Single molecule electron transport junctions: charging and geometric effects on conductance

Area of Science:

• Solid State Physics
• Materials Science
• Nanotechnology

Background:

• The p-benzenedithiolate (BDT) molecule on gold electrodes is a model for molecular transport junctions.
• Previous studies show varied conclusions on transport mechanisms and magnitudes.
• Discrepancies are linked to charge transfer, Fermi energy accuracy, geometry, and switching.

Purpose of the Study:

• Compare transport codes TRANSIESTA-C and HUCKEL-IV for BDT molecular junctions.
• Investigate the effect of Au-S bond lengthening on molecular conductance.
• Explain variations in reported conductance values.

Main Methods:

• Computational transport simulations using TRANSIESTA-C and HUCKEL-IV.
• Analysis of molecular orbital energies and charge distribution.
• Modeling of extended BDT systems with added gold atoms.

Main Results:

• Low bias conductance increases up to 30-fold upon Au-S bond lengthening.
• Conductance increase is mediated by the highest occupied molecular orbital (HOMO).
• Charging of sulfur atoms and reduced Fermi level-HOMO gap contribute to increased conductance.
• Adding gold atoms introduces new molecular states near the Fermi energy.

Conclusions:

• Au-S bond length is a critical factor influencing molecular conductance.
• Electronic structure modifications, particularly the HOMO level alignment, dictate transport properties.
• The presence of additional molecular states can explain experimental variability in BDT junctions.