Investigations of dc electrical properties in electron-beam modified carbon nanotube films: single- and multiwalled
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Investigations of dc electrical properties in electron-beam modified carbon nanotube films: single- and multiwalled S. Gupta1,∗, N. D. Smith1, R. J. Patel2, and R. E. Giedd2 1 Department of Physics, Astronomy, and Materials Science 2 Center for Applied Science and Engineering, Missouri State University, Springfield, MO 65897 (USA). ABSTRACT Carbon nanotubes (CNTs) in the family of nanostructured carbon materials are of great interest because of several unique physical properties. For space applications, it needs to be shown that CNTs are physically stable and structurally unaltered when subjected to irradiation becomes indispensable. The CNT films were grown by microwave plasma-assisted chemical vapor deposition (MWCVD) technique using Fe as catalyst. Synthesis of both single- and multiwalled CNTs (SW and MW, respectively) were achieved by varying the thickness of the Fe catalyst layer. To investigate the influence of electron-beam irradiation, CNTs were subjected to low and/or medium energy electron-beam irradiation continuously for a few minutes to several hours. The CNT films prior to and post-irradiation were assessed in terms of their microscopic structure and physical properties to establish property-structure correlations. The characterization tools used to establish such correlations include scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy (RS), and current versus voltage (I-V) measuring resistance (two-probe) and conductivity (four-probe) properties. Dramatic improvement in the I-V properties for single-walled (from semiconducting to quasi-metallic) and relatively small but systematic behavior for multi-walled (from metallic to more metallic) with increasing irradiation hours is discussed in terms of critical role of defects. Alternatively, contact resistance of single-walled nanotubes decreased by two orders of magnitude on prolonged E-beam exposures. Moreover, these findings provided onset of saturation and damage/degradation in terms of both the electron beam energy and exposure times. Furthermore, these studies apparently brought out a contrasting comparison between mixed semiconducting/metallic (single-walled) and metallic (multiwalled) nanotubes in terms of their structural modifications due to electron-beam irradiation. INTRODUCTION Advanced nanostructured carbon-based materials continue to attract attention because of their unique combination of structural and physical properties and therefore offering multifunctionality. CNTs, in particular, exhibit exceptional structural (high aspect ratio of ~ 1000), physical (electrical, electronic, mechanical), and chemical properties, which have resulted in their continued investigations since they were discovered [1, 2]. One of the exciting behaviors of SW is their dual electronic character (semiconducting and metallic) which is based on their chirality. In other words, SW can be used for semiconducting devices as well as conducting nano-wires if sought for [2]. In either case i.e.
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