Free-Standing and Circular-Polarizing Chirophotonic Crystal

Free-Standing and Circular-Polarizing Chirophotonic Crystal Reflectors: Photopolymerization of Helical Nanostructures Dae-Yoon Kim,† Changwoon Nah,† Shin-Woong Kang,‡ Seung Hee Lee,‡ Kyung Min Lee,§ Timothy J. White,§ and Kwang-Un Jeong*,† †

BK21 Plus Haptic Polymer Composite Research Team & Department of Polymer-Nano Science and Technology, and ‡Department of BIN Convergence Technology, Chonbuk National University, Jeonju 54896, Korea § U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7750, United States S Supporting Information *

ABSTRACT: The preparation of materials exhibiting structural colors has been intensively studied in biomimetic science and technology. Utilizing a newly synthesized cholesteric liquid-crystal (CLC) monomer (abbreviated as BP1CRM), we have prepared CLC films. Photoinitiated copolymerization of this monomer with a common achiral liquid-crystalline monomer produced free-standing films with homogeneous and nanoscale pitch distributions. Employing the thermal sensitivity of the CLC monomer, chirophotonic crystal reflectors were prepared exhibiting a range of colors. The free-standing and circular-polarizing chirophotonic crystal films maintain excellent thermal, mechanical, and chemical stabilities, and the composition can readily be applied as polarized optical films and smart paints.

KEYWORDS: photonic crystal, helical nanostructure, free-standing, circular-polarizing, photopolymerization

C

standing the fundamental nature of coloration as well as motivating synthetic efforts to realize and exploit them in applications relating to color control.13 Here, we focus on materials that form the cholesteric liquidcrystal (CLC) phase. The CLC phase self-organizes into a onedimensional (1D) photonic band gap material.14 The CLC phase can be obtained by doping chiral molecules to nematic media, producing a Bragg reflection which is expressed by λ0 = na × P0, where λ0, na, and P0 are notch position, average refractive index, and pitch length, respectively.15 The static bandwidth of CLCs is a product of LC birefringence and cholesteric pitch, Δλ = Δn × P. It is noted that Δn = (ne − no), where ne and no are the extraordinary and the ordinary refractive index, respectively.16 Helical nanostructures of a CLC can be either a right-handed or a left-handed rotation of the LC director along the helical axis. When irradiated with unpolarized light, a CLC will reflect 50% of this light with a circular polarization matching the handedness of helical nanostructure. The remaining portion of the light is transmitted.17

olor is a powerful and often rapid means for intuitive communications.1 One notable area in which animals use color change is when they are threatened.2 A variety of animals can rapidly change their coloration to either hide-in-plain-sight or intimidate any potential predator.3 Color is visualized in a variety of ways including transmission, diffraction, absorption, and reflection of light.4 Reflective materials are particularly interesting.5 The reflection can arise in periodic media with a variation in the refractive index to produce a photonic band gap.6 With an appropriate thickness regime, the reflection is described by Bragg’s law, and accordingly, the position as well as the bandwidth of the photonic band is dependent on the periodicity as well as birefringence of the media.7 When the periodicity is proportional in length to the wavelength of visible light, these materials exhibit iridescent and opalescent colors observable with naked eyes.8 Nature widely employs these structures to enable vision enhancement, species recognition, and camouflaging described above.9 Scales of Koi fish exhibit multilayer reflectors that produce the characteristic silvery luster.10 Helical nanostructures of the exoskeleton of the Glorious beetle reflects circularly polarized reflection lights.11 Epicarps of Pollia fruits show vivid colors from Bragg reflection of light from helicoidally stacked cellulose fibers.12 These and other examples in nature have inspired considerable research into under© 2016 American Chemical Society

Received: July 25, 2016 Accepted: September 19, 2016 Published: September 19, 2016 9570

