Biomedical Engineering Reference
In-Depth Information
ordered crystal is determined to be
∼
1.46
D
by
the first peak of the pair correlation function
(PCF), which is calculated from a low-magnifica-
tion SEM image
[110]
.
controlled by the syringe pump. As the wafer is
withdrawn, the floating monolayer colloidal
crystal is transferred onto the substrate.
Figure 12.8
a shows a photograph of a 5-in.
solar-grade (mc-Si) wafer with the right half
(yellowish region) covered by a uniform mon-
olayer of 250-nm silica particles. The typical top-
view SEM images in
Figure 12.8
b illustrate the
right part of the wafer. The high surface rough-
ness of the wafer as evidenced by the randomly
distributed, micrometer-sized pits, and the uni-
form coverage of the rough surface by hexago-
nally close-packed silica particles is clearly
evident.
Figure 12.8
c shows a photograph of a
5-in. single-crystal silicon (sc-Si) wafer covered
by a high-quality monolayer of 200-nm silica
spheres assembled using the LB method. The
long-range ordering of the colloidal array is
demonstrated by the top-view SEM image in
Figure 12.8
d. This simple colloidal self-assembly
technology does not require sophisticated equip-
ment (e.g., a Langmuir-Blodgett trough)
[127]
to
organize silica microspheres with diameter
ranging from
∼
70 nm to
∼
30
μ
m over wafer-
sized areas. In addition, extensive experimental
results show that this technique is compatible
with roll-to-roll processing, promising for scal-
ing up to large-volume production.
12.2.2 Langmuir-Blodgett Particle
Assembly at Air-Water Interface
The spin-coating technology described in Sec-
tion
12.2.1
is suitable for large-scale produc-
tion of highly ordered colloidal crystals with
unusual non-close-packed crystalline struc-
ture. However, spin coating can only be per-
formed on flat substrates with low surface
roughness. It is almost impossible for spin
coating to create colloidal crystals on curved
surfaces (e.g., optical lenses) and substrates
with rough surface, such as a solar-grade mul-
ticrystalline silicon (mc-Si) wafer. To enable
the formation of colloidal crystals on curved
and/or rough surfaces, a simple but scalable
Langmuir-Blodgett particle-assembly technol-
ogy has been developed
[111]
.
Monodispersed silica particles synthesized
by the standard Stöber method
[126]
are puri-
fied by repeated centrifugation/redispersion
cycles in ethanol and then redispersed in ethyl-
ene glycol with particle volume fraction of 0.20.
With a clamp attached to a syringe pump, vari-
ous substrates (e.g., mc-Si wafer) can be verti-
cally immersed in a crystallizing dish containing
deionized water. The silica/ethylene glycol sus-
pension is then added dropwise to the surface
of the water. The suspension is spread to form
a thin layer floating on the surface of the water.
With the gradual dissolving of ethylene glycol
in water, silica microspheres accumulate at the
water-air interface due to the high surface ten-
sion of water (72.75 mN/m at 20°C). The capil-
lary action between neighboring silica
microspheres can then organize the floating
particles into close-packed monolayer colloidal
crystals that exhibit striking iridescence caused
by light diffraction
[80]
. The substrate is then
slowly withdrawn at a rate of
∼
0.5 mm/min
12.3 TEMPLATED BROADBAND
MOTH-EYE ANTIREFLECTION
COATINGS ON SEMICONDUCTOR
WAFERS
The self-assembled colloidal arrays can be used
as structural templates to create broadband
moth-eye ARCs on a large variety of techno-
logically important inorganic semiconductor
wafers. The high refractive indices of semi-
conductors lead to severe surface reflection,
greatly impeding the efficiencies of many opto-
electronic devices ranging from solar cells to
photodiodes.
