Interplay of Hydrogen-Bond and Coordinate Covalent-Bond Interactions in Self-Assembly ofNH3Molecules on the Si(001) Surface

An exchange of hydrogen-bond and coordinate covalent-bond (dative-bond) interactions is found to play a critical role in the self-assembly of ${\mathrm{NH}}_{3}$ molecules on the Si(001) surface. An ${\mathrm{NH}}_{3}$ molecule in the height of $\ensuremath{\sim}3--10\text{ }\text{ }\AA{}$ above the surface is attracted toward the preadsorbed ${\mathrm{NH}}_{2}$ moiety through the long-range H-bond interaction. Within $\ensuremath{\sim}3\text{ }\text{ }\AA{}$, the H-bond interaction becomes repulsive, and instead the dative bond with the buckled-down Si atom governs the adsorption process. The interplay of the two interactions induces the clustering and the zigzag feature of the dissociatively adsorbed ${\mathrm{NH}}_{3}$ molecules on the Si(001) surface.