| Abstract
| - Density gradient centrifugation is a high-resolution technique for the separation and characterization of largemolecules and stable complexes. We have analyzed various nanotube structures by preparative centrifugationin sodium metatungstate−water solutions. Bundled, isolated and acid-treated single-walled nanotubes (SWNTs)and multiwall nanotubes (MWNTs) formed sharp bands at well-defined densities. The structure of the materialin each band was confirmed by transmission electron microscopy and Raman spectroscopy. Our data suggestrespective densities of 1.87, 2.13, 1.74, and 2.1 g/cm3 for bundled, isolated, and acid-treated SWNTs andMWNTs. These measured results compare well with their calculated densities.
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