Kinetics of Electrochemical Derivatization of the Silicon Surface by Grignards

The hydrogenated silicon surface can be derivatized with alkyl groups using anodization in a Grignard reagent. The derivatized surfaces have been characterized using infrared spectroscopy and X-ray photoelectron spectroscopy, and the kinetics of the reaction have been investigated using in situ infrared spectroscopy. The initial reaction rate is found to be on the order of one grafted alkyl group per two elementary charges passed through the interface, corresponding to a faradaic efficiency on the order of unity. The kinetics are modeled assuming that the derivatization proceeds through electrochemically generated alkyl radicals. For the case of a flat (111) Si surface, the results are accounted for by a reaction rate proportional to current density and to radical concentration at the surface, leading to a fast reaction up to completion of monolayer coverage. The detailed shapes of the kinetic curves, and their variations with experimental conditions, are well reproduced by the model. At an atomically rough surface, the kinetics exhibit a power-law behavior. These nonexponential kinetics can be ascribed to a distribution of rate constants associated with steric-hindrance effects, as quantitatively confirmed by numerical simulations. In practice, these results show that the maximum theoretical coverage may be hard to reach. They also indicate that the electrochemical techniques are intrinsically much faster than the available chemical techniques, which is probably favorable for reaching this maximum coverage. In the case of a Grignard from bromide and iodide, the role of the halogen in improving the electronic passivation of the surface is also demonstrated. This indicates that halogenation of the surface can be a side reaction in the derivatization process. However, the dominant reaction pathway appears to be abstraction of surface hydrogen by an electrochemically generated alkyl radical and reaction of the resulting Si dangling bond with the Grignard or another alkyl radical.