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A flexible self-cleaning broadband antireflective film inspired by the transparent cicada wings Zhiwu Han, Ze Wang, Bo Li, Xiaoming Feng, Zhibin Jiao, Junqiu Zhang, Jie Zhao, Shichao Niu, and Luquan Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01948 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Materials & Interfaces

A flexible self-cleaning broadband antireflective film inspired by the transparent cicada wings Zhiwu Han,† Ze Wang,† Bo Li,†,‡ Xiaoming Feng,† Zhibin Jiao,† Junqiu Zhang,†,§ Jie Zhao,† Shichao Niu,†,* and Luquan Ren† †Key

Laboratory of Bionic Engineering, Ministry of Education, Jilin University, 130022, People's Republic of China.

‡Department of Chemistry, University of California-Riverside, California 92521, USA. §Department of Mechanical Engineering, Columbia University, New York 10027, USA. *Corresponding author: [email protected] KEYWORDS: Bio-inspired antireflection, self-cleaning, template manufacturing, PMMA film, cicada wings

ABSTRACT Cicada wings, covered with arranged nanostructure, were widely studied owing to their highly transparency and low reflection. However, limited by technologies, its exquisite surface structures and multifunctional features were not inherited and applied by most artificial materials adequately. Here, the excellent optical properties of cicada wing were investigated in details experimentally and theoretically. Besides, a flexible self-cleaning broadband antireflective film inspired by the cicada wing has been successfully fabricated by a well-designed biological template method and sol-gel process. The cicada wing (Megapomponia intermedia) was selected as the original template directly, and a SiO2 negative replica was obtained by a sol-gel process. Then, chemical corrosion was used to remove the original template, remaining the pure negative replica. Subsequently, the polymethyl methacrylate (PMMA) positive replica could be rebuilt after another sol-gel process. Compared with flat PMMA film, the 1 ACS Paragon Plus Environment

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average reflectivity of structured PMMA film over the visible region was reduced from 10% to 2%. Besides, the bio-inspired film with a thickness of 0.18mm exhibited satisfactory comprehensive performances with low reflectance ( ≤ 2%) in most of visible region, as well as superhydrophobic property and perfect flexibility. Our results offered a quick and simple method to rebuild the nanostructured functional materials, promoting the practical applications of the bionic nanostructured materials. Meanwhile, the modified biomimetic fabrication method provides a solution for rebuilding exquisite biological materials and designing multifunctional surfaces. Moreover, the multifunctional antireflective film with wider universality will exhibit enormous potential application value in optical communications, photoelectric devices, flexible display screens, and anti-dazzle glasses.

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INTRODUCTION Nature has provided diversified strategies to solve their survival challenges, such as self-cleaning, antifogging, antireflection from the lotus, moth eye and cicada wing.1-4 Compared with artificial functional materials, these natural adaptive surfaces seemed more practical and splendid. Antireflective nanostructures distributing on the surfaces of creatures have been used to overcome the dazzling on indicators, screens, and displays, meanwhile, reduce the loss of reflection in the field of optoelectronic devices, photovoltaic industry, and optical equipments. Nevertheless, more performance requirements were proposed to meet the practical applications. Excessive reflection usually brought many problems, for example, the loss of solar energy, strong glare, and the bad readability of display device. In view of this, available measures must be taken to solve these matters. Actually, suppressing the surface reflection was always mentioned, and many researchers had spared no effort on it. Moreover, antireflective functional materials fabricated by etching,5 self-assembly technique,6 and nanoimprint7 were surely effectual to some degree. However, most of these antireflective materials paid close attention to light absorption, and the requirements of light transmittance and the multifunctional properties which were necessary for the applications were sometimes neglected. Nowadays, various electronic products were closely related to our daily life. The images of mobile phones, computers, instrument panels and the screens of display were expected to be much clearer. Therefore, the readability of images under the sun or strong light has drawn the attention of researchers. Generally speaking, the ‘clear’ we said means the contrast of light and shadow. For most transmissionbased displays, light received by eyes is consisted of ambient light and the transmission light of display. Hence, the shadow would be weakened when the images were exposed under strong light or outside, which meant a decrease in contrast. As a result, the quality of the image was not ideal. In fact, the readability could be indicated by the ambient contrast ratio (ACR) as blow,8-9

