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Recent advances in antireflective surfaces based on
nanostructure arrays
Jinguang Cai
ab
and Limin Qi*
a
Reducing the reflection and improving the transmission or absorption of light from wide angles of incidence
in a broad wavelength range are crucial for enhancing the performance of the optical, optoelectronic, and
electro-optical devices. Inspired by the structures of the insect compound eyes, nanostructure arrays
(NSAs) have been developed as effective antireflective surfaces, which exhibit promising broadband and
quasi-omnidirectional antireflective properties together with multifunctions. This review summarizes the
recent advances in the fabrication and performance of antireflective surfaces based on NSAs of a wide
variety of materials including silicon and non-silicon materials. The applications of the NSA-based
antireflective surfaces in solar cells, light emitting diodes, detection, and imaging are highlighted. The
remaining challenges along with future trends in NSA-based antireflective surfaces are also discussed.
1. Introduction
Generally, the basic science underlying our colorful world
results from the interactions between light and surfaces of the
objects, which comprise reection, diffraction, interference,
scattering, absorption and emission. In particular, reducing the
light reection and improving the light absorption or trans-
mission are crucial for some creatures in nature. For example,
the compound eyes of nocturnal insects (e.g., moths and some
butteries) have effective antireective (AR) functions, which
can increase light transmission under dark conditions signi-
cantly, thus raising the sensitivity of light vision.
1
It is the sub-
wavelength-size nanoarray structure of insect compound eyes
with tapered proles that provides a gradient in refractive index
(RI) between air and the surface of cornea, suppressing the light
reection and increasing transmission at the surface towards a
large range of wavelengths and incident angles. The antire-
ective principle of the insect compound eyes is largely
different from that of traditional layered antireective coatings
(ARCs), which are based on destructive interference at the layer
interfaces. Generally, the layered AR coatings can reduce the
reection at only one or several wavelengths and specic
Jinguang Cai received his PhD
degree in Physical Chemistry
from Peking University in 2013
under the supervision of Prof.
Limin Qi. Currently, he is
working at China Academy of
Engineering Physics as an assis-
tant professor. His present
research interest is mainly
focused on the development of
novel nanostructured materials
for energy conversion.
Limin Qi received his PhD degree
from Peking University in 1998.
He then went to the Max Planck
Institute of Colloids and Inter-
faces to work as a postdoctoral
fellow. In 2000, he joined the
College of Chemistry at Peking
University, where he has been a
full professor since 2004. He is
an advisory board member of
Advanced Functional Materials
and ACS Applied Materials &
Interfaces. His current research
is focused on the controlled synthesis and assembly of functional
micro- and nanostructures by colloidal chemical methods and bio-
inspired approaches, with particular attention paid to their
applications in energy conversion and storage.
a
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for
Structural Chemistry of Unstable and Stable Species, College of Chemistry, Peking
University, Beijing 100871, P. R. China. E-mail: liminqi@pku.edu.cn
b
China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan,
P. R. China
Cite this: Mater. Horiz.,2015,2,37
Received 11th August 2014
Accepted 10th September 2014
DOI: 10.1039/c4mh00140k
rsc.li/materials-horizons
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incident angles, which is determined by their intrinsic
destructive interference mechanism. Therefore, it is difficult for
the layered AR coatings to satisfy the high requirements of
broadband and omnidirectional antireection.
Inspired by the structures of the insect compound eyes, a
large quantity of nanostructure arrays (NSAs) have been devel-
oped as antireective coatings or surfaces, which exhibit
promising broadband and quasi-omnidirectional antireective
properties.
2–4
In practical applications, reducing the reection
and improving the transmission or absorption of light from
wide angles of incidence in a broad wavelength range are crucial
for enhancing the performance of the optical, optoelectronic,
and electro-optical devices. For example, the broadband and
omnidirectional AR coatings on solar cells could suppress the
reection and improve the absorption of light within the
absorption wavelength band for nearly all incident directions,
which would not only boost the optoelectronic conversion effi-
ciency, but also leave out the rotating control unit following the
solar radiation angles.
5
As for the photodetectors, bringing
down the reection can effectively enhance the sensitivity and
accuracy of the devices.
6
In camera lens, light-emitting diodes
(LEDs), surface-emitting lasers, and at panel display applica-
tions, AR coatings are usually employed to increase light
extraction efficiency and transmission, eliminate ghost images
or veil glare caused by reection from the optical surfaces, and
enlarge the visual angles of the devices.
7–9
In laser desorption
ionization mass spectrometry (LDI-MS), lower laser uence can
be used to desorb and ionize the analytes on an antireective
surface due to the effectively suppressed reection towards the
incident laser.
10
It is noteworthy that many NSA-based AR
surfaces exhibit useful properties in addition to the high AR
performance, such as self-cleaning capability due to the high
fraction of air trapped in the trough area between arrays.
11
Several comprehensive reviews on AR coatings have been
published in the literature.
2–4,11
In this review article, we focus
on the recent developments in the fabrication and applications
of AR surfaces based on nanostructure arrays, which are
essentially inspired by the sub-wavelength structure arrays of
the insect compound eyes. First, the basic principle of antire-
ection based on NSAs and the requirements for perfect AR
coatings are briey described. Then, the fabrication and
performance of the AR surfaces based on NSAs of silicon and
non-silicon materials are introduced, respectively. Finally, some
current practical applications and perspectives on AR surfaces
based on NSAs are briey discussed.
2. Theoretical aspects of
antireflection based on nanostructure
arrays
2.1 Basic principles
For the traditional layered AR coatings widely used in many
optical and opto-electronic devices, the basic principle of a
single layer dielectric thin lm with a low RI (n) on a substrate
with a different RI (n
s
), where n
s
>n, follows the lm interfer-
ence law (Fig. 1a). Two interfaces are created in this thin lm
conguration, which produces two reected waves, and
destructive interference occurs when these two waves are out of
phase. Minimal reection loss can be achieved for the opti-
mized thickness and RI of the AR coating, which are dependent
on the wavelength, angle, and polarization of the incident light.
Therefore, single layer AR coatings can only obtain good AR
performance towards the incident light with specic wave-
length, angle, and polarization.
2,4
AR coatings based on micro- and nanostructure arrays
inspired by “moth's eye”structures follow an alternative way of
reducing reectance. Depending on the characteristic scale of
the structures, there are two different ways of the interaction
between the arrayed structures and the incident light.
4
If the
size of the individual unit is much larger than the wavelength,
namely a macrostructure unit, the incident light would nor-
mally be reected and scattered aer being absorbed partly. If
the depth and space between individual structure units are in
the same scale of light wavelength, light rays are trapped in the
gaps leading to multiple internal reections (Fig. 1b). Thus, the
incident radiation can be absorbed obviously, reducing the
reection in the visible range to a very low level. However, when
the AR structures have dimensions less than the wavelength, i.e.
located in the sub-wavelength scale or nanoscale, an alternative
way is employed. Light is insensitive to the AR structures and
tend to bend progressively as if the AR surface has a gradient
refractive index (Fig. 1c and d). Even though the angle of inci-
dence is changed, the coating still exhibits a relatively smooth
change of RI towards the incident direction of light, thus sup-
pressing the reection of light for a broad range of wavelength.
Besides, natural light always shows some degree of polarization,
including s- and p-polarizations, which have the electric eld
perpendicular and parallel to the incidence plane, respectively.
For the sub-wavelength-scale or nanoscale arrays with smoothly
graded RI from air to the substrate, the reection of light with
either s- or p-polarization can be suppressed to a very low level,
because the transmission of light with different polarizations is
insensitive to the media with extremely low disparity of RI.
2,4
Therefore, this type of AR coatings based on NSAs with gradient
Fig. 1 (a) Propagation of incident light through a single layer film on a
substrate (n
s
>n). (b) Multiple internal reflections of incident light in a
microstructure array. (c) Interaction of incident light with the sub-
wavelength-size nanoarray. (d) Schematic illustration of the refractive
index change corresponding to (c).
38 |Mater. Horiz.,2015,2,37–53 This journal is © The Royal Society of Chemistry 2015
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RI can realize broadband, omnidirectional, and polarization-
insensitive AR performance, which is superior to that of layered
AR coatings.
Generally, different NSAs may have different refractive index
prole curves, such as linear, parabolic, cubic, quintic, expo-
nential, and exponential-sinusoidal, thus exhibiting different
AR performance. For example, Xi et al. demonstrated a low-
refractive-index optical thin lm with the unprecedented
refractive index of 1.05 using an inclined SiO
2
nanorod array,
and they proposed a quintic prole as the best AR prole.
