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; its production process is inefficient; and it is expensive and becoming more so because of increasing demand. One potential alternative is to use tiny copper nanowires and researchers have now perfected a simple way to make these in quantity. The cheap conductors are small enough to be transparent, making them ideal for thin-film solar cells, flat-screen TVs and computers, and flexible displays.
Benjamin Wiley, an assistant professor of chemistry at Duke University, and his students, PhD candidate Aaron Rathmell and undergraduate Stephen Bergin, 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 said. 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, Wiley thinks 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. Nanowires made of copper are also a better replacement for ITO than carbon nanotubes, and are much cheaper than silver nanowires, Wiley said.
“If we are going to have these ubiquitous electronics and solar cells,” Wiley said, “we need to use materials that are abundant in the earth’s crust and don’t take much energy to extract.” He points out that there are very few materials that are known to be both transparent and conductive, which is why ITO is still being used despite its drawbacks. However, Wiley’s new work shows that copper, which is a thousand times more abundant than indium, can be used to make a film of nanowires that is both transparent and conductive.
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