Stretchable transparent conductors using nano accordian structure for flexible electronics

Stretchable, transparent conductors suited to a range of applications, including flexible electronics, have been developed by a research team from North Carolina State University. "There are no conductive, transparent and stretchable materials in nature, so we had to create one," said researcher Abhijeet Bagal. "Our technique, which uses geometry to stretch brittle materials, is inspired by springs that we see in everyday life. The only thing different is that we made it much smaller." The researchers began by creating a 3D polymer template on a silicon substrate. The template, shaped like a series of identical, evenly spaced rectangles, was coated with a layer of aluminum doped zinc oxide, followed by an elastic polymer. The team then flipped the composite and removed the silicon and the template. The result is a series of symmetrical zinc oxide ridges on an elastic substrate. Because the zinc oxide and polymer layers are clear, the structure is transparent. It can also be stretched because the ZnO ridges allow the structure to expand and contract, like the bellows of an accordion. Fellow researcher Erinn Dandley added: "We can control the thickness of the ZnO layer and have done extensive testing with layers ranging from 30 to 70nm thick. This is important because ZnO thickness affects the structure's optical, electrical and mechanical properties." The 3-D templates used in the process are precisely engineered, using nanolithography, because the dimensions of each ridge directly affect the structure’s stretchability. The taller each ridge is, the more stretchable the structure. This is because the structure stretches by having the two sides of a ridge bend away from each other at the base – like a person doing a split. The structure can be stretched repeatedly without breaking. And while there is some loss of conductivity the first time the nano-accordion is stretched, additional stretching does not affect conductivity. “The most interesting thing for us is that this approach combines engineering with a touch of surface chemistry to precisely control the nano-accordion’s geometry, composition and, ultimately, its overall material properties,” says Chih-Hao Chang, an assistant professor of mechanical and aerospace engineering at NC State and corresponding author of the paper. “We’re now working on ways to improve the conductivity of the nano-accordion structures. And at some point we want to find a way to scale up the process.” The researchers are also experimenting with the technique using other conductive materials to determine their usefulness in creating non-transparent, elastic conductor