Simultaneous Measurement of Force and Conductance Across Single Molecule Junctions

Measurement of electronics and mechanics of single molecules provides a fundamental understanding of conductance as well as bonding at the atomic scale. To study the mechanics at these length scales, we have built a conducting atomic force microscope (AFM) optimized for high displacement and force resolution. Here, we simultaneously measure conductance and force across single Au-molecule-Au junctions in order to obtain complementary information about the electronics and structure in these systems. First we show that single-atom Au contacts, which have a conductance of G0 (2e /h), have a rupture force of about 1.4 nN, in excellent agreement with previous theoretical and experimental studies. For a series of amine and pyridine linked molecules which are bound to Au electrodes through an Au-N donoracceptor bond, we observe that the rupture force depends on the backbone chemistry and can range from 0.5 to 0.8 nN. We also study junctions formed with molecules that bind through P-Au and S-Au interactions. We find that both the conductance signatures and junction evolution of covalent S-Au bond (thiolate) and a donor-acceptor S-Au bond (thiol) are dramatically different. Finally, we perform density functional theory based adiabatic molecular junction elongation and rupture calculations which give us an insight into the underlying mechanisms in these experiments.