Artificial Compound Eyes Prepared by a Combination of Air-Assisted

Jan 9, 2019 - Furthermore, the BIC eyes reduced the imaging distortion by 1.5/3.4 and 2.3/3.1 times along the x and y axes, respectively, under 10° a...
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Artificial Compound Eyes Prepared by a Combination of Air-Assisted Deformation, Modified Laser Swelling, and Controlled Crystal Growth Jiang Li,†,‡ Wenjun Wang,*,†,‡ Xuesong Mei,†,‡ Aifei Pan,†,‡ Xuefeng Sun,†,‡ Bin Liu,†,‡ and Jianlei Cui†,‡ †

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710054, China Shaanxi Key Laboratory of Intelligent Robots, Xi’an Jiaotong University, Xi’an 710049, China

‡

S Supporting Information *

ABSTRACT: This study presents the manufacturing process of bioinspired compound (BIC) eyes. The hierarchical eyes were accomplished by a combination of (i) modified laser swelling, (ii) air-assisted deformation, and (iii) controlled crystal growth. The results show that the addition of nanostructures on the surface effectively improved the water repellent performance with a contact angle (CA) of ∼160° and generally decreased the reflection by ∼25% in the wavelength range of 400−800 nm than the planar surface. Apart from these properties, the BIC eyes showed good optical performance. The convex structure has a circular shape and aspherical profile; this provides optical uniformity and constant resolution (full width at half-maximum = 1.9 μm) in all the directions. Furthermore, the BIC eyes reduced the imaging distortion by 1.5/3.4 and 2.3/3.1 times along the x and y axes, respectively, under 10° and 20° incident lights than a single lens. In the light acceptance range, the image displays almost no distortion. KEYWORDS: bioinspired compound eyes, hierarchical structures, antireflection, superhydrophobicity, optical performance

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swelling also caused distribution variation due to elastic deformation, making it difficult to evaluate the overall properties of hierarchical surfaces.13 In addition, compared with natural eyes with a tertiary configuration, the reported BIC eyes actually have a macromicro- or micronanohierarchy.18−26 However, the achievement of a tertiary configuration was mainly hindered by the difficult integration of micronanohierarchy on the macrobase. This is because the nanostructures were easily distorted or even destroyed during the transformation, and the absence of structures at any level severely degrades the normal function of BIC eyes. Thus, a curvature-compatible hierarchy is clearly required for the actual implementation of BIC eyes. The crucial characteristics required for the development of BIC eyes are low surface reflection and superhydrophobicity. Nanostructures not only suppress the reflection of incidence light by offering a smooth refractive index gradient but also

nlike mammals with two eyes each having a single lens that focuses the images, the compound eyes of insects and crustaceans have a completely different architecture: Thousands of individual optical elements known as ommatidia are distributed on a curved surface, and nipple arrays are present on the top of ommatidia.1 This special configuration endows the compound eyes with many advantages than the ordinary single aperture eyes, such as a much wider field of view, higher sensitivity to light, and stronger detection of moving objects. Moreover, the on-top nanoripples provide both antireflective and water-repellent properties.2−4 Based on the understanding of nature, many bioinspired compound (BIC) eyes with hierarchical structures have been fabricated using reactive-ion etching, self-assembly of colloidal particles, nanoimprint lithography, soft lithography, laserassisted etching, and swelling methods.5−17 Some of the methods suffer from a complex process and high cost. Furthermore, most of the BIC eyes suffer from inconsistent performance of secondary structures due to structural deformation. Even with the initial uniform nanoripples obtained by imprinting technology, the subsequent laser © XXXX American Chemical Society

Received: May 29, 2018 Accepted: December 11, 2018

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DOI: 10.1021/acsnano.8b04047 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano create a superhydrophobic surface by forming air pockets.27−29 Various methods have been proposed for the fabrication of nanostructures; however, these methods are limited by inherent planar substrate. Recently, Shi et al. reported the formation of ZnO nanorods (NRs) on the surface of micropillar arrays using soft replication and crystal growth methods and confirmed the possibility of well-aligned NR growth on a curved surface.30 Therefore, application of the growth method to the preparation of BIC eyes would potentially solve the integration problem of hierarchical structures onto a macrobase. Thus, we envisioned a scheme for the practical implementation of curvature-compatible, hierarchical compound eyes. To mimic the function and structure of compound eyes in nature, BIC eyes with a tertiary configuration were developed. A facile technique was developed to generate curvaturecompatible, hierarchical compound eyes by successively using laser swelling technology, gas-assisted method and NR growth method. Note that each single lens covered with wellcontrolled NRs was omnidirectionally distributed on a curved polymer dome. Unlike traditional methods, the proposed method allows the fabrication of nanostructures on preprepared millimicrocomposite structures. This fabrication method for artificial compound eyes has diverse potential optical applications such as surveillance imaging, light-field photography, and wide-angle communication antenna.

