Measuring Single-Molecule Conductance at An Ultra-Low Molecular Concentration in Vacuum.

We report on systematic investigation of single-molecule detection mechanisms in break junction experiments in vacuum. We found molecular feature in the conductance traces at an extremely low concentration of molecules of 10 nM. This was attributed to condensation of the molecular solution on the junction surface upon evaporation of the solvent during evacuation. Furthermore, statistical analyses of the temporal dependence of molecular junction formation probabilities suggested accumulation effects of the contact mechanics to concentrate molecules absorbed on a remote area to the tunneling current sensing zone, which also contributed to the capability of molecular detections at the low concentration condition. The present findings can be used as a useful guide to implement break junction measurements for studying electron and heat transport through single molecules in vacuum.