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𝐿𝑂𝑁 + 𝑅𝐿 ∙ 𝐿𝑎𝑚𝑏𝑖𝑒𝑛𝑡

𝐿𝑂𝑁 ― 𝐿𝑂𝐹𝐹

ACR = 𝐿𝑂𝐹𝐹 + 𝑅𝐿 ∙ 𝐿𝑎𝑚𝑏𝑖𝑒𝑛𝑡 = 1 + 𝐿𝑂𝐹𝐹 + 𝑅𝐿 ∙ 𝐿𝑎𝑚𝑏𝑖𝑒𝑛𝑡

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(1)

𝜆

𝑅𝐿 =

∫𝜆2𝑉(𝜆)𝑅(𝜆)𝑆(𝜆)𝑑𝜆 1

(2)

𝜆

∫𝜆2𝑉(𝜆)𝑆(𝜆)𝑑𝜆 1

In Eq. (1), LON refers to the luminance of the pixels when they are on. Similarly, LOFF means the luminance of the off pixels, and Lambient is the ambient luminance. In Eq. (2), RL is defined as the luminous reflectance of the indicators.8 It was obtained by the normalized integrated product of V(λ)·R(λ)·S(λ). Here, V(λ) refers to the standard photonic curve, R(λ) refers to the total reflectance, and S(λ) refers to the spectral power distribution of the source. a·R(λ)·S(λ) is defined as the reflection caused by ambient light, and a geometric factors α converts radiant existence to radiance. Apparently, a higher value of ACR meant a clearer vision.9 Since it was not easy to change the surroundings, there were two ways to improve the readability of images according to the Eq. (1) and Eq. (2): increasing the intensity of LON or decreasing R(λ), in other words, improving screen brightness or minimizing the surface reflection. The former sounds easier to achieve. However, screen with high luminance consumes more electric energy and it will cause serious damage to vision. Consequently, minimizing the surface reflection is more practical. In general, glass is chosen as an important material for the screen of indicators, and an average 4% reflection on air-glass interface matters so much.10 This surplus reflection, which leads to an obvious damage to the image readability, should also be depressed, and the reflection here usually refers to the Fresnel reflection. The reflection appears when the light spreads between different media, and the biggish difference in the refractive index between the two media is usually thought to be the major reason of strong Fresnel reflection.11-12 It is obvious that the Fresnel reflection can be decreased if the abrupt change of refractive index is eliminated. Since air, with a refractive index of 1, is one of the most common medium, it’s hard to find materials with a refractive index close to air. Currently, this problem was solved by eliminating the biggish difference in the refractive index of the two media by building structures or textures with a graded index on the surface of substrate.10,13-16 However, the optimization of structure parameters is still a challenge. Recently, the functional surfaces triggered by nature have appealed plenty 4


