Charge transport through molecular ensembles: Recent progress in molecular electronics

This review focuses on molecular ensemble junctions in which the individual molecules of a monolayer each span two electrodes. This geometry favors quantum mechanical tunneling as the dominant mechanism of charge transport, which translates perturbances on the scale of bond lengths into nonlinear electrical responses. The ability to affect these responses at low voltages and with a variety of inputs, such as de/protonation, photon absorption, isomerization, oxidation/reduction, etc., creates the possibility to fabricate molecule-scale electronic devices that augment; extend; and, in some cases, outperform conventional semiconductor-based electronics. Moreover, these molecular devices, in part, fabricate themselves by defining single-nanometer features with atomic precision via self-assembly. Although these junctions share many properties with single-molecule junctions, they also possess unique properties that present a different set of problems and exhibit unique properties. The primary trade-off of ensemble junctions is complexity for functionality; disordered molecular ensembles are significantly more difficult to model, particularly atomistically, but they are static and can be incorporated into integrated circuits. Progress toward useful functionality has accelerated in recent years, concomitant with deeper scientific insight into the mediation of charge transport by ensembles of molecules and experimental platforms that enable empirical studies to control for defects and artifacts. This review separates junctions by the trade-offs, complexity, and sensitivity of their constituents; the bottom electrode to which the ensembles are anchored and the nature of the anchoring chemistry both chemically and with respect to electronic coupling; the molecular layer and the relationship among electronic structure, mechanism of charge transport, and electrical output; and the top electrode that realizes an individual junction by defining its geometry and a second molecule-electrode interface. Due to growing interest in and accessibility of this interdisciplinary field, there is now sufficient variety in each of these parts to be able to treat them separately. When viewed this way, clear structure-function relationships emerge that can serve as design rules for extracting useful functionality. (C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)