Centre for Nanomaterials in nanOntario Manufacturing Innovation Learning Nanotechnology from Ontario’s Great Outdoors Year 2 | Anti-Reflective Surfaces

1. The Nature www.naba.org Both the Monarch and the Mourning animals.nationalgeographic.com Message from the Publisher Cloak are popular North Professor Uwe Erb American . While the Monarch has one of the largest Welcome to nanOntario’s second year research theme: wingspans, the Mourning Cloak enjoys “Bio-designed Anti-reflective Surfaces.” Inspired by a long lifespan of up to 11 months. the spectacular design found in some of Ontario’s native butterflies, these antireflective surfaces are based The significant difference between on the Eye Effect. This natural phenomenon, these two butterflies is found within created by the precise nano-nipple structure on certain their eye surface structure. The mourning cloak’s eye surface structure moth and butterfly , reduces the reflection of light The Monarch Butterfly by the eye. The effect creates improved night vision for shows the moth eye effect while the and better for some butterflies. monarch’s eye does not. In examining the structure of the Mourning Cloak Butterfly and the Monarch Butterfly eyes, the What is in the Mourning scanning electron microscope was used to obtain Cloak’s Eye? high magnification images of their surfaces. The images show a relatively flat surface on • There are approximately 15,000 the Monarch eye and an amazing nanostructure surface ommatidia per compound eye on the Mourning Cloak eye. This nano-nipple covered • There are approximately 13,000 surface creates a refractive index gradient that reduces nano-nipples per the reflection of light by the eye. Therefore, only the • In total, this adds up to Mourning Cloak eye has an anti-reflective surface due to www.britannica.com this Moth Eye Effect. approximately 200 million nano- nipples per compound eye The Mourning Knowing that a nano-structured surface pattern can • It takes the butterflies only 10 days to Cloak Butterfly reduce light reflection, many scientists and industries create all these ommatidia and nano- are now developing new technologies to improve the nipples during its cocoon stage. optical properties of current products and designing totally new products as well. The significant application possibilities include less glare on display devices as well as increased efficiency for solar cells. Such applications Your fingernails grow approximately are examples as to what we can learn from nature and how nature can hold innovative solutions to engineering 1 nanometre per second design.

2.The Structure The structure of an eye is The scanning electron microscope vastly different than that of a images reveal millions of nano- ; have a type nipples on the surface of each of eye called compound eye. Mourning Cloak eye that are Unlike the human eye structure responsible for the “Moth Eye that consists of a single Effect.” spherical , insect eyes have 50µm a structure that consists of thousands of repeating units called ommatidia .These Close examination of the surface ommatidia are packed closely of the compound eye under the together and typically form the scanning electron microscope spherical or semi-spherical reveals an interesting surface facet structure of an insect structure on the Mourning Cloak 2000nm www.boston.com compound eyes. eye. 3. The Theory The Moth Eye Effect

The outer layer of moth and However, the nano-nipples create a butterfly eyes is made of a gradient in which the refractive (a) (b) substance called chitin. In index increases gradually from the addition, certain moth and refractive index of air to the butterfly species have eyes that refractive index of chitin. As a are covered with arrays of nano-nipples (nano- result of n1 increasing as light scale bumps). Therefore, for these species, light passes through the nano-nipple must pass through a layer of chitin nano-nipples structure and enters the eye, the before it reaches the corneal lens of each reflectivity gradually decreases. ommatidium. The reflectivity, R, for light This leads to greater transmission traveling from air to chitin with a flat surface is of light into the eye which 0.049 according to the equation translates into improved vision for the butterfly. This effect of a a)The Monarch butterfly eye surface is flat which structured, nano-nippled surface results in a high value of light reflection. b)The Mourning Cloak butterfly eye surface is because n =1.57 (the refractive index of chitin) resulting in decreased reflectivity is 2 covered with millions of nano-nipples which can and n = 1.00 (the refractive index of air). called the Moth Eye Effect. 1 greatly reduce reflection of light.

Moth Eye Structure for Reduced Solar Cell Reflectivity 4. The Applications Conventional solar cells can be inefficient, with average solar cells having efficiencies of 6-20%. This is partially due to the Moth Eye Structure for light reflection of silicon, a substance widely used in solar cells and electronic devices. The moth eye nipple Reflection-Free Displays structure can be adapted for use in solar cell design, “creating a www.iof.fraunhofer.de STOReadability Lotusan of display devices situation in which most under sunlight conditions is an of the light from the sun ongoingHouse challenge Paint in is absorbed and efficiently manufacturing. Anti-reflection utilized instead of reflected properties for polymer surfaces uselessly” [1]. are required in modern optical applications. www.nanotex.com ® AR-plas developed by 1] M. Marquit, “Moth Eyes May Hold Key to More Efficient Solar Cells”, Physorg. Fraunhofer Institute for Applied www.iof.fraunhofer.de Available: http://www.physorg.com/news122899685.html. Optics and Precision Engineering (IOF) has excellent anti- reflective properties for oblique incidence of light. Collaborators and Sponsors The ME Series anti-reflective films, developed by Ontario Research MacDermid Autotype, replicate an anti-reflection Fund for Research nanostructure into the hard Excellence (ORF-RE) surface of the films and have excellent anti-reflection properties with < 1% reflectivity over the entire

www.autotype.com visible spectrum.

nanOntario, Volume 2 Contributing Editors Materials Science & nanOntario Lewis P. F. Chen Engineering Project Team 2010 Publisher Jeffrey Ho University of Toronto Professor Uwe Erb David D. Y. Kim T: 416.978.3012 L to R: N. Wang, D. Y. Editor-In-Chief Robert Pallotta Kim, R. Pallotta, P. F. Nick Wang September 2010 Luke Y. H. Ng Chen, J. Ho, K. Lee, Patrick Woo U. Erb External Relations Officer www.mse.utoronto.ca