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Enhanced transmission and self-cleaning of patterned sapphire substrates prepared by wet chemical etching using silica masks Gui-Gen Wang, Zhao-Qing Lin, Dong-Dong Zhao, and Jiecai Han Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01486 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Enhanced Transmission and Self-cleaning of Patterned Sapphire Substrates Prepared by Wet Chemical Etching using Silica Masks

§

Gui-Gen Wang†,‡,∗, Zhao-Qing Lin†, Dong-Dong Zhao†, Jie-Cai Han†, †

Shenzhen Key Laboratory for Advanced Materials, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, P.R. China

‡

Centre for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore §

Center for Composite Materials, Harbin Institute of Technology, Harbin 150080, P.R. China

Keywords: Sapphire; Sub-wavelength structure; Antireflection; Self-cleaning; Optical window

ABSTRACT Highly-transparent and super-hydrophilic sapphire with surface antireflective sub-wavelength structures were prepared by wet etching using colloidal monolayer silica masks. The film thicknesses of the silica masks were adjusted by the volume concentrations of polystyrene spheres. The evolution of etching morphologies of sapphire was studied, and antireflective concave pyramid nanoarrays on sapphire substrates were designed by calculation and were then prepared. The transmission and wettability of as-obtained patterned sapphire substrates were also investigated. As to

∗

Corresponding author (G.G. Wang). Tel: +86-755-26629471, fax: +86-755-26033504. E-mail: [email protected] 1

ACS Paragon Plus Environment

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sapphire with optimum surface concave micro-pyramid arrays, average visible transmittance can reach 91.7%, which is apparently higher than that of flat sapphire (85.5%). Moreover, the concave pyramid arrays can increase significantly surface hydrophilicity of sapphire, exhibiting a water contact angle of 12.6° compared with 62.7° of flat sapphire. The proposed method can be an excellent strategy for preparing antireflective and self-cleaning concave micro-pyramid sub-wavelength structures on sapphire without complicated equipment and expensive raw materials.

INTRODUCTION It has been known that suppressing the reflection and improving the transmission are very necessary to enhance the performance of optical and optoelectronic devices, including photovoltaic cells [1], electronic display devices [2], LEDs [3] and optical detectors [4]. During the past decade, antireflective coatings (ARCs) with gradient refractive indices have been proved an efficient way to reduce the Fresnel interface reflections [5]. However, the preparation of such multiple-layer coatings is in trouble due to some difficulties of optimizing many coating parameters and controlling the homogeneity of the coatings. Besides, it can only suppress the reflection at certained wavelengths and incident angles [6]. Inspired by the sub-wavelength structures (SWSs) of the compound moth eyes of some insets and the wings of some butterflies which both have gradual changes of the effective refractive indices as light propagates across the air-substrate interface [7, 8], various biomimetic antireflective structures have been fabricated on silicon and fused silica substrates, respectively [9-12]. 2


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Sapphire, which has excellent thermal and chemical stabilities, and high optical transparency from UV to mid-IR region, is widely used for optical windows, displays and touch screen planes. However, due to its high surface reflectance, the light transmittance of sapphire is not high enough for many special applications. Many techniques, such as nano-imprint lithography [13], inductively coupled plasma (ICP) etching [14] and even solid phase epitaxy [15] have been applied to fabricate SWSs on silicon and even sapphire substrates. Recently Li and Cao demonstrated the fabrication of sub-wavelength structures on sapphire substrates by femtosecond laser direct writing showing greatly enhanced transmittance [16], where the SWSs feature dimensions are the same order as the wavelength of the incident light (λ=4 µm). However, for ultraviolet and visible light applications, feature length of SWS should be

below

visible

wavelength.

