Perspective
pubs.acs.org/cm
Materials Chemistry in 3D Templates for Functional Photonics
Paul V. Braun*
Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, Beckman Institute for Advanced
Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
ABSTRACT: This Perspective overviews many of the
developments in templated porous three-dimensional photonics, with a particular focus on functional architectures, and
provides suggestions for future opportunities for research. A
significant diversity of 3D structures is available today with
characteristic dimensions appropriate for providing strong
light−matter interactions, in no small part due to recent
advances in 3D patterning techniques. However, the optical
functionality of these structures has generally remained limited.
Advances in materials chemistry have the opportunity to
dramatically increase the function of templated 3D photonics,
and a few examples of highly functional templated 3D
photonics for sensing, solar energy harvesting, optical
metamaterials, and light emission are presented as first
examples of success.
KEYWORDS: photonic crystal, photonic band gap, self-assembly, three-dimensional, optoelectronics, metamaterials
benefits.12 Real and meaningful advances in materials
chemistry, which yield device quality materials when performed
within complex 3D templates, would provide a substantive
advance in the state-of-the-art in functional 3D photonics for a
number of these applications.
For the purposes of this Perspective, I take a rather narrow
definition of “functional photonics” to include structures or
devices which enable energy transduction between photons and
electrons, significantly narrow thermal emission, enhance
photocatalysis, enable chemical sensing, or enable manipulation
of light over small dimensions. I will not discuss passive optical
elements, which function primarily as dielectric stacks, optical
filters, or any similar element, unless the form and function is
considerably different than dielectric lenses and other
comparable optical elements. The rationale for not including
these passive optical elements is that, aside from transformation
optics, most three-dimensionally templated solids are likely to
have greater losses due to optical scattering than traditional
dielectric elements (e.g., lenses and dielectric stacks) formed via
conventional routes.
Recently, there have been several exciting developments in
templated 3D functional photonics that demonstrate templating can be used to form optically functional structures, which
may provide inspiration for additional research. Via hydrogen-
1. INTRODUCTION
Over the past 20 years, the diversity of templated porous threedimensional photonics has exploded, as has the diversity of
functional photonics (e.g., light-emitting diodes (LEDs), solar
cells, optical modulators, optical detectors); however, the
intersection of these fields has remained rather surprisingly
small. In this Perspective, I discuss some of the recent advances
in templated photonics, why perhaps the number of functional
3D photonic systems has remained limited, and opportunities
for the future. Fundamentally, the limitation in functional 3D
systems appears to be very much due to limitations in materials
chemistry. Most of the materials chemistries which operate
within a 3D template do not yield what I define as “functional”
materials. Rather, with only a few exceptions, they yield
polycrystalline or amorphous, often highly defective, materials
that are not acceptable for high performance optics. While this
may be acceptable for some fields, e.g., energy storage,1,2 this is
not acceptable for the field of optics, where the materials often
need to be optoelectronically active and both the real and
imaginary parts of the refractive index need to be precisely
controlled. Many 3D photonic crystal devices have been
proposed, including zero-threshold lasers,3,4 low-loss waveguides,5−7 high-efficiency light-emitting diodes (LEDs), and
solar cells,8−10 but such devices have generally not been realized
because of material limitations. Exciting concepts in metamaterials, including negative refraction and cloaking, could be
made practical using 3D structures formed from high quality
metals or, as has been suggested, entirely new materials to
minimize optical losses.11 Metamaterial devices which incorporate electrically pumped gain elements to balance the
inherent optical loss of such devices would provide additional
© 2013 American Chemical Society
Special Issue: Celebrating Twenty-Five Years of Chemistry of
Materials
Received: July 13, 2013
Revised: October 1, 2013
Published: October 11, 2013
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plasma passivation, chemical vapor grown (CVD) polycrystalline silicon inverse opals were formed with intrinsic properties
similar to conventional polycrystalline silicon, and both p- and
n-type doping was demonstrated.13,14 Growth of single crystal
GaAs 15 and GaN 16 within a 3D template has been
demonstrated. For the GaAs system, modulation of electrically
pumped light emission by the photonic band structure of the
structure was observed. The colloidal crystal templated growth
of 3D structures with significant thermal emission spectral
narrowing has been proposed17 and fabricated.18−20 In the field
of sensors, a number of systems have been demonstrated21,22
and may represent the most mature of the functional photonic
systems. Competing with templating methods to form 3D
photonic structures are semiconductor processing methods
such as deep reactive ion etching, which have been used to form
3D photonic crystals.23 Such strategies have the advantage that
they start with a high quality semiconductor grade substrate,
but the number of symmetries that can be formed is limited, as
is the thickness of the resultant 3D structure.
