jp106961w

Sep 8, 2010 - Present address: Functional and Interactive Polymers, DWI RWTH ... on an underlying substrate via gravity sedimentation of TiO2-coated ...
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J. Phys. Chem. C 2010, 114, 16389–16394

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Three-Dimensional Colloidal Crystal Arrays Exhibiting Stop Band in Near-Infrared Region Mukesh Agrawal,*,† Dieter Fischer,† Smrati Gupta,†,‡ Nikolaos E. Zafeiropoulos,†,§ Andrij Pich,|,⊥ Elefterios Lidorikis,# and Manfred Stamm*,† Leibniz-Institut fu¨r Polymerforschung Dresden e.V., Hohe Strasse-6, 01069, Dresden, Germany, Institut fu¨r Makromolekulare Chemie, Technische UniVersita¨t Dresden, D-01062 Dresden, Germany, and Department of Materials Science & Engineering, UniVersity of Ioannina, 45110 Ioannina, Greece ReceiVed: July 26, 2010; ReVised Manuscript ReceiVed: August 25, 2010

We report on the fabrication of three-dimensional colloidal crystal arrays (CCAs) on an underlying substrate via gravity sedimentation of TiO2-coated polystyrene (PS) colloidal particles. The beauty of the described system lies in the fact that obtained CCAs, for the first time, display a photonic band gap in the near-infrared (NIR) region with as much bandwidth (∆λ/λ) as 54-61%. Interestingly, stop band position and bandwidth have been found to be modulated with structural parameters of building blocks such as particle size and thickness of TiO2 shell, etc. Moreover, no significant change in stop band position was observed with the variation in incidence angle of the light. Theoretical calculations from the simulation studies have been found in agreement with the experimental findings. Introduction 1a

Since the inspiring pioneering work of Yablonovitch and John,1b photonic crystals have garnered ever-increasing scientific interest and emerged as a potentially powerful platform for the fabrication of advanced optoelectronic devices. Photonic crystals are periodic structures that affect the behavior of photons in much the same way that crystalline semiconductors affect the properties of electrons. They have a periodic modulation in their refractive index and diffract the light of wavelengths commensurate to their periodicity. This results in the localization of photons,2 thus providing a mechanism for controlling and inhibiting spontaneous light emission that can be exploited for photonic device fabrication. Owing to the important optical effects of photonic crystals, it is necessary to identify versatile fabrication materials and methods that can accommodate diverse applications. Any method to make photonic crystals that offers control over their optical properties, such as spectral band position or bandwidth (∆λ/λ), is a significant step toward engineering these materials for specific applications. A wide spectrum of strategies has been employed for the fabrication of three-dimensional photonic crystals, including colloidal self-assembly,3 colloidal crystal templating,4 holographic patterning,5 stacking logs of a dielectric material,6 layerby-layer lithography,7 electron beam lithography,8 and nanolithography.9 Among these protocols, a great deal of work has * To whom correspondence should be addressed. E-mail: [email protected] ((M.A.) or [email protected] (M.S.). † Leibniz-Institut fu¨r Polymerforschung Dresden e.V, Hohe Strasse-6, 01069, Dresden, Germany. ‡ Present address: Institut fu¨r Makromolekulare Chemie, Technische Universita¨t Dresden, 01062, Dresden, Germany. § Present address: Department of Materials Science & Engineering, University of Ioannina, Greece. | Institut für Makromolekulare Chemie, Technische Universität Dresden, 01069, Dresden, Germany. ⊥ Present address: Functional and Interactive Polymers, DWI RWTH Aachen University, Pauwelsstr. 8, 52056 Aachen, Germany. # Department of Materials Science & Engineering, University of Ioannina, Greece.

