Research Article www.acsami.org
Polymeric Inverse Glasses for Development of Noniridescent Structural Colors in Full Visible Range Gun Ho Lee, Jae Young Sim, and Shin-Hyun Kim* Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: Amorphous colloidal array with short-range order displays noniridescent structural colors due to the isotropic nature of the colloidal arrangement. The low angle dependence renders the colloidal glasses, which is promising for various coloration applications. Nevertheless, the colloidal glasses are difficult to develop red structural color due to strong cavity-like resonance from individual particles in the blue region. To suppress the cavity mode and develop the colors in the full visible range, we prepare inverse glasses composed of amorphous array of air cavities with short-range order. To produce the structures in a simple and reproducible manner, monodisperse silica particles are dispersed in a photocurable resin of poly(ethylene glycol) dimethacrylate (PEGDMA) at a volume fraction of 0.3. The particles spontaneously form the amorphous array with short-range order, which is rapidly captured in polymeric films by photopolymerization of the resin. Selective removal of silica particles from the polymerized resin leaves behind amorphous array of air cavities. The inverse glasses display structural colors with negligible backscattering in blue due to short optical path and low index in each cavity. Therefore, the colors can be tuned in full visible range by simply controlling the cavity size. The photocurable suspensions of silica particles can be patterned by photolithography, which enables the production of freestanding films containing patterned inverse glasses with noniridescent structural colors. KEYWORDS: structural color, short-range order, inverse glass, noniridescence, micropattern
1. INTRODUCTION Colloidal arrays develop structural color through constructive interference.1,2 The color never fades as long as the arrays persist and the color can be tuned by adjusting interparticle distance.3 These unique features of structural color make colloidal arrays appealing as new types of color pigments for a wide range of applications, including aesthetic coatings, displays, security materials, colorimetric sensors, and optical barcodes.4−8 The color property depends on colloidal arrangement. Colloidal crystals with long-range order exhibit brilliant color, of which wavelength is strongly angle-dependent.9 Such an iridescence is indeed undesired in most applications, although it sometimes benefits aesthetic coatings. By contrast, colloidal glasses with only short-range order provide relatively dim, but orientation-independent structural color due to their isotropic nature.10,11 Although there is a trade-off between color brightness and angular dependence, the importance of noniridescent colors makes colloidal glasses appealing for many applications. To suppress colloidal crystallization and prepare amorphous array of colloids, three different approaches have been used. Colloidal particles are rapidly concentrated by either fast solvent evaporation or centrifugation, which kinetically arrests the quasi-amorphous colloidal arrays.12−15 Alternatively, two different sizes of colloids can be used for concentration, which © XXXX American Chemical Society
disturbs the crystallization, leading to glassy packing while maintaining consistent interparticle distance.16−18 The amorphous array can also be prepared by minimizing repulsive interparticle interaction;19,20 interparticle repulsion drives the crystallization even at low concentration. However, these methods yield bulk glassy packing of colloids in liquid medium, which is mechanically unstable and difficult to be patterned; this severely restricts their applications. More importantly, the glassy packing of colloids is difficult to produce red, even though relevant particle size is employed.21,22 This is because a cavity-like mode from individual particles yields strong backscattering in blue, overwhelming the resonance in red from the glassy array of colloids.22 To develop red structural color, optical length within individual particles should be shortened. For example, the use of polymeric core−hydrogel shell particles decouples scatter size and interscatter distance and produces red color by locating the cavity resonance in ultraviolet (UV).23 However, water medium is a prerequisite to maintain the structures, which makes only capsule format available. Received: March 16, 2016 Accepted: April 28, 2016
A
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration for preparation of polymeric inverse glasses. Silica particles dispersed in polyethylene glycol dimethacrylate (PEGDMA) form amorphous array, which is captured by photopolymerization. Selective removal of silica particles leaves behind amorphous array of air cavities with short-range order in the polymer films. (b, c) Scanning electron microscopy (SEM) images showing the film cross sections for the composite (b) and porous structures (c). Inset in panel c shows two-dimensional fast Fourier transform (2D FFT) image that is converted from the SEM image. A diffused ring indicates that the array of cavities possesses short-range order. (d) Set of optical microscopy (OM) images showing film surfaces of inverse glasses with three different structural colors. The films are prepared using silica particles with the diameters of 205, 185, and 168 nm (from the top panel) at a volume fraction of 0.3. (e) Reflectance spectra of three films with different colors in panel d.
inverse glasses exhibit noniridescent structural colors under diffusive light. The colors can be tuned in full visible range by adjusting particle diameter. Under illumination of directional light, the inverse glasses show no rotation dependence and relatively low angular dependence in comparison with inverse crystals. Inverse glasses are made of monolithic and highly cross-linked polymers, enabling the formation of freestanding films. Moreover, the inverse glasses can be further patterned by selectively polymerizing the suspensions with a photomask, providing new opportunity for practical applications of noniridescent structural colors.
