Mar 4, 2008
Nature offers up photonic nanostructure
Researchers in China have had more success in using an insect's wing to produce nanostructured anti-reflective film. This time, the team from Peking University and the Chinese Academy of Nanotechnology and Engineering has made a negative image of the cicada wing in gold and then used the biotemplate to pattern PMMA material.
Nature has much to offer when it comes to anti-reflective structures and scientists have taken inspiration from the corneas of moth and butterfly eyes as well as the wings of hawkmoths and cicadas. Engineers are keen to apply photonic structures to lenses, light-sensitive detectors, solar cells, displays and a host of other applications.
The anti-reflective property of the cicada wing, which offers camouflage to the insect, is due to a gradual refractive index profile at the interface between the wing and the air. It is thought that the presence of the sub-wavelength nipple array reduces the reflectivity at broad angles or frequency ranges by as much as a factor of 10. Each nipple has a height of around 400 nm and average diameters of about 65 and 150 nm at the nipple top and base.
To create their biotemplate, the researchers thermally deposit a gold film onto a cicada wing that has been cleaned in acetone. Cicada wings are well suited to the moulding process. They consist mainly of chitin a high molecular weight crystalline polymer with a Young's modulus that is high enough to retain the wing's original structure during casting. The wing even has a layer of wax on its outer layer that serves as an anti-sticking agent.
The second step is to transfer the pattern onto the PMMA film. Here, temperature is key. The polymer is first cast on to the gold template and then heated in an oven to 90 °C for 30 minutes. Heating to 60 °C was shown to be insufficient to replicate the structure in the PMMA film. To remove the polymer, the researchers simply peel off the material using tweezers.
"The gold mould can be used continuously for more than tens of times," Fen Lin of the Centre for Nanoscale Science and Technology (Peking University) told nanotechweb.org. "Now in our lab, we are searching for materials to replace the PMMA in this work to reduce the cost."
The team, which also includes Jin Zhang and Zhongfan Liu, is busy following up applications for the anti-reflective film. "We have already applied for a patent in China and have contacted some manufacturers to develop a practical product," added the researchers.
The group presented its work in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org.