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Flexible Near-Field Nanopatterning with Ultrathin, Conformal Phase Masks on Non-Planar Substrates for Biomimetic Hierarchical Photonic Structures Young Woo Kwon, Junyong Park, Taehoon Kim, Seok Hee Kang, Hyowook Kim, Jonghwa Shin, Seokwoo Jeon, and Suck Won Hong ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00816 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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Flexible Near-Field Nanopatterning with Ultrathin, Conformal Phase Masks on Non-Planar Substrates for Biomimetic Hierarchical Photonic Structures Young Woo Kwon,†,§ Junyong Park,‡,§,¶ Taehoon Kim,‡ Seok Hee Kang,† Hyowook Kim,‡ Jonghwa Shin,‡ Seokwoo Jeon,*,‡ and Suck Won Hong*,† †

Department of Cogno-Mechatronics Engineering, Department of Nano-Fusion Technology,

Department of Optics and Mechatronics Engineering, College of Nanoscience and Nanotechnology, Pusan National University, Busan 609-735, Republic of Korea ‡

Department of Materials Science and Engineering, KAIST Institute for The Nanocentury,

KAIST, Daejeon 305-701, Republic of Korea §

These authors contributed equally to this work

¶

Present address: Department of Chemical Engineering, Massachusetts Institute of Technology,

Cambridge, MA 02139, USA *Correspondence should be addressed to S.J. (email: [email protected]) and S.W.H. (email: [email protected]) KEYWORDS: nanofabrication, near-field phase shift lithography, conformal phase masks, hierarchical structures, antireflective diffusers, biomimetics

ACS Paragon Plus Environment

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ABSTRACT: Multilevel hierarchical platforms that combine nano- and micro-structures have been intensively explored to mimic superior properties found in nature. However, unless directly replicated from biological samples, desirable multiscale structures have been challenging to efficiently produce to date. Departing from conventional wafer-based technology, new and efficient techniques suitable for fabricating bioinspired structures are highly desired to produce three-dimensional (3D) architectures even on non-planar substrates. Here, we report a facile approach to realize functional nanostructures on uneven microstructured platforms via scalable optical fabrication techniques. The ultrathin form (~3 µm) of a phase grating composed of poly(vinyl alcohol) (PVA) makes the material physically flexible and enables full-conformal contact with rough surfaces. The near-field optical effect can be identically generated on highly curved surfaces as a result of superior conformality. Densely packed nanodots with sub-micron periodicity are uniformly formed on microlens arrays with a radius of curvature that is as low as ~28 µm. Increasing the size of the gratings causes the production area to be successfully expanded by up to 16 in2. The ‘nano-on-micro’ structures mimicking real compound eyes are transferred to flexible and stretchable substrates by sequential imprinting, facilitating multifunctional optical films applicable to antireflective diffusers for large-area sheetillumination displays.


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Exceptional properties in nature, such as anisotropic adhesion, super hydrophobicity, structural color, and antireflection, provide far-reaching inspiration for the design of advanced material systems.1-4 The realization of hierarchical structures at the nano- and micro-scales is of great importance for bio-inspired technologies.5-7 In particular, the eyes of insects contain nanolenson-microlens hierarchical structures that enable unique optical features, such as a wide field-ofview and high motion sensitivity with excellent visible acuity.8,9 Successful mimicry of compound eyes offers tremendous potential in developing novel high-performance optoelectronic devices.9-11 Unfortunately, the current mimicry strategies still depend on conventional wafer-based technologies such as photolithography and imprint techniques. These technologies can generally produce non-hierarchical, single-level bumpy structures on ultra-flat rigid substrates and are limited to providing an antireflective function that captures only one feature of a real insect’s eyes.12,13 Only a few studies have attempted to develop fabrication techniques based on two-step thermal imprinting or point-by-point laser swelling for mimicking real compound eye structures.14,15 These techniques could provide full coverage of nanodots on microlens arrays or even cover flat gaps between microlenses, which is a feature of insect eyes that is experimentally difficult to achieve except by direct replication from biological samples.16 However, such thermal post-treatment processes are intrinsically limited by the achievable radius of curvature of the bottom microlenses and do not satisfy several industrial prerequisites in terms of resolution, throughput, reproducibility, and large-area uniformity. Furthermore, the superior optical functions enabled by ‘nano-on-micro’ hierarchical structures have not been fully demonstrated. Here, we propose a high-throughput and scalable optical fabrication technique advanced from near-field phase-shift lithography (NFPSL),17,22 which can generate high-resolution