been done on the first two approaches because of their simplicity in ordering a dielectric material in three dimensions. These methods involve the “natural” self-assembly of polymer or silica microspheres from a colloidal suspension into solid, threedimensionally periodic opal structures. In colloidal crystal templating, the voids of the opal are filled with the precursor capable of solidification and then the template particles are removed via thermal treatment, yielding the macroporous inverse opal structures.10 Unfortunately, this approach has been reported to be accompanied by some drawbacks such as infiltrating species deposited predominantly either on the surface or bottom of the colloidal crystal template, which produces severe light scattering.11 In addition, the resulting inverse opal structures are not stable due to poor mechanical strength. In the second approach, colloidal nanospheres hybridized with metal/metal oxide particles are used as building blocks in fabrication of photonic crystals.12 Apart from being free from the abovementioned drawbacks, the use of colloids, in particular core-shell particles, offers some additional advantages: for example, the composition of the core and shell can be controlled. For example, a high refractive index material such as a metal oxide can be coated on the core of the low refractive index material. In addition, shell thickness and hence the final particle size of these building blocks can be easily tuned by controlling the reaction parameters during their synthesis. Modulation of these properties will greatly affect the performance of final CCAs, enabling extensive control over the spectral position and magnitude of the optical stop band. A great deal of work has been done on the fabrication of coated particles, for example, core-shell latex particles with semiconductors,13 dyes incorporated into shell,14 zinc sulfide-coated polystyrene (PS)15 or silica particles,16 and gold-coated silica particles,17 suggesting that such particles would have potential application as building blocks of photonic crystals. From the application point of view, the spectral positions of the stop band and bandwidth are considered as the most crucial properties of a photonic crystal. The motivation of this study was to achieve a wide photonic stop band in the near-infrared

10.1021/jp106961w  2010 American Chemical Society Published on Web 09/08/2010

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(NIR) region. While the position of the stop band depends on the lattice constant (size of building blocks) and the effective average refractive index, bandwidth greatly depends on the refractive index contrast between spheres and medium.18 Most of the research efforts have been focused on the CCAs that exhibit a photonic stop band in the UV-vis region, but there are very few reports that describe the fabrication of CCAs operating in the near-infrared region. In order to achieve the stop band in the NIR region, the array spacing in CCAs should be large enough, but three-dimensional arrangement of the large colloidal particles is far from straightforward. In addition, it is of utmost necessity for the refractive indices to be much larger than those of silica or polymer particles. In this context, semiconductors are considered as the most promising candidate owing to their high refractive index and low absorbance in the NIR region.19 Recently, Caruso and co-workers20 described the fabrication of CCAs composed of polystyrene particles coated with polyelectrolyte layers via a self-assembling process and reported the variation in stop band position in the NIR region with change in size of building blocks. Reese and Asher21 have developed NIR photonic crystals in colloidal solution by employing highly charged PS spheres as building blocks. Kumacheva and co-workers11 reported self-assembly of poly(methyl methacrylate) (PMMA) colloidal particles infiltrated with PbS quantum dots to achieve the stop band in the NIR region. But in all these studies and others reported on photonic crystals, the bandwidth has been found to be less than 40%.22 The bandwidth decides the strength of a photonic crystal. An increase in the bandwidth of a stop band undoubtedly extends the range of applications of the photonic crystal.23 It is still a challenge for the researchers to fabricate CCAs that can exhibit a wide stop band (∆λ/λ > 40%) in the NIR region. In the present study, we report on the fabrication of threedimensional CCAs derived via the self-assembly of TiO2-coated polystyrene colloidal particles. The protocol involves three steps: (1) preparation of 540 nm PS nanospheres; (2) synthesis of complete, smooth, and 100-130 nm thick TiO2 shell of polystyrene beads; and finally (3) three-dimensional assembling of PS-TiO2 core-shell particles into CCAs on quartz by the gravity sedimentation method. Optical measurements of resulting CCAs revealed a wide and deep stop band in the NIR region. In addition, we found that the spectral position of the stop band and bandwidth can be modulated with structural parameters (size and shell thickness) of the employed building blocks. Interestingly, we observed that the prepared photonic crystals are insensitive to the incidence angle of the light in terms of the stop band position. However, obtained CCAs have not been found to have a complete photonic band gap. In order to achieve stop bands in the NIR region, TiO2-coated polystyrene particles with 740-800 nm sizes (100-130 nm TiO2 shell thickness) were prepared by sol-gel process24 and employed as building blocks for the fabrication of photonic crystals. The large particle size of building blocks and the presence of TiO2, which has a remarkably high refractive index (>2.5), as the shell ensure that our CCAs exhibit a wide stop band in the NIR region. PS colloidal particles were prepared by surfactant free emulsion polymerization as described elsewhere,25 and three-dimensional CCAs were grown on quartz substrate by exploiting the gravity sedimentation method.26 Experimental Section Materials. Styrene (ST) (Fluka) and acetoacetoxyethyl methacrylate (AAEM) (97%) (Aldrich) were passed through an inhibitor removal column and then vacuum-distilled under