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of interests, such as the self-cleaning surface inspired by lotus leaves, the antireflection surface inspired by moth eyes, and the structural color inherited from the butterfly wings. These natural surfaces have provided us with new ideas and solutions to the problem. Here, the cicada Megapomponia intermedia was investigated as a prototype because of its transparent wings with remarkable self-cleaning properties and quite low reflection. Meanwhile, the consistency between nanostructures and its multifunction of cicada wings was explicated adequately. Besides, this work developed a simple method via a modified sol-gel process to achieve the biomimetic fabrication, and the structured PMMA film was obtained with a 2% average reflectivity over the whole visible region. Moreover, the structured PMMA film was modified for further improvement on hydrophobicity. This work mainly focused on biological multifunctional surfaces and its nano-micro cross-scale fabrication. And it might be helpful to shorten the gap between bio-inspired theoretical design and practical applications. RESULTS AND DISCUSSION Periodical nanostructures in Original Cicada Wings. Here, cicada Megapomponia intermedia (Distant, 1905), spread in tropical regions all over the world, was selected as a biomimetic prototype. Compared with other typical “moth-eye structure” on compound eyes, nanostructures on cicada wings displayed more advantages besides meeting the basic need for antireflection (Table S1).17-18 The cicada wings shown in Figure 1(a) were thin and transparent in appearance, and there was fibrous support network distributing on it. The words on paper could be seen clearly through the wings. Meanwhile, the intense light beam was weakened when the laser passed through the transparent region of the wing, leaving an enlarged light spot on the background (Figure S1). Besides, a 131.3° water contact angle (CA) shown in Figure 1(b) confirmed its excellent hydrophobicity performance and the motion dynamics of droplets in Movie S1 and Movie S2 indicated its self-cleaning performance. What’s more, the 5 ACS Paragon Plus Environment


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reflectance spectrum of cicada Megapomponia intermedia wing was shown in Figure 1(c), and the average reflectivity ratio of cicada wings was about 2% from 500 nm to 800 nm, forming a broadband antireflection. These characteristics formed an effective way to suppress the glare. In a word, the cicada wings exhibited outstanding antireflection and self-cleaning performances.

Figure 1. Cicada Megapomponia intermedia wing. (a) Digital photograph of cicada Megapomponia intermedia wing placed on a sheet of paper. (b) Droplets on the cicada wing and the static water contact angle is 131.3°. (c) The reflectivity spectrum of cicada wing. (d) The image and enlarged view of cicada wing observed from top view. (e) The cross section of cicada wing observed from side view. (f) The atomic force microscope image of cicada wing.

As seen in Figure S2, the thickness of transparent wing was about 8.25 µm. Then, it also proved that the nanostructures arranging on the surface of ventral and back were totally consistent. As shown in Figure 1(d), the exquisite structures observed with the help of field emission scanning electronic microscopy (FESEM) confirmed its complete hexagonal arrangement. The distance between adjacent vertexes was about 200 nm. Similarly, the side view was shown in Figure 1(e), and it was obvious that the nanostructures distributing on the cicada wing presented a hexagonal arrangement with 180 nm in base 6


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diameter and 325 nm in height. It could be found that the shape of the nanostructures was not a simple cone or pillar with large aspect ratio. It was better to be described as streamline with a hemispherical top according to the Figure 1(f), which was obtained with atomic force microscope (AFM).

Figure 2. The FDTD simulations results of the cicada wing and its antireflective mechanism. (a) The antireflective model of the cicada wing surface and its parameters for simulation. (b) The simulation results of the cicada optical wings. (c) Electric field (E) profile for the cicada wing. (d) Bearing area distribution curve of nanocones on the cicada wing. (e) Schematics of mechanism for antireflection of the cicada wing.

Then, the simulation based on FDTD was carried out. As shown in Figure 2(a), the model was established by 3D modeling software with the structure parameters obtained from the FESEM images and the AFM images. The normal incident plane wave was used to the simulation process, and the distance between the illuminant and the top of nanocones was 500 nm. The reflectivity monitor was placed behind 7 ACS Paragon Plus Environment