Figure 2. SEM images of templates for 3D photonic structures. (a)
Self-assembled silica opal (Reprinted with permission from ref 41.
Copyright 2001 Nature Publishing Group.); (b) polymer woodpile
formed by direct ink writing (Reprinted with permission from ref 42.
Copyright 2004 Nature Publishing Group.); (c) polymer woodpile
formed by laser direct writing, schematic of the body-centered
tetragonal (bct) (left) and face-centered cubic (fcc) (right) unit cell is
shown in the inset (Reprinted with permission from ref 43. Copyright
2012 The Optical Society.); (d) top and side views of a polymer
photonic crystal formed via multibeam interference lithography
(Reprinted with permission from ref 44. Copyright 2009 Royal
Society of Chemistry.).
2. TEMPLATING
The inherent value of templating is to decouple patterning
strategies from the final material. For two-dimensional systems,
direct lithographic patterning of functional materials is
common, and the demand for templating strategies is limited.
However, lithographic patterning of materials on the nanometer to micrometer scale in three-dimensions is quite
challenging, in particular for structures with complex or broken
symmetry, and there is a limited subset of available materials
which can be effectively patterned. Templating strategies enable
the three-dimensional structure to be formed in the most
patternable material. Then, the structure of this template is
imprinted into a functional but difficult to directly pattern
second material (Figure 1).
assembly of silica or polystyrene colloids or by the
aforementioned laser or ink direct writing of polymers.
The structural quality of readily available 3D templates has
now become quite impressive. Via direct laser writing (DLW),
nearly completely arbitrary 3D structures containing minimum
feature sizes as small as ∼100 nm have been formed using
advanced stimulated emission depletion (STED)-based methods (Figure 3), and slightly larger features can be obtained on
Figure 1. Schematic of the templating process, in this case starting
with a self-assembled colloidal crystal, which is filled with a secondary
material (green), followed by template removal, providing a final
structure which is the inverse of the starting template.
Figure 3. Oblique-view SEM image of a 52 layer woodpile photonic
crystal with a lattice constant of 350 nm fabricated with STED-DLW.
Reprinted with permission from ref 46. Copyright 2011 The Optical
Society.
The most common templates for 3D photonic structures are
provided by colloidal self-assembly silica or polymer opals,24−29
direct laser writing of photoresist,30−32 direct ink writing,32,33
and multibeam (holographic) interference lithography,34−37
although other templates are certainly possible (Figure 2). In
the aggregate, these template generation methods provide
extraordinary structural diversity but with a rather limited
materials diversity. The template materials available consist
mostly of polymers (photoresists and common polymers such
as polystyrene), the ceramics titania and silica, and isolated
metals. While there are examples of other template materials
including carbon colloids38 and photopatternable chalcogenide
glasses,39,40 the quality and structural regularity of these
templates is not nearly as good as those formed by self-
commercial systems.32 Via interference lithography, large-area
periodic 3D structures with features on the order of 200 nm
have been formed by a number of groups,35−37 although it is
not yet clear if the minimum feature size provided by
interference lithography can be as small as that provided by
direct laser writing with STED, and formation of aperiodic
structures by interference lithography is challenging. Colloidal
templating also enables features with quite small dimensions
(on the order of 10% of the colloid diameter),29 but there are
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deposition strategies based on chemical vapor deposition
(CVD), which was first demonstrated through the growth of
Si 3D photonic crystals using a colloidal crystal template,41,49
and the related technique of atomic layer deposition (ALD).50
The diversity of material which can now be grown via CVD and
ALD is impressive, including many metals, polymers, semiconductors, and oxides, and is continually increasing.50,51 Both
CVD and ALD have two similar challenges. Because the growth
is conformal, any 3D structure which contains a narrow throat
cannot be fully filled. As material deposits, the channels
required for the precursors to diffuse into the structure simply
pinch-off.52 The second challenge is that the deposited material
is polycrystalline or amorphous, which significantly reduces the
possibility that the material can be optoelectronically active.