In this work, we use inverse glassy structures to develop noniridescent structural colors in the full visible range; air cavities have short optical lengths, which yield weak cavity resonance out of visible range, as theoretically suggested by Magkiriadou et al.22 At the same time, to provide easy material processing and high mechanical stability of inverse glasses, we employ photocurable suspensions containing amorphous array of concentrated colloids, which serve as templates to make amorphous array of air cavities in polymerized matrix. To prepare the suspensions, silica particles are dispersed in the photocurable resin, which forms thin solvation layers on the surface.24 Although the silica particles are spontaneously crystallized due to the repulsive force exerted by solvation layers when they are highly concentrated, they form non-closepacked amorphous array for a certain range of concentration, because the repulsion diminishes even for small interparticle separation. The colloidal array can be rapidly captured by photopolymerizing the medium, which is then removed by selective etching, thereby yielding inverse glassy structures. The
2. RESULT AND DISCUSSION 2.1. Formation of Amorphous Array of Air Cavities in Polymeric Films. Colloidal particles with repulsive interparticle potential in long-range spontaneously form a crystalline phase even at low volume fraction. For example, silica particles dispersed in a photocurable resin of ethoxylated trimethylolpropane triacrylate (ETPTA) form a face-centered cubic (fcc) B
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a−c) Three sets of OM images showing film surfaces and SEM images showing film cross sections for porous films prepared by three different volume fractions of silica particles in PEGDMA: 0.2 (a), 0.3 (b), and 0.4 (c). Insets in the bottom panels are 2D FFT images obtained from the corresponding SEM images. Diffusive circle in 2D FFT image of panel a indicates no order, whereas a ring in panel b and a hexagonal pattern in panel c indicate short-range and long-range order, respectively. (d) Reflectance spectra of three films in panels a−c.
lattice above a volume fraction of 0.1;25 ETPTA molecules form a dense solvation layer on the surface of silica particles due to hydrogen bonds, which render the silica particles repulsive. The crystalline structures can be permanently stabilized by photopolymerizing the resin to provide stable films or micropatterns with brilliant structural colors. By contrast, colloidal particles that insignificantly interact with their neighbors form crystalline structures only when they are highly concentrated. At the volume fraction immediately below the minimum for crystallization, the colloids distribute to be quasi-amorphous without long-range order, while maintaining consistent interparticle distance. Such an arrangement therefore possesses short-range order and can provide noniridescent structural colors when the interparticle distance is approximately half the wavelength of visible range. Although quasi-amorphous array of colloids with consistent interparticle distance, davg, can provide structural colors at wavelength of 2neffdavg, individual colloidal particles cause significant cavity resonance, which usually overwhelms the structural resonance; neff is effective refractive index of materials. The cavity resonance occurs at the wavelength of 2npd, which is at blue when d is selected to make structural resonance at red, where d and np are diameter and refractive index of colloidal particles, respectively. Therefore, red structural color from amorphous array of colloids is difficult
to develop. Furthermore, the quasi-amorphous array of colloids in liquid medium is unstable, restricting practical uses. To develop noniridescent structural colors in full visible range with mechanically stable structures, we use non-closepacked amorphous array of air cavities embedded in a solid matrix. Air cavities with np = 1 makes weak cavity resonance at the wavelength of 2d, which is in the UV range if the diameter of cavities is smaller than 200 nm. Moreover, the monolithic matrix can be mechanically stable. To prepare the inverse glasses, we disperse monodisperse silica particles in a photocurable resin of poly(ethylene glycol) dimethacrylate (PEGDMA), and use the suspensions to make composite films; silica particles are highly monodisperse as shown in the scanning electron microscopy (SEM) image of Figure S1. Inverse glasses are then prepared by selectively removing silica particles from polymerized matrix. The silica particles dispersed in PEGDMA at a volume fraction of 30% form non-closepacked quasi-amorphous array, while maintaining high suspension stability. This is in contrast to silica particles dispersed in ETPTA;26 silica particles in other photocurable resins, including trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA), form non-close-packed crystals at the volume fraction of 30%, as shown in Figure S2 of the Supporting Information, and it has been reported that silica particles in poly(ethylene glycol) diacrylate (PEGDA) also form crystalline structures.27 We attribute the formation of C
