Insulating Properties of Self-Assembled Monolayers Monitored by Impedance Spectroscopy

Abstract

The ionic insulating properties of n-alkylthiol self-assembled monolayers (SAMs) and a related fluorinated SAM are characterized by electrochemical ac impedance spectroscopy. CH3(CH2)nS/Au (n = 7−15) SAMs prepared via prolonged incubation in a thiol solution obey the Helmholtz ideal capacitor model and thus are virtually defect-free. The electrochemical properties of SAM-coated electrodes are compared with those of a bare gold electrode. The phase angle at an ion-diffusion-related frequency (1 Hz) is ≥ 88° and is independent of the electrolyte concentration. The thiol chains act as a dielectric material whose impedance (modulus) is 105 Ω cm2 at 1 Hz. Both the phase angle and the complex dielectric constant in the low-frequency region are good indicators of the film's ability to act as an ionic insulator.