Under the right conditions, both ALD and CVD can deposit
epitaxial thin films but only on single crystal or highly textured
substrates. However, since the initial 3D templates currently
available are polycrystalline or amorphous, the resulting
deposited film is also polycrystalline or amorphous. I do not,
however, want to discount the importance of ALD and CVD.
Both techniques are being increasingly used due to the many
optically interesting materials which can be formed, potential
high purity of the deposited materials, and very good control
over the thickness of the deposited layer. For a few systems, it
has been shown that a templated polycrystalline material can
have optoelectronic properties which approach that of a
conventional polycrystalline thin film.13,14,53
CVD can be used to grow an epitaxial thin film on a single
crystal substrate, in which case the process is often termed
metal−organic vapor-phase epitaxy (MOVPE) or metal organic
CVD (MOCVD).54 MOCVD is widely used on the semiconductor growth industry, and its importance for making
semiconductor devices (e.g., semiconductor lasers based on
GaAs) cannot be overstated. Until recently, epitaxial MOCVD
had not been used in conjunction with a 3D template. Two
reports15,16 now indicate the possibility of using epitaxial
MOCVD as a bottom-up template infilling method and will be
discussed in greater detail in Section 3 of this Perspective.
A highly versatile generally bottom-up infilling method is
electrodeposition (if the template is conductive, the growth can
be conformal rather than bottom-up). This approach can be
used to infill templates with almost any material that can be
electroplated, which includes many metals and semiconductors,
and a few conductive oxides and polymers.19,20,26,36,55,56 Figure
5 shows a schematic of the electrochemical process for
templating the growth of CdSe with a self-assembled colloidal
crystal, as well as SEM images of colloidal crystal templated
CdS and Ni structures after removal of the colloidal crystal
template. A challenge in templated electrodeposition is that the
template needs to be on top of a conducting substrate which
limits some of the optical template fabrication methods (e.g.,
holography on a reflective substrate is challenging), and the
growth front of the electrochemically deposited material must
remain planar which can require the use of complicated pulse
sequences.20,45
The most trivial method to completely infill a template is to
simply fill the template with a monomer solution which is
subsequently polymerized. Then, if desired, the colloidal
particles are removed. Despite the simplicity, this remains a
powerful approach for forming chemical and pressure
sensors,21,22,57 as it enables the direct patterning of a soft
responsive polymer into a 3D optically active photonic crystal.
If the dimensions or refractive index of the structure changes in
some limitations. It is difficult to crystallize colloids into
anything except a face-centered-cubic structure, colloidal
templates always contain some defects, and the defect density
tends to increase significantly as the colloid diameter decreases
below ∼250 nm. Templates can also be created via electrochemical etching of silicon or aluminum, which provides a
disordered pore network in the x−y plane, and the possibility of
modulating the pore size and volume fraction in the z-direction.
However, more complex highly ordered structures are
challenging.45 As expected, there is a trade-off between
complexity and template fabrication speed. Serial techniques
such as direct laser writing have fabrication rates limited by the
available laser power and beam rastering optics. Holographic
strategies can pattern larger areas in a single laser exposure but
with limitations on the structural complexity. The self-assembly
methods can cover nearly limitless areas but only with a small
set of crystal symmetries, and as the rate of self-assembly
increases, the level of random defects also tends to increase.
