June 25, 2001
Volume 79, Number 26
CENEAR 79 26 pp. 13
ISSN 0009-2347
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Single molecules switch on and off via matrix-controlled mechanism


Building computers or other high-tech devices out of molecules may sound like creative science fiction, but based on new findings at Pennsylvania State University and Rice University, such futuristic inventions are moving toward reality.

ON/OFF Isolated in an alkanethiolate matrix, a single phenylene ethynylene oligomer appears tall and bright when switched into a highly conducting "on" state and dull and short when switched "off."
Researchers have demonstrated that individual molecules can function as the active elements in electronic switches and that the molecules can remain switched on or switched off for hours at a time [Science, 292, 2303 (2001)]. In addition, the Penn State-Rice team has discovered key factors that govern the switching rate and has made progress in understanding the switching mechanism.

Switches are among the most basic components of memory and logic devices, says Penn State chemistry professor Paul S. Weiss, a coleader of the research group. Accordingly, it is essential to understand what causes switching in single molecules if molecule-based technologies are to play a role in reducing the size of today's computer circuits.

To probe the switching process, the scientists examined the relationship between order in the environment surrounding the molecules and the switching rate.

"Essentially, we tightened the noose around individual molecules and showed that once their motion was reduced, the switching rate went way down," Weiss explains. Specifically, the researchers used scanning tunneling microscopy (STM) to study phenylene ethynylene oligomers embedded in self-assembled monolayers of dodecanethiolate.

In well-ordered monolayers, the conjugated molecules switch slowly between a strongly conducting "on" state and a nonconducting "off" state. But when isolated in a less ordered matrix, the molecules are seen to switch rapidly. Due to the physics of STM imaging, molecules that are switched on appear as bright protrusions. In the off state, they appear darker and approximately 3 Å shorter. The researchers concludes that switching results from conformational changes in the oligomers. These motions are hindered in densely packed and well-ordered matrices.

The group reports that one of the molecules investigated, a nitro-derivatized phenylene ethynylene (shown), can be switched from on to off states by applying an electric field to the molecule. But so far the reverse operation has been difficult to control.

The study was conducted by Weiss and several Penn State colleagues including chemistry and materials science and engineering professor David L. Allara, graduate students Zachary J. Donhauser and Brent A. Mantooth, postdoctoral associate Kevin F. Kelly, and others. Also part of the team are chemistry professor James M. Tour and others at Rice University.

Tomihiro Hashizume, a senior research scientist at Hitachi's Advanced Research Laboratory in Japan, notes that a number of steps need to be taken for the field of molecular electronics to produce usable, large-scale integrated circuits.

Included on the list is measuring properties of individual molecules and studying switching characteristics of individual molecular-scale devices. A third requirement is being able to control microscopic circuits by applying current or voltage.

Hashizume points out that some of the key steps have already been reported in earlier papers. For example, molecular-device characterization has been carried out previously in cases where molecules were part of monolayer films. In those cases, the single-molecule aspect was missing. "But this paper is significant," he says, "because it shows all three requirements together."

The present study builds on work published earlier this month by Tour and Mark A. Reed, an applied physics and electrical engineering professor at Yale University, that showed that bundles of several thousand of these types of molecules can be fashioned into functioning random-access-memory devices [Appl. Phys. Lett., 78, 3735 (2001)]. Reed and Tour demonstrated that the tiny circuits can be switched controllably between conductivity states in a way that allows data to be written, read, and erased from the bundles. This work moves the field toward the single-molecule limit.

Weiss stresses that his coworkers have not worked out architectural designs for single-molecule computing "or anything close to that. But tackling the very small end of things has been interesting and very exciting."

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