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Multifunctional Moth-eye TiO2/PDMS Pads with High Transmittance and UV Filtering Segeun Jang, Seong Min Kang, and Mansoo Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15502 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Applied Materials & Interfaces

Multifunctional Moth-eye TiO2/PDMS Pads with High Transmittance and UV Filtering Segeun Jang†, ‡, Seong Min Kang*,§ and Mansoo Choi*,†, ‡ †

Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 151-744, Republic of Korea. ‡

Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, Republic of Korea. §

Department of Mechanical Engineering, Chungnam National University, Daejeon 34134, Republic of Korea

CORRESPONDING AUTHOR EMAIL ADDRESS: [email protected] (M. Choi) [email protected] (S. M. Kang)

KEYWORDS: moth-eye structure, ultraviolet protection, titanium dioxide, anti-reflection, multiscale synthesis

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ABSTRACT This work reports a facile fabrication method for constructing multifunctional moth-eye TiO2/PDMS pads using soft nano-imprinting lithography and a gas-phase deposited thin sacrificial layer. Mesoporous TiO2 nanoparticles acted as an effective UV filter, completely blocking high-energy UVB light and partially blocking UVA light, forming a robust TiO2/PDMS composite pad by allowing the PDMS solution to easily fill the porous TiO2 network. The paraboloid shaped moth-eye nanostructures provided high transparency in the visible spectrum and also have self-cleaning effects due to nano-roughness on the surface. Furthermore, we successfully achieved a desired multiscale-patterned surface by partially curing select regions using TiO2/PDMS pads with partial UVA ray blockers. The ability to fabricate multifunctional polymeric pads is advantageous for satisfying increasing demands for flexible and wearable electronics, displays, and solar cells.


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1. Introduction A large amount of recent experimental and theoretical research has focused on antireflection (AR) fields for improved optical properties using various nano/micro technologies1-8 and computational calculation methods.9-10 In general, moth-eye inspired subwavelength structures have been widely used in AR coating applications due to their superior structural anti-reflectivity over the broadband spectrum.11 Conical-shaped nanostructures on the surface can generate a smooth refractive index gradient at the interface of two different media, which effectively suppresses Fresnel reflections.12 Diverse fabrication processes such as conventional lithography, self-assembly approaches, and physical/chemical etching methods have been developed in order to make these functional nano-textures.2, 10, 12-13 These fabricated AR structures are used in many application fields within flat panel display and photovoltaic energy conversion systems.1-2, 14-17 However, the high transparency of moth-eye structures is not always favorable, especially in the ultraviolet (UV) spectrum. The UV spectrum consists of three regions: UVA (315–400 nm), UVB (280–315 nm) and UVC (100–280 nm). Among these, considering that UVC radiation is blocked by dioxygen and ozone in the atmosphere, UVB is the most harmful to human body as it can induce skin cancer and eye damage due to its high radiation energy, and can also accelerate the degradation of perovskite and organic polymer solar cells.18-19 While UVA, which has less energy than UVB, can also damage the human body, this light can be employed for UVcurable polymer patterning processes to fabricate micro-/nano-polymeric structures. To mitigate the harmful effects of UV light, titanium dioxide (TiO2) nanoparticles are frequently used as UV filters in sunscreens and cosmetic products because of their proper band gap position for UV absorption, good photocatalytic effect, and bio-compatibility.20-22 However, a drawback of the use of inorganic UV shielding materials is that embedded TiO2 nanoparticles reduce transmittance in the visible spectrum at the interface. Therefore, it is critical that a 3 ACS Paragon Plus Environment


