Fractional coverage of defects in self-assembled thiol monolayers on gold

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (da) and average fractional coverage of defect (θa), we have obtained the θa∼da plot. The influence of the apparent standard rate constant on the shape of the θa∼da plot has been discussed. In our experiments, Fe(CN)63−/4− is used as a redox probe to study the θa∼da plot of an octadecanethiol monolayer. The θa versus da plot indicates that the defects with da<6 methylene groups and θa<0.1 can increase the apparent standard rate constant from 1.9×10−10 cm s−1, which is the theoretical value calculated from electron tunnelling theory, to 2.9×10−7 cm s−1. The average thickness of the whole monolayer (ATWM), which is obtained from the θa versus da plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.