The set of materials chemistries available to be templated still
remains limited. Although a diversity of materials can be
deposited as thin films via gas-phase techniques, many of the
deposition methods do not effectively deposit material deep
within a three-dimensional template. Any technique that
deposits material line-of-sight (e.g., sputtering or molecular
beam epitaxy) will only significantly coat the top or top few
layers of the template. Any technique in which the volume of
the material changes significantly during materials deposition,
for example, sol−gel-based template infilling strategies, will
have limited use for photonic structures due to the defects
formed by the volume change. Nanoparticle infilling has been
effectively used to imbed fluorescent emitters within 3D
photonic crystal crystals,47 but this approach is only somewhat
effective to form 3D templated structures (Figure 4).48 Even
Figure 4. SEM cross-section of a Ge nanoparticle inverse opal. The
spaces between the nanoparticles have been filled with polymer to
increase the effective refractive index of the inverse opal to 2.05. The
glass substrate is at the bottom of the image. Reprinted with
permission from ref 48. Copyright 2007 American Chemical Society.
when the nanoparticles fully fill the template, the resulting
structure suffers from a lower than might be expected effective
refractive index due to the maximum volume fraction that can
be occupied by packed particles (there is still considerable void
volume between the nanoparticles),48 and the presence of a
very high density of interfaces due to the large number of
nanoparticle−nanoparticle contacts limits the possibility of
good electrical performance.
The most commonly applied approaches for template
infilling are the highly successful gas-phase conformal
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Figure 5. (a) Schematic of the conditions used to electrodeposit CdSe
inside a self-assembled colloidal crystal. Reprinted with permission
from ref 26. Copyright 1999 Nature Publishing Group. (b)
Electrodeposited CdS inverse opal templated by a silica colloidal
crystal. Reprinted with permission from ref 26. Copyright 1999 Nature
Publishing Group. The colloidal crystal has only been partially
removed. (c) Ni inverse opal formed by electrodeposition through a
colloidal crystal. After removal of the colloidal template, the Ni
structure was electropolished to increase the structural openness.
Reprinted with permission from ref 20. Copyright 2007 John Wiley &
Sons, Inc.
Figure 6. (a) Optical micrograph of a glucose responsive polymer
inverse opal. The lines are cracks in the structure that were present in
the initial colloidal template. (b) SEM image of the top surface of the
polymer inverse opal. Reprinted with permission from ref 22.
Copyright 2004 American Chemical Society.
the presence of an analyte or the photonic crystal is
mechanically deformed, the resulting change in the lattice
constant of the photonic crystal can be read out optically. In
one example, we formed a glucose sensor using this approach
(Figure 6). Here, the polymer was functionalized with a
boronic acid which, when it bonds with glucose, changes its
state of charge, which in turn results in swelling of the polymer
with water. Because the structure is porous, glucose can diffuse
rapidly into the interior of the photonic crystal, which is
important if a fast response (order of minutes) is required.
While not the subject of this Perspective, the reader is also
encouraged to investigate the subject of polymerized colloidal
crystal arrays (PCCAs) if interested in stimuli-responsive
photonic crystals.58−61 PCCAs consist of a 3D nontouching
periodic array of colloidal particles suspended in a polymerized
matrix. When the matrix expands or contracts, the wavelength
of light shifts accordingly, providing a facile read-out
mechanism.
It is particularly important to understand the chemistry of the
template. The template must withstand the conditions required
for infilling (e.g., the high temperatures often required for CVD
or the often corrosive conditions of an electroplating bath), and
generally, it is desirable for there to remain a path to remove
the template. Except for colloidal-based templates, which can be
formed from thermally and chemically stable materials such as
silica or carbon, and a few of the direct ink written templates,
which can be formed from metals or ceramics, the majority of
the templates are based on polymers (e.g., photoresists), which
are not thermally stable and have limited chemical stability. A
common approach to overcome this apparent shortcoming is a
double templating (double inversion) strategy.37,40,62 In this
strategy, the template is used to pattern a thermally and/or
chemically robust material which can be formed at lower
temperatures, for example, CVD or ALD grown silica or
alumina. Then, the template is removed, often via oxygen
plasma, leaving behind an inverse of the template. Now, this
new and more robust template is used to impart a structure into
a material which can only be formed at high temperatures. This
double inversion strategy has been used to convert structures
formed by direct ink writing,62,63 laser direct writing,64 and
optical interference lithography37 into semiconductors such as
silicon (Figures 7 and 8). The product of the double inversion
is a direct replica of the starting structure.
