Cicada wing ‘nano-nipples’ inspire anti-reflective material

Using an insect’s wings as a template – literally – researchers in China have made a material that snares visible light coming in from different angles and suppresses reflections.

Imran Zada and colleagues at Shanghai Jiao Tong University emulated the tiny “nano-nipples” that cover cicada wings to produce an anti-reflective titanium dioxide that could be used into maximise photon input in solar cells.

The work was published in Applied Physics Letters.

Scientists often borrow ideas from nature to manipulate light – just last year, for instance, a team in Saudi Arabia unveiled an ultra-black coating inspired by a bright white beetle.

Cicadas’ wings are of interest for their anti-reflective properties. Their transparent segments are finely tuned to nab photons that hit their surface and stop them bouncing off.

The reason: arrays of ordered “nano-nipples”, around 200 nanometres high, that cover the top and bottom wing surfaces. (These nano-nipples also bestow anti-bacterial properties, because they bust open bacteria, and keep wings dry by holding water droplets on their points.)

If a clear material’s surface is completely smooth, a photon might have a 50% chance of bouncing off rather than passing through.

But with the nano-nipples, the odds drop considerably. This boils down to the material’s refractive index, which describes how light propagates through a medium.

Not only does the refractive index dictate how much light bends as it passes into a material, but also how much of that light is reflected at its interface. 

In the case of cicada wings, the nano-nipples create a refractive index gradient, which gradually blurs the interface between air and wing, and more effectively keeps photons from bouncing away.

The problem for Zada and colleagues was making the tiny, finicky nano-nipples.

They could have used any number of techniques such as atomic layer deposition, which builds structures layer by layer, but they were all too slow, complicated and expensive – especially for mass production.

So they dipped real cicada wings in various solutions, baked them to burn off the wings, and were left with faux-wings made of titanium dioxide covered with nano-nipples.

When they shone different wavelengths of light on their titanium dioxide wings, only 1.4 to 7.8% was reflected. This is in contrast to non-nippled titanium dioxide, which reflected 50 to 80% of light.

Such anti-reflective materials “show great potential for photovoltaic devices such as solar cells”, study co-author Wang Zhang says.

And because the material was heat-treated at 500C for an hour, it’s able to withstand harsh environments, the researchers write – perfect for long-term applications.