An electrochemical immittance analysis of the dielectric properties of self-assembled monolayers

Abstract

The ability of organic self-assembled monolayers (SAMs) to act as insulating barriers to electron transfer, ion transport, or molecular diffusion is critical to their application in a variety of technologies. The use of appropriate analytical tools to characterize the dielectric properties of these molecular thin films is important for the control of structural defects and establishing structure–property relations. In this context, we analyze the ionic permeability and dielectric response of SAMs formed from a homologous series of n-alkanethiolates (CH3(CH2)nS, where n = 9, 11, 13, 15, 17, and 19) on gold using the immittance quantities of the complex impedance, capacitance, and permittivity available from the same electrochemical impedance spectroscopy (EIS) measurement. The most sensitive parameters and frequency range for characterizing the capacitive behavior and assessing the ion-blocking quality of the SAMs under non-Faradaic conditions are identified. We also investigate the effect of chain length on the interfacial capacitance and dielectric constant of ionic insulating SAMs. The advantages of the capacitance quantity and related permittivity data over traditional impedance representations and equivalent electric circuit modeling are discussed.