  We show by first-principles calculations that the electronic properties of
zigzag graphene nanoribbons (Z-GNRs) adsorbed on Si(001) substrate strongly
depend on ribbon width and adsorption orientation. Only narrow Z-GNRs with even
rows of zigzag chains across their width adsorbed perpendicularly to the Si
dimer rows possess an energy gap, while wider Z-GNRs are metallic due to
width-dependent interface hybridization. The Z-GNRs can be metastably adsorbed
parallel to the Si dimer rows, but show uniform metallic nature independent of
ribbon width due to adsorption induced dangling-bond states on the Si surface.