Accordingly, they fabricated a hybrid system of multi-layer
quintic RI coatings consisting of three layers of TiO
2
nanorods
and two layers of SiO
2
nanorods, which registered RI values of
2.03 for the bottom layer and 1.05 for the top layer, exhibiting a
reectivity as low as 0.1%.
12
Theoretical computation is very important in developing and
optimizing high-performance AR surfaces. In the view of the
effective medium theory (EMT), which is an essential concept
for many computational models in the area of antireectivity,
the RI of the surface depends on the topology and the compo-
sition and can be calculated as a function of f, the volume
fraction of inclusion for a material mixture.
4
As for a NSA lm,
the effective refractive index can be determined by considering
the surface consisting of layers of homogeneous mixtures of the
nanostructured materials and the air in the interspaces. The
rigorous coupled-wave analysis (RCWA), rst proposed by
Moharam in 1981,
13
is widely used in the theoretical calcula-
tions to optimize the AR coating design. This analytical model is
a relatively straightforward technique to exactly solve Maxwell's
equations to get the accurate analysis of the diffraction of
electromagnetic waves. In RCWA, the cross-section of the
structures is treated as consisting of a large number of thin
layers parallel to the surface. A particular formulation without
any approximations has been developed to analyze both the
transmission and reection from planar and surface-relief
structures accurately and efficiently. This model can predict the
performance of AR structures and conduct the structure opti-
mization of the AR coatings.
2.2 Requirements for perfect antireective coatings
A perfect AR coating should meet the requirements of excellent
AR properties, namely, broadband, omnidirectional, and
polarization-insensitive antireectivity.
4
The fact that there may
be difference in the refractive index matching or optical
impedance matching required for different wavelength regions,
such as visible, ultraviolet, and near infra-red regions, impairs
the broadband antireective performance of the AR coatings.
The angles of incidence have a signicant impact on the
reectance. For example, most glass and plastics with RI around
1.5 exhibit a 4% reectance at normal incidence, but a much
higher reectance; even 100% reectance can be reached as the
angles of incidence are increased.
4
This causes large difficulties
in the case of solar cells which need to be mechanically oriented
to face the sun throughout the day, which needs additional
control devices and energy consumption. Therefore, omnidi-
rectional antireectivity is very important for the practical
applications of AR coatings in solar cells. Moreover, the AR
coatings have to be insensitive towards light polarization due to
the fact that light reecting at shallow angles has the p-polar-
ized light reecting to the maximum. Therefore, a perfect
antireective coating should own broadband, omnidirectional,
and polarization-insensitive antireectivity. It is very difficult
for a traditional layered AR lm to satisfy all the requirements
due to the fundamental interference destructive principles.
Moreover, the stability of AR coatings, including mechanical
stability or integrity, thermal stability, and chemical stability, is
essential for their long-term usage in the practical applications,
especially for the devices operating under extreme conditions.
When contamination or fogging takes place on the AR surfaces,
the AR performance would dramatically deteriorate, and addi-
tional maintenance would be needed. This problem may be
addressed, if the AR surfaces have additional functionalities,
such as self-cleaning, self-healing, antimicrobial, and super-
amphiphobic functions.
11
Furthermore, it would be highly
desirable that AR coatings be easily produced in an industry
scale at low cost. To summarize, the perfect antireective
coatings should exhibit broadband, omnidirectional, and
polarization-insensitive anti-reectivity, and are associated with
high stability, multi-functionality, and low cost (Fig. 2). It is
noteworthy that there has been considerable progress towards
the perfect AR coatings based on nanostructure arrays in recent
years. Some recent advances in the fabrication and applications
of promising AR surfaces based on nanostructure arrays will be
summarized in the following sections.
3. Silicon nanostructure arrays
Silicon is the most important material in modern semi-
conductor industry, and is also one of the most widely investi-
gated materials used as antireective coatings because it has
been broadly applied in the photovoltaic eld for reection loss
minimization and good compatibility with Si photovoltaics.
Therefore, in this section, the fabrication and performance of
AR surfaces based on silicon NSAs will be reviewed in detail as
representative NSA-based AR structures. For the fabrication of
silicon NSAs, several effective strategies have been adopted,
including vapor-phase growth or deposition, plasma etching or
dry etching, and wet etching. As for AR coatings with graded
Fig. 2 Requirements for perfect antireflective coatings.
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refractive index, most silicon NSAs have been fabricated by the
etching methods due to the convenience and low cost.
3.1 Si nanostructure arrays by dry etching
Masks are always adopted during the fabrication process in
order to obtain controllable NSAs. According to the fabrication
methods, several types of masks have been used, such as
photoresists, monolayer colloid crystals (MCCs), metal nano-
particles, and self-masks, as shown in Fig. 3. When the photo-
resist nanopillars are formed by the laser interference
lithography method (Fig. 3a) or nanosphere lithography based
on self-assembled MCCs (Fig. 3b), ordered nanostructure arrays
can be fabricated on the substrates. On the other hand, dry
etching using thermally dewetted metal nanoparticles and
other randomly distributed nanostructures as etching masks
would produce disordered nanostructure arrays (Fig. 3c).
Inspired by the moth-eye structures, two dimensional (2D)
MCCs have been widely used as masks to fabricate antireec-
tive, periodic NSAs due to their period arrangement and the
easy operation,
3
and the method is also called nanosphere
lithography or colloidal lithography. The schematic procedure
is shown in Fig. 3b, which indicates that the NSAs are produced
as the colloidal spheres and uncovered substrates are etched.
The NSAs obtained by this method always have high periodicity.
The periodicity and the height of the NSAs can be controlled by
the size of the colloidal spheres and etching time, respectively.
14
Zhu et al. reported the fabrication of a-Si:H nanowire and
nanocone arrays, through a reactive ion etching (RIE) process
using close-packed silica monolayers as masks.
15
These a-Si:H
nanostructures display greatly enhanced absorption over a large
range of wavelengths and angles of incidence, due to sup-
pressed reection. The enhancement effect is particularly
strong for a-Si:H nanocone arrays, which provide nearly perfect
impedance matching between a-Si:H and air through a gradual
reduction of the effective refractive index. Besides silica
colloidal crystals, self-assembled polymer sphere monolayers
were also used as etching masks for generating antireective Si
nanopillar arrays combining RIE methods. For example, Park
et al. proposed a novel method to extend the antireection
spectral range shorter than the lattice constant of the nano-
structure by combining AR coatings and the moth's eye struc-
ture without a complicated process to fabricate a feature size of
sub-300 nm (Fig. 4a).
16
Hexagonal close-packed monolayer
polystyrene (PS) nanosphere crystals were used as etching
masks to construct novel graded-index nanostructures inte-
grating AR nanoisland coating arrays on top of silicon nano-
conical-frustum arrays (Fig. 4b). These complex structures not
only exhibit good antireection properties in the visible wave-
length, but also decrease the average reectance in the near-UV
spectral range (300–400 nm) from 9.2% for sharp-tipped
nanocone structures to 3.8% (Fig. 4c).
In addition to the periodic NSAs, there have been some
remarkable advances in the fabrication of aperiodic NSAs with
excellent AR properties. For example, an impressing work on
high-ratio Si nanotip array-based AR coatings was reported by
Huang et al. using a self-masked dry etching technique, where
SiC cluster caps formed on the surface acted as masks in the
following etching process.
17
The fabricated Si nanotip array
characterized by an apex diameter in the range 3–5 nm, a base
diameter of about 200 nm, and a length of 16 mm, provides a
smooth refractive index change, and thus can effectively
suppress the reection of light at a range of wavelengths from
the ultraviolet, through the visible, to the terahertz region.
Meanwhile, this structure shows excellent AR performance for a
wide range of angles of incidence and both s- and p-polarized
light. Another study on high-aspect-ratio NSAs was performed
by Cho et al. using a deep reactive-ion etching (DRIE) process
without masks (Fig. 5a).
18
The surfaces exhibited good AR
properties over the UV-vis-IR range (<0.04% for certain ranges),
which can be tuned simply by controlling the length and
morphology of the high-aspect-ratio nanostructures on a wafer-
scale surface (Fig. 5b). Meanwhile, these AR surfaces possessed
superhydrophobic properties. Furthermore, Si nanocone arrays
were fabricated on Si(100) substrates by Qiu et al. using Ar
+
ion
sputtering combined with metal ion co-deposition.
