Probing Potential-Tuned Resonant Tunneling through Redox Molecules with Scanning Tunneling Microscopy

We have studied electron transfer through redox molecules adsorbed on a conductive substrate with scanning tunneling microscopy (STM) in aqueous solution. By adjusting the substrate potential, the Fermi levels of the substrate and tip can be easily shifted relative to the energy levels of the molecules. Aligning the Fermi levels to an energy level, a nearly tenfold increase in the tunneling current that flows between the substrate and the STM tip via the molecules, due to resonant tunneling is observed. This experiment also shows that structurally similar molecules may be identified with STM based on their different redox properties.