Potential-induced defects in n-alkanethiol self-assembled monolayers monitored by impedance spectroscopy

The ionic permeability of alkanethiol self-assembled monolayers (SAMs) chem isorbed on gold is studied using ac impedance spectroscopy in the absence o f redox active species. CH3(CH2)(n)S/Au (n = 7-15) SAMs behave as ionic ins ulators until a critical potential, V-c, is reached or exceeded. At potenti als more cathodic than V-c, SAMs are no longer ionic insulators and a signi ficant change in the phase angle is associated with ion penetration in the low-frequency region. Ve is chain length dependent and is observed at poten tials (-0.15 to -0.35 V vs Ag/AgCl) that are considerably more anodic than the alkanethiol electrodesorption potential. The relaxation frequency of tr ans-SAM ion migration (4-100 Hz) can be calculated from fitting of the impe dance data to an appropriate equivalent circuit or from Bode phase plots.