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the plane wave while the transmission monitor was placed behind the nanocones. The medium around the nanocones was air with a constant refractive index of nair = 1 while that of photonic structure material was nchitin= 1.7. In this work, only two dimensional spatial simulations were considered. Periodic boundary condition (PBC) and perfectly matched layer (PML) absorbing boundary condition has been applied to the boundary surrounding in the computational domain in the x-direction and y-direction respectively. The result of optical simulation in Figure 2(b) indicated that the reflectivity of cicada wing at wavelengths between 350 nm and 800 nm was less than 5%, and the average reflectivity over the visible wavelength range was about 3%. At the same time, the optical wave distribution changed when going through the nanocones ， and almost all of the plane light wave transmitted the film (Movie S3 and Figure S3). Besides, as shown in Figure 2(c), the intensive electric field enhancement occurred around the nanocones, and the maximum amplitude of electric field was 1.4, indicating that the reflection between the nanocones was strong. However, this enhancement was only in a small area. Meanwhile, electric field was distorted significantly when the incident light approaching the nanocones. (Figure S4) As it known to all, the reflection of the interface could be suppressed if the abrupt refractive index change was eliminated. The nanocones covering on cicada wing is projecting, and the area between them was filled with air, forming a mixed media area, which meant that the interface of different media could be blurred. The effective refractive index of the mixed media area could be explained by effective medium theory, and it could be expressed as follows,19-21 𝑛2𝑒𝑓𝑓 = 𝑛2𝑎𝑓 + 𝑛2𝑠 (1 ― 𝑓)

(3)

Where neff is the effective refractive index of air-substrate mixed media, and na is the refractive index of air, as well as ns is the refractive index of substrate. The f in this Eq. (3) means the volume percentage of the nanocones arraying on the cicada wings surface, and the value of f is available via bearing analysis with the help of AFM as shown in Figure 2(d).12, 22 Generally speaking, the refractive index of solid is larger than that of air. So, the curve of neff could be obtained by the Eq. (3) as shown in Figure 2(e). It was easy to see that the effective refractive index of mixed media changed smoothly along the nanocones 8


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height, forming a remarkable contrast to that of the interface without nanostructures. Thus, the nanocones achieved a smooth transition from air to substrate. Meanwhile, it reduced the Fresnel reflection and exhibited excellent anti-reflection performance.

Figure 3. Schematic diagram of biomimetic fabrication, showing cicada-wing original template, fabrication of SiO2 negative replica, and transfer of the pattern onto the PMMA positive replica. (a) Original template obtained from the cicada wing. (b) The precursor solution added on the original template. (c) Heating curing process. (d) The SiO2 negative replica. (e) The mixture of PMMA/ chloroform was added on the negative replica. (f) Evaporation solvent process. (g) Solidified PMMA film. (h) The PMMA positive replica.

Biomimetic Fabrication. At present, many technologies have been used to duplicate the exquisite and ordered nanostructures, such as electroless plating and electroplating,5, 23 self-assembled technologies,24 9 ACS Paragon Plus Environment


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template method,25-27 and so on. Although these technologies can reach the point, the operation is always so complicated and the instruments they need are unavailable. What’s worse, the inheritance between original organism and biomimetic replica in structures and functions is not extremely satisfactory. This work developed a simple way to mimic the prototype and fabricate the biomimetic nanocones with a modified sol-gel progress.28-29 The schematic diagram was shown as Figure 3. The wings of Megapomponia intermedia (Distant, 1905) were chosen as biological prototype for further experiments. Figure 3(a)-3(d) shown the preparation of the SiO2 negative replica and has been explained in detail in our previous work.30 Firstly, the prototype was pretreated to remove the impurities on the wing. Then, the biological prototype was sandwiched between two pieces of glass and the precursor reacted by tetraethylortho-silicate and attenuat hydrochloric acid was added to the edge of the glass slide. After that, the sandwiched assemblies were put in a vacuum drying oven to solidify the precursor. Finally, the biological prototype was removed in acid and the SiO2 negative replica was left on the glass slide. Since the diameter of hole was tiny enough to prevent the fluid with higher surface energy expanding, it was necessary to choose a material that could soak into the holes distributing on the SiO2 negative replica. In this work, the PMMA dissolved in chloroform was used to manufacture the positive replica. It is worth to note that the PMMA with smaller relative molecular weight is more favorable for dispersion in organic solvent and the formation of nanostructures (Figure S5). First, the PMMA was dried in vacuum drying oven to guarantee the quality of the generated film (because of its hygroscopicity). Then, the PMMA powder was dissolved in chloroform, followed by coating on the SiO2 negative replica with a spin coater. Afterwards, it was put on a constant temperature heating table to remove the chloroform and a structured PMMA film was formed. At last, after taking the thin film off carefully, the bio-inspired antireflective film was obtained.

