Biomimetic Structural Color Films with a Bilayer Inverse

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Applications of Polymer, Composite, and Coating Materials

Biomimetic Structural Color Films with Bilayer Inverse Heterostructure for Anti-Counterfeiting Application Yao Meng, Jinjing Qiu, Suli Wu, Benzhi Ju, Shufen Zhang, and Bingtao Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14146 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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ACS Applied Materials & Interfaces

Biomimetic Structural Color Films with Bilayer Inverse Heterostructure for Anti-Counterfeiting Application Yao Meng, Jinjing Qiu, Suli Wu, Benzhi Ju, Shufen Zhang, and Bingtao Tang* State Key Laboratory of Fine Chemicals, Dalian University of Technology, P.O. Box 89, West Campus, 2# Linggong Rd, Dalian 116024, China. E-mail: [email protected]; Tel: +86-411-84986267 KEYWORDS: heterostructure, colloidal crystal, quasi-amorphous array, bilayer inverse structure, anti-counterfeiting, structural color

ABSTRACT

The unique brilliant and angle-independent structural colors of Morpho butterfly wings were derived from the multilayer interference, diffraction, and scattering of light with the composite structure including ordered and quasi-amorphous arrays. Inspired by the biological heterostructure of ordered and quasi-amorphous arrays in the wings, bilayer inverse heterostructure (BLIHS) containing ordered (OALIS) and quasi-amorphous arrays layer inverse structures (Q-AALIS) of polyvinylidene fluoride (PVDF) were successfully prepared through the template method. The BLIHS films selectively displayed iridescent structural color derived from Bragg diffraction of OALIS, whereas the color states 1

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transform to non-iridescent color due to Q-AALIS just by rotating the sample. Furthermore, the patterning process could be realized by using spray coating method onto the BILIS films as quasi-amorphous array layers (Q-AALs). By virtue of this novel photonic structure, the switch between hiding and displaying patterns could be easily realized by changing the viewing angles, and the as-prepared films exhibited inherent excellent durability, which is crucial to their potential for practical application as anti-counterfeiting materials. INTRODUCTION Structural colors, which are produced by the interaction of light with nanoarchitectures of many organisms in nature, have attracted increasing attention in recent years1-5 and are classified into iridescent and non-iridescent colors.6 Mallards,7 sea mice,8 and peacocks9 are representative natural creatures that exhibit iridescent structural colors due to the reflection or interference of light with ordered arrays embedded in their wings or feathers. Inspired by the regular structures, an artificial material, named photonic crystals (PhCs),10 with lattice spacings on the scale of the wavelength of light, was prepared and studied extensively.11 In PhCs, light cannot be propagated in a certain frequency range located in the photonic band gap (PBG).12 Therefore, iridescent structural colors derived from Bragg diffraction can be achieved and tuned by the PBG via changing the lattice spacing or effective refractive index.13-16 Nevertheless, non-iridescent structural colors17 with wide viewing angles are also found in the feathers of many birds (Cotinga cotinga),18 longhorn

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beetles,19 and chameleons20-21 and are generated from the coherent scattering of light by the quasi-amorphous arrays in the nanostructure.22-24 In contrast to the above natural creatures that can produce structural colors, the Morpho butterfly from Central and South America is one of the species that exhibits a unique optical performance.25 The wings of M. didius butterflies produce angle-independent structural colors with iridescence effect and high reflectivity, and the colors can be detected and remain a brilliant bluish in wide viewing angles due to the heterostructure of ordered and quasi-amorphous arrays in their wings.26 The multilayered and similarly shaped ridges are closely packed into ordered arrays on the wing scales to produce iridescent structural color with high reflectivity because of the multilayer interference.27-28 Meanwhile, the quasiamorphous arrays attributed to variations in height and the narrow width of the ridges will break the periodicity and the continuous multilayer interference of the ridges,26, 29 which results in diffuse and uniformly distributed reflectance at various angles.30 Moreover, the pigments beneath the wings reduce background reflection and enhance color saturation by absorbing unnecessary light. Thus, M. didius butterflies produce unique iridescent structural colors through multilayer interference, diffraction, and the scattering effect of light by the unique heterostructure, which combine seemingly conflicting features of ordered and quasi-amorphous arrays.25 Inspired by the hierarchical structure of the M. didius butterflies’ wings, we designed polyvinylidene fluoride (PVDF) structural color films with bilayer inverse heterostructure 3



