Insulating properties of self-assembled monolayers monitored by impedance spectroscopy

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)(n)S/Au (n = 7-15) SAMs prepared via prolo nged incubation in a thiol solution obey the Helmholtz ideal capacitor mode l and thus are virtually defect-free. The electrochemical properties of SAM -coated electrodes are compared with those of a bare gold electrode. The ph ase angle at an ion-diffusion-related frequency (1 Hz) is greater than or e qual to 88 degrees and is independent of the electrolyte concentration. The thiol chains act gsa dielectric material whose impedance (modulus) is 10(5 ) Omega cm(2) at 1 Hz. Both the phase angle and the complex dielectric cons tant in the low-frequency region are good indicators of the film's ability to act as an ionic insulator.