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Gold 'nanowalls' ensure greater transparency for touchscreens

08 January 2016

Researchers at ETH Zurich have manufactured highly transparent, conductive electrodes for use in touchscreens using a novel nano-printing process.

With a special mode of electrohydrodynamic ink-jet printing scientists can create a grid of ultra fine gold walls (illustration: Ben Newton/Digit Works)

Researchers, led by Professor Dimos Poulikakos at ETH Zurich, have used 3D print technology to create a new type of transparent electrode for touchscreens, which takes the form of a grid made of gold or silver 'nanowalls' on a glass surface. The walls are so thin that they can hardly be seen with the naked eye.

It is believed to be the first time that scientists have created nanowalls like these using 3D printing. The new electrodes have a higher conductivity and are more transparent than those made of indium tin oxide, the standard material currently used in smartphones and tablets.

“Indium tin oxide is used because the material has a relatively high degree of transparency and the production of thin layers has been well researched, but it is only moderately conductive,” says Patrik Rohner, a PhD student in Poulikakos’ team.

In order to produce more conductive electrodes, the ETH researchers opted for gold and silver. But, of course, these metals are not transparent. “If you want to achieve both high conductivity and transparency in wires made from these metals, you have a conflict of objectives, says Poulikakos. "As the cross-sectional area of gold and silver wires grows, the conductivity increases, but the grid’s transparency decreases.”

The solution was to use metal walls only 80 to 500 nanometres thick, which are almost invisible when viewed from above. Because they are two to four times taller than they are wide, the cross-sectional area - and thus the conductivity - is sufficiently high.

The researchers produced these tiny metal walls using a printing process known as Nanodrip, which Poulikakos and his colleagues developed three years ago. Its basic principle is a process called electrohydrodynamic ink-jet printing. In this process scientists use inks made from metal nanoparticles in a solvent; an electrical field draws ultra-small droplets of the metallic ink out of a glass capillary. The solvent evaporates quickly, allowing a three-dimensional structure to be built up drop-by-drop.

What is special about the Nanodrip process is that the droplets that come out of the glass capillary are about ten times smaller than the aperture itself. This allows for much smaller structures to be printed. “Imagine a water drop hanging from a tap that is turned off," Poulikakos explains. "And now imagine that another tiny droplet is hanging from this drop – we are only printing the tiny droplet. The researchers managed to create this special form of droplet by perfectly balancing the composition of metallic ink and the electromagnetic field used.

The next big challenge will now be to upscale the method and develop the print process further so that it can be implemented on an industrial scale. To achieve this, the scientists are working with colleagues from ETH spin-off company Scrona.

Another possible future application could be in solar cells, where transparent electrodes are also required. The more transparent and conductive they are, the more electricity that can be harnessed.


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