Single-Molecule Electronic Measurements with Metal Electrodes

The process of transferring electrons from one metal electrode to another via an intervening molecule differs from electrochemically or optically-induced electron transfer in important ways. The simplest process of ballistic electron transport with no charge localization on the molecule (tunneling) should be easy to understand quantitatively, yet experimental reports and theoretical calculations can differ by orders of magnitude. Here, we show how many of the discrepancies are resolved with measurements carefully constructed to provide data on single molecules connected to electrodes in well-defined ways. Furthermore, a number of these results are in good agreement with first-principles calculations (no adjustable parameters). The more interesting (and theoretically challenging) process of charge transport with intermediate localization by redox states can be measured in the same ways, but present theories do not yet describe these processes fully.