Probing the Binding Mechanics of Single-Molecule Junctions Using Atomic Force Spectroscopy
We investigate the electronic and mechanic properties of single molecule junctions using a custom high-resolution atomic force microscope (AFM). We perform AFM-based break-junction measurements using a gold substrate and gold-coated AFM cantilever on a series of methyl-sulfide terminated alkane chains. We measure, simultaneously, two independent quantities for each junction, force and conductance. We use conductance as a signature of the junction structure and electronic characteristics and use the measured force to get insight into the junction elongation and rupture processes. We find that molecular junctions form with one or two molecules bridging the gap between the cantilever and substrate, with the two-molecule junction having roughly twice the conductance of the one-molecule junction. More importantly, we find that the probability to form a two-molecule junction is higher for alkanes with an odd number of carbon atoms. We attribute this to the different orientation of the methyl-sulfide group relative to the molecular backbone axis in the odd and even numbered alkane chains. Based on the measured mechanical properties, we conclude that there is no significant molecule-molecule interaction in these two-molecule junctions formed with alkane chains that have an odd number of carbon atoms. We also find, that the force required to rupture the molecular junction shows a decreasing trend with increasing molecular length. By utilizing a hybrid model fit on the measured force traces, we show that this trend is the result of the decrease of the Au-SMe binding energy. This result is in good agreement with density functional theory based calculations.