Nanopattern-Embedded Micropillar Structures for Security

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Surfaces, Interfaces, and Applications

Nano-patterns Embedded Micro-pillar structures for Security Identification Zhi-Jun Zhao, SoonHyoung Hwang, Moonjeong Bok, Hyeokjung Kang, Sohee Jeon, Sanghu Park, and Jun-Ho Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07308 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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ACS Applied Materials & Interfaces

Nano-patterns Embedded Micro-pillar structures for Security Identification Zhi-Jun Zhaoa, SoonHyoung Hwanga, Moonjeong Boka, Hyeokjung Kanga, Sohee Jeona, Sang-Hu Parkb, and Jun-Ho Jeonga* aDepartment

of Nano Manufacturing Technology, Korea Institute of Machinery and

Materials, Daejeon 305-343, South Korea bSchool

of Mechanical Engineering, Pusan National University, Busandaehak-ro 63beon-gil,

Geumjeong-gu, Busan 609-735, Republic of Korea KEYWORDS Nano-patterns, micro-pillar, nanowelding, metal nanostructures, security identification

Abstract A novel method was developed for fabricating nanopatterns embedded on micropillarstructured surfaces using nanowelding technology for security identification. Commonly used substrates, i.e., polyethylene film, glass wafer, Si wafer, and curved surface, were employed and their characteristics were evaluated. Cr was deposited onto the selected substrate to strengthen the adhesion force, and an adhesive layer of ultra-thin metal was deposited on top of the Cr layer. Lastly, nanopatterns were embedded on the substrates by nanowelding. The morphologies, cross-sections, and three-dimensional (3D) images of the fabricated nanostructures were evaluated, and their crystalline structures and compositions were 1

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analyzed. Using the same method, nanopatterns embedded on micropillar-structured surfaces were fabricated for the first time as security patterns to improve security identification. The fabricated security patterns were characterized in three stages. First, micropillar structures and structural color were simply observed via optical microscopy to achieve a preliminary judgment. The appearance of structural color was due to the nanostructures fabricated on the micropillar surface. Next, the designed nanopatterns on the micropillar-structured surfaces were observed by scanning electronic microscopy (SEM). Lastly, the changes in spectral peaks were precisely observed using a spectrometer to achieve an enhanced security pattern. The fabricated security patterns can be suitable for valuable products, such as branded wines, watches, and bags. In addition, the proposed method offers a simple approach for transferring metal nanopatterns to common substrates. Moreover, the fabricated security patterns can have potential applications in semiconductor electrodes, transparent electrodes, and security identification codes.

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1. Introduction With the development of nanotechnology, nanofabrication has become increasingly important in a wide range of applications, such as in optics,1–3 biosensing,4–7 metamaterials,8– 11

and medicine.12,13 The existing nanofabrication technologies, including e-beam

lithography,14–19 focused ion beams (FIBs),20–24 nanotransfer,25–30 and nanoimprint lithography,31–34 have been widely used to fabricate nanopatterns, which have been successfully applied in various fields. Thus, nanopatterning is indispensable for the development of nanotechnology. Thus far, the nanopatterns fabricated via e-beam lithography have predominantly been used in optical filters, metastructures, biotechnology, and medicine. However, following are the disadvantages of e-beam lithography: (i) very high fabrication cost, (ii) complicated and time-consuming operation, (iii) no potential for re-use, and (iv) difficult fabrication over large surface areas. To solve these issues, many researchers have adopted nanoimprint lithography, evaporation, and nanotransfer to fabricate nanopatterns. These nanofabrication technologies can overcome the above-mentioned disadvantages of nanopatterning. Jung et al. proposed a solvent-assisted nanotransfer method to implement the transfer of Ag, Au, and Pd.35 Hwang et al. developed covalent-bonding-assisted nanotransfer lithography, in which a chemical adhesion layer was utilized to transfer nanostructures onto substrates.36 More recently, a research group developed a sub-50-nm replication technique by combining the advantages of template-stripping and nanotransfer printing, in which poly(vinyl alcohol) dissolved in water was utilized.37 Seo et al. reported a materialindependent nanotransfer method for fabricating nanowires on flexible substrates using a dryremovable nano sacrificial layer located between the master mold and nanowires.38 These methods have also provided important support for nanotechnology. Typically, the transfer of nanopatterns is achieved using adhesion layers, including chemical adhesive coating, surface 3



