Published: April 18, 2011 r2011 American Chemical Society 1988 dx.doi.org/10.1021/nl200324e | Nano Lett. 2011, 11, 1988–1992 LETTER pubs.acs.org/NanoLett Environmental Control of Single-Molecule Junction Transport V. Fatemi, †,§ M. Kamenetska, † J. B. Neaton,* ,‡ and L. Venkataraman* ,† † Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States ‡ Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States b S Supporting Information T he past decade has seen rapid progress toward the understanding and control of molecular-scale transport phenomena 13 While advances have been made in understanding how intrinsic factors, such as intramolecular configuration and binding geometry, affect the molecular junction conductance, 49 uncertainties regarding extrinsic factors remain. In particular, the role of the liquid or solvent envi- ronment, which are often necessary in nanoscale electronic transport measurements, is generally not addressed in experiments and often neglected in theoretical calculations. 10 Understanding the impact of solvents on molecular junction conductance will serve to elucidate ways in which nanoscale device properties may be manipulated through control of their environment. Here, we report and explain the modi fication of the conductance of a single molecule junction through the solvent environment. We use 1,4-benzenediamine (BDA)Au molecular junctions as a test bed, as past measurements and calculations have shown that its conductance is well-defined and reproducible and originates with speci fic metal molecule contact geometries. 4,11,12 BDA junction conductances (G BDA ) are measured with a scanning tunneling microscope-based break junction technique 13 in 13 di fferent ambi- ent-solvated environments. We observe that G BDA varies by more than 50%, depending on the solvent. First-principles density func- tional theory (DFT) calculations indicate that like BDA, select solvent molecules can also bind to undercoordinated Au atoms on the electrodes. The smaller surface dipoles induced by a solvent molecule (relative to BDA) lead to a larger Au contact work function and shift the BDA HOMO toward the Au Fermi level (E F ), resulting in a larger conductance. We show that solvent-induced shift in conductance depends on (1) the affinity of the solvent to Au binding sites, which affects surface coverage, and (2) the induced dipole upon adsorption. With this mechanism, molecular junction level alignment and transport properties can be controllably altered by solvent molecule binding to the contact surface. Single molecule junctions are created by repeatedly forming and breaking an Au point-contact in a 1 mM solution of the BDA under a 25 mV bias with a pulling rate of 15 nm/s in a home- built scanning tunneling microscope. 14 Conductance (current/ voltage) is measured as a function of tip/sample displacement to generate conductance traces (Figure 1B inset). The process of breaking an Au point-contact in a solution of BDA creates numerous undercoordinated Au atoms on each electrode, where the amine-terminal groups can bind to form single-molecule junctions. Conductance traces show plateaus at multiples of the quantum of conductance, G 0 =2e 2 /h, as the tipsurface contact area is reduced, followed by either a tunneling background or an additional con- ductance plateau due to the formation of a BDA junction. Normalized histograms of the conductance traces in three re- presentative solvents—chlorobenzene (ClPh), bromobenzene (BrPh), and iodobenzene (IPh)—are shown in Figure 1A (mea- surements for all solvents are shown in Figure S1 in the Supporting Information). We see that the peak positions, which give us the most probable junction conductance, are highly solvent-dependent; the Br- and I-based solvents shift the conductance peak to larger values relative to ClPh. Furthermore, experiments in BrPh, which evapo- rates after about 5000 traces, show a peak at 8.2  10 3 G 0 when solvent is present and ∼7  10 3 G 0 immediately after solvent evaporation. The lower conductance value compares well with the Received: January 27, 2011 Revised: April 10, 2011 ABSTRACT: The conductance of individual 1,4-benzenediamine (BDA)Au molecular junctions is measured in different solvent environments using a scanning tunneling microscope based point- contact technique. Solvents are found to increase the conductance of these molecular junctions by as much as 50%. Using first principles calculations, we explain this increase by showing that a shift in the Au contact work function is induced by solvents binding to undercoordinated Au sites around the junction. Increasing the Au contact work function reduces the separation between the Au Fermi energy and the highest occupied molecular orbital of BDA in the junction, increasing the measured conductance. We demonstrate that the solvent-induced shift in conductance depends on the affinity of the solvent to Au binding sites and also on the induced dipole (relative to BDA) upon adsorption. Via this mechanism, molecular junction level alignment and transport properties can be statistically altered by solvent molecule binding to the contact surface. KEYWORDS: Metalorganic interface, single-molecule conductance, tunnel coupling, solvent effects, density functional theory, F-phenylenediamine