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Selected Subfields Nanomaterials Nanomedicine Molecular self-assembly Molecular electronics Scanning probe microscopy Nanolithography Molecular nanotechnology

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.

Contents 1 Concept genesis and theory 1.1 Charge transfer complexes 1.2 Conducting polymers 2 C60 and carbon nanotubes 2.1 From graphite to C60 2.2 Carbon nanotubes 3 Recent Progress 4 See also 5 Further reading 6 References

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. The first experiment on measuring the conductance of a single molecule was reported in 1997 by Mark Reed and co-workers.

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.

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 [1] iodine-doped oxidized polypyrrole blacks with resistivities as low as 1 ohm/cm. Subsequent papers [2][3] 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 " [4]. The Nobel citation made no reference to Weiss et al's similar earlier work. Also see Nobel Prize controversies.

Similarly, in 1974, John McGinness and his coworkers described [5] the putative first molecular electronic device, 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.

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 theoretical 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.

See Carbon nanotubes and fullerenes

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.

The first widely reported result was using a mechanical break junction approach to connect two gold electrodes to a sulfur-terminated molecular wire by Mark Reed and James Tour.

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.

An important issue in molecular electronics is the determination of the resistance of a single molecule (both theoretical and experimental). For example, Bumm, et al[6] used STM to analyze a single molecular switch in a self-assembled monolayer to determine how conductive such a molecule can be. 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.

• Single-molecule magnet
• Stereoelectronics
• Organic Semiconductors
• Conductive polymers

• For the history of the field, see the following references:
  • Cassoux, P. “Molecular Metals: Staying Neutral for a Change” Science Sciencce 2001 volume 291, pages 263-264. [DOI: 10.1126/science.291.5502.263.
  • "An Overview of the First Half-Century of Molecular Electronics" by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003) and
  • Bendikov, M; Wudl, F; Perepichka, D. F. “Tetrathiafulvalenes, Oligoacenenes, and Their Buckminsterfullerene Derivatives: The Brick and Mortar of Organic Electronics” Chemical Reviews 2004, volume 104, 4891-4945.

1. Aviram, A. & Ratner, M.A. Molecular Rectifiers. Chem. Phys. Lett. 29, 277 (1974).
2. Aviram, A. & Ratner, M.A. Molecular Rectifiers. Chem. Phys. Lett. 29, 277 (1974).
3. BA Bolto, R McNeill and DE Weiss, Electronic Conduction in Polymers. III. Electronic Properties of Polypyrrole, Australian Journal of Chemistry 16(6) 1090 - 1103 (1963) [7]
4. John McGinness, Corry, P, Proctor, P.H. Amorphous Semiconductor Switching in Melanins,Science, vol 183, 853-855 (1974) [8]
5. S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R. E. Smalley, L. J. Geerligs, & C. Dekker, Nature, vol 386, 474 (1997).
6. H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl & R. E. Smalley, Nature, vol 318, 162 (1985)
7. H. W. Kroto, Nature, vol 329, 529 (1987)
8. T. Oberlin, M. Endo, & T. Koyama, Journ. of Crystal Growth, 32, 335 (1976).
9. Geoffrey J. Ashwell and Daniel S. Gandolfo, J. Mater. Chem. 12
10. M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin, and J.M. Tour, “Conductance of a molecular junction”, Science 278, 252 (1997).