Keywords: electrochemistry, silicon surfaces, organic monolayers, ferrocene, pyrrole, conducting polymers, tetrathiafulvalene, nanotechnology
Redox-active organic monolayers deposited on silicon surfaces for the fabrication of molecular scale devices
Different Si(111) surfaces covalently derivatised with alkyl monolayers terminated by redox-active centres, namely reversibly oxidisable ferrocene and tetrathiafulvalene (TTF), and electrochemically polymerisable pyrrole, have been prepared. Ferrocene and pyrrole were attached to the silicon surface using carbodiimide coupling between an amino-substituted derivative and a pre-assembled acid-terminated monolayer. TTF monolayers were produced from the direct reaction of an alcohol-substituted TTF derivative with hydrogen-terminated Si(111). The electrochemical oxidation of the pyrrole-modified surface in the presence of pyrrole yielded strongly adherent and smooth conducting polypyrrole films, the thickness of which could be easily controlled from the consumed electrical charge. The hybrid polypyrrole/pyrrole monolayer/Si(111) junction showed the expected metal-insulator-semiconductor (MIS) diode behaviour with a rectification factor at ?2 V of ca. 280. The electrochemical characteristics of the ferrocene- and TTF-modified surfaces evidenced a single and two one-electron system(s) respectively, located at potentials very close to those observed with the electroactive compounds in solution. The surface coverages of ferrocene and TTF were 0.23 and 0.15 per surface silicon atom respectively. Moreover, the rate constants of electron transfer were found to be 50 s−1 and 25 s−1 for the ferrocene and TTF monolayers respectively.