mechanism is as follows: The photolysis of underlying doped material results in the formation of gaseous products, and the molecular relaxation of surface-transparent material leads to the heating of matrix. The formation of a convex structure is possible only when the high focused laser spot just reaches inside the polymer. This prevents the leakage of gaseous products and guarantees the formation of convex structures with good morphology. In the experiments, the morphology of convex structures was controlled by adjusting the laser power and irradiation time. Notably, the focus position should be carefully tuned; otherwise, ablation occurs, causing surface defects. Next, the convex array template was fixed onto a 500 μm-thick slab of 184 elastomer (Figure 1b). After peeling (Figure 1c), a concave array template was obtained. For reconfigurable microtemplating, a home-built mold with 5 mm in diameter circular chamber was used to attach a vacuum pump. An annular groove was designed to fix the membrane. The macrostructures were obtained using air-assisted technology. Under the action of negative pressure tensile force, tearing and rupture occurred if the soft mold is too thin. If it is too thick, this affects the adsorption of soft mold and also produces irregular deformation. Ultimately, hierarchical structures were obtained using the crystal growth method. By introducing the nanostructures, the surface reflectance of the material was effectively reduced; moreover, better surface hydrophobicity was obtained, as shown in Figure 1e. Performance of Nanostructures Obtained Using Crystal Growth Method. Morphology of NRs and XRD Study. Figure 2a−d shows the typical SEM images of NRs prepared by crystal growth method with different reaction times of 20−60 min with an increment of 10 min (marked with a−d) in details. The NRs are relatively uniform in size except in Figure 2a, exhibiting a sparse distribution. For Figure 2a−d, the nanostructures have different lengths of 202, 504, 577, and 621 nm, respectively, and diameters of 127, 108, 97, and 90 nm, respectively. To further examine the distribution of NRs, high-magnification SEM images (insets in Figure 2a−d) were obtained. Most of the NRs grow normally to the substrate. Figure 2e shows typical XRD patterns of the four nanostructured surfaces. The sharp diffraction peaks indicate that the prepared films have high crystallinity; no impurity peaks were detected in the spectrum. A stronger ZnO (002) peak showed the preferential growth of NRs in the [0001] orientation and the highly oriented c-axis alignment of largearea NRs.28,31 With the increase in reaction time, the diffraction peaks became sharper and indicated a more preferential growth, consistent with the SEM images. Hydrophobic Property. Geometrical structure of solid surface and chemical composition are the main factors that affect the surface wettability. Hou et al. reported that an increase in the surface roughness or decrease in the surface energy improved the hydrophobic effect.31 The surface wettability was improved by increasing the surface roughness over a specific size range. To evaluate how the optimal NRs enhance the superhydrophobic properties, the contact angle (CA) on planar and nanostructured surfaces was measured, as shown in Figure 2f. By introducing nanostructures, improved hydrophobic effect is expected after the C4F8 gas plasma treatment. For a planar surface, the measured static angle is ∼113°, whereas for a nanotextured surface, an obvious improvement in the water repellent performance was observed with a higher CA. The CA could be tuned in the range 115− 160° experimentally, and the best CA was 160°.

RESULTS AND DISCUSSION Hierarchical BIC eyes are mainly acquired through multiple polymer replication by reconfigurable microtemplating, that is, dome-like arrays by laser swelling technology, pattern conversion of microlens from a planar distribution to a curved distribution by membrane deformation, and growth of ZnO NRs. The microstructure array was prepared on a substrate (Figure 1a). The microstructure array was first prepared by using a femtosecond laser by dual-layer fabrication. The

Figure 1. Fabrication process of BIC eyes. (a) A convex structure was formed by front-side irradiation of dual-layer PMMA. The inset shows that the laser beam is transmitted through the upper transparent layer and focused on the underlying layer. (b−d) A reconfigurable microtemplating polymer process: (b) PDMS molding, (c) PDMS membrane deformation, (d) second molding. (e) NRs were fabricated using crystal growth method. (f) Formation of fully covered hierarchically tertiary structures. B

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Figure 2. Morphology and characterization of NRs. (a−d) SEM images of ZnO NRs fabricated with a bath time of 20−50 min with an increment of 10 min. The insets show partial magnification. (e, f) Characterization of fabricated structures: (e) XRD patterns, (f) CA of fabricated structures.

