Ab initio simulation of the grafting of phenylacetylene on hydrogenated surfaces of crystalline silicon catalyzed by a lewis acid

Car-Parrinello simulations have been carried out to identify the grafting mechanism of phenylacetylene, a prototypical alkyne, on the hydrogenated surfaces of crystalline silicon, catalyzed by a Lewis acid (AlCl3). To this purpose, we have made use of a new technique, metadynamics, devised recently to deal with complex chemical reactions in first principles simulations. The reaction mechanism, leading to a styrenyl-terminated surface, turns out to be equivalent to the corresponding gas-phase hydrosilylation reaction by silanes that we have identified in a previous work. The activation energies for the surface reactions (0.43, 0.42, 0.35 eV, for H-Si(111), H-Si(100) 2 x 1, and H-Si(100)1 x 1, respectively) are very close to that of the corresponding gas-phase reaction (0.37 eV). The estimated activation free energy at room temperature is sufficiently low for the grafting reaction to be viable at normal conditions and at low coverage on the crystalline silicon surfaces, as already well documented to occur on the surface of porous silicon. However, the conformation of the transition state shadows a large area of the surface, which might contribute to making the grafting process self-limiting.