Photoinduced Self-Epitaxial Crystal Growth of a Diarylethene

May 18, 2013 - ... National Institute of Advanced Industrial Science and Technology (AIST), Kansai Centre, Japan for the measurement of refractive ind...
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Photoinduced Self-Epitaxial Crystal Growth of a Diarylethene Derivative with Antireflection Moth-Eye and Superhydrophobic Lotus Effects Naoki Nishikawa,† Shingo Sakiyama,† Seiji Yamazoe,†,△ Yuko Kojima,‡ Ei-ichiro Nishihara,§ Tsuyoshi Tsujioka,∥ Hiroyuki Mayama,⊥ Satoshi Yokojima,# Shinichiro Nakamura,▽ and Kingo Uchida*,† †

Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan Science and Technology Research Center, Mitsubishi Chemical Group, 1000 Kamoshida, Yokohama 227-8502, Japan § DNP Fine Chemicals, Midori-ku Yokohama 226-0022, Japan ∥ Department of Arts and Sciences Faculty of Education, Osaka Kyoiku University, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582 (Japan) ⊥ Department of Chemistry, Asahikawa Medical University, 2-1-1-1 Midorigaoka-higashi, Asahikawa, Hokkaido 078-8510, Japan # School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan ▽ RIKEN Research Cluster for Innovation, Nakamura Laboratory, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡

S Supporting Information *

ABSTRACT: We identified the mechanism of the formation of needle-shaped microcrystals on which the contact angle of a water droplet exceeds 170° [Nishikawa, N. et al. Langmuir, 2012, 28, 17817−17824]. The standing needle-shaped crystal of the closed-ring isomer of a diarylethene 3c grew at a much lower temperature than the eutectic temperature by irradiation of UV light on the thin films of the open-ring isomer 3o, due to the epitaxial growth of the 013 plane of 3c over the 110 plane of the crystal lattice of 3o in the subphase. Therefore, the new crystal-growth mechanism triggered by the photoisomerization does not require special inorganic single-crystal substrates and may be called self-epitaxial crystal growth. The needle-shaped crystals appeared well-ordered and stood inclined at an angle of about 60° to the surface. Consequently, the photo-induced rough surface shows not only the superhydrophobic lotus effect, but also the antireflection moth-eye effect, and these effects were switchable by alternate irradiation with UV and visible light.

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INTRODUCTION Surface roughness control permits the modification of multifunctional surface properties, superhydrophobicity, and antireflection.1−6 Such surface functional materials have recently been inspired by natural plants and insects, with the former referred to as the lotus effect, and the latter the moth-eye effect (the effect’s mechanism is described in the Supporting Information, SI).1−6 These effects have been the subject of many research papers, because the unique properties of bioinspired, smart materials are attracting more and more interest related to many new real-world applications, such as self-cleaning, anti-reflection film on mobile displays, and gecko adhesion tape.7−10 These phenomena, as well as other natural ones including the attachment mechanism of geckos, are all related to unique micro- and nanostructures on surfaces.11−16 The creation of such complex functionalities in bioinspired materials depends on well-ordered multiscale structures. For the fabrication of superhydrophobic surfaces, micro- to © XXXX American Chemical Society

nanostructures are required, while the formation of antireflection surfaces requires rough surfaces whose roughness has a regularity at the size of light wavelengths.11−16 We have reported the surface topographical changes of the microcrystalline surface of diarylethenes 1o and 2o, accompanied by wettability changes, which are controlled by alternate irradiation with UV and visible light.17−22 However, the direction of the photogenerated needle-shaped crystals was randomly oriented, and their sizes were larger than 1 μm in diameter and 10 μm in length. Compared with superhydrophobic surfaces with high contact angle (CA),23−26 these crystals should be smaller (e.g., less than 1 μm in diameter) and ordered alignment was required in order to attain superhydrophobicity with the CA of a water droplet that Received: April 18, 2013 Revised: May 15, 2013

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were carried out on a Mettler Toledo FP90, to which an FP82HT hot stage was attached to keep the temperature at 30 °C. The static CA and SA were measured on an optical contact angle meter (Kyowa Interface Science Co., Ltd., Drop Master 500) with a capillary [outer diameter: 30 μm (straight)] at ambient temperature. Deionized water (1.5 μL) was dropped carefully onto the surface. An average CA value was obtained by measuring the samples at eight different positions. An atomic force microscope (AFM, Shimadzu SPM-9600) was used for measuring the force curve on the single-crystalline surfaces. Reflection spectra were measured on a JASCO V-670 Spectrophotometer equipped with a specular reflection ARMN-735, integrating sphere ISN-723, and large integrating sphere ILN-725.

