Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

TL;DR

This study measures high conductance in single pi-conjugated molecules covalently bonded to gold electrodes, revealing near-resonant transmission and tunneling behavior, with implications for molecular electronics.

Contribution

It demonstrates direct covalent Au-C bonds in molecular junctions with high conductance, using in situ cleaving of end groups, and provides detailed conductance measurements and theoretical analysis.

Findings

Conductance approaches one quantum (G0) in certain junctions.

Longer oligophenyls show exponential conductance decay, indicating tunneling.

Density functional theory reveals near-resonant transmission and crossover to tunneling.

Abstract

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ resulting in formation of a direct covalent sigma bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated pi-system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the pi-system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls with two to four phenyl units show a hundred-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling as they exhibit an exponential dependence of conductance with oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission with a cross-over to tunneling for the longer oligomers.