Biomedical Engineering Reference
In-Depth Information
Amazingly, the polymer replica attracted insects,
even without the pigment. ALD was applied to
the rose petal to fine-tune the reflectance in the
lower-ultraviolet regime. By coating with thin
films of alumina (Al
2
O
3
), a shift of the reflectance
peak as a function of the coating thickness was
observed. The shift may be related to two effects:
the modification of the structure's effective
refractive index by adding alumina on top, and
changing structural parameters by adding an
inorganic film on top. Presumably, both of these
effects have an influence on the shift of the
reflectance peak.
A more exciting form of coloration is found in
natural photonic crystals. Excellent examples are
furnished by many butterfly wings with over-
lays of micro- and nanostructures. In the work
of Huang
et al
.
[74]
, the wings of a blue-colored
butterfly (
Peleides morpho
) were coated with
Al
2
O
3
by ALD. This technique for such appro-
aches is attractive because of the non-line-of-sight
deposition and the extreme conformity of the
coating. The deposited films had thicknesses of
10-40 nm in steps of 10 nm. With increasing
thickness of the coating, the optical appearance
of the butterfly wing shifted from the original
blue coloration toward pink (
Figure 16.7
). The
optical behavior is influenced by the addition of
Al
2
O
3
, which has a different refractive index
than the chitin that the wing consists of. A
removal of the chitin by thermal treatment
resulted in an Al
2
O
3
replica, which, again due to
the change of the refractive index, showed a blue
shift of the main reflectance peak from 550 nm.
The fascination with the colors of butterfly
wings as well as the possibility of replicating
and/or tuning those colors by ALD resulted in
several investigations by other research groups.
Gaillot
et al
. investigated the wings of another
species of butterfly,
Papilio blumei
, which shows
green iridescence with blue-colored tails
[75]
.
The work was focused on the accessibility of the
air pockets in the photonic structures to the
metal-organic precursors. TiO
2
was the material
of choice for the ALD process because of its
refractive index being higher than that of Al
2
O
3
.
For the coating, two differing cases—(1) deposi-
tion on the top and bottom surfaces of the wing
only, and (2) coating of the surfaces of the
embedded air pockets—were implemented. For
distinguishing those two cases, the shift of the
main reflectance peak at 524 nm was observed
in detail. When the structure is coated externally
only, this peak shifts toward larger wavelengths
by about 1 nm per nm TiO
2
coating thickness.
When the air pockets are also coated, the peak
shifts by 12 nm per nm TiO
2
coating thickness.
The control of the coating, however, still seems
difficult. The accessibility of the air pockets for
the precursor is perhaps related to structural
defects or cracks in the chitin, or it proceeds
from the edges of the structure through inter-
connected channels. Thus, coating of only the
top and bottom surfaces can be guaranteed.
Further tuning of the ALD deposition on but-
terfly wings led to additional functionalities. In
a recent work, Liu
et al
. showed that besides the
optical features, the antiwetting properties of
the butterfly wings could also be replicated
[76]
.
The hydrophobicity is an important effect for
the survival of a butterfly, since any humidity on
the wing will make flying impossible. The
nanoscale roughness of the structure appears to
be more detrimental for hydrophilicity than the
material the structure is composed of and there-
fore can be easily reproduced by ALD. More
recently, even photovoltaic applications have
been proposed. Through replication of the struc-
ture of the butterfly wing (here
Hypochrysops
polycletus
), antireflective structures were pro-
duced in a solar cell stack
[77]
.
Antireflective properties are also found in the
eye of a household fly. The structure of the eye
is complicated and consists of small lenslets
(
ommatidia
), which contain further nanoscale
protuberances. This combination results in anti-
reflective properties above a wavelength of 400
nm with a reflectance peak at a wavelength of
330 nm. Similar to the butterfly wings, the fly
eye was coated with Al
2
O
3
at 100 °C
[78]
.
