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

Au electrodes are alkylated by self-assembled organic monolayers of octadec anethiol from alcohol solution. The electron tunnelling resistance of a mon olayer-coated gold electrode has been investigated by ac impedance. The rel ation between the fractional coverage of different defects and the correspo nding film thickness at these 'collapsed' sites has been deduced from elect ron tunnelling theory. By using the concepts of average film thickness at d efect (d(a)) and average fractional coverage of defect (theta(a)), we have obtained the theta(a) similar to d(a) plot. The influence of the apparent s tandard rate constant on the shape of the theta(a) similar to d(a) plot has been discussed. In our experiments, Fe(CN)(6)(3-/4-) is used as a redox pr obe to study the theta(a) similar to d(a) plot of an octadecanethiol monola yer. The theta(a) versus d(a) plot indicates that the defects with d(a) < 6 methylene groups and theta(a) < 0.1 can increase the apparent standard rat e constant from 1.9 x 10(-10) cm s(-1), which is the theoretical value calc ulated from electron tunnelling theory, to 2.9 x 10(-7) cm s(-1). The avera ge thickness of the whole monolayer (ATWM), which is obtained from the thet a(a) versus d(a) plot and which can indicate the blocking property of the m onolayer, is 11 methylene groups. (C) 2000 Elsevier Science S.A. All rights reserved.