Indium tin oxide (ITO) is commonly used to make transparent electrodes today but it is unsuitable for flexible electronic applications because it loses its conductivity when folded. ITO is also brittle – and expensive – so researchers are busy looking for real alternatives. So far, they have put forward conducting polymers, carbon nanotubes and graphene as possible contenders, but while these materials are more flexible, their low conductivity remains a problem.

Metal nanowires are promising too, but to be of any real use, the wires must be made with fewer defects, have high conductivities, be as long as possible (to minimize the number of wire-to-wire contact junctions) and have small junction resistances to boot.

Yi Cui and colleagues are now saying that they have come up with an easy fabrication process that satisfies all of these criteria. Indeed, the team has succeeded in making a new kind of transparent electrode that has a sheet resistance of around just 2 ohms at 90% light transmittance. The electrode is also remarkably flexible and can be both stretched and bent at will without suffering much damage to its electronic properties.

Nanotrough networks

The Stanford researchers produced their electrode, which is made up of freestanding metallic nanotrough networks, using electospinning and metal deposition techniques. They used an electrostatic force to draw continuous polymer nanofibres from a viscous solution and then metallized the fibres to form a highly transparent and conducting network of long nanowires. The process can be used to make continuous nanotrough networks from a wide variety of functional materials, they say, including gold, copper platinum, aluminium, chromium, nickel and their alloys.

Scanning electron microscopy revealed that the networks are made of intertwined metal nanotroughs with widths of around 400 nm and lengths greater than 1 mm. The individual nanotroughs also appear to become naturally interconnected at their junctions during the metal deposition step. The networks can easily be attached to different substrates, such as glass slides, plastic, paper, textiles and even curved glass without any particular surface treatment, thus transforming these non-conducting substrates into highly conducting ones. What is more, the nanonetworks are firmly attached to the substrates and cannot be peeled off easily – even with sticky tape.

Highly transparent

Metal films that are thicker than 80 nm do not usually let light pass through, but the new metal nanowire networks made by Cui’s team are highly transparent. Nanowire electrodes made from copper appear to be the best of all the metal electrodes tested with a sheet resistance of just 2 ohms at 90% optical transmittance, 10 ohms at 95% transmittance and 17 ohms at 97% transmittance. These values are comparable to those seen in state-of-the-art device-grade ITO and better than those of other transparent conducting electrodes, made from graphene, carbon-nanotube films and conducting polymers.

"This new type of transparent electrode can be made easily and cheaply," says Cui, "and we hope that it will replace ITO in next-generation, flexible and high-performance optoelectronics devices."

The team is now busy trying to improve the optical transmittance and electrical conductance of its electrodes. "We are also trying to produce very uniform nanotrough networks over a large area," Cui told nanotechweb.org.

The present work is reported in Nature Nanotechnology.