A moth’s eye, showing the highly structured nature of its antireflective eye. [Credit: Ian Lindsay on Pixabay.]
A moth’s eye, showing the highly structured nature of its antireflective eye. [Credit: Ian Lindsay on Pixabay.]
Scanning electron microscopy images of an anti-reflective thin film produced using the bio-inspired nanostructured mold. [Credit: Jun Taniguchi, Tokyo University of Science.]
Scanning electron microscopy images of an anti-reflective thin film produced using the bio-inspired nanostructured mold. [Credit: Jun Taniguchi, Tokyo University of Science.]

Moths’ eyes have inspired researchers to design new, highly effective antireflective coating that could be useful for solar panels, smartphones and tablet computers [Yano et al., Micro and Nano Engineering (2020), https://doi.org/j.mne.2020.100077 ]. These mainly nocturnal creatures have evolved eyes that are non-reflective to protect them from the notice of predators.

The non-reflective nature of moths’ eyes arises because of the periodic nanoscale structure of the surface. The roughly patterned structure causes incident light to bounce around in random directions and be transmitted into the eye rather reflect off it, as it would be from a smooth surface. Researchers at Tokyo University of Science and Geomatec are attempting to mimic the nanoscale arrayed surface of moths’ eyes to create antireflection structures (ARSs) for coatings. Jan Taniguchi and his team have already demonstrated an ARS able to suppress reflection over a wide range of light wavelengths and incident angles. But the mold to produce the highly structured Velcro-like layer was fabricated by irradiating a glassy carbon substrate with an oxygen ion beam in an electron-cyclotron resonance-type ion source system. For large scale production of cost-effective antireflective coatings, this approach just isn't practical.

“Producing glassy carbon substrates requires the use of powder metallurgy technology, which is difficult to use to produce molds with a large area,” explains Taniguchi.

Now he and his colleagues have come up with a promising approach that could produce molds for fabricating moth-eye inspired antireflective coatings at larger scales. Instead of a glassy carbon structure, the researchers opted for a thin layer deposited on a regular glass substrate. They also switched from an electron-cyclotron resonance-type ion source to an inductively coupled plasma (ICP) system, which produces a wider beam irradiation range and is more suitable for large-area structures. In fact, a two-step ICP etching process yielded the best quality nanostructured mold.

Once the team had created the nanostructured mold, they used it to produce a transparent film with a moth-eye-like surface structure from a UV-curable resin. The optical properties of the transparent film extremely promising, boasting both reflectance as low as 0.4% in the visible light range and increased transmittance. The fact that the reduction in reflected light does not come at the expense of other optical properties is extremely promising, suggest the researchers.

Ultimately, if these antireflective nanostructured films can be produced at the meter scale, there could be many possible applications.

“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,” points out Hiroyuki Sugawara, Chief Technical Officer at Geomatec. “Moreover, antireflective coatings could also be an efficient way to improve the performance of solar panels,” he says.