Figure 3. AFM images of structured surface for reaction time of (a) 20 min, (b) 30 min, (c) 40 min, and (d) 50 min, respectively. (e) Average roughness, root-mean-square roughness of structured surfaces as a function of reaction time.

min). With further increase in reaction time, the height slightly increased, and the space slightly decreased (for the reaction of 40 min). This is why the roughness increased with reaction time. When the reaction time exceeded 40 min and reached 50 min, the NRs showed a dense distribution. The too dense distribution results in a downward trend of surface roughness. This hypothesis is supported by the SEM and AFM images, providing insight into the surface morphology of nanostructured surface. However, further increase in reaction time will decrease the light transmittance of substrate. Therefore, considering the focusing property of microstructures, the nanostructures obtained with reaction time of 20−50 min were studied in the experiment. This phenomenon can be explained by Cassie and Baxter theory. The theory is favorable because it provides a higher CA. This situation can be described as follows:32−34

In this study, the air trapped inside the gaps of well-aligned individual NRs significantly increased the air/water interface. The increased interface effectively prevented the penetration of water droplets into the grooves and finally improved the hydrophobic effect of nanostructured surface. Notably, the CA showed a decreasing trend when the bath time exceeded 40 min. This can be attributed to the fact that a smaller pitch between NRs affects the filling of trapped air, thus eventually affecting the hydrophobicity. It was observed that the CA with reaction time in the range of 40−50 min reaches almost 160°, that is, the nanostructures obtained by the crystal growth method turned the surface into a superhydrophobic surface. For the quantification of surface roughness, Figure 3a−d shows the atomic force microscopy (AFM) scanning of surface morphology as a function of reaction time. Figure 3e shows the roughness values obtained from the AFM measurements. The roughness first increased until a reaction time of 40 min and then decreased. This trend can be attributed to the following reasons: The nanostructures showed a sparse distribution when the reaction time is 20 min. The space between NRs is very large. With the increase in reaction time, the space decreased significantly, and the height obviously increased, and thus the surface roughness increased (for the reaction of 30

cos θ = r f cos θY + f − 1

(1)

where θ is the Cassie−Baxter CA, θY is the Yong CA as defined for an ideal surface, f is the fraction of solid surface wet by the liquid, and rf is the ratio between the actual and projected solid surface area. The NRs could prevent the complete contact between the droplet and unwetted surface, promoting the C

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Figure 4. (a) Reflectance and (b) correlation of neff and depth of planar and nanostructured surfaces. The inset shows the refractive index of a flat NOA-65 film for reference.

Figure 5. (a−c) Characterization of fabricated structures with inclination angle of 0−75° by an increment of 25° with 40 min bath time: (a) XRD patterns, (b) reflectance, and (c) CA of fabricated structures. The results show a slight difference with each other, indicating the possibility to produce omnidirectionally and functionally uniform compound eyes using this crystal growth method.

Cassie−Baxter state.35−37 This is why the surface hydrophobic performance was improved after the introduction of NRs on the planar surface. In this study, both surface roughness and the fraction of solid surface wet by the liquid are main factors that affect the surface wettability.38−40 However, these two factors have opposite effect on the surface wettability.41−43 It can be deduced from the Cassie−Baxter equation that an increase in roughness results in an increase in Cassie−Baxter CA, whereas f is negatively correlated with the CA. In the experiment, the distribution and morphology of NRs affect not only the surface roughness but also the fraction of solid surface wet by the liquid. The surface roughness increased with an increase in the reaction time (for reaction time of 20−30 min, as shown in Figure 2f). However, at this time range, the diameter of NRs significantly decreased, increasing the fraction of solid surface wet by the liquid. Owing to the negative effect of f in this process, the CA remains almost unchanged even if the surface roughness increased significantly. With the increase in reaction time (for the reaction time of 30−40 min), the surface roughness further increased. Meanwhile, f slightly decreased (the space between NRs slightly decreased, as shown in Figure 2b,c). As a result, the hydrophobic effect significantly improved, and a larger CA was observed. For the reaction time of 40−50 min, the NRs became closely packed. This decreased both the surface roughness and f. However, the effect of surface roughness variation dominates the surface wettability at this time range. Therefore, although f decreased, a smaller CA was observed. Generally, the change in surface wettability was a competing result of roughness and the fraction of solid surface wet by the liquid. Therefore, by tuning