treatment, and soft substrate. However, by using these methods, successful transfer cannot be achieved in health, nanopatterning on curved surfaces, nanopatterning without defects, nanopatterns embedded on the micropillar-structured surfaces, and stacking of 3D metaloxide nanostructures. To improve the applicability of methods, we recently published reports on nanoimprint lithography, evaporation, and nanowelding, whereby three-dimensional (3D) metal-oxide nanostructures were fabricated and applied to photocatalytic and polarization color filters.39 In addition, we proposed shape-controlled metal–metal nanostructures with potential applications in optics, displays, and biosensors.40 Although simple nanotransfer was achieved in these studies using the nanowelding method and without the use of chemical adhesive coating, the polymer patterns located on the substrate cannot be removed. If existing methods are used to transfer nanostructures onto designated substrates, chemical adhesive coating must be applied. In addition, nanowelding cannot be used to transfer nanopatterns onto curved surfaces and micropillar-structured surfaces. In this study, we attempted to solve these problems by developing a method, based on the nanowelding technique, for simple nanotransfer onto an arbitrary surface without the use of chemical adhesive coating. An ultra-thin metal film was deposited as an adhesive layer to achieve simple nanopattern transfer. Ag and Au nanowires and dots were selected for transfer onto arbitrary surfaces, including PET film, glass, Si, and curved surfaces. Importantly, we developed novel security patterns composed of the designed nanopatterns embedded onto micropillar-structured surfaces by the proposed fabrication method. The fabricated security patterns were characterized according to the following three stages: optical microscopy was first performed to observe the microstructures and structural color to obtain a preliminary judgment. The appearance of structural color is due to the nanostructure fabricated on the microstructured surface. For further authentication, scanning electronic microscopy (SEM) 4


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was conducted to observe the designed nanostructures on the microstructured surfaces. Lastly, the spectral changes were precisely observed using a spectrometer to determine the authenticity of the goods. The fabricated security patterns can be suitable for valuable products such as branded wines, watches, and bags. Since it is difficult to use conventional transfer methods to fabricate nanopatterns on microstructured surfaces, we believe that our method can be applied for nanotransfer onto arbitrary surfaces in various fields, such as biosensors, security identification, and metamaterials.

2. Results and Discussion Figure 1 shows the nanostructure fabrication process based on nanoimprint lithography, ebeam evaporation, nanotransfer, and nanowelding. The polymer patterns were easily obtained by nanoimprint lithography (Figure 1a-(i-iii)). This process has some obvious advantages: (1) nanoimprinting over a large surface area using the existing roll-to-roll method is easy, (2) reproduction is possible using a designated stamp, and (3) operation is easy and short. Ag nanowires were fabricated via e-beam evaporation (Figure 1a-iv). Because it was impossible to transfer the fabricated Ag nanowires onto a curved surface using the existing nanotransfer method, our proposed nanowelding method was employed. An ultra-thin Cr film (thickness < 5 nm) was deposited on the curved surface to strengthen the adhesion force (the film thickness was adjusted using the Ag film thickness), and Ag film was fabricated by e-beam evaporation for use as a nanowelding layer (Figure 1a-v). Then, the fabricated Ag nanowires were transferred onto the curved surface by nanowelding (Figure 1a-vi). This process was easily controlled by adjusting the heating temperature, pressure, and time. After nanowelding, the polyethylene (PET) film with polymer patterns was separated from the curved surface with nanopatterns (Figure 1a-vii) and the Ag nanowires were fabricated on the curved surface 5



(Figure 1a-viii). Using the same method, various patterns were fabricated on the surfaces of arbitrary substrates.