Agrawal et al. nitrogen before use. Titanium ethoxide (85%) (Acros), sodium peroxydisulfate (SPDS) (97%) (Aldrich), ammonium hydroxide (28-30% NH3 in water) (Acros), and acetic acid (100%) (Merck) were used without additional purification. Distilled water was employed as the polymerization medium. Synthesis of PS Beads. Monodisperse and negatively charged polystyrene particles were synthesized by surfactant free emulsion polymerization.25 In a typical process, 170 g of water, 20 g of styrene, and 1 g of AAEM were introduced into a doublewalled glass reactor equipped with mechanical stirrer, reflux condenser, nitrogen inlet, and temperature controller. The reaction mixture was deoxygenated by passing nitrogen gas through it at room temperature for 30 min along with gentle stirring. Subsequently, the temperature was increased to 70 °C, followed by addition of SPDS solution (0.3 g in 10 g of water) into the reaction mixture to start the polymerization process. The reaction was allowed to proceed for 24 h, and finally polystyrene latex particles were collected with ca. 10% solid content. Synthesis of PS-TiO2 Core-Shell Particles. Variable amounts of titanium ethoxide (0.5-2 mM) were mixed into 10 g of extra-pure ethanol in different reaction sets and the mixture was refluxed at 70 °C for 2 h.24 Subsequently, 1 g of latex containing 10 wt % polystyrene particles was added into the reaction medium, followed by the addition of 4-5 drops of glacial acetic acid as precipitating agent. After reaction for 20 h, hybrid particles were cleaned by three cycles of centrifugation/ redispersion in ethanol and water, respectively, and finally dried in a vacuum oven at 30 °C. Fabrication of Colloidal Crystal Assemblies. To assemble the PS-TiO2 particles into three-dimensional CCAs, a glass substrate without particular pretreatments except for gentle cleaning with water and methanol, respectively, was placed on the bottom of a Petri dish (diameter 5 cm). Thereafter, 0.5 wt % aqueous solution of PS-TiO2 particles was poured into the Petri dish. The dish was placed in a convection oven at 60 °C for 2 h. Characterization Methods. Scanning electron microscopy (SEM) images were taken on a Gemini microscope (Zeiss) at a voltage of 4 kV. Prior to analysis, samples were coated with a thin layer of gold to increase the contrast and quality of the images. To reveal the assembly of building blocks into the bulk through the cleaved edges, samples were tilted at 30-40° angles during SEM analysis. Optical properties of the obtained CCAs were evaluated by measuring their NIR spectra in diffuse reflectance mode at normal incidence angle, on NIR diode-array process spectrometers Sentro Proc (Sentronic GmbH, Dresden, Germany), equipped with a single NIR probe. The angledependent measurements were performed on the same spectrometer but in specular reflectance mode with two NIR probes at different incidence angles in the range of 45-75°. Results and Discussion Figure 1 shows SEM images of CCAs made of 540 nm uncoated polystyrene particles as well as 740 and 760 nm TiO2 coated polystyrene core-shell particles. Insets of these images illustrate high-magnification views of the top surfaces of CCAs. These micrographs clearly reveal the hexagonal ordered packing of PS or PS-TiO2 particles with the (111) face parallel to the quartz substrates. After evaporation of the solvent, sufficiently rigid CCAs extending over hundreds of square micrometers have been formed. Figure 1 also shows the cleaved edges of the samples, revealing the 3D architecture of the particles in the bulk of colloidal crystals. It should be noted that we deliberately

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Figure 1. SEM images of colloidal crystalline arrays (CCAs) composed of (a) 540 nm PS spheres and (b) 740 and (c) 760 nm PS-TiO2 spheres. Insets show high-magnification views of the top surface of CCAs (scale bar in inset images is 500 nm). The self-organized colloidal crystals form as close-packed planes arranged along the (111) direction.