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nanostructures with the aid of conformal phase masks, to produce combined multiscale nanoand micro-structures in a single-exposure step. Unlike thick conformal masks composed of highly viscous elastomers such as poly(dimethylsiloxane) (PDMS) or perfluoropolyether(PFPE) generally used for NFPSL,18-20 the masks developed here consist of poly(vinyl alcohol) (PVA) which enables them to take a sufficiently thin (~3 µm) and flexible form and to fully cover nonflat substrates even when the radius of curvature is extremely low (~28 µm). Only a few studies have been demonstrated for flexible NFPSL on curved substrates with relatively large radii of curvature (i.e., cylindrical lens coated with photoresist) by use of established elastomeric phase masks.22

To the best of our knowledge, we are the first to demonstrate submicron scale

conformal lithography on microscale non-flat, curved objects and wafer-scale optical films (~16 in2, the largest area reported to date) with hierarchical structures mimicking a real insect’s eye, which cannot be achieved by conventional optical mask and lithography systems. The state-ofthe-art technique and structures presented here enable a new class of multi-functional optical films that may be applicable to sheet-illumination displays as antireflective diffusers. RESULTS AND DISCUSSION Biomimetic Strategy. Figure 1 presents the basic concept and experimental procedures of this study. The process begins with the replication of ultrathin phase masks that contain square and hexagonal arrays of holes with periodicities ranging from 600 to 1,500 nm. The duty cycle of the masks is fixed at ~50 % and the height of the masks is designed to satisfy the π phase shift at given conditions (i.e., refractive index and wavelength). In the scalar regime, the phase shift (φ) can be expressed as,21 ߮=

ଶ஠ ఒ

∆݊ ∙ ݀

(1)


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where ∆n is the difference in refractive index between the phase mask and surrounding medium, d is the grating height, and λ is the incident wavelength of light. If a photopolymer film is placed under the phase mask, the transmission of light through the phase mask generates a unique light distribution in the photopolymer as a result of the interference of near-field diffraction. Various arbitrary patterns with nanoscale resolution can be generated as a function of the periodicity (p) of the grating and the incident wavelength (λ).23 Despite the usefulness of the near-field effect on surface nanotexturing, we are not aware of any previous reports regarding NFPSL on non-flat, curved surfaces with micron-scale radii of curvature. We prepare a half-covered PVA blanket with submicron gratings and imprinted photopolymer in the shape of microlens arrays as a nonplanar substrate (Figures 1a and S1). The sufficiently low bending stiffness of the thin PVA film makes it flexible and allows for conformal contact on non-planar substrates without any external field of pressure. After UV exposure (~365 nm) and development, the photopolymer film is divided into two areas at the middle boundary of the PVA blanket. Under the region of the hole gratings, nanodot arrays with sub-micron periodicity are uniformly formed on the surface of the microlens arrays with a hedgehog-like appearance; the other half of the film without gratings remains smooth (right panel in Figure 1a). The result proves that the near-field effect is valid even on the micron-sized curved surfaces, if only conformal contact between the flexible phase mask and photopolymer can be guaranteed. The production area can be simply expanded by increasing the size of the phase mask because this method employs reliable optical components that are compatible with conventional photolithography. Our inch-scale product contains densely packed nanodots on microlens arrays with a low radius of curvature (~35 µm) (Figures 1b). The nanodots fully cover both the highly protruded microlenses and narrow flat valleys between microlenses. This structural coverage and resolution, which are similar to those of a biological