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ACS Nano Low molar mass LC materials do not have sufficient structural integrity to prepare free-standing films.18 Previously, free-standing films have been prepared from glass-forming precursors, but these materials are delicate to handle and brittle.19 The approach we take here is to prepare a polymerizable CLC monomer to yield films with the desired optical properties and the robustness needed for flexible devices. Similar approaches have been taken to prepare polymeric films.20 This contribution distinguishes the optical activity of the monomer employed and the use of thermal sensitivity of this monomer in mixtures with an achiral LC monomer to prepare CLC polymeric films with homogeneous distribution in the pitch.21 Polymer-based photonic crystals have been widely discussed due to their orientational properties of anisotropic liquid-crystal molecules and elastic properties of polymeric networks.22 Broer and co-workers demonstrated a photonic crystal polymer film by stabilizing CLCs via photopolymerization of reactive mesogen (RM) as broadband polarizers.23 Castles et al. also fabricated a photonic crystal polymer film with three-dimensional (3D) periodic lattice as a tunable gel.24 Compared with conventional photonic crystals made from hard materials, soft materials have attracted much attention due to good processability, cost effectivity, and lightweight operation.25 Since the diffusion-controlled phase separation during the in situ photopolymerization is difficult with RM in the nonreactive host mixture, we used the fully reactive CLC monomers. During the photopolymerization process, RM is diffused toward a source of UV light, inducing the pitch variation across the sample thickness.26 The notch position and bandwidth of CLC could be controlled by the curing conditions, such as polymerization temperature and UV intensity.27 To securely preserve the homogeneous helical pitch distribution in the robust polymer network, acrylate functions with an identical reactivity are introduced both to chiral guest and to nematic host for suppressing the phase separation via photopolymerization.28 Fast photopolymerization freezes the original helical nanostructures and original notch position. In this contribution, we report the reflective and flexible CLC polymer films with red (R), green (G), and blue (B) reflection colors, which are prepared by the photopolymerization of CLC monomers. The free-standing and circular-polarizing reflectors exhibit robust thermal, mechanical, and chemical stabilities. Full color patterned chirophotonic crystal films are also demonstrated by the selected area photopolymerization processes for a flexible multicolor display.

Figure 1. (a) Chemical structure of chiral monomer designed and synthesized in this study. (b) Polarizing optical microscopy images of CLC phase obtained from the mixture of 10 wt % BP1CRM and 90 wt % RM257 at different temperatures. Oily streak texture of CLC phase is observed. (c) Red shifting of reflection wavelength by increasing temperature.

well-aligned by introducing a planar anchoring condition on the surface of alignment cell substrates.30 As the content of BP1CRM increases from 7 to 10 wt %, the reflection wavelength of the CLC phase shifts from 700 to 400 nm (Figure S6). Helical twisting power (HTP) of BP1CRM is estimated to be 22 μm−1 by using the reflection spectra in Figure S7. The helical twisting originating from the chiral BP1CRM monomer can be effectively transferred to the nematic LC medium by strong intermolecular interactions between chiral naphthyl groups of BP1CRM and nematic LC molecules.31 Since the pitch of CLC material systems can be controlled by temperature, the CLC reflection color is conveniently tuned by temperature.32 The initial notch position of the CLC phase prepared from 10 wt % BP1CRM is around 450 nm (Figure S8). As shown in Figure 1c, increasing the temperature red shifts the reflection notch to longer wavelengths. The initial blue color turns to green at 70 °C (λmax = 550 nm) and red at 90 °C (λmax = 650 nm). This result indicates that the HTP of BP1CRM is inversely proportional to the temperature.33 The reduction of HTP interrupts the twisted molecular packings with neighboring molecules (Figure 1b), and the subsequent lengthening of the helical pitch results in the red shift of the reflection spectrum (Figure 1c). The finite step change of reflection depending on temperature, so-called “pitch-hopping”, is attributed from the competition between a molecular twisting force and a surface anchoring force.34 Helical nanostructures

RESULTS AND DISCUSSION To develop the free-standing and circular-polarizing chirophotonic crystal reflectors, we newly synthesize a polymerizable CLC monomer, which is abbreviated as BP1CRM (Figure 1a). BP1CRM chiral monomer is specifically designed to show a strong thermochromic relationship between reflection wavelength and temperature. BP1CRM consists of a photopolymerizable methacrylate unit and a chiral R-configured naphthyl group (Figure 1a). Chemical structures of BP1CRM and their intermediates are confirmed by 1H and 13C NMR spectroscopy and MALDI-ToF mass spectrometry (Figures S1−S5). Nematic diacrylate monomer (RM257) doped with a BP1CRM chiral monomer produces the oily streak texture, which is often expected in cholesteric phases (Figure 1b).29 The spontaneously constructed CLC helical nanostructures are 9571

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Figure 2. (a) Free-standing chirophotonic crystal films obtained by the in situ photopolymerization. (b) Selective reflection spectra for BF, GF, and RF, respectively. (c) Cross-sectional scanning electron microscopy images of the fractured (c) blue-color film (BF), (d) green-color film (GF), and (e) red-color film (RF) show the periodic stacked layers of the helical nanostructures with well-controlled helical pitches.

