Published: March 02, 2011 r2011 American Chemical Society 1518 dx.doi.org/10.1021/nl1042903 | Nano Lett. 2011, 11, 1518–1523 LETTER pubs.acs.org/NanoLett Mechanics and Chemistry: Single Molecule Bond Rupture Forces Correlate with Molecular Backbone Structure Michael Frei, † Sriharsha V. Aradhya, † Max Koentopp, ‡,|| Mark S. Hybertsen,* ,§ and L. Venkataraman* ,†,‡ † Department of Applied Physics and Applied Mathematics, ‡ Center for Electron Transport in Molecular Nanostructures, Columbia University, New York, New York, United States § Center for Functional Nanomaterials, Brookhaven National Laboratories, Upton, New York, United States b S Supporting Information U nderstanding physical properties of single molecule junc- tions is of fundamental importance to nanoscale electronics. 1 While electrical and thermal properties have been probed in a variety of organic molecules bound to metal electrodes, 2-8 measurements of rupture forces of single metal-molecule-metal junctions are new, 9,10 and simple predictions relating the me- chanics of these junctions to the backbone chemistry have not been tested. Here, we use a modified atomic force microscope (AFM) to form single molecule junctions between a gold sub- strate and a gold-coated cantilever. The simultaneously measured conductance and force between the AFM tip and substrate are analyzed to determine bond rupture forces. We use a new technique to analyze force data to obtain bond rupture forces from a large, statistically significant set of individual junction elongation traces. Using this method, we first show that the force required to break a gold-gold bond is 1.4 nN, based on over 38 000 measurements and in good agreement with previous published results. 11 We then show that for single molecule junctions, the N-Au bond-rupture force depends on the molec- ular backbone, and varies from 0.8 nN for 4,4 0 bipyridine to 0.5 nN in 1,4 diaminobenzene. There results are supported by density functional theory calculations for adiabatic junction elongation trajectories. Bond rupture forces determined from these calcula- tions agree quantitatively with the experimental results. We simultaneously measure the conductance and force of molecular junctions by repeatedly forming Au point contacts with a modified home-built conductive atomic force microscope (AFM) described in detail in the Supporting Information. All molecules were obtained from Sigma-Aldrich and were used without further purification. Conductance is determined by measuring current through the junction at a constant applied bias of 25 mV for short molecules and 75 mV for 1,6-hexane- diamine and for 4,4 0 bipyridine. Simultaneous measurements of cantilever deflection relate to the force applied across the junction (Figure 1A). The AFM is operated in ambient condi- tions at room temperature. For each measurement, a gold point-contact is first formed between the substrate and cantilever. It is then pulled apart and broken in an environment of molecules while conductance and force are measured as a function of sample displacement. This process is repeated thousands of times to obtain large data sets of conductance and simultaneously acquired force traces. Before adding a molecule to the substrate, at least 1000 conductance traces were collected to ensure that no contamination was present in the setup. Individual conductance traces for a gold point-contact show stepwise decrease in conductance until a single atom contact is formed with a conductance of G 0 =2e 2 /h, the quantum of conductance (Figure 1B). The simultaneously acquired force traces show a characteristic sawtooth pattern (Figure 1B) indicating two distinct force regimes: gradual linear increases due to elastic (reversible) elongations and sharp drops due to permanent deformations of the junction. Upon further Received: December 8, 2010 Revised: January 21, 2011 ABSTRACT: We simultaneously measure conductance and force across nanoscale junctions. A new, two-dimensional histogram technique is intro- duced to statistically extract bond rupture forces from a large data set of individual junction elongation traces. For the case of Au point contacts, we find a rupture force of 1.4 ( 0.2 nN, which is in good agreement with previous measurements. We then study systematic trends for single gold metal- molecule-metal junctions for a series of molecules terminated with amine and pyridine linkers. For all molecules studied, single molecule junctions rupture at the Au-N bond. Selective binding of the linker group allows us to correlate the N-Au bond-rupture force to the molecular backbone. We find that the rupture force ranges from 0.8 nN for 4,4 0 bipyridine to 0.5 nN in 1,4 diaminobenzene. These experimental results are in excellent quantitative agreement with density functional theory based adiabatic molecular junction elongation and rupture calculations. KEYWORDS: Molecular conductance, force spectroscopy, gold point-contact, bond rupture, break-junction