Molecular electronics
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Molecular electronics (sometimes called moletronics) is a branch of applied physics which aims at using molecules as passive (e.g. resistive wires) or active (e.g transistors) electronic components.
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 such minute components. It is an entincing alternative to extend Moore's Law beyond the foreseen limits of small-scale conventional silicon integrated circuits. As a result, molecular electronics is currently a very active research fied, sometimes marked by controversy since many fundamentals questions concerning both theory and experiments are left open.
Among the important issues is the determination of the resistance of a single molecule (both theoritical and experimental). Another problem faced by this field is the difficulty to perform direct characterization since imaging at the molecular scale is often impossible in many experimental devices.
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Concept genesis and Theory
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 single-molecule field-effect transistor in 1988. Further concepts were proposed by Forrest Carter of the Naval Research Laboratory, including single-molecule logic gates.
Conducting polymers
Apart from the Aviram and Ratner proposal, molecular electronics received an initial boost from the experimental discovery of conducting polymers in the mid-seventies. Before this date, organic molecules (which form crystals or polymers) were considered insulating or at best weakly conducting semi-conductors. McGinness, Corry, and Proctor reported the first molecular electronic device ( a voltage-controlled bistable switch ) in Science magazine. As its active element, this device used melanin, a mixed polymer of polyacetylene, polypyrrole, and polyaniline. This device is now in the Smithsonian's collection of historic electronic devices. As Hush notes, their material also showed negative differential resistance, "a hallmark of modern advances in molecular electronics". Polymeric materials are solids made of molecular chains (macromolecules) which are bound to each other via weak electrostatic interactions. Each chain is a regular repetition of a small ensemble of atoms called a monomer. In polyacetylene, the monomer is made of three carbon atoms linked alternatively by a single sigma bond and a double bound. Because of this alternate structure, the polymer is said to be conjugated. In conjugated structures, electrons are more delocalized than in other organic stuctures, which a-posteriori explains the experimental properties found by Mc Giness et al.
However, conjugation is not sufficient to obtain useful conduction from polymers. A few years later, in 1977, through the introduction of dopants (such as halogen atoms), chemists started to greatly improve the conductance of conjugated polymers. These findings opened the door to plastic electronics and optoelectronics which are beginning to find extensive commercial application.
C60 and Carbon nanotubes
From graphite to C60
In polymers, classical organic molecules are composed of both carbon and hydrogen (and sometimes additional compounds such as nitrogen, chlorine or sulfur). They are obtained from petrol and can often be synthethized in large amounts. Most of these molecules are insulating when their length exceeds a few nanometers. However, naturally occurring carbon is conducting. In particular, graphite (recovered from coal or encountered naturally) is conducting. From a theoritical point of view, graphite is a semi-metal, a category in between metals and semi-conductors. It has a layered structure, each sheet being one atom thick. Between each sheet, the interactions are weak enough to allow an easy manual cleavage.
Tailoring the graphite sheet to obtain well defined nanometer-sized objects remains a challenge. However, by the close of the twentieth century, chemists were exploring methods to fabricate extremely small graphitic objects that could be considered single molecules. After studying the interstellar conditions under which carbon is known to form clusters, Richard Smalley 's group (Rice university, Texas) set up an experiment in which graphite was vaporized using laser irradiation. Mass spectrometry revealed that clusters containing specific "magic numbers" of atoms were stable, in particular those clusters of 60 atoms. Harry Kroto, an English chemist who assisted in the experiment, suggested a possible geometry for these clusters - atoms covalently bound with the exact symmetry of a soccer ball. Coined buckminsterfullerenes, buckyballs or C60, the clusters retained some properties of graphite, such as conductivity. These objects were rapidly envisioned as possible building blocks for molecular electronics.
Carbon nanotubes
- See Carbon nanotubes and fullerenes
Self Assembled Monolayers (SAMs)
Recent progress in nanotechnology and nanoscience has facilitated both experimental and theoretical study of molecular electronics. In particular, the development of the scanning tunneling microscope (STM) and later the atomic force microscope (AFM) have facilitated manipulation of single-molecule electronics.
A collaboration of researchers at HP and UCLA, led by James Heath, Fraser Stoddart, R. Stanley Williams, and Philip Kuekes, has developed molecular electronics based on rotaxanes and catenanes.
Work is also being done on the use of single-wall carbon nanotubes as field-effect transistors. Most of this work is being done by IBM.
The Aviram-Ratner model for a molecular rectifier, which until recently was entirely theoretical, has been confirmed experimentally and unambiguously in a number of experiments by a group led by Geoffrey J. Ashwell at Cranfield University, UK. Many rectifying molecules have so far been identified, and the number and efficiency of these systems is expanding rapidly.
Supramolecular electronics is a new field that tackles electronics at a supramolecular level.
See also
Further reading
- "An Overview of the First Half-Century of Molecular Electronics" by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003).
References
- Aviram, A. & Ratner, M.A. Molecular Rectifiers. Chem. Phys. Lett. 29, 277 (1974).
- Aviram, A. & Ratner, M.A. Molecular Rectifiers. Chem. Phys. Lett. 29, 277 (1974).
- S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R. E. Smalley, L. J. Geerligs, & C. Dekker, Nature, vol 386, 474 (1997).
- H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl & R. E. Smalley, Nature, vol 318, 162 (1985)
- H. W. Kroto, Nature, vol 329, 529 (1987)
- T. Oberlin, M. Endo, & T. Koyama, Journ. of Crystal Growth, 32, 335 (1976).
- Geoffrey J. Ashwell and Daniel S. Gandolfo, J. Mater. Chem. 12es:Electrónica molecular