Researchers at Northwestern University have developed a new nanomanufacturing technique which can be used to produce nanostructures measuring tens of square centimeters. This new technique, dubbed 'soft interference lithography' (SIL), can lead to nanomaterials with optical properties mimicking some metamaterials in the natural world such as peacock feathers and butterfly wings. As said the researchers, their SIL technique 'combines the ability of interference lithography to produce wafer-scale nanopatterns with the versatility of soft lithography and used it to create plasmonic metamaterials.'
This research, funded by the U.S. National Science Foundation (NSF), was performed at Northwestern University by Teri Odom, Associate Professor in the Department of Chemistry, and other researchers in her group. You can see on the left a diagram showing what is soft interference lithography (SIL): "the fabrication procedure of infinite nanohole arrays and finite-sized arrays (patches) of holes" (Credit: Teri Odom's group at Northwestern University).
Here is how NSF describes SIL. "The optical nanomaterials in this research are called 'plasmonic metamaterials' because their unique physical properties originate from shape and structure rather than material composition only. Two examples of metamaterials in the natural world are peacock feathers and butterfly wings. Their brightly colored patterns are due to structural variations at the hundreds of nanometers level, which cause them to absorb or reflect light. Through the development of a new nanomanufacturing technique, Odom and her co-workers have succeeded in making gold films with virtually infinite arrays of perforations as small as 100 nanometers -- 500-1000 times smaller than a human hair. On a magnified scale, these perforated gold films look like Swiss cheese except the perforations are well-ordered and can spread over macroscale distances. The researchers' ability to make these optical metamaterials inexpensively and on large wafers or sheets is what sets this work apart from other techniques."
In the research section of her site, Odom describes plasmonic materials and their optical properties. "Plasmonics is an exciting and emerging area that uses metal nanostructures to manipulate light on the nanoscale. Depending on their size, shape, and materials properties, noble metal nanoparticles can scatter and absorb light to produce colors ranging from the ultra-violet to the near-infrared. In addition, significantly more light can be transmitted through metal films perforated with subwavelength hole arrays than is permitted by geometric optics, a phenomena known as enhanced optical transmission."
And she gives some additional details. "We focus primarily on the optical properties of two different but complementary systems that can control light on the nanometer scale: (i) metallic films of nanohole arrays and (ii) pyramidal nanoparticles. The former have properties dominated by SPPs, and the latter have properties dominated by LSPs. Such nanostructures are easily made by our innovative fabrication scheme, PEEL, for preparing large-area, free-standing films of nanoscale holes and particles. PEEL is a simple procedure which combines Phase-shifting photolithography, Etching, Electron-beam deposition, and Lift-off of the metal film."
For more information, this research work has been published in Nature Nanotechnology under the name "Multiscale Patterning of Plasmonic Metamaterials" (Volume 2, Number 9, Pages 549-554, September 2007) and was even featured on the cover. Here are two links to the abstract and to the article (PDF format, 6 pages, 1.22 MB), from which the above illustration has been extracted.
Sources: U.S. National Science Foundation news release, September 11, 2007; and various websites
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