Abstract
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.
Highlights
Molecular electronics (ME) describes the field of research in which single molecules or ensembles of molecules are utilized as functional elements in electronic 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
The primary experimental hurdle was the attachment of electrodes to individual molecules; while Mann and Kuhn used Hg to contact Langmuir–Blodgett films, it was the invention and development of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in the 1980s that laid the foundation for modern research on molecular ensemble junctions (MEJs) by allowing the interrogation of individual molecules on surfaces
Summary
Molecular electronics (ME) describes the field of research in which single molecules or ensembles of molecules are utilized as functional elements in electronic circuits. Nuzzo and Allara[7] contemporaneously demonstrated the robust selfassembly of thiols on Au, which enabled simple r-bonded (e.g., alkanes) and p-bonded molecules (e.g., phenylenes) bearing thiols to be assembled into molecular ensembles; the combination of these breakthroughs enabled the foundational techniques of modern ME, the trapping of single molecules between two electrodes,[8] or contact by a conductive tip in densely packed monolayers, allowing the measurement of (tunneling) currents under applied bias.[9,10,11,12] These pioneering studies advanced the methodology for determining the electrical conductivity of different molecules and provided insight into their charge-transport properties in experimental platforms that were readily reproducible in different laboratories These early experiments drove broad interest in cross-disciplinary physical scientists, which has led to the acceleration of fruitful discoveries of recent decades.
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