Figure 1. Proposed nanofabrication process and morphologies for nanostructure fabrication on a curved surface. (a) Nanofabrication process: (i) fabrication of polymer mold with desired patterns, (ii) UV curing to completely polymerize the polymer mold, (iii) detachment process, (iv) metal deposition via e-beam evaporation, (v) metal film deposited on the curved surface as an adhesive layer for transfer, (vi) nanowelding, (vii) separation, and (viii) formation of 6


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the nanostructure on the curved surface. (b) Photographs of a (i) curved surface, (ii) bottle, (iii) glass wafer, and (iv) flexible PET film. (c) SEM and FIB cross-sectional images of Ag nanowires with dimensions of (i) 100 nm (width) × 100 nm (space) × 30 nm (thickness), (ii) 200 nm × 200 nm × 30 nm, and (iii) 400 nm × 200 nm × 30 nm. (d) SEM and FIB crosssectional images of dots and cross nanostructures: (i) 100 nm (diameter) × 300 nm (pitch) × 30 nm (thickness); (ii) 256 nm × 400 nm × 30 nm; and (iii) three layers with dimensions of 200 nm × 200 nm (width). All scale bars are 1 µm.

The images of various substrates after nanotransfer are shown in Figure 1b (curved surface, glass bottle surface, glass wafer, and flexible PET), which reveals nanotransfer with no defects. The morphologies and cross-sectional images of Ag nanowires with different sizes are shown in Figure 1c. Our results demonstrate that different-sized nanopatterns were easily transferred onto various surfaces (Figure 1c(i-iii)). In this experiment, thin Au and Ag layers were deposited onto the 5-nm-thick Cr layer as a nanowelding layer to achieve good transfer. Notably, the melting temperatures of noble metals increase with increasing size on the nanoscale. Therefore, Au and Ag nanowelding layers of different thicknesses were chosen to analyze and compare the corresponding morphologies. The roughnesses of Au and Ag thin films of different thicknesses were measured using atomic force microscopy (AFM), the results of which are provided in the supporting information (Figure S1). The dependence of size on melting temperature has been previously reported.42–44 During nanowelding at 100 °C, the Ag thin film became island-shaped due to the lower melting temperature. However, good transfer is possible with the proposed method, as shown in Figure S2(a-c). Additionally, the nanopatterns were transferred onto the surfaces of Ag islands. To examine the changes in the Ag nanofilm, a 30-nm-thick Ag nanowelding layer was fabricated for nanotransfer under 7



the same conditions. A fine surface was observed because of the higher melting temperature. The morphology results are displayed in Figure S2(d). To confirm that the proposed method can be applied to other nanopatterns, dots with different dimensions were fabricated (Figure 1d(i-ii)). Large-scale images of the fabricated nanostructures are provided in Figure S3, confirming the effectiveness of nanofabrication. In addition, stacked nanostructures with three layers were fabricated (Figure 1d-iii). We observed cross nanostructures measuring 200 nm × 200 nm (Figure 1d-iii), indicating that this method can provide important support for nanofabrication.

Figure 2. Cross-sectional TEM and EDS-mapping images of nanostructures fabricated on Au film (10 nm thick). (a-c) Low-magnification TEM images (scale bars: 200 nm, 50 nm, and 20 nm), (d-e) high-resolution TEM images (scale bars: 10 nm and 5 nm), and (f) EDS-mapping images (red, green, and blue represent Ag, Au, and Cr, respectively).

