Using a new nanodroplet printing method, 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.
Solvent containing nanoparticles (yellow dots) flows out of a capillary and forms controllably ultra-small droplets. The solvent evaporates rapidly from the droplets, leaving a structure made of accumulated nanoparticles in its wake (credit: Patrick Galliker/ETH Zurich)
Applications – Better solar energy, sensors, nanoantennas and Camoflage suits
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!
Using the new method, researchers can print dots, small towers, lines and other structures at the nanoscale (SEM image) (credit: Patrick Galliker / ETH Zurich)
Nanotechnology, with its broad impact on societally relevant applications, relies heavily on the availability of accessible nanofabrication methods. Even though a host of such techniques exists, the flexible, inexpensive, on-demand and scalable fabrication of functional nanostructures remains largely elusive. Here we present a method involving nanoscale electrohydrodynamic ink-jet printing that may significantly contribute in this direction. A combination of nanoscopic placement precision, soft-landing fluid dynamics, rapid solvent vapourization, and subsequent self-assembly of the ink colloidal content leads to the formation of scaffolds with base diameters equal to that of a single ejected nanodroplet. The virtually material-independent growth of nanostructures into the third dimension is then governed by an autofocussing phenomenon caused by local electrostatic field enhancement, resulting in large aspect ratio. We demonstrate the capabilities of our electrohydrodynamic printing technique with several examples, including the fabrication of plasmonic nanoantennas with features sizes down to 50 nm.