Detailed growth mechanism of single-wall nanotube bundles and of multi-wall nanotubes of carbon is investigated using ab initio and parametrized calculations. Our results show that single-wall tubes grow only in presence of a catalyst, whereas multi-wall tubes, stabilized at the growing edge by a covalent "lip-lip" interaction, may form in a pure carbon atmosphere. The individual tubes in these systems are likely to exhibit a low-frequency twisting motion. The weak, partly anisotropic inter-wall interaction, present in single-wall nanotube bundles and in multi-wall nanotubes, may cause significant changes in the density of states near the Fermi level.
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