Half-metallicity in graphene nanoribbons with topological defects at edge

We report first principles studies of zigzag edged graphene nanoribbons (ZGNR) with one edge partially covered by topological defects. With increasing coverage of an edge by pentagons and heptagons, which are two of the simplest topological defects possible in a graphenic lattice, ZGNRs evolve from a magnetic semiconductor to a ferromagnetic metal. This evolution can be intermediated by a narrow bandgap half-metallic phase, upon suitable concentration and conformation of defects at the edge. Spin-frustration induced by topological defects lead to substantial lowering of magnetic ordering and localization of defect-states in the vicinity of the defects. Dispersion of bands constituted by the defect-states within the bandgap of the corresponding unmodified ZGNR, leads to availability of energy windows for spin-polarized electron transport. Driven primarily by exchange interactions, the energy window for transport of electrons near Fermi energy, is consistently wider and more prevalent for the minority spin, in the entire class of ZGNRs with discontinuous patches of topological defects at an edge. Such defects have been widely predicted and observed to be naturally present at the interfaces in polycrystalline graphene, and can even be formed through chemical and physical processes. Our approach thus may lead to a feasible strategy to manifest workable half-metallicity in ZGNRs without involving non-carbon dopants or functional groups.