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To more clearly observe the nanowelding layer after transfer, transmission electron microscopy (TEM) was implemented. The high-resolution TEM images reveal good junctions within the interfaces between layers (Figure 2). The arrangement of atoms can also be observed in the inset of Figure 2e. To confirm the compositions of the nanostructures, energy dispersive X-ray spectroscopy (EDS) mapping was implemented. Figure 2f reveals the composition, including the presence of Ag as nanowires (red), Au as the nanowelding layer (green), and Cr as the adhesive layer (blue). When a 10-nm-thick Ag film was used as the nanowelding layer, good transfer was achieved; however, the morphology of the film became island-shaped because of the lower melting temperature (Figure S1). As the melting temperatures of noble metals increased with thickness, we analyzed the morphologies of Ag nanowelding layers with different thicknesses. A 30-nm-thick Ag film was fabricated to implement nanotransfer. The TEM and EDS mapping images (Figure S4) illustrate the crystallinity and composition of the fabricated nanostructures, while Figure 3 shows their 3D morphologies analyzed using AFM. Figures 3a-c depict Ag nanowires of different sizes transferred to the surfaces of 10-nm-thick Au and Cr films deposited on the Si substrate. The sizes and shapes of the nanostructures in Figure 3a-c are the same as those in Figure 1c(i-iii). Figure 3d shows Ag dots with a diameter of 256 nm and on the Au film with thickness of 30 nm. To evaluate the morphologies of Ag and Au films as adhesive layers, their roughnesses were measured. Figures 3e and 3f display the roughnesses of Ag and Au film, which are 3.645 nm and 0.610 nm, respectively. The difference is because the melting temperature of the Au film is lower than that of the Ag film during nanowelding. In addition, the 3D morphologies of the Ag and Au nanowires using Ag film as the nanowelding layer are depicted in Figure S5 for comparison of the roughnesses of the nanostructures and nanowelding layers. 9



Figure 3. AFM images of nanostructures fabricated using Ag nanowires and dots with different dimensions: (a) 100 nm (width) × 100 nm (space) × 30 nm (thickness), (b) 200 nm × 200 nm × 30 nm, (c) 400 nm × 200 nm × 30 nm, and (d) 256 nm (diameter) × 30 nm

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(thickness), (e) and (f) show roughnesses of Ag film (Ra: 3.645) and Au film (Ra: 0.610), respectively.

Figure 4. Fabrication process and optical microscope images and SEM images of nanopatterns embedded on the micropillar-structured surfaces. (a) Fabrication process (b) Photograph of Au micro dot patterns with a thickness of 30 nm. (c) and (d) Optical microscopy images. (e) SEM image of Ag nanowires measuring 100 nm × 200 nm × 30 nm fabricated onto the surface of Au micropillar patterns. (f) and (g) SEM images of the critical surface (the bold red dotted line represents the edge of the micropillar and nanowires).

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To confirm the broad applicability of the proposed method to fabricate various patterns, it was employed to fabricate Ag nanowires on the surfaces of Au micropillar structures. Until now, nanostructures on the surfaces of micropillar structures based on required designs have been rarely fabricated. Figure 4 shows the optical microscopy and SEM images of the micropillar structures and nanostructures. Au micropillar structures with diameters of 200 μm and thicknesses of 30 nm were fabricated on the glass wafer by e-beam evaporation. The fabrication process of the nano-micro nanostructures is depicted in Figure 4a. The designed shadow mask with microstructures was prepared, and 30-nm-thick Au micropillar structures were fabricated via e-beam evaporation. Lastly, Ag nanowires were embedded onto the micropillar-structured surfaces. An image of the fabricated sample is provided in Figure 4b. Figures 4c and 4d present optical microscopy images of patterns of 200-μm-diameter dots at different magnifications (Figure 4c: 400 μm, Figure 4d: 50 μm). Ag nanowires deposited onto the polymer patterns were embedded onto the surfaces of Au micropillar structures with diameters of 200 μm using the proposed method. Figure 4e shows the morphologies of the Ag nanowires with dimensions of 100 nm (width) × 200 nm (pitch) × 40 nm (thickness) fabricated on the micropillar-structured surfaces. Figure 4f depicts the critical surfaces of the nanopatterns and micropillar structures. Figure 4g shows a holistic SEM image of the fabricated structure.

