Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 216

inspiration from moth eye corneas, which consist of a hexagonal non-close-packed
arrays of conical nipples ( Min et al. 2008 ). The latter have subwavelength sizes
so that they do not scatter the incident light but rather modify the refractive index
profile, inducing a gradual variation of this parameter at the interface between
cornea and air. This principle has been demonstrated experimentally by fabricating
a similar pattern of GaSb nipple arrays, by first inserting a hexagonal non-close-
packed colloidal layer of silica particles in a polymer matrix deposited on a GaSb
wafer followed by removing the polymer matrix by oxygen plasma etching and
using the released colloidal array, which is removed in the final step, as etching
mask in a subsequent chlorine RIE process. As a result, the effective refractive index
increases with the depth z in the material as

n f. z /n 2=3

air o

3=2

GaSb C Œ1 f. z /n 2=3

n. z / D ; (8.5)

where f is the fraction of GaSb, and n GaSb and n air are the refractive indices of
the wafer material and air, respectively. The nipple geometry, and thus the form of
the refractive index and of the f change with z depends on the etching conditions,
simulations showing that pyramidal and paraboloid nipples are more efficient as
broadband antireflection coatings than half-ellipsoid nipples. These structures can
also be used in thermophotovoltaic cells since they have excellent thermal stability.
Although most bioinspired materials and devices reproduce a single dominant
characteristic of a biological system, multifunctionality can be achieved combining
characteristics of different life-forms. For example, an optical coating that is at
the same time antireflective and superhydrophobic/self-cleaning can be fabricated,
reproducing the functionalities of moth eyes and cicada wings, respectively ( Sun
et al. 2008 ). The antireflection effect of nocturnal moths is determined by the
refractive index gradient across the interface between their cornea and air, while the
self-cleaning property of cicada wings is due to surface microstructures. These bio-
logical structures are similar, consisting of hexagonally ordered, non-close-packed
protrusions in the shape of nipples with dimensions of about 300 nm. Synthetic
structures that mimic the biological ones and fabricated by a scalable templating
technique, can be used as broadband antireflection coatings. More precisely, a
process that involves transferring with the help of a mold of the periodic surface
protrusions of non-close-packed 360-nm colloidal silica spheres to perfluoroacrylate
polymers, which have a low index of refraction and a low surface energy, was
used to fabricate self-cleaning broadband antireflection coatings. The fluoropolymer
nipple arrays were then replicated on glass substrates and shown to exhibit Bragg
diffraction, i.e., angle-dependent colors, specific for periodic structures. In addition,
the reflectivity decreased with increasing nipple heights, which can be controlled
by oxygen plasma etching, the hemispherical nipple arrays showing relatively high
reflectivities (around 1%) at long wavelengths if templated from small particles
(with diameters in the 200-250 nm range) and at short wavelengths if templated
from particles with a diameter between 450 and 500 nm.