XPS and electrochemical studies of ferrocene derivatives anchored on n- and p-Si(100) by Si-O or Si-C bonds

High-coverage functionalization of H-terminated n- and p-Si(1 0 0) with vinylferrocene (VFC) and ferrocenecarboxaldehyde (FCA) has been obtained by wet chemistry methods, via the formation of a covalent bond between silicon atoms and the carbon (VFC) or the oxygen (FCA) termination of the molecules. The resulting functionalized electrodes have been analyzed by XPS, before and after cyclic voltammetry and capacitance-voltage measurements in an electrochemical cell. The very high quality of the hybrid species Si-CH2-CH2-C5H4-Fe2+-C5H5, resulting from VFC, has been certified by the negligible presence of silicon oxide and ferrocenium ions, and by the correct carbon/iron atomic ratio, accounting for the molecular species. The hybrid produced from FCA, Si-O-CH2-C5H4-Fe2+-C5H5, presents a higher amount of silica, carbon and ferrocenium, as a consequence of the higher temperature of the functionalization procedure and of the higher reactivity of FCA. The use of two closely similar redox molecules has allowed to compare the behaviour of the Si-C-Y vs. Si-O-Y bond (Y being the redox moiety), with respect to their electrochemical reactivity in an organic solution. Both hybrids behave similarly on n- and p-Si substrates, in terms of redox potentials, stability to more than 1000 voltammetric cycles, linearity of the current intensity with the scan rate. The vinyl derivative, however, showed a faster and more reversible electron transfer kinetics than the carboxaldehyde derivative and an enhanced stability to long-term electrochemical experiments. VFC/p-Si is the first reported ferrocene derivative anchored to monocrystalline silicon via a C-Si bond. It is also the only large-area hybrid capacitor, to date, to be frequency-independent up to several hundred Hz, having a redox capacitance of 10(-4) F cm(-2), the highest reported so far for a monolayer on a Si(1 0 0) face.