the distribution and morphology characteristics of NRs, the surface wettability can be optimized. Antireflective Property. NRs not only effectively created a superhydrophobic surface but also significantly suppressed the spectral reflection. Moreover, the ability to control surface reflection is of great importance for the fabrication of BIC eyes in actual applications. To evaluate the AR characteristics of NRs, the spectral reflection over the visible spectrum was observed (Figure 4a). The nanostructures significantly affected the reflectance, and the surface showed a better antireflective performance with the increase in reaction time. In general, the nanostructured surfaces (for Figure 4b−d) generally decreased the reflection by almost 25% in the wavelength range 400−800 nm compared with the planar surface, demonstrating their broadband AR characteristics. Fresnel reflection results from the index discontinuity at the interface of the two media. In this study, the growth of nanostructures on the surface produces an intermediate layer, producing a stepped refractive index variation. Therefore, the large discontinuity at the interface is broken into smaller steps, resulting in a lower reflectivity. For ideal antireflection arrays (ARs) with near-zero reflectivity, the nef f value should be gradually increased from the interface of air/ARs to the interface of ARs/substrate. According to the effective medium theory, the effective refractive index of surface with a composite layer can be calculated as follows:29 neff =

n12f + n22(1 − f )

(2)

where n1 and n2 are the refractive index of air and the substrate, respectively, and f is the fill factor of antireflective NRs. The relationship between nef f and the height of AR is shown in Figure 4b. The nef f value abruptly changes from the air to the D

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Figure 6. (a) SEM images of convex structures obtained by laser swelling technology. (b) Cross-section of convex structures acquired by LSCM. (c) SEM images of micronanohierarchy. (d) Partial magnification of hierarchical structures. (e) Measured static CA on different surfaces. (f) Spectral intensity of textured surface. The cross-section of convex structure showed the para-profile, indicating better optical performance. Moreover, the NRs are fully and uniformly distributed on the convex dome. The high CA and lower reflectance demonstrate the high performance of hierarchy.

substrate (inset of Figure 4b), leading to a high reflection of flat substrate at the interface. To decrease the reflection, the nef f should be gradually changed. For nanostructured surfaces, the neff changes continuously from 1.0 to 1.5; this is the reason why a nanostructured surface exhibits a lower surface reflection (Figure 4a). Furthermore, with the increase in reaction time, the height and fill factor of NRs increased; this offered a more gradual index gradient (Figure 4b). Therefore, the NRs decreased the surface reflectance over the visible spectrum. The antireflective property improved, and a lower reflection was observed with the increase in reaction time. The NRs were actually developed on the curved surface. Therefore, it was essential to study the angle dependence of reflectance and water repellent performance. The inclination angle slightly affected both the reflectance and water repellent performance of nanostructures, as shown in Figure 5a−c. This confirms the possibility to produce omnidirectionally and functionally uniform compound eyes. Fabrication of Micronano Hierarchical Structures and Their Performance. Microstructures were fabricated using the laser swelling technology. The formation of microstructures is based on two concepts: First, the surface layer of sample was transparent, allowing a high light transmittance. The laser beam focused on the underlying layer, ensuring the formation of convex structures. Second, the material of

underlying layer decomposed when it interacted with the laser beam, guaranteeing the height of convex structures. Figure 6a shows the SEM images of convex structures obtained by laser swelling technology. Figure 6b shows the cross-section of convex structures acquired by LSCM. The height and diameter of each convex structure were 11 and 40 μm, respectively. The distance between two convex structures is 60 μm, as set by the laser swelling process. The surface profile of convex structures was almost a parabola, and the focal length of each microlens can be described as follows:5,19 f=

d2 8h(n − 1)

(3)

where h and d are the height and diameter of lens, respectively, and n is the refractive index of NOA-65. When h = 11.0 μm, n = 1.524, and d = 40.0 μm, the focal length is 34.7 μm. Because both micro- and nanostructures were fabricated, the proposed method could be used to create micronanohierarchy. As discussed above, the morphology of nanostructures could be controlled by changing the reaction time. Moreover, the morphology of convex structures could be controlled by changing the laser power and irradiation time of laser beam. Figure 6c,d shows an array of convex structures covered with NRs with a reaction time of 40 min. E

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Figure 7. (a) SEM images of convex arrays with nanostructures on nine surfaces with a reaction time of 30 min (marked with S1−S3, H1−H3, and D1−D3) in details. On S1, S2, and S3 surfaces, the micronanostructures have different spaces of 80 μm, 60 μm, and 40 μm, respectively, but almost the same diameter and height of ≈42 μm and ≈11 μm on average. On H1, H2, and H3 surfaces, the micronanostructures have different heights of ≈11 μm, ≈16 μm and ≈21 μm, respectively, but almost the same diameters of ≈42 μm, ≈40 μm, and ≈41 μm, respectively, and the same space of 60 μm. On D1, D2, and D3 surfaces, the micronanostructures have different diameters of ≈25 μm, ≈42 μm, and ≈60 μm, respectively, on average, but almost the same heights of ≈10.7 μm, ≈11 μm, and ≈11.5 μm, respectively, and the same space of 60 μm. P1, P2, and P3 are partially enlarged images of three different positions in H3. (b) Measured CA and (c) spectral intensity on the surface of different micronanostructures.