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Figure 4. The characteristic results of SiO2 negative replica. (a) The cross section of the SiO2 negative replica. (b)-(c) The FESEM image of SiO2 negative replica observed from the side view and top view respectively. (d) The AFM image of SiO2 negative replica. (e) The spectrum of SiO2 negative replica obtained with the help of energy dispersive spectroscopy.

Structure Characterization. After rebuilding the nanostructures, the morphologies and structures of SiO2 negative replica and PMMA positive replica were characterized by atomic force microscope and field emission scanning electronic microscopy. The detailed data for replica including the single nanocone’s height, diameter, and the spacing between nanocones would be expressed respectively, and the analysis of elements and components was carried out subsequently. The morphologies and structures of the SiO2 negative replica were shown in Figure 4. The area marked with red line could be seen clearly in Figure 4(a). It was covered by negative structure. The FESEM 11 ACS Paragon Plus Environment


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images of SiO2 negative replica in Figure 4(b)-(c) were observed from the side view and top view respectively. It could be seen that the shape of the negative structure was similar to the inverted cones. The diameter of the tapered bottom was about 180 nm while the distance between inverted cones was about 200 nm, which was consistent with the biological prototype. From the cross section (Figure S6), it could be found that the height of negative structure was 290 nm. The arrangement of the inverted nanocones was also consistent with prototype shown in Figure 4(d). In a word, the negative SiO2 replica basically inherited the features of the biological prototype. The spectrum of the SiO2 negative replica obtained with energy dispersive spectrometer (EDS) was used to characterize the element types. As shown in Figure 4(e), the peaks of silicon and oxygenium were obvious. The SiO2 negative replica was mainly composed by silicon (24.28% in atomic percentage) and oxygenium (62% in atomic percentage). Meanwhile, some small peaks were also contained (Table S2). Since the main composition of cicada wing is chitin, yet there is no carbon peak in the EDS spectrum. Hence, it was obvious that the original template has been removed completely and the SiO2 negative replica provided a favorable condition for the next step.

Figure 5. The characteristic results of PMMA positive replica. (a) Digital photograph of PMMA positive replica. (b) The model of PMMA positive replica. (c) The FESEM image of PMMA positive replica observed from the top view. (d) The FESEM image of the side view of PMMA positive replica. (e) The spectrum of PMMA positive replica obtained with fourier transform infrared spectroscopy (FTIR).

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The morphologies and structures of the positive PMMA replica were shown in Figure 5. It was a thin film with good transparency and the range of thickness was about 180-190μm as shown in Figure 5(a) and Figure S7. There were nanocones distributing on the positive replica surface. The model was established in Figure 5(b). The nanostructures were characterized with FESEM, and the regular arrangement was easy to be seen in top view in Figure 5(c). It was obvious that the ordered arrangement in positive PMMA replica could correspond to the hexagonal arrangement distributing on the original prototype. Besides, the side view (45 degree inclination) of positive PMMA replica observed in Figure 5(d) expressed the morphologies of the nanostructures on it clearly. Its morphologies approximated taper, which was almost the same as the original template. Meanwhile, the height of positive structures was about 280 nm, which was almost the same with the SiO2 negative replica. The distance between nanocones is also the same with the SiO2 negative replica and the original prototype. Consequently, the morphologies and structures of the positive PMMA replica inherited the original prototype well. The materials analysis of the positive PMMA replica was depicted with the help of FTIR spectrum shown as Figure 5(e). The peaks at 2995 cm1

and 2951 cm-1 were caused by -CH and -CH2- stretching vibration bond. The peak at 1730 cm-1

represented C=O stretching vibration bond. Other peaks from 1100 cm-1 to 1300 cm-1 were attributed to C-O-C stretching vibration bond. The FTIR spectrum of the PMMA positive replica matched the FTIR spectrum of PMMA.31 Consequently, the component of the positive replica was indeed PMMA.



