Models to describe tunnelling charge-transport through molecular junctions must be able to produce trends across large series of molecules in order to be useful to chemists designing new molecules. Although sophisticated DFT calculations can often reproduce non-intuitive observations about trends in conductance, their sophistication makes it difficult to draw straightforward conclusions that are relevant to the molecular design. In this work, we present a simple single-level model (SLM) that readily produces trends of a molecule/electrode coupling parameter from measured J-V data for more than 50 self-assembled monolayers (SAMs) in MTS/SAM//EGaIn (eutectic Gallium-Indium) junctions. The input parameters, which are low-bias current-density (J for V→0) and transition voltages (Vtrans), are readily measured and can be easily extracted from J-V data, which facilitated the application of SLM model across data from different labs. We examined data from aliphatic to conjugated molecules with varied functional groups at the electrode interfaces and the backbones. For conjugated molecules, the trends produced by the SLM help us investigate the effect of the chemical identity of the functional groups on the molecule-electrode interface and therefore the SAM's transport behaviour. Furthermore, SLM was applied to SAMs of molecular rectifiers measured in our lab and other research groups that rectify current through the Stark effect, intramolecular gating, or by molecular orbitals tracking EFermi, demonstrating the usefulness of the model. SLM was able to predict trends hinting at further insight into rectification mechanisms.

Empirical model for molecule-electrode coupling in large-area EGaIn tunnelling junctions

ACS Spring Meeting 2021

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