Surface Reaction of Acetylene with H-Terminated Silicon Surfaces. A Theoretical Study from Hybrid DFT-D2 Periodic Simulations

The addition of unsaturated molecules on H-terminated silicon surfaces is a way to functionalize them. Usual procedures are initiated by an H–Si bond cleavage, yielding an Si dangling bond, which in turn reacts with unsaturated molecules to form a carbon-centered radical intermediate (process called hydrosilylation). This intermediate can evolve according to two different channels: (i) via H-abstraction of a neighboring Si–H surface group, thus forming a stable adsorbed species and leading to a new Si dangling bond, which is able to newly undergo the same processes with other incoming unsaturated molecules; or (ii) by reacting with a second unsaturated molecule hence elongating the organic chain. In this work, these processes (i.e., hydrosilylation, surface H-abstraction, and alkyne addition) have been studied by means of hybrid B3LYP-D* and BHLYP-D* periodic calculations considering the reaction of acetylene with the (111) and (100) H-terminated silicon surfaces. The computed potential energy surfaces indicate that both processes are fairly competitive, as the energy barriers are similar and the reaction energies are large and negative. However, the computed free energy profiles indicate that polymerization has a significantly higher energy barrier than the H-abstraction reaction due to disfavored entropic effects, in agreement to what is observed to occur experimentally. Moreover, significant differences in the computed energy profiles between the two surfaces are observed, the kinetics and thermodynamics of these reactions being more favorable on H–Si(111).