Atomic and electronic structure of divacancies in carbon nanotubes

We present atomic and electronic structure of divacancies in carbon nanotubes, which is calculated using the density functional theory. Divacancies in carbon nanotubes self-heal by spontaneous reconstructions, which consist of concerted bond formations. Divacancy formation energies E(DV), which strongly depend on the divacancy orientation with respect to the tube axis, are in the range of 2.8-4.3 eV for favorable orientations in the nanotubes of 4-9 angstrom diameter, making divacancies more probable than monovacancies in carbon nanotubes. Defect related states lead to a higher density of states around the Fermi level. Semiconducting nanotubes develop midgap levels that may adversely affect the functionality of carbon nanotube based devices. Our spin polarized density functional calculations show that the exchange splitting of defect-related bands in nonsemiconducting defective nanotubes leads to net spin polarizations of rho up arrow-rho down arrow <= 0.5 mu(B) per divacancy for some divacancy orientations.