Figure 7. SEM images of a silicon woodpile fabricated using DLW and
double-inversion. (a) Top view. (b) FIB cross-section. (c) and (d)
compare the double inverted Si woodpile and the original SU-8
woodpile and show that both the structure and low surface roughness
are maintained. Reprinted with permission from ref 64. Copyright
2006 John Wiley & Sons, Inc.
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dimensions of the polymer change as the glucose concentration
changes due to a change in the water volume fraction in the
polymer.21,22 Although the field of polymer inverse opals is
reasonably mature, greater challenges exist when deterministic
polymeric templates are used (e.g., via interference lithography
or direct laser writing), as these templates are often cross-linked
by the writing process, making it challenging to remove them
without degrading the templated polymer. Development of
photoresists appropriate for 3D patterning that can be removed
with solvent (the best resists today for 3D patterning are crosslinked and insoluble) or the double inverse process could be
used to overcome these issues.
There have been a number of promising reports on
templated 3D photonic structures for solar energy harvesting.
There are now examples of dye sensitized solar cells where the
templated structure provides both the porosity required for
electrolyte transport and enhancements in the photocollection
efficiency (Figure 9).65 There has been a steady interest in
Figure 8. (a) Schematic illustration (inset is a low-magnification
scanning electron microscopy (SEM) image of a 250 μm × 250 μm
woodpile structure) and (b) SEM image of direct-write-assembled
polymer woodpile structure. Polymer woodpile is a 16-layer structure.
(c) Process sequence for templated assembly of such structures. (d)
Low- and (e) high-magnification SEM images of a Si/SiO2/Si hollowwoodpile structure after focused ion beam milling (8-layer structure).
Contrast is enhanced in the inset in (e) to reveal the trilayer Si/SiO2/
Si tube wall. Reprinted with permission from ref 62. Copyright 2006
John Wiley & Sons, Inc.
Figure 9. (a) Schematic of a DSSC, with a porous silicon photonic
crystal attached to the back side of the light harvesting layer. The light
enters from the top of the structure. (b) Cross-sectional SEM image of
the structure. The titania nanoparticle layer is at the top and the
photonic crystal at the bottom. Panel b is reprinted with permission
from ref 65. Copyright 2011 John Wiley & Sons, Inc.
3. FUNCTIONAL TEMPLATED PHOTONICS:
STATE-OF-THE-ART
Despite the great strides that have been made in template-based
fabrication methods for 3D periodic and aperiodic structures,
the number of functional photonics (by my narrow definition)
is rather small. The primary reason is not due to limitations in
template fabrication. After all, via direct laser writing, 3D
structures with ∼100 nm features are possible, via holographic
methods, patterning of large areas with similarly high resolution
features is possible, and self-assembly of submicrometer
colloidal particles is practiced by many groups. Rather, the
fundamental stumbling block is how to fill templates with
functional materials. As a result, the number of publications on
functional photonics has been rather limited, and the current
state-of-the-art is limited to a small, yet promising, set of
examples which are highlighted here.
As previously mentioned, the most mature of the templated
functional photonics is in the area of polymer-based sensors.
There are many examples of chemically responsive polymers,
and as long as the constituent monomers, or the polymer itself,
can be infilled into a 3D template, there exists the possibility to
form a photonic-based sensor. Of course, the polymer must
respond in such a way to enable an optical signal. This could be
either a change in its optical constants or a change in dimension
of the polymer, both of which will result in a movement of the
diffraction peak generated by the 3D structure. In the glucose
sensor example, both the refractive index and the physical
thermophotovoltaic (TPV) systems based on 3D metallodielectric structures,17−19,66 and as discussed, templating is a
particularly effective way to realize these structures (Figure
10).66 It remains to be seen, however, whether structures with
the combination of the required optical and thermal properties
can be realized. The required operating temperatures may be as
high as 2000 K, and the best structures today are only stable to
about 1500−1700 K.66 There is also considerable work on 2D
TPV structures,67 and it remains to be determined whether the
potentially enhanced optical response of the 3D structures over
a 2D structure will outweigh the required additional complexity
of a 3D system. This will very much depend on the materials
which can be templated into 3D structures that meet the
required thermal and optical requirements. Both the dye
sensitized and thermophotovoltaic solar systems are rather
interesting, if only because unlike many other functional
photonics (e.g., LEDs and lasers), where semiconductor-grade
material is required, it is acceptable for the material comprising
the photonic crystal to be rather defective. In fact, defects, e.g.,
polycrystallinity and dopants, may actually serve to enhance the
optical properties and thermal stability of some materials.
