Electron Spin Resonance on Carbon Nanofibers
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Electron Spin Resonance on Carbon Nanofibers Mircea Chipara1, Robert H. Hauge2, Hua Fan2, Richard Booker2, Haiqing Peng2, Wen Fang Hwang2, J. M. Zaleski1, Richard Smalley2 1
Indiana University, Bloomington, IN 47405
2
Chemistry Department, University of Rice, Houston, TX
INTRODUCTION The discovery of carbon nanotubes (CNT) and the subsequent evaluation of their outstanding properties (huge Young modulus, high electric conductivity, and large thermal conductivity) incited the research of composite materials obtained by dispersing CNT in different polymeric matrices. An increase of the Young modulus and tensile strength of the polymeric matrix, due to the dispersion of low amounts of CNT (typically below 10%) was reported [1-3]. Extremely low percolation thresholds (even smaller than 1 % wt CNT in polymers) have been reported in various composites based on polymeric matrices [1, 3]. It was speculated [1-3, 8, 9] that the high aspect ration of nanotubes would trigger anisotropic properties in polymer-carbon nanotubes materials that would allow a further enhancement of their mechanical, electrical, and thermal properties. To obtain a composite with excellent properties is required to have uniform dispersion of CNT (for isotropic composites) or controlled orientation of CNT (anisotropic composites). The agglomeration of CNTs makes difficult their dispersion in polymer solutions or melts [3]. Carbon nanofibers (CNF) is a collection of SWNTs oriented parallel dispersed in a polymeric matrix. This is a cheaper approach [8] to anisotropic composites, lightweight composites, high-strength composites, and electrical (thermal) conducting composites than the standard dispersion of randomly oriented and agglomerated SWNTs. The as prepared (crude) CNF shows weak metallic properties resulting from the p doping (presumably due to bisulfate ions [8, 9]). Electron spin resonance (ESR) consists in the resonant absorption of energy by an ensemble of uncoupled electronic spins located in an external magnetic field from the magnetic component of an electromagnetic field. The resonance
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condition equals the Zeeman splitting (due to the external magnetic field) with the frequency of the electromagnetic field (in microwaves the typical resonance field is about 0.35 T-for a free uncoupled electronic spin). Hence, ESR is a versatile technique that allows the study of conducting materials. Some authors [6] use a different terminology defining by conduction electron spin resonance the ESR spectroscopy of conducting materials.
EXPERIMENTAL METHODS Crude CNF obtained from purified HiPco SWNT [8, 9] were investigated by ESR spectroscopy using a Bruker EMX spectrometer operating in X-band ( 9 GHz). The angular dependence of the resonance line has been investigated. The preferential orientation of SWNTs is confirmed by the angular dependence of the g-factor and of the resonance line width.
EXPERIMENTAL RESULTS AND DISCUSSIONS A typical ESR spectrum of crude CNF is shown in Fig. 1. It is observed that the resonance
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