Benjamin Wiley, PhD
Written by Whitney L.J. Howell
Someday, the same copper used to make U.S. coins could be painted across the faces of hand-held touchscreen devices.
Although it’s not visible to the naked eye, every smartphone, tablet, and e-reader is coated with a thin conductive layer necessary for information transfer. Most of these films are made of indium tin oxide, a solid solution that easily transmits information and is both colorless and transparent in slight thickness. But, with a price tag of up to $1,000 per kilogram, this rare-element compound isn’t cost efficient.
Some manufacturers have turned to using cheaper ($630 per kilogram) silver nanowires as an indium tin oxide replacement. But Duke University researcher Benjamin Wiley, Ph.D., has discovered how to drop the cost by roughly 100 times. The answer: copper nanowires.
“Copper nanowires are very transparent, but they’re still conductive, allowing for electrical conduction,” said Wiley, assistant professor of chemistry, adding that copper sells for only $6 per kilogram. “And, copper is only 6 percent less conductive than silver – copper and silver are, by quite a fair amount, the most conductive metals. That was another piece of information that made us think that what works with silver has to work with copper.”
To bring his discovery to market, Wiley launched the start-up company NanoForge in 2011. Since then, the company has shuttered its doors, but the science and research behind perfecting the technology continues. Work is still ongoing, he said, to bring create copper nanowires that perform to the level of indium tin oxide.
In addition to smartphones and tablets, copper nanowires can be useful in touchscreen organic LEDs and solar cells. They’re also easier and less time-consuming to produce than either indium tin oxide or silver nanowires. When you boil the process down, Wiley said, it’s akin to working in the kitchen.
“Constructing nanowires is a lot like cooking soup,” he said. “You add in the ingredients and heat up the solution. It might look clear at first, but after 30 minutes, the nanowires start to grow tiny crystals.”
How the nanowires actually grow, however, is still not fully understood. But researchers do know that molecules react with metal ions when the solution is heated, and they become metals themselves. Based on the reaction conditions, they can form metal nanowires, as well as nanocubes or nanotriangles.
The nanowires are, then, dipped into acetic acid to separate them from the reaction solution. This also makes it easier to infuse them into an ink that will coat a glass or plastic substrate. In contrast, silver nanowires must be heated to 100C for five minutes before they are ready for use.
The nanowire-filled ink doesn’t coat the entire substrate surface, however. According to Wiley, they take on a more scattered appearance.
“When you look at pine needles on the ground, they’re kind of touching. But you can still see the ground underneath,” he said. “That’s why nanowire films are transparent. All of them are covering the film and touching, but they don’t take up much space. Less than 5 percent of a surface is actually covered with metal.”
Although NanoForge shut down its operations, Wiley credits Duke’s Office of Licensing and Ventures with pointing him in the right direction from the beginning.
“They were very helpful in guiding me to get the initial intellectual property covered,” he said. “I also had the good fortune of talking to a number of different people around Duke who had formed start-ups, and I got a sense of what their experience was. It was all memorable.”
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