adjusting the laser power and irradiation time (as shown in Figures S1a,b). Figure 7a shows the SEM images of hierarchy with nanostructures on nine surfaces with reaction time of 30 min (marked with S1−S3, H1−H3, and D1−D3) in details. On S1, S2, and S3 surfaces, the micronanostructures have different spaces of 80 μm, 60 μm, and 40 μm, respectively, but almost the same diameter and height of ≈42 μm and ≈11 μm on average. On H1, H2, and H3 surfaces, the micronanostructures have different heights of ≈11 μm, ≈16 μm, and ≈21 μm, respectively, but almost the same diameters of ≈42 μm, ≈40 μm, and ≈41 μm, respectively, and the same space of 60 μm. On D1, D2, and D3 surfaces, the micronanostructures have different diameters of ≈25 μm, ≈42 μm, and ≈60 μm on average, but almost the same heights of ≈10.7 μm, ≈11 μm, and ≈11.5 μm, respectively, and the same space of 60 μm. Wettability of Micronanohierarchy. To evaluate the effect of distribution and morphology of micronanostructures on water repellent property, a reference drop of 5 μL was placed on various surfaces, and the CAs were measured. Figure 7b shows the CAs on S, H, and D surfaces. For S1, S2, and S3 surfaces, the CAs were ≈152°, ≈157°, and ≈158°, respectively. For H1, H2, and H3 surfaces, the CAs were ≈156°, ≈159°, and ≈164°, respectively. For D1, D2, and D3 surfaces, the CAs were ≈157°, ≈156°, and ≈153°, respectively. The microstructured surface did not exhibit superior airtrapped ability compared with a flat surface (the CAs for flat and microstructured surface were 106° and 109°, respectively, as shown in Figure 6e). However, when the microstructures

In the experiment, to evaluate the effect of hierarchical structures on hydrophobicity, the CAs on different surfaces were measured after chemical modification, as shown in Figure 6e. The CA is ∼106° for a flat NOA-65 surface. When a microstructure was formed, the CA is ∼109°. However, for a surface with nanostructures, a much higher CA of 158° was observed. The CA for micronanohierarchy was ∼161°. The study confirms the potential of turning a surface into a superhydrophobic surface by using the proposed method. In addition to hydrophobicity, the light-absorbing properties of structure are also very important. The reflectivities of various substrates, that is, planar substrate, nanostructured film, and micronanohierarchy were measured. Figure 6f shows the dependence of reflectance on nanostructure morphology within the wavelength range of 400−800 nm. It was observed that the nanostructured surfaces have a much lower reflectance compared with a planar surface, and the reduction ratio is ∼25%. Besides, the hierarchical structured surface has a slight lower reflectance compared with a nanostructured surface. This is because the height of hierarchy is higher than that of NRs, resulting in a more gradual index change. With the same variation in nef f, hierarchy provides a smaller slope than the NRs and thus providing a better antireflective performance. Wettability and Antireflectivity of Various Micronanohierarchy. To further evaluate the effect of microstructures (i.e., distribution and morphology) on the antireflection and water repellent properties, microstructures with different spaces and morphologies were fabricated by F

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Figure 8. (a) SEM image of fabricated BIC. The inset shows partial magnification. (b) Schematic image of the optical characterization system. (c) Microscopic images of the BIC eyes illuminated with a tungsten lamp and the intensity distribution obtained along the orange and blue line. (d) Arrays of miniaturized “A” letters on the focal plane of BIC eyes illuminated with tungsten lamp.

With the decrease in space (for S1, S2, and S3 surfaces), the surface reflection shows a downward trend, as shown in Figure 7c. A similar trend was observed in the case of both increase in height (for H1, H2, and H3 surfaces) and diameter (for D1, D2, and D3 surfaces). According to the effective medium theory, the effective refractive index of the composite layer can be expressed as follows:48,49

