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.
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