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24 Nov 2006
Scientists from Peking University, China, talk to optics.org about how the intricate wings of cicadas can be used to replicate nanostructures that function as anti-reflective coatings.
Scientists at China's National Laboratory for Molecular Sciences and the College of Chemistry, Peking University, have found a way of using the wings of cicadas as nanoscale printing blocks Small 2 1440). Their method, based on nanoimprint lithography (NIL), could be used to make anti-reflective lens coatings and improved substrates for surface-enhanced Raman spectroscopy (SERS).
Cicadas are insects that live mainly in tropical climates, easily recognizable by their large size (adults can be 2 inches long) and loud "singing" noises. Their wings are covered with thousands of tiny pillar-like projections that scatter incident light and prevent it from being reflected. Each conical pillar is 400 nm tall and inclined at a 30° angle to the surface.
The Beijing team exploited NIL to make highly precise moulds of these patterns. In their technique, a section of the wing is thoroughly cleaned and then pressed against a heated layer of poly-methyl methacrylate (PMMA), a substance often used to make moulds. When the wing is removed, the PMMA mould has a series of nanowell arrays (ie. the reciprocal of the structures on the cicada wings). These moulds can then be used to replicate the nanostructures in a variety of surfaces.
"By using cicada wings as stamps, we were able to replicate natural nanostructures which are very difficult to fabricate using traditional techniques," Dr Jin Zhang of Peking University told optics.org."The cicadas can be obtained from our own institute's backyard, making for a very cheap and easily available resource," he added with a chuckle.
Cicada wings have a natural advantage for the NIL technique, since their surface is coated with a wax-like substance that refuses to stick to the plastic mould. This eliminates the need for applying an anti-adhesive layer prior to imprinting, as is the practice in traditional NIL techniques.
After the PMMA mould has set firmly, its pattern can be transferred to a silicon substrate using processes such as Reactive Ion Etching (RIE). The pock-marked silicon substrates obtained after etching display an ordered array of nanowells on their surface, and previous research has shown that such surfaces can suppress light from being reflected (Appl. Phys. Lett. 78 142). They are therefore ideal for anti-reflective layers in lenses and optical instruments.
Alternatively, metallic nano-pillars, which have applications in near-field optics and SERS, can be fabricated by coating a layer of metal over the mould. In particular, recent studies by an Australian group have shown that gold and silver nano-pillars made from cicada wings could be ideal for producing SERS substrates (Nanotechnology 17 680).
The process is reasonably cost effective, but challenges still remain. For instance, the silicon substrate produced by etching is not uniform, with some nanowells even touching one another. This may indicate that PMMA is not an ideal mask for such an etching process, and other materials and etching times still need to be investigated.
In addition, the wings can only be used a few times. "A key limitation of this process is that the (wing) stamps can only be used a few times, as the heat and pressure applied during the imprinting process can alter the underlying structures." said Dr Zhang. The technique has been patented, and further research is aiming to bring the idea closer to market.
This is not the first time that such natural templates have been used to fabricate microstructures. Nanotechnologists have long been studying the microscopic structures present on lotus leaves, which keep the leaf clean and dry at the same time. Replicating such designs could lead to a new generation of self-cleaning paints, roof tiles and fabrics (see Switchable lotus effect creates self-cleaning surfaces), while material scientists believe than mimicking other natural structures could lead to significant commercial applications.
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