(BLIHS) which including an ordered array layers inverse opal structure (OALIS) and a quasi-amorphous array layers inverse opal structure (Q-AALIS) beneath the OALIS. BLIHS is a simplified way to combine the characteristics of ordered and quasi-amorphous arrays similar to the unique hierarchical structure of M. didius butterfly wings’, and the materials were endowed with a different optical property. The iridescent structural color was produced by the OALIS through Bragg diffraction, whereas the non-iridescent colors were derived from light scattering by the Q-AALIS. Under the specular reflection angle, BLIHS films selectively display vivid iridescent colors of the OALIS because the scattered light of the Q-AALIS is suppressed by the strong Bragg diffraction. While the scattered light of the Q-AALIS will pass through the ordered layers under diffuse reflection conditions, BLIHS films exhibit the non-iridescent colors of the Q-AALIS. Meanwhile, the quick response code (QR code) patterns can be transferred onto the PVDF films as quasi-amorphous array layers (Q-AALs) by using the spray coating method. A switch between hiding and displaying the patterns of this photonic heterostructure can be easily realized by changing the viewing angles. Multiple color states switching makes the asprepared films a reliable anti-counterfeiting material. At the same time, the as-prepared PVDF films possess excellent durability and can be bent freely, which are crucial for anticounterfeiting applications. RESULTS AND DISCUSSION

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First, SiO2 ordered array layers (OALs) were assembled using the dip-coating method with 240, 280, and 320 nm SiO2 nanospheres (Figure 1a). Q-AALs were fabricated via the spray coating method, and the SiO2 ethanol dispersions were sprayed onto the surface of the OALs using an airbrush (Figure 1b).31 The bilayer template, which is composed of OALs (green part) and Q-AALs (blue part), was prepared (inset in Figure 1b) after the spray coating process step. A solution containing PVDF polymer and surface-modified carbon black (CB) dispersed in hexamethyl phosphoramide (HMPA) was dropped onto the surface of the bilayer template with the assistance of driving forces, including strong capillary and high vacuum pressures (Figure 1c). The background and incoherent scattering light could be absorbed by CB, and the color saturation of the structural color films was enhanced significantly. The PVDF polymer solution could infiltrate the template interstices completely and efficiently, replacing air (0.003 MPa and heated to 135 °C, Figure 1d). The PVDF/HMPA solution could adhere and wet the surface of SiO2 nanospheres very well because they possess similar wettability. Moreover, the viscosity of the PVDF/HMPA dispersion remained low at 135 °C; therefore, the external polymer solution would replenish the pores that resulted from the evaporation of the solvent among the nanospheres throughout the drying process. When HMPA evaporated completely after 30 min, the PVDF/SiO2 film could be peeled off from the glass substrate. After the SiO2 template was corroded with 4 wt.% hydrofluoric (HF) acid, the PVDF BLIHS film was fabricated and displayed brilliant structural color (Figure 1e). The surface of the BLIHS film was an OALIS, whereas a Q-AALIS was embedded underneath the OALIS (inset in Figure 1e). 5



Figure 1. Schematic illustration of preparation of the BLIHS PVDF colored films. (a) SiO2 ordered arrays template. (b) Quasi-amorphous array layers were spray-coated onto the surface of the SiO2 ordered array layers (OALs), the upper part for Q-AALs and the lower part for OALs (inset). (c) The driving forces leading PVDF polymer dispersion infiltrate into the interstice of the template. (d) PVDF/CB/SiO2 composite film with SiO2 bilayer template. (e) The BLIHS PVDF films was formed after removal of the SiO2 template. The wings of M. didius butterflies exhibited iridescent structural color with high reflectivity and wide viewing angles (insets of Figure 2a, magnified images in Figure S1). The wing is composed of curved cover scales (Figure 2a). The ridges in the wing run parallel along the long side of the scale (Figure 2b) and are closely packed with high 6