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Figure 6. The performance analysis of the positive PMMA replica. (a) Reflectivity curves of cicada wing, chitin, flat PMMA and structured PMMA. (b) Transmittance of flat PMMA, structured PMMA, glass slide, glass slide covered with structured PMMA, and glass slide covered with flat PMMA. (c) The result of bending test. (c)-1 The contrast of frosted area, structured area and flat area under an illuminant. (d) The water contact angle of flat PMMA, structured PMMA, and silylanized structured PMMA. (d)-1 The water sliding angle of silylanized structured PMMA.

Optical and Self-cleaning Performance of the Replica. The reflectance spectrum was measured and shown as Figure 6(a). It was clear that the average reflectivity of the flat PMMA film was about 11.5%, much higher than that of the film of bioinspired PMMA with nanostructures. As it known to all, the main component of the cicada wing is chitin,32 while the reflectivity of these two is quite different. It has been 14


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approved that the especial nanostructures truly promoted the antireflection performance. In addition, the reflectivity curves of the antireflective film with different incident angles (30°, 40°, 50°, 60°) were measured in Figure S8, indicating an angle-dependent antireflection performance. Moreover, the reflectance took on a raising trend when the incident angle increased. Besides, the transmittance curve of the PMMA positive replica was also shown in Figure 6(b). The trend of these curves was roughly the same for most of the wavelengths considered (For a clearer comparison, magnifying some of the images in the dotted frame). As shown in Figure 6(b), the order of transparency from high to low is as following: Structured PMMA>Glass slid with structured PMMA>Flat PMMA>Glass slide>Glass slide with flat PMMA. The enlarged view in dashed wireframe indicated that nanostructure PMMA shows higher transmittance than flat PMMA. It can be also confirmed that the glass slide covered with structured PMMA, averaging 92.5% for 450-950nm, shows higher transmittance than both a single glass slide, averaging 89.8%, and a glass slide with flat PMMA, averaging 87.2%. So, the existence of the bioinspired antireflection structure not only does not reduce the transparency of PMMA, but also increases its transmittance. Besides, the presence of this bioinspired antireflective structure also significantly increases the transparency of the glass slide. In brief, it was obvious that the nanocone arrays had an influence on the light raying on the surface, increasing light transmission and reducing light reflection. Moreover, the replica with mimetic nanocone arrays has inherited excellent optical properties from the biological prototype. At present, the antireflective films were mainly used in optical field.33-34 The flexibility has become an important indicator. In this work, a pen tube with diameter of 6 mm was chosen to check the flexibility of replica shown as Figure 6(c) and the results of bending test in Movie S4 are gratifying. At the same time, the Figure 6(c)-1 displayed the differences in appearance between structured PMMA area (blue frame), frosted PMMA area (Green frame) and flat PMMA area (the rest part). It was obvious that the flat PMMA glared seriously, while the frosted PMMA area which meant the rough surface with irregular microstructures looked much better but decreased the readability. In general, structured PMMA area seemed to be gratifying in ensuring the clarity. Besides, the structured PMMA film covered by nanocones 15 ACS Paragon Plus Environment


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could also change the surface wettability. The water contact angle of structured PMMA film was 86.4°, higher than that of flat PMMA film (Figure S9). What’s more, to further improve the self-cleaning properties, the surface of structured PMMA film was silylanized.35 As shown in Figure 6(d), the average water contact angle of the modified film was 152°, meanwhile, a dynamic water contact angle measurement test was also carried out and the sliding process was recorded in Figure 6(d)-1 and Movie S5. It was gratifying that the sliding angle (SA) of the surface was about 3°, meeting the requirement of superhydrophobic state well and better strengthening its self-cleaning performance.