The most sophisticated of the functional photonics published
to date have been formed by epitaxial MOCVD.15,16 These are
the only examples to date of the templated growth of a single
crystal, and in one case, even electrically pumped light emission
was observed.15 Both examples rely on what is termed selective
area epitaxy (SAE).54 Initially, it may not seem possible to grow
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Figure 11. Schematic of the fabrication of a single crystal GaN logpile
photonic crystal from a Si/SiO2 logpile template. Reprinted with
permission from ref 16. Copyright 2011 American Chemical Society.
materials which not only have the potential for optoelectronic
activity but also can be grown using processes compatible with
forming optoelectronically active devices. Epitaxial MOCVD is
particularly attractive in this regard. Without removing the
template from the growth chamber, it is possible to deposit not
only p- and n-type doped materials but also materials with
different bandgaps which can serve to trap or guide electron−
hole pairs to enable efficient electrically stimulated photoemission. Figure 13 shows a schematic of a device structure
formed via this process, as well as images of the electrically
pumped emission and the spectra of the emission under various
conditions.
Figure 10. (a) SEM micrograph and (b) reflectance properties of
tungsten (W) inverse colloidal crystal after annealing to 1000 °C for
12 h. The red spectrum is the measured reflectance, and the dashed
red spectrum is the reflectance calculated using the FDTD method.
Reprinted with permission from ref 66. Copyright 2013 Nature
Publishing Group.
4. NEAR-TERM OPPORTUNITIES
Some of the near-term materials opportunities for templated
functional photonics are rather obvious and are reasonably
linear extensions of the current examples in the areas of sensors,
solar energy harvesting, and light emitting structures. Clearly,
sensors with increased structural and chemical sophistication
can be formed via templating. Any material which can be filled
as a monomer solution into a 3D template and subsequently
polymerized can in principle be templated, and the number of
templates available is continually increasing. For the templated
architecture to be effective as an optically responsive sensor, it
must respond by either changing dimensions or changing its
optical constants. In the sensing arena, the key question to ask
is if the design will be more effective than a simple 1D or 2D
photonic structure, 3D photonic sensors formed from
polymerized colloidal crystal arrays,61 or an already commercially available sensor. If the answer to all parts of this question
is yes, there are few if any fundamental issues preventing
template-based sensor fabrication methodologies. I also expect
there will be continuing improvements in templated
architectures for solar energy harvesting for both DSSCs and
TPVs. For DSSCs, research on optimization of the photonic
response, including through the incorporation of new absorber
materials,68 while maintaining the required ion and/or chargecarrier transport characteristics, continues, and for TPV-based
solar cells, structures designed for optimized optical properties
could certainly be applied using the current templating
methodologies. However, for both DSSC and TPV systems,
there are a number of critical needs that are not as simple as
a semiconductor from the gas phase starting at the substrate−
template interface. After all, this requires the semiconductor
precursors to diffuse through the thickness of the template
without reacting with the template and then efficiently nucleate
and grow in an epitaxial fashion on the underlying template. In
fact, in a somewhat modified way (using a much simpler
template), this is exactly what is already performed in the
semiconductor industry, where an oxide stripe is patterned on a
semiconductor substrate and then the growth conditions are set
such that the growing semiconductor only nucleates and grows
on the exposed semiconductor and not on the oxide stripe.54
Under the appropriate conditions, many CVD reactants will
only nucleate on specific substrates and will not nucleate on an
oxide even after multiple hours in the CVD reactor. This is
exactly how the growth of both GaAs and GaN single crystals
was performed. The GaN 3D photonic crystal was grown on a
GaN substrate, and the GaAs photonic crystal was grown on a
GaAs substrate. An example of the process flow for the GaN
system is shown in Figure 11. A similar process was used for
growing the GaAs system, although the process involved a
number of additional steps since both p- and n-type GaAs was
grown, as well as an InGaAs quantum well layer, an AlGaAs
capping layer to minimize surface recombination, and the
necessary electrodes for driving light emission. Images of the
GaAs structure at various steps in the process and the final GaN
structure are shown in Figure 12.