Traditional

lithographic

technologies

are

time-consuming and expensive for large-area fabrication [17]. In contrast, wet etching is a simple and low-cost preparation method. However, traditional wet etching is usually assisted by additional methods to fabricate masks, such as plasma reactive ion etching [18] and standard photolithography [19]. Hence, in this study, it was demonstrated for SWSs on sapphire fabricated by chemical wetting assisted with facile periodical silica masks. The surface antireflection and hydrophobicity were also both analyzed.

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EXPERIMENTAL Materials and substrate. The regents used for the synthesis of polystyene microspheres, including styrene (St) and potassium persulfate, were purchased from Aladdin Corp. Polymerization inhibitor was removed from the styrene by 10.0 wt% NaOH. Absolute ethyl alcohol (99.9%) and tetraethyl orthosilicate (TEOS) (98%) were obtained from Shanghai Macklin Corp. Deionized water was used directly from a Millipore A-10 water purification system. Sapphire wafers with C (0001) crystal planes were purchased from Hefei Kejing Materials Technology Co. Ltd. (China). Concentrated sulfuric acid, concentrated phosphoric and hydrofluoric acid (HF) were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), respectively. Preparation of PS colloidal suspensions. Using emulsifier-free polymerization method, mono-disperse colloidal polystyrene (PS) spheres of different diameters were synthesized [20], in which there is a standard size deviation of less than 5%. In this study, 9 g styrene were purified and were stirred with 100 ml deionized water in a three-necked round bottom flask equipped with a stopper, and nitrogen inlet & outlet. The reaction mixture was warmed up to 80 °C. Then, 0.22 g potassium persulfate initiator dissolved in 20 ml water was added and was reacted for 10 h. The PS spheres were washed with absolute ethyl alcohol for three times by repeated centrifugation and was re-dispersed in deionized water diluted with an equal volume of ethanol allowed to keep at 45 °C for standby.

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Preparation of silica etching masks. In order to obtain periodical masks for wet etching, we proposed the evaporative co-assembly of a sacrificial PS colloidal sphere template and sol-gel derived silica in one step. Typically, colloidal suspension with 2.5 w/v% PS was prepared firstly, which was the pre-prepared PS spheres diluted by a mixture solution of 8 ml of absolute ethyl alcohol and 12 ml of deionized water. The standard silica precursor consisted of 1:1:1.5 ratios by the weight of TEOS, 0.1M hydrochloric acid solution, and absolute ethyl alcohol, respectively. Then it was stirred at room temperature for 1 h prior to use. Sapphire substrate (15 × 15 mm), ultrasonic cleaned in a solution by mixing concentrated sulfuric acid and hydrogen peroxide with a volume ratio of 3:1, were vertically suspended in a vial containing the PS colloid/TEOS suspension. The solvent content was evaporated in an electric vacuum drying oven at 45 °C for 24 h. The as-prepared samples were then calcined in air at 800 °C for 5 h in order to remove the PS template. Meanwhile, the sol-gel silicate was transformed to periodical silica array which can be served as the mask for wet etching of sapphire. Periodic silica masks were prepared according to co-assembly procedures as presented in Figure 1. Our approach combined sol-gel process and self-assembly methods, relies on the assembling of polystyrene by evaporative deposition in the precursor of sol-gel derived silica, in order to obtain a large-area, defect-free and single-layer pore-like silica film. Some initial experiments of the syntheses of SiO2/PS composite films were performed with varying amounts of PS colloidal suspensions, in order to determine an optimal single-layer of silica film for well-ordered PS 5


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deposition. By choosing suitable PS colloidal volume fraction (a ratio of PS to silica precursor and solvent), the final film thickness can be precisely controlled with a single colloidal layer, which act as etching masks to fabricate periodic nanostructures on sapphire substrate.

Figure 1. Schematic illustration of the preparation process of the mask for etching silica. (a) One-step colloidal co-assembly of PS using sol-gel silicate precursor. (b) Calcination in air at 500 °C for 5 h to remove the PS template.

