One-way molecules channel electric current

Individual molecules acting as wires and switches offer a promising avenue to a world of integrated-circuit chips crammed with electronic devices.

 Researchers now report success in synthesizing and manipulating a type of molecule in which electric current flows much more easily from one end of the molecule to the other than it does in reverse. The asymmetry means that the molecule acts as a rectifier and could be used as an electronic component known as a diode.

 Robert M. Metzger of the University of Alabama in Tuscaloosa and his coworkers report their findings in the Oct. 29 Journal of the American Chemical Society. "A great deal is happening in molecular electronics," says James C. Ellenbogen of MITRE Corp. in McLean, Va. It appears that Metzger's group has taken "a significant step forward by demonstrating diode behavior in a molecule or a molecular monolayer."

 The results also represent experimental verification of a rectification effect predicted in 1974 by Ari Aviram of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., and Mark A. Ratner of Northwestern University in Evanston, Ill. "They have finally demonstrated the concept," Aviram says.

 Metzger and his coworkers used molecules of hexadecylquinolinium tricyanoquinodimethanide, originally studied a few years earlier by J.R. Sambles of the University of Exeter in England and his colleagues. The Metzger team succeeded in sandwiching a one-molecule-thick layer of the material between aluminum electrodes. The molecules are all oriented with the negative charge adjacent to the same electrode.

 Applying a potential of about 1 volt across the molecular film, the researchers observed much more current flow in one direction than in the other. "It's certainly not the sturdiest device, but it is stable," Metzger says. Indeed, it was possible to do repeated measurements on the same molecular layer.

 However, the setup "raises the question of how one can be certain that just one molecule is responsible for the behavior," Ellenbogen says. "I would like to see this molecular diode behavior exhibited by a single molecule embedded in a circuit made by a single molecular wire."

 Further experiments are needed to confirm and extend the results. "It's not very hard to do, once you know what you're doing," Metzger says.

 Mark A. Reed of Yale University and his colleagues have recently reported an alternative approach to fabricating molecular wires and measuring electric conductivity directly through only a small number of molecules. "Just by slapping two different metals on different sides of a molecule, you have a rectifier," Reed says.

 Researchers hope these discoveries will be borne out by additional experiments. "We are attempting here in my group at MITRE to build upon such work by fabricating a molecular electronic adder circuit that uses such diodes for switching," Ellenbogen says.

 The MITRE team has prepared a design and expects to start fabrication experiments next year. Says Ellenbogen, "If [the Reed and Metzger] experimental results are accurate, then the molecule we have designed should be able to add two numbers."