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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therefore optical properties of resulting films. To investigate this, we prepare the suspensions with three different volume fractions of 20%, 30%, and 40% and produce porous films with the same protocol; silica particles with the diameter of 210 nm are used for all suspensions. The film prepared from 20% suspension exhibits no color and no order as shown in Figure 2a. At such a low volume fraction, particles are inhomogeneously distributed over the volume, thereby losing short-range order. Therefore, there is no meaningful peak in reflectance spectrum, as denoted with the gray curve in Figure 2d. The films prepared from 30% suspensions display red colors due to formation of amorphous array of air cavities with short-range order, as shown in Figure 2b. This yields a single peak at 634 nm as denoted red curve in Figure 2d, which is consistent with 3.041d. The reflectivity is approximately 15% and full width at half-maximum (fwhm) is 123 nm. At a volume fraction of 40%, the particles form a crystal lattice with long-range order and the resulting porous films show green color, as shown in Figure 2c. The formation of crystal lattice significantly increases the reflectivity to 74% and reduces fwhm to 57 nm, as denoted with the green curve in Figure 2d; these are typical contrasts between crystals and glasses. The reflectance peak of the crystal films is located at 555 nm, which is consistent with Bragg’s law expected from (111) planes of non-close-packed fcc with void fraction of 40%; Bragg’s law provides the structural resonance at 548 nm.30 This indicates that the air cavities are aligned to form hexagonal array along planar surface of films. As volume fraction of particles is increased above 30%, average interparticle distance is shortened, making the solvation layers of neighboring particles overlapped. This can provide interparticle repulsion, and therefore lead to the formation of crystal lattice; nevertheless, the structure involves many defects and the reflection color is slightly heterogeneous as shown in Figure 2c, because the solvation layer is not dense enough to provide strong repulsive force between the particles. Therefore, to produce amorphous array only with short-range order, the volume fraction of silica particles in PEGDMA should be selected to be approximately 30%. 2.3. Angle Dependence of Structural Colors Developed by Inverse Glasses and Inverse Crystals. To evaluate angle dependence of structural colors developed by inverse glass films, we measure reflectance spectra by adjusting two different angles: (1) rotation angle of the films, while maintaining parallel paths for incident light and detection and (2) angles of incident light and detection, while maintaining the film angle; the second is specular reflection. To compare the angle dependence with a crystal counterpart, we also prepare inverse crystal films using silica−ETPTA suspensions with 200 nm silica particles at volume fraction of 30%; internal structures of the composite and inverse crystals are investigated with the films prepared using silica−ETPTA suspensions with 210 nm silica particles at a volume fraction of 30%, as shown in Figure S5. The inverse glass films prepared from silica particles with diameter of 220 nm show reflectance peak at 653 nm for normal incident light and normal detection, as denoted with the black curve in Figure 3a; the angle between film surface normal and incident light is 0°. As the angle increases from 0° to 40° by rotating the films, reflection intensity significantly decreases because the reflection dominantly occurs at the angle identical to that of the incident light. Nevertheless, reflection peak position remains almost unchanged; variation of peak position is smaller than 9 nm. This angle independence is contributed from isotropic nature of arrangement of air cavities. The
amorphous array in PEGDMA to relatively weak interparticle repulsion. In all resins except PEGDMA, strong hydrogen bonds between carbonyl groups in acrylate of resin molecules and silanol groups on the silica surface lead to the formation of robust solvation layers, which render the particles repulsive.24,28 By contrast, methyl groups of PEGDMA interrupt the formation of the hydrogen bonds, thereby resulting in a less dense solvation layer.28 This possibly leads to weak interparticle repulsion, which is effective only for very small interparticle separation. Therefore, the silica particles dispersed in PEGDMA have high dispersion stability, but form amorphous array at the volume fraction. We prepare the composite films by two steps: Infiltration of the silica−PEGDMA suspensions into a gap between two parallel slide glasses separated by 50 μm using a capillary force and photopolymerization of PEGDMA through UV irradiation. The composite films possess amorphous array of silica particles embedded in polymerized