system can achieve both UV filtering and high transmittance when using nanoscale TiO2 in order to find use in a wider application space. Herein, we propose a novel fabrication method to create multifunctional polymer pads with bioinspired moth-eye 300-nm structures with embedded TiO2 nanoparticles, which demonstrate both high transmittance and UV-filtering effects. Using soft nano-imprinting lithography and a thin octafluorocyclobutane (C4F8) sacrificial layer, TiO2 nanoparticles are successfully embedded in well-ordered periodic polydimethylsiloxane (PDMS) 300-nm moth-eye structures with high fidelity and no defects. This fabricated moth-eye TiO2/PDMS (MTP) pad is expected to partially and perfectly block UVA and UVB rays respectively, maintaining high transparency (maximum %T ~97.6%) in the visible spectrum. Attaching this bioinspired TiO2/PDMS pad on both the front and back side of glass considerably reduces the reflectance compared to a flat glass substrate. The superior optical properties, combined with a structural self-cleaning effect, suggest diverse application possibilities for this system. In addition, based on the partial blockage of UVA rays, we demonstrate a facile fabrication method of constructing a multiscale-patterned surface by selectively curing certain regions using this TiO2/PDMS pad.

2. Experimental details Fabrication of Moth-Eye Silicon Master. An 8-inch silicon wafer (LG Siltron, Korea) was cleaned using SC-1 solution (a mixture of deionized (DI) water, NH4OH and H2O2). To fabricate hexagonal moth-eye structures with a 300 nm pattern size, a 1-µm-thick photoresist layer (LX-429, Dongjin Semichem, Korea) was spin coated onto the silicon wafer. Next, 170 nm pillar arrays were formed using annular photolithography using a KrF laser. The silicon wafer was then anisotropically etched to a depth of 180 nm by means of an ICP system using Cl2/HBr mixture gas phase plasma and 13.56 MHz radio-frequency power 4 ACS Paragon Plus Environment

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generators. After cleaning and ashing the PR residue layer, the silicon wafer was blanketetched on the surface. To make a compact distribution of hemispherical nanodomes with near zero spacing, a 30-nm-thick SiO2 layer was deposited on the surface of the etched silicon wafer by thermal oxidation at atmospheric pressure with H2 (4 slm) and O2 (12 slm) gas flow, followed by nitride deposition to a thickness of 10 nm. Fabrication of mesoporous MTP. The sacrificial C4F8 layer used in this study was deposited onto the as-prepared moth-eye silicon wafer by ICP at a pressure of 22 mTorr with C4F8 gas (100 slm) for 30 s. A commercial TiO2 paste (18NR-T, Dyesol) with a 20 nm particle size was diluted in anhydrous ethanol at a weight ratio of 5:1. The diluted solution was then spin-coated onto the C4F8 deposited moth-eye silicon master at 3500 rpm for 60 s and the substrate was dried at 70 °C for 10 min and annealed at 500 °C for 1 h. During the annealing process, the C4F4 layer and the ethyl cellulose binder (in the TiO2 solution) was degassed. After, a diluted hard PDMS (h-PDMS; Gelest) solution was casted onto the motheye silicon wafer with the mesoporous TiO2 layer and a doctor-blade method was performed using a microscopy glass slide. The diluted h-PDMS was prepared using 1.7 g of a vinyl PDMS pre-polymer (VDT-731, Gelest Corp.), 0.5 g of a hydrosilane prepolymer (HMS-301, Gelest Corp.), 10 µL of a platinum catalyst (SIP6831.2, Gelest Corp.), 5 µL of a modulator (2,4,6,8-tetramethyltetravinylcyclotetrasiloxane, Sigma-Aldrich), and 8.8 g of toluene (Daesung Chemical Co. Ltd., Korea), which were mixed and degassed for several minutes. The coated h-PDMS layer was thermally cured at 80 °C for 20 min, and standard PDMS with a mixing ratio of 10:1 (precursor:curing agent) was casted onto the cured h-PDMS layer and thermally cured at 70 °C for 1 h. Finally, the PDMS(h-PDMS/s-PDMS) pad with embedded mesoporous moth-eye TiO2 was detached from the moth-eye silicon wafer. A reference sample (flat TiO2) without moth-eye structures underwent the same procedure without



