Chemistry Reference
In-Depth Information
The most widely used bottom-up synthesis technique for AR
nanostructures is the CVD route. Yang et al. reported impressive
AR phenomena from extremely long (>300
m) vertically aligned
carbon nanotube (VA-CNT) arrays (Fig. 2.9c-d) synthesised
by catalyst-assisted CVD [49]. The optical properties of CNTs
were calculated assuming an effective RI (
µ
) following the MG
approximation, which holds for very low density of the array (0.01-
0.02 g/cm
n
eff
3
), that is, when the volume fraction of air trapped in the
array is high. For spacing of 50 nm and diameter of 10 nm, an
n
eff
−1
of 1.026 and absorption constant of 0.12
µ
m
was estimated. Low
R
< 0.07% were demonstrated over 450-650 nm spread under
normal incidence (Fig. 2.9e,f) [49]. However, such high-layer
thickness makes it unsuitable as an ARC for most device applications.
Industrially preferred techniques like the plasma-enhanced
CVD has been successfully employed for multi-purpose coating
that combines ARC (TiO
/SiO
), anti-scratch layer (SiO
), easy-
2
2
2
to-clean coating (of SiO
), and quite interestingly it
showed to be tailor-made for polymers also, for example, PMMA
and PC (polycarbonate) [50]. Fabrication of GRIN RI is also feasible
through atmospheric pressure CVD (APCVD) as found in GRIN layer
comprising SiO
C
H
and TiO
x
y
z
2
and TiO
duo that has an effective reflectance of
2
2
0.5% [51].
2.4.1.2 Top-down technology
Top-down technology is preferred over the bottom-up technique
when precise control of structure dimension or spacing is required;
however, it has a limitation on the smallest structures it could
achieve.
Porous silicon (PSi), widely used on active solar cell devices, has
been prepared by electrochemical etching of silicon in hydrofluoric
(HF) acid solution: an example of mask-less wet-etching process [52].
Here the pore-volume fraction, hence density, or
, was controlled
by simply the etching current and duration. Instead of conventional
constant current etching, a dynamical etching procedure, where a
higher current density in the beginning imparting high porosity and
then gradually decreasing the current density which would, in turn,
lower the porosity, could be used to achieve gradient-RI in PSi film
[53].
n
eff
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