Determining the conductance of single molecular wire

We have designed a method for determining the conductance of an isolated molecular wire from the I-V characteristic of molecular junctions. The molecular diodes were 1 mm$^{2}$ in area and consist of self-assembled monolayer (SAM) from a mixture of the molecular wires and non-conducting molecules that are used as spacers; coupled to two opposite gold electrodes. We studied the I-V characteristic dependence of the fabricated diodes on the ratio, $r$ of wires/spacers. To obtain the number of molecular wires in the device we used multiple self-assemblies and titration techniques, as well as AFM of a small portion of the SAM surface. Our method was applied to a mixture of Me-BDT (methyl-bezenedithiol) molecules as wires and PT (pentathiol) molecules as spacers. For 10$^{-8 }<$ r $<$ 10$^{-3}$ we found that the device conductance is dominated by the molecular wires. From the current and obtained number of Me-BDT molecules in the device we determined the molecular conductance of Me-BDT to be 600 M$\Omega $, in good agreement with a theoretical tunneling model.