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2.15) [73]. The 16
µ
m-long SiNTs show reflectance below 1% at AOI
values between 5
for both polarisations (Fig. 2.15b,c) in
the UV-VIS-NIR region. In the mid-IR range, the reflectance remains
below 20% within the AOI range of 30
°
and 60
°
(Fig. 2.15e,f).
SiNTs do provide significant broadband advantage in comparison
to natural 'moth-eye' structures that are most effective in the
VIS range (300-800 nm) [73]. In addition, most sub- or quarter-
wavelength structures, independent of the material used, show
a suppression of reflectance below an AOI of 60° (in an extended
VIS to IR region) [20,30,62,63,78,79], whereas SiNTs could easily
suppress the reflectance up to 70
°
to 80
°
. Reflectance from aperiodic
SiNTs demonstrates a minimal sensitivity to the AOI compared with
the periodic multi-layer coatings and SWS, as well as non-periodic
porous materials. Notably, the 'moth-eye' nipples demonstrated a
hexagonal packing arrangement, whereas the SiNTs are random.
Besides, the rounded pillar-like nipples is quite different from the
tapered conical shape of the SiNTs.
°
Figure 2.14
(a) Cross-section SEM image of SiNTs, inset showing tilted
top view. (b) Photograph comparing a shiny polished Si and
black SiNTs wafers. (c-f) Wavelength dependence of the
hemispherical reflectance (using IS) at (c) UV-VIS-NIR and
(d) mid-IR region for an AOI of 30
°
, for SiNTs with
L
= 1.6
m (red), with a reference
planar Si (solid line, black). (e-f) Comparison of specular
reflectance for planar Si (black line) and SiNTs with
µ
m (green), 5.5
µ
m (blue) and 16
µ
µ
L
= 16
m
(symbols, red) in far-IR (e) and THz (f) regions (AOI of 30
°
).
Inset in (e) shows the SEM image of the
m SiNTs. Inset
in (f) compares the reflectance in SiNTs and a planar Si with
unpolarised light and an AOI of 30
L
= 16
µ
°
°
(filled squares) and 45
(open squares) [73].
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