19
The aspect
ratio, height and base diameter of the Si cone can be tuned by
controlling the sample temperature and ion dose. The absor-
bance increases in general with increasing aspect ratio and
height. A close to unity and all-solar-spectrum absorption by the
Fig. 3 Schematic illustration of the fabrication procedures of Si
nanostructure arrays by dry etching: (a) photo-interference lithog-
raphy, (b) nanosphere lithography based on monolayer colloidal
crystals, and (c) dry etching using dewetted metals or deposited
nanostructures as masks.
Fig. 4 (a) Schematic diagram showing the detailed fabrication process
of AR nanoislands in silicon nano-conical-frustum arrays. (b) SEM
image of PS nanoislands on top of Si nano-conical-frustum arrays. (c)
Photograph of bare silicon and surface textured silicon.
16
Reproduced
with permission from ref. 16, Copyright 2011, Wiley-VCH.
40 |Mater. Horiz.,2015,2,37–53 This journal is © The Royal Society of Chemistry 2015
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nanostructured Si was nally achieved, with the absorbance for
l¼350 to 1100 nm being higher than 96%, and that for l¼
1100 to 2000 nm higher than 92%. Besides, other antireective
nanostructure arrays have been fabricated using the dry etching
method without masks.
20
In addition, as shown in Fig. 3c,
dewetted metal nanoparticles were also used as masks for dry
etching to construct antireective, aperiodic silicon NSAs.
21,22
3.2 Si nanostructure arrays by wet etching
Since Pent et al. reported the facile metal–nanoparticle-assisted
catalytic etching technique to produce large area and highly
oriented 1D silicon NSAs with desirable axial crystallographic
orientation,
23
wet etching methods have been widely used to
construct silicon antireective NSAs. Compared with dry
etching methods, wet etching methods are relatively simple and
inexpensive, because the etching process takes place only in
reaction solutions and does not need special equipment.
Moreover, combining the Ag deposition technique with the
masks of different shapes, silicon nanowire arrays with
controlled diameter, length, and density, can be easily fabri-
cated.
24
The geometrical proles of Si nanowire arrays, such as
the densities of nanowire arrays, the interfaces of air–nanowire
array layers and nanowire array layers–substrate, can be tuned
by the concentration of AgNO
3
(Fig. 6a), resulting in an inter-
mediate transition region containing a favorable gradient of
effective refractive index (Fig. 6b–d).
25
The specular reectance
of these nanowire array layers is suppressed to an extremely low
(<0.1%) level in the wavelength range of 200–850 nm (Fig. 6e).
Moreover, the AR layers show other favorable AR properties,
including omnidirectionality and polarization-insensitivity, and
are also effective in suppressing the undesired diffuse reec-
tion. As another example, PS nanosphere templating followed
by Ag catalyst layer deposition was used for chemical electroless
etching to fabricate biomimetic silicon nanowire arrays with a
high aspect ratio (100) and high density. An ultra-low reec-
tance of approximately 0.1% was achieved for Si nanowires
longer than 750 nm.
26
Porosity in the silicon nanowires can
further tune and optimize the refractive index proles. Najar
et al. demonstrated that the high density vertically aligned
porous silicon nanowires fabricated on a silicon substrate using
a metal assisted chemical etching process exhibited excellent
AR performance, with the 35% reectivity of the starting silicon
wafer dropping to 0.1% for more than 10 mm long porous silicon
nanowire arrays.
27
However, the results obtained by To et al.
show that the mesoporous Si nanowires vertically standing on a
mesoporous silicon layer trap less light than solid Si nanowires
over a wavelength range of 400–800 nm, due to porosication-
enhanced optical scattering in the mesoporous silicon skele-
tons.
28
Besides, many other NSAs have also been successfully
fabricated via electroless chemical etching methods and used as
effective AR coatings.
29,30
3.3 Hierarchical Si nanostructure arrays by multistep
etching
Besides the above-mentioned relatively simple silicon antire-
ective NSAs, some hierarchical antireective NSAs have been
fabricated through multistep etching methods. For instance,
high aspect ratio silicon hollow-tip arrays were obtained by a
metal catalytic wet etching process using slightly etched non-
close-packed MCCs as masks and the following short time RIE,
Fig. 5 (a) Schematic of the fabrication of Si nanograss using a deep
reactive ion etching (DRIE) process. (b) Relative reflectance of the
nanostructure arrays with different lengths.
18
Reproduced with
permission from ref. 18, Copyright 2011, Royal Society of Chemistry.
Fig. 6 (a) Schematic of the fabrication of Si nanowire array layers at
different AgNO
3
concentrations. (b–d) Cross-sectional SEM images
and (e) specular spectra of Si nanowire array layers fabricated at
different AgNO
3
concentrations.
25
Scale bars are 1 mm. Reproduced
with permission from ref. 25, Copyright 2011, Royal Society of
Chemistry.
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which can suppress surface reection to less than 1% in the
250–1600 nm range.
31
Hierarchical NSAs can make change of
the refractive index of the coatings smoother, thus achieving
better AR performance. Corrugated silicon nanocone arrays on
a silicon wafer by two PS-sphere monolayer-masked dry etching
steps can reduce the reection from above 35% of at silicon to
less than 0.7% in the range 400–1050 nm, and the reectance
remains below 0.5% at incidence angles up to 70at 632.8 nm
for both s- and p-polarized light.
32
Silicon pyramidal structure
arrays are ideal primary structures to construct hierarchical AR
coatings. The silver nanoparticle-assisted electroless etching
process carried out on silicon pyramidal structures obtained by
etching the silicon wafer in a solution of KOH can produce
hierarchical structures, which exhibit strong AR properties, with
areectance less than 4% in a large wavelength range.
33
Recently, a very interesting biomimetic articial silicon
compound eye structure was fabricated by integrating nano-
structures into periodically patterned microstructures via
thermal dewetting of gold and the subsequent dry etching
(Fig. 7a). The desirable shape and density of the nanostructures
could be obtained by controlled dewetting (Fig. 7b and c). The
compound eye structure can further reduce the surface total
reectance over a wide wavelength range of 300–1030 nm at
near normal incidence from an average reectance of 4.9% for
the only nanostructures on the at Si surface to 2.5% (Fig. 7d).
7
Optimized double-layer NSAs may provide smoother graded
refractive index change, thus leading to better AR performance.
Ravipati et al. reported a near-perfect optical absorber
comprising thin (<680 nm), double-layered, sub-50 nm a-Si
nanograss/Si nanofrustum nanostructures prepared using a
simple, one-step, maskless plasma etching process.
34
These
hybrid structures exhibited the high average total absorption of
99.6%, with an average total reectance of 0.34%, at wave-
lengths between 300 and 800 nm. The superior blackness of this
near-perfect optical absorber was attributed to the smooth
transitions of the effective refractive index from both the low
density of its moth-eye-like nanograss prole and the presence
of the nanofrustum transition underlayer. In addition, quasi-
ordered moth-eye arrays were fabricated on a Si substrate using
a colloidal lithography method comprising two step reactive ion
etching (RIE), which showed highly efficient, broadband, and
omnidirectional transmission of mid- and far infrared
radiation.
35
4. Non-silicon nanostructure arrays
Besides silicon materials, antireective NSAs composed of
many other materials have been explored as AR coatings, in
spite of relatively less reports compared to those of silicon NSAs.
In this section, we will summarize the recent developments in
the fabrication and performance of AR surfaces constructed by
the non-silicon NSAs, mainly including silicon oxide, metal
oxides, group III–V compounds, polymers, carbon, plasmonic
metals, etc.
4.1 Silicon oxide based
Silicon oxide (SiO
2
) is a very common but important material
used in everyday lives and optic units, such as glass windows,
automobile windows, glass, laser windows, and camera lenses.
Antireective SiO
2
surfaces are usually employed to increase
transmission and eliminate ghost image or veil glare due to
reection from the optical surfaces for glass-based devices.
Silicon oxide NSAs used as AR coatings were mostly fabricated
by dry etching methods, with or without masks.
Tubelike SiO
2
pillars with a structural period of 110 7nm
and a height of 116 8 nm were fabricated by self-assembly
based nanolithography and reactive ion etching using quasi-
hexagonal arrays of gold nanoparticles as an etching mask.