Figure 2. Diffuse reflectance spectra of CCAs composed of (a) 740, (b) 760, and (c) 800 nm PS-TiO2 composite particles, taken at normal incidence angle. The red shift in the stop band position from 1352 to 1448 nm and increase in the bandwidth from 54% to 61% can be observed as thickness of TiO2 shell is increased from 100 to 130 nm in the employed building blocks.

focused on the cracks during the SEM analysis of colloidal crystals to have the impression of the three-dimensional assembly of hybrid colloidal particles in the bulk. One can observe that the ordered assemblies produced from the PS-TiO2 composite particles are not as perfect as the ones from bare PS particles, most probably due to the increased surface roughness of the building blocks after the TiO2 coating.27 As will be shown later with numerical simulations, however, this disorder, instead of averting the formation of the stop band, actually facilitates it. As a result, the obtained CCAs show the wide stop band in the NIR region. A hallmark feature of the prepared photonic crystals is that they are capable of displaying tunable optical properties over a large range of NIR region. Figure 2 shows the diffuse reflectance spectra of CCAs taken with an optical fiber spectrometer at normal incidence angle [incident light aligned perpendicularly to the (111) plane of as prepared CCAs]. For these measurements, a diffuse reflectance probe with NIR fiber bundles was used in backscattering (180°) arrangement. For better clarity of the stop band position and bandwidth, spectra are shown in separate panels (Figure 2a-c). One can observe that a size-dependent optical stop band exists in the three dimensionally structured CCAs, which red-shifts over a large range, that is, 1352-1448 nm, with an increase in size of building blocks from 740 to 800 nm. For the quantitative analysis, the exact peak positions of the stop bands were estimated by use of GRAMS software,28 taking into consider-

ation the same boundaries for all spectra. The CCAs composed of the 740, 760, and 800 nm PS-TiO2 particles display stop bands at 1352, 1399, and 1448 nm, respectively. It is noteworthy that these results are quite reproducible. Since polystyrene as well as TiO2 has no absorption in this spectral range, it can therefore be concluded that the observed peaks are attributed to the CCAs. This modulation in terms of the stop band position is undoubtedly important in both applications and fundamental research. Jeong and Xia29 reported temperature-induced modulation in stop band position in the NIR region for photonic crystals composed of Se-coated Ag2Se colloidal particles. Asher and coworkers30 recently developed photochemically controlled photonic crystals, which can be operated in a large spectral range. These results are consistent with Bragg’s law at normal incidence, λpeak ) 1.632dneff. As the TiO2 shell thickness increases, the effective refractive index (neff) of the film as well as the size of the building blocks and thus the spacing (d) also increase, leading to the red shift in stop band position.20,31 As expected, in comparison to the stop band position λpeak ) 843 nm for the CCAs composed of pure polystyrene particles (not shown here), a red shift in all cases of PS-TiO2 composite particles has been observed.11,20 In addition, one can see that obtained stop bands are remarkably wide and intense in nature. These bands start at approximately 1000 nm and extend up to approximately 1700 nm depending on the size of the building blocks. Interestingly, bandwidth ∆λ/λ was also found to increase with increasing TiO2 shell thickness. It was estimated as 54%, 58%, and 61% for the CCAs fabricated by employing 740, 760, and 800 nm PS-TiO2 particles with 100, 110, and 130 nm TiO2 shell thickness. Increase in the TiO2 content of the building blocks causes an increase in the refractive index contrast of CCAs, which in turn leads to the increase in bandwidth of the stop band.32 However, we do not exclude the fact that disorders in colloidal crystal assemblies, caused by the presence of a rough titania layer in the case of PS-TiO2 colloidal crystals, may also partly play a role in widening of the bandwidth. To the best of our knowledge, these values are at least 4-5-fold higher than those reported in previous studies on photonic crystals.22 Such a large 3D gap will allow strong photon localization2,33 as well as a detailed manipulation of photonic defect states.34 To investigate the effect of the incidence angle θ (the angle between the incident light beam and the normal to the sample plane) on the position of the stop band, we took the reflectance spectra of the CCAs (composed of 760 nm PS-TiO2 composite particles) via irradiating the samples at different incidence angles, and the results are shown in Figure 3. It is worth mentioning here that these angle dependence measurements were carried out in specular reflectance mode with an optical fiber spectrometer equipped with two NIR single fiber probes (one probe was used to incident the light on sample at a given angle and another one to collect the specularly diffused light).