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periodicity (Figure 2c). The ordered multilayered ridges generate an iridescent structural color through multilayer interference. The differences in the exact shapes between the ridges and the random variation in spatial positions such as the ridge height (Figure 2d) destroy the regularity of the ridge arrays to form quasi-amorphous structures, which produce the angle-independent structural colors. Based on the advantages of the hierarchical structure of M. didius butterfly wings, a novel bilayer heterostructure was prepared combining the characteristics of ordered and quasi-amorphous arrays. The OALs of a SiO2 template were assembled by the dip-coating method, and the face-centered cubic closely packed structure is clearly seen from the scanning electron microscope (SEM) images in Figures 2e (top-view) and 2f (cross-section). The 2D fast Fourier transform (2DFFT) pattern of the OALs show sharp hexagonal peaks in the inset, indicating the presence of a long-range crystalline order. The Q-AALs that were fabricated by spray coating are demonstrated in the SEM images in Figures 2g (top-view) and 1h (cross-section). The 2DFFT pattern (inset in Figure 2g) shows bright clusters of white pixels that are concentrated in symmetrical, circular patterns around the origin, indicating that the SiO2 nanospheres were isotropically distributed with short-range order in Q-AALs. The bilayer template including Q-AALs and OALs (separated by the red dotted line) was very smooth, as shown in the cross-section SEM image in Figure 2i. The pores that formed after the removal of the SiO2 template were distributed uniformly on the surface of the OALs (Figure S2). The BLIHS composed of OALIS and Q-AALIS is denoted x/y BLIHS in the subsequent discussion, where x stands for the SiO2 diameter of OALs, and y stands for the diameter of 7



the Q-AALs. The 280/280 BLIHS composed of OALIS and Q-AALIS (Figures 2j, 2k and 2l) demonstrates the regularity and irregularity of BILIS. The thicknesses of the OALs and Q-AALs were 5.1 µm and 4.5 μm, respectively.

Figure 2. (a), (b) SEM images of wings scales of M. didius, inset of (a) is digital image of M. didius. (c), (d) SEM images of the ground scale of M. didius, and the multilayered ridges cross-section of the M. didius butterfly. (e) SEM image of the top-view of the SiO2 OALs, the inset picture is the 2D-FFT patterns of corresponding SEM images of OALs. (f) The cross-section SEM images of the OALs (dip-coated for two times). (g) SEM image of the top-view of the SiO2 Q-AALs, the inset picture is the 2D-FFT patterns of corresponding SEM images of Q-AALs. The cross-section SEM images of the (h) QAALs, (i) bilayer template including OALs and Q-AALs array layers and the (j) 280/280 8


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BLIHS PVDF film, respectively. (k) and (l) Magnification of the SEM image of the OALIS and Q-AALIS parts of (j). The upper OALIS in BLIHS exhibits iridescent structural color. The angle dependence of 280 OALIS and 280/240, 280/280, and 280/320 BLIHS PVDF films was evaluated using their specular reflective spectra as the angle of incident light varied from 5° to 50° (Figures 3a-d). The peak shapes, peak heights, full width at half maxima (FWHM), and baseline positions of the reflective spectra were very similar to each other at the corresponding incidence angles. And the data about the detailed information of reflective spectra of the above four structural films were listed in Table S1 when the incidence angle at 50o. This results demonstrated the consistency of the peak shape of the reflectance spectra at different incident angles. The four digital photos (insets in Figures 3a-d) show vivid green with high brightness and saturation at 5°. When the angle of incidence varied from 5° to 50°, the blueshifts of the peak positions of the four samples were very close (Δλ280=147-150 nm, Figure 3e). All reflective peak positions of 280/240, 280/280, and 280/320 BLIHS (λBLIHS) films were blueshifted less than a dozen nanometers compared with that of 280 OALIS (λOALIS) film at 5° incidence angle (Figure 3f). The reflective spectra of the 240 and 320 nm series samples were also in accordance with these principles, and the blueshift of the reflective peak positions were also restricted to a fairly limited range (Δλ240 = 120–125 nm and Δλ320 = 170–174 nm, Figure S3). Although the Q-AALIS was embedded underneath the OALIS (Figure 2j), the scattered light by the Q-AALIS was negligible compared with 9



the reflected light by the OALIS, indicating that Bragg diffraction plays a dominant role for the optical performance of the BLIHS PVDF films. Thus, the Q-AALIS has almost no influence on the iridescent structural colors and angle dependence of OALIS under specular reflection angles. However, BLIHS will exhibit the color of the Q-AALIS under diffuse reflection conditions.

