Besides Noisy Summer - Antireflective Films from Zhi Liao Wings

Zhi Liao is the Chinese nickname for cicada, a noisy, fat, clumsy but cute insect that has amused thousands of boys in their crazy boyhoods (no offense here but girls, as I have observed, unexceptionally scream out loud at every obedient, well-behaved cicada in sight). Cicadae are not held in good repute in stories, a well known version of which is The Cicada and the Ant, the poem by Jean de La Fontaine (its situation in the modern version is worse). Jean-Henri Fabre (1823-1915) had restore the image of cicada a bit in Souvenirs Entomologiques by pointing out the gang behavior of ants and the darkness a youth cicada has to suffer in its early years. And now, Chinese researchers from Peking University have proved further their Zhi Liaos are useful in nanotechnology, in their interesting paper on Small (2006, 2, 1440–1443. DOI: 10.1002/smll.200600255).

The researchers captured the cicadas locally for their experiments (and perhaps recalled their childhoods for a while). Under a scanning electron microscope (SEM), they found the surface of cicada wings are covered by a hexagonal array of pillars with space between each other about 190 nm. This structure is utilized as a stamp in the nanoimprint lithography (NIL) (see also the pioneer report by Stephen Y. Chou on Science, 1996, 272, 85-87. DOI: 10.1126/science.272.5258.85), a novel but simple procedure to fabricate nano-pattern on surface. Patterns of the cicada wing were imprinted on a layer of organic glass, or polymethyl methacrylate (PMMA), at its processing temperature, and a nanowell array has formed.

This negatively patterned PMMA layer subsequently acted as a mask on a silicon (Si) surface under a reactive ion etching process (RIE) so that the patterns were copied to the Si surface. Although it showed that PMMA is not an ideal mask of such an RIE process, an antireflective property of the resulting Si surface was obtained due to the subwavelength character of the surface, which is the surface-relief grating with the period smaller than the light wavelength. Gold pillar arrays similar to the structures in the cicada wings were also fabricated by using the patterned PMMA as a mold, and may be useful in optical imaging or in surface-enhanced Raman scattering (SERS).

One of the most important feature of a stamp in NIL is low surface tension. If the surface tension is too high, the patterned polymer will be destroyed during stamp release due to conglutination after imprinting. Fortunately the surface tension of the cicada wing is low enough, with a wax layer on the surface which contains esters, acids alcohols and hydrocarbons. The experiment also showed that cicada wings have sufficient stillness and chemical stability to carry out NIL. These properties originate from the special composition of cicada wings. The main component of cicada wings consist of an arrangement of highly crystallined chitin nanofibers embedded in a matrix of protein, polyphenols, and water, with a small amount of lipid. Crystalline chitin interacts with the protein matrix via hydrogen bonding, which imparts stillness and chemical stability to the structure.