However, the conductance variation from junction to junction has made it difficult to verify even the simplest predictions about how molecules should behave in unimolecular devices. Here, using amine link groups 13 to form single molecule junctions, we show a clear correlation between molecule conformation and junction conductance in a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine squared relation predicted theoretically for transport through π-conjugated systems 14 .We recently demonstrated that metal-molecule-metal junctions, formed by breaking Au point contacts in a solution of molecules, exhibit more reliable and reproducible conductance values when amine groups rather than thiols or isonitriles are used to attach the molecules to the junction contacts 13 . Because of this reduced variability, we can 2 determine statistically meaningful average conductance values for specific singlemolecule junctions; this capability in turn allows us to study the impact of molecular properties on junction conductance.We present our experimental results as conductance histograms, where peaks indicate the most prevalent molecular junction conductances while the width of the conductance distributions reflects the microscopic variations from junction to junction.(For details on experimental and data analysis procedures, see SupplementaryInformation.) Figure 1A shows the histograms for 1,4-diaminobenzene (1) and 2,7-diaminofluorene (2), each constructed from over 10000 conductance traces without resorting to any data selection or processing. Many of our conductance traces reveal stepwise changes in conductance not only at conductance values that are multiples of the fundamental quantum of conductance G 0 (2e 2 /h), but also below G 0 (see Fig. 1C and Supplementary Information Figure S1). These steps are due to conduction through a single molecule bridging the gap between the two Au point-contacts. As seen in the histograms of 1 and 2 (Fig. 1A), for each junction type the additional step occurs within a narrow distribution of conductance values. The most prevalent value is determined by a fit to the peak below G 0 in the conductance histograms using a Lorentzian line shape, which we find to fit our peaks more accurately than a Gaussian line shape (see inset of Figure 1A and also Supplementary Information).When the experiment is repeated using a solution containing an equimolar mixture of 1 and 2 (as indicated in Figure 1B), the resulting histogram (red curve in Figure 1A) shows two distinct peaks below G 0 at nearly the same conductance values as those of the peaks seen in the histograms for the individual molecules (green and yellow curves).Individual traces using the solution mixture show conductance plateaus corresponding to either molecule 1 or 2, and sometimes a plateau corresponding to 1 followed by a plateau 3 corresponding to 2 (see Figure 1C). But ...

Abstract:We measure the conductance of amine terminated molecules by breaking Au pointcontacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule-Au junctions is much less than the variability for diisonitrile and dithiol-Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2-8 carbon atoms in the hydrocarbon chain our results show a systematic trend in the conductance from which we extract a tunneling decay constant of 0.91 ± 0.03 per methylene group. We hypothesize that the diamine link binds preferentially to under-coordinated Au atoms in the junction. This is supported by density functional theory based calculations that show the amine binding to a gold adatom with sufficient angular flexibility for easy junction formation but well-defined electronic coupling of

Molecular electronics is often limited by the poorly defined nature of the contact between the molecules and the metal surface. We describe a method to wire molecules into gaps in single-walled carbon nanotubes (SWNTs). Precise oxidative cutting of a SWNT produces carboxylic acid-terminated electrodes separated by gaps of

We have measured the conductance of a hepta-aniline oligomer attached to gold electrodes held under potential control in electrolyte. It increases fifteen-fold (to 5.3+/-0.4 nS) on oxidation from the leucoemeraldine form to the emeraldine salt. The single-molecule current-voltage characteristic, linear in toluene, displays negative differential resistance in an acidic electrolyte. The negative differential resistance is accounted for by modification of the local surface potential by the applied bias. These results connect electrochemical data directly to molecular electronic behavior in a two-terminal device.

Abstract.A comprehensive review is presented of single molecule junction conductance measurements across families of molecules measured while breaking a gold point contact in a solution of molecules with amine end groups. A theoretical framework unifies the picture for the amine-gold link bonding and the tunnel coupling through the junction using Density Functional Theory based calculations. The reproducible electrical characteristics and utility for many molecules is shown to result from the selective binding between the gold electrodes and amine link groups through a donor-acceptor bond to undercoordinated gold atoms. While the bond energy is modest, the maximum force sustained by the junction is comparable to, but less than, that required to break gold point contacts. The calculated tunnel coupling provides conductance trends for all 41 molecule measurements presented here, as well as insight into the variability of conductance due to the conformational changes within molecules with torsional degrees of freedom. The calculated trends agree to within a factor of two of the measured values for conductance ranging from 10 -7 G 0 to 10 -2 G 0 , where G 0 is the quantum of conductance (2e 2 /h).

We used a combination of dip-pen nanolithography and scanning optical confocal microscopy to fabricate and visualize luminescent nanoscale patterns of various materials on glass substrates. We show that this method can be used successfully to push the limits of dip-pen nanolithography down to controlled deposition of single molecules. We also demonstrate that this method is able to create and visualize protein patterns on surfaces. Finally, we show that our method can be used to fabricate polymer nanowires of controlled size using conductive polymers. We also discuss the factors that influence the size of these nanowires.

This study details a modular and general synthesis of a new class of molecules consisting of cruciform pi-systems. The key to synthesizing these molecules was an unprecedented double Staudinger cyclization. Once formed, these rigid compounds assemble into ordered monolayer films on metal and metal oxide surfaces to orient their conjugated, bis-phenyloxazole subunits upright. This surface orientation is enforced by the external phenyl substituents that are out of the ring plane, thus preventing the prone conformation.

I. Preparation of moleculesThe molecules used in this study were prepared by published methods. The bisoxazole was prepared by method of Klare and coworkers,(S1) and the terpyridyls were prepared according to a published procedure.(S2) The thiols were protected as their acetate esters and deprotected using the method of Tour and coworkers.(S3) II. Monolayer characterizationA. Monolayer formation. The monolayers and the imine-forming surface chemistry of the bisoxazole was previously described.(S1) The formation of the terpyridyl monolayers and their subsequent reaction with cobalt ions is described below.A few drops of ammonium hydroxide were added into a 1 mM solution of acetate protected terpyridyl thiol in THF (5 mL). Into this solution was placed three fresh Pt substrates for 24 hrs. Then the Pt substrates were taken out and rinsed with THF, CH 2 Cl 2 , acetone and isopropanol thoroughly and dried under a stream of Argon. One of the three was used for characterization experiments, and the other two were placed into 10mM aqueous solution of