ARTICLE pubs.acs.org/Langmuir
Biomimetic Antireflective Hierarchical Arrays Hongbo Xu,† Nan Lu,*,† Gang Shi,† Dianpeng Qi,† Bingjie Yang,† Haibo Li,† Weiqing Xu,† and Lifeng Chi*,†,‡ † ‡
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, PR China Physikalisches Institut and Center for Nanotechnology (C198eNTech), Westf€alische Wilhelms-Universit€at M€unster, D-48149 M€unster, Germany
bS Supporting Information ABSTRACT: We report a simple method for the fabrication of biomimetic antireflective hierarchical arrays based on the combination of self-assembled polymer spheres and nanoimprint lithography (NIL). The hierarchical structures are fabricated by creating nanopillars on the microscale round protrusion arrays, which are similar to natural mosquito eyes consisting of combined micro- and nanostructures. The hierarchical arrays dramatically suppress the surface reflection from visible to near-infrared regions with an angle of incidence of up to 70°.
’ INTRODUCTION Some natural structures exhibit unique functions, such as selfcleaning lotus leaves,1,2 colorful butterfly wings,3-6 photoreceptors in brittlestar,7 and antireflective moth eyes.8-15 For example, the outer surface of a moth’s cornea consists of conical protuberance arrays, providing pixelated images for individual receptors, and can suppress surface reflections.16 Similar nipple arrays have also been observed on the wings of cicadas to produce superhydrophobic surfaces for water-repellent applications.17 Generally speaking, the nipple arrays on the insects’ compound eyes and wings result in both antireflective and water-repellent properties. Traditionally, coatings with gradient refractive indices have been widely utilized to suppress undesired Fresnel reflections that severely limit the performance of the devices.18-22 However, the coatings are limited by stability problems induced by adhesion and thermal mismatch. Inspired by moth eyes, surface-relief arrays with dimension smaller than the wavelength of incident light are an alternative to thin film coatings, which are more stable and durable than surface coatings because only one material is involved. The basic principle of this technique is to introduce a gradient refractive index between air and a substrate material by creating a structured layer.23,24 Reflection can be substantially suppressed for a wide spectral bandwidth and over a large field of view.25,26 It is known that silicon nanopillars with a high aspect ratio can result in a gradual increase in the effective refractive index (neff) from air to the substrate. However, the fabrication of silicon cones with a high aspect ratio requires special etching technology, such as chlorine-based RIE or electron cyclotron resonance plasma etching.27,28 Although natural mosquito eyes consist of combined micro- and nanostructures, the most commonly fabricated structures are limited to 2D microhemisphere arrays, which result in different reflectivity from natural reflectivity. Very recently, artificial compound eyes were generated using soft lithography and their antifogging properties were r 2011 American Chemical Society
investigated.14 However, the fabrication of artificial compound eyes has still been less-frequently addressed.29,30 Herein, we demonstrate an alternative method to creating hierarchical arrays based on the combination of self-assembly and nanoimprint lithography (NIL).31 The fabricated hierarchical arrays, which are similar to natural mosquito eyes,14 show a lower reflection, a broader spectral region, and a higher contact angle than do the 2D nanopillars.
