Transparent conducting electrodes are crucial for optoelectronics devices, such as displays, photovoltaics, touch panels and electrochromic windows. Although indium tin oxide (ITO) has been the transparent electrode material of choice for a long time, the material is relatively expensive and brittle, and absorbs light in the near-infrared region of the electromagnetic spectrum, which makes it unsuitable for photovoltaic and photodetector applications.

Metal nanowire films could make good replacements for ITO in next-generation electronics, thanks to their excellent electrical and optical properties and the fact that they can be easily processed in solution.

Metal nanowires good

Researchers recently succeeded in synthesizing silver nanowires that measure 50–100 nm across and around 100 µm long. Thin films made from these wires boast a low sheet resistance of less than 20 Ω/sq and are transparent to 90% of the light passing through them (at a wavelength of 550 nm). These values are similar to those of commercial ITO substrates.

However, silver nanowires do suffer from two main problems. The first is that silver is an expensive metal. Second, the diameter of silver nanowires made thus far is quite large, which means that they strongly scatter light. This light scattering produces a lot of haze, which means that pixels in a display behind the transparent conductor start to blur.

Protecting copper nanowires

Copper nanowires could come into their own here – for one, they are as electrically conducting as silver while being 20 times cheaper. And because copper nanowires with diameters of less than 20 nm can be produced, they scatter light a lot less than their thicker silver nanowire cousins. Copper nanowire-based transparent films also show good optical and electrical properties.

However, it is not all plain sailing, because thin copper nanowires are unstable in air and rapidly oxidize, something that dramatically reduces their conductivity. Researchers have tried to improve their stability by, for example, growing a layer of nickel on the copper wires or coating them with a thin layer of alumina. Although successful to some extent, these techniques either decrease the total transparency of thin films made from the nanowires or their overall conductivity.

Room-temperature solution-based technique

An alternative approach involves wrapping graphene around the nanowires. This not only improves their stability in air but also the electrical and thermal conductivity of the metal wires. However, plasma chemical vapour deposition at temperatures as high as 500–700°C is required to grow the thin layer of graphene on the nanowires – a method that is just too expensive for producing large quantities of the material.

A team led by Peidong Yang has now overcome this problem with a room-temperature solution-based technique to produce high-quality ultrathin copper reduced-graphene-oxide-core-shell nanowires.

The researchers prepared graphene-oxide nanosheets with an average diameter of around 10 nm and diluted in methanol. They then added the copper-nanowire suspension to the dilute graphene-oxide solution and placed the mixture in an ultrasonic bath for a few minutes. During this step, the graphene-oxide nanosheets self-assemble onto the surface of the copper nanowires. “We can tune the coverage and shell thickness by simply varying the ratio of copper nanowires to graphene oxide,” explains team-member Fan Cui.

Good for display applications

“Films made from these coated copper nanowires are highly transparent and at the same time have low electrical resistivity,” she told nanotechweb.org. “They are very stable in air and perform well at high temperatures and in humid environments. What's more, they can be mechanically deformed and are thus ideal for making flexible electrodes. Last but not least, they do not scatter light, so could be used in display applications.”

The core−shell conducting films are as good in terms of performance as ITO and silver nanowire thin films, adds team-member Letian Dou. “Our work demonstrates a new approach to improve and stabilize ultrathin metal nanowires, and takes us one step further towards commercializing copper nanowires as low cost transparent conductors for optoelectronic devices.”

Full details of the research are reported in ACS Nano DOI: 10.1021/acsnano.5b07651.