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

We have fabricated a variety of isolated molecule diodes based on self-assembled monolayers (SAMs) of solid-state mixture (SSM) of molecular wires [1,4-methane benzene dithiol (Me-BDT)], and molecular insulator spacers [penthane 1-thiol (PT)] with different concentration ratios r of wires/spacers, which were sandwiched between two gold (Au) electrodes. We introduce two specialized methods borrowed from surface science to (i) confirm the connectivity between the Me-BDT molecules with the upper Au electrode, and (ii) count the number of isolated molecular wires in the devices. The electrical transport properties of the SSM SAM diodes were studied at different temperatures via the conductance and differential conductance spectra. We found that a potential barrier caused by the spatial connectivity gap between the PT molecules and the upper Au electrode dominates the transport properties of the pure PT SAM diode (r=0). The transport properties of SSM diodes with r values in the range 10(-8)< r < 10(-4) are dominated by the conductance of the isolated Me-BDT molecules in the device. We found that the temperature dependence of the SSM diodes is much weaker than that of the pure PT device, indicating the importance of the Me-BDT simultaneous bonding to the two Au electrodes that facilitates electrical transport. From the differential conductance spectra we also found that the energy difference between the Au electrode Fermi level and the Me-BDT highest occupied molecular-orbital (or lowest unoccupied molecular-orbital) level is similar to 1.5 eV; where it is similar to 2.5 eV for the PT molecule. The weak temperature-dependent transport that we obtained for the SSM diodes reflects the weak temperature dependence of Delta. In addition, our measurements reveal that the conductance of SSM diodes scales linearly with r, showing that the charge transport in these devices is dominated by the sum of the isolated Me-BDT molecular conductance in the device. Based on this finding, and the measured number of the Me-BDT molecules in the device we obtained the "single molecule resistance," R-M. We measured R-M=6x10(9) Omega for isolated Me-BDT molecules, which is consistent with previous measurements using other transport measuring techniques. A simple model for calculating R-M, where the transport is governed by electron tunneling through the Me-BDT molecule using the WKB approximation, is in good agreement with the experimental data, thus validating the procedures used for our measurements. (c) 2005 American Institute of Physics.