’ EXPERIMENTAL SECTION Chemicals and Materials. All solvents and chemicals were of reagent quality and were used without further purification. Ethanol, acetone, chloroform, and tetrahydrofuran (THF) were purchased from commercial sources in the highest available purity. Ultrapure water (18.2 MΩ 3 cm) was used directly from a Millipore System (Marlborough, France). The monodisperse PS spheres with less than 5% diameter variation were obtained from Sigma-Aldrich. The Si wafers (n type (100)) were obtained from Youyan Guigu (Beijing, China). Substrate Preparation. Silicon slices (approximately 4 4 cm2 cut from a Si wafer) were treated with an oxygen plasma system (100, PVA Tepla) at 300 W and 660 mTorr for 3 min and thoroughly cleaned with acetone, chloroform, and ethanol in an ultrasonic bath for 3 min. Then, the substrates were rinsed with deionized water and dried under a flow of nitrogen gas. A spin-coated PMMA layer on a silicon substrate was prepared at a rotational speed of 3000 rpm (rpm) for 60 s. After the 380 nm PMMA layer was spin coated onto the silicon slices and baked at 120 °C for 5 min, NIL was performed on the freshly prepared PMMA layer under a pressure of 40 bar and a temperature of 170 °C to fabricate the PMMA patterns on the silicon substrate. To change the imprinted PMMA squares to hemispheres, the imprinted substrate was subjected to heating treatment at 110 °C for 10-30 min. Then, the Si substrate with a microstruture array was subjected to the Received: March 31, 2010 Revised: January 11, 2011 Published: March 25, 2011 4963
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Langmuir RIE process using a Plasmalab Oxford 80 plus (ICP 65) system (Oxford Instrument Co., U.K.). The processing gases were SF6 (30 sccm), CHF3 (6 sccm), and O2 (5 sccm), the rf power was 100 W, and the chamber pressure was 30 mTorr. Then the substrate was kept in a 10% SDS solution for 10 min. Fabrication of the biomimetic self-cleaning antireflective hierarchical arrays: The 10 wt % aqueous suspension of PS microspheres was diluted with an equal volume of ethanol and subjected to ultrasonication for 10 s to improve the mixing. A 2% SDS solution was introduced to consolidate the particles. First, the prepared substrate was tilted into a glass Petri dish that was filled with 80 mL of water, thus constructing the system for fabricating the PS monolayer. Five microliters of the PS mixed suspension was dispersed onto the substrate. Some domains of the PS sphere monolayer were initially formed on the water surface, and after standing for a certain period of time, the highly ordered PS monolayer was dip coated onto the substrate from the water surface by drawing back the substrate. A monolayer of 580 nm PS spheres was prepared according to ref 32. To transfer the pattern of PS spheres onto the Si substrates, RIE was carried out in a similar process mentioned in the preceding section. Characterization. AFM images were recorded with a Nanoscope scanning probe microscope Dimension 3100 system (Digital Instru-
Figure 1. Scheme of the procedure for creating biomimetic self-cleaning antireflective hierarchical arrays.
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ments, Santa Barbara, CA) under ambient conditions. Si cantilevers (Nanosensors, Digital Instruments) with spring constants of 250350 kHz were used. AFM images were flattened and shown without any further image processing. Nanoimprint lithography was performed on a 2.5 in. nanoimprinter (Obducat AB, Malm, Sweden). Spectra were collected on a spectroscopy meter (Shimadzu UV3600, Shimadzu, Japan).
’ RESULTS AND DISCUSSION Figure 1 schematically illustrates the fabrication procedure used to create biomimetic antireflective hierarchical arrays on a silicon substrate. A thin film of poly(methylmethacrylate) (PMMA) was initially spin-coated onto the silicon substrate and imprinted with the stamp shown in Figure 2A. Figure 2B presents the imprinted PMMA microstructures. As illustrated in Figure 1B, the PMMA squares were changed into hemispheres by heating to 110 °C for 10-30 min (Figure 2C). The PMMA hemispheres were transferred to the Si substrate by subjecting them to the RIE process with SF6 and CHF3 at an rf power of 100 W. The higher-magnification image (inset of Figure 3A) reveals that a single layer of 580 nm PS spheres was assembled on the silicon microstructure (Figure 3A). Finally, the Si substrate covered with the PS sphere array was etched again with SF6, CHF3, and O2 at an rf power of 100 W (as illustrated in Figure 1E), which resulted in non-close-packed Si nanopillars. The etching selectivity of PS to Si is about 1:1.6. Figure 3B shows SEM images of the biomimetic antireflective hierarchical arrays. After etching for 5 min and lifting off the polymer resist, the silicon nanopillars of 580 nm periodicity were constructed on the microstructures, as shown in Figure 3B. The enlarged SEM image
Figure 3. (A) SEM image of the 580 nm PS spheres monolayer prepared on the patterned substrate. (B) Cross-sectional SEM image of the biomimetic silicon antireflective hierarchical arrays fabricated with 580 nm spheres masked etching. The insets showing the highermagnification images, the scale bars of the images and insets are 10 and 2 μm, respectively.
