Nuclear magnetic resonance study of highly concentrated metallic and semiconducting single-walled carbon nanotubes

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Single-walled carbon nanotubes (SWCNTs) can be metallic or semiconducting, depending on their structural parameters. Up to now, nuclear magnetic resonance (NMR) studies have been performed on samples with bundles of mixtures of metallic and semiconducting SWCNTs. In this study, we have succeeded in observing the distinct 13C NMR spectra in the metallic and semiconducting SWCNT bundles prepared by density gradient ultracentrifugation technique. We found substantial differences in the observed spectra between the metallic and semiconducting SWCNTs. These were explained by difference contributions from Knight shift and current shift for metallic and semiconducting SWCNTs. Furthermore, we studied the mechanism for transformation of SWCNTs encapsulating C60 fullerene into double-walled CNTs.