Theory of the site-selective reaction of ${\mathrm{NH}}_3$ with $\mathrm{Si}\mathrm{}\left(111\right)-(7\mathrm{}\times 7)$

Adsorption of ${\mathrm{NH}}_3$ molecules on the $\mathrm{Si}\mathrm{}\left(111\right)\mathrm{}-(7\mathrm{}\times 7)$ surface has been studied by density-functional theory calculations. We find that the molecular adsorption is strongly site selective: the active site is Si adatoms. Si restatoms appear repulsive towards ${\mathrm{NH}}_3$ molecules, contrary to experimental suggestions and a recent theoretical prediction. The adsorbed molecule on an adatom undergoes a low-barrier dissociation process, which is readily activated by an adatom backbond breaking and ends up with capping an adjacent restatom by the dissociated H species. We also find that the 2-to-1 adatom-restatom population ratio of $\mathrm{Si}\mathrm{}\left(111\right)\mathrm{}-(7\mathrm{}\times 7)$ makes the precursor-mediated dissociation process self-limiting and becomes the origin of the intriguing experimental reaction behavior, an early saturation of restatom features and a partial saturation of adatom features.