Figure 3. The angle-dependence of the (a) PVDF structural color film with 280 nm OALIS, (b) 280/240 BLIHS, (c) 280/280 BLIHS, and (d) 280/320 BLIHS, specular reflection spectra were measured with the angles of incident light vary from 5o to 50o, and the digital photos of the corresponding samples in the inset were taken under 5o of the incident light. (e) Changes of the reflection spectra peak positions of the four samples as the incidence angles increased from 5o to 50o. (f) Changes of the reflection spectra peak

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positions of the representative twelve colored PVDF plastic films when the incident angle was 5o. Scale bars: (a) 1 cm. The effect of the Q-AALIS on OALIS is evaluated in Figure 4 under the diffuse reflection mode (Figure S4). Three types of colored PVDF films with an OALIS, Q-AALIS, and BLIHS were prepared as the steps listed in Figure 1. OALIS PVDF films (prepared from 240, 280, and 320 nm OALs SiO2 templates) showed blue, green, and red with a high visibility and iridescence effect (Figure 4a, column i), and a high reflectivity and narrow peak width were demonstrated by their reflection spectra (black curves in Figure 4b). In contrast, 240, 280, and 320 Q-AALIS PVDF films exhibited non-iridescent blue, green, and red (Figure 4a, column ii) with a low reflectivity and wide peak width in their reflectance spectra (blue curves in Figure 4b).32 This finding is attributable to the color derived from the scattering effect of coherent light with low intensity by the Q-AALIS.33 As a result of the synergy of light with the Q-AALIS and OALIS, 240/240, 280/280, and 320/320 BLIHS PVDF films still exhibited vivid structural color (Figure 4a, column iii), which became slightly pale than that of OALIS films, and the decrease resulted from the effect of the Q-AALIS. Therefore, we can see increment in baseline of BLIHS films as the absolute peak height drops, accompanied with slight increase in peak width (red curves in Figure 4b). The reflectance spectra of BLIHS contains the light scattered by the Q-AALIS in the whole visible region and the light reflected by OALIS (as shown in Figure 4c), so the high baseline for BLIHS is a result of the synergy of light with Q-AALIS and OALIS. 11



And the thin OALIS is transparent enough for the resonant wavelengths of the Q-AALIS which demonstrated by the transmittance spectra (Figure S5). The reflective spectra were transformed into Commission Internationale de L’Eclairage (CIE) chromaticity value for a more standardized expression (Figure 4d), and the color coordinates of PVDF films with OALIS, BLIHS, and Q-AALIS gradually became close to the white light point in accordance with their optical performances. The above digital photos and spectra of the PVDF films demonstrated that the Q-AALIS can influence the optical performance of the OALIS. Under diffuse reflection conditions (the incident angle differing from the viewing angle), the non-iridescent blue of 240 Q-AALIS in 320/240 BLIHS film was observed, whereas 320/240 BLIHS film shows iridescent red under specular reflection angles (Figure 4e).

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Figure 4. (a) The digital photos of PVDF structural color films with Q-AALIS (column i), OALIS (column ii), and BLIHS (column iii) prepared by 240 nm, 280 nm, and 320 nm, respectively. (b) Reflective spectra of the Q-AALIS, OALIS, and BLIHS structural color films measured under integrating sphere mode. (c) Schematic illustration of the reflectance and scattering of lights interacted by the BLIHS. (d) A CIE chromaticity diagram obtained by the spectra of (b). (e) Color switching of 320/240 BLIHS film. Scale bars: (a) and (e) 1 cm. By virtue of the spray-coating method, some precise patterns could be easily printed on the BLIHS film. To prepare the bilayer template, 240 nm SiO2 nanoparticles were sprayed 13