exceeds 170°. In such a surface with microcrystals whose sizes are less than the wavelengths of light, the optical effect is also expected. Additionally, such inorganic single crystals are expensive; therefore, ordered surface formation without using special crystal substrates is strongly desired. Recently, we by chance discovered a surface made of a diarylethene in which the CA of a water droplet exceeds 170°.22 The superhydrophobic surface was covered with needle-shaped crystals (less than 0.2−0.3 μm in diameter and 2.2−2.5 μm in length) that grew at 30 °C, much lower than the eutectic temperature of diarylethene 3.22 A scanning electron microscope (SEM) easily confirmed that the needle-shaped crystals grew at 30 °C standing on the surface, unlike the crystals grown at the eutectic temperature of 142 °C that lie flat on the surface. This is quite unexpected since the results were obtained on a glass substrate. In addition, needle-shaped crystals always stand in good order on any substrate, in contrast with our previous diarylethene derivatives,17−21 e.g., the needle-shaped crystals of 2o on the (110) surface of a SrTiO3 single-crystal substrate, formed by photoinduced epitaxial crystal growth never stood and superhydrophobicity was not achieved.21 We here report that the mechanism of growth of the standing needle-shaped 3c crystals on the surface are due to the epitaxial growth over the crystal lattice of 3o without using special inorganic single-crystal substrates [we call this Self-Epitaxial Crystal Growth (SECG)]. We also found that the surface displayed not only excellent superhydrophobicity, but also the anti-reflective moth-eye effect. The reversible generation of both the lotus and moth-eye effects was observed upon alternative irradiation to the microcrystalline surface of 3o on a normal glass substrate with UV and visible light (Scheme 1).

Figure 1. SEM images of the surface topographical changes by UV irradiation: (a) flat surface of 3o before UV irradiation ( ×3000, scale bar: 3.33 μm); (b) rough surface covered with needle-shaped 3c microcrystals ( ×10 000, scale bar: 1.00 μm); and (c) cross-section of the surface of (b), and (d) a water droplet on the surface showing CA = 172.0°.

Scheme 1. Molecular Structures of Diarylethenes

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SEM Observation of Cross-Section of the Film (Figure 1b). The chloroform solution containing 3c was coated on a cover glass plate (for an optical microscope) that was etched on the backside using a glass cutter, and from this the film was prepared. After the film was formed, the glass with the film attached was broken along the etched line, and thus a cross-section was prepared for examination. Grazing-Incidence X-ray Diffraction (GIXD) of Microcrystalline Surface of 3c. The preferred orientation of the microcrystalline surface of 3c was studied by in-plane and out-of-plane X-ray diffraction using a GIXD diffractometer (Rigaku RINT2000) with CuKa radiation at an incident angle of 0.2° (Figure 2). Measurements of Refractive Indices of Polystyrene−Diarylethene 3o or 3c Composites. The composites of styrene− diarylethene 3o or 3c were prepared by mixing open (3o) or closedring (3c) isomers of diarylethene 3 with polystyrene in chloroform while changing the composite ratio in the dark (details are described in the SI). Then the solutions were used for solution casting to prepare polymer−diarylethene composite films on a quartz substrate (25 × 25 × 2.0 mm). The average thickness of the film was measured by using a surface profiler (KLA Tencor, Alpha-Step IQR) and obtained as 60 μm. The refractive indices of the composite films on quartz substrates were measured on a Prism Coupler (Metricon Model 2010) at 1545 nm in bulk/substrate mode.

