Spin-asymmetric graphene nanoribbons in graphane on silicon dioxide

Hydrogenated graphene, graphane, is studied on an oxygen-terminated silicon dioxide substrate using ab initio calculations. The two lowest-energy structures with quarter and half monolayer hydrogen coverage are presented. We form zigzag graphene nanoribbons by selectively removing hydrogens from the epitaxial graphane layer. In these ribbons the spin degeneracy of the free-standing antiferromagnetic zigzag ribbons is broken, and band gaps of different magnitude emerge for the opposite spin species. This degeneracy breaking is due to a charge imbalance in the substrate below the ribbon, introduced through the asymmetric alignment of the substrate atoms with respect to the edges of the graphene ribbon. As the edge geometry is restricted by the neighboring graphane, the zigzag edges are robust to reconstructions that are suggested to destroy edge magnetism in free-standing graphene ribbons.