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2007-04-27T08:08:00.000-07:00

Molecular electronics

For quantum mechanical study of the electron distribution in a molecule, see stereoelectronics.

Molecular electronics (sometimes called moletronics) is an interdisciplinary theme that spans physics, chemistry, and materials science. The unifying feature of this area is the use of molecular building blocks for the fabrication of electronic components, both passive (e.g. resistive wires) and active (e.g transistors). The concept of molecular electronics has aroused much excitement both in science fiction and among scientists due to the prospect of size reduction in electronics offered by molecular-level control of properties. Molecular electronics provides a means to extend Moore's Law beyond the foreseen limits of small-scale conventional silicon integrated circuits.

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Study of charge transfer in molecules was advanced in the 1940s by Robert Mulliken and Albert Szent-Gyorgi in discussion of so-called "donor-acceptor" systems and developed the study of charge transfer and energy transfer in molecules. Likewise, a 1974 paper from Mark Ratner and Avi Aviram ¹ illustrated a theoretical molecular rectifier. Later, Aviram detailed a theoretical single-molecule field-effect transistor in 1988. Further concepts were proposed by Forrest Carter of the Naval Research Laboratory, including single-molecule logic gates.

Unfortunately, the direct measurement of the electronic characteristics of individual molecules awaited the development of methods for making molecular-scale electrical contacts. This was no easy task. Thus, the first experiment measuring the conductance of a single molecule was only reported in 1997 by Mark Reed and co-workers. Since then, this branch of the field has progressed rapidly. Likewise, as it has become possible to measure such properties directly, the theoretical predictions of the early workers have been mostly confirmed.

Voltage-controlled switch, an molecular electronic device from 1974.

However, while mostly operating in the quantum realm of less than 100 nanometers, "molecular" electronic processes often collectively manifest on a macro scale. Examples include quantum tunneling, negative resistance, phonon-assisted hopping, polarons, and the like. Thus, macro-scale active devices were described decades before molecular-scale ones. E.g., in 1974, John McGinness and his coworkers described [1] the putative "first experimental demonstration of an operating molecular electronic device"[2]. This was a voltage-controlled switch. As its active element, this device used DOPA melanin, an oxidized mixed polymer of polyacetylene, polypyrrole, and polyaniline. The "ON" state of this switch exhibited almost metallic conductivity.

Since the 1970's, scientists have developed an entire panoply of new materials and devices. These findings have opened the door to plastic electronics and optoelectronics, which are beginning to find commercial application.

Charge transfer complexes

The first highly-conductive organic compounds were the Charge transfer complexes. In 1954, researchers at Bell Labs and elsewhere reported Charge transfer complexes with resistivities as low as 8 ohms-cm ^[1] ^[2]. In the early 1970's, salts of tetrathiafulvalene were shown to exhibit almost metallic conductivity, while superconductivity was demonstrated in 1980. Broad research on charge transfer salts continues today.

Conducting polymers

The linear-backbone "polymer blacks" (polyacetylene, polypyrrole, and polyaniline) and their copolymers are the main class of conductive polymers. Historically, these are known as Melanins. In 1963 Australians DE Weiss and coworkers reported [3] iodine-doped oxidized polypyrrole blacks with resistivities as low as 1 ohm/cm. Subsequent papers [4][5] reported resistances as low as 0.03 Ohm/cm. With the notable exception of Charge transfer complexes (some of which are even superconductors), organic molecules had previously been considered insulators or at best weakly conducting semiconductors.

Beginning in 1977, Shirakawa, Heeger, and MacDiarmid reported equivalent high conductivity in similarly oxidized, iodine-doped polyacetylene. They later received the 2000 Nobel prize in chemistry for " The discovery and development of conductive polymers " [6]. The Nobel citation made no reference to Weiss et al's similar earlier work. Also see Nobel Prize controversies.

Tom Tom

Molécule

Molecular electronics

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Charge transfer complexes

Conducting polymers