Figure 3. (a) TGA thermograms of BF and its macroscopic images at different temperatures. (b) Optical texture of the carbon pencil scratched BF. (c) Photograph of BF after the chemical stability test in organic solvents.

finally disappear at the phase transition from the CLC to the isotropic phase (Figure S9). Free-standing CLC films are obtained by photopolymerizing the 10 wt % BP1CRM-doped cholesteric mixture with UV exposure. As noted in Figure 1, the selective light reflection of the CLC mixture with BP1CRM is shifted to a longer wavelength upon increasing the temperature. The temperature sensitivity of BP1CRM allows us to adjust the pitch of a given CLC mixture and subsequently fix it by photopolymerization. After sufficient photopolymerization at different temperatures, we can prepare a series of photonic crystals as free-standing solid films with lateral dimensions of several centimeters. Therefore, the resulting free-standing film can be easily handled using a tweezer when the cholesteric polymer is peeled off from the substrate, as shown in Figure 2a. Moreover, the CLC films

exhibit the vivid colors as a result of the selective reflection of visible light. The spectra taken at room temperature confirm the retentions of thermochromic helical nanostructures by photopolymerization (Figure 2b). The CLC film prepared at 50 °C shows a strong reflection band at λmax = 445 nm with a bandwidth of Δλ = 54 nm (measured at half of the maximum reflection) corresponding to the blue-color film (BF). The reflection spectra of CLC films photopolymerized at 70 and 90 °C show λmax = 535 nm for the green-color film (GF) and 645 nm for the red-color film (RF), respectively. The small but detectable broadening of the reflection band results from the contraction of pitches near the substrates during the photopolymerization of chiral RM and nematic RM.35 The contracted pitches also account for the decrease in transmission 9572

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ACS Nano at lower wavelengths (400−450 nm), as shown in Figure 2b.36 The transmission decrease in this region of the spectrum is due to the light scattering from the boundaries of polymer networks. Fingerprint defects are observed by polarizing optical microscopy in the photopolymerized CLC phase (Figure S10). However, the peak positions of selective reflections are almost identical before and after the photopolymerization of CLC mixtures. Therefore, it is concluded that three samples have the targeted helicoidal periodicities of their nanostructures, resulting in the selective light reflections of blue, green, and red colors. Scanning electron microscopy (SEM) examinations of the chirophotonic crystal reflectors confirm the constructions of helical nanostructures and the pitch retentions in the freestanding RGB films.37 Figure 2c shows the cross-sectional image along the film thickness direction of the BF with the pitch (PBF) of 280 nm. Figure 2d,e shows the pitches of CLC films for GF with PGF = 350 nm and RF with PRF = 420 nm. The static bandwidth of CLC is a few tens of nanometers in the visible wavelength and can be obtained by the product of birefringence and pitch.38 Therefore, the helical pitches are well-matched to the selective reflections in Figure 2b according to Bragg’s law. This kind of helical nanostructure and chirophotonic property can be found in nature, such as iridescent beetles having spiral nanostructures.39 From the experimental results, it is confirmed that the long-range molecular orientational order with chiral organization is successfully preserved in the free-standing CLC films during the in situ photopolymerization. Thermal stability of the free-standing CLC film is investigated by thermogravimetric analysis (TGA).40 The representative BF shows about 5% weight loss at 315 °C (Figure 3a). The overall thermal degradation profile is monitored above 550 °C. The reflection color of BF is stable up to 315 °C, as expected for a heavily cross-linked polymer network (Figure S11). The mechanical robustness of CLC films is estimated by a hardness test (ASTM D3363).41 Resistance of BF against pencil scratch is judged by microphotographs taken from the reflection mode of optical microscopy, as shown in Figure 3b. By increasing the pencil hardness from 4B to 4H, the CLC film resists against the scratching up to HB (Figure S12). BF starts to be scratched above 2H. This robust mechanical stability of the CLC film should be due to the stabilization of ordered helical nanostructures by polymer networks.42 Chemical stabilities are also evaluated by immersing the freestanding CLC films in the organic solvents (Figure 3c).43 The blue color reflection remains in both nonpolar and polar solvents (Figure S13). Cross-linked polymer films provide a good dimensional stability. Thermomechanical properties of the CLC films are further examined by a dynamic mechanical analyzer (DMA).44 Flexible characteristics of robust free-standing photonic crystal films are represented in Figure 4a. Storage modulus (E′) and glass transition temperature (Tg) of BF are summarized in Figure 4b. The photopolymerized BF shows Tg = 97 °C obtained by the peak position of loss tangent (tan δ) and E′ = 0.7 GPa at 25 °C. Differential scanning calorimetry (DSC) shows a broad transition region at 90 °C (Figure S14), which agrees well with the DMA results.45 Due to the excellent thermal stability, mechanical properties, and solvent resistances, the materials and methods described here could be employed in the applications ranging from displays, architectural coatings, and optoelectronics.

