Biomedical Engineering Reference
In-Depth Information
are superhydrophobic and the measured appar-
ent water contact angle (CA) is
∼
172° for the
former and
∼
160° for the latter, significantly
enhanced from
∼
108° to
∼
105° on fluorinated
flat silicon and glass substrates.
Figure 12.21
c shows the dependence of the
measured water CA on RIE duration. It is appar-
ent that longer etching duration (i.e., larger
aspect ratio for the resulting nanopillars) leads
to a more hydrophobic surface. This agrees well
with previous studies on microstructure-induced
dewetting
[144-146]
. The volume shrinkage
during the sol-gel glass solidification process
could explain the reduced CA for glass pillar
arrays over the corresponding silicon arrays.
state, which is promising for developing self-
cleaning antireflection coatings.
The bioinspired moth-eye ARCs can be
directly used in improving the performance of
many optical and optoelectronic devices (e.g.,
optical lenses and photodiodes). However, on
some occasions, the low reflection rendered by
the moth-eye structure may not be sufficient in
enhancing the final device performance. A
prominent example is furnished by crystalline-
silicon solar cells. One key technical risk that
could limit the conversion efficiency of solar
cells with integrated moth-eye ARCs is the sur-
face recombination of charge carriers
[5]
. The
high surface area enabled by nanostructured
moth-eye gratings could greatly increase the
surface recombination rate of electrons and
holes, impacting the final conversion efficiency
of the solar cells. The moth-eye nanostructure
(e.g., size, shape, and depth of nanopillars)
therefore needs to be optimized to balance the
optical and optoelectronic properties of the
resulting coatings.
12.6 CONCLUSIONS
Biomimetic moth-eye ARCs exhibit much-
improved broadband antireflection and thermal
stability than traditional quarter-wavelength
ARCs. Colloidal lithography-based templat-
ing nanofabrication provides a much simpler,
cheaper, and faster alternative to complex top-
down nanolithography in creating subwave-
length-structured moth-eye ARCs.
To improve the scalability and compatibility
of current colloidal self-assembly, a versatile
spin-coating technological platform and a Lang-
muir-Blodgett assembly technology that enable
large-scale production of colloidal template for
patterning moth-eye ARCs have been devel-
oped. The structural parameters of the resulting
antireflection gratings, such as crystalline struc-
ture, pitch size, pillar depth, aspect ratio, and
shape, can be easily adjusted by tuning the tem-
plating conditions. Excellent broadband antire-
flection has been achieved on a large variety of
substrates for important technological applica-
tions ranging from improving the efficiency of
PV cells to reducing glare from optoelectronic
devices. The templated periodic arrays of nano-
pillars with high aspect ratios can also facilitate
the achievement of superhydrophobic surface
Acknowledgments
This work was supported by the U.S. National Science
Foundation (NSF) under Grants Nos. CBET-0744879 and
CMMI-1000686.
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