Nanostructures in
Nature
If we look closely, we can notice that many plants and animals around us
have developed special features that are at the nano scale level. Let's examine
some of the ways in which nature has used nano structures.
A moth’s eye has very small bumps on its surface. They have a hexagonal
shape and are a few hundred nano meters tall and apart. Because these patterns
are smaller than the wavelength of visible light (350-800nm), the eye surface
has a very low reflectance for the visible light so the moth’s eye can absorb
more light.
The moth can see much better than humans in dim or dark conditions
because these nano structures absorb light very efficiently.
In the lab, scientists have used similar man-made nanostructures to enhance
the absorption of infra-red light (heat) in a type of power source ( a
thermo-voltaic cell) to make them more efficient!
On the surface of a butterfly’s wings are multilayer nanoscale patterns.
These structures filter light and reflect mostly one wavelength, so we see a
single bright color. For instance the wings of the male Morpho Rhetenor appear
bright blue. But the wing material is not, in fact, blue; it just appears blue
because of particular nanostructures on the surface.
More precisely, the nanostructures on the
butterfly’s wings are about the same size as the wavelength of visible light
and because of the multiple layers in these structures optical interferences
are created. There is constructive interference for a given wavelength (around
450nm for the Morpho Rhetenor) and destructive interferences for the other
wavelengths, so we see a very bright blue color.
In the laboratory, many scientific
instruments use this same phenomena to analyze the color of light.
The edelweiss (Leontopodium nivale) is an alpine flower which lives at high
altitudes, up to 3000m / 10,000 ft, where UV radiation is strong. The flowers
are covered with thin hollow filaments that have nanoscale structures
(100-200nm) on their periphery.
They will absorb ultraviolet light, which wavelength is around the same
dimension as the filaments, but reflect all visible light. This explains the
white color of the flower. Because the layer of filaments absorbs UV light, it
also protects the flower’s cells from possible damage due to this high-energy
radiation.
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