EXPERIMENTAL SECTION

Preparation of Films and Characterization. The solutioncoated films were prepared according to the previous paper.22 The film thickness was approximately 10 μm by laser microscope (KEYENCE VK-8550) after scratching the surface. A scanning electron microscope (KEYENCE VE-8800) and an optical microscope (Leica DMLP) were used to study the surface microstructure. Photoirradiation (visible light: λ > 500 nm) was carried out using a Ushio 500-W xenon lamp with a cutoff filter (Toshiba color filter Y-50), and UV irradiation was carried out with a Spectroline Hand-Held UV lamp, E-series (λ = 313 nm, 820 mW/cm2 (distance: 15 cm)). Photoirradiation experiments

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RESULTS AND DISCUSSION Self-Epitaxial Crystal Growth upon UV Irradiation. It has been previously reported that the needle-shaped crystals of 3c were generated at 30 °C, which is much lower than the eutectic temperature of the 3o and 3c mixtures (141 °C; see

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substrate. It is also expected that the alignment will effectively enhance the CA. The thin films of 3o were also prepared on several kinds of substrates by solution coating, and similar surfaces were observed. The as-prepared 3o thin film had a 110 orientation from the X-ray diffraction (XRD) pattern (Figure S5 of the SI). The 013oriented needle-shaped 3c grew on the surface of the 110oriented 3o thin film from the GIXD pattern (Figure 2a). We compared the crystal lattice of 3o and 3c. The 110 plane of 3o and 013 plane of 3c are shown in Figure 3a,b. The height and

Figure 2. (a) In-plane and out-of-plane GIXD profiles. (b) Illustration of the 3c crystal shape over the microcrystalline surface of a 3o substrate unit cell of 3c (black lines) and 013 plane of 3c (sky blue plane in crystal 3c).

phase diagram in Figure S1 of the SI),22 and that the CA of a water droplet on the surface increased to 172° (Figures S2 and S3 of the SI).22 Such a high CA has been observed only for perpendicular nanopin film or densely packed nanofibers.5 Unlike other surface topographical changes for diarylethenes 1 and 2, where each isomer’s crystal growth was observed at the eutectic temperatures,17,20 the surface of 3o became soft upon UV irradiation, which resulted in the generation of the needleshaped crystals of 3c at such a low temperature.22 The diameters and lengths of the needle-shaped 3c crystals were less than average, at 0.2−0.3 μm in diameter and 2.2−2.5 μm in length,22 which are much smaller values than those for the 1c fibrils.17,18 Consequently, the CA of the film of 3 was extraordinarily high. Observing the surface at different SEM scales, each fiber appears to stand inclined at an angle of about 60°. To ascertain that the fine needle-shaped crystals consist of 3c, we carried out grazing incidence X-ray diffraction (GIXD) analysis for the film where 3c crystals are standing on a 3o subphase prepared by solution casting.27 Due to the rough surface of 3o, small peaks other than 110 and 220 reflections were observed (the largest peaks around 7.5° and 15° are attributed to the 110 and 220 peaks of 3o, respectively: see Figure S4 of the SI). The strong reflection attributed to the 013 peak of the 3c crystals appeared only in the out-of-plane but not in the in-plane profiles (denoted by the green arrow in Figure 2a) among the diffraction peaks of 3o after UV irradiation, in contrast to the out-of-plane profiles. This result indicates that the 013 surface (the sky blue surface in Figure 2b) of 3c is located almost parallel to the substrate; hence, the needle-shaped crystals stand all together at about 60° to the

Figure 3. (a) The 110 plane of the 3o crystal. (b) The 013 plane of the 3c crystal. (c) The size and shape of the 110 plane of 3o. (d) The size and shape of the 013 plane of 3c. (e) The overlapping planes of 110 of 3o and 013 of 3c.

width of the 110 plane of 3o are 23.609 and 11.956 Å, respectively, while those of the 013 plane of 3c are 23.847 and 9.974 Å, respectively (Figure 3c,d). The height of the 013 plane of 3c is in accordance with that of the 110 plane of 3o. Although the width is different between them, 10 pieces of the 110 plane of 3o and 12 pieces of the 013 plane of 3c overlap similar regions (Figure 3e). Therefore, the needle-shaped crystal of 3c epitaxially grows on the crystal lattice of 3o. In the above results, the nano crystals of 3c always formed by UV irradiation standing at 60° regardless of which substrate was applied. The lower activation energy of formation of 3c crystals (58 kJ/mol)22 compared to that (143 kJ/mol)19 of crystal growth of 1c on the eutectic melted surface of the mixture of C

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flat surfaces were calculated according to eqs 1 and 2 using Fresnel’s coefficient.35,36

