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 SiO2 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 SiO2 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.