Molecular Scale Electronics - History and Recent Progress

History and Recent Progress

In their 1940's discussion of so-called "donor-acceptor" complexes, Robert Mulliken and Albert Szent-Gyorgi advanced the concept of charge transfer in molecules. They subsequently further refined the study of both charge transfer and energy transfer in molecules. Likewise, a 1974 paper from Mark Ratner and Ari Aviram 1 illustrated a theoretical molecular rectifier. In 1988, Aviram described in detail a theoretical single-molecule field-effect transistor. Further concepts were proposed by Forrest Carter of the Naval Research Laboratory, including single-molecule logic gates. A wide range of ideas were presented, under his aegis, at a conference entitled Molecular Electronic Devices in 1988. These were all theoretical constructs and not concrete devices. 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 directly-measuring the conductance of a single molecule was only reported in 1995 on a single C60 molecule by C. Joachim and J. K. Gimzewsky in their seminal Physical Revie Letter paper and later in 1997 by Mark Reed and co-workers on a few hundred molecules. 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 substantially confirmed.

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. In addition, theoretical advances in molecular electronics have facilitated further understanding of non-adibatic charge transfer events at electrode-electrolyte interfaces.

The concept of molecular electronics was first published in 1974 when Aviram and Ratner suggested an organic molecule that could work as a rectifier. Having both huge commercial and fundamental interest much effort was put into proving its feasibility and 16 years later in 1990 the first demonstration of an intrinsic molecular rectifier was realized by Ashwell and coworkers for a thin film of molecules.

The first measurement of the conductance of a single molecule was realised in 1994 by C. Joachim and J. K. Gimzewski and published in 1995 (see the corresponding Phys. Rev. Lett. paper). This was the conclusion of 10 years of research started at IBM TJ Watson, using the scanning tunnelling microscope tip apex to switch a single molecule as already explored by A. Aviram, C. Joachim and M. Pomerantz at the end of the 80's (see their seminal Chem. Phys. Lett. paper during this period). The trick was to use an UHV Scanning Tunneling microscope to allow the tip apex to gently touch the top of a single C
60 molecule adsorbed on a Au(110) surface. A resistance of 55 MOhms was recorded together with a low voltage linear I-V. The contact was certified by recording the I-z current distance characteristic, which allows the measurement of the deformation of the C
60 cage under contact. This first experiment was followed by the reported result using a mechanical break junction approach to connect two gold electrodes to a sulfur-terminated molecular wire by Mark Reed and James Tour in 1997.

A single-molecule amplifier was implemented by C. Joachim and J.K. Gimzewski in IBM Zurich. This experiment involving a single C
60 molecule demonstrated that a single C
60 molecule can provide gain in a circuit just by playing with through C
60 intramolecular quantum interference effects.

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.

Some specific reports of a field-effect transistor based on molecular self-assembled monolayers were shown to be fraudulent in 2002 as part of the Schön scandal.

Until recently entirely theoretical, the Aviram-Ratner model for a unimolecular rectifier has been unambiguously-confirmed in experiments by a group led by Geoffrey J. Ashwell at Bangor 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. 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 of performing direct characterization since imaging at the molecular scale is often difficult in many experimental devices.

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