(268c) Monolayer and Electrode Geometry Effects On the Formation, Structure, and Conductance of Molecular Junctions

The construction of basic circuit elements (e.g., diodes, transistors, switches) from molecular building blocks constitutes a potential route to smaller, faster, and cheaper electronics.  One method for wiring individual molecules between metallic electrodes is the mechanically controllable break junction (MCBJ) technique, in which molecules self-assemble between two fractured nanowire tips.  Here, we report atomistically detailed simulations of the MCBJ technique for forming molecular junctions composed of benzene-1,4-dithiolate (BDT) chemically attached between two Au nanotips.  The simulations utilize a hybrid molecular dynamics-Monte Carlo (MD-MC) approach and account for the influence of important environmental factors (e.g., monolayer effects) on the behavior of the junctions.¹ Using this tool, we demonstrate the impact of monolayer density and tip geometry on the likelihood of molecular junction formation, and on the bonding geometry and tilt angle of bridged molecules.  Since the primary property measured in experiment is conductance, we next calculate the low-bias conductance of the structures resulting from the MD-MC simulations.  We employ a combined density functional theory and Green’s function approach to calculate conductance; self-interaction corrections² are included to accurately describe the energy-level lineup between BDT and the Au leads.  Using this approach we are able to systematically study structure-conductance relationships in Au-BDT-Au junctions, helping to further inform and interpret experimental results.              

[1] W. R. French, C. R. Iacovella and P. T. Cummings, “Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions”, ACS Nano, 2012, 6 (3), 2779-2789.

[2] C. Toher and S. Sanvito, “Effects of Self-Interaction Corrections on the Transport Properties of Phenyl-Based Molecular Junctions”, Phys. Rev. B, 2008, 77, 155402.