Transport studies of isolated molecular wires in self-assembled monolayer devices

We have fabricated a variety of novel molecular diodes based onself-assembled-monolayers (SAM) of solid-state mixture of molecular wires (1,4benzene-dimethane-thiol), and molecular insulator spacers (1-pentanethiol) withdifferent concentration ratios r of wires/spacers, which were sandwichedbetween two gold (Au) electrodes. We introduce two new methods borrowed fromSurface Science to (i) confirm the connectivity between thebenzene-dimethane-thiol molecules with the upper Au electrode, and (ii) countthe number of isolated molecular wires in the devices. The electrical transport properties of the SAM diodes were studied atdifferent temperatures via the conductance and differential conductancespectra. We found that a potential barrier caused by the spatial connectivitygap between the pentanethiol molecules and the upper Au electrode dominates thetransport properties of the pure pentanethiol SAM diode (r = 0). The transportproperties of molecular diodes with low r-values are dominated by theconductance of the isolated benzene-dimethane-thiol molecules in the device. Wefound that the temperature dependence of the molecular diodes is much weakerthan that of the pure pentanethiol device indicating the importance of thebenzene-dimethane-thiol simultaneous bonding to the two Au electrodes thatfacilitate electrical transport. From the differential conductance spectra wealso found that the energy difference, Delta between the Au electrodeFermi-level and the benzene-dimethane-thiol HOMO (or LUMO) level is ~1.5 eV;whereas it is ~2.5 eV for the pentanethiol molecule. The weak temperaturedependent transport that we obtained for the SSM diodes reflects the weaktemperature dependence of Delta.Comment: 38 p 8 Fi