Transport Studies of Isolated Molecular Wires in Self-Assembled Monolayer Devices

TL;DR

This study investigates the electrical transport properties of self-assembled monolayer molecular diodes composed of molecular wires and insulators, introducing new surface science methods to analyze connectivity and wire count, revealing key transport mechanisms and energy level differences.

Contribution

It presents novel methods to confirm molecular connectivity and count isolated wires in SAM diodes, advancing understanding of their transport properties and energy level alignments.

Findings

Transport dominated by connectivity gap in pure pentanethiol SAM diodes.

Low wire ratio diodes show conductance mainly from molecular wires.

Weak temperature dependence indicates stable energy level differences.

Abstract

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.