PEGDMA (pPEGDMA) matrix, as illustrated in the left panel of Figure 1a,b; silica particles with the diameter of 183 nm are used. The films released from the glasses are then treated by hydrofluoric acid to selectively etch silica particles out from pPEGDMA matrix. The resulting porous films have quasi-amorphous array of air cavities, as illustrated in the right panels of Figure 1a,c. The consistent distance between neighboring air cavities is confirmed by the two-dimensional fast Fourier transform image in the inset of Figure 1c, which shows a ring whose radius corresponds to the cavity-to-cavity distance of 205 nm. With three different diameters of silica particles of d = 205, 185, and 168 nm, three different structural colors of red, green, and blue are developed from the porous films, as shown in Figure 1d. In the pPEGDMA matrix, dye molecules of Sudan black b are included at 0.05 w/w% to reduce undesired whitening. Sudan black b absorbs all the wavelengths of visible light, as shown in Figure S2 of the Supporting Information, which reduces Mie scattering. The films show pronounced red and green colors as much as blue. This is because reflectance spectra for all three films have a single peak from structural resonance at 624, 569, and 503 nm, as shown in Figure 1e. The wavelength of structural resonance is linearly proportional to the pore diameter, d, with a proportion constant of 3.041; we assume no structural deformation during silica removal. From the constant, we have a relation of davg = 1.132d, where value of neff is estimated as 1.3433 from the Maxwell−Garnet average for a composite of 30 v/v% air cavities in pPEGDMA matrix;29 the distance between neighboring (111) planes of fcc lattice with same volume fraction of air cavities in pPEGDMA matrix, d111 is 1.104d. There is no significant rise of reflectivity in blue region for all films. For d = 168 and 185 nm, the cavity resonance is expected at the wavelengths of 336 and 370 nm, which are out of visible range. Although the resonance can occur around the wavelength of 410 nm for d = 205 nm, the intensity is very low. We attribute the low intensity to high refractive index of the matrix, at which light is more concentrated than at low index regions of the air cavities, thereby suppressing cavity resonance. On the other hand, the composites with amorphous silica arrays do not show pronounced color or reflectance peak due to very low refractive index contrast between silica and pPEGDMA, as shown in Figure S3. 2.2. Influence of Particle Volume Fraction on Structural Order. The volume fraction of silica particles in PEGDMA significantly influences the particle arrangement, and D
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a, b) Series of reflectance spectra of inverse glass (a) and inverse crystal films (b) taken along the angle of the film rotation, as illustrated in the insets, where the angle is varied from 0° to 10°, 20°, 30°, and 40°. Inverse glasses show consistent reflectance-peak position, whereas inverse crystals show a reflectance dip for the angle lager than 0°, which blue shifts with the angle.
arrangement remains unchanged during the film rotation, providing the same value of davg for structural resonance. By comparison, we measure the reflectance spectra of the inverse crystal films, as shown in Figure 3b; for this comparison, we use inverse crystal film prepared from silica−ETPTA suspensions with 200 nm silica particles at volume fraction of 30%. The films show a reflectance peak at 592 nm for normal incident light and reflectivity is as high as 87%; the fwhm normalized by bandgap wavelength is 0.094 for the inverse crystals, which is consistent with previous reports.6 When the angle increases from 0° to 10°, reflectance dramatically falls and a reflectance dip appears. This is because the incident light is strongly reflected at the angle of 10°, as expected by the law of reflection. In particular, reflection at the resonance wavelength is even stronger than at other wavelengths, yielding a dip in reflectance spectra. The dip position is blue-shifted when the angle further increases to 40°, as expected from Bragg’s diffraction; this blue-shift of dip position is clearly observed in normalized spectra, as shown in Figure S6. Therefore, the color of inverse crystal films strongly depends on the rotation. We further investigate the angle dependence with angleresolved specular reflection. As both angles of incident light and detection are simultaneously increased from 0° to 40°, reflectance peak position of the inverse glass films is blueshifted from 622 to 588 nm, as shown in Figure 4a. During the
Figure 4. (a,b) Series of specular reflectance spectra of inverse glass (a) and inverse crystal films (b) taken along the angle of light incidence, as illustrated in the inset, where the angle of detection is set to be the same as that of incidence, which is varied from 0° to 15°, 20°, 30°, and 40°. The spectra are normalized. (c) Angle dependence of reflectance peak shift from 0°, which is normalized by peak position at 0°.
