In a recent experiment, Derycke et al. have made the exciting observation that ${\mathrm{H}}_2$ molecules readily adsorb dissociatively on the $c(4\times 2)$ but not on the $3\times 2$ surface of SiC(001) at room temperature. To unravel this spectacular reactivity difference, we have investigated a variety of ${\mathrm{H}}_2$ reaction scenarios within density-functional theory using the generalized gradient approximation. It turns out that intradimer adsorption is unlikely at both surfaces, while interdimer adsorption depends crucially on the distinct spatial arrangement and dangling-bond topology of the Si dimers at the surfaces. The results clearly reveal barrierless reaction pathways for dissociative ${\mathrm{H}}_2$ adsorption on the $c(4\times 2)$ surface as opposed to pathways with significant energy barriers on the $3\times 2$ surface. The latter finding also allows us to explain the inertness of self-organized Si addimer nanolines on the $c(4\times 2)$ surface to ${\mathrm{H}}_2$ uptake.

• Received 10 January 2007

DOI:https://doi.org/10.1103/PhysRevB.75.073409