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Figure 3. Specular reflectance spectra of CCAs composed of 760 nm PS-TiO2 composite particles at different incidence angles. No significant change in peak position of the stop band is observed with variation in the incidence angle of the light.

Therefore, one can see that the shape of the stop band is quite different than that observed in the diffuse reflectance spectrum (shown in Figure 2b). However, the peak position of the stop band in both cases was observed at same wavelength, which further proves the reproducibility of the investigated CCAs. We note that the diffuse reflectance set up used in previous measurements (shown in Figure 2b) does not allow a change in incidence angle. In addition to the primary stop band at 1399 nm, one can observe the presence of a secondary stop band at 1680 nm,35,36 which disappears as the angle of incidence increases. In a marked difference from previously reported studies,37 the center of the primary stop band of our CCAs hardly shifts from its position at 1399 nm with a change in the incidence angle from 45° to 75°. Similar results have been reported by Duan et al35 for the stop band from ordered arrays of Ni(OH)2 hollow spheres. It is well-known that the θ dependence of the stop band position is taken as a disadvantage of 3D photonic crystals in applications. If one wants to prevent transmission of light from different incidence angles, a full photonic crystal with a θ-independent stop band is needed. This is a challenge for 3D photonic crystals. The PS-TiO2 CCAs attained here could be good candidates for these types of applications as they show θ-independent stop bands. It is remarkable that our CCAs exhibit a θ-independent stop band, a feature typically associated with a complete photonic band gap, given that close-packed arrays of dielectric particles are known not to have one.37 In order to understand this behavior, we performed band structure and reflectivity calculations for the 760 nm PS-TiO2 spheres. At the frequency range of interest, both materials show very little dispersion, so for simplicity we assume a constant index of refraction n ) 1.57 for polystyrene and n ) 2.6 for TiO2, which is the average of TiO2’s ordinary (∼2.72) and extraordinary (∼2.46) indices. For the simulation studies, we used the MPB software38 and the resulting photonic band structure is shown in Figure 4a. We assumed a close-packed face-centered cubic (fcc) lattice of the PS-TiO2 composite spheres for the colloidal crystal structures. As expected, there is no full photonic band gap between 1100 and 1500 nm (i.e., between 0.67 and 0.91 µm-1). It is wellknown that fcc lattices can yield small gaps between the eighth and ninth bands, only for close-packed air spheres inside a high dielectric background,39 which is not the case here. To get more insight, we performed reflectivity calculations using the finite-difference time-domain (FDTD) method.40 We assume a close-packed fcc lattice with normal incidence along the [111] direction. The simulation system has a thickness of 10 stacking layers (6.2 µm), where each (111) stacking layer

Figure 4. (a) Photonic band structure for colloidal crystals composed of 760 nm PS-TiO2 composite particles. Inset shows fcc Brillouin zone and the k-points traced by the band structure. The Γ-point is at the center of the zone, while the L-point is along the [111] direction, which is the one relevant to the experiment. (b) Reflectance of a perfect fcc arrangement of the composite spheres, 10 stacking layers thick (6.2 µm). The gray shaded area marks the second region and extends over to the L-point for ease of comparison.

has a thickness of c ) 2j/3a ) 620 nm, and a ) 760 nm is the nearest-neighbor distance. We used a discretization of 20 nm per grid. As shown in Figure 4b, one can observe three main regions: below 0.73 µm-1 (i.e., below the diffraction limit), where reflectance is dominated by Fabry-Perot oscillations; between 0.73 and 0.9 µm-1, where reflectance is generally very large but also has a great number of sharp resonances; and above 0.9 µm-1, where it has a generally low value with a lot of rapid oscillations. The gray shaded area marks the second region, which is the one relevant to the large reflections measured experimentally. For comparison, we extend it to the L-point band structure, which is related to the [111] direction (it should be noted that there may be a small, up to a few percent, offset between the two methods because of different discretization errors: MPB typically overestimates frequencies, while FDTD underestimates them). The band structure at the L-point exhibits many flat bands. These are laterally guided states that couple with the incident radiation, resulting in sharp guided (Fano) resonances.41 The net result is high reflection superimposed with a large number of sharp reflection dips. The exact position of each resonance depends on the colloidal structure. This immediately brings up the question of what happens when disorder is present, as it is the case in the experiments. To this end we performed supercell FDTD calculations, where the colloid spheres are randomly displaced from equilibrium. Each supercell has lateral dimensions 5.28 µm × 4.56 µm (see insets in Figure 5), while in the vertical direction the total thickness of 6.2 µm [i.e., 10 (111) stacking layers] is still maintained. For creating the disorder, we displace each composite sphere by (rd along each Cartesian direction, where r is a uniform random number in the range [-1, 1] and d ) 20, 60, and 100 nm as shown in Figure 5 panels a, b, and c, respectively. The spheres at the top and bottom faces are displaced vertically by a tenth of what they are displaced in the bulk in order to maintain a relatively flat profile, while at the side faces spheres are repositioned in order to satisfy periodic boundary conditions. It should be noted here that because of the large lateral size of the computation cell, diffraction is allowed, and thus while the incidence is strictly