36
The surfaces exhibited effective AR performance for wave-
lengths ranging from deep UV to IR at oblique angles of inci-
dence. Because of the simplicity and tunability of the
preparation of the metal–nanoparticle masks, especially when
Fig. 7 (a) Schematic diagram of process steps for fabricating the compound eye architectures consisting of nanostructures (NSs) on p-MS/Si
substrates. SEM images of (b) the thermally dewetted Au nanopatterns (top view) and (c) the fabricated NSs (30
o
-tilted oblique view) on p-MS/Si
substrates for different Au film thicknesses. (d) Measured total reflectance spectra of the fabricated NSs on p-MS/Si substrates for different Au film
thicknesses.
7
Reproduced with permission from ref. 7, Copyright 2013, Royal Society of Chemistry.
42 |Mater. Horiz.,2015,2,37–53 This journal is © The Royal Society of Chemistry 2015
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combining with the dewetting method, metal–nanoparticle
layers were frequently used as RIE masks to construct NSAs as
AR coatings.
37–40
Colloidal lithography is considered as the most
useful technique in mass production of moth eye surfaces and
in geometrical regulation of the nanopillars: height, shape, and
diameter.
41,42
Silica cone arrays obtained on a glass slide
through a short time RIE using 2D PS colloidal crystals as masks
could introduce a gradient in the refractive index between air
and the silica substrate, thus dramatically suppressing the
surface reective loss (reectivity < 2%) in a wavelength region
of 300–800 nm with an incidence angle up to 45.
41
Ji et al.
successfully fabricated a series of nanopillars of various heights
and shapes (i.e. cylindrical, bullet-like, truncated, round, and
pointed cone) through colloidal lithography by adjusting the
masking area of PS MCCs, the reactive plasma species, and the
post-cleaning process (Fig. 8a).
42
The nanopillar arrays with a
pointed cone shape show better optical performance in the
visible range than those with the rounded cone shape (Fig. 8b),
which is of typical AR nanostructure in nature. Based on the
observed antireective behaviors, the low reectance in the
visible wavelength range is demonstrated by moth eye
mimicking nanostructures on both sides of a glass surface.
It is a unique strategy to realize a broadband spectrum in the
visible range showing 99% transparency via the appropriate
matching of nanopillar height on the front and back sides of
glass. Recently, they numerically and experimentally investi-
gated effects of the geometrical parameters such as height,
periodic distance, shape, and arrangement on the AR perfor-
mance of a transparent quartz substrate, demonstrating that
the height of nanopillars around 400 nm with a pointy shape is
optimal for the best antireection performance, and the peri-
odic nanostructure array provides better antireection perfor-
mance than the random array due to low light scattering.
43
Antireective sub-wavelength structure (SWS) arrays could also
be fabricated by a wet chemical etching method using self-
assembled SiO
2
nanospheres as masks.
44
Besides, the anodic
aluminum oxide lms directly formed on the silica substrates
were used as etching masks to fabricate AR surfaces with
nanohole arrays.
45
In addition, other methods have been
recently used to construct antireective silica NSAs, such as
chemical vapor deposition
46
and nanoparticle self-assembly.
47
4.2 Metal oxide based
ZnO is a very attractive material used for AR coatings, because of
its low cost, good transparency, appropriate refractive index (n
¼2), and ability to form textured coating via anisotropic growth.
To date, various ZnO NSAs have been synthesized as AR coatings
through either vapor deposition or solution growth method.
Tapered ZnO nanorod arrays fabricated by Lee et al. using a
solution-growth method on seeded substrates display broad-
band reection suppression from 400 to 1200 nm.
48
Nanorod
parameters, such as nanorod tip diameter, diameter of the
nontapered region, thickness of the fused base layer, overall
nanorod length, and length of the tapered region, have a great
effect on the macroscopic AR performance. ZnO pyramidal array
structures on zinc substrates with a gradient change of refrac-
tive index, which were prepared through a hydrothermal
method, can effectively suppress the reection of light at a
range of wavelength from ultraviolet through the visible part to
the near-infrared region, with reectivity <2.5%, resulting in its
black color.
49
The alignments of ZnO nanoarray structures
provide different surface proles, resulting in different behav-
iors of scattered light. In this regard, Chao et al. fabricated three
types of ZnO nanoarray structures with different alignments
(well-aligned, quasi-aligned, and ower-like) using the hydro-
thermal process, and investigated their antireective perfor-
mance. The results demonstrated that the nanorod arrays with
large alignment variation (i.e. ower-like ZnO nanoarrays)
exhibit broadband and omnidirectional AR properties due to
the gradual index prole.
50
ZnO NSAs are always used as AR
coatings in thin lm solar cells due to their easy anisotropic
growth via either the solution method or vapor deposition.
Recently, several NSAs including nanorod, nanotree, dandelion-
shaped, branched nanorod, and branched nanoower arrays
have been hydrothermally fabricated as AR coatings on
Cu(In,Ga)Se
2
(CIGS) or Cu
2
ZnSnS
4
solar cells, all of which
exhibit the efficient AR performance, thus largely enhancing the
short-circuit current density.
51–54
ZnO nanostructured arrays are
also used to construct hierarchical structures combined with
other materials because of the facile fabrication of ZnO NSAs on
arbitrary substrates. Notably, Qi et al. adopted silicon micro-
pyramids, which were prepared by photolithography followed
Fig. 8 (a) Schematic of the fabrication procedures for the shape control of nanopillars (left) and SEM images of the resultant surfaces (right) (from
top: cylindrical, bullet-like, truncated, round, and pointed cone, scale bars: 500 nm). (b) Reflectance spectra of different shapes of nanopillar
arrays.
42
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by the anisotropic etching process in KOH solution, as the
starting substrate for the hydrothermal growth of ZnO nanorods
(Fig. 9a), obtaining a hierarchical hetero–nanojunction struc-
ture (Fig. 9b).
55
This hierarchical structure can suppress the
light reection more effectively than ZnO nanorods either on
at silicon or pyramid silicon (Fig. 9c), resulting in a high
photocurrent response and good charge separation simulta-
neously. Similarly, the performance of Si nanowire solar cells
was largely enhanced when the surfaces were covered by ZnO
nanorod arrays because of the efficient antireection caused by
multiple scattering.
56
The AR performance of the ZnO NSAs was
also enhanced when they were combined with Si
3
N
4
-coated Si
microgrooves and microcones.
57
Using a similar strategy, ZnO
nanowire arrays can suppress the surface reection more
effectively aer deposition of a MgO shell or branch, due to the
favorable refractive-index modulation.
58
As another important metal oxide material, TiO
2
has been
widely used as AR coatings in silicon photovoltaics, due to its
wide energy bandgap, matching refractive index (2.49 for
anatase and 2.9 for rutile), excellent chemical and thermal
stability, and low deposition cost. However, only a few reports
were published on antireective coatings based on TiO
2
nano-
structure arrays. The rst work on the antireective NSAs using
TiO
2
materials was reported by Xi et al., who fabricated TiO
2
and
SiO
2
graded-index lms constructed with nanorod layers via
oblique-angle deposition.
12
The lm with a well controlled
graded-index prole virtually eliminates Fresnel reection over
a broad wavelength and angle of incidence range. Subsequently,
co-sputtered (SiO
2
)
x
(TiO
2
)
1x
layers have been frequently used
to nely tune the refractive index of the lm.
59
In 2012, we
reported excellent broadband and quasi-omnidirectional AR
structures based on rutile TiO
2
nanorod arrays, which were
grown by a facile hydrothermal synthesis directly on Ti foils.
60
The thickness of typical nanorod lms is about 4 mm and each
hierarchical nanorod is a single-crystal-like rutile TiO
2
mesocrystal comprising many [001]-oriented nanotips about
10–30 nm in diameter grown on the top of a [001]-oriented stem
nanorod about 100–400 nm in diameter (Fig. 10a–c). These
novel hierarchical mesostructures exhibit efficient suppression
of reection towards wavelengths ranging from visible to near
infrared (NIR) regions, with reection <0.5% in the visible
region and <2% in the NIR region, at a wide range of incident
angles ranging from nearly normal to 45(Fig. 10d and e). These
excellent AR properties could be attributed to an optimized
graded refractive index prole resulting from the hierarchical
tips-on-rod structure of the mesocrystalline nanorods (Fig. 10f).
Moreover, the AR structures exhibit high chemical and thermal
stability, and useful self-cleaning properties. Because of the
light scattering, TiO
2
NSAs can generally suppress the light
reectance and enhance light absorption in dye- and quantum
dot-sensitized solar cells,
61–63
and light extract in electrochromic
devices based on TiO
2
nanowire arrays.
