Measurement of electron transport and mechanical properties of single molecules

We have developed a method to determine the conductance of single molecules covalently bonded to gold electrodes by repeatedly forming a large number of molecular junctions. We create each molecular junction by separating two Au electrodes from contact in a solution containing the sample molecules. During the separation, we simultaneously measure the conductance and the force between the two electrodes. The conductance decreases in discrete steps, corresponding to the breakdown of individual molecules covalently bonded to the electrodes. Each discrete drop in the conductance is accompanied by a discrete drop in the force, which allows us to determine the bonding strength of the molecule to the electrodes. We have also determined the effective spring constant of a single molecule and the dependence of the conductance on the applied force.