Electrochemical gate-controlled conductance of single molecules

The ability to measure and control current through a single molecule is a basic requirement towards the ultimate goal of building an electronic device using single molecules. This ability also provides one with a rather unique opportunity to study charge transport, a phenomenon that plays vital roles in many chemical, electrochemical and biological processes, on a single molecule basis. To reliably measure the current, one must: 1) provide a reproducible contact between the molecule and two probing electrodes; 2) find a signature to identify that the measured conductance is due to not only the sample molecules but also a \textit{single} sample molecule; 3) provide a third gate electrode to control the current. The method that we have used to create individual molecular junctions is to bring two electrodes into and out of contact with each other in the presence of sample molecules terminated proper linkers that can bind covalently to the electrodes. The individually created molecular junctions vary in the atomic scale details of the contact configurations, and statistical analysis is used to extract the conductance of the molecular junction with the most probable configuration. When several configurations occur with comparable probabilities, the method may result in multiple conductance values. In order to control the current through a molecule, we use an electrochemical gate in which the molecular junction is immersed in an electrolyte and biased with respect to a reference. We have studied three types of molecules: electrochemically inactive molecules, electroactive molecules that undergo irreversible redox reactions, and electroactive molecules that undergo reversible redox reactions. These molecular systems exhibit rather different electrochemical gating behaviors.