Design and Fabrication of antireflective structures. The sapphire substrate covered with silica mask was etched in a mixture of concentrated sulfuric acid (98%) and concentrated phosphoric acid (85%) with the volume ratio of 3:1 at 300 °C. After the desired time, the silica etching mask was removed by immersion in 8.0 wt% hydrofluoric acid solution, followed by washing in deionized water for 15 min. Characterization. Field emission scanning electron microscopic (FE-SEM, Hitachi, S4700) images were all recorded with a low incident electron voltage of 5 kV. 6


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The depths of SWSs were measured by atomic force microscopy (Bruker, Dimensional Icon). The Zeta potentials of PS spheres were tested by using Malvern Zetasizer Nano ZS and MPT-2 titrator. Optical transmission spectra were characterized by using UV-VIS spectrophotometer (Shimadzu, UV-2600) in the wavelength range of 400~800 nm. The wettability was characterized by recording the contact angles between droplets and substrates in which the volume of the individual deionized water droplet was kept constant at 5 uL.

RESULTS AND DISCUSSION Formation of nanobowl-array-like silica masks. Before the co-assembling experiment, PS colloidal spheres were prepared by controlling the stirring speed and the amount of polymerization initiator, respectively. The diameters of PS spheres increase with the increase of initiator concentration (Figure S1). When the stirring speed is 650 r/min, the mono-dispersity of PS spheres is the best, and the sizes of the PS spheres have good uniformity (Figure S2). Finally, the PS spheres with the diameters of 100 nm, 200 nm, 300 nm and 400 nm were prepared, respectively. The self-assembly process of microspheres requires a certain amount of surface charges, which mainly influences the interaction between microspheres. So the Zeta potentials of PS spheres with diameters of 100 nm and 400 nm were tested. The anions formed by the thermal decomposition of the initiator are attached to the surface of PS spheres, leading to negative charge on the surface of PS spheres (ccure S3). Then the PS colloidal crystal film is formed after co-assembling of PS colloidal spheres template 7


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using sol-gel silicate precursor, as shown in Figure 2. It is evident that the number of layers increases when the volume fraction of PS colloidal is increased from 0.4 vol% to 0.7 vol% (Figure 2). When the PS colloidal volume fraction reaches 0.7%, there are multiple layers of PS spheres deposited on the surface, as shown in Figure 2d. As is known, the drying tension in thin colloidal films may cause some cracks. Larger residual stress is accumulated in the multiple-layer film, resulting in more cracks [21].

Figure 2. Surface SEM images of PS colloidal films prepared using PS suspensions with different volume fractions: (a) 0.4 vol%, (b) 0.5 vol%, (c) 0.6 vol%, (d) 0.7 vol%, (e) Low-magnification SEM image of SiO2 mask after removing PS spheres, (f) High-magnification SEM image of SiO2 mask after removing PS spheres. 8


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As shown in Figure 2b, the co-assembled single-layer PS/SiO2 film can be prepared when the PS colloidal volume fraction is 0.5%. No cracks are found, and the well-ordered PS colloidal crystal film is obtained. By selective removing the PS spheres, the morphology of the periodic silica etching masks is shown in Figure 2e. The silica masks are hexagonally arranged on the bare sapphire substrate and there is a circular region (A) on the bottom of each PS position (Figure 2f). The circular region originates from the planar contact between PS spheres and sapphire substrate, which is caused by the deformation of PS spheres during thermal annealing. A closer view reveals that the silica mask looks like nanowell-structured array (the inset in Figure 2f). Morphology evolution of sapphire after etching. The morphologies of sapphire with silica mask were examined using a mixed solution with H2SO4 and H3PO4 by the volume ratio of 3:1 at 300 °C after different etching time. When the etching time is short, the bottom of silica mask begins to be etched, so the corroded pits appear on the sapphire after 4 min, as shown in Figure 3a. Then, the silica mask is peeled and the exposed sapphire parts in the circular region are etched at the same time. With the increase of etching time, the thickness of silica mask decreases, and the exposed area of sapphire which severs as the original active region is expanded (Figure 3b-d). The etching velocities of sapphire depend on the crystal orientations. Due to the different etching velocities of different crystal planes (C-plane＞R-plane＞ M-plane) [22], the C-plane is etched firstly until the concave pyramid is formed (Figure 3d). After etching for 12 min, almost all the silica masks are removed and the 9


