Electronic and magnetic properties of boron nitride nanoribbons with square-octagon (4 | 8) line defects.

The electronic and magnetic properties of boron nitride nanoribbons (BNNRs) with square-octagon (4 | 8) line defects (LD-(4 | 8)), parallel to their edges, have been investigated by first-principles calculations. It is found that the zigzag type LD-(4 | 8) BNNR with boron terminated edges is an antiferromagnetic (AFM' + - + -') insulator with an indirect bandgap, but that with nitrogen terminated edges has two degenerate ground states with the same energy, among which one is ferromagnetic (FM ' + + + +') half-metallic and the other is AFM ' + + - -' metallic. In contrast, it is more interesting to find that the bare and fully-hydrogen terminated armchair edge BNNRs with unsymmetric (4 | 8) line defects have an indirect and direct gap, respectively, both of which show a characteristic three family oscillation behavior, depending only on the ribbon width in its narrower part, but not the whole BNNR's width. Finally, the stabilities of a two-dimensional h-BN sheet with a zigzag or armchair type LD-(4 | 8) in it are further confirmed, among which the one with the armchair type LD-(4 | 8) is much more stable than that with the zigzag type counterpart.