shift, the fwhm remains almost unchanged as 149 nm and the variation is as small as 13 nm. By contrast, the reflectance peak position of the inverse crystal films is significantly shifted from 593 to 516 nm, as shown in Figure 4b. To compare quantitatively the angle dependence of reflection peak position between glassy and crystalline structures, the shift of peak position relative to the position at angle of 0°, Δλ/λ0, is plotted as a function of angle, θ, as shown in Figure 4c. Resonance wavelength of the inverse glasses is blue-shifted only by 6% at E
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. (a) Schematic diagram illustrating the production procedure of a freestanding polymeric film containing micropatterned inverse glasses. The micropatterns are photolithographically prepared, which are then captured in a transparent polymer film by depositing a particle-free resin and subsequent UV curing. Silica particles in the micropatterns are finally etched away to create inverse glasses. (b) Series of optical images of the films containing micropatterns of inverse glasses under diffusive light illumination taken at three different angles, as denoted in each panel. The micropatterns are composed of small K’s to form a large K. The OM image in the inset of the leftmost panel shows the array of small K’s. The patterns show a consistent green color, regardless of observation angle. Under directional light illumination, the patterns display blue-shifted color when the angle is increased, as shown in the inset of the rightmost panel.
with inverse glassy structures. For example, green-colored patterns, composed of small “K” arrays that form a large “K”, are captured in transparent films, as shown in Figure 5b. Each small K exhibits a structural green color under diffusive light illumination. The structural green color remains unchanged when the angle of observation is changed from 0° to 30° and 60° due to isotropic arrangement of air cavities in each small K. Under directional illumination, the colors turn blue as both angles of incident light and observation is simultaneously changed to 60°, as shown in the inset of Figure 5b.
the angle of 40°, whereas that of the inverse crystals is blueshifted as much as 13% at the same angle; variation of resonance wavelength is insignificant for each angle of measurement. This clear contrast in angle dependence indicates that the glassy structures are less iridescent than the crystal counterpart even in specular reflection under directional lightening. Angle dependence is inevitable even for glassy structures due to intrinsic angle-dependence of path difference between two lights scattered by adjacent air cavities. Nevertheless, the isotropic arrangement of the air cavities can reduce the angular dependence, rendering the glassy structures lessiridescent. 2.4. Freestanding Films Containing Noniridescent Color Patterns. The inverse glassy structures can be patterned using photolithography. Because the silica−PEGDMA suspensions are photocurable, selective UV irradiation through a photomask locally cross-links the PEGDMA resin, resulting in patterns with the same design to the photomask after washing out uncured resin. Moreover, the patterns can be captured in transparent films by infiltrating particle-free PEGDMA resin into a gap between two slide glasses and subsequently polymerizing the resin, where the bottom slide glass contains the patterns. The resulting films can be released from the slide glasses to provide freestanding films containing the patterns. The procedure to prepare the films is schematically illustrated in Figure 5a. The films are then treated by hydrofluoric acid to remove silica particles in the patterns, yielding color patterns
3. CONCLUSION We have shown that silica particles dispersed in a photocurable resin form amorphous array with short-range order. The array can be captured by rapid photocuring of the resin, which is then selectively removed from the polymerized matrix to provide inverse glassy structures. The inverse glasses have low-index air cavities, which can suppress the cavity resonance or exclude the resonance out of visible range, thereby yielding pronounced structural colors at selected wavelengths from the entire visible range. The structural colors developed from the inverse glasses exhibit no angle dependence for the sample rotation due to isotropic nature of air cavities arrangement. In addition, angle dependence of the colors by the inverse glasses is reduced from that by the inverse crystals even for specular reflection. The inverse glasses are made of monolithic and highly cross-linked polymers, thereby providing high mechanical stability. MoreF
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces over, no delicate procedure for formation of amorphous array is required and the structures are highly reproducible. More importantly, noniridescent structural colors can be readily patterned by photolithography and the color patterns can be embedded in freestanding films. This novel but simple approach to create inverse glasses and their patterns will benefit various optical applications, which require noniridescent and nonfading coloration.
