In-situ TEM-STM Observations of SWCNT Ropes/tubular Transformations
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In-situ TEM-STM Observations of SWCNT Ropes/Tubular Transformations F. Solá1a, M. Lebrón-Colón1b, P.J. Ferreira2, L. F. Fonseca3, M.A. Meador1b and C. Marín3 1 National Aeronautics and Space Administration (NASA), Glenn Research Center, Structures and Materials Division, 21000 Brookpark Road, Cleveland, OH 44135, U.S.A a Advanced Metallics Branch; bPolymers Branch 2 Materials Science and Engineering Program, University of Texas at Austin, Austin, TX, 78712, U.S.A 3 Institute for Functional Nanomaterials and Department of Physics of the University of Puerto Rico, Rio Piedras, PO Box 23343, San Juan, PR 00931, U.S.A.
ABSTRACT Single-walled carbon nanotubes (SWCNTs) prepared by the HiPco process were purified using a modified gas phase purification technique. A TEM-STM holder was used to study the morphological changes of SWCNT ropes as a function of applied voltage. Kink formation, buckling behavior, tubular transformation and eventual breakdown of the system were observed. The tubular formation was attributed to a transformation from SWCNT ropes to multi-walled carbon nanotube (MWCNT) structures. It is likely mediated by the patching and tearing mechanism which is promoted primarily by the mobile vacancies generated due to currentinduced heating and, to some extent, by electron irradiation. INTRODUCTION Since the discovery of carbon nanotubes (CNTs) by Iijima [1], CNTs have been extensively studied due to their unique electrical, mechanical and thermal properties [2]. Among the various experimental techniques used to investigate these properties, in-situ transmission electron microscopy (TEM) has contributed significantly to the fundamental understanding of CNT’s behavior. The main reason for this is that in-situ TEM generally allows real-time observation and manipulation of structures at atomic scale resolution [3]. To perform in-situ TEM experiments several TEM holders are available [3]. One of these holders, named STM holder, has led to the: 1. Discovery of superplasticity in CNTs [4], 2.Demonstration of cap-to-cap joining of two SWCNT, which may be used as a route to reconstruct a failed nanotube during electrical device performance [5], 3.Creation of tunable nanoresonators [6], 4.Observation of vacancy migration [7], 5.Demonstration of the nanopipette behavior for metal transport [8] and 6.Systematic comparative studies of transport measurements with boron nitride nanotubes [9]. In addition, the C-K energy-loss near-edge structure [10] has been used to extract bonding information of irreversibly deformed SWCNT ropes induced by the STM tip. Results show a correlation between the reduction of π bonding (by the introduction of permanent nonhexagonal defects) and the current capacity of the tubes [11]. However, most of the TEM-STM work has focused on single nanotubes rather than ropes. In this study (using the TEM-STM holder) we present evidence of current-induced morphological modifications of purified SWCNT ropes made by the HiPco (high pressure carbon monoxide) process, which are relevant to underst
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