Revealing physical insight from current-voltage response of molecular junctions

If molecular electronics would ever reach technological realization it must carry information, most probably within its current – voltage response. Nevertheless, the molecular-electronics community lacks an accepted framework to report, compare and discuss the current – voltage (I-V) response. The major obstacle is the huge variability of details, such as single / numerous molecules, molecular length, conjugation, presence of redox moieties and so on. Nature’s irony is that in practice, I-V traces are rather ‘boring’: more rounded than linear but no striking jumps, plateaus or other fingerprints, at least for the large majority of molecular junctions. As a result, a given experimental I-V trace can be described equally-well by vastly different charge-transport models.[1-2]

Acknowledging this limitation calls for using generic Taylor expansion as a transparent bridge between theory and observation. Specifically, the ratio between the first and third terms of G∼V provides a robust measure of the voltage response, dubbed as ‘scaling-bias’. The scaling-bias reflects the energy landscape of the junction, yet differs from the energy-barrier as the latter is specific to an assumed model.[2] Model-fitting beyond simplistic expansion rarely adds information, unless differentiation is used to magnify minute variations in I-V response and reveal hidden fingerprints. Normalized differential conductance (NDC) is a powerful tool which provides a quantitative measure of the deviation of a given trace from a simple linear relation (Ohmic → NDC ≡ 1).[3]

The suggested generic analyses cannot answer ‘what is the transport mechanism?’ yet it helps clarifying the mist between theory and experimental observation.

References

1. Vilan et al., Chem. Rev., 117, 4248 (2017).
2. Vilan et al., Acs Nano, 7, 695 (2013).
3. Vilan, Phys. Chem. Chem. Phys., 19, 27166 (2017).