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concave pyramids are joined together, so the pyramids begin to emerge (Figure 3e). At the same time, M points of sapphire expose in the air, and their C-plane begin to be corroded until new concave pyramid is formed on the top of pyramid (Figure 3g). Further increasing corrosion time, the corrosion of C-plane and R-plane are both ended, and the M-plane begins to be corroded, so the shape of the pyramid is changed from triangle to hexagon (Figure 3h).

Figure 3. SEM images of sapphire after wet etching for different time: (a) 4 min, (b) 6 min, (c) 8 min, (d) 10 min, (e) 12 min, (f) 14 min, (g) 16 min, (h) 18 min.

According to the influences of etching time on the morphologies of sapphire during the above etching process, it can be concluded that: (1) The morphology of SWSs are mainly concave pyramidal (Figure 3a-d) or convex pyramidal (Figure 3e-h); 10


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(2) Etching time has a great influence on the morphologies, including the depth and filling ratio. According to the effective medium theory, the SWS on sapphire can influence its refractive index. The filling ratio is important to guarantee homogeneous transmission for different optical materials [23]. However, the depth and filling ratio are difficult to be controlled for pyramid structures after etching for 12 min. So the concave pyramid structures with different periods and filling ratios are designed to calculate the theoretical reflectance using COMSOL Multiphysics software, as shown in Figure S4 and Figure S5. Preparation of concave pyramid nanoarrays. According to the calculation results, the reflectance decreases with the decrease of the period when fixing the filling ratio, and increases with the increase of filling ratio when fixing the period (Figure S4). It is desirable for the PS spheres with the diameter of 100 nm, and the filling ratio is needed to be increased as possible. Figure 4 shows SEM images of fabricated sub-wavelength structure on sapphire after wet etching for different time from 4 min to 7 min. After removing the silica mask, the concave pyramids appear on the surface of sapphire. By using silica mask with the period of 100 nm, there is concave pyramid SWS with 100 nm period. When the sample is etched for 7 min, the concave pyramids are almost all connected, and the filling ratio is almost the largest. The depth of SWS is an important parameter influencing optical performance. As well known, the theoretical minimum depth of the sub-wavelength structure is determined by the formula (dmin

) [24], where λ is the propagating wavelength, and n1

and n2 are the refractive indice of air and sapphire substrate, respectively. For sapphire 11


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(n2＝1.76) at visible wavelength range, we have the optimum depth range from 75 nm to 150 nm (Figure S2). The AFM images in Figure 5 show the morphology of the concave pyramid on sapphire substrate after wet etching for 7 min. The line-height profile (Figure 5b) clearly indicates that the depth of concave pyramid approximately reaches 80 nm within the optimum depth range.

Figure 4. Surface SEM images of sapphire after wet etching for different time: (a) 4 min (b) 5 min (c) 6 min (d) 7 min.

Performance characterization. As to surface antireflection, with an appropriately designed gradient, the reflection can be minimized over a broad spectral range. The gradient distribution of the refractive index is realized by increasing the material density from air to sapphire concave pyramid arrays, finally merging them into bulk sapphire.

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Figure 5. (a) Surface AFM image and (b) The line-height profile of the concave pyramid arrays on sapphire substrate, respectively. Position A(C) and B indicate the locations of the edge and bottom of the concave pyramid on sapphire substrate, respectively.