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J. Phys. Chem. C, Vol. 114, No. 39, 2010 16393 Conclusions In summary, we have demonstrated the fabrication of threedimensional CCAs by gravity sedimentation of TiO2-coated polystyrene particles. The resulting CCAs exhibit a wide photonic stop band in the NIR region with a bandwidth (∆λ/λ) as wide as 54-61%. We showed that the spectral position and bandwidth of the photonic stop bands of obtained CCAs can be tuned with the size and thickness of the TiO2 shell of the building blocks. Interestingly, stop band position was found to be insensitive to the angle of incidence. Simulation results have been found in agreement with the experimental findings. Since a wide range of coated colloidal particles with different sizes and core-shell compositions can be synthesized by the sol-gel process, a variety of the advanced CCAs with tailored properties can be fabricated by employing the described approach. Acknowledgment. We are thankful to Mrs. Ellen Kern for helping us out in SEM analysis of the samples. We acknowledge computing time at the Research Center for Scientific Simulations (RCSS) at the University of Ioannina.

Figure 5. Simulation results for colloidal crystal arrays with three disorder strengths, (a) d ) 20 nm, (b) d ) 60 nm, and (c) d ) 100 nm, where spheres are displaced from the fcc lattice sites by ( rd, with r a uniform random number in the range [-1, 1]. Two different disordered configurations of the same strength are averaged in each case. The top inset is the radial distribution function of the disorder lattice, while the bottom inset is a cross section of the top face. A wide reflection peak between 1100 and 1400 nm (∼25%) is observed.

normal, the reflection may include many diffraction orders. Therefore, the calculated reflectance includes both specular and diffuse reflectivities. For better statistics, we perform the simulation twice for each disorder strength and take the average reflectance of the two different configurations. Figure 5 summarizes the results. In the bottom inset is a cross section of the top face, while the top inset is the simulated system’s radial distribution function. For the smallest disorder coefficient of d ) 20 nm, we obtain a reflectivity that is very similar to the perfect case, except that the multitude of resonances has now been smoothed out into a wide band of high reflectance, extending approximately from 1100 to 1400 nm (∼25%). As disorder increases, the overall reflectivity smoothes out more, while it drops in overall magnitude. At the largest disorder case, the high reflectance band is only a narrow and short peak but still distinguishable from the background. Comparing Figure 5 with Figure 2, we see that the experimental results fall somewhere between Figure 5b and 5c, with the simulations just underestimating slightly the long wavelength edge of the reflection band. It is worth mentioning that these results remain practically unchanged as we reduce the number of layers, even down to three stacking layers or a total thickness of 1.86 µm. The role of disorder is thus quite interesting: it smoothes out all sharp states into a wide “effective” stop band. Also, as the disorder increases, any strong angular dependence disappears, which explains the θ-independence observed in Figure 3. As a result, a very thin (∼2 µm) film is indeed capable of wideband large omnidirectional reflectance in the NIR region. While it does not exhibit the high (∼100%) reflectance expected from a full photonic band gap, it may still be an excellent solution for NIR shielding or other relevant applications. Furthermore, such strong scattering in a somewhat disordered medium may have application in observing Anderson localization of light42 and the modification of spontaneous emission rates due to local density of states (LDOS) spatial fluctuations.43

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