It was shown, by using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial C atom right beside a vacancy, which were present in irradiated graphite, could also exist in bundles of C nanotubes. Due to the geometrical structure of a nanotube, however, this defect had a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that were created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizable amount, clearly indicating its importance for the mechanical properties of nanotube bundles.

Bundling Up Carbon Nanotubes through Wigner Defects. A.J.da Silva, A.Fazzio, A.Antonelli: Nano Letters, 2005, 5[6], 1045-9