Bundling up carbon nanotubes through Wigner defects

We show, using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right beside a vacancy, which are present in irradiated graphite, can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are 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.