Scanning electron microscopy images of an anti-reflective thin film produced using the bio-inspired nanostructured mold. Image: Jun Taniguchi, Tokyo University of Science.
Scanning electron microscopy images of an anti-reflective thin film produced using the bio-inspired nanostructured mold. Image: Jun Taniguchi, Tokyo University of Science.

A huge number of technologies, from Velcro to Japan's famous bullet trains, have been inspired by mechanisms utilized by other lifeforms. Thus, it should come as no surprise to discover that many remarkable advances in anti-reflective coatings have been inspired by the peculiar biostructures found in moth eyes.

As mainly nocturnal animals that wish to stay hidden from predators, the eyes of moths have evolved to be non-reflective. Their eyes have a periodic nanometric structure that makes the eye surface graded, as opposed to polished. This causes most incident light to bend at the surface and thus be transmitted through the eye instead of being reflected off it. This nanoscale structure is so effective that researchers have tried to mimic it in other materials to create anti-reflective coatings, with varying degrees of success.

In spite of the recent progress in nanoscience that allows the adoption of this idea for various practical applications, there are still barriers to overcome in terms of scalability and cost of manufacturing. To tackle these problems, scientists from Tokyo University of Science and Geomatec, both in Japan, have been working on a novel strategy to produce moth-eye nanostructures and transparent films. Now, in a paper in Micro and Nano Engineering, they report a promising method for fabricating moth-eye molds and films at large scales.

In an earlier study, the scientists had succeeded in creating moth-eye molds made of glassy carbon etched with an oxygen ion beam, but this approach is not scalable. "Producing glassy carbon substrates requires the use of powder metallurgy technology, which is difficult to use to produce molds with a large area," explains Jun Taniguchi from Tokyo University of Science, "To overcome this limitation, we tried using only a thin layer of glassy carbon deposited on top a large regular glass substrate."

Moreover, to make this new strategy feasible, the team opted to use an inductively coupled plasma (ICP) system, as opposed to the electron-cyclotron resonance ion source used in the earlier study. While both techniques employ a concentrated beam of oxygen ions to etch the glassy carbon, ICP technology produces a wider ion beam that is more suitable for working on large-area structures.

After testing different ICP parameters, the researchers determined that a two-step ICP etching process was best for obtaining a high-quality nanostructured mold. They then used this mold to produce a transparent film with a moth-eye nanostructure using a UV-curable resin.

The optical properties of this film were remarkable: its reflectance toward light in the visible range was only 0.4%, 10 times lower than that of a similar film without the moth-eye nanostructure. What's more, the transmittance of light through the material was also increased, meaning that no trade-off in optical properties occurred as a result of using the nanostructure to reduce reflected light.

Hiroyuki Sugawara, chief technical officer at Geomatec, highlights the many possible applications of such anti-reflective films, if they can be produced at the meter scale. "We could use these films to improve visibility in flat panel displays, digital signs and the transparent acrylic plates used everywhere since the start of the COVID-19 pandemic. Moreover, anti-reflective coating could also be an efficient way to improve the performance of solar panels."

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