The effect of van der Waals interactions on the properties of intrinsic defects in graphite

The formation energies and geometries of intrinsic interstitial and vacancy point defects in graphite are calculated using density functional theory and the generalized gradient approximation with a semi-empirical van der Waals (vdW) correction for dispersion interactions. Inclusion of the vdW terms in the simulations leads to energetically unfavorable out of plane distortions and inter-layer bonding for isolated vacancies (V) and interstitials (I), respectively. Conversely, despite changes in the calculated formation energies, inclusion of vdW terms only slightly affects the optimized inter-layer bonding in di-vacancies (VV), di-interstitials (I2) and close interstitial–vacancy (I–V) pairs. On this basis, the interaction between defects is the origin of the inter-layer bonding in unsheared graphite. Partial shearing of graphite layers is shown to significantly lower (by 0.5 eV) the I–V recombination barrier with respect to the value calculated in perfectly AB-stacked hexagonal graphite (1 eV). We also find that the defect–defect interaction acts to cancel defect magnetic moments and excludes contributions of VV, I2 and I–V pairs to the measured magnetic signal of irradiated graphite.