64
4.3 Group III–V semiconductor based
Group III–V semiconductors are widely used in optoelectronics,
such as solar cells, light-emitting diodes, and lasers, due to their
high carrier mobility and direct energy gaps. However, there are
some challenges in the fabrication of broadband AR coatings of
most group III–V semiconductors, due to their bandwidth
disparity. Nanostructure arrays of group III–V materials with
gradient refractive index directly grown on the same substrates
may be able to address the problems. Nanostructure arrays of
group III–V semiconductors were usually fabricated through
vapor deposition growth or reactive ion etching methods, rather
than solution methods due to their intrinsic physicochemical
properties.
Vapor deposition is a very effective method to fabricate
various antireective NSAs. For example, Diedenhofen et al.
prepared GaP nanorod arrays by chemical vapor deposition
using the vapor–liquid–solid (VLS) method, and the produced
surfaces exhibited enhanced transmission and reduced reec-
tion properties over a broad wavelength and angular range,
owing to a graded refractive index in the nanorod layer.
65
Then,
base-tapered InP nanowires on the top of an InP substrate were
prepared by combining patterned gold catalyst and VLS growth
by the same group.
66
The specic geometry of the nanowires
endowed them with a broad band and omnidirectional
absorbing medium, and more than 97% of the light was
absorbed in the system. Aerwards, they fabricated tapered GaP
nanowires on the top of AlInP/GaAs substrates using the VLS
method, which could reduce the reection over a broad spectral
range, still due to the graded refractive index of the layer.
67
An
interesting randomly branched vertical array of InSb nanowires
was obtained by Mohammad et al. via electrodeposition within
branched porous anodic alumina membranes, which exhibited
low reectance over the visible and IR regions as well as wave-
length-dependent absorbance in the IR region.
68
GaAs nano-
pyramids prepared by using a combination of nanosphere
lithography, nanopyramid metal organic chemical vapor depo-
sition (MOCVD) growth, and gas-phase substrate removal
processes showed excellent optical absorption over a broad
Fig. 9 (a) Schematic of the fabrication process of the ZnO/silicon
hetero-junctions. (b) SEM image of ZnO nanorods on the silicon
pyramids. (c) Reflectance spectra of flat silicon, pyramidal silicon, ZnO/
F–silicon and ZnO/P–silicon.
55
Reproduced with permission from ref.
55, Copyright 2013, Royal Society of Chemistry.
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range of wavelengths, at various incident angles and at large-
curvature bending.
69
Besides the vapor growth method, the top-down etching
(mainly the dry etching technique) was also a very effective
approach for fabricating NSAs of group III–V semiconductors as
AR coatings. Various dry etching methods similar to those used
for silicon NSAs were employed to fabricate NSAs of group III–V
semiconductors. A one-step and maskless plasma process was
developed by Ravipati et al. to fabricate GaAs nanograsses
consisting of nanowires with diameters less than 20 nm.
70
These nanograsses exhibited remarkable broadband AR prop-
erties and enhanced absorption, arising from the graded
refractive index between air and the GaAs substrate by the
nanograss layer. Patterned photoresists produced by a laser-
interference lithography were adopted by Song et al. as etching
masks to fabricate parabola subwavelength nanoarrays using an
ICP etcher, and the resultant structures can signicantly
suppress the surface reection in the longer-wavelength range,
compared to SWSs with a cone shape.
71
Dewetted Au and Ag
nanostructures were also adopted as masks for dry etching to
fabricate antireective nanostructures of group III–V mate-
rials.
72,73
Similar to the fabrication of silicon nanostructure
arrays, close-packed or non-close-packed MCCs are frequently
used as etching masks. The shape and height of the NSAs could
be controlled by the diameter of the nanospheres and etching
time when using a method combining nanosphere lithography
and RIE. The average reectance of the tapered GaAs structures
and sub-micron conical GaAs structures was reduced from
35.1% for a polished GaAs substrate to 0.6% and 2.3%,
respectively, due to graded-refractive index and multi-scattering
effects.
74,75
4.4 Polymer based
Antireective polymer lms have undergone intense investiga-
tion because of their advantageous characteristics compared
with inorganic materials, such as easily controllable
morphology and porosity, adherence to the exible substrate,
and ease of large-area processing; however, polymer AR coatings
usually suffer from poor wear-resistance and poor photo-
stability. To date, a wide variety of polymer thin lms for AR
coatings have been reported, which can be referred to a feature
article recently published by Han et al.
76
In this section, we will
focus on the fabrication and AR properties of polymer nano-
structure arrays, especially the moth-eye structures.
Template imprinting is the most widely used technique in
preparing polymer AR coatings based on nanostructure arrays.
In a recent report, polyhedral oligomeric silsesquioxane-based
(POSS) antireective moth's eye nanostructures on glass were
developed by a double-side nanoimprint lithography (NIL)
using a Ni mold (Fig. 11a–c).
77
These AR nanostructures
exhibited excellent broadband and quasi-omnidirectional anti-
reective properties, improving the transmittance of the
resulting glass to 98.2% (Fig. 11d). Daglar et al. fabricated large
area nanostructured polymer lms using a drop casting tech-
nique on the tapered silicon molds and the polymer lm with
optimized single-side nanostructures can reach 92% trans-
mission.
78
A polyethylene terephthalate (PET) lm with similar
double-side nanopores was fabricated by a solithographic
approach with a modied polyurethane acrylate mold, and its
average transmission efficiency can be enhanced to 98.7% at
normal incidence and 92.5% at an incident angle of 60over a
range of 400–800 nm.
9
Lee et al. fabricated conical sub-wave-
length gratings of an SU8 polymer at one- and both-sides of the
Fig. 10 SEM (a and b) and TEM (c) images of rutile TiO
2
nanorod arrays grown at 150 C on Ti foil for 20 h. The inset in (c) shows the corre-
sponding SAED image. Specular reflectance spectra of rutile TiO
2
nanorod arrays grown on Ti foils at different times at an incident angle of 6(d)
and rutile TiO
2
nanorod arrays grown on Ti foils for 20 h at different angles of incidence (e). (f) Schematic diagram of the graded refractive index
originating from hierarchical TiO
2
mesostructures.
60
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sapphires by a similar solithography technique with polydi-
methylsiloxane (PDMS) as the mold, enhancing the trans-
parency of sapphires over a wide wavelength region of 400–
1000 nm.
79
Self-assembled porous lms were used as templates
to fabricate PDMS replica molding, which were covered by a
compact distribution of hemispherical nanodomes about
250 nm in diameter and about 100 nm in height.
80
The trans-
mission can be enhanced largely towards wavelengths ranging
from visible to near IR regions when these nanostructures are
applied on a glass surface. Wu et al. fabricated an interesting 3D
moth-eye structure via a one-step imprinting process, which
exhibited better AR performance than 2D arrays over most
wavelengths from 400 to 2400 nm, with the reectivity lower
than 5.7% at all of the wavelengths.
81
Anely controlled AAO
template structure could be transferred with exactitude to the
polymer surface, and the resulting 5-level lm exhibited the best
AR properties, with an average reectance of 0.64% at a wave-
length range of 400 nm to 800 nm.
82
Then, they developed a roll-
to-roll thermal nanoimprint lithography to fabricate nano-
porous honeycomb structures with high throughput and AR
performance.
83
Recently, Li et al. fabricated moth-eye nano-
structures of PMMA via the hot-press imprinting method using
alumina as templates, and optimal broadband wide-angle
antireection performance (average specular reectance of
0.5% at 8–45for wavelengths of 400–2000 nm) was
obtained.
84
The AR lms aer depositing a thin layer of
hydrophilic silica nanoparticles showed superhydrophilic anti-
fogging function.
Biotemplates are natural and interesting molds, which can also
be replicated to fabricate biomimetic polymer AR nanostructures.
For example, Ko et al. reproducibly replicated an original eye
surface with nanoscale delity of the surface topography of the
Attacus atlas moth's compound eye using a solithographic
technique.
85
Both the natural moth eye and its replica showed less
than 1% reectivity in the visible region. Zhang et al. reported the
replication of an entire cicada wing by using a low-surface-energy
orin polymer.
86
The replicated polymeric coatings are capable of
high-performance antireection and light diffraction, which can
enhance light transmission and change the light pathway.
4.5 Carbon based
As a good light absorber because of the p-band's optical tran-
sitions, carbon has been used in many conventional black
materials such as carbon black and graphite. However, the light
absorption is limited due to the moderate reection at the air–
dielectric interface, which can be addressed by using carbon
nanostructure arrays, especially vertically aligned carbon
nanotubes (CNTs). For example, Mizuno et al. fabricated a
forest of vertically aligned single-walled carbon nanotubes
(SWNTs), which behaves similarly to a black body and can
absorb light almost perfectly across a very wide spectral range
(0.2–200 mm).