CONCLUSIONS In summary, the microstructures arraying on cicada wings surface was investigated in details due to its excellent natural optical and other performance. Based on FDTD simulations and basic optical theories, the interaction mechanism between the subwavelength structures on cicada wings surfaces and its optical and multiple properties were interpreted. What’s more, a simple and efficient manufacturing method was developed and a flexible self-cleaning antireflective film inspired by the cicada wing was successfully fabricated. Thanks to the ordered nanocones inspired by cicada wing, the reflectivity of the positive PMMA replica was much lower than flat PMMA film, reducing from 10% to 2%. Besides, the bioinspired film, with a thickness of 0.18 mm, exhibited satisfactory and comprehensive performances with low reflectance ( ≤ 2%) and high transmittance ( ≥ 93%) in a broadband visible region. It could also be confirmed that the nanocones were effective for the wettability of the surface (CA=152°, SA=3°). Furthermore, its perfect flexibility was also explored, showing great application potential in flexible display. Inspired but not limited to biology, multifunctional structured materials were obtained through a biomimetic fabrication and modification method. The results in this work were also promised for plenty of practice applications in curved screen, flexible displays, optoelectronic devices, solar cells and even antidazzle glasses. 16


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EXPERIMENTAL SECTION Biomimetic prototype. The cicada Megapomponia intermedia wing was selected as a prototype. The support network and edges were cut to leave its transparent region for further research. Clarity Experiment. A simple clarity experiment was carried out to confirm that the glass-wing of the cicada with good transparency was suitable as a template to fabricate a replica. The wing was put on a paper printed with Jilin University, and the words could be seen clearly. Morphology and Structure Characterization. The morphologies and structures of the prototype, the SiO2 negative replica, and the structured PMMA were characterized by AFM (BRUKER DIMENSION ICON) and ultrahigh-resolution FESEM (ZEISS MERLIN Compact). Besides, the detail structured parameters of the arrays containing diameter, height, structural period could be obtained under increasing magnifications. 3D Models of Nanostructures. The 3D models of the nanostructures were established with the help of 3D modeling software according to the parameters obtained from the FESEM images and AFM images. The models displayed the morphologies and dimension of the nanostructures on the cicada wing, and it was used for FDTD simulations. Spectrum Characterization. The reflectivity spectra and transmission spectra of all the surfaces in 450-900nm were measured with the help of miniature fiber-optic spectrometer (Ocean Optics USB 4000). The standard white board was used for calibration. Wettability Characterization. Static water contact angle and dynamic water sliding angle were measured by water drop angle measuring instrument (KRUSS DSA25), and they were used to characterize the wettability of the structured PMMA. Chemical Component Analysis and Element Distribution Characterization. The element distribution of the SiO2 negative replica was obtained with an EDS (OXFORD X-MaxN 150) to illustrate the prototype was removed completely. Besides, the chemical components of the PMMA positive replica 17 ACS Paragon Plus Environment


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were confirmed by analyzing the functional groups and chemical bonds with the help of Fourier Transform Infrared Spectrometer (SHIMADZU IRAffinity-1S WL).

The Source of Reagents. TEOS, ethanol, acetone, hydrochloric acid and hydrogen nitrate were provided by Beijing Chemical Works (analytical reagent) , deionized water were provided by Sangon Biotech (Shanghai) Co., Ltd. Chloroform was provided by Sheng Tongyue Chemical Industry (analytical reagent) , and pulverous PMMA is provided by Mitsubishi Rayon. The PMMA powder we used for fabrication was produced by Sumitomo chemistry in Japan, and the model is G5098. The relative molecular weight of this PMMA powder with 200 meshes was about 100000. Biomimetic Fabrication. Preparation process. The wings of cicada Megapomponia intermedia (Distant, 1905) were chosen as biological specimen used for further experiment. First, the wing was cut to eliminate the fibrous support network and edges. Then, the wings were pretreated with acetone and ethylalcohol for 10 min respectively to remove the impurities attaching to the surface. After that, keep the biological specimen dry naturally. SiO2 negative replica preparation: First, the biological specimen was sandwiched between two pieces of glass and fixed with clips to make sure no change in position. Next, the precursor was reacted by tetraethyl ortho-silicate and hydrochloric acid with a mass ratio of 3:1. Subsequently, the precursor was added to the edge of the glass slide with a micro pipette. It is worth pointing out that the concentration of hydrochloric acid aqueous solution was 0.1mol/L, and it should be hydrolyzed adequately. Ensuring the precursor just soaked the specimen, and too much precursor might result a difficult separation of sandwich. After that, the sandwich was put in a vacuum drying oven with a constant temperature at 120℃ for 30 min to solidify the precursor. After that, the sandwich was soaked in a mixed solution with concentrated nitric acid and isometric perchloric acid and put in the vacuum drying oven with a constant temperature at 130℃ for 30 min to get the biological specimen removed. Finally, the sandwich architecture could be separated, and the SiO2 negative replica was left on the glass slide after a 18