were combined with nanostructures, the surfaces exhibited superhydrophobic property. For the same reaction time of 30 min, the water repellent property can be attributed to different distribution and morphology of microstructures with different air-trapping abilities. With the decrease in space (for S1, S2, and S3 surfaces), the status of micronanostructures changed from “significant separation” to “mild separation” to “tangency”, as shown in Figure 7a. Moreover, the density of micronanostructures gradually increased. Therefore, a decrease in space increases the roughness of obtained surface and improves the water repellent property.34 With the increase in height (for H1, H2, and H3 surfaces), the micro/nanocomposite structure can trap a larger amount of air. This decreases the contact area between water and the solid surface, and a higher CA is observed. With the increase in diameter (for D1, D2, and D3 surfaces), the fraction of surface wetted by the liquid increases. It can be deduced from the Cassie−Baxter equation that an increase in the liquid−solid composite interface will lead to an increase in CA. The above-mentioned results indicate that the droplets on different hierarchical surfaces were in the Cassie−Baxter regime. For hierarchical structures, the air trapped between the solid and liquid interface decreases the solid−liquid contact and thereby decreases the CA.44−47 Reflectivity of Micronanohierarchy. The antireflection property of micronanohierarchy is better than that of nanostructured surfaces for most wavelengths. To evaluate the effect of distribution and morphology of microstructures on antireflective performance, the surface reflection on these surfaces was measured. Figure 7c shows the spectral reflectance of various surfaces in the range of 400−800 nm.

2 2 neff = [nZnO f + nAir (1 − f )]1/2

(4)

where, nZnO and nAir are the refractive index of ZnO film and air, respectively, and f is the filling factor. For micronanohierarchy, the value of f gradually changes from the top to the bottom of the film, thus leading to a gradual change in nef f. The gradual change in refractive index could suppress the surface reflection.50,51 Second, because of the coexistence of micro- and nanoscale features, the hierarchy can induce multiple scattering of incident light, thus reducing the surface reflection over a broad range of wavelength. Another advantage of the hierarchy is that the antireflection properties are less dependent on the angle.52 By introducing micronanostructures on a flat surface, the antireflective performance can be significantly improved owing to a gradual change in nef f.53,54 In the experiment, the fill factor increased with the decrease in space (for S1, S2, and S3 surfaces), increase in height (for H1, H2, and H3 surfaces), and increase in diameter (for D1, D2, and D3 surfaces). Therefore, the nef f changed more gradually for S3, H3, and D3 surfaces. This explains why S1, H1, and D1 surfaces have a relatively large surface reflection, whereas S3, H3, and D3 surfaces have a small surface reflection. Optical Characterization of BIC Eyes. Imaging and Optical Uniformity of BIC Eyes. The planar distributed microlens arrays were transformed into a spherical configG

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Figure 9. FOV Characterization of single lens and BIC eyes. (a,c) and (b,d) are PSF comparison of the single microlens and BIC eyes for x and y directions, respectively. To clearly observe the evolution of spot morphology, the light intensity of (a,b) is much higher than (c,d). Compared with single lens, the BIC eyes have better focusing and imaging ability. The intensity distribution is systematic and narrower, when a tilted light is incident on the BIC eyes. However, with the increase in tilted angle, the central light intensity became lower.

achieved by the reduction of lens pitch by controlling the laser fabrication process. Notably, some decrease in intensity and distortion occurred at the outer parts of the BIC eyes. The distortion can be attributed to the curved distribution of microlens, and thus the incident light was not perpendicular to the single microlens. The slight decrease in the observed distributions of relative intensity on the edge was due to the inherent planar photoelectric detector. Furthermore, a mask with a letter “A” was inserted between the bright light source and BIC eyes to characterize the optical performance of BIC eyes. Figure 8d shows the real image of compound eyes of the red-labeled area when focused on the top of BIC eyes. Near the center of compound eyes, the microlens exhibited some but not very uniform imaging property. This can be partly attributed to the imperfect uniform distribution and morphology of NRs obtained by the crystal growth method. Moreover, the inhomogeneous elastic deformation under the adsorption force using air-assisted technology that results in asymmetric deformation along the x and y axes also plays a role. On the other hand, at the edge, the image showed significant distortion. This can be mainly attributed to the exceeding of acceptance angle of a single microlens owing to the curved surface. On one hand, a variation in focus plane also resulted in the distortion of image. Characterization and Comparison of Optical Performance of Single Lens/BIC Eyes. Point spread functions were used to characterize the performance of optical systems. Optical microscope images were then acquired and measured. Figure 9a−d shows the PSF along the x/y axis for a single lens and BIC eyes. For a single lens, the light spot has almost a systematic distribution for normal incidence. With a tilted incident light, the spot produces obvious distortion along the symmetry axes. With increasing incidence angle, the distortion

