A line appears on the monitor and becomes longer within seconds. It bends off at a right angle, changes direction several times and crosses itself on a couple of occasions until a tangle of lines emerges. Then the line grows more slowly, appears darker, stops and darkens further in a dot of a consistent size. Then it continues: a line, another dot, line, dot, line, dot.

What may sound a bit like Morse code is actually a demonstration of a new technique that ETH-Zurich researchers have developed at the Laboratory of Thermodynamics in Emerging Technologies. The method enables them to print the tiniest of structures on a micro- and nanoscale.

Using this printing method, ultrafine particles are transferred onto a surface from a capillary in a targeted fashion with the aid of an electrical field. Depending on how long material accumulates at the same spot, the structure grows taller, producing a nano-tower. If doctoral student Patrick Galliker, who was instrumental in developing the printer, allows them to get ever taller, they can clearly be seen toppling over on account of their proximity to the capillary. For this simple demonstration, Galliker uses controls similar to those found in computer games. When the researchers automatize the nano-printer using special software, it can produce the little towers autonomously, uniformly and without any connecting lines whatsoever. They can also make towers that are slightly bent or lean two of the towers against each other to form a sort of tiny arch, explains Galliker using photos he took of the structures.

The printing takes place with nano-particles of a wide variety of materials that are placed in solvents to form inks. During printing, the nano-particles self assemble next to each other according to the related physics, which was explained by the researchers. The solvent evaporates and the nano-structures, which can be markedly smaller than 100 nanometres, are ready.

Manipulating light with nano-structures as antennas

The ETH-Zurich researchers envisage a wide range of possible applications for their new method. It paves the way for applications in optics, they explain. After all, light interacts differently with nano-structures than with larger objects. Surfaces that have been modified with nano-structures “manipulate the light”, as Galliker puts it. These surfaces can absorb, concentrate and transmit light instead of reflecting it. Acting as mini-antennae and absorbers, the minuscule structures thus soak up and amplify the light, which falls into a kind of trap before ideally being transmitted to where it is needed.

This could be used to increase the efficiency of thin-film solar cells by capturing the light and channelling it directly towards the active layer, for instance. Until now, such solar cells did not use all the incident light as they reflected part of it and let another part to escape unused. Camouflage suits with such surfaces are conceivable, explains Dimos Poulikakos, professor of thermodynamics and head of the research group.

Moreover, using such nanostructures, new kinds of faster, more selective and highly sensitive detectors and sensors might be feasible. The nanostructures could also be used in special light microscopes in which light nanoantennas trigger fluorescence, Poulikakos adds, enabling the tiniest of objects, such as individual molecules, to be observed. And, of course, the nano-printer could be employed wherever material needs to be applied on a nanoscale in a targeted fashion, such as in the production of modern microprocessors: imagine, a CPU printed on the spot!

Affordable, robust and reproducible method

Using the new printing method, the tiny structures can be applied to different surfaces in a quick and reproducible manner. It is fast because the printer can be programmed in such a way that material is applied precisely where it is needed. The removal of excess material, as is necessary with other methods on a micro- and nanoscale structuring, is no longer required, saving precious resources.

Moreover, compared to established methods that perform similar functions at the nanoscale, the new technique is considerably less expensive. It does not need large-scale facilities, high calssification cleanrooms, exceedingly high temperatures or special pressure ratios. It works perfectly without laborious and time-consuming vacuum steps needed in many other processes.

As a result, the throughput and size of the printed surfaces may be increased considerably during industrial production, says Poulikakos. Additionally, prototyping at the smallest scale could be performed fast and affordably. All this will make the method considerably more attractive than the alternatives already available.

Spin-off in the cards

The researchers still have a lot of work ahead of them. For example, they target the development of a print head containing several individually addressable capillaries. On the one hand, such an approach will lead to an increase of throughput. On the other hand, it will enable to stack layers of different materials on top of each other opening further avenues to future products and science projects.

According to the researchers, the prospects for the new method are promising. A patent application has already been filed and the first interested parties from industry have already shown interest. Even the founding of a spin-off is in the pipeline. Currently, the ETH-Zurich researchers are also involved in several projects with other scientists who need facile nanostructures, otherwise producible with much more complex, expensive and time consuming methods.

Reference

Galliker P et al. Direct Printing of Nanostructures by Electrostatic Autofocusing of Ink Nanodroplets. Nature Communications, Published online, 12th June 2012. DOI: 10.1038/ncomms1891