Cornell University chemists have created the world's smallest wires and encased them in a plastic polymer, an accomplishment that could lead to a host of new electrical or optical uses at the nanometer scale.
An electrical cord only a few atoms thick? That's about the size of it. The wires, only 6 angstroms in diameter, or just several atoms wide, could be kept separate or bunched together to make cables inside a polymer matrix, depending on the intended purpose, the researchers say. The wires can be up to at least 10,000 angstroms in length.
"No one has ever made wires this small before, so we're not sure what all the uses are going to be," said Francis J. DiSalvo, Cornell professor of chemistry who led the work with his Cornell colleague, Jean M.J. Frechet, the Peter J. Debye Professor of Chemistry.
The chemists published their report in the journal Science (Aug. 9, 1996), with Josh H. Golden, former doctoral student in both their laboratories now at Cytek Industries in Stamford, Conn.; John Silcox, professor of applied and engineering physics and director of Cornell's Materials Science Center; Malcolm Thomas, a research support specialist in the Materials Science Center; and Jim Elman of Eastman Kodak Co. in Rochester, N.Y. The recent work was funded by the National Science Foundation, with earlier work funded by the U.S. Department of Energy.
Here is how they did it: They took atoms of the metallic substances molybdenum and selenium separated by lithium. By putting them in a solvent of ethylene carbonate -- which polymerizes into polyvinylene carbonate -- the lithium was separated out, leaving long strings of the metals. Then they added an agent to make the polymer. By doing so quickly, the organic polymers gelled before the wires had a chance to clump together.
"It's like trapping a small, skinny sausage in a big bowl of spaghetti," DiSalvo said. "We trapped the wires in the solution. The trick is to do it very fast, before they have a chance to clump."
The result: A plastic block laced with subnanometer-sized wires. To make cables of more than one wire held together, the researchers just increased the amount of metallic grains.
"We polymerize it very quickly using light. It freezes the wires in whatever orientation they are in," said Frechet, a polymer chemist. "What is remarkable is that this is so tiny -- the size of a molecule -- and we can do that. We can't do anything very useful with them yet, but this is the way science progresses. In time, we will. For now we can study their interaction with light."
The researchers did not know for sure that they had succeeded until they gave the samples to Cornell's Materials Science Center, where Silcox and Thomas subjected them to scanning transmission electron microscopy (STEM). The images confirmed that the wires were in place. The microscopists used an imaging technique in which the atomic number of the metallic ions are distinguished from the polymer's organic materials, which have lower atomic numbers.
The images showed single wires from 6 angstroms diameter, and double wires of about twice that size, and groups of wires, or cables, up to 40 angstroms diameter, all of which can act as electrical conductors.
"We could not have done this without the Materials Science Center," Frechet said. "It brings together scientists from different disciplines who otherwise might not be collaborating." Also making the feat possible was having a graduate student, Golden, who was skillful enough to work in both solid-state and polymer chemistry labs.
Now that they have shown it is possible to make such materials, the researchers are turning their attention to what they can do with them. The chemists are trying to use the new structures as membranes, in which the wires act as a solid-state catalyst. Other possibilities, they say, include, anti-static polymeric materials for microelectronics, such as in the packaging of chips or for computer housings, and anti-static agents for film. In many cases, static discharges can destroy sensitive electronic equipment or leave a blotch on film.
"Part of the problem is in the basic science," DiSalvo said. "We can make these perfect wires 6 angstroms in diameter. How do you make electrical contacts for wire that thick? We have more basic science to think about. What happens to the properties when you go from bulk to a single thin wire? Maybe now we can test some theories that propose unusual behavior of such narrow wires."
The scientists also would like to understand how these wires might behave under different conditions, such as high and low temperatures.