To date, the focus of such single molecule studies has been on symmetric junctions, where both ends of the molecule are contacted to the respective metal terminals with chemically identical end groups. However, systems where the contacting groups at either end of the molecule are different (contact asymmetry) are also of potential interest in molecular electronics.

In such cases a "polar" orientation of mono-molecular films in devices can result, which has significance in molecular electronics because it has been previously shown that it can lead to electrically rectifying junctions with diode-like behaviour. This inspired our team to examine experimentally and theoretically how contact asymmetry influences the conductance of single molecules in an electrical junction, a study that has now appeared in Nanotechnology.

We measured and calculated the single molecule conductance of molecular wires with different combinations of end groups; either thiol groups at each end, carboxylic acids groups at each end, or mixed end group systems with a thiol group at one end and a carboxylic acid group at the other. We found that for molecular wires with mixed functional groups (X-bridge-Y) the single molecule conductance decreases with respect to the comparable symmetrical (X-bridge-X) molecules.

These differences are confirmed by theoretical computations based on a combination of density functional theory and the non-equilibrium Green's functions formalism and also by a heuristic tight-binding model, which captures the essential quantum mechanics. This study demonstrates that the apparent contact resistance, as well as being highly sensitive to the type of contacting group, is strongly influenced by contact asymmetry of the single molecular junction. This highlights that contact asymmetry is a significant factor to be considered when evaluating nano-electrical junctions incorporating single molecules.