Organic Functionalization at the Si(001) Dimer Vacancy Defect – Structure, Bonding and Reactivity

In this density functional theory study, the influence of the dimer vacancy on the reactivity of the Si(001) surface is investigated. To this end, electronic and structural properties of the defect are analyzed. Band structure calculations reveal a higher-lying valence band which would suggest increased reactivity. However, the opposite is found when organic molecules for interface formation (acetylene, ethylene and cyclooctyne) are adsorbed at the defect. Significant reaction barriers have to be overcome in order to form bonds with defect atoms, while adsorption on the pristine surface is mostly direct. This suggests the presence of a, rather weak, Si-Si bond across the defect which must be dissociated before organic adsorbates can react. A rich adsorption and reaction network is found in addition to the structures known from the pristine surface. All three investigated adsorbates show different bonding characteristics. For acetylene and ethylene, the preferred thermodynamic sink is the insertion into the defect, with the latter molecule even dissociating. Bulky cyclooctyne on the other hand avoids reaction with the defect due to steric demands imposed by the small defect cavity. The DV has no effect on reactivity of neighboring dimers. A combination of defect creation and hydrogen-precoverage could be a promising approach for selective surface functionalization. We thus show the influence of a non-ideal surface on organic functionalization and interface build-up reactions for a prototypical interface.