Carbon nanotubes can have more than one shell: from double-walled (two shells) and triple-walled (three shells) to multi-walled (ten or more shells). Shells are coupled by weak van der Waals forces. The properties of multi-shell tubes are determined by the distribution of diameters, chiral indices and the number of walls .
None of the current synthesis methods for carbon nanotubes produces just one type of tube. The growth product is a mixture of semiconducting and metallic tubes with various diameters and structures. Nanotubes aggregate into bundles during growth. Bundles differ from single tubes, e.g. they do not emit light. Individual tubes can be obtained by breaking the bundles and dispersing the tubes in micelle-like suspension [2,3]. Currently, the key challenge for nanotube technology is to select or produce tubes with specific properties.
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M. J. O Connell et al. (2002): Band gap fluorescence from individual single-walled carbon nanotubes. Science 278, 593-596
S. M. Bachilo et al. (2003): Structure-assigned optical spectra of single-walled carbon nanotubes. Science 298, 2361-2366