Tuning the conductance of molecular junctions: transparent versus tunneling regimes

TL;DR

This theoretical study investigates how molecular junctions with diatomic molecules attached to gold and platinum electrodes exhibit conductance behaviors, revealing a transition from tunneling to transparent regimes characterized by wide transmission plateaus.

Contribution

The paper demonstrates that the transparent regime in molecular junctions is a generic property driven by strong coupling, contrasting with the usual tunneling regime with sharp resonances.

Findings

All studied junctions have conductance near the quantum unit G0.

Transmission coefficients show wide plateaus in the transparent regime.

Gold/Benzene junctions are highly conductive, unlike gold/Benzene-dithiolate (BDT).

Abstract

We present a theoretical study of the transport characteristics of molecular junctions, where first-row diatomic molecules are attached to (001) gold and platinum electrodes. We find that the conductance of all of these junctions is of the order of the conductance quantum unit $G_0$, spelling out that they belong to the transparent regime. We further find that the transmission coefficients show wide plateaus as a function of the energy, instead of the usual sharp resonances that signal the molecular levels in the tunneling regime. We use Caroli's model to show that this is a rather generic property of the transparent regime of a junction, which is driven by a strong effective coupling between the delocalized molecular levels and the conduction channels at the electrodes. We analyse the transmission coefficients and chemical bonding of gold/Benzene and gold/Benzene-dithiolate (BDT) junctions to understand why the later show large resistances, while the former are highly conductive.