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. Growth of transistor counts for Intel processors (dots) and Moores Law (upper line=18 months; lower line=24 months) Moores law is about the empirical observation, that at our rate of technological development, the complexity of an integrated circuit, with respect to minimum component cost, will double in... An integrated circuit (IC) is a thin chip consisting of at least two interconnected semiconductor devices, mainly transistors, as well as passive components like resistors. ...

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.

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. Robert Sanderson Mulliken (June 7, 1896-October 31, 1986) was an American physicist and chemist, primarily responsible for the elaboration of the molecular orbital method of computing the structure of molecules. ... Albert Szent-Györgyi (September 16, 1893 - October 22, 1986) was a Hungarian physiologist who won the Nobel Prize in Physiology or Medicine in 1937. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) arranged for converting alternating current to continuous current. ... The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape of the nonconductive depletion layer within a semiconductor material, thus controlling the conductivity of a channel in that material. ... -1... A logic gate is an arrangement of controlled switches used to calculate operations using Boolean logic in digital circuits. ...

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. Almost all organic polymers are electrical insulators. ...

Carbon nanotubes and 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. A polymer is a long, repeating chain of atoms, formed through the linkage of many molecules called monomers. ... Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Together with the metals and nonmetals, the metalloids (in Greek metallon = metal and eidos = sort - also called semimetals) form one of the three categories of chemical elements as classified by ionization and bonding properties. ...

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. Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Richard Errett Smalley (June 6, 1943 – October 28, 2005) was the Gene and Norman Hackerman Professor of Chemistry and a Professor of Physics and Astronomy at Rice University, in Houston, Texas. ... Sir Harold Walter Kroto KBE FRS (born October 7, 1939) is an English chemist. ... Fullerene C540 The Fullerenes are recently-discovered allotropes of carbon. ...

 See Carbon nanotubes and fullerenes 

An electronic device known as a diode can be formed by joining two nanoscale carbon tubes with different electronic properties. ... Buckminsterfullerene (C60) Fullerenes are molecules composed entirely of carbon, taking the form of a hollow sphere, ellipsoid, or tube. ...

Self Assembled Monolayers (SAMs)

See Self-assembled monolayer Self assembled monolayers are surfaces consisting of a single layer of molecules on a substrate. ...

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 mite next to a gear set produced using MEMS, the precursor to nanotechnology. ... Image of substitutional Cr impurities (small bumps) in the Fe(001) surface. ... The atomic force microscope (AFM) is a very powerful microscope invented by Binnig, Quate and Gerber in 1986. ...

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. The Hewlett-Packard Company (NYSE: HPQ), commonly known as HP, is a very large, global company headquartered in Palo Alto, California, United States. ... The University of California, Los Angeles, popularly known as UCLA, is a public, coeducational university situated in the neighborhood of Westwood within the city of Los Angeles. ... A rotaxane is a molecular structure consisting of a dumbbell shaped molecule that is threaded through a macrocycle or ring-like molecule. ... Catenanes are interlocked macrocyclic rings. ...

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. International Business Machines Corporation (IBM, or colloquially, Big Blue) (NYSE: IBM) (incorporated June 15, 1911, in operation since 1888) is headquartered in Armonk, New York, USA. The company manufactures and sells computer hardware, software, and services. ...

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. Cranfield University is a university based on three campuses at Cranfield, Silsoe and Shrivenham. ...

Supramolecular electronics is a new field that tackles electronics at a supramolecular level. Supramolecular electronics is the experimental field of supramolecular chemistry that bridges the gap between molecular electronics and bulk plastics in the construction of electronic circuitry at the nanoscale 1. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

See also

• single-molecule magnet

A single-molecule magnet is an object that is composed of molecules each of which behaves as a magnet. ...

Additional 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

• [1] 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)
• [http://www.rsc.org/CFmuscat/intermediate_abstract.cfm?FURL=/ej/JM/2002/b110591b/b110591b.PDF&TYP=003 Geoffrey J. Ashwell and Daniel S. Gandolfo, J. Mater. Chem. 12