uration by air-assisted technology, and the BIC eyes were then obtained by subsequent crystal growth. The convex microlenses were spherically distributed on the curved surface. Using the proposed technology, the height could be easily controlled by adjusting the negative pressure. Figure 8a shows the SEM image of the BIC eyes. The diameter and height of BIC eyes fabricated in the experiment are 5.0 mm and 2.2 mm, respectively. The inset of Figure 8a shows a partially enlarged magnification. The NRs have a uniform and full coverage distribution. However, during the deformation, the microlens distributed on the spherical surface would also gradually deform both in height and diameter. Therefore, the negative pressure should also be carefully controlled. To characterize the optical property of BIC eyes, a system consisting of a microscope and a CCD camera was set up. Figure 8b shows a schematic image of the optical characterization system. A tungsten lamp was used as the light source. A mechanical stage was used to adjust the angle illuminated on the BIC eyes. The CCD camera captured the confocal image. Owing to the curved distribution of microlens, the focus of each optical unit is not in the same imaging plane. A clear image was obtained from the apex of BIC eyes. Figure 8c shows the confocal images of BIC eyes illuminated with normal incident light. A tungsten lamp was used as the light source to investigate the focusing ability. The distributions of relative output intensity measured along the orange and blue lines are shown at the left and bottom sides of the main image, respectively. The sharp intensity distribution of focusing spots and the almost systematic distribution along x/y directions show that the BIC eyes have high optical uniformity. Note that the light beam could be transmitted through all the regions of the dome, both through the ommatidium and between. This could be avoided by introducing the waveguide into the BIC and improving the filling factor. The filling factors were H

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ACS Nano along x axis became larger. However, for y axis, the spot showed some distortion, but almost systematic light distribution. The results indicate a weaker focusing ability of single lens under tilted light. For comparison, the BIC eyes showed systematic light distribution, even when the incident light was increased to 40°. Moreover, almost no distortion occurred along the x and y axes. The fwhm (x/y axis) increased from 1.8/1.8 μm to 2.7/4.1 μm and 6.2/5.5 μm when the light was changed from 0° to 10° and 20° for a single lens (Figure 9a,b), respectively. For the BIC eye, the fwhm (x/y axis) remained almost constant and is ∼1.9 μm under different incident angles. As a result, the BIC eyes reduced the imaging distortion by 1.5/3.4 and 2.3/3.1 times along the x and y axes, respectively, under 10° and 20° incident light. The slight variation in fwhm for the BIC eyes under normal incident light can be attributed to lens deformation while carrying out the air-assisted technology, affecting the focusing ability to a small extent. Figure 10 shows the central intensity under different incident lights; the red line shows the fitted curve of

Based on the numerical deduction (Figure S2), the FOV of BIC eyes is as follows: FOV = 2 arcsin 2Rh/R2 + h2

(6)

The theoretical FOV was ∼180°, whereas the measured FOV was ∼80°. The large difference between the test results and theoretical results can be attributed to the planar incident light of photodetector and the relatively small intensity of light source used in the experiment. On the other hand, the hierarchical structures affect the light transmittance to some extent while suppressing the surface reflection.

CONCLUSIONS In conclusion, an effective method is proposed for the manufacturing of large-area hierarchical structures by modified laser swelling, air-assisted deformation, and controlled crystal growth methods. Using this approach, hierarchical structures were fabricated on a curved surface with well-controlled nanostructures and defined convex structures, similar to natural eyes. The addition of nanostructures endows the BIC eyes with antireflection and water repellent properties. Furthermore, the BIC eyes significantly reduced the imaging distortion by 1.5/3.4 and 2.3/3.1 times along the x and y axes, respectively, under 10° and 20° light incidence. In addition, the BIC eyes showed high-directional optical stability with a slight distortion in the light acceptance range. MATERIALS AND METHODS Preparation of Samples. PMMA crystals (20 g, available on Aladdin) were slowly added to chlorobenzene (80 g) and then stirred at intermediate speed for 12 h using a magnetic stirrer. Subsequently, the obtained PMMA solution was uniformly deposited on a black acrylic plate (20 mm × 20 mm) (available on Taobao), and the thickness of sample was controlled by the volume of solution. Finally, the sample was placed in a sealed environment and dried at room temperature for 12 h. Preparation of Microstructures. The light source used in the experiment was a femtosecond laser operating at a wavelength of 800 nm and repetition frequency of 1 kHz. The accurate control of sample movement was achieved using a motorized translation stage (DaHeng photoelectric technology). The laser beam was successively passed through an attenuator, a diaphragm, a half-wave plate, a beam splitting prism, and an objective lens and finally focused on the sample. Structures with controllable diameter and height were obtained by adjusting the laser power (Figure S1a) and irradiation time (Figure S1b). Fabrication of Macromicro Secondary Structures. Transparent elastomeric PDMS material (184) and curing agent with a certain mass ratio (10:1) are widely used for the pattern transfer process. A vacuum oven was used to eliminate bubbles from the mixing process. Then, 184 compounds were prepared on the sample obtained in the first step using the spin coating method, with a speed of 500 rpm for 18 s. Subsequently, the sample was placed in an oven at 80 °C for 1 h. After fully curing, the replica was peeled off and fixed on a home-built mold. Then, a negative air pressure (0.7−1.0 MPa) was applied to form a deformed elastomer membrane. After the pressure became stable, the solvent-free UV curable epoxy resin was filled in the formed cavity, covered with a coverslip, and then fully cross-linked for 5 min under UV light irradiation. The master mold was released, affording the secondary structures. Preparation of Macromicronano Tertiary Structures Over Secondary Structure Surfaces. The nanostructures were prepared as follows: A 20 nm ZnO seeder layer was first sputtered on the sample surface. Then, 7.00 g of hexamethylenetetramine and 14.87 g of zinc nitrate were added to 500 mL deionized water and then mixed and stirred using a magnetic stirrer for 20 min. Subsequently, the

