Optical and transport studies of single molecule tunnel junctions based on self-assembled monolayers

We have fabricated a variety of novel molecular tunnel junctions based on self-assembled-monolayers (SAM) of two-component solid-state mixtures of molecular wires (1,4-methane benzene-dithiol; Me-BDT with two thiol anchoring groups), and molecular insulator spacers (1-pentanethiol; PT with one thiol anchoring group) at different concentration ratios, r of wires/spacers, which were sandwiched between two metallic electrodes such as gold and cobalt. FTIR spectroscopy and surface titration were used, respectively, to verify the formation of covalent bonds with the electrodes, and obtain the number of active molecular wires in the device. The electrical transport properties of the SAM devices were studied as a function of (i) r-value, (ii) temperatures, and (iii) different electrodes, via the conductance and differential conductance spectra. The measurements were used to analyze the Me-BDT density of states near the electrode Fermi level, and the properties of the interface barriers. We measured the Me-BDT single molecule resistance at low bias and gold electrodes to be 6×109 Ω. We also determine the energy difference, Δ between the Me-BDT HOMO level and the gold Fermi level to be about 1.8 eV. In addition we also found that the temperature dependence of the SAM devices with r<10−4 is much weaker than that of the pure PT device (or r=0), showing a small interface barrier.