  Hydrogenated graphene, graphane, is studied on oxygen-terminated silicon
dioxide substrate using ab initio calculations. A structure with hydrogenation
only on one side of the graphene layer is found stable and its hydrogen
configurations are presented. Additionally, we form zigzag graphene nanoribbons
by selectively removing hydrogens from the epitaxial graphane layer. In these
ribbons, the spin degeneracy of the freestanding 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.
