Probing temperature-dependent behaviour in self-assembled monolayers by ac-impedance spectroscopy

Gold electrodes modified by monolayers of three different long-chain thiol derivatives were studied by electrochemical impedance spectroscopy with a [Fe(CN)(6)](4-/3-) redox probe over a wide temperature range. The impedance characteristics of a bare gold electrode are compared to those of the same electrode modified with n-hexadecanethiol, n-octadecanethiol, and 11-mercaptoundecanoic acid, all between 15 degreesC and 65 degreesC. A strong temperature dependence of the films' electrical features was observed. All films show markedly different behaviour below and above a characteristic temperature specific to each film. It was possible to model bare gold and the thiol-modified electrodes with the Randles circuit over the whole temperature range examined, except for the 11-mercaptoundecanoic acid-modified electrode below its characteristic temperature, which required a different equivalent circuit. Cyclic voltammetry was used to verify the characteristic temperature of each film. The results are interpreted in terms of a two-dimensional phase transition in these monolayers. The possible nature of this phase transition is discussed.