Figure 4. (a) Macroscopic images of a free-standing chirophotonic crystal film exhibit the reversible mechanical bending property. (b) DMA analysis shows the thermomechanical behaviors of BF.

Figure 5a presents a schematic diagram to illustrate the method employed to fabricate a patterned RGB reflector. The

Figure 5. (a) Schematic illustration of the fabrication process for a multicolor reflective CLC film. (b) Macroscopic image of the optical cell exhibiting three primary colors. (c) Photograph of the free-standable and color-patternable chirophotonic crystal reflector.

initial blue reflection pitch is produced by adding CLC mixture of 10 wt % BP1CRM. First, the optical cell filled with the CLC mixture is exposed to UV light at 50 °C through a photomask to locally fix the helical pitch. Subsequently, the temperature is increased to 70 °C to shift to the green color. Second, the unmasked area is photopolymerized to retain the green color. Finally, the sample temperature is increased to 90 °C to create the red reflection, after which the sample is exposed to UV light to polymerize the final region and to retain the red reflection. As a result, chirophotonic materials exhibiting the brilliant colors covering the visible wavelength region of red, green, and blue reflections are successfully obtained from the single optical cell with a single cholesteric monomer content (Figure 5b). The flexible nature of the full color patterned film is evident in Figure 5c. The CLC films fabricated by a simple process can be effectively applied in next-generation displays and sensors because they do not require a color filter and battery.46 As 9573

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molecule (abbreviated as BP1CRM) was newly designed and synthesized. By doping BP1CRM with a high helical twisting power (HTP = 22 μm−1) into the nematic diacrylate reactive mesogen, helical nanostructures were spontaneously induced. The subsequent photopolymerization stabilized the helical nanostructures in the CLC films, which could be used as circular-polarizing chirophotonic crystal films. Upon tuning the temperature of the helical nanostructure, we can precisely control the structural colors from blue to green and to red, in which the selected colors can be further fixed by photopolymerization. The effectively cross-linked free-standing chirophotonic crystal reflectors exhibited excellent mechanical, thermal, and chemical stabilities. The development of flexible chirophotonic crystal reflectors can be applied in smart paints and photocommunication materials.

previously discussed, the intrinsic chiroptical properties of flexible CLC films could be used in the reflective devices.47 It is well-known that the reflective light obtained from a CLC film is circularly polarized with the same handedness of the CLC film, and the opposite handedness circularly polarized light is transmitted (polarization selective reflection).48 Therefore, the color film prepared by the BP1CRM reflects the right-handed circularly polarized light, as shown in Figure 6a. In Figure 6b,