Figure 6 shows the total transmission spectra for sapphire and patterned sapphire at normal incidence. For bare sapphire substrate, the poor total transmittance of 85.5% is observed over visible wavelength range. For the planar sapphire substrate, the refractive index abruptly changes from 1.0 to 1.76 (at 400 nm) across the air/sapphire interface, resulting in Fresnel reflection. According to the effective medium theory, periodic nanostructures can be replaced by the equivalent medium, which refractive index is closely related to the filling ratio of the periodic structure. For the sapphire substrate with concave pyramid arrays, the effective index is almost continuously changed from 1.0 to 1.76, leading to minimal Fresnel reflection. So the concave micro-pyramid arrays can effectively increase the total transmittance. The sub-wavelength surfaces exhibit the transmittance of about 91.7% over a spectral range from 400 nm to 800 nm when the etching time is 7 min. The transmission 13


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spectra are basically consistent with the given photographs. It can be seen that the transmittance of the surface SWSs increases gradually with increasing filling ratio, which coincides with the calculated results.

Figure 6. The transmission spectra and photographs of sapphire before and after etching for different etching time.

Due to excellent mechanical properties, sapphire may be used as optical windows in more extreme environments including raining and fogging. Then, the water droplets on the surface may decrease the transmittance of sapphire. So, the wetting properties of the concave pyramid sapphire with different etching time were systematically investigated by measuring water contact angles. The measured apparent water contact angles (Figure 7) are 62.7°, 56.2°, 44.3°, 33.6°, and 12.6°, respectively. Apparently, the water contact angle gradually decreases after longer etching treatments, in which the 7 min-etched sample (Figure 7e) has the smallest water contact angle. Due to the shallow depth of the nanostructures, the water fills the concaves on the surface during the wetting experiment, so the wettability can be 14


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evaluated according to the Wenzel model. The apparent contact angle θ* is defined by the following equation: cosθ* = r (γSG-γSL)/γLG = r cosθe

(1)

where r is the roughness ratio; γij is the surface energy between the two indicated phases; θe is the Young contact angle as defined for an ideal flat surface. The surface of sapphire is hydrophilic, so cos θe > 0, and 0 < θe < 90o. The rough surface makes the actual contact area larger than the macroscopic area, so r ≥ 1, and then cos θ* > cos θe and θ* < θe. So the nanostructure enhances the hydrophilicity. The concave structure decreases water contact angle significantly. Due to the small water contact angle, the water drops can be flat on the sapphire surface. Although the water-film may result in the light scattering, the thin film will evaporate or slid down quickly due to the heat or gravity. At the same time, the water can take away the dusts on the surface, so the sapphire shows the benefit of self-cleaning [25].

Figure 7. Wettability of the concave pyramid structures on sapphire prepared after different etching time: (a) Sapphire (b) 4 min (c) 5 min (d) 6 min (e) 7 min. 15


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CONCLUSIONS In conclusion, we have developed a simple up-bottom technology using facile monolayer PS sphere masks for preparing a variety of periodic concave pyramid nanostructures on sapphire which exhibit high transmittance in visible wavelength range and good superhydrophilicity. The final film thickness can be precisely controlled as a single colloidal layer when the volume fraction of the PS colloidal is 0.5 vol%, which acts as the etching masks to prepare nanostructures on sapphire. The formation of concave pyramid is caused by the different etching velocities of different crystal orientations of sapphire. The filling ratio of sub-wavelength has great influences on the antireflection and wetting properties. For 400~800 nm spectral regions, the average transmittance of SWS is 91.7%, which increases by 6.0%. The superhydrophilic surfaces with the water contact angle of 12.6° can be achieved by etching the sapphire for 7 min. Our proposed strategy provides a simple, high-efficiency and low-cost nanofabrication route to obtain highly-transparent and self-cleaning sapphire for practical applications.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 50902028), Natural Science Foundation of Guangdong Province (Grant No. 2016A030313663), Shenzhen Science and Technology Plan Supported Project (Grant Nos. JCYJ20140616172915497, JCYJ20170413105844696), and China Scholarship Council (Grant No. 201606125092).

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