Figure 2. AFM images of (A) the stamp, (B) imprinted PMMA structure with the stamp (A) and (C) the PMMA structure after annealing at 110 °C. 4964
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Figure 4. (A) Optical microscopy image of a hierarchical array (right) and a polished Si wafer (left). (B) Hemispherical reflection a normal incidence for flat polished silicon (-), a microstructure with 10 μm width (---), 2D nanopillars prepared with the 580 nm PS spheres as a mask ( 3 3 3 ), and hierarchical arrays (-).
Figure 5. Simulated correlation of the refractive index and the structure height (λ = 632.8 nm). The top left inset shows the refractive index of a planar Si wafer for reference.
in Figure 3B reveals that the nanopillars are nonclosed with a height of 800 nm, which makes the total height of the biomimetic antireflective hierarchical arrays 2.4 μm. A photograph of a polished Si substrate and a hierarchical array is presented in Figure 4A, which shows that the reflection on the Si substrate structured with the hierarchical array (right: 4 4 cm2) is much lower than that on the polished silicon substrate (left). The reflection of different substrates was evaluated using visible-near-IR reflectivity measurement at 8° (Shimadzu UV3600). Figure 4B presents the spectra of the polished silicon (solid line), micropatterned silicon (dashed line), 2D 580 nm nanopillars (dotted line), and hierarchical arrays (bold solid line). The reflection of the polished silicon (solid line) is very high (about 33%) for the visible and near-infrared regions, and the micropatterned silicon exhibits similar behavior. The reflectivity of the nanopillars is lower than 10% at 300-1100 nm, and the lowest reflectivity is shown in the range of 600-900 nm, which is around 8%. For the hierarchical arrays (bold solid line), the reflectivity is further reduced to less than 5% in the range of 300-1100 nm, and the minimal reflection is further suppressed to 1% in the wavelengths range of 600-900 nm. The spectra show that the reflection of the hierarchical arrays is lower that that of the 2D nanopillar in a broader waveband and a larger range of the angle of incidence. As revealed in Figure 5, the hierarchical array is higher than the 2D array, which means that neff of the hierarchical arrays changed more gradually than
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Figure 6. Correlation of the angle of incidence and the reflectivity. The incident light is (A) p-polarized and (B) s-polarized at 632.4 nm.
that of the 2D array, resulting a lower reflection as a broader waveband.25 The roughness of the hierarchical arrays is higher than that of the 2D nanopillars, which is why the contact angle of the hierarchical array is larger that of the 2D nanopillar.14 The Fresnel reflection of incident light comes from the large refractive index discontinuity at the interface of two media. By inserting a layer with an intermediate index or multilayer with a stepped refractive index, this large refractive index discontinuity is broken into smaller steps, resulting in a lower reflectivity.33,34 According to the effective medium theory, the antireflection array (AR) here is referred to an inserted layer with effective refraction index neff. For an ideal AR with nearly zero reflectivity, the neff value gradually increases from 1 to nsubstrate from the boundary of air/AR to the boundary of AR/substrate.34-36 Only the real part of the numbers are calculated for the correlation of the reflective index and H to simplify the calculation. In this case, the reflectivity decreases dramatically with increasing aspect ratio of the structure. The effective index for normal incidence is36 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 - f þ fnsi 2 Þff þ ð1 - f Þnsi 2 g þ nsi 2 ð1Þ nef f ¼ 2ff þ ð1 - f Þnsi 2 g where nsi is the refractive index of the silicon structure and f is the filling factor of the antireflection array, which is the volume percentage of the antireflection array in the film.36 We calculate the f of the antireflection array based on information from the cross-sectional SEM, where f is function of the height of the antireflection array. For surface antireflective properties, the gradual transition of the effective refractive index (neff) from air to the bulk is crucial. The correlation of neff and the height (H) of antireflection arrays is presented in Figure 5. The abrupt change in neff from air (1.0) to silicon (nsi = 3.82 at 632.8 nm) across the air/silicon interface results in a high reflection on the polished silicon substrate (inset). The minimum neff of the 580 nm nanopillars and hierarchical structures is 1 (air), and the maximum neff values of these two kinds of structures are almost equal to the