Figure 10. Central intensity under different incident lights of BIC eyes. Red line shows the fitted curve of normalized intensity dependence on incident angle. The images captured at different incident lights are shown on the top. When the angle exceeds 40°, the CCD camera shows an almost dark screen.

normalized intensity dependence on incident angle. The results show that although no distortion was produced for BIC eyes, the light intensity decreases with the increase in incident angle. When the tilted angle exceeded 40°, the light was not transmitted through the dome lens. Therefore, the CCD camera showed a dark screen. Notably, the controllable FOV could be easily achieved by adjusting the height of BIC eyes using the air-assisted technology. The red lines in Figure 10 show the relationship between normalized intensity and incident angle; this can be fitted by the following equation: y = −0.0096x 2 + 4 × 10−16x + 1.0306R2 = 0.9

(5)

Therefore, the intensity of the central lens decreased to 0.5 when the incident angle was ca. 36.5°. I

DOI: 10.1021/acsnano.8b04047 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano hexamethylenetetramine solution was added to a zinc nitrate solution, mixed, and stirred for 15 min. Then, the sample was carefully added to the mixture. The sample with ZnO seed layer was immersed in the mixture and kept at 90 °C for a certain time. After the reaction was completed, the sample was removed, washed repeatedly with excess deionized water to remove any unreacted materials on the ZnO surface, placed in a clean oven, and dried at 50 °C for 30 min. A compound lens with a tertiary structure was obtained. Measurement and Characterization. The structures were characterized using a scanning electron microscope (HITACHI). The morphology of microstructures and their cross sections were characterized using a laser scanning confocal microscope (Olympus OLS4000, Japan). The AFM images were obtained using an atomic force microscope (Innova, Veeco, USA). The CA of water droplet on hierarchical surfaces was measured using a Data Physics OCA 20 system at ambient temperature. The CA of each sample is the average of measurements at five different locations. Before the measurement, a layer of fluorocarbon was deposited with C4F8 using a Plus 100 system (Oxford Instruments, UK). Reference water droplets of 5 μL were used for CA measurements. The surface reflectance of samples was measured using a spectrophotometer (Shimadzu UV3600).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04047. Structures with controllable diameter and height were obtained by adjusting both the laser power and irradiation time. The dependence of diameter and height of fabricated microstructures on laser power and irradiation time. The FOV was determined with the height (h) and the radius (R) of the macro-spherical dome. The theoretical calculation of the FOV of the BIC eye which was mainly determined by the spherical macrobase (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Wenjun Wang: 0000-0002-2562-4077 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 51475361), National Key Research and Development Program of China (grant no. 2017YFB1104602), and Program for Changjiang Scholars and Innovative Research Team in University (grant no. IRT_15R54). REFERENCES (1) Fitzgerald, R. J. Artificial Compound Eyes. Phys. Today 2006, 59, 21−21. (2) Huang, C.-C.; Wu, X.; Liu, H.; Aldalali, B.; Rogers, J.-A.; Jiang, H. Large-Field-of-View Wide-Spectrum Artificial Reflecting Superposition Compound Eyes. Small 2014, 10, 3050−3057. (3) Xu, H.; Lu, N.; Shi, G.; Qi, D.; Yang, B.; Li, H.; Xu, W.; Chi, L. Biomimetic Antireflective Hierarchical Arrays. Langmuir 2011, 27, 4963−4967. (4) Ko, D.-H.; Tumbleston, J. R.; Henderson, K.-J.; Euliss, L. E.; DeSimone, J.-M.; Lopez, R.; Samulski, E. T. Biomimetic Microlens Array with Antireflective “Moth-Eye” Surface. Soft Matter 2011, 7, 6404−6407. J

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DOI: 10.1021/acsnano.8b04047 ACS Nano XXXX, XXX, XXX−XXX