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

Abstract

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

Enhanced stability of short- and long-chain diselenide self-assembled monolayers on gold probed by electrochemistry, spectroscopy, and microscopy

“…Thermal stability studies of thiol monolayers have been reported by various groups [19,20,[27][28][29][30][31][32][33][34][35]. Bensebaa and coworkers observed two kinds of transitions; the first transition is between two ordered phases that differ in the tilt direction [19b,19c].…”

Current understanding of the structure, phase transitions and dynamics of self-assembled monolayers on two- and three-dimensional surfaces

“…Recently AC impedance spectroscopy has been used to probe the temperaturedependent phase transition of the SAMs on a gold surface [143]. This technique involves the measurement of the frequency dispersion of the impedance or admittance of a system.…”

Phase transitions of an ionic liquid self-assembled monolayer on Au

“…The formation of a disordered monolayer at deposition or observation temperature higher than the phase transition is manifested by a change in wettability, thickness of the monolayer, of vibrational frequency, and electron transfer properties. 21,22,32,33,[35][36][37] Also, the phase transition induces a nonreversible change of molecular structure in the timescale of the experiments, previously reported to be at least two weeks. 22 This article aims at answering these questions for IL-SAM: do IL-SAM undergo the same phase transition as organic SAM and how does deposition temperature influence the properties of resulting IL-SAM?…”

“…The electrochemical properties of a neutral alkyl thiol selfassembled monolayer depend on the deposition temperature. 21,22 For example, C 16 and C 18 alkyl chain thiols are insulating below a critical deposition temperature. Above the critical deposition temperature, the monolayers exhibited increased electron transfer as measured with Fe(CN) 6 4À/3À…”