An Arizona State University professor and graduate student have developed a way to measure electrical resistance in single molecules, a feat that may pave the way for molecule-sized sensors and electronic circuits.
Nongjian Tao, an electrical engineering professor, and his graduate student, Bingqian Xu, believe they have achieved a breakthrough in the precision and repeatability of their process.
"A lot of people are doing this, but we are using a different technique," Tao said. "We can tell exactly how many molecules we have, we know we have connected individual molecules to two wires, and we can do a lot of measurements in milliseconds. We have a lot of statistical confidence that we have created a circuit and measured the resistance."
Tao and Xu published their research in the Aug. 29 issue of Science magazine.
Molecular electronics is seen as a possible advance over electronic circuits using silicon. The demand for faster electronic devices and fears that physical limits in the use of silicon may be fast approaching has spurred research in the study of very small structures, a field called nanotechnology. Researchers believe that molecularbased electronic devices could allow ever-larger number of transistors to be crammed into ever-smaller spaces.
"As electronic devices get smaller and smaller, it is more difficult technically and economically to continue to push that limit," said Ray Tsui, a fellow of the technical staff at Motorola Labs in Tempe, who has collaborated with Tao. "So we are looking at molecular-scale components to extend the shrinking of electronics."
Tao’s research is focused on answering some basic questions before practical circuits can be produced, including determining the electrical resistance of single molecules.
Tao and Xu created their circuits using Alkanedithiol, a carbon-chain molecule with sulfur atoms on either end of the chain that act as a "glue" to attach the molecule to the two electrodes. They took their measurements by forming thousands of junctions in which the molecules are directly connected to two gold electrodes. The molecular junction is created by repeatedly moving a gold scanning tunneling microscope tip into and out of contact with a gold substrate in a solution containing the carbon-chain molecule.
"Our idea is fairly straightforward," Tao said. "Because of its simplicity, it can be done repeatedly and provide a quality of data that has been missing in many other experiments."
It may seem simple to Tao, but it involves working on an scale that is almost unimaginably small. A single molecule is only a few nanometers wide. By comparison, a human hair is about 50,000 nanometers wide. The most advanced microchips have circuit lines that are about 100 nanometers wide.
Despite advances, Tao believes molecular electronic circuits for use in microprocessors that run personal computers may not be practical for 10 years or more. But molecular sensors may be practical much sooner, he said.
Because some molecules react to lead or recognize heavy-metal ions, devices could be built that detect contaminants in air or water and send off an electronic signal, he said. Also such sensors could detect diseases in humans, he said.
Such sensor technology could be available within a few years, he said.
Both Tao and Motorola’s Tsui said molecules are unlikely to totally replace silicon-based electronics, but they will extend and supplant some of the functions of silicon.
Tsui said silicon can’t be counted out because "the folks working in silicon are extremely resourceful. There is still a lot of shrinking to be done."