sensitivity single-nanotube devices for strain and magnetic field sensing. Based upon these phenomena, electron beam lithography procedures have been developed to study the transport properties of purified HiPCO single walled carbon nanotubes for development into sensory materials for nondestructive evaluation. Purified nanotubes are dispersed in solvent suspension and then deposited on the device substrate before metallic contacts are defined and deposited through electron beam lithography. This procedure produces randomly dispersed ropes, typically 2 – 20 nm in diameter, of single walled carbon nanotubes. Transport and scanning probe microscopy studies have shown a good correlation between the junction resistance and tube density, alignment, and contact quality. In order to improve transport properties of the junctions, a technique has been developed to align and concentrate nanotubes at specific locations on the substrate surface. Lithographic techniques are used to define local areas where high frequency electric fields are to be concentrated. Application of the fields while the substrate is exposed to a nanotube-containing solution results in nanotube arrays aligned with the electric field lines. A second electron beam lithography layer is then used to deposit metallic contacts across the aligned tubes. Experimental measurements are presented showing the increased tube alignment and improvement in the transport properties of the junctions. INTRODUCTION Carbon nanotube based materials have been heavily researched lately due to the promising electronic properties, high strength, and low density of carbon nanotubes [1]. Multifunctional materials capitalizing on these attributes have the potential to enable the development of high strength, lightweight, materials with embedded sensing capabilities. In particular, NASA LaRC is studying single wall nanotube based materials for structural health monitoring applications including strain sensing and electromagnetic nondestructive evaluation [2]. In order to fabricate such materials systems, a significant effort has been placed on carbon nanotube dispersion, alignment, and nanotube/metallic junction quality. In this work, a method for directed placement of solution dispersed single wall carbon nanotubes across a surface is described. Nanotube alignment in localized positions across a 2 mm2 area has been
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demonstrated, and transport and scanning probe microscopy measurements show improved junction quality over randomly aligned systems. ELECTRIC FIELD INDUCED ALIGNMENT OF S
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