METHODS Synthesis. The compound of 4′-[6-(2-methacryloyloxy)hexyloxy]biphenyl-4-carboxylic acid (0.25 mmol) was added to a solution of N(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (1.25 mmol) and 4-(dimethylamino)pyridine (1.25 mmol) in 50 mL of tetrahydrofuran/methylene chloride mixed solvent (v/v = 1/4). After being stirred at room temperature for 15 min, (R)-1-(2-naphythyl)ethanol (1.0 mmol) was introduced as a solid, and the solution was stirred at room temperature for 48 h. After distillation, the residue was dissolved in chloroform and washed with water several times. The crude product was purified by column chromatography on silica gel using ethyl acetate/methylene chloride = 1:4 to yield BP1CRM as a white powder (yield 72%). 1H NMR (400 MHz, CDCl3, δ): 8.15 (d, 2H), 7.85 (d, 2H), 7.81 (d, 1H), 7.68 (d, 3H), 7.55 (d, 3H), 7.49 (d, 2H), 6.98 (d, 2H), 6.32 (m, 1H), 6.07 (s, 1H), 5.58 (s, 1H), 4.11 (t, 2H), 4.01 (t, 2H), 1.91 (s, 3H), 1.72 (m, 7H), 1.45 (m, 4H) ppm. 13C NMR (400 MHz, CDCl3, δ): 167.8, 166.1, 159.5, 145.4, 139.4, 136.8, 133.1, 131.9, 130.0, 128.8, 128.5, 128.4, 126.5, 126.4, 126.1, 124.9, 124.8, 124.3, 124.2, 115.5, 73.4, 68.1, 64.7, 29.4, 27.2, 22.1, 18.2 ppm. MALDI-ToF: m/z = 559.0 [M + Na]+ (calcd m/z = 536.2). Preparation. The mixture of 10 wt % BP1CRM and 90 wt % RM257 was homogeneously mixed at 100 °C and capillary-filled into the planar aligned glass cell with 10 μm thickness. Irgacure 651 was used as a photoinitiator. For the photopolymerization, the samples were placed on the preset heating stage (LINKAM LTS 350) and irradiated with a 100 mW/cm2 UV light (USHIO SP9) for 10 min. The polymerized cells were carefully opened to obtain the freestanding chirophotonic crystal film. The optical textures of CLC phases were monitored using cross-polarized optical microscopy (Nikon Eclipse E600). The reflection color changes were determined by a spectrophotometer (Ocean Optics QP1000) equipped with the reflection mode of optical microscopy. For the SEM (Carl Zeiss SUPRA 40VP) observation, the CLC film was frozen in the liquid nitrogen for 5 min and then rapidly cut with a razor blade. The macroscopic photographs were taken using a digital camera (Canon EOS 5D). Characterization. Proton (1H) and carbon (13C) NMR (JEOL JNM EX400), MALDI-ToF MS (Voyager DE STR), and elemental analysis (Vario EL) were conducted to identify the chemical structure and purity of BP1CRM. The polarized reflective light of chirophotonic reflector was measured by UV−vis spectroscopy (JASCO ARSN 733) with linear polarizer and circular polarizer. Storage modulus (E′) and tan δ of CLC films were determined by thermomechanical analysis (TA Q800) with operating cantilevers of 7 mm × 5 mm × 10 μm at a strain of 1% and frequency of 1.0 Hz over the temperature range of 25 to 175 °C. Thermal properties were monitored using TGA (TA Q50) and DSC (TA Q20) at 10 °C/min. The mechanical stability was also estimated by the pencil hardness test using STAEDTLER Mars Lumograph 100 G12 (ASTM D3363), and the results were observed by optical microscopy.

Figure 6. (a) Transmittance change of the chirophotonic crystal reflector which is probed with left- or right-handed circularly polarized lights. (b) BF shows a blue color with a right-handed circular polarizer, while the reflection color is mostly lost with a left-handed circular polarizer. (c) Reflection intensity of the BF plotted at the different angles of the polarizer coupled with a retarder. (d) Chirophotonic crystal reflectors with a quarter-wave plate exhibit linearly polarized lights.

the circularly polarized light and the helical nanostructure have opposite handedness and no light is reflected. To utilize the CLC film in current display industries, the conversion of circularly polarized lights to linearly polarized ones is required.49 In an attempt to generate the linearly polarized light from right-handed circularly polarized light of the CLC film, a quarter-wave plate is arranged, where the direction of polarized light is at an angle of 45° with respect to the optical axis.50 The color patterned CLC film is illuminated by light with a black background. As shown in Figure 6c, the intensity distribution of reflected light exhibits linear polarization characteristics that refer to the rotation of the polarizer axis.51 Compared with a conventional polarizer, coatable CLC films can allow us to fabricate electro-optical devices with excellent mechanical flexibility and design diversity. When the quarterwave plate is rotated to 45°, total reflection with brilliant color is observed (Figure 6d). When the linearly polarized light and optic axis are parallel, there are no reflective colors observable from the CLC film.