1o and 1c is attributed to 3o crystals acting as the seeds of formation of 3c crystals. Photoinduced Reversible-Appearance Moth-Eye Effect near IR Region. Due to the highly ordered structure, the rough surfaces are expected to show not only superhydrophobicity, but also the moth-eye anti-reflection effect in the infrared region. The moth-eye effect is the antireflective3,28−31 capabilities of functional surfaces found in nature, especially in the eyes of moths. The deep blue-colored closed-ring isomer 3c formed by UV irradiation has an absorbance whose band tail extends to around 800 nm (Figure S6 of the SI). Therefore, the anti-reflectance of the microcrystalline surface was monitored in a region larger than 800 nm wavelengths, where 3c has no absorbance. The results are summarized in Figure 4. The reflection spectrum of the initial microcrystalline surface of 3o averages

⎛ cos θ − R s = ⎜⎜ ⎝ cos θ + ⎛ n2cos θ − R p = ⎜⎜ ⎝ n2cos θ +

2 n2 − sin 2 θ ⎞⎟ ⎟ n2 − sin 2 θ ⎠ 2 n2 − sin 2 θ ⎞⎟ ⎟ n2 − sin 2 θ ⎠

(1)

(2)

The calculated average reflectance over S and P polarizations for the ideal flat surfaces are 4.94 and 5.63 for 3o and 3c, respectively. Therefore, the reductions of the reflectance due to the roughness of the surfaces, i.e., the moth-eye effect, were found to be 80.34% for the relatively flat surface of 3o, but 6.68% for the rough surface of 3c (Table S4 of the SI). Reflectance was recovered by regeneration of the 3o surface by 2 h visible light (λ > 500 nm) irradiation, maintaining the eutectic temperature (141 °C). As is expected from the previous observation of the reversible formation of a superhydrophobic surface by alternate irradiations with UV and visible light accompanied by temperature control,22 the surface showing the moth-eye effect was also switchable under the same conditions.

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CONCLUSIONS We clarified the formation mechanism of a highly superhydrophobic surface whose water CA droplet on the surface is larger than 170° and whose SA is around 0.5°. Specifically, the needle-crystal of photogenerated 3c grew epitaxially on the open-ring isomer 3o by the SECG mechanism without using special crystals. This epitaxial crystal growth could be repeated. The mechanism requires two conditions. (1) The one (open-ring) isomer expands on the subphase showing only one crystal plane. (2) A lattice of the other (closed-ring) isomer matches the above lattice of the plane of the former (open-ring) isomer crystal. Then photoinduced SECG formation is expected. Such stable formation of a hydrophobic surface is attributed to the submicroscale needle-shaped crystals standing in a densely packed situation. Due to the ordered structure, the switchable moth-eye effect was also performed. These surface functions are due to the SECG mechanism. Such self-organized crystal growth will be a candidate for the formation of the photonic crystals.

Figure 4. Reversible reflection changes accompanied by reversible topographical changes of the surface. (a) Initial microcrystalline surface of 3o: black line. (b) UV-generated rough surface covered with needle-shaped 3c crystals showing moth-eye effect: red line. (c) Regenerated microcrystalline surface of 3o: green line. (d) Regenerated rough surface of 3c: blue line.

3.5% reflectance in the 800−1800 nm wavelength region. By UV irradiation followed by maintaining the film in the dark at 30 °C for 3 days, the surface became covered with needleshaped 3c crystals whose diameters and lengths were around 0.2−0.3 μm and 2.2−2.5 μm, and almost no reflectance was observed (less than 0.5%), thus demonstrating the moth-eye effect. Reflectance was measured by a spectrophotometer via a mirror reflection arrangement of 5° incidence and 5° reflection in an 800 to 1800 nm wavelength region. Incident light has 45° polarization to the plane of incidence. Reflectances of 3.97% and 0.38% were obtained for the 3o and 3c surfaces, respectively. In order to estimate the efficiency of the reflectance (observed reflectance/theoretical reflectance), the theoretical reflectance values were evaluated. The refractive indices of 3o and 3c were measured by changing the concentration of the polystyrene-3o or 3c composite (Tables S1 and S2 of the SI). The results are shown in Table S3 and Figure S7 of the SI.32 Refractive indices at 1545 nm were estimated to be 1.572 for 3o and 1.622 for 3c by extrapolation of the concentration of 3o or 3c. These values are reasonable compared to the refractive indices n of other diarylethene derivatives.33,34 Using the measured values of refractive indices n and the incident angle θ of reflectance measurement (θ = 5°), the theoretical reflectances in P and S polarization for the ideal