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moth eye sample (Alcides Orontes) and are difficult to be achieved by wafer-based conventional lithographic techniques, can maximize light transmission by an omnidirectional antireflective effect (Figure 1c). Conformal Contact Behavior of Ultrathin Diffractive Optical Elements. Figure 2 explains the mechanism of realizing conformal contact on rough surfaces with rigid PVA phase masks. In general, the interfacial mechanics and soft adhesion between the film and substrate govern the conformal contact property. Assuming that the substrate characteristics are non-variable, the two key elements determining interfacial contact are the thickness and elastic modulus of the film. The total energy associate with interfacial contact is represented as,24 Uinterface = Uphase mask_bending+Usubstrate_elasticity+Uadhesion

(2)

where Uphase mask_bending, Usubstrate_elasticity, and Uadhesion are the bending energy of the phase mask, the elastic energy of the substrate, and the adhesion energy of the contact, respectively. When the adhesion energy overcomes the sum of the bending energy of the phase mask and the elastic energy of the substrate, conformal contact occurs by the Van der Waals force. This phenomenon implies that the total energy is positive. An analytical solution can provide the ideal film thickness, which is referred to as the critical thickness, generating conformal contact with the experimental conditions.24 Assuming that the surface morphology of the substrate is sinusoidal, the elastic energy of the substrate is determined by the roughness of the microlenses and the elastic modulus of the photopolymer (E~2 GPa). The bending energy of the phase mask is strongly related to the effective bending stiffness, considering the thickness of the PVA film and an estimated effective work of adhesion of ~0.1 N/m. As the amplitude (h) of the microlens arrays increases, signifying that the radius of curvature (r) decreases, the total energy sharply


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decreases and the critical thickness is gradually down-shifted according to the thickness of the PVA phase mask (Figure 2a). Consequently, a thinner and more flexible phase mask is required to satisfy conformal contact on the substrate with higher roughness. When the amplitude is 40 µm, a thickness of the PVA phase mask of under ~3 µm is expected to provide conformal contact. The experimental results are consistent with these calculations. When the thickness of the PVA phase mask is greater than ~10 µm, the relatively high bending stiffness of PVA allows minimal deformation without conformal contact (Figures 2b and 2c). By contrast, the ultrathin PVA phase mask with a thickness of ~3 µm is relatively flexible and fully adheres to the microlens arrayed photopolymer substrate with amplitude of ~8 µm (Figures 2d and 2e). The surface hole gratings of the PVA phase mask maintain full conformal contact on both the uphill ridges and flat valleys of the microlenses after a few minutes because of sufficient adhesion energy overcoming shape recovery force of the phase mask (Figure 2f). On the basis of these experiments, we confirmed the PVA phase mask yielded elastic responses to applied strains in a manner that provides perfect conformal contact to the hemi-spherical microlenses without mechanical constraint. For more information in detail, parametric study was performed using the finite element modeling (FEM) on the plastic deformation of PVA phase mask (Figure S3), which indicates that the PVA thin film can withstand a radial elongation along the radius of curvature of ~32 µm associated with the maximum normal force (~1 pN/m) caused by the Van der Waals force between the film and the substrate as the film thickness becomes thinner as low as ~3 µm. It is worth noting that these thin film PVA phase masks were reusable multiple times for the successive UV exposure step without the loss of critical dimension when the larger radii of curvature substrates (> 32 µm) were prepared. However, the amplitude of the microlenses exceeds over this threshold level (h ~20 µm), despite of being fully conformed to the curved


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substrates, the permanent plastic deformations of PVA masks were experimentally observed. The superior deformation capability of the ultrathin PVA phase mask can be proved by rolling up with a slender hex key (5/64 inch) (Figure 2g) and placing on rough surfaces of a leaf and a jujube fruit (Figures 2h and 2i). The sub-micron gratings on the surface of the phase mask generate clear diffraction orders when a blue laser (~447 nm) passes through the phase mask (Figure 2j). Flexible Near-Field Phase Shift Lithography on Non-Planar Substrates. Figure 3 presents a collective set of hierarchical structures formed by ultrathin, flexible PVA phase masks. When the angle of incident light, whose wavelength is comparable to the periodicity of the grating, is normal to the flat substrate, the interference of diffracted beams generates a periodic intensity distribution of light along the depth direction in the proximity-field up to a few hundreds of microns.25-29 This exceptional optical effect, which is referred to as the Talbot or self-imaging effect,30 is identical in the case of non-planar substrates (Figure S2). However, the solid porous structures after photopolymer development act as scattering centers for light, resulting in a dramatic decrease in transmittance.31,32 We chose a UV lamp with a short coherence length as the incident light and a high-absorption negative-tone resist as a structuring layer to remove the unwanted lower porous layers and leave only surface antireflective nanotextured surfaces. When the absorption coefficient (α) of the photopolymer is ~0.6 µm-1 at 365 nm, the effective penetration depth of incident light is restricted to just under 500 nm from the surface (Figure S4). The compound eye structures of moth eyes are successfully mimicked via near-field lithography with the aid of the ultrathin PVA film with nanohole gratings and the high-absorption photopolymer with imprinted microlens arrays (r~58 µm) (Figure 3a). The exposure dose and developing time for the surface textures are experimentally optimized at ~20 mJ/cm2 and 120