87
Lehman et al. prepared vertically aligned
multiwall carbon nanotubes (MWCNTs) on a large-area lithium
tantalate pyroelectric detector, and the reectance is uniformly
less than 0.1% from 400 nm to 4 mm and less than 1% from 4 to
14 mm.
88
Recently, vertically aligned MWCNTs on metallic
substrates with a high site-density were synthesized using a
plasma-enhanced CVD method, which exhibit ultra-low reec-
tance (0.02%) over a wide spectral range from UV-to-IR for
relatively thin (<10 mm) absorber ensembles.
89
By adjusting the
thicknesses of the CNTs and designing the bottom tandem
absorber, Selvakumar demonstrated an interesting transition of
a CNT-based tandem absorber (Ti/AlTiO/CoO/CNTs) from a
near-perfect blackbody absorber to a solar selective absorber.
90
In addition to CNTs, carbon nanowalls (CNWs) have also been
fabricated to reduce reectance and improve absorption. For
example, Krivchenko et al. rst studied and reported the optical
properties of CNW lms in the visible range, which can reach an
ultra-low total reectance up to 0.13% depending on the lm
structure.
91
Recently, Evlashin et al. reported the fabrication and
optical absorption of multilayer CNWs. The absorption of a
single CNW layer with the average thickness of 1 mm shows
record values of 96–99% in the wavelength range from 0.4 to
10 mm.
92
4.6 Plasmonic metal based
Photons would be coupled into metal nanostructures, inducing
collective oscillation of valence electrons, when the frequency of
photons matches the natural frequency of surface electrons
Fig. 11 (a) Schematic of the steps involved in the fabrication of POSS moth's eye antireflective nanostructures by NIL. SEM images of POSS
antireflective nanostructures imprinted on top side (b) and bottom side (c) of the glass. (d) Transmission spectra of POSS moth's eye antireflective
glass (single- and double-side) and plane glass.
77
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oscillating against the restoring force of positive nuclei. The
resonant photon wavelength can be tuned by manipulating the
composition, shape and size of the nanostructures. Therefore,
plasmonic nanostructures were used for light trapping in solar
cell applications over the visible spectrum to enhance the
performance of photovoltaic devices.
93
To some extent, this
could be called another type of AR coatings, namely, plasmonic
antireection. Recently, some interesting works on plasmonic
AR coatings were reported in the literature. Toma et al. fabri-
cated 2D periodic gold nanocone arrays on exible Teon lms
by combination of colloidal lithography and oxygen plasma
etching, and the periodicity and size of the nanocone arrays can
be tuned by changing the bead diameter and the etching time
(Fig. 12a).
94
Aer deposition of a thin gold lm, the nanocone
arrays show a strong black color, exhibiting strong suppression
of light (reectivity below 1%) through the entire visible spec-
trum and over a wide range of incident angles (0–70) (Fig. 12b).
The excellent AR performance can be attributed to a combina-
tion of diffractive scattering loss from the periodic structure and
localized plasmonic absorption from the rough gold lm. A
buried nanoantenna array composed of a subwavelength metal
array and a dielectric cover layer (superstrate) as an AR coating
was designed and investigated by Kabiri et al.
95
The phase and
intensity of the wave circulating inside the superstrate cavity
can be controlled by the material properties and the thickness
of the superstrate and nanoantennas' geometry and periodicity.
Thus, this buried nanoantenna array can be used as a tailoring
layer to match any substrate to free space in selected narrow and
broadband spectral response across the entire visible and
infrared spectrum. A minimum reectance of 0.02% is achieved
in the mid-infrared from a silicon substrate. A novel two layer
AR coating based on a plasmonic metamaterial and a dielectric
was designed and fabricated, showing a broadband reduction of
reection that is less sensitive against oblique incidence when
compared to traditional antireective coatings.
96
Furthermore,
this metamaterial on a metal reector can be used for the
visualization of different coloration such as plasmonic rainbow.
Besides, Oh et al. fabricated an antireective double-side glass
nanopillar array using reactive ion etching, and the double-side
glass nanopillar array can be translated to biophotonic surfaces
with self-antireection aer metal layer deposition on one side,
which exhibits highly sensitive uorescence detection and
surface-enhanced Raman scattering (SERS) as well as high
contrast optical imaging.
97
4.7 Other materials
Silicon carbide (SiC) is a wide bandgap semiconductor material
with excellent optical properties as well as outstanding chem-
ical and thermal stability. Moreover, SiC can be used as an
appropriate substrate for the growth of gallium nitride (GaN),
which is the key material for high efficiency visible and ultra-
violet LEDs. The implementation of antireective sub-
wavelength structures on the SiC surface could increase
drastically the efficiency of such devices. Ou et al. have done
some work on the fabrication of antireective SiC nanoarray
structures. First, they presented an approach to fabricate the
periodic cone-shaped AR structures on the N–B doped uores-
cent 6H-SiC by using the RIE with a periodic Au nanoparticle
array as an etch mask, and the surface reectance over the
whole visible spectral range was dramatically suppressed from
20.5% to 1.01%; thus, the luminescence intensity of the SiC
sample was enhanced by more than 91% in a very large emis-
sion angle range (up to 70).
98
Then, they reported an approach
of fabricating pseudoperiodic antireective structures on uo-
rescent SiC by using a self-assembled dewetting Au etch mask.
By applying the pseudoperiodic AR structures, the average
surface reectance at 6incidence over the spectral range of
390–785 nm was dramatically suppressed from 20.5% to 1.62%,
and a considerable omnidirectional luminescence enhance-
ment with an integral intensity enhancement of 66.3% was
obtained.
99
Recently, they systematically investigated the inu-
ence of the RIE conditions and the Au lm thickness on the SWS
prole and its corresponding surface reectance.
100
Under the
optimal experimental conditions, the average reectance of the
SiC AR structures in the range of 390–784 nm was dramatically
Fig. 12 (a) Schematic of the fabrication process of gold nanocone arrays: (1) fabrication of a PS bead monolayer on a flexible Teflon film; (2 and 3)
formation of nanocone arrays by simultaneous plasma etching of PS beads and Teflon film; (4) deposition of a gold thin film on the Teflon
nanocone array. (b) Reflectance spectra of gold nanocone arrays at different angles of incidence.
94
Reproduced with permission from ref. 94,
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suppressed from 21.0% to 1.9% aer introducing the pseudo-
periodic nanostructures.
Sapphire has been widely used in optical and optoelectronic
components due to its good thermal/chemical stability,
mechanical durability, and optical transparency in a wide range
of wavelengths. Leem et al. reported the wafer-scale highly
transparent and superhydrophilic sapphires with antireective
SWSs, which were fabricated by dry etching using thermally
dewetted gold nanomasks.
101
For the sapphire with both-side
SWSs at 5 nm of Au lm, the average total transmittance of
96.5% at 350–800 nm was obtained, indicating a higher value
than those of the at sapphire (85.6%) and the sapphire with
one-side SWSs (91%). Then, they fabricated the broadband
highly transparent sapphires with biomimetic antireective
compound submicrometer structures (c-SMSs) made of disor-
dered nanocone arrays on closely packed ordered hemi-
spherical submicrometer gratings (o-SMGs) using the spin-
coated silica spheres and thermally dewetted gold nanoparticles
as the etch masks, respectively.
102
For the c-SMSs with taller and
larger nanocone arrays at 7 nm of Au lm, an average total
transmittance of 90.7% was obtained, exhibiting haze ratios
of 33.3 and 26% at l¼525 and 635 nm, respectively, much
higher than those of bare sapphire. Besides, a free standing
alumina nanotube array showed good AR properties with a
minimum reectance of 0.1%.
103
Indium tin oxide (ITO), which is one of the common trans-
parent conducting oxide materials, can be widely used as an
antireective and transparent electrode layer in solar cells due
to its relatively low refractive index as well as good optical and
electrical properties. Yu's group demonstrated the fabrication
of highly oriented ITO nanocolumns using electron-beam
evaporation with an obliquely incident nitrogen ux, which
exhibits broadband and omnidirectional antireective charac-
teristics up to an incidence angle of 70for the 350–900 nm
wavelength range for both s- and p-polarizations.
