Nanowires pave way for cheap flexi screens
As flexible display technology gathers momentum researchers at Duke University in North Carolina, USA claim to have solved problems that previously impeded the low cost, high volume manufacture of bendable screens. The team, led by Benjamin Wiley, claim they can produce transparent and conductive copper nanowires simply and in quantity. The cheap materials are said to be “ideal” for flat-screen TVs and flexible displays.
Last week, Wiley, an assistant professor of chemistry at Duke, reported his team’s findings online in Advanced Materials.
“Nanowires made of copper perform better than carbon nanotubes, and are much cheaper than silver nanowires,” Wiley said.
Modern 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, Wiley argus, 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 continued, “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.
Wiley’s 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.
Silver nanowires also perform well as a transparent conductor, and Wiley contributed to a patent on the production of them as a graduate student. But silver, like indium, is rare and expensive. Other researchers have been trying to improve the performance of carbon nanotubes as a transparent conductor, but in Wiley’s view, without much luck.
“The fact that copper nanowires are cheaper and work better makes them a very promising material to solve this problem,” Wiley said.
Wiley 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 crystallises, 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.
Wiley said the process will need to be scaled up for commercial use, and the technology comes with a few issues that still need to be straightened out. For successful large-scale production a solution must be found to prevent the nanowires from clumping, which reduces transparency, and prevents the copper from oxidising, which decreases conductivity. Once the clumping problem has been worked out, Wiley believes the conductivity of the copper nanowires will match that of silver nanowires and ITO.
Wiley, who has applied for a patent for his process, expects to see copper nanowires in commercial. He notes that there is already investment financing available for the development of transparent conductors based on silver nanowires.