molding process. The moth-eye PDMS pad without TiO2 layer was fabricated using the same procedure without deposition and annealing of the TiO2 solution. Fabrication of multiscale hierarchical structures. A 20 µm and 800 nm hole PDMS mold was fabricated using the 20 µm and 800 nm pillar array-patterned silicon masters. First, the UV-curable prepolymer resin (PUA MINS 301 RM, Minuta Tech, Korea) was poured on the flat PET film (50 µm thickness) and the as-prepared 20 µm hole PDMS mold was placed on the resin. After, flat PDMS and mesoporous MTP pad was position on the 20 µm hole PDMS mold, followed by UV exposure for ~60 s (λ = 300–420 nm) under ambient conditions. The PDMS molds (flat, mesoporous moth TiO2, and 20 µm hole) were then successively peeled off from the PET film, yielding a fully cured region under the flat PDMS and a partially cured region under the mesoporous TiO2/PDMS due to the UV absorption characteristics of TiO2. Finally, we achieved micro-/nano-multiscale structures by placing an 800 nm hole PDMS on the partially cured region with sufficient UV exposure for 3 min under slight pressure. Characterization. Surface images of the mesoporous MTP pad, moth-eye PDMS pad, and the multiscale structures were obtained from using field emission scanning electron microscopy (FE-SEM, AURIGA, Carl Zeiss). Transmittance and reflectance spectra were measured by means of UV-visible spectroscopy (Cary 5000, Agilent Technologies) at 300– 800 nm. Static water contact angles on the various substrates were measured by a contact angle analyzer (KRUSS DSA 100, Germany). Self-cleaning experiment was carried out with various samples using 5 g of sand dust which consist of 100-to 300 µm-sized grains.


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3. Results and discussion 3.1. Fabrication of moth-eye TiO2/PDMS pads Figure 1a–d demonstrate a procedure for fabricating mesoporous MTP pad (see Figure S-1 for more details with cross-sectional illustrations of the experimental procedures). First, a C4F8 layer was deposited onto an as-prepared moth-eye silicon master using an inductively coupled plasma (ICP) system. This hexagonal array of 300 nm moth-eye master structures was fabricated using ICP etching and deposition of SiO2 and nitride based on previously-reported methods.12 Generally, C4F8 gas is utilized to form a passivation layer during the anisotropic etching of silicon. In our study however, we utilized the C4F8 layer as a sacrificial thin layer to create a mesoporous MTP pad. Next, diluted TiO2 paste, with ~20 nm TiO2 particles and a polymeric binder, was casted onto the C4F8-deposited moth-eye silicon master and spin-coated. Although the surface of the moth-eye silicon master is hydrophobic due to the C4F8 layer, the TiO2 layer was uniformly coated on the master with ~300 nm thickness on average thanks to the polymeric binder and well-dispersed TiO2 nanoparticles. The TiO2 coated moth-eye substrate was then annealed at 500 °C for 1 h, which also degassed the polymeric binder in TiO2 solution and the C4F8 layer. A diluted hard-PDMS (hPDMS) solution was then casted onto the silicon master with the mesoporous TiO2 layer using a simple doctor-blade coating technique, which was thermally cured at 80 °C for 20 min. Removal of the polymeric binder created a sponge-like mesoporous TiO2 network, allowing the h-PDMS solution to easily fill the TiO2 network and form a robust TiO2/PDMS composite. The degassed C4F8 layer also formed voids at the interface between the surface of silicon master and the TiO2 layer. This allowed the h-PDMS solution to infiltrate the interface space, making the TiO2 layer perfectly embedded with h-PDMS. In this study, h-PDMS is required to prevent deformation and distortion of the nanoscale moth structures and TiO2 network.1, 12 Casting and curing of standard PDMS (s-PDMS) pre-polymer onto the h-PDMS 7 ACS Paragon Plus Environment