onto the surface of the 320 nm OALs with the assistance of a mask of QR codes (official website of Dalian University of Technology). The 320/240 BLIHS PVDF film with a QR code pattern was prepared as the process shown in Figure 5a. The switch between hiding and displaying of the pattern in the 320/240 BLIHS film could be realized just by rotating the samples (Figure 5b), because the films exhibited distinguishable optical performances through the synergy of light with the OALIS and Q-AALIS. When the angle of incident light θ (equals to viewing angles) deviated from the normal of the PVDF film at 5°, the 320/240 BLIHS PVDF film showed vivid iridescent red (Figures 5c). Meanwhile, the QR code pattern (Q-AALIS) was hidden completely by the OALIS because the Bragg diffraction of light by the OALIS would suppress the scattering effect by the Q-AALIS under the symmetric reflection angles. Rotating the sample slightly (Figures 5d), the scattered light could pass through the OALIS and dominate the color performance, and the blue QR code pattern (240 nm Q-AALIS) displayed and showed non-iridescent blue at β values of 5° (Figures 5d).34 The normal daylight lamp was used as the light source in the switching process of the pattern, and no special flashlight is needed. A smartphone can extract QR code and access the website (pattern displayed) by scanning the QR codes (Video S1 and Figure S6). The abundant optical states, including three iridescent colors (Figure S7) and switching between the invisible and visible of the QR code pattern (Figures 5c and 5d, Video S1), make the PVDF BLIHS flim a reliable anti-counterfeiting materials. The optical state change required no external stimuli that would not damage the original porous structure and shorten the lifetime. 14


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Tensile and bending tests were designed to evaluate the mechanical strength of the BLIHS PVDF structural color films. The BLIHS film (with a thickness of approximately 60 μm) was clamped tightly by the test fixtures, and the fixtures stretched the film gradually at a rate of 5 mm/min (inset of Figure 5e). The BLIHS film was only stretched to 7% when the stress reached 20 MPa, which demonstrated the excellent tensile strength of the BLIHS PVDF films. In addition, the breaking stress of the BLIHS PVDF films reached 29.7 MPa. Similar to the schematic of the bending test in the inset (Figure 5f i), the film was bent into a curved surface at a curvature of 3 mm (Figure 5f ii). The reflective spectra were almost unchanged after 1000 bending tests (Figure 5f). The PVDF/PMMA structural color film was prepared using the heat-pressed method, and the composite film showed great resistance to friction (Figure S8). Good durability was derived from the inherent covalent bond between molecular and close-packed macromolecule chains of the PVDF resin. The excellent toughness (29.7 MPa) and flexibility of the BLIHS films are important for their practical applications.

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Figure 5. (a) Schematic diagram of the preparation of the patterned 320/240 BLIHS PVDF film, and 240 nm SiO2 nanoparticals were sprayed out as angle-independent colored pattern “QR code”. (b) The patterned 320/240 BLIHS film displayed and hidden the QR code under different viewing modes. (c) The diagram of specular reflection mode and the corresponding digital photos of the BLIHS film at the incident angles θ of 5o for red. (d) The diagram of diffuse reflection mode and the corresponding digital photos exhibits the blue pattern as the β of 5o. (e) The tensile curve of the BLIHS PVDF films (29.7 MPa), schematic of the tensile test (inset). (f) The reflectance spectra of the BLIHS 16


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PVDF film before and after 1000 times bending tests, (i) Schematic diagram of the bending test, (ii) digital photos of the bended BLIHS PVDF film. CONCLUSION Inspired by the Morpho butterfly wings, a new PVDF film with BLIHS containing QAALIS and OALIS was successfully prepared. The PVDF BLIHS film combined the paradoxical features of ordered and quasi-amorphous arrays. A number of complex patterns (QR codes) could be printed on the BLIHS films. The synergistic effect of light with ordered and quasi-amorphous arrays in BLIHS films was demonstrated in this study. The patterns were hidden by brilliant structural color derived from the interaction of light with the OALIS because of the Bragg diffraction, which would suppress the scattering effect from the Q-AALIS. The non-iridescent structural color originating from the scattered light of the Q-AALIS dominated the optical performance, and the QR code pattern could be read by a smartphone. The excellent mechanical strength (29.7 MPa) and flexibility (1000 bending tests) of the BLIHS PVDF films are crucial for their practical application in anti-counterfeiting materials. METHODS Materials. Carbon black (CB) M-900 was purchased from the Cabot Corporation. The average Mw of polyvinylidene fluoride resin (PVDF, refractive index 1.42, Sigma Aldrich) was 180000 as measured using GPC. 4-Nitroaniline, sodium nitrite, absolute ethanol (AR), 17



