Electrochemical characterization of self-assembled monolayers (SAMs) of thiophenol and aminothiophenols on polycrystalline Au: Effects of potential cycling and mixed SAM formation

We report the results of our study on the structural integrity and barrier properties of self-assembled monolayers (SAMs) of some aromatic thiols namely thiophenol (TP), o-aminothiophenol (o-ATP) and p-aminothiophenol (p-ATP) on polycrystalline gold surface using electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The blocking ability of these monolayers is evaluated using both the positive and negative redox probes namely [Ru(NH3)6]2+|3+ and [Fe(CN)6]3−|4−. The results of CV and EIS measurements suggest that these aromatic thiols form stable but moderately blocking monolayers. We have shown that the barrier property of these monolayers can be significantly improved by potential cycling in the case of the TP and o-ATP monolayers and by mixed SAM formation in the case of p-ATP. For studies involving mixed SAM formation, we have used 1-octanethiol and 1,6-hexanedithiol having a similar chain length to that of aromatic thiols analyzed in this work. From our experimental results we find that the electron transfer reaction of [Fe(CN)6]3−|4− redox couple occurs predominantly through the pinholes and defects present within the monolayer in contrast to tunneling of electrons for [Ru(NH3)6]2+|3+ redox species. From the impedance data, we have determined a surface coverage value of ∼0.999 for these SAMs on polycrystalline Au surface.