Electron Transfer Initiated Formation of Covalently Bound Organic Layers on Silicon Surfaces

An electron transfer initiated organic modification of silicon surfaces, using a donor/acceptor pair of ferrocene (Fc) and N-bromosuccinimide (NBS) (or N-bromophthalimide (NBP)), is reported for the first time. This modification method is efficient and does not require the use of heat or irradiation. Multilayer structures are formed on the surfaces through radical reactions, similar to the reduction of aryl diazonium salts. The modified surfaces were characterized by infrared reflection–absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). IRRAS spectra clearly indicate a strong carbonyl stretch and the presence of a ferrocene moiety for each of the modified surfaces. XPS data show the presence of all elements from the donor/acceptor pair and provide strong evidence that the samples can be further modified by nucleophilic substitution. AFM imaging indicates the formation of dense layers that can be scratched away to reveal depth information on approximately 2 nm and the underlying terraces of the silicon substrate. A mechanism is proposed where radical abstraction of a surface hydrogen takes place leading to the formation of a covalently bound imidyl group, which is followed by the formation of polymeric structures through radical chain reactions.