layer was then performed. Finally, a mesoporous MTP (h-PDMS/s-PDMS) pad was obtained by gentle peel-off from the silicon master. It is noteworthy that the gas-phase deposited thin sacrificial layer resulted in perfect replication of moth-eye structures and no TiO2 particle residue after peeling off. For comparison, a moth-eye PDMS pad without TiO2 layer was fabricated using the same procedure without deposition and annealing of TiO2 solution (Figure S-2). Surface and cross-sectional scanning electron microscopy (SEM) images of mesoporous flat and MTP pads are shown in Figure 1e–f. Both the flat and moth-eye TiO2 layers were well embedded in the PDMS pad. The morphology of the MTP pad was a “negative copy” of the original silicon master, and showed high pattern fidelity of wellordered TiO2 moth-eye structures over a large area (larger than 3 cm × 3 cm) with relatively thin thickness (~300 nm on average). Comparing these with digital camera images of the flat TiO2/PDMS surface, the MTP surface shows visual effects of optical interference because of the existence of hexagonal periodic moth-eye nanostructures (inset of Figure 1e and Figure 1f). The moth-eye PDMS pad without TiO2 layer also shows high pattern fidelity, and its surface SEM images are shown in Figure S-3.

3.2. Optical properties of the moth-eye TiO2/PDMS pads Firstly, to elucidate the optical characteristics of the moth-eye, the total transmittance and reflectance spectra were measured using a UV-Vis-NIR spectrophotometer (see Supporting Information, Figure S-4 and Figure S-5). After confirmation of optical characteristics of moth-eye structures, we measured absorbance spectra of (flat/moth) TiO2/PDMS pads and glass to figure out the UV-absorption characteristics of embedded mesoporous TiO2 layer in the PDMS (Figure S-5d). The differences in absorbance spectra between the (flat/moth) TiO2/PDMS pads and the glass were not pronounced in the visible region, but at UV wavelengths (< 400 nm), the absorption spectra of (flat/moth) TiO2/PDMS 8 ACS Paragon Plus Environment

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pads rapidly increase compared to that of glass, due to the UV absorption ability of TiO2 particles. By comparison with the absorbance spectra of flat PDMS, we also confirmed that bare PDMS hardly affects the absorbance spectra in our measured wavelength range (300– 800 nm). Next, we measured the transmittance spectra for (flat/moth) TiO2/PDMS pads and bare glass (Figure 2a). The TiO2 embedded moth/PDMS pads showed UV filtering characteristics as well as enhanced transmittance in the visible spectrum, due to the combined properties of the moth-eye structure and the mesoporous TiO2 material. In comparing the low transmittance properties of flat TiO2/PDMS pads, the dual side moth-eye TiO2/PDMS (DMTP) pads show a high maximum transmittance (~97.6%) around 550 nm. Notably, they can perfectly block UVB light (280–315 nm) and allow UVA light to partially pass through. The reflectance was also significantly reduced compared to the flat glass substrate at all measured wavelengths (Figure 2b), corresponding to the trend shown by transmittance measurements. In case of UVB light, it can have harmful effects on biological systems such as human skin or eye tissue.20, 23 Many studies have shown that exposure of human cells to UVB light can lead to DNA damage or modification.20, 24 Therefore, being able to block UVB exposure in common environments is highly desirable. By utilizing the perfect UVB absorption characteristic of our MTP pads, we could make flexible and transparent UVB shielding pads with high transmittance for various applications such as displays, healthcare electronics, wearable devices, and solar cells. As displayed in Figure 2c, we can observe the clear visual images owing to the anti-reflection effect of the DMTP pads which allow more incident light to pass through in the visible spectrum, while perfectly blocking UVB light. The paraboloid shaped moth-eye TiO2 structure in MTP pad can generate a graded distribution of refractive indices, and thus it can effectively suppress the interfacial Fresnel reflection as shown in Figure 2d–f. The refractive index of mesoporous TiO2 particle/PDMS



composite is calculated based on the ellipsometry measurements of the mesoporous TiO2 thin film.

