Addition of hydrocarbons to H-Si(100) in extra-mild conditions: A novel mechanism valid for single and multiple C-C bonds

The establishment of strong C-Si bonds connecting a molecular moiety to the Si surface has been widely reported with different synthetic recipes, but general and reliable reaction mechanisms have not been described yet for the distinct chemical routes. The coupling of a suitable functional group in the molecule with a reactive termination of the Si surface is a prerequisite for the reaction to happen, and the presence of a C-C multiple bond has long been thought to be necessary for an extra-mild attachment, as in the visible light induced organics-Si anchoring reaction. In this paper, the addition of saturated and unsaturated hydrocarbons to the hydrogenated Si(100) surface has been modeled by density functional theory calculations. The aim is to describe a mechanism allowing for the addition of single C-C bonds to Si(100) and the addition of C=C bonds with preservation of the unsaturation. In fact, both these reactions have been observed recently, but they are not explained by radical-initiated hydrosilylation, the more commonly invoked mechanism for this class of processes. The mechanism proposed here is described by computing the reaction path in the ground state and recomputing the energies in the first excited state. Both for saturated hydrocarbons and for unsaturated hydrocarbons we found that the activation barriers in the excited state reduce to about 60-65% of their ground state value. The barrier lowering is explained in terms of the frontier orbital change along the reaction path. These findings can explain why visible light can activate the formation of a C-Si bond, even if it is not energetic enough to break a H-Si bond.