Function is more than just structure. For many applications,
function requires the conversion of electrical energy into light
or the conversion of light into electrical energy. This requires
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Figure 12. (a) Schematic and SEM of a 3D photonic crystal template partially filled with single crystal GaAs by epitaxy. Inset highlights the bottomup growth of the GaAs. Schematic and focused ion-beam cross-section SEM images of (b) GaAs filled template and (c) inverted (template removed)
GaAs structure. Panels a−c reprinted with permission from ref 15. Copyright 2011 Nature Publishing Group. (d) Cross-section TEM image of a five
layer single crystal GaN logpile structure. Inset on the right-hand side shows an electron diffraction pattern from the region indicated by the dotted
red circle, and the left-hand side inset shows the diffraction pattern from the white dotted circle region. (e) Cross-section SEM image of a nine layer
GaN logpile. The top right-hand side image shows an enlarged top view, and the bottom image shows an enlarged section of the perspective view of
regions indicated by the dotted black box. Panels d and e reprinted with permission from ref 16. Copyright 2011 American Chemical Society.
Figure 13. (a) Schematic of a GaAs 3D photonic crystal (blue) containing an InGaAs light-emitting layer (red). The structure is lithographically
patterned into the form of a cylindrical mesa with a ring electrode on the top surface (gold). (b) An optical micrograph of a device under white light
illumination showing the Ti/Au ring electrode and the mesa surrounded by the etched GaAs. (c−e) Upon current injection (c, 2 mA; d, 4 mA; e, 6
mA) light is emitted. The light output increases with current. (f) Electroluminescence (EL) spectra from 3D photonic crystal (PhC) LEDs with
lattice constants of 735 and 1030 nm, respectively. The shape of the EL spectrum from the 735 nm lattice constant structure does not change when
the pores are filled with dodecane, whereas the EL spectrum from the 1030 nm lattice constant structure changes significantly when the pores are
filled with o-xylene. The EL spectra from the 1030 nm lattice constant structure are shifted up for clarity. (g) EL spectra from a 3D PhC LED where
the emission is not modified by the 3D structure (a = 735 nm). The EL intensity increases linearly with current. Reprinted with permission from ref
15. Copyright 2011 Nature Publishing Group.
structural optimization using current materials, which will be
discussed in the following section. For the epitaxially grown
optoelectronically active 3D structures, the selective area
epitaxy process has been demonstrated for a few semi283
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increase? TPV systems require even more significant materials
developments. A viable TPV material will probably need to
maintain its structure at temperatures well in excess of 1500 K
for months or even years. Today, refractory metals are the most
popular emitter materials for TPV systems, but must this be so?
Are there other refractory compounds which have the necessary
optical properties and better thermal stabilities? If so, can these
materials be filled into a 3D template?
3D optical metamaterials is an area where some work has
been performed to date, but there remain significant
opportunities for advances. Starting from DLW structures,
several groups have now formed chiral optical metamaterials by
coating the structure with metal via electrodeposition.71−74
However, functional 3D metamaterials are quite challenging.
Issues of optical loss are significant, and the required structural
perfection is much greater than that required for 3D dielectric
structures. Pathways to new materials that have acceptable loss,
elements which provide optical gain to compensate loss, and
structures with the required perfection are all necessary but
currently not available. Advanced materials chemistries will play
an important role to realize advanced functional 3D optical
metamaterials.
Finally, functional devices contain many elements. While
development of discrete components is important, real systems
require integration of many elements, and all the processing
steps must be compatible. 3D porous structures are particularly
challenging, as they are not readily compatible with conventional photoresist processing, and are more susceptible to
oxidation and other environmental constraints due to their high
surface area. Characterization can also be challenging, in
particular when the 3D templated material is integrated into a
larger system since the material to be characterized is often
deeply imbedded within the structure.
conductors, but from our experience, GaAs was significantly
easier to epitaxially grow than for example AlGaAs; so,
expanding this list may not be trivial. The AlGaAs tended to
nucleate on the oxide phase, limiting successful epitaxial
template infilling. Expanding the diversity of materials available
through better process controls should be possible. The
number of templates could also be significantly expanded. To
date, only a silica colloidal crystal, a holographically defined
polymer, and a lithographically defined silica template have
been used. No attempts have been made to introduce optically
active defects, e.g., waveguides or optical cavities, nor have the
structures been optimized for optical function. Both these
directions should be possible using direct laser writing template
fabrication strategies.