3.3. Multiple usages of moth-eye TiO2/PDMS pads Next, we demonstrate multiple usages of the MTP pads (Figure 3 and Figure 4) in order to propose different applications for this system. As shown in Figure 3a, UVB radiation is perfectly blocked by our prepared MTP pads. On the contrary, UVA radiation partially passes through the fabricated UV-shielding filter of the MTP pads, which is used for partial curing studies in this work. Partial curing is a representative fabrication approach able to engineer multiscale structures that contain both nano- and micro-scale features. It is well known that multiscale, hierarchical structures are widely relevant in biomimetic,25 photonic,26 nano/microfluidic27 and energy conversion devices.28 In general, UV-assisted partial curing methods using UV-curable polyurethane acrylate (PUA) polymer and oxygen permeable PDMS mold have been used to make such hierarchical structures.25 As shown in Figure 3b, the UV lamp used for curing the PUA polymer demonstrates a maximum energy at approximately ~350 nm, which corresponds to the UVA area in Figure 3a. In other words, we can control the energy density of UV rays in the UVA region using the MTP pads, and inhibit full curing of PUA polymer under UV lamp exposure (Figure S-6). In general, the fabrication methods for manufacturing multiscale structures using UVcurable PUA polymer consist of three steps: UV-partial curing of an initial pattern using prepared micro PDMS mold (Figure 3c), conformal contact of the prepared nano-PDMS (or rigid substrate) mold on top of the pattern (Figure 3d), and full UV-curing to fabricate robust hierarchical structures (Figure 3e) (see Figure S-7 for more details with cross-sectional illustrations of the experimental procedures). However, the main problem of these conventional UV-partial curing fabrication methods is that hierarchical patterning on partially 10 ACS Paragon Plus Environment

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cured local regions is difficult due to uniform illumination of UV rays on the initial fabricated micro-patterned region. Alternatively, using the prepared UV-shielding effect of the MTP pads (which allows UVA rays to partially penetrate the covered region), we successfully fabricated both individual length-scale and hierarchical dual-scale structures on a single substrate, as shown in Figure 3c–g. In the first step (Figure 3c), the MTP pad is smoothly placed on the prepared micro PDMS mold to partially block penetration of UVA rays on the PUA polymer. Then, only the region indicated by the red-dot box in Figure 3d is partially cured due to the selective UV-shielding ability of the MTP pads. As a result, both a micropatterned area (blue-dot box in Figure 3e) and a locally well-defined multiscale-patterned area (orange-dot box in Figure 3e) are developed using PUA polymer on a single substrate. Fully-cured PUA micro patterns (blue-dot box in Figure 3e) and hierarchical nano/microcombined structures made using this partial curing method (orange-dot box in Figure 3e) are clearly shown in representative magnified SEM images in Figure 3f and g, respectively. The digital images in Figure 3e and SEM images in Figure 3f–g prove that the morphology of the multiscale structures fabricated using this local partial curing method are well defined with high pattern fidelity. The fabricated micro pillars are 20 µm in diameter and 20 µm in height, while the nano-patterns are 800 nm in diameter and 600 nm in height. The inset SEM images in Figure 3f–g demonstrate definite structural differences between the micro pillars and hierarchical pillars. More notably, a transient region of microscale and hierarchical patterns exists at the interface between the flat PDMS pad and the moth-eye TiO2 pad (Figure S-8). This demonstrates that we can fabricate versatile polymer surfaces with partially multiscale-patterned areas in select locations. This fabricated MTP pad also shows self-cleaning properties due to nano-roughness on the surface, which can be readily understood by the Cassie-Baxter equation.29 Because a self-cleaning surface requires a high static water contact angle, we measured the contact 11 ACS Paragon Plus Environment


