Control of Molecular Switches Increased by Tailored Molecular Interactions

29 September 2004—A means to stabilize molecular switches based on chemical interactions with surrounding molecules has been developed at by a research team lead by Penn State Professor of Chemistry and Physics Paul S. Weiss. While molecules known as OPEs (oligo phenlylene-ethynylene molecules) previously have been shown to switch randomly or with applied electric fields between conductive (ON) and non-conductive (OFF) states, their potential use as switches in computers and other electronic devices depends on the ability to control these states. Such switches could advance nanoscale computer applications, decreasing the size and energy costs of memory.

A paper describing the research results, titled "Mediating Stochastic Switching of Single Molecules Using Chemical Functionality," will be published in the Journal of the American Chemical Society on 6 October 2004.

"If we can stabilize and control the conductance state, we are closer to developing molecular memory components," says Weiss, whose research team includes James E. Hutchison, professor of chemistry at the University of Oregon and James M. Tour, professor of chemistry at Rice University. "The chemical interactions that we observed reduce random switching, which could decrease the refresh rate needed for a random-access-memory device and significantly reduce power usage." Weiss points out that this research is providing basic information about the mechanism of switching and that its application in computers is not imminent.

The researchers varied the local chemical environment of the molecules by inserting OPE molecules into the matrix of a self-assembled monolayer of amide-containing alkanethiol molecules attached to a gold surface. The monolayer consists of long molecules extending outward from the surface. The OPE molecules physically extend beyond the monolayer and can be detected with a scanning tunneling microscope. Interactions between functional chemical groups on the OPE molecule and groups on the molecules of the monolayer stabilize the electronic state after it changes. A key observation is that the change can be induced when an electric field of the correct polarity is applied by the tip of the scanning tunneling microscope. "This reversibility supports our hypotheses about the mechanism of the switching and demonstrates that the chemical environment is crucial to the function of the switches," Weiss says. Reversibility is an essential factor in any application of OPE molecules as components in electronic devices.

The chemical interaction was based on hydrogen bonding between a nitro group attached to the OPE and amide groups attached to the surrounding molecules. Additional research is ongoing to measure the effects of other combinations of functional groups. "By engineering tailored intermolecular interactions into our molecular designs, we have introduced control to electronic switching of single molecules," says Weiss. The research is an essential step toward molecular engineering of computer components at the nanoscale.

This research was funded, in part, by the Army Research Office (ARO), Defense Advanced Research Projects Agency (DARPA), Office of Naval Research (ONR), National Institutes of Standards and Technology (NIST), National Science Foundation (NSF), and Department of Energy (DOE).

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CONTACTS:
Paul S. Weiss: (+1) 814-865-3693, <stm@psu.edu>
Barbara K. Kennedy (PIO): (+1) 814-863-4682, <science@psu.edu>