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hydrofluoric acid (40 wt.%) and ammonia solution (25 wt.%) were purchased from Fuyu Chemical Reagent Factory. Tetraethyl orthosilicate (TEOS, wSiO2> 28%, Xilong Chemical Reagent Factory), hexamethylphosphoramide (HMPA, wHMPA > 98.5%, Sinopharm Chemical Reagent Co., Ltd) were used as received. A butterfly specimen was supplied by the Shanghai Qiuyu Biotechnology Co., Ltd. Preparation

of

Monodispersed

SiO2

Nanospheres,

SiO2

arrays

and

PVDF/CB/HMPA dispersion. SiO2 nanospheres were prepared under a modified Stöber method.35 The volume of Mixture A was 50 mL with various concentration of ammonia solution. Mixture B was composed of 80 mL of ethanol and 20 mL of TEOS. Mixture A was added to 100 mL of ethanol under vigorous stirring conditions (1500 r/min), and Mixture B was poured quickly into the system, 600 r/min after two minutes. The preparation process of SiO2 nanospheres finished after 3 h (detailed information on the SiO2 nanospheres is listed in Table S2), and washed with ethanol. 8 wt.% SiO2 ethanol dispersion was prepared for further use. A SiO2 photonic crystal template was prepared via the dip-coating method (schematic in Figure S9). The pulling rate of dip-coating assemble process was 2 μm/s for two times. The Q-AALS template was sprayed from an airbrush uniformly driven by an air pump under a pressure of 100 kPa, and usage of SiO2 emulsion was 10 μL/cm2. The surface of CB was modified by grafting nitrobenzene free radicals from the decomposition of diazonium salt (Figure S10). The modified CB (0.02 g) was dispersed in HMPA (20 mL) under ultrasonication for 30 min, and then the PVDF (2 g)

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was added and stirred for 24 h until the solution seemed uniform (65 °C and 20 r/min). The usage of the PVDF dispersion was 0.13 mL/cm2 for the template. ASSOCIATED CONTENT Supporting Information. The digital photos of M. didius butterfly. The top-view image of BLIHS film. The specular reflective spectra of 240 nm and 320 nm series samples. The integrating sphere mode. The transmittance spectra of the OALIS film. The printscreens and video of the extracting QR code process. Digital photos of 320/240 BLIHS film under different angles. The friction test of PVDF/PMMA composite film. Characterization section. The schematic illustration of the dip-coating method. The characterization of the SiO2 nanoparticles. The surface-modified process of CB. AUTHOR INFORMATION Corresponding Author Bingtao Tang,* E-mail: [email protected]. Tel: +86-411-84986267 Present Addresses † State Key Laboratory of Fine Chemicals, Dalian University of Technology, West Campus, 2# Linggong Rd, Dalian 116024, China. Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (21576039, 21276042, 21536002, 21421005, U1608223). The Fundamental Research Funds for the Central Universities DUT18ZD218. REFERENCES (1). Zhong, K.; Li, J.; Liu, L.; Cleuvenbergen, S. V.; Song, K.; Clays, K. Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals. Adv. Mater. 2017, No. 1707246. (2). Li, Q.; Zhang, Y.; Shi, L.; Qiu, H.; Zhang, S.; Qi, N.; Hu, J.; Yuan, W.; Zhang, X.; Zhang, K.-Q. Additive Mixing and Conformal Coating of Noniridescent Structural Colors with Robust Mechanical Properties Fabricated by Atomization Deposition. ACS Nano 2018, 12, 3095-3102. (3). Hou, J.; Li, M.; Song, Y. Patterned Colloidal Photonic Crystals. Angew. Chem., Int. Ed. 2018, 57, 2544-2553. (4). Xiao, M.; Li, Y.; Allen, M. C.; Deheyn, D. D.; Yue, X.; Zhao, J.; Gianneschi, N. C.; Shawkey, M. D.; Dhinojwala, A. Bio-Inspired Structural Colors Produced via SelfAssembly of Synthetic Melanin Nanoparticles. ACS Nano 2015, 9, 5454-5460. (5). Boyle, B. M.; French, T. A.; Pearson, R. M.; McCarthy, B. G.; Miyake, G. M. Structural Color for Additive Manufacturing: 3D-Printed Photonic Crystals from Block Copolymers. ACS Nano 2017, 11, 3052-3058. (6). Zhang, C.; Wu, B.-H.; Du, Y.; Ma, M.-Q.; Xu, Z.-K. Mussel-Inspired Polydopamine Coatings for Large-Scale and Angle-Independent Structural Colors. J. Mater. Chem. C 2017, 5, 3898-3902. 20