5. CRITICAL NEEDS AND LONG-TERM
OPPORTUNITIES
For templated functional photonics to grow in significance, a
number of critical needs need to be addressed. There is the very
limited subset of materials which can be templated with defect
densities rivaling conventional electronic materials. Currently,
only two semiconductor materials have been templated in
single crystal form, GaAs and GaN, and only the GaAs structure
was demonstrated to be electrically active. The X-ray pole
figure of the GaAs structure indicated it contained stacking
faults and other defects, and considerable reduction in the
defect density is probably required for practical functional
devices. Significant expansion of the list of materials will also
require major advances. While better process control might
enable growth of some materials using known precursors, e.g.,
AlGaAs, for fast and effective selective area epitaxy through 3D
templates, new precursors, which are particularly robust against
secondary nucleation on the template, will probably be needed.
It is also a question if selective area epitaxy is the best route
for forming large single crystal 3D templated structures. Most
large single crystal semiconductors are formed via crystallization
from the melt (the standard approach for forming single crystal
silicon) and not gas-phase growth, which is generally seen as
more appropriate for fabrication of thin films. Intrinsically then,
might it be possible to directionally solidify a semiconductor
from the melt within a template? The challenges would be
significant. Not only would the template have to withstand the
required temperatures (1414 °C for silicon), but also the
template would need to be chemically stable to the molten
semiconductor. For many applications, there would need to be
a path to remove the template without damaging the
semiconductor. Finally, there is the specific challenge of getting
the semiconductor into the template. Unless the surface energy
of the template is greater than the surface energy of the molten
semiconductor, the required pressures will be significant. While
low melting temperature, low surface tension semiconductors
such as selenium have been imbibed into a colloidal
template,69,70 a similar process using a material such as silicon
would be challenging. Some semiconductors can be directly
electrodeposited into the template,56 but it is not clear if the
semiconductor will remain in the template once it is melted.
For solar energy harvesting, both DSSCs and TPV systems
continue to receive attention, and the efficiency of both may be
enhanced by 3D structuring. For DSSC systems, might
templating enable the current collector and the solar absorber
to be the same material? Or, if the current collector could be
templated in single crystal form, while maintaining the high
surface area required for high dye loading, might the efficiency
6. CONCLUSIONS
Developments in fabrication of templated porous threedimensional structures now enable formation of an exceptional
diversity of structures from a wide subset of materials.
However, the optical functionality of these structures still
remains limited, in large part due to limitations in filling
structures with functional materials. Advances in template
infilling strategies, which provide very low defect density high
purity semiconductors, chemically responsive materials, and
thermally stable materials, are required to advance the optical
functionality of these structures. Advances in template
fabrication, which yield structurally complex templates which
are chemically stable against infilling yet are easily removed, are
also needed. Recent advances in materials chemistry have
enabled a few specific successful examples of highly functional
templated 3D photonics for sensing, solar energy harvesting,
optical metamaterials, and light emission, but with further
materials chemistry developments, there are opportunities for
many additional and significant advances.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: pbraun@illinois.edu.
Notes
The authors declare no competing financial interest.
Biography
Paul V. Braun is the Ivan Racheff Professor of Materials Science and
Engineering at the University of Illinois at Urbana−Champaign and a
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■
ACKNOWLEDGMENTS
The recent work from my group reported in this Perspective
was primarily supported by the US Department of Energy
“Light Material Interactions in Energy Conversion” Energy
Frontier Research Center under Grant DE-SC0001293. I would
also like to acknowledge the many excellent students, postdocs,
and colleagues I have had the opportunity to work with on the
subject of templated 3D photonics.
■
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