CONCLUSIONS For the fabrication of free-standing and circular-polarizing chirophotonic crystal reflectors, a chiral reactive monoacrylate 9574

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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/acsnano.6b04949. Synthesis of BP1CRM, sample preparation for optical and morphological studies, and characterization methods (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was mainly supported by BRL (2015042417), MOTIE (10047806), MOTIE-KDRC (10051334), and MidCareer Researcher Program (2016R1A2B2011041) of Korea. REFERENCES (1) Li, Q.; Green, L.; Venkataraman, N.; Shiyanovskaya, I.; Khan, A.; Urbas, A.; Doane, J. W. Reversible Photoswitchable Axially Chiral Dopants with High Helical Twisting Power. J. Am. Chem. Soc. 2007, 129, 12908−12909. (2) Land, M. F.; Nilsson, D.-E. Animal Eyes; Oxford University Press, 2012. (3) White, T. J.; Bricker, R. L.; Natarajan, L. V.; Tondiglia, V. P.; Green, L.; Li, Q.; Bunning, T. J. Electrically Switchable, Photoaddressable Cholesteric Liquid Crystal Reflectors. Opt. Express 2010, 18, 173−178. (4) Tilley, R. J. D. Colour and the Optical Properties of Materials; Wiley-VCH, 2010. (5) Wang, G.; Chen, X.; Liu, S.; Wong, C.; Chu, S. Mechanical Chameleon through Dynamic Real-Time Plasmonic Tuning. ACS Nano 2016, 10, 1788−1794. (6) Jeong, K.-U.; Jang, J.-H.; Kim, D.-Y.; Nah, C.; Lee, J. H.; Lee, M.H.; Sun, H.-J.; Wang, C.-L.; Cheng, S. Z. D.; Thomas, E. L. ThreeDimensional Actuators Transformed from the Programmed TwoDimensional Structures via Bending, Twisting and Folding Mechanisms. J. Mater. Chem. 2011, 21, 6824−6830. (7) Sharma, A.; Mori, T.; Lee, H.-C.; Worden, M.; Bidwell, E.; Hegmann, T. Detecting, Visualizing, and Measuring Gold Nanoparticle Chirality Using Helical Pitch Measurements in Nematic Liquid Crystal Phases. ACS Nano 2014, 8, 11966−11976. (8) Dumanli, A. G.; van der Kooij, H. M.; Reisner, E.; Baumberg, J. J.; Steiner, U.; Vignolini, S. Digital Color in Cellulose Nanocrystal Films. ACS Appl. Mater. Interfaces 2014, 6, 12302−12306. (9) Mitov, M.; Dessaud, N. Going Beyond the Reflectance Limit of Cholesteric Liquid Crystals. Nat. Mater. 2006, 5, 361−364. (10) Gur, D.; Leshem, B.; Oron, D.; Weiner, S.; Addadi, L. The Structural Basis for Enhanced Silver Reflectance in Koi Fish Scale and Skin. J. Am. Chem. Soc. 2014, 136, 17236−17242. (11) Sharma, V.; Crne, M.; Park, J. O.; Srinivasarao, M. Structural Origin of Circularly Polarized Iridescence in Jeweled Beetles. Science 2009, 325, 449−451. (12) Vignolini, S.; Rudall, P. J.; Rowland, A. V.; Reed, A.; Moyroud, E.; Faden, R. B.; Baumberg, J. J.; Glover, B. J.; Steiner, U. Pointillist Structural Color in Pollia Fruit. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15712−15715. (13) Bisoyi, H. K.; Li, Q. Light-Directing Chiral Liquid Crystal Nanostructures: From 1D to 3D. Acc. Chem. Res. 2014, 47, 3184− 3195. (14) Wang, B.; Walther, A. Self-Assembled, Iridescent, CrustaceanMimetic Nanocomposites with Tailored Periodicity and Layered Cuticular Structure. ACS Nano 2015, 9, 10637−10646. 9575

DOI: 10.1021/acsnano.6b04949 ACS Nano 2016, 10, 9570−9576

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DOI: 10.1021/acsnano.6b04949 ACS Nano 2016, 10, 9570−9576