Tuning the conductance of molecular junctions: transparent versus tunneling regimes

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.