Charge Transfer across Self-Assembled Nanoscale Metal−Insulator−Metal Heterostructures

The kinetics of charge transfer across a metal−insulator−metal architecture is investigated by electrochemical impedance spectroscopy. The insulating component of the architecture is composed by a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA), polyelectrolyte multilayers, and a monolayer of 22 nm SiO 2 nanoparticles. The charge transfer to the hexacyanoferrate couple is strongly hindered by the MUA monolayer. The blocking properties effectively vanish with the adsorption of a diluted monolayer of Au nanoparticles (19 nm). Atomic force microscopy and scanning electron microscopy analyses demonstrate that the Au nanoparticles are physically separated from the Au surface by the SiO 2 monolayer. The strong electronic communication between the metal nanoparticles and the electrode is rationalized by a nonthermalized transport process involving redox species trapped in the multilayer assembly.