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sec, respectively. The nanodot height and pattern shape are not strongly influenced by the developing time (Figure S5). The density of nanodot arrays on the photopolymer substrate is precisely controlled by changing the periodicity of the PVA phase mask grating from 600 to 1,500 nm. In all cases, the nanodot arrays are uniformly produced on the microlens-arrayed platform including curved surfaces and deep valleys. If the PVA phase mask is not sufficiently thin to be physically flexible, then the nanodot patterns cannot be uniformly produced over the entire area, causing some partial nanodots to remain on top of the small ridges of the microlens arrays because the conformal contact is localized (Figure S6). Over the periodicity of 1,000 nm, the surface textures are transformed from simple dots to arbitrary shapes because the higherorder diffractions (i.e., the 3rd- or 4th-order diffractions) produce more complex interference.30 The contact fringes clearly appear on the curved surfaces of the microlens arrays when a periodicity of ~1,000 nm is used (Figure 3b). The surface profile image obtained by atomic force microscopy (AFM) reveals the roughness of the nanotextures in detail (Figure 3c). The scan area and radius of curvature of the microlenses used here are 1,764 µm2 and ~58 µm, respectively. We measured the surface profiles in detail at three major points placed on the valley, the middle, and the ridge. The average height of nanodots for each region is ~100 nm, and the distribution ranges from 70 to 120 nm (Figure S7). The height and structural morphology are well consistent with optical simulation conducted by FDTD (Figure S8). Despite the sparse contact fringes, the height and the shape of the nanodots are not significantly changed. To prove superior flexibility of the ultrathin phase mask and patterning capability, various radii of curvature of microlens arrays are tested (Figure S9). The uniformity of the structures is well maintained as the radius of curvature decreases (Figures 3d and S10). When a highly curved surface (r~28 µm) is used, the


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nanodots fully cover the deep valley between microlenses, demonstrating the superior conformal contact and flexibility of this ultrathin PVA phase mask (inset of Figure 3d and S10d). Photonic Application of Biomimetic Hierarchical Structures. Figures 4 and 5 demonstrate the versatility and usability of these hierarchical structures as multifunctional optical films. In the hierarchical structures, the periodic arrays of upper nanodots and bottom microlenses can optically induce antireflection and diffusion, respectively. The patterned photopolymer structures can serve as semi-permanently reusable templates to produce flexible and stretchable optical films through the sequential imprinting processes (Figure 4a). As an initial step, an intaglio structure is inversely replicated from the template using polyurethane acrylate (PUA), which has a low surface energy (~25 dyne/cm) because of the incorporated fluorine releasing agent.33 Consecutive molding with PDMS produces a semi-transparent and attachable optical film with an embossed hierarchical structure that is perfectly identical to the original template (Figure 4b). The area of the optical film is successfully expanded up to ~16 in2 with high uniformity, which can be useful in various optoelectronic applications, such as light trapping units for solar cells, diffuser sheets for sheet-emitting displays and mood lighting, and transparent projection screens (Figure 4c).34-37 The incident laser (λ~633 nm), which is normally incident to the optical film, is effectively diffused in practice because the film consists of hexagonal arrays of microlenses with a periodicity of ~63 µm (Figures 4d and S11). When the screen is relatively close (~5 cm) to the film, a focused image on the screen results in enlarged uniform illumination because of an overlap of diffractions. When the distance exceeds ~10 cm, the discrete Fraunhofer diffractions are clearly focused on the screen. The important difference between the conventional diffuser and the moth-eye-inspired film referred to as an antireflective diffuser is the optical transparency. The latter exhibits improved diffuse transmittance (>3%) and correspondingly reduced reflection


