Moth eye-inspired anti-reflective coating could mean better digital displays

Moths, being largely nocturnal animals which must stay hidden from predators, have non-reflective eyes. These have a distinctive nanostructure which makes the surface graded, causing most light falling on the eye to bend at the surface and transmit through the eye, rather than reflecting from it.

This arrayed structure is so effective at preventing reflection that researchers have attempted to mimic it in an effort to create anti-reflective coatings, with varying degrees of success.

Despite progress in nanoscience which could pave the wave to the adoption of this structure in practical applications, there remain barriers regarding scalability and cost of manufacturing.

Researchers from Tokyo University of Science – working with scientists from thin-film manufacturer Geomatec – have developed a novel technique to produce nanostructures and transparent films inspired by moth eyes, which could allow for manufacturing at large scales.

The team of researchers had previously created moth-eye moulds from glassy carbon etched with an oxygen ion beam, although the approach they used was not scalable.

“Producing glassy carbon substrates requires the use of powder metallurgy technology, which is difficult to use to produce moulds with a large area,” said Professor Jun Taniguchi. “To overcome this limitation, we tried using only a thin layer of glassy carbon deposited on top a large regular glass substrate.”

The researchers decided to replace the electron-cyclotron resonance ion source (using electron cyclotron resonance to ionise a plasma) with an inductively coupled plasma system (in which energy is supplied by induced electric currents). While both devices can etch glassy carbon using a concentrated beam of oxygen ions, the latter produces a wider ion beam irradiation range; this is more suitable for working on large-area structures.

Testing with various parameters determined that a two-step inductively coupled plasma etching process was optimal for obtaining a high-quality mould. This mould was used to produce a transparent film with moth-eye nanostructure, using a UV-curable resin.

The reflectance of the film towards visible light was 0.4 per cent: ten times lower than a similar film without the moth-eye nanostructure. Light transmittance through the material also increased, indicating no trade-off in optical properties as reflectance is reduced.

“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,” said Hiroyuki Sugawara, CTO at Geomatec.