Electrochemical characterization of Si(111) modified with linear and branched alkyl chains

A simple chemical strategy is described to produce branched alkyl chains on Si(111) from the reaction of an ester-terminated silicon surface. The stability of the silicon surfaces with linear and branched monolayers is characterized by electrochemical impedance, Kelvin probe, and high-resolution electron energy loss spectroscopy (HREELS). The direct observation of surface states in capacitance-voltage plots can be used to monitor the growth of electrically active defects associated with oxidation of the silicon substrate. We find that the total surface state density of the freshly made surfaces increases in the following order: Si-B < Si-UDE < Si-C10 (where B is the branched structure, UDE is ethyl undecanoate, and C10 is decyl) in aqueous and organic solvent/electrolyte systems. After 24 h in the electrolyte solution, the surface state densities increase but the order remains the same. The branched structure is significantly more resistant to oxidation. These observations are consistent with the results of other characterization techniques including HREELS and surface photovoltage and indicate that the branched alkyl chain-modified surfaces are considerably more stable, especially in aqueous electrolytes, making them suitable for future use in biological sensor applications.