104
Then, they
applied the ITO columns on the GaAs solar cell, whose
conversion efficiency increases by 28% compared to a cell
without any AR treatment.
105
Moreover, nearly 42% enhance-
ment was achieved for photocurrents generated at wavelengths
that are transparent to the window layer. In addition, Leem et al.
fabricated the parabola-shaped ITO SWSs using the laser
interference lithography, inductively coupled plasma (ICP)
etching, and subsequent radio frequency (RF) magnetron re-
sputtering techniques, which exhibit broadband and wide-
angle antireective properties.
106
It may be noted that the nanostructure arrays of some
other inorganic materials have also been fabricated as AR
coatings. For example, ZnS conical pillar arrays using mask-
less laser interference ablation enhanced the transmission at
infrared wavelength, over 92% at normal incidence.
107
Recently, GaOOH nanopillars were synthesized on III–V
InGaP/GaAs/Ge triple junctionsolarcellsbyanelectro-
chemical deposition method, showing a good antireective
performance for a wide wavelength range of 300–1800 nm,
with an efficiency enhancement by 3.47% compared to that of
the bare solar cell.
108
5. Applications
Construction of AR coatings on the substrates is aimed to
reduce the reection and improve the transmission or absorp-
tion of light, thus enhancing the performance of the optical,
optoelectronic, and electro-optical devices, such as glasses,
mirrors, camera lens, solar cells, LEDs, photodetectors, surface-
emitting lasers, at panel display, optical sensing and imaging,
and laser desorption/ionization mass spectrometry. In this
section, we will briey introduce the main progress in the
applications of the AR coatings based on nanostructure arrays.
5.1 Solar cells
The sufficient absorption of light is very important for
increasing the short-circuit current (I
sc
), which transforms into
an increase in efficiency of the solar cells. Traditionally, the
coatings of AR layers on silicon solar cells are made of SiO
2
and
TiO
2
,orSi
3
N
4
, which may face some practical problems in
optical match, surface passivation or angle-dependent antire-
ection. The NSAs would be suitable for the next-generation AR
coatings of silicon solar cells, because the 3D architectures of
silicon nanoarrays are cost-effective and can provide broadband
and omnidirectional antireection, short diffusion length of
minority charge carriers, and potential self-cleaning properties.
Silicon solar cells based on nanowire arrays have been widely
investigated, as summarized in a review article.
5
Recently, a
review article by Brongersma et al. discussed the recent devel-
opments in the design and implementation of photonic
elements in thin lm photovoltaic cells, including nano-
structured antireective surfaces.
109
As a typical example,
nanodome solar cells with a 280 nm thick hydrogenated
amorphous silicon layer can absorb 94% of the light of 400–800
nm, signicantly higher than the 65% absorption of at lm
devices, and the corresponding efficiency of nanodomes reach
5.9%, which is 25% higher than the at lm control.
110
A similar
subwavelength nanopatterned Si structure was fabricated by Liu
et al. using a nanosphere lithography technique.
111
The short
circuit current density increased to 37.2 mA cm
2
and the power
conversion efficiency was obviously improved compared to the
reference cell on a at Si substrate. Wang et al. reported a
double-sided grating design, where the front and back surfaces
of the cell are separately optimized for antireection and light
trapping, respectively. The optimized structure yielded a
photocurrent of 34.6 mA cm
2
at an equivalent thickness of 2
mm, close to the Yablonovitch limit.
112
Nanohole arrays and
hybrid nanocone arrays also exhibited good AR properties and
high photo-conversion efficiency.
113,114
Recently, the Schottky
junction solar cells with modied graphene lms and antire-
ective silicon pillar arrays were fabricated by Lin, which gave a
conversion efficiency of up to 7.7% following the theoretical
optimization.
115
Chan et al. implemented density-graded
surface nanostructures on ultrathin silicon solar microcells by
silver-nanoparticle-catalyzed wet chemical etching (Fig. 13a).
116
Compared to the devices of the bare silicon, the black silicon
surface can enhance the energy conversion efficiency of the thin
lm microcells by 148% and 50% with and without a diffuse
48 |Mater. Horiz.,2015,2,37–53 This journal is © The Royal Society of Chemistry 2015
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backside reector, respectively, while the combination of the
bare silicon and a conventional one-layer antireective coating
can only lead to a smaller increase in the energy conversion
efficiency (Fig. 13b). Besides, SiO
2
and ZnO nanoarray struc-
tures were also directly fabricated on the surface of solid solar
cells as effective antireective coatings.
37,38,52,56,117
However, there are still some challenges to the fabrication of
high-efficiency silicon solar cells based on antireective NSAs.
The high surface area leads to high surface recombination
velocity, which acts as a large drawback for achieving high-
performance devices. Effective passivation methods should be
developed to reduce the surface recombination.
5,118
The fabri-
cation of high-quality silicon nanostructure arrays at lower
temperature and low-cost substrates is still a big challenge. For
the heterostructures of the AR coating and the absorption layer,
the adhesion stability is another important issue that should be
considered.
In addition, with the development of liquid-junction solar
cells, such as dye-sensitized and quantum dot-sensitized solar
cells, the NSAs of ZnO and TiO
2
have been investigated as
photoanodes due to the good AR properties, short diffusion
length of minority, and good electron conductivity. Since the
rst report on the fabrication of ZnO nanowire arrays and
their application in sensitized solar cells,
119
dye-sensitized
and quantum dot sensitized solar cells based on ZnO NSAs
have been widely studied. The similar NSAs of rutile and
anatase TiO
2
were fabricated as AR photoanodes in sensitized
solar cells, including rutile TiO
2
nanowires and anatase TiO
2
nanotubes and nanowires. For solar cells based on ZnO
nanoarrays or TiO
2
nanoarrays, there is a big problem i.e. lack
of enough surface area for dye adsorption and photo-
absorption. Some novel nanoarray structures, such as multi-
layer ZnO nanorod arrays,
120
ultralong TiO
2
nanotubes,
121
and
ZnO nanoforest,
122
have been fabricated to improve the pho-
toabsorption, but the photo-conversion efficiency remains to
be increased. In the following research, the study on hierar-
chical nanoarrays and nanoarray–nanoparticle complex
structures with good AR properties, large surface area, and
good conductivity, should be emphasized to improve the
efficiency of liquid-junction solar cells.
5.2 Light emitting diodes
Light extraction efficiency is a very important criterion for the
performance of LEDs. The interface between air and the
conductive layers or LED materials largely reduces the light
extraction efficiency. Integrating NSA-based AR layers on the
LED devices is an efficient way to reduce the reection at the
interface, thus improving the light extraction efficiency. LEDs
made of III–V semiconductors are the most widely studied, and
the nanoarray-based AR coatings usually use stable materials
with matched refractive index. A six-layer coating made entirely
of ITO was fabricated by Kim et al. on GaInN LEDs via oblique-
angle deposition, with each layer having a tailored refractive
index for optimum AR performance.
123
The light extraction
efficiency of the GaInN LEDs with the ITO AR coating achieved
an enhancement of 24.3% over LEDs with conventional ITO
contact. Light extraction efficiency of GaN-based green LEDs
with antireective non-periodic and periodic SWSs shows an
increased light output power of the LEDs by 18% and 39% at a
20 mA input current, respectively, compared to LEDs with at
ITO layers.
124
Periodic ITO SWSs own a larger enhancement of
output power because of their smoother graded refractive index
proles. Since ZnO and GaN materials have similar refractive
indices and lattice matching, ZnO nanoarrays have been
adopted as AR coatings to enhance the light extraction effi-
ciency of GaN-based LEDs. GaN LEDs with ZnO nanorod arrays
grown on the top layer using catalyst-free metal organic vapor
phase epitaxy showed an increase up to 50% and 100% at
applied currents of 20 and 50 mA, compared to conventional
GaN LEDs.
125
Then, periodic aluminum-doped zinc oxide (AZO)
subwavelength nanostructure arrays fabricated on the surface
of indium tin oxide electrodes of LEDs improved the light
output power by 19% at 100 mA.
126
GaN-based LEDs with
articial compound eye structures using SiN
x
, consisting of
microlens arrays and antireective SWSs, exhibited a light
extract efficiency enhancement of 93%, compared to convent
ional GaN LEDs with a at surface.
127
Besides, silica cone arrays directly etched on the opposite
side of an ITO coated fused silica substrate can dramatically
reduce the reection loss of the white organic light-emitting
devices, resulting in a luminance efficiency increase by a factor
of 1.4 compared to that of the device using a at silica
substrate.