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over the entire visible spectrum compared to the conventional diffuser without an upper nanodot layer because of its antireflective functionality (Figure 4e). This enhancement is reasonable compared to previously reported values measured from well-mimicked structures similar to real compound eyes.15,16 This type of diffusive films can be used to transform a point emission into a sheet emission as a transparent diffuser screen. As a proof of concept, the antireflective diffusers supported with PDMS slabs are attached on 8 by 8 light emitting diode (LED) matrices with five different colors (red, green, blue, yellow, and white) that cover the broadband visible wavelengths (Figures 5a and S12). The thickness of the PDMS slabs and the area of the LED matrices are fixed at ~1 cm and 0.64 in2, respectively. In all cases, the focused images on the diffuser screens are successfully changed to sheet illumination which is distinct from the original discrete point illumination. The antireflective diffuser also exhibits higher electric power efficiency (~4%) defined as the ratio of the output power divided by the input power compared to the conventional diffuser, which means that the power consumption can be reduced (Figure 5b). The sheet-illumination from large-area flexible LED arrays covered with the wafer-scale, moth-eye-inspired film demonstrates the scalability of the technique and its applicability to flexible electronics or improved pixel-based displays (Figure 5c). CONCLUSIONS In summary, the collective set of results reported here present a state-of-the-art lithographic technique for realizing hierarchically structured ‘nano-on-micro’ structures biologically inspired by insect eyes. The key element of this technique is that physically flexible, ultrathin (~3 µm) phase masks enable conformal contact even on rough surfaces with an extremely low radius of curvature of ~28 µm. The most important outcome is that artificially produced wafer-scale optical films mimicking real moth eye structures are demonstrated via


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mass-producible optical technology for the first time. The large-area platform (~16 in2), which consists of fully covered nanodots on microlens arrays, can serve as a template to produce attachable and stretchable moth-eye-inspired structures via sequential imprinting. The resulting new class of multifunctional optical films is may be applicable to various optoelectronic devices, as we demonstrate here by realizing sheet-illumination displays.


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MATERIALS AND METHODS Preparation of Ultrathin Phase Masks. Silicon masters, which contained square and hexagonal arrays of holes with a height of ~420 nm and various sub-micron diameters and periodicities, were prepared by conventional photolithography and etching processes. A small amount (~20 µl) of polyurethane acrylate (PUA) (MINS-311RM) monomer was dropped onto the Si master and was carefully covered with a flexible film without trapped air. After few seconds, the PUA monomer was spread out and perfectly filled into holes because of capillary forces. The PUA monomer was fully cured during UV exposure (~365 nm) with an energy dose of ~200 mJ/cm2. The solid PUA replica was then carefully peeled off from the Si master. A small amount (~3 ml) of dissolved poly(vinyl alcohol) (PVA) (MW: 89,000-98,000, Sigma Aldrich) solution (4 wt% in water) was dropped onto the PUA replica which act as a second master. A thickness was controlled by varying an applied spin-speed from 500 to 600 rpm for 30 sec. During mild annealing in an oven at 80°C for 10 min, the solvent was fully evaporated without air-bubbles. The solid PVA film with original hole patterns was carefully peeled-off from the master and served as a flexible and conformal phase mask. Preparation of microlens arrayed photopolymer substrates. Hexagon arrays of photopolymer (AZ 9245, Microchemicals) on a 4 inch Cr-coated glass substrate were patterned by conventional photolithography. The reflow of the patterned sample was conducted on a hot plate at 140 °C for 1 min for fabricating hemispherical microlens arrays. A radius of curvature of microlenses was controlled

by an

initial

thickness

of photopolymer. A large

amount

(~80g) of

poly(dimethylsiloxane) (PDMS) (Sylgard184, Dow Corning) was then poured onto the patterned substrate and cured at 65 °C for overnight. After full crosslinking, the solid PDMS with intaglio was detached from the substrate and served as a template mold. A small amount (~20 ml) of


