Many animals, including birds, beetles, fish and butterflies, get their beautiful
colors not only from pigments, but also from nano-scale structures that manipulate
light. Now, such natural wonders have inspired Hong Yee Low and colleagues at
the Institute of Materials
Research and Engineering of A*STAR, Singapore*, to make polymer structures
similar to butterfly wings. Their structures produce optical effects that could
prove useful in various electronic devices.
 | | Micrograph, with an image of a butterfly, showing that a butterfly wing has hair-like nanostructures arranged in larger scales.
Micrograph © 2009 Hong Yee Low
Photograph © 2009 istockphoto/tcp
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“I was inspired by the possibility of creating color without using chemical
dyes and without altering the chemical composition of the pristine engineering
polymer,” explains Low. “The use of surface architecture to impart
functionality could be a more green or sustainable approach.”
The surfaces of butterfly wings, for example, are made of long and thin parallel
hair-like structures just 150 nanometers thick, separated by air spaces. The
fibers and air have different refractive effects on light striking the wings,
causing interference and producing bright iridescence.
“The butterfly wing structure is actually very challenging to mimic,”
says Low, explaining that previous approaches, though quite successful, have
required expensive equipment and time-consuming deposition methods. To address
this problem, she and her co-workers took a new approach based on a simple and
inexpensive technique called nano-imprint lithography.
First, the researchers fabricated a silicon-oxide mold containing evenly spaced
nanometer-sized trenches. They then heated and softened a polymer before pressing
it onto the mold. Finally, they pulled the polymer off the mold at an angle
to produce hair-like pillars on the polymer surface. The pillars, each just
200 nanometers in diameter and around 1.2 micrometers long, resemble a collapsed
array of dominoes.
Low and co-workers observed either rainbow patterns or single colors on their
polymer surface, depending on the angle of incident light. This is because the
collapsed pillars are parallel, creating ridge lines that look like a diffraction
grating when viewed from one direction, but resemble a multilayered reflective
surface when viewed from another direction.
Eventually, the researchers hope they will be able to fine tune the structure
of their butterfly surfaces and use different materials to produce a variety
of optical properties. Low thinks the finished products could have some interesting
applications.
“Initially I explored the possibility of using this structural color
effect for contact lenses, but we are now seeing more promise in using it as
a color indicator for gas sensing. This has attracted interest from food and
drug packaging industries,” she says. “Another potential application
is in electronic displays with restricted viewing angles.”
The A*STAR affiliated authors in this highlight are from the Institute of Materials
Research and Engineering
* Kustandi, T.S., Low, H.Y., Teng, J.H., Rodriguez, I. & Yin, R. Mimicking
domino-like photonic nanostructures on butterfly wings. Small 5, 574–578
(2009).
Posted November 24th, 2009
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