Bendy copper nanowire akin to ‘foldable iPad’

DUKE (US)—Chemists have perfected a simple way to make inexpensive copper nanowires in quantity that are so small they are transparent, making them ideal for thin-film solar cells, flat-screen TVs, computers, and flexible displays.

“Imagine a foldable iPad,” says Benjamin Wiley, assistant professor of chemistry at Duke University. Nanowires made of copper perform better than carbon nanotubes, and are much cheaper than silver nanowires, he says.

Details are published online in Advanced Materials.

The latest flat-panel TVs and computer screens produce images by an array of electronic pixels connected by a transparent conductive layer made from indium tin oxide (ITO).  ITO is also used as a transparent electrode in thin-film solar cells.

But ITO has drawbacks: it is brittle, making it unsuitable for flexible screens; its production process is inefficient; and it is expensive and becoming more so because of increasing demand.

“If we are going to have these ubiquitous electronics and solar cells,” Wiley says, “we need to use materials that are abundant in the earth’s crust and don’t take much energy to extract.”

Very few materials are known to be both transparent and conductive, which is why ITO is still being used despite its drawbacks.

Copper—a thousand times more abundant than indium—can be used to make a film of nanowires that is both transparent and conductive.

Silver nanowires also perform well as a transparent conductor, but silver, like indium, is rare and expensive. Attempts have been made to improve the performance of carbon nanotubes as a transparent conductor, but without much luck, Wiley says.

“The fact that copper nanowires are cheaper and work better makes them a very promising material to solve this problem.”

Wiley and colleagues grew the copper nanowires in a water-based solution. “By adding different chemicals to the solution, you can control the assembly of atoms into different nanostructures,” Wiley explains.

In this case, when the copper crystallizes, it first forms tiny “seeds,” and then a single nanowire sprouts from each seed. It’s a mechanism of crystal growth that has never been observed before.

Because the process is water-based, and because copper nanowires are flexible, the nanowires could be coated from solution in a roll-to-roll process, like newspaper printing, which would be much more efficient than the ITO production process.

Other researchers have produced copper nanowires before, but on a much smaller scale.

“We think that using a material that is a hundred times cheaper will be even more attractive to venture capitalists, electronic companies, and solar companies who all need these transparent electrodes,” he says.

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