Cambridge, England--"Moth-eye" antireflection (AR) thin-film optical coatings, which create an index gradient (and thus AR properties) using 2D periodic arrays of tapered subwavelength structures, have been created in many forms. One of the perceived problems of these coatings is that their structures can get clogged with contaminants—fingerprint oil, for example—and thus lose their AR properties.
Now, researchers from the University of Cambridge, Cornell University (Ithaca, NY), CSM Instruments (Peseux, Switzerland), and the University of South Carolina (Columbia, SC) have created a self-cleaning AR moth-eye coating by incorporating a photocatalytic material in the structure.1 The group created a polymer moth-eye structure with a larger-than normal array period that allowed them to incorporate preformed titanium dioxide (TiO2) nanocrystals within the array.
When light falls on the photocatalytic TiO2 nanocrystals, they break down the dirt clogging the structure until all that is left is carbon dioxide and water (which evaporates off the surface). In early tests of the material, the TiO2 nanoparticles were able to break down all of the oils contained in a fingerprint within 90 minutes. The coating is capable of breaking down most of the standard hydrocarbons that clog most porous AR coatings, the researchers assert.
The coating, which can be deposited onto flexible plastic substrates, adheres to the substrate through sol-gel chemistry, resulting in a durable bond and a coating that will not flake off, say the researchers. The material is currently only suitable for outdoor applications. as it requires UV light for photocatalysis to occur; however, the researchers are planning more tests to see if the material could be adapted in future for indoor light.
Potential applications include building glass and solar cells.
1. Stefan Guldin et al., Nano Letters 2013 13 (11); doi: 10.1021/nl402832u