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photopolymer (AZ nLOF 2035, Microchemicals) was dropped onto the PDMS mold. The sample was spin-coated at 500 rpm for 30 sec and was applied vacuum of ~0.1 MPa for 1 min in a chamber for removing microbubbles. The flattened photopolymer surface was then covered with a thick glass substrate. Following two-step thermal annealing at 80 °C for 30 min and 110 °C for 25 min solidified the photopolymer without delamination. After cooling down, the PDMS mold was gently removed from the microlens arrayed photopolymer on the glass substrate. Flexible Near-Field Phase Shift Lithography (F-NFPSL). A PVA phase mask with thickness of ~3 µm was placed on top of an imprinted photopolymer substrate. Within few seconds, the mask spontaneously generated full conformal contact with the substrate because of Van der force interaction. Then, collimated UV (~365 nm) was exposed through the phase mask with an controlled energy dose from 15 to 40 mJ/cm2. After detaching the mask from the substrate, full cross-linking of exposed regions was achieved by post-baking on a hot-plate at 110 °C for 30 sec. Unexposed regions were selectively dissolved using a developer solution (AZ 300MIF, Microchemicals) for 2 min. When higher radius of curvature used, the developing time was increased up to 4 min. The wet sample was gently dried using N2 gas. Pattern Transfer to Flexible Substrates via Sequential Imprinting. A small amount (~20ml) the PUA monomer was dropped onto the patterned photopolymer as a template. Under UV exposure (~365 nm) with an energy dose of ~350 mJ/cm2, the PUA was fully cross-linked and remained as a solid film. The PUA replica with intaglio was then carefully peeled off from the template. A large amount (~80 g) of PDMS was dropped onto the PUA replica which act as a second template. After curing at 65 °C for overnight, the solid PDMS film with original surface patterns was carefully detached from the template.


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Application of Moth-Eye-Inspired Films to Sheet-Illumination. Commercially available 8by-8 LED dot matrices and flexible large-area LED arrays were purchased from Adrafuit Industries. A circuit board with resistance supported LEDs and electrically connected to a power supply. An operating current ranged from 10 to 50 mA. The output power of LEDs was carefully measured by an optical detector at a monochromatic wavelength (Thorlabs) before and after attaching diffuser films supported by a thick PDMS slab (1 cm). Characterization. Microscopic images were captured via optical microscopy (OM) and fieldemission scanning electron microscopy (FESEM) (S-4800, Hitachi) operated at an accelerating voltage of 5-10 kV. Surface images and profiles of moth eye structures were analyzed using atomic force microscopy (AFM) (LV100POL, Bruker). Radii of curvature were measured by color 3D laser microscope (VK-9700K, Keyence). Diffraction patterns were measured by customized laser setup with a wavelength of ~633 nm (He-Ne, Uniphase). Optical and Mechanical Modeling. Large-scale simulation of 3D intensity distribution in microlens arrayed photopolymer was conducted using a finite-difference time-domain (FDTD) method (Lumerical). A penetration depth of incident light in photopolymer with different absorption coefficient was calculated by a finite elements modeling (FEM) method (COMSOL Multiphysic V3.5). The deformation behavior of phase masks with various thicknesses was also simulated by FEM.


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Figure 1. Strategy for fabricating moth-eye-inspired structures. (a) Schematic describing flexible near-field phase-shift lithography (NFPSL) with an ultrathin phase mask made of poly(vinyl alcohol) (PVA). (b) Digital and SEM images of a 1-inch optical film produced by flexible NFPSL. (c) Digital and SEM images of real moth-eye structures of Alcides orontes.


