"If you want to achieve both high conductivity and transparency in wires made from these metals, you have a conflict of objectives. As the cross-sectional area of gold and silver wires grows, the conductivity increases, but the grid's transparency decreases."Dimos Poulikakos, ETH Zurich

From smartphones to the operating interfaces of ticket machines and cash dispensers, every touchscreen requires transparent electrodes. The glass surface of these devices is coated with a barely visible pattern of conductive electrodes, which, by monitoring changes in conductivity, can recognize whether and exactly where a finger is touching the surface.

Now, researchers under the direction of Dimos Poulikakos, professor of thermodynamics at ETH Zurich in Switzerland, have used three-dimensional (3D) print technology to create a new type of transparent electrode, comprising a grid of gold or silver ‘nanowalls’ on a glass surface. The walls are so thin that they can hardly be seen with the naked eye, and this is the first time that scientists have created nanowalls like these using 3D printing. The research is reported in a paper in Advanced Functional Materials.

The new electrodes have a higher conductivity and are more transparent than those made of indium tin oxide (ITO), the standard material used in smartphones and tablets today. This is a clear advantage: the more transparent the electrodes, the better the screen quality; and the more conductive they are, the more quickly and precisely the touchscreen will work.

"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, which conduct electricity much better.

Because these metals are not transparent, however, the scientists had to make use of the third dimension. "If you want to achieve both high conductivity and transparency in wires made from these metals, you have a conflict of objectives," explains 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 80nm to 500nm 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. Nanodrip is based on electrohydrodynamic ink-jet printing, which utilizes inks made from metal nanoparticles in a solvent and an electrical field to draw 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 emerging from the glass capillary are about ten times smaller than the aperture itself, allowing much smaller structures to be printed. "Imagine a water drop hanging from a tap that is turned off. And now imagine that another tiny droplet is hanging from this drop – we are only printing the tiny droplet," Poulikakos explains. The researchers managed to create this special form of droplet by perfectly balancing the composition of the metallic ink and the strength of the electromagnetic field

The next big challenge will be to scale up 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.

The scientists have no doubt that once it is scaled up, the technology will bring a host of advantages compared with existing methods. In particular, it will likely be more cost-efficient, as Nanodrip printing, unlike the production of ITO electrodes, does not require a cleanroom environment. The new electrodes should also be more suitable for large touchscreens due to their higher conductivity. And finally, this is the first process that allows the height of the nanowalls to be varied directly while printing, says Rohner.

Another possible future application of this technology could be in solar cells, where transparent electrodes are also required. The more transparent and conductive they are, the more electric power can be harnessed. And lastly, the electrodes could also play a role in the further development of curved displays using OLED technology.

This story is adapted from material from ETH Zurich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.