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Figure 5. Images of designed letter art composed of micropillar structures and nanopatterns. (a) Photographs and optical images: (i) all letters fabricated on a 4-inch glass wafer, (ii) magnified image of one letter (many dots can be observed), and (iii) optical images of the micropillar structures (scale bar: 300 μm). (b) Images of Ag nanowires fabricated on micropillar-structured surfaces with dimensions of 100 μm (diameter) × 30 nm (thickness): (i) letters after the nanopatterns were transferred onto the micropillar-structured surfaces (the remaining PET film without Ag nanowires is shown in the inset), (ii) optical image of the fabricated micro-nanopatterns (the Ag nanowires fabricated on the microstructure surface are 13



observable), (iii) SEM image (scale bar: 300 μm), (iv) SEM image of Ag nanowires measuring 100 nm (width) × 200 nm (pitch) × 30 nm (thickness), (v) magnified SEM image of the micro–nanopatterns, and (vi) SEM image of the critical surface. (c) Optical and SEM images of PET film with the remaining nanopattern after transfer: (i) photograph of PET film after transfer, (ii) and (iii) SEM images (the nanopatterns were transferred onto the microstructure surfaces), (iv) SEM image of the remaining Ag nanowires, (v) and (vi) optical images, and (vii) and (viii) FIB sectional images (the Ag nanowires were embedded onto the Au dot microstructure surfaces). To evaluate the applicability of our proposed nanofabrication method for security patterning, letter art composed of Au micro-nanostructures with different designs was fabricated by e-beam evaporation. Figure 5 presents the resulting letter art with micronanostructures, in which the letters KIMM were fabricated on a 4-inch glass wafer (Figures 5a-i). To observe the dots clearly, the image of the letter “K” was magnified, as shown in Figure 5a-ii. Figure 5a-iii display optical images of dots with 100 µm diameter and 150 µm pitch (the scale bar is 300 µm). Using the proposed method, Ag nanowires with dimensions of 100 nm (width) × 200 nm (pitch) × 30 nm (thickness) were fabricated on Au micropillarstructured surfaces with 100 µm diameter, 150 µm pitch, and 30 nm thickness (Figure 5b). Optical microscopy and SEM images are presented in Figure 5b(ii)-(vi). Interestingly, not only were the Ag nanowires transferred onto the Au micropillar-structured surfaces, but the Au and Au micropillar structures were also connected to one another like bridges by the Ag nanowires. To confirm that the Ag nanowires were transferred onto the microstructured surfaces, optical microscopy and SEM images of a PET film after transfer were obtained (Figure 5c). We can observe that KIMM with nanowires was transferred onto the glass wafer. It should be noted that the distance between the Au and Au microstructures is important (~25 14


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µm); as the distance increases, the Au and Au micropillar-structured surfaces will no longer be connected because of the weight and pressure of the Ag nanowires on the nanoscale. For comparison, we established a large distance between the Au and Au micropillar-structured surfaces (Figure 4, the distance between the Au and Au pillars is ~700 µm). Figure 5b-iv shows SEM images of Ag nanowires with dimensions of 100 nm (width) × 200 nm (pitch) × 30 nm (thickness), which reveals nanowires with no defects. In contrast, broken Ag nanowires are observable at the critical surface in Figures 5b-v and vi (red dotted lines). Figure 5c shows photographs, optical images, and SEM images of the PET film with nanostructures after the transfer process. From Figure 5c(i, v, vi), we can observe that the nanostructures were perfectly transferred onto the microstructure surfaces. Figure 5c(vii) shows a cross-sectional image of the hole part of the PET substrate after transfer. Figure 5c(viii) presents a cross-sectional image of the Ag nanowires fabricated on the Au microstructure surface. Ag nanowires fabricated on the surface of microstructures were observed and analyzed by TEM and EDS mapping. To confirm the good junction and composition of the Ag nanowire and Au dot microstructures, TEM and EDS mapping images are provided in Figure S6.

