Fereiro Lab - Molecular electronics

What are molecular electronics? " Essentially, every technology you have ever heard of, where electrons move from here to there has the potential to be revolutionized, molecular electronics could become reality" Richard Smalley

Background: The term molecular electronics describes applied paradigms involving a single molecule or a single layer of molecules oriented in parallel between two conductors ( or "contacts"), with the critical dimension between contacts in the range of one to a few tens of nanometers. The origin of molecular electronics is co-related to the pioneering work by Aviram and Ratner entitled " Molecular Rectifiers" (1974). Their paper generated significant excitement by outlining a theoretical model indicating that a single molecule could exhibit preferential electronic conduction in one direction along its molecular axis. The development of SAMs and scanning probe microscopy (SPMs) provided the initial experimental tool (the 1980s) for investigating electron transport across molecules.

A particularly strong attribute of research in molecular electronics is the highly interdisciplinary nature that results from the diverse chemical, physical and electronic phenomena involved. Contributions from physicists, chemists, materials scientists, electrical engineers, etc. are all invaluable to progress. For example, from the perspective of a physicist, a molecular junction might be considered a parallel plate capacitor with an unusual dielectric layer, while a chemist may focus on the structure of the molecule between the plates. Depending on the structure and thickness of the ‘‘dielectric,’’ the device may have a variety of current-voltage (I–V) signatures and may operate under large electric fields (>10^6 V/cm). Alternatively, an electrochemist might view a molecular junction as a very thin electrochemical cell, possibly with redox-active components or even mobile ions.

Molecular junctions are readily divided into two types: single-molecule and ensemble. They are further categorized by the method of making the electrical contact between the conductors and molecule(s) since the conductor-molecule bonds have major effects on the electronic properties and stability of the finished junction.

Single-molecule experiments have provided important fundamental insights into how electron transport occurs within molecules and their contacts. However, there are some limitations imposed by the nature of the paradigm. It is generally recognized that the details of contact geometry and conformation are critical to the electronic behavior of the junction, and these may vary significantly for each molecule studied. Except for the special cases of inelastic electron tunneling spectroscopy (IETS) and tip-enhanced Raman spectroscopy, single-molecule devices are not amenable to the spectroscopic characterization of working junctions, so it is difficult to determine the precise conformation and contact geometry. In addition, thermal fluctuations of molecular conformation can create stochastic variations in electron transport, resulting in noise in the i–V curves, or even in apparent conductance switching. Finally, it is not clear how single-molecule junctions might be integrated with commercially viable microelectronics. Even if a useful single-molecule junction could be made reliably, there would remain the problem of ‘‘wiring up’’ and addressing a large number of such devices into a commercially viable product.

If molecular electronics is ever to be realized in practical devices, the electronic coupling between the contacts and the molecules must be understood or at least controlled reproducibly. If, for example, there is a large injection barrier between the contact and molecule, the overall electronic behavior may be interface-dominated, and lead to frustrated attempts to modify device characteristics by changes in molecular structure.

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