4. EXPERIMENTAL SECTION
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4.1. Preparation of Photocurable Suspensions. Monodisperse silica particles are synthesized by two steps: Seed particles are prepared by a two-phase method, which are then grown to a desired diameter by the Stöber method.31,32 The silica particles are washed several times and dried in a convection oven at 70 °C to measure their weight. The dried silica powders are redispersed in 2 mL of ethanol, at which time we add the photocurable monomer, either polyethylene glycol dimethacrylate (PEGDMA, Mw 550, Sigma-Aldrich) or trimethylolpropane ethoxylate triacrylate (ETPTA, Mw 428, Sigma-Aldrich), containing 1 wt % 2-hydroxy-2-methyl-1-phenyl-1-propaneone (Darocur 1173, Ciba chemical) as a photoinitiator; the amount of monomer is determined from the measured weight of the particles to have target volume fraction of silica particles in ethanol-free basis. Typically, 1.5 g of silica powder was redipsersed in 2 mL of ethanol, at which time 1.92 g of PEGDMA is added to make the silica volume fraction of 30% in ethanol-free basis. To enhance the color contrast, chemical pigment of (2,2-dimethyl-1,3-dihydroperimidin-6-yl)-(4phenylazo-1-naphthyl)diazene (Sudan black b, Sigma-Aldrich) is added to the PEGDMA suspensions to have concentration of 0.05 w/w% in ethanol-free basis. Ethanol is selectively vaporized from the suspensions in a convection oven at 70 °C for 24 h. 4.2. Film Casting and Photolithography. The photocurable suspensions are infiltrated by capillary force into a gap between two parallel glass slides that are separated by a 50 μm-thick spacer of polyimide tape (Kapton); typical dimensions of films are 2 × 2 cm. The infiltrated suspensions are exposed to UV light. The polymerized films are detached from the glasses and washed with ethanol. To remove selectively silica particles from the polymerized matrix, the film is treated with 2 w/w% hydrofluoric acid for 12 h. After the treatment, the films are washed with distilled water several times and completely dried. To make freestanding films of micropatterns, the photocurable suspensions are infiltrated into a gap between a glass slide and an amorphous silicon photomask, which are separated by 50 μm; the amorphous silicon photomask is prepared by following a protocol we reported.33 The suspensions are selectively irradiated by collimated UV light through the photomask. After detaching top glass slide, uncured suspensions are washed out by ethanol. With new top glass slide separated by 100 μm from bottom one containing micropatterns, particle-free photocurable monomer is infiltrated into the gap, which is then polymerized by UV exposure without a photomask. The resulting films are released, which are finally subjected to hydrofluoric acid treatment. 4.3. Characterization. The amorphous photonic films are observed using optical microscopy in reflection mode (Nikon, Eclipse L150) and scanning electron microscopy (SEM, Hitachi, S-4800); samples are coated with osmium tetroxide for SEM analysis. To take SEM images of film cross sections, the films are cleaved by a razor blade and washed out with ethanol before osmium tetroxide coating. The reflection spectra are measured using fiber-coupled spectrometer (Ocean Optics Inc., USB 4000) equipped in the optical microscope. For measurement of the specular reflection spectra, a variable-angle reflection sampling device (Ocean Optics Inc., RSS-VA) is used.
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SEM images of three different silica nanoparticles (Figure S1), OM and SEM images of inverse crystals prepared by three different resins (Figure S2), set of OM, SEM images and reflectance spectra of composite glasses and inverse glasses (Figure S3), absorbance spectrum of Sudan black b-dissolved PEGDMA (Figure S4), set of OM, SEM images, and reflectance spectrum of inverse crystals (Figure S5), normalized reflectance spectra of inverse crystals taken at various angles of sample orientation (Figure S6) (PDF).
AUTHOR INFORMATION
Corresponding Author
*S.-H. Kim. E-mail:
[email protected]. Author Contributions
G.H.L and J.Y.S carried out all experiments and S.-H.K. supervised the research. All authors have discussed and interpreted the results and given approval to the final version of the paper. G.H.L and J.Y.S contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Midcareer Researcher Program (2014R1A2A2A01005813) and Global Research Laboratory (NRF-2015K1A1A2033054) through the National Research Foundation (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP). We thank Dr. Sofia Magkiriadou and Dr. Jin-Gyu Park in Harvard University for useful discussion and Jae Hyung Cho in KAIST for a proofreading.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03217. G
DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.6b03217 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
