Single-Molecule Circuits with Well-Defined Molecular Conductance

TL;DR

This study demonstrates that amine-terminated molecules form stable, well-defined single-molecule junctions with gold, enabling precise conductance measurements and revealing a systematic tunneling decay trend across alkane diamines.

Contribution

It introduces amine linkages as a reliable method for consistent single-molecule conductance measurements, supported by experimental data and density functional theory calculations.

Findings

Amine-Au junctions show less conductance variability than other linkers.

Conductance decreases systematically with increasing chain length.

Density functional theory supports amine binding to gold adatoms.

Abstract

We measure the conductance of amine-terminated molecules by breaking Au point contacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule-Au junctions is much less than the variability for diisonitrile and dithiol-Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2-8 carbon atoms in the hydrocarbon chain, our results show a systematic trend in the conductance from which we extract a tunneling decay constant of 0.91 +/- 0.03 per methylene group. We hypothesize that the diamine link binds preferentially to undercoordinated Au atoms in the junction. This is supported by density functional theory-based calculations that show the amine binding to a gold adatom with sufficient angular flexibility for easy junction formation but well-defined electronic coupling of the N lone pair to the Au. Therefore, the amine linkage leads to well-defined conductance measurements of a single molecule junction in a statistical study.