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Carbon nanotubes can be thought of as graphite sheets with a hexagonal lattice that have been wrapped into a tube and capped at each end with half of a fullerene sphere (see left figure). Oxidation leads to removal of the fullerene half spheres (right figure). The electronic properties of nanotubes depend sensitively on tube diameter and helicity. Depending on the degree of twist along its length, nanotubes encompass many structural types from the achiral "zigzag" (semi-conducting) tube, through several chiral tubes, to the achiral "armchair" (metallic) tube. In other words, very similar molecules consisting of only carbon behave very differently electronically. |
| The properties of multi-wall carbon nanotubes purified by oxidation in air and with potassium permanganate have been probed using 13C and 129Xe NMR spectroscopy under continuous flow optical pumping conditions. Some of the 129Xe NMR spectra for the nanotubes oxidized in air are shown on the right. The tall peak arises from xenon in the interparticle space, while the smaller peak which shifts downfield with decreasing temperature is due to xenon in the adsorbed phase. Xenon is shown to penetrate the interior of the nanotubes. A distribution of inner tube diameters gives rise to chemical shift dispersion. No exchange between these different adsorption sites or with the gas phase is observable. In the case of the permanganate-oxidized sample, rapid xenon relaxation is attributed to interaction with residual MnO2 nanoparticles in the interior of the tubes. The effect of gas circulation rate on chemical shift is mainly due to a change in temperature, while optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate affects signal intensity ratios and may lead to the complete suppression of signals. |
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