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Figure 1. Schematic illustration of preparation of the BLIHS PVDF colored films. (a) SiO2 ordered arrays template. (b) Quasi-amorphous array layers were spray-coated onto the surface of the SiO2 ordered array layers (OALs), the upper part for Q-AALs and the lower part for OALs (inset). (c) The driving forces leading PVDF polymer dispersion infiltrate into the interstice of the template. (d) PVDF/CB/SiO2 composite film with SiO2 bilayer template. (e) The BLIHS PVDF films was formed after removal of the SiO2 template. 153x103mm (300 x 300 DPI)



Figure 2. (a), (b) SEM images of wings scales of M. didius, inset of (a) is digital image of M. didius. (c), (d) SEM images of the ground scale of M. didius, and the multilayered ridges cross-section of the M. didius butterfly. (e) SEM image of the top-view of the SiO2 OALs, the inset picture is the 2D-FFT patterns of corresponding SEM images of OALs. (f) The cross-section SEM images of the OALs (dip-coated for two times). (g) SEM image of the top-view of the SiO2 Q-AALs, the inset picture is the 2D-FFT patterns of corresponding SEM images of Q-AALs. The cross-section SEM images of the (h) Q-AALs, (i) bilayer template including OALs and Q-AALs array layers and the (j) 280/280 BLIHS PVDF film, respectively. (k) and (l) Magnification of the SEM image of the OALIS and Q-AALIS parts of (j). 162x109mm (300 x 300 DPI)


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Figure 3. The angle-dependence of the (a) PVDF structural color film with 280 nm OALIS, (b) 280/240 BLIHS, (c) 280/280 BLIHS, and (d) 280/320 BLIHS, specular reflection spectra were measured with the angles of incident light vary from 5o to 50o, and the digital photos of the corresponding samples in the inset were taken under 5o of the incident light. (e) Changes of the reflection spectra peak positions of the four samples as the incidence angles increased from 5o to 50o. (f) Changes of the reflection spectra peak positions of the representative twelve colored PVDF plastic films when the incident angle was 5o. Scale bars: (a) 1 cm. 134x79mm (300 x 300 DPI)



Figure 4. (a) The digital photos of PVDF structural color films with Q-AALIS (column i), OALIS (column ii), and BLIHS (column iii) prepared by 240 nm, 280 nm, and 320 nm, respectively. (b) Reflective spectra of the Q-AALIS, OALIS, and BLIHS structural color films measured under integrating sphere mode. (c) Schematic illustration of the reflectance and scattering of lights interacted by the BLIHS. (d) A CIE chromaticity diagram obtained by the spectra of (b). (e) Color switching of 320/240 BLIHS film. Scale bars: (a) and (e) 1 cm. 145x113mm (300 x 300 DPI)


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Figure 5. (a) Schematic diagram of the preparation of the patterned 320/240 BLIHS PVDF film, and 240 nm SiO2 nanoparticals were sprayed out as angle-independent colored pattern “QR code”. (b) The patterned 320/240 BLIHS film displayed and hidden the QR code under different viewing modes. (c) The diagram of specular reflection mode and the corresponding digital photos of the BLIHS film at the incident angles θ of 5o for red. (d) The diagram of diffuse reflection mode and the corresponding digital photos exhibits the blue pattern as the β of 5o. (e) The tensile curve of the BLIHS PVDF films (29.7 MPa), schematic of the tensile test (inset). (f) The reflectance spectra of the BLIHS PVDF film before and after 1000 times bending tests, (i) Schematic diagram of the bending test, (ii) digital photos of the bended BLIHS PVDF film. 125x134mm (300 x 300 DPI)



Table of Contents 79x47mm (300 x 300 DPI)


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Biomimetic Structural Color Films with a Bilayer Inverse

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