refractive index of the substrate, which is the reason that the structured surfaces exhibit lower antireflection. However, the total height of the hierarchical structures is higher than that of the 580 nm nanopillars, which results in the neff of hierarchical structures changing more gradually than that of 580 nm nanopillars. Therefore, the spectra suggest that the hierarchical arrays exhibit stronger antireflective properties within a broader band and a wider angle property than do 2D nanopillars. Figure 6 shows the correlation of the angle of incidence and the measured reflectivity. It was measured with a He-Ne laser (632.8 nm) at an angle of incidence from 25 to 85° for p and s polarization, where p and s denote planes of incidence parallel 4965
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Figure 7. (A) SEM image of the silicon nanopillars created with 580 nm spheres masked etching. (B) SEM image of the silicon hierarchical structures fabricated with 2.5 μm square protrusions etched with 580 nm spheres as the mask; the spacing of the squares is 3.5 μm. (C) Hemispherical reflection at the angle of normal incidence.
Figure 8. (A) SEM image of the silicon nanopillars obtained by 220 nm spheres masked etching. (B) SEM image of the silicon hierarchical arrays created by etching on 1 μm protrusions with 220 nm spheres as the mask. (C) Hemispherical reflection at the angle of normal incidence.
Figure 9. (A) SEM image of the silicon nanopillars fabricated with 220 nm PS masked etching. (B) SEM image of the silicon structures fabricated by etching on the 580 nm silicon hemispheres with 220 nm spheres as the mask. (C) Hemispherical reflection at the angle of normal incidence.
and perpendicular to the electrical field of the light waves, respectively. Figure 6a,b shows that the measured reflectance of the polished flat Si wafer varies strongly with angles of incidence (AOI) for both p- and s-polarized light. By contrast, neither the 2D nanopillars nor the hierarchical arrays are highly sensitive to polarization. Furthermore, the performance of the hierarchical arrays is better than that of the 2D nanopillars for both s- and ppolarized light. For both p and s polarization, the reflectance of the 2D nanopillars (the dashed line) reaches a value of 5% when the AOI is 60°, whereas that of the hierarchical arrays remains below 1% at values of AOI ranging from 25 to 70° (dotted line). The range of angles of incidence for which hierarchical arrays suppress the reflectivity is larger than that of the 2D nanopillars because of the neff of the hierarchical arrays changing more slowly than that of the 2D array.25After fluorosilane modification, the water contact angle (CA) of silicon is ca. 104° and the CA and sliding angle of the hierarchical arrays are ca. 163° and less than 2°, respectively. As shown in Figure S2, the hierarchical structures significantly enhance the hydrophobicity of the silicon surface compared to that of the microscale arrays (ca. 114°) and nanopillars (ca. 144°). The roughness of the hierarchical arrays is higher than that of the 2D nanopillars, which is why the contact angle of the hierarchical array is larger than that of the 2D nanopillar.14
The effect of the periodicity on the reflection was investigated. Figure 7A shows the 580 nm nanopillars. Figure 7B shows the SEM images of the 6 μm and 580 nm hierarchical arrays. Figure 7C presents the reflection spectra of the polished silicon (solid line), micropatterned silicon (dashed line), 2D 580 nm nanopillars (dotted line), and hierarchical arrays (bold solid line). The spectra suggest that the hierarchical arrays exhibit a stronger antireflective property and a broader band antireflective property than the 2D nanopillars from 300 to 2400 nm with the proper periodicity; the antireflective band range can be broadened. In the same way, we can achieve hierarchical arrays with microstructures and nanostructures having different periodicities. Figures 8 and 9 show the hierarchical structures fabricated with 220 nm spheres masked etching on the 1 μm and 580 nm protrusions, respectively. The results show that the reflection of the hierarchical arrays is lower than that of the 2D nanopillars. In summary, we have demonstrated a method of creating biomimetic self-cleaning antireflective hierarchical arrays based on the combination of self-assembly and nanoimprint lithography. The biomimetic self-cleaning antireflective hierarchical arrays exhibit lower reflection and higher contact angles than do the 2D arrays. The lower reflection can be achieved by further 4966
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Langmuir increasing the height of the primary arrays, but this will make it difficult to assemble spheres on the arrays.
