Experiment results lead to technology in nanoscale structures

The image above is a false-color transmission electron microscope image of self-assembled silver nanowires that were produced in the laboratory of Heinrich Jaeger, Professor in Physics. The center-to-center spacing between neighboring wires is 50 nanometers.

An experiment that University physicists conducted just for fun has unexpectedly led them to a new technique for producing nanoscale structures. The physicists have built simple electronic devices using the new technique, which precisely controls the growth of metal wires along tiny scaffolds that automatically assemble themselves, following nature’s own tendencies.

“This is perhaps the first time that it has been possible to assemble large numbers of parallel, continuous wires that are truly nanometer scale in cross-section,” said Heinrich Jaeger, Professor in Physics. Jaeger and Ward Lopes of Arryx Inc. in Chicago described the technique in the Thursday, Dec. 13 issue of the journal Nature.Self-assembly is a hot research field today because of the promise it holds for producing new technology at the nanoscale––the scale of atoms and molecules. Conventional methods for building smaller, faster computer components involve chiseling ever-finer structures out of a larger piece of material. Self-assembly, in contrast, builds up larger structures from smaller building blocks.

The nanowires that Lopes fabricated during the course of his Ph.D. research at the University measure 30 nanometers by 10 nanometers in diameter. A nanometer is a billionth of a meter, or the width of a double strand of DNA. Lopes also fabricated “nanochains,” tiny strings of metal beads of similar size that could serve as switches.

The most perfect wirelike structures are formed with silver, Jaeger said. “Silver is unique in that it forms the wires. Essentially all other metals––gold, copper, tin, lead and bismuth––form nanochains under normal conditions.

“We can also form nanochains with silver, but the exciting advance of Ward’s research is that he was able to combine experimental results with computer simulations to get a feeling of what it is about a particular metal that makes it behave in a wirelike fashion or a chainlike fashion.”

This productive line of research started on a lark.

“In Heinrich’s lab we had a tradition on Friday afternoons of doing experiments that you

couldn’t justify spending time on, that you would only do because you wanted to have fun and try things out,” Lopes said.

In his experiment, Lopes attempted to see if silver would chemically react to certain copolymers––synthetic compounds––the way gold did, as would be expected. But Lopes noticed that the silver exhibited strange behavior. All other metals formed balls on the copolymers and if he added too much metal, the balls would bond to each other and ignore the template. When he added enough silver, he expected the silver to ignore the copolymer template, but the silver spheres had become long and thin.

“I just followed my nose and said, how long can I get these things to be?”

Potential applications for the technique include the production of high-density computer disks, and to make lenses for X-ray lithography, a process for transferring ultra-small patterns to silicon computer chips.

The Chicago physicists used commonly available copolymers and simple methods with an eye toward easing the transfer of their results to potential applications.

“The plastics in the copolymer we used are standard, everyday plastics,” Lopes said. “One was polystyrene, which is used to make Styrofoam, and the other, polymethylmethacrylate, is familiar from Plexiglas.”

Added Jaeger, “The technology for making these structures is extremely straightforward. It’s not high technology in some sense. That’s the beauty of this.”