We perform temperature dependent conductance measurements on sub-nanometer sized single molecules bound to gold electrodes using a scanning tunneling microscope-based break junction technique in Ultra-High Vacuum (UHV). We find a threefold increase in the conductance of amine-terminated conjugated molecules when the temperature increases from 4 K to 300 K in UHV. Furthermore, the conductance measured at 300 K in UHV is consistent with solution-based measurements under ambient conditions where the transport mechanism corresponds to off-resonant electron tunneling across the molecule. Our measurements indicate that at 300 K, conductance is largely independent of pressure or solvent around the junction. In addition, our data unambiguously show that temperature can affect the tunneling conductance of single molecule-metal junctions. We show that the structure of the metal electrodes that form in these junctions varies systematically with temperature, and hypothesize that this changing structure of the interface alters electron tunneling probability and propose a mechanism to explain our findings.

We measure simultaneously force and conductance of Ag metal point-contacts under ambient conditions at room temperature. We observe the formation of contacts with a conductance close to 1 G0, the quantum of conductance, which can be attributed to a single-atom contact, similar to those formed by Au. We also find two additional conductance features at ∼0.4 G0 and ∼1.3 G0, which have been previously ascribed to contacts with oxygen contaminations. Here, using a conductance cross-correlation technique, we distinguish three different atomic-scale structural motifs and analyze their rupture forces and stiffness. Our results allow us to assign the ∼0.4 G0 conductance feature to an Ag-O-Ag contact and the ∼1.3 G0 feature to an Ag-Ag single-atom contact with an oxygen atom in parallel. Utilizing complementary information from force and conductance, we thus demonstrate the correlation of conductance with the structural evolution at the atomic scale.

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 (2e2/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 donor-acceptor 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.

Van der Waals interactions are critical to the understanding of functional metal/molecule interfaces in catalysis, molecular electronics and self-assembly. Such interactions have now been characterized at the single-molecule level through a combination of measurements of the stretching mechanics of molecular junctions and atomistic simulations.

I. EXPERIMENTAL DETAILS .................................................................................................. 2 Experimental Setup and Procedures: ......................................................................................... 2 Force calibration: .......................................................................................................................... 2 Results from Control Measurements: ......................................................................................... 4

We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au-S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au-S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.

The break-junction technique is widely used to measure electronic properties of nanoscale junctions including metal point-contacts and single-molecule junctions. In these measurements, conductance is measured as a function of electrode displacement yielding data that is analyzed by constructing conductance histograms to determine the most frequently observed conductance values in the nanoscale junctions. However much of the rich physics in these measurements is lost in this simple analysis technique. Conductance histograms cannot be used to study the statistical relation of distinct junction configurations, to distinguish structurally different configurations that have similar conductance values, or to obtain information on the relation between conductance and junction elongation. Here, we give a detailed introduction to a novel statistical analysis method based on the two-dimensional cross-correlation histogram (2DCH) analysis of conductance traces and show that this method provides new information about the relation of different junction configurations that occur during the formation and evolution of metal and single-molecule junctions. We first illustrate the different types of correlation effects by using simulated conductance traces. We then apply this analysis method to several different experimental examples. We show from break-junction measurements of different metal point-contacts that in aluminum, the first conductance histogram peak corresponds to two different junction structures. In tantalum, we identify the frequent absence of adhesive instability. We show that conductance plateaus shift in a correlated manner in iron and vanadium junctions. Finally, we highlight the applicability of the correlation analysis to single-molecule platinum-CO-platinum and gold-4,4'-bipyridine-gold junctions.

We simultaneously measure conductance and force across nanoscale junctions. A new, two-dimensional histogram technique is introduced 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' 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.