128
Recently, Ho et al. fabricated polycarbonate nano-
structure lms nanoimprinted by density-graded nanoporous
silicon, which reduce the reectivity from 10.2% to 4.8% in the
visible wavelength region (Fig. 14a).
8
Aer attaching on the
display panel to reduce the light reection on the substrate, the
brightness enhancement and decrease of ambient light reec-
tion were observed (Fig. 14b), and the apparently luminous
enhancement was observed for RGB colors at wide incident
angles (Fig. 14c).
It has been demonstrated that reducing the reection
between the interface of air and LED surfaces plays an
Fig. 13 (a) Schematic illustration of fabrication procedures for ultra-
thin (6mm), black silicon solar microcells, where density-graded
surface nanostructures were incorporated on their front surface
through silver-catalyzed wet chemical etching. (b) Representative
current density (J)–voltage (V) curves of an individual black silicon
microcell measured under an AM 1.5 G standard solar spectrum with
and without a diffuse backside reflector (BSR) together with the J–V
curves from microcells with only bare silicon and with bare silicon
covered by a conventional single-layer ARC.
116
Reproduced with
permission from ref. 116, Copyright 2014, AIP Publishing LLC.
This journal is © The Royal Society of Chemistry 2015 Mater. Horiz.,2015,2,37–53 | 49
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important role in improving the light extraction efficiency.
However, much effort is needed to realize the wide applications
of the NSA-based AR coatings on LEDs. The design of optimized
device structures, the choice of suitable antireective materials,
and the controllable fabrication methods are key issues.
Besides, the stability of the material and the interface also has
to be considered.
5.3 Detection and imaging
In addition to solar cells and LEDs, NSAs can be applied in
detection and imaging, such as matrix assisted laser desorp-
tion/ionization mass spectrometry (MALDI-MS), photodetec-
tors, uorescence sensing, surface enhanced Raman scattering
(SERS), and high contrast imaging to improve the sensibility
and detection level. Particularly, MALDI-MS is an important
method in the analysis of biological and synthetic macromole-
cules. However, it is difficult to analyze low weight molecules by
using MALDI-MS due to intense chemical noise from the
matrix. High photoabsorption has been considered to play a
vital role in leading to a successful desorption/ionization of
analytes for a MALDI substrate. The NSA-based AR coatings
could be applied to improve MALDI performance because of the
low photo reection. For example, Wang et al. reported the
MALDI-MS analysis of small molecules by using a biomimetic
antireective silicon nanocone array.
10
The desorption/ioniza-
tion process was more efficient owing to the increase in the
analyte ion signal and decrease in the silicon cluster ion signal
on the Si nanocone array, because the absorbed laser energy was
more effectively channeled into analyte ion production rather
than silicon cluster ejection. The optimized Si nanocone array
showed excellent performance in detecting small molecules,
including peptides, amino acids, drug molecules, and carbo-
hydrates, with little or no interference in the low mass range.
Reducing light reection can increase light absorption of
photodetectors, thus improving the responsibility. NSAs with
broadband and omnidirectional AR performance would endow
the photodetectors with broadband and high-responsibility
properties. Tsai et al. developed the high-responsibility broad-
band Si-based photodetectors using ZnO nanorod arrays grown
by a low-temperature hydrothermal method.
129
The ZnO NRAs
can effectively absorb the photons in the UV region, and provide
RI matching between Si and air for the long-wavelength region,
leading to 3 and 2 orders of magnitude increase in the
responsibility of Si metal–semiconductor–metal photodetectors
in the UV and visible/NIR regions, respectively.
Moreover, antireective nanostructure arrays could be
employed to construct biophotonic surfaces for highly sensitive
optical biosensing and bioimaging. Recently, Oh et al. demon-
strated novel biologically inspired biophotonic surfaces with
self-antireection for highly sensitive uorescence detection
and SERS as well as high contrast optical imaging.
97
The glass
nanopillar arrays with 0.5 ll factor contributed to self-anti-
reection for various media with different RIs, which could
reduce optical mismatches in a broadband visible range by
spatial averaging of RI. Thus, the nanopillar arrays improved
both the excitation and collection of light, leading to substantial
increase of SERS and uorescence signals by about 20 percent.
In addition, the spontaneous RI modulation of the nanopillar
arrays could effectively remove the reection at the solution–
substrate interface and provide exceptionally high contrast
images.
6. Conclusions and perspectives
Many effective fabrication techniques have been developed to
construct high-performance bio-inspired NSA-based AR coat-
ings of various materials. First, dry etching by reactive ion
etching can produce AR surfaces based on both aperiodic and
periodic NSAs of Si, III–V compounds, and SiO
2
with or without
an etching mask. Second, an electroless wet chemical etching
method is very useful and simple for fabricating NSA-based AR
surfaces of single-crystalline, poly-crystalline, and amorphous
silicon with or without patterned metal catalysts. Third, solu-
tion growth has been widely employed to construct NSA-based
AR surfaces of metal oxides, such as ZnO and TiO
2
. Fourth,
nanoimprinting is a scalable and low-cost method to fabricate
polymer NSA-based AR surfaces using articial or natural
templates. Fih, CVD growth is frequently used to fabricate
vertically aligned carbon nanostructures including carbon
nanotubes and nanowalls. Additionally, two or more methods
can be combined to construct hierarchical NSA-based AR coat-
ings. These synthesized NSA-based AR surfaces have exhibited
excellent AR properties (e.g., broadband, omnidirectional, and
polarization-insensitive antireectivity) together with multi-
functions, and they have shown promising applications in a
wide range of elds including solar cells, LEDs, detection, and
optical imaging.
However, there are still some challenges in the fabrication
and practical applications of the NSA-based AR coatings. First,
the development of the cost-effective and large-scale fabrication
methods should keep pace with the theoretical calculation and
structure design, especially for the ideal AR structures. Second,
Fig. 14 (a) Reflectance spectra of a patterned thin film with different
average heights. Images of single color frame under dark ambient (b)
and luminous enhancement (c) of a commercial OLED panel with and
without polycarbonate AR film attachment.
8
Reproduced with
permission from ref. 8, Copyright 2013, Optical Society of America.
50 |Mater. Horiz.,2015,2,37–53 This journal is © The Royal Society of Chemistry 2015
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the NSA-based AR coatings should have chemical, thermal, and
mechanical stability for long-term usage in various environ-
ments. That is, the NSAs should have the ability to resist acid or
alkaline corrosion, temperature-change induced stress or
morphology change, and high mechanical strength and abra-
sion resistance without reducing the AR performance. As for the
heterogeneous interface of the AR coatings and substrates, the
refractive indices of each material should match with each
other, and the NSA-based AR coatings should tightly adhere to
the surfaces of the substrates. Third, the design and fabrication
of multifunctional coatings with excellent AR performance in
combination with other useful functions (e.g., self-cleaning
properties) should be highly efficient, durable and cost effec-
tive. Therefore, in the near future, the research in the area of
NSA-based AR coatings may be focused on the following
aspects. First, ideal nanostructure arrays made of suitable
materials should be theoretically designed as effective AR
surfaces or coatings in useful devices with specic require-
ments. Second, it is important to develop facile, low-cost, and
large-scale fabrication methods to construct the ideal NSAs;
moreover, the fabrication methods should be compatible with
the current device production process. Third, for practical long-
term applications, the investigation of the mechanical stability
and abrasion resistance of the NSA-based AR coatings should be
paid more attention. Fourth, the AR coatings together with the
modules should be easily integrated into a complete device that
exhibits the same AR performance. Fih, high-performance AR
surfaces based on novel AR mechanisms, such as the emerging
plasmonic AR surfaces, can be rationally designed and devel-
oped. Besides, the development of new strategies to fabricate
exible NSA-based surfaces with excellent AR performance and
high stability is worthy of consideration due to the fast evolu-
tion of new exible optical or optoelectronic devices. Moreover,
multi-functional smart surfaces with excellent AR properties
would attract increasing interest owing to their promising
applications in practical devices. Last but not the least, nature
has given us much inspiration on the design and fabrication of
the high-performance antireective structures. The ndings in
the future research on biological surfaces might provide new
concepts and ideas for the development of high-performance
bio-inspired antireective surfaces.
Acknowledgements
This work was supported by NSFC (grant no. 21173010,
21473004, and 51121091), MOST (grant no. 2013CB932601),
China Academy of Engineering Physics (item no. TP201302-3),
and the Fundamental Application Research of the Department
of Science and Technology of Sichuan Province (grant no.
2014JY0137).
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