Estimating the Number of Molecules in Molecular Junctions Merely Based on the Low Bias Tunneling Conductance at Variable Temperature

TL;DR

This paper demonstrates that temperature-dependent conductance data in molecular junctions can be explained by a single-step tunneling mechanism, challenging the common interpretation of two-step hopping processes, and provides formulas to estimate junction areas.

Contribution

The paper introduces analytical formulas for G(T) that distinguish tunneling from hopping mechanisms and applies them to estimate junction areas from conductance data.

Findings

Single-step tunneling can produce Arrhenius-like conductance behavior.

Analytical formulas accurately fit experimental G(T) data.

Estimated effective area ratio f matches previous studies.

Abstract

Temperature ($T$) dependent conductance $G = G(T)$ data measured in molecular junctions are routinely taken as evidence for a two-step hopping mechanism. The present paper emphasizes that this is not necessarily the case. A curve of $\ln G$ versus $1/T$ decreasing almost linearly (Arrhenius-like regime) and eventually switching to a nearly horizontal plateau (Sommerfeld regime), or possessing a slope gradually decreasing with increasing $1/T$ is fully compatible with a single-step tunneling mechanism. The results for the dependence of $G$ on $T$ presented include both analytical exact and accurate approximate formulas and numerical simulations. These theoretical results are general, also in the sense that they are not limited, e.g., to the (single molecule electromigrated (SET) or large area EGaIn) fabrication platforms, which are chosen for exemplification merely in view of the available experimental data needed for analysis. To be specific, we examine in detail transport measurements for molecular junctions based on ferrocene (Fc). As a particularly important finding, we show how the present analytic formulas for $G=G(T)$ can be utilized to compute the ratio $f = A_{\text{eff}} / A_n$ between the effective and nominal areas of large area Fc-based junctions with an EGaIn top electrode. Our estimate of $f\approx 0.6 \times 10^{-4}$ is comparable with previously reported values \ib{based on completely different methods} for related large area molecular junctions.