Printed 3D Nanostructures Against Counterfeiting

Security features are to protect bank notes, documents, and branded products against counterfeiting. Losses caused by product forgery and counterfeiting may be enormous. According to the German Engineering Association, the damage caused in 2016 in its branch alone amounted to EUR 7.3 billion. In the Advanced Materials Technologies journal, researchers of Karlsruhe Institute of Technology (KIT) and the ZEISS company now propose to use printed 3D microstructures instead of 2D structures, such as holograms, to improve counterfeit protection.

Today, optical security features, such as holograms, are frequently based on two-dimensional microstructures,” says Professor Martin Wegener, expert for 3D printing of microstructures at the Institute of Nanotechnology of KIT. “By using 3D-printed fluorescent microstructures, counterfeit protection can be increased.” The new security features have a side length of about 100 µm and are barely visible with the eye or a conventional microscope. For their production and application, Wegener and his team have developed an innovative method that covers all processes from microstructure fabrication to the readout of information.

The microstructures consist of a 3D cross-grid scaffold and dots that fluoresce in different colors and can be arranged variably in three dimensions within this grid. To produce and print such microstructures, the experts use a rapid and precise laser lithography device developed and commercialized by the Nanoscribe company, a spinoff of KIT. It enables highly precise manufacture of voluminous structures of a few millimeters edge length or of microstructured surfaces of several cm² in dimension. The special 3D printer produces the structures layer by layer from non-fluorescent and two fluorescent photoresists. A laser beam very precisely passes certain points of the liquid photoresist. The material is exposed and hardened at the focus point of the laser beam. The resulting filigree structure is then embedded in a transparent polymer in order to protect it against damage.


Photovoltaics: Light Absorption Enhanced by Up to 200 Percent

Sunlight reflected by solar cells is lost as unused energy. The wings of the butterfly Pachliopta aristolochiae are drilled by nanostructures (nanoholes) that help absorbing light over a wide spectrum far better than smooth surfaces. Researchers of Karlsruhe Institute of Technology (KIT) in Germany, have now succeeded in transferring these nanostructures to solar cells and, thus, enhancing their light absorption rate by up to 200 percent.

 “The butterfly studied by us is very dark black. This signifies that it perfectly absorbs sunlight for optimum heat management. Even more fascinating than its appearance are the mechanisms that help reaching the high absorption. The optimization potential when transferring these structures to photovoltaics (PV) systems was found to be much higher than expected,” says Dr. Hendrik Hölscher of KIT’s Institute of Microstructure Technology (IMT).


The scientists of the team of Hendrik Hölscher and Radwanul H. Siddique (formerly KIT, now Caltech) reproduced the butterfly’s nanostructures in the silicon absorbing layer of a thin-film solar cell. Subsequent analysis of light absorption yielded promising results: Compared to a smooth surface, the absorption rate of perpendicular incident light increases by 97% and rises continuously until it reaches 207% at an angle of incidence of 50 degrees. “This is particularly interesting under European conditions. Frequently, we have diffuse light that hardly falls on solar cells at a vertical angle,” Hendrik Hölscher says. However, this does not automatically imply that efficiency of the complete PV system is enhanced by the same factor, says Guillaume Gomard of IMT. “Also other components play a role. Hence, the 200 percent are to be considered a theoretical limit for efficiency enhancement.

The scientists have reported their results in the journal Science Advances. (DOI: 10.1126/sciadv.1700232.)


How To Generate Wonderful Colors

Colors are produced in a variety of ways. The best known colors are pigments. However, the very bright colors of the blue tarantula or peacock feathers do not result from pigments, but from nanostructures that cause the reflected light waves to overlap. This produces extraordinarily dynamic color effects.

blue-tarantulaScientists from Karlsruhe Institute of Technology (KIT) in Germany, in cooperation with international colleagues, have now succeeded in replicating nanostructures that generate the same color irrespective of the viewing angle.

In contrast to pigments, structural colors are non-toxic, more vibrant and durable. In industrial production, however, pigments have the drawback of being strongly iridescent, which means that the color perceived depends on the viewing angle. An example is the rear side of a CD. Hence, such colors cannot be used for all applications. Bright colors of animals, by contrast, are often independent of the angle of view. Feathers of the kingfisher always appear blue, no matter from which angle we look. The reason lies in the nanostructures: While regular structures are iridescent, amorphous or irregular structures always produce the same color. Yet, industry can only produce regular nanostructures in an economically efficient way. Radwanul Hasan Siddique, researcher at KIT in collaboration with scientists from USA and Belgium has now discovered that the blue tarantula does not exhibit iridescence in spite of periodic structures on its hairs. First, their study revealed that the hairs are multi-layered, flower-like structure. Then, the researchers analyzed its reflection behavior with the help of computer simulations. In parallel, they built models of these structures using nano-3D printers and optimized the models with the help of the simulations. In the end, they produced a flower-like structure that generates the same color over a viewing angle of 160 degrees. This is the largest viewing angle of any synthetic structural color reached so far.

Apart from the multi-layered structure and rotational symmetry, it is the hierarchical structure from micro to nano that ensures homogeneous reflection intensity and prevents color changes. Via the size of the “flower,” the resulting color can be adjusted, which makes this coloring method interesting for industry. “This could be a key first step towards a future where structural colorants replace the toxic pigments currently used in textile, packaging, and cosmetic industries,” says Radwanul Hasan Siddique of KIT’s Institute of Microstructure Technology, who now works at the California Institute of Technology. He considers short-term application in textile industry feasible. Dr. Hendrik Hölscher thinks that the scalability of nano-3D printing is the biggest challenge on the way towards industrial use. Only few companies in the world are able to produce such prints.


3-D Nanoscale Printing

Thanks to a novel laser lithography method, Nanoscribe GmbH, a spin-off of Karlsruhe Institute of Technology (KIT) – Germany, presents the world’s fastest 3D printer of micro- and nanostructures. With this printer, smallest three-dimensional objects, often smaller than the diameter of a human hair, can be manufactured with minimum time consumption and maximum resolution.
nanoscale spacecraft
Miniature-spacecraft printed with a Photonic Professional GT system in less than one minute.

Nanoscribe systems are used to print polymer waveguides reaching data transfer rates of more than 5 terabits per second.
Biosciences produce tailored scaffolds for cell growth studies among others. In materials research, functional materials of enhanced performance are developed for lightweight construction to reduce the consumption of resources.