Researchers create reversible molecular computer switch

November 18, 1999

HOUSTON--The world of molecular computing, with its ultrafast speeds, low power needs and inexpensive materials, is one beaker closer to reality. Using chemical processes rather than silicon-based photolithography, researchers at Rice University and Yale University have created a molecular computer switch with the ability to be turned on and off repeatedly.

Such a switch, or logic gate, is a necessary computing component, used to represent ones and zeros, the binary language of digital computing.

As far as building the basic components of molecular computing is concerned, "50 percent of the job is done," said James Tour, the Chao Professor of Chemistry at Rice and designer of the synthetic molecule used in the switch. "The other 50 percent is memory." Rice and Yale researchers plan to announce a molecular memory device at the International Electronic Device Meeting in Washington, D.C. on Dec. 6.

Tour said the cost of the molecular switches would be at least several thousand times less expensive than traditional solid state devices. They also promise continued minitaturization and increased computing power, leapfrogging the limits of silicon.

The findings are reported in the Nov. 19 issue of Science, in a paper titled, "Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device." The authors are graduate student Jia Chen and professor Mark Reed of Yale, and graduate student Adam Rawlett and Tour of Rice.

The switch works by applying a voltage to a 30 nanometer wide self-assembled array of the molecules, allowing current to flow in only one direction within the device. The current only flows at a particular voltage, and if that voltage is increased or decreased, it turns off again, making the switch reversible. In other previous demonstrations of a molecular logic gate, there was no reversibility.

In addition, the difference in the amount of current that flows in the on/off state, known as the peak to valley ratio, is 1000 to 1. The typical silicon device response is, at best, 50 to 1. The dramatic response from off to on when the voltage is applied indicates the increased reliability of the signal.

The active electronic compound, 2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiol, was designed and synthesized at Rice. The molecules are one million times smaller in area than typical silicon-based transistors.

"Not only is it much smaller than any switch that you could build in the solid state, it has complementary properties, which in this case, if you want a large on/off ratio, it blows silicon away," Tour said.

The measurements of the amount of current passing through a single molecule, taken at Yale, occurred at a temperature of approximately 60 Kelvin, or about -350 degrees Fahrenheit.

The researchers note that since submission of their findings to Science, they have observed the reversible switch behavior in a similar molecule at room temperature, with a current peak to valley ratio of 1.5 to 1, which is still sufficient for numerous electronic applications. More efficient systems are now being synthesized.

In addition to logic gates, potential applications include a variety of other computing components, such as high frequency oscillators, mixers and multipliers.

"Integrating these switches into a full-blown system where we need to address perhaps 10 million of these devices remains a big challenge," Tour said. "A big piece of the puzzle has been solved. We're looking at properties of single and very small packets of molecules, and now we need to learn how to string them together.

"It really looks like we're going to have hybrid molecular- and silicon-based computers within five to 10 years," Tour said.
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Rice University

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