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ASSOCIATED CONTENT

S Supporting Information *

Phase diagram of open- and closed-ring isomers of diarylethene 3 by DSC, the contact angle profile of a water droplet after UV irradiation to the microcrystalline surface of 3, force curves obtained by AFM on single crystalline surfaces of 3o, assignments of diffraction peaks of in-plane and out-of-plane of GIXD profiles, fabrication of polystyrene−diarylethene composite film and the refractive indices (at 1545 nm) of the composites diarylethene and polystyrene, and an explanation of the moth-eye effect. This material is available free of charge via the Internet at http://pubs.acs.org. D

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(14) Parker, A. R.; Townley, H. E. Biomimetics of photonic nanostructures. Nat. Nanotech. 2007, 2, 347−353. (15) Zhou, B. L. Bio-inspired study of structural materials. Mater. Sci. Eng., C 2000, 11, 13−18. (16) Feng, X.; Jiang, L. Design and creation of superwetting/ antiwetting surfaces. Adv. Mater. 2006, 18, 3063−3078. (17) Uchida, K.; Izumi, N.; Sukata, S.; Kojima, Y.; Nakamura, S.; Irie, M. Photoinduced reversible formation of microfibrils on a photochromic diarylethene microcrystalline surface. Angew. Chem., Int. Ed. 2006, 45, 6470−6473. (18) Uchida, K.; Nishikawa, N.; Izumi, N.; Yamazoe, S.; Mayama, H.; Kojima, Y.; Yokojima, S.; Nakamura, S.; Tsujii, K.; Irie, M. Phototunable diarylethene microcrystalline surfaces: Lotus and petal effects upon wetting. Angew. Chem., Int. Ed. 2010, 49, 5942−5944. (19) Nishikawa, N.; Uyama, A.; Kamitanaka, T.; Mayama, H.; Kojima, Y.; Yokojima, S.; Nakamura, S.; Tsujii, K.; Uchida, K. Photoinduced reversible topographical changes on diarylethene microcrystalline surfaces with bio-mimetic wetting properties. Chem. Asian J. 2011, 6, 2400−2406. (20) Uyama, A.; Yamazoe, S.; Shigematsu, S.; Morimoto, M.; Yokojima, S.; Mayama, H.; Kojima, Y.; Nakamura, S.; Uchida, K. Reversible photocontrol of surface wettability between hydrophilic and superhydrophobic surfaces on an asymmetric diarylethene solid surface. Langmuir 2011, 27, 6395−6400. (21) Sakiyama, S.; Yamazoe, S.; Uyama, A.; Morimoto, M.; Yokojima, S.; Kojima, Y.; Nakamura, S.; Uchida, K. Photoinduced reversible heteroepitaxial microcrystal growth of a photochromic diarylethene on (110) surface of SrTiO3. Cryst. Growth Des. 2012, 12, 1464−1468. (22) Nishikawa, N.; Kiyohara, H.; Sakiyama, S.; Yamazoe, S.; Mayama, H.; Tsujioka, T.; Kojima, Y.; Yokojima, S.; Nakamura, S.; Uchida, K. Photoinduced formation of superhydrophobic surface on which contact angle of a water droplet exceeds 170° by reversible topographical changes on a diarylethene microcrystalline surface. Langmuir 2012, 28, 17817−17824. (23) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. Superhydrophobic perpendicular nanopin film by the bottom-up process. J. Am. Chem. Soc. 2005, 127, 13458−13459. (24) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Super-hydrophobic surface of aligned polyacrylonitrile nanofibers. Angew. Chem., Int. Ed. 2002, 41, 1221−1223. (25) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Superhydrophobic and lipophobic properties of self-organized honeycomb and pincushion structures. Langmuir 2005, 21, 3235−3237. (26) Dorrer, C.; Ruhe, J. Wetting of silicon nanograss: From superhydrophilic to superhydrophobic surfaces. Adv. Mater. 2008, 20, 159−163. (27) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Design and characterization of crystalline thin film architectures at the air−liquid interface: Simplicity to complexity. Chem. Rev. 2001, 101, 1659−1696. (28) Kanamori, Y.; Sasaki, M.; Hane, K. Broadband antireflection gratings fabricated upon silicon substrates. Opt. Lett. 1999, 24, 1422− 1424. (29) Lalanne, P.; Morris, G. M. Anti-reflection behavior of silicon subwavelength periodic structures for visible light. Nanotechnology 1997, 8, 53−56. (30) Barnes, G. A.; Flaim, T. D.; Jones, S. F.; Bruce, J.; Dudley, W.; Koester, D. A.; Peters, C. P.; Bobbio, S. M. Anti-reflective coating for deep UV lithography process enhancement. Polym. Eng. Sci. 1992, 32, 1578−1582. (31) Xie, G.; Zhang, G.; Lin, F.; Zhang, J.; Liu, Z.; Mu, S. The fabrication of subwavelength anti-reflective nanostructures using a biotemplate. Nanotechnology 2008, 19, 095605. (32) Tanio, N.; M. Irie, M. Refractive index of organic photochromic dye-amorphous polymer composites. Jpn. J. Appl. Phys. 1994, 33, 3942−3946. (33) Chauvin, J.; Kawai, T.; Irie, M. Refractive index change of an amorphous bisbenzothienylethene. Jpn. J. Appl. Phys. 2001, 40, 2518− 2522.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address △

