  We have fabricated a variety of novel molecular diodes based on
self-assembled-monolayers (SAM) of solid-state mixture of molecular wires (1,4
benzene-dimethane-thiol), and molecular insulator spacers (1-pentanethiol) with
different concentration ratios r of wires/spacers, which were sandwiched
between two gold (Au) electrodes. We introduce two new methods borrowed from
Surface Science to (i) confirm the connectivity between the
benzene-dimethane-thiol molecules with the upper Au electrode, and (ii) count
the number of isolated molecular wires in the devices.
  The electrical transport properties of the 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 pentanethiol molecules and the upper Au electrode dominates the
transport properties of the pure pentanethiol SAM diode (r = 0). The transport
properties of molecular diodes with low r-values are dominated by the
conductance of the isolated benzene-dimethane-thiol molecules in the device. We
found that the temperature dependence of the molecular diodes is much weaker
than that of the pure pentanethiol device indicating the importance of the
benzene-dimethane-thiol simultaneous bonding to the two Au electrodes that
facilitate electrical transport. From the differential conductance spectra we
also found that the energy difference, Delta between the Au electrode
Fermi-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 temperature
dependent transport that we obtained for the SSM diodes reflects the weak
temperature dependence of Delta.
