NDC analysis of current-voltage response reveals voltage partition across 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 largely ignores the effect of applied voltage on transmission probability, due to both technical (stability) and fundamental (non-equilibrium) challenges. Furthermore, molecular I-V traces often lack characteristic features that can support or reject modeling assumptions. Normalized differential conductance (NDC) reveals only the out-of-equilibrium response (V≠0) while eliminating the orders-of-magnitude variation in the net conductance (at V=0); NDC can be used both quantitatively and qualitatively as it magnifies unique voltage-driven features.¹ Two examples for NDC analysis will be presented. First, for molecular junctions with semi-metallic carbon contacts (R.L. McCreery lab, Alberta), NDC reveals that the voltage response is dominated by the limited density-of-states of carbon; this yields an exponential I-V dependence, in contrast to tunneling-limited current that has a much weaker voltage effect. Second, NDC of Au-S-alkyl // EGaIn junctions reveals the internal voltage partition between the contacts and the molecular.² Rough top-contacts withdraw most of the applied voltage to the –CH₃ // EGaIn interface and therefore no field is developed across the molecules and the NDC is independent of molecular length. In contrast, under smooth EGaIn contacts, the voltage drops on the alkyl chains yielding a highly non-linear response as the chains grow longer.² In summary, NDC helps resolving the potential drop within molecular junctions, yielding a critical insight toward realization of highly non-linear molecular I-V response.

1) Vilan, Chem. Chem. Phys., 19, 27166 (2017)

2) Karuppannan et al., Funct. Matr., 1904452 (2019)