A single-walled tube is a cylinder made by rolling-up a graphene sheet. The tubes exist in various sizes and structures, because graphene can be wrapped in various ways. Nanotubes are uniquely specified by a chiral index (n,m) [1]. Depending on the chiral index a tube is metallic, semi-metallic or semiconducting. Carbon nanotubes emit light [2]: the absorption and emission lines are strongly structured due to singularities in the density of states and exciton formation [1]. Each (n,m) tube has a different set of singularities and transition energies. Thus, carbon nanotubes are prime candidates as building blocks for nano-devices.

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 [1].

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.

1. 1. S. Reich, C. Thomsen and J. Maultzsch (2004): Carbon Nanotubes: Basic concepts and Physical Properties. Wiley-VCH, 215 pp.

2. M. J. O Connell et al. (2002): Band gap fluorescence from individual single-walled carbon nanotubes. Science 278, 593-596

3. S. M. Bachilo et al. (2003): Structure-assigned optical spectra of single-walled carbon nanotubes. Science 298, 2361-2366