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Figure 2. Relationship between the thickness and flexibility of PVA phase masks. (a) Relationship between the interfacial energy and thickness of PVA phase masks. (b) Tilted-view SEM image of the interface between a PVA phase mask with a thickness of ~52 µm and a microlens arrayed photopolymer with a radius of curvature of ~58 µm. (c) Tilted-view SEM image of the interface between a PVA phase mask with a thickness of ~12 µm and a microlens arrayed photopolymer with a radius of curvature of ~58 µm. (d) Top-view optical image of a microlens arrayed photopolymer half-covered with a PVA phase mask with a thickness of ~3 µm. (e) Tilted-view SEM image of the interface between a PVA phase mask with a thickness of ~3 µm and a microlens arrayed photopolymer with a radius of curvature of ~58 µm. (f) Tilted-view SEM images of the interface between a PVA phase mask with a thickness of ~800 nm and microlens arrayed photopolymer with a radius of curvature of ~58 µm. (g) Digital image of an inch-scale, ultrathin PVA phase mask rolled up with a slender hex key (5/64 inch). (h) Digital image of an ultrathin PVA phase mask placed on a leaf. (i) Digital image of an ultrathin PVA phase mask placed on a jujube fruit. (j) Diffraction image of a blue laser (~447 nm) generated by an ultrathin PVA phase mask with a periodicity of ~600 nm.


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Figure 3. Production of ‘nano-on-micro’ hierarchical structures via flexible near-field phaseshift lithography. (a) SEM images of the surface structures with a radius of curvature of ~58 µm patterned by phase masks with grating periodicities ranging from 600 to 1,500 nm. (b) Magnified SEM image of the surface structures patterned by a phase mask with a grating periodicity of 1,000 nm. (c) AFM image and height profile of the surface structures patterned by a phase mask with a grating periodicity of 600 nm. (d) SEM images of patterning results for various radii of curvature ranging from ~28 to ~58 µm.


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Figure 4. Large-area pattern transfer to flexible and stretchable substrates for attachable, antireflective optical films. (a) Schematic illustration of sequential imprinting processes to produce attachable and stretchable moth-eye-inspired optical films. (b) SEM images of optical films for each transfer step. (c) Wafer-scale (~16 in2) optical film after pattern transfer to a PDMS substrate. (d) Diffraction and diffusion of incident light (λ~633 nm) passing through the optical film as a function of distance. (e) Transmittance and reflectance spectra of optical films in the visible wavelength.


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Figure 5. Realization of sheet-illumination displays with enhanced power efficiency by using antireflective diffuser films attached on point LED arrays. (a) Digital images of illumination before and after attaching antireflective diffusers on the 8 by 8 LED matrices (0.64 in2). (b) Comparison of electric power efficiencies of sheet-emitting LEDs after attaching conventional diffusers and moth-eye-inspired antireflective diffusers. (c) Digital images of flexible sheetemitting displays converted from the point LED arrays by large-area antireflective diffuser films (~16 in2).


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ASSOCIATED CONTENT Supporting Information. Additional figures and captions for schematic illustration of preparation steps for microlens arrayed photopolymer, optical simulation results for propagating light intensity distribution and expected structures, SEM and AFM images for hierarchical structures in detail, and customized experimental setup for photonic application. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]. Present Addresses ¶

Present address: Department of Chemical Engineering, Massachusetts Institute of Technology,

Cambridge, MA 02139, USA. Author Contributions §

These authors contributed equally. J.P., Y.W.K., S.J., and S.W.H. conceived the idea and

designed the experiments. Y.W.K., J.P., T.K., and S.H.K. performed the experiments and characterization. J.P. conducted the mechanical analysis. H.K., J.P., and J.S. conducted the optical simulation. J.P., Y.W.K., S.J., and S.W.H. wrote the paper. Funding Sources Ministry of Science, ICT & Future Planning (MSIP) (2014M3A6B3063708) and NRF Grant funded by the Korean Government (MSIP) (2015R1A5A7036513)


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Global Frontier Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2014M3A6B3063708). This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (2015R1A5A7036513).

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Insert Table of Contents Graphic and Synopsis:

Flexible near-field phase shift lithography (F-NFPSL) facilitates the large-area, conformal patterning of nanoscale structures on non-planar substrates. The ultrathin form (~3 µm) of polymeric phase grating makes the material physically flexible and enables conformal contact with rough surfaces. The resultant ‘nano-on-micro’ structures mimicking real compound eyes are transferred to flexible and stretchable substrates by sequential imprinting, facilitating multifunctional optical films applicable to antireflective diffusers for large-area sheetillumination displays.


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Flexible Near-Field Nanopatterning with Ultrathin ... - ACS Publications

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