Several reports in the last year have shown plasticity in nanotubes under stress and at high temperatures. But the mechanism by which they retain their perfect structure has only just been elucidated. Boris I. Yakobson and colleagues at Rice University have explored a discrete mobile defect that sews up damage to the carbon lattice as it moves along [Ding et al., Phys. Rev. Lett. (2007) 98, 075503; Ding et al., Nano Lett. (2007) doi: 10.1021/nl0627543].
Yakobson identified the nano-seamstress in 1998 as a 5/7 defect – a pentagon joined at one side to a heptagon. Using molecular dynamics and Monte Carlo simulations, the recent reports outline just how the 5/7 defect works.
After the passage of the defect, the atoms rearrange themselves back into a perfect hexagonal lattice – a seemingly orchestrated behavior. The simulations show two cooperative mechanisms at work. In ‘gliding’, the defects inch up the length of nanotubes by swapping bond order. The result is a lengthening, thinning, and change in helicity of the nanotube. ‘Climbing’ occurs perpendicular to the glide direction and results in the release of pairs of carbon atoms from the nanotubes. Climbing 5/7s cause kinks between the differing local diameters, an effect shown experimentally last year.
Yakobson suggests that the defects act as a local safety valve allowing the local structure to reorganize and release excess energy or atoms. And they are tireless; in the simulations, 5/7s carry on to the ends of nanotubes before turning around to continue their work.
5/7 defects (red) progressively result in kink formation and thinning of a CNT. (Courtesy of Kun Jiao.)
The findings could be applicable in other effectively two-dimensional systems that demonstrate kinking behavior, such as micelles or cellular microtubules.