angle (CA) for DI water exposure on five different substrates (glass, flat PDMS, flat TiO2/PDMS, moth PDMS and MTP) as shown in Figure 4a. DI droplet (~5 µl volume) was used for the CA measurement, which was carried out 10 times per each sample. The CA on flat PDMS (without any surface structures) is ~101°, which is the same as flat TiO2/PDMS (Figure 4a). However, the CA increased to ~130° on both moth PDMS and MTP due to the moth-eye patterns presented on the PDMS surface. It is noted that the CAs are identical with and without TiO2 nanoparticles, which means that the inorganic nanospheres are wellembedded in the PDMS surface with high structural integrity and fidelity. Figure 4b demonstrates the self-cleaning effect of the fabricated MTP pad. Compared to the glass and the flat PDMS surfaces that is contaminated by sand dust even after dropping DI water (see Supporting Information, Figure S-9), the dust particles on the MTP pad surfaces are clearly removed by simply dropping DI water (see also Supplementary Video), and such a selfcleaning effect would be useful for diverse optical devices with distinct optical properties. The multifunctional effects of MTP pad with high transmittance, UV filtering, and selfcleaning effect are briefly illustrated in in Figure 4c.

4. Conclusion We have presented moth-eye inspired PDMS structures with embedded TiO2 nanoparticles to develop multifunctional polymer pads with high transmittance and UV protection via a novel fabrication method. This TiO2/PDMS pad, fabricated using soft nanoimprinting lithography and a thin C4F8 sacrificial layer method had a robust structural morphology with high fidelity. The fabricated multifunctional polymer pad demonstrated superior optical properties with both high transparency and high UV-shielding effect. Its ability to completely block harmful UVB rays can be applied to diverse wearable devices, displays and solar cell panels. Its ability to partially block UVA rays can also be used to 12 ACS Paragon Plus Environment

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fabricate hierarchical micro-/nano-structures, and using such a multi-step UV-curing process is useful in a wide range of applications. It is envisioned that the novel and facile fabrication method for these multifunctional MTP pads could yield various insights in the development of optical, energy, architecture and human-compatible devices.

Acknowledgement This work was supported by the Global Frontier R&D Program of the Center for Multiscale Energy Systems funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2012M3A6A7054855). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017R1C1B1005834).

Supporting Information Fabricating process of moth-eye PDMS pad without TiO2 layer, additional SEM images, detailed optical characteristics of the moth-eye with various PDMS pads, transient region of microscale and hierarchical patterns exists at the interface between the flat PDMS pad and the moth-eye TiO2 pad, and a short video clip of the self-cleaning on the dusted moth-eye TiO2/PDMS pad. This material is available free of charge via the Internet at http://pubs.acs.org.



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Figure 1. (a–d) Schematic illustrations of mesoporous moth-eye TiO2/PDMS pad fabrication. (e,f) Cross sectional and surface SEM images of flat TiO2/PDMS pad (e) and moth-eye TiO2/PDMS pad (f).



Figure 2. (a) Transmittance and (b) reflectance spectra of bare glass and TiO2/PDMS pads on glass with and without moth-eye structures. (c) Anti-reflection effect using dual side motheye TiO2/PDMS (DMTP) pad on glass. (d–e) Refractive index profiles of (d) flat TiO2/PDMS pad, (e) single-side moth-eye TiO2/PDMS pad (SMTP) and (f) dual side moth-eye TiO2/PDMS (DMTP) pad on glass.


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Figure 3. (a) Transmittance spectra of the moth-eye TiO2/PDMS pad in the UV spectrum. (b) Relative irradiation energy of the UV curing lamp. (c–e) Digital camera images of the successive process for fabricating multiscale hierarchical structures using moth-eye TiO2/PDMS

pad. (f–g) Surface SEM images of unmodified micro-pillar array using flat PDMS

and multiscale nano-/micro-pillar array using moth-eye TiO2/PDMS pad.



Figure 4. (a) Contact angle for DI water on glass, flat PDMS, flat TiO2/PDMS, moth PDMS and moth TiO2/PDMS (The gap between upper and lower bounds represents the error). (b) Images of self-cleaning effect using the moth-eye TiO2/PDMS pad. (c) Schematic illustrations of multifunctional abilities of the moth-eye TiO2/PDMS pad.


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