Single-component and mixed ferrocene-terminated alkyl monolayers covalently bound to Si(111) surfaces

Self-assembled ferrocene monolayers covalently bound to monocrystalline Si(111) surfaces have been prepared from the attachment of an amine-substituted ferrocene derivative to a pre-assembled acid-terminated alkyl monolayer using carbodiimide coupling. This derivatization strategy yielded nanometer-scale clean, densely packed monolayers, with the ferrocene units being more than 20 A from the semiconductor surface. The amount of immobilized electroactive units could be varied in the range 2 x 10(-11) to similar to 3.5 x 10(-10) Mol cm(-2) by diluting the ferrocene-terminated chains by inert n-decyl chains. The highest coverage obtained for the single-component monolayer corresponded to 0.25-0.27 bound ferrocene per surface silicon atom. The electrochemical characteristics of the mixed n-decyl/ferrocene-terminated monolayers were found to not depend significantly on the surface coverage of ferrocene units. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at E-o' = 0.50 +/- 0.01 V vs SCE, and the rate constant of electron transfer k(app) was about 50 s(-1).