Revealing the role of anchoring groups in the electrical conduction through single-molecule junctions

Using the mechanically controllable breakjunction technique we have performed transport experiments through single ethyne molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups. We have measured current-voltage characteristics inside a liquid cell. By fitting the experimental curves to a single-level resonant tunneling model we are able to extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal-molecule coupling. We compare the results to ab initio calculations which give further insight into the transport properties. The dependence of the I-V characteristics on the various anchoring groups shows clearly that these groups affect the coupling strength between metal and the molecules as well as the position of the molecular energy levels.