Nitrogen in graphite and carbon nanotubes: Magnetism and mobility

We use ab initio methods to study the atomic, electronic, and magnetic structure of nitrogen impurities in graphite and carbon nanotubes. We show that the probable configurations for the N impurity atoms are bridgelike adatoms on graphene sheets/nanotubes, substitutional sites and intercalated N-2 molecules. We further calculate the migration energy barrier for an N adatom on the graphite surface and the activation energy for the coalescence of two N adatoms into a molecule, which are 1.1 eV and 0.8 eV, respectively. We also find that the N adatom has a magnetic moment of 0.6 mu(B) and that substitutional N enhances the magnetism of C adatoms on graphite and nanotubes, acting as a general promoter of magnetism in carbon systems.