’ ASSOCIATED CONTENT
bS
Supporting Information. SEM image of the silicon nanopillars. Water drop profiles. Details of the experimental process. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected],
[email protected].
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’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC 20773052), the Program for New Century Excellent Talents in University of China, the National Basic Research Program (2007CB808003 and 2009CB939701), and Program 111. ’ REFERENCES (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (2) Blossey, R. Nat. Mater. 2003, 5, 301. (3) Clapham, P. B.; Hutley, M. C. Nature 1973, 244, 281. (4) Prevo, B. G.; Hon, E. W.; Velev, O. D. J. Mater. Chem. 2007, 17, 791. (5) Kumazawa, K.; Tabata, H. Zool. Sci. 2001, 18, 1073. (6) Kinoshita, S.; Yoshioka, S. ChemPhysChem 2005, 6, 1419. (7) Aizenberg, J. Nature 2001, 412, 819–822. (8) Xu, H.; Lu, N.; Qi, D.; Hao, J.; Gao, L.; Zhang, B.; Chi, L. F. Small 2008, 4, 1972. (9) Qi, D.; Lu, N.; Xu, H.; Yang, B.; Huang, C.; Xu, M.; Gao, L.; Wang, Z.; Chi, L. F. Langmuir 2009, 25, 7769. (10) Watson, G. S.; Watson, J. A. Appl. Surf. Sci. 2004, 235, 139. (11) Grann, E. B.; Moharam, M. G.; Pommet, D. A. J. Opt. Soc. Am. A 1995, 12, 333. (12) Watson, A.; Schulz, X. U.; Kaiser, P. N. Surf. Coat. Technol. 2005, 200, 58. (13) Chen1, H. L.; Chuang, S. Y.; Lin, C. H.; Yang, H. Opt. Express 2007, 15, 14793. (14) Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K. J.; Yang, B.; Jiang, L. Adv. Mater. 2007, 19, 2213. (15) Miller, W. H.; Bernard, G. D.; Allen, J. L. Science 1968, 162, 760. (16) Watson, G. S.; Watson, J. A. Appl. Surf. Sci. 2004, 235, 139. (17) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (18) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (19) Chen, D. G. Sol. Energy Mater. Sol. Cells 2001, 68, 313. (20) Heine, C.; Morf, R. H. Appl. Opt. 1995, 34, 2476. (21) Nikolajeff, F.; Lofving, B.; Johansson, M.; Bengtsson, J.; S.; Heine, C. Appl. Opt. 2000, 39, 4842. (22) Striemer, C. C.; Fauchet, P. M. Appl. Phys. Lett. 2002, 81, 2980. (23) Zhang, G.; Zhang, J.; Xie, G. Y.; Liu, Z. F.; Shao, H. B. Small 2006, 2, 1440. (24) Wu, C. T.; Ko, F. H.; Lin, C. H. Appl. Phys. Lett. 2007, 90, 171911. (25) Kanamori, Y.; Sasaki, M.; Hane, K. Opt. Lett. 1999, 24, 1422. (26) Kanamori, Y.; Roy, E.; Chen, Y. Microelectron. Eng. 2005, 7879, 287. (27) Huang, Y. F.; Chattopadhyay, S.; Jen, Y. J.; Peng, C. Y.; Liu, T. A.; Hsu, Y. K.; Pan, C. L.; Lo, H. C.; Hsu, C. H.; Chang, Y. H.; Lee, C. S.; Chen, K. H.; Chen, L. C. Nat. Nanotechnol. 2007, 2, 770. 4967
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