Using the Electrode Potential to Orient Single Molecules in Nanoscale Electrical Junctions

Charge transport through and between molecules is central to important processes in nature. Studying the conductivity of single molecules can contribute to a better understanding of charge transport, and also help to develop building blocks of molecular electronics, light harvesting devices, etc. We measure the conductivity of molecules using the Scanning Tunneling Microscope break-junction (STM-BJ) method that utilizes repeatedly formed circuits where one or a few molecules are trapped between two electrodes, at least one of which has nanoscale dimensions. The statistical analysis of thousands of measurements yields the conductance of single molecules. One particular interest is the role of the molecule-electrode contact and orientation in charge transport. In the simplest analysis this contact can present a substantial barrier to charge injection, which can have important consequences in devices such as dye sensitized semiconductor nanoparticle solar cells. Our most recent developments include controlling the orientation of the molecule in the junction using the electrode potential so that we can measure conductance along different molecular axes, accessing elements of the anisotropy of the molecular conductance tensor. Specifically, when the molecule is oriented flat, with the molecular plane parallel to the electrode surface, charge transport is measured perpendicular to the molecular plane. This approach can be applied to molecules that do not have suitable molecular anchoring groups for traditional STM-BJ measurements, and allows us to determine the importance of anchoring groups in charge transport in molecular junctions.