Department of Chemistry, School of Science, the University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 221-0033, Japan (S.Y.). Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Sekisui Chemical Foundation “Innovations Inspired by Nature” Research Support Program and A-STEP (Adaptable & Seamless Technology Transfer Program through Target-driven R&D) FS stage: Exploratory Research from the Japan Science and Technology Agency (JST). The authors express great thanks to Dr. Koji Ohta and Dr. Kenji Kintaka of the Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Kansai Centre, Japan for the measurement of refractive indices of polystyrene−diarylethene composites.

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REFERENCES

(1) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (2) Xia, F.; Jiang, L. Bio-inspired, smart, multiscale interfacial materials. Adv. Mater. 2008, 20, 2842−2858. (3) Vukusic, P.; Sambles, J. R. Photonic structures in biology. Nature 2003, 424, 852−855. (4) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Reversibly erasable nanoporous anti-reflection coatings from polyelectrolyte Multilayers. Nat. Mater. 2002, 1, 59−63. (5) Cho, S. J.; An, T.; Kim, J. Y.; Sunga, J.; Lim, G. Superhydrophobic nanostructured silicon surfaces with controllable broadband reflectance. Chem. Commun. 2011, 47, 6108−6110. (6) Liu, M.; Zheng, Y.; Zhai, J.; Jiang, L. Bioinspired superantiwetting interfaces with special liquid-solid adhesion. Acc. Chem. Res. 2010, 43, 368−377. (7) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Transparent superhydrophobic thin films with selfcleaning properties. Langmuir 2000, 16, 7044−7047. (8) Zhang, F.; Zhao, L.; Chen, H.; Xu, S.; Evans, D. G.; Duan, X. Corrosion resistance of superhydrophobic layered double hydroxide films on aluminium. Angew. Chem., Int. Ed. 2008, 47, 2466−2469. (9) Sun, T.; Tan, H.; Han, D.; Fu, Q.; Jiang, L. No platelet can adherelargely improved blood compatibility on nanostructured superhydrophobic surfaces. Small 2005, 1, 959−963. (10) Cottin-Bizonne, C.; Barrat, J. L.; Bocquet, L.; Charlaix, E. Lowfriction flows of liquid at nanopatterned interfaces. Nat. Mater. 2003, 2, 237−240. (11) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem. Soc. Rev. 2007, 36, 1350−1368. (12) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. Superhydrophobic surfaces: From structural control to functional application. J. Mater. Chem. 2008, 18, 621−633. (13) Lee, Y.; Park, S.-H.; Kim, K.-B.; Lee, J.-K. Fabrication of hierarchical structures on a polymer surface to mimic natural superhydrophobic surfaces. Adv. Mater. 2007, 19, 2330−2335. E

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(34) Bertarelli, C.; Bianco, A.; Castagna, R.; Pariani, G. Photochromism into optics: Opportunities to develop light-triggered optical elements. J. Photochem. Photobiol. C, Photochem. Rev. 2011, 12, 106− 125. (35) Fowles, G. R. Introduction to Modern Optics, 2nd ed.; Dover: New York, 1989. (36) Born, M.; Wolf, E. Optics, 5th ed.: Pergamon Press: New York, 1975.

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