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washing with ethanol. PMMA positive replica preparation: The PMMA was used for the manufacture of positive replica, and it could be accomplished by three steps. It was worth noting that the pretreatment of PMMA was kept under 70~80℃ for 2~4 hours in draught drying cabinet. First, a mixture of powdered PMMA and chloroform (7:100 in weight) was stirred by a magnetic stirrer under 40℃ for 30 min. Next, the colloidal mixture was coated on the fabricated SiO2 negative replica evenly, followed by evaporation of the solvent under 0.08 MPa, 60 ℃ for 30 min. At last, taking the thin film off carefully, and the antireflective film was obtained. FDTD Simulations. In this work, only two dimensional spatial simulations were considered. Periodic boundary condition (PBC) and perfectly matched layer (PML) absorbing boundary condition have been applied to the boundary surrounding in the computational domain. Plane wave was chosen as light source with normal incidence over a wavelength range of 350-1000 nm. The optical simulation software used for electric field analysis was provided by Lumerical, and the version was 8.15.736.



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. More details about the experimental and simulation methods; supplemental results of material morphology characterization and chemical elemental analysis; sol-gel reaction mechanism and FDTD simulation. (PDF) (AVI) AUTHOR INFORMATION Corresponding Author *Email:

[email protected]

ORCID Junqiu Zhang: 0000-0002-2896-9740 Shichao Niu: 0000-0003-0208-9996 Zhiwu Han: 0000-0002-5035-1795 Author Contributions Z. W., S. N. and Z. H. conceived the idea and designed the experiments. Z. W., B. L. X. F. and Z. J. performed the experiments and characterization. J. Z. and S. N. helped with data analysis and theoretical simulation. Z. W., S. N. and Z. H. proposed the mechanism of the bioinspired materials’ property. Z. W., S. N., J. Z., and Z. H. wrote the paper. S. N., Z. H. and L. R. conceived the project. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51835006, 51875244, 51325501, 51505183), JLU Science and Technology Innovative Research Team (No. 2017TD-04), China Postdoctoral Science Foundation Funded Project (2018T110246), Joint 20


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Construction Project of Jilin University and Jilin Province (SF2017-3-4), Scientific and Technological Development Program of Changchun City (Double Ten Project-19SS001), Science and Technology Development Program of Jilin Province (Technology R&D Project-20190302021GX), Graduate Innovation Fund of Jilin University (No. 101832018C007).



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Table of Contents A flexible, self-cleaning, and broadband antireflective film, inspired by the cicada wings, is successfully fabricated. Compared with flat PMMA film, the reflectivity of the bio-inspired PMMA film is reduced from 10% to 2%. Besides, this bio-inspired film with only a 0.18 mm thick, exhibiting lower reflectance ( ≤2%) in a broadband visible light, better superhydrophobic property (CA=152º) and perfect flexibility. The results offer a quick and simple method to rebuild the bio-inspired optical functional nanostructures. Furthermore, the multifunctional antireflective film also has great application potential in optical communications, photothermal equipment, solar cells, and anti-dazzle glasses.

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A flexible self-cleaning broadband antireflective ... - ACS Publications

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