Orbital Interaction Mechanisms of Conductance Enhancement and Rectification by Dithiocarboxylate Anchoring Group

TL;DR

This study computationally investigates how dithiocarboxylate anchoring groups enhance conductance and enable rectification in molecular junctions, revealing microscopic mechanisms behind these effects.

Contribution

It introduces a microscopic understanding of conductance enhancement and rectification mechanisms involving dithiocarboxylate groups in molecular electronics.

Findings

Interaction of LUMO of anchoring group with molecular orbitals causes resonances near Fermi energy.

Asymmetrical anchoring groups lead to a new rectification mechanism.

Transmission peaks respond differently to applied voltage depending on their orbital origin.

Abstract

We study computationally the electron transport properties of dithiocarboxylate terminated molecular junctions. Transport properties are computed self-consistently within density functional theory and nonequilibrium Green's functions formalism. A microscopic origin of the experimentally observed current amplification by dithiocarboxylate anchoring groups is established. For the 4,4'-biphenyl bis(dithiocarboxylate) junction, we find that the interaction of the lowest unoccupied molecular orbital (LUMO) of the dithiocarboxylate anchoring group with LUMO and highest occupied molecular orbital (HOMO) of the biphenyl part results in bonding and antibonding resonances in the transmission spectrum in the vicinity of the electrode Fermi energy. A new microscopic mechanism of rectification is predicted based on the electronic structure of asymmetrical anchoring groups. We show that the peaks in the transmission spectra of 4'-thiolato-biphenyl-4-dithiocarboxylate junction respond differently to the applied voltage. Depending upon the origin of a transmission resonance in the orbital interaction picture, its energy can be shifted along with the chemical potential of the electrode to which the molecule is more strongly or more weakly coupled.