Patterns of Reacted Adatoms in Adsorption of Acetonitrile on Si{111}-(7 × 7)

We report on the covalent binding of acetonitrile (CH₃CN) on Si{111}-(7 × 7) at ∼300 K studied by scanning tunneling microscopy, thermal desorption spectroscopy, and first-principles theoretical calculations. The site-specific study makes it possible to unravel the site-by-site and step-by-step kinetics. A polarized CH₃CN prefers to adsorb on the faulted half more frequently compared to on the unfaulted half. Moreover, a molecular CH₃CN adsorbs four-times more preferably on the center adatom-rest atom (CEA-REA) pair than on the corner adatom-rest atom (COA-REA) pair. Such site selectivity, the number ratio of reacted-CEA/reacted-COA, depends on the number of reacted adatoms in the half-unit cell. The site selectivity and the resulting reacted-adatom patterns are understood well by considering a simple model. In this simple model, the molecular adsorption probability changes step-by-step and site-by-site with increasing reacted adatoms. Furthermore, our theoretical calculations are overall consistent with the experimental results. The site-selectivity of the adsorption of CH₃CN on Si{111}-(7 × 7) is explained well by the chemical reactivity depending on the local conformation, the local density of states, and the interaction between polarized adsorbates.