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Figure 6. Images and optical analysis of dot nanopatterns fabricated on the surface of micropillar structures. (a) Fabrication process. (b) Optical, SEM, and EDS mapping images of dot nanopatterns measuring 256 nm in diameter × 30 nm thick: (i) photograph of Ag dot nanopattern fabricated on the surface of a Au micropillar structure on a 4-inch glass wafer, 16


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(ii) optical image, (iii) SEM image of Ag dot nanopattern, (iv-vii) EDS mapping images (red represents the Ag nanopattern, green represents the Au micropillar structure, and the scale bars are 70 nm). (c) Optical analysis of the fabricated samples employed as security patterns: (i) schematic of the security pattern, (ii) SEM image (the FIB and EDS mapping images are shown in the inset), and (iii) reflection results for five types of samples (black curve: Au filmAg dot pattern, red curve: Au film-Ag dot pattern-SiO2-5 nm, blue curve: Au film-Ag dot pattern-SiO2-10 nm, pink curve: Au film-Ag dot pattern-SiO2-15 nm, green curve: Au filmAg dot pattern-SiO2-20 nm). To prove that it is possible to fabricate different nanopatterns and evaluate their optical properties for use in security patterns, Ag dot nanopatterns were fabricated onto micropillarstructured surfaces using the proposed method. Figure 6 depicts the fabrication process and provides photographs, optical microscopy images, and SEM images of the Ag dot nanopatterns fabricated on the Au microstructured surfaces. Using the method shown in Figure 4a, Ag dot nanopatterns could be transferred onto the Au micropillar-structured surfaces (Figure 6a). The images and microstructures of the letter art “KIMM” are observable in Figure 6b(i-ii). Ag dot nanopatterns with dimensions of 256 nm (diameter) × 30 nm (thickness) are also observable (Figure 6b-iii). Figure 6b(iv-vii) shows the EDS mapping images of Ag dot nanopatterns fabricated on the Au microstructured surfaces to facilitate observation of the Ag-Au junction. To confirm that the Ag dot nanopatterns were transferred onto the micropillar-structured surfaces, optical microscopy and SEM images of the PET film after transfer were obtained (Figure S7). Due to the specificity of the proposed nanostructure, we believe that this technology will have important applications in various fields. For security pattern applications, we fabricated Ag dot nanopatterns with dimensions of 256 nm (diameter) × 30 nm (thickness) on Si substrates. Figure 6c depicts the evaluation of security 17



identification. In this study, the proposed security patterns could be characterized in three stages, as schematically shown in Figure 6c(i). Optical microscopy was conducted to observe the micropillar structures to obtain a preliminary judgment. For further authentication, SEM was performed to observe the designed nanostructures on microstructured surfaces. Lastly, the spectral changes were precisely observed using a spectrometer to determine the authenticity of the goods (Figure 6c(i)). To improve the security of the nanopatterns, 5-nmthick SiO2 layers were deposited on the nanostructures. In addition, SiO2 layers of thicknesses 10 nm, 15 nm, and 20 nm were fabricated to compare the resonance peaks. To confirm the good junctions and compositions of the fabricated samples, TEM and EDS mapping images are provided in Figure S8 for reference. The SEM and FIB cross-sectional images are displayed in Figure 6c-ii (the EDS mapping image is shown in the inset). Figure 6c-iii presents the reflection results for five types of samples. The nanostructures fabricated without SiO2 layers exhibit two types of resonance peaks at wavelengths of ~520 nm and ~610 nm. With the addition of a SiO2 layer, the surrounding refractive index increased, the resonance peak at 610 nm could not be observed at the range of visible wavelength. With increasing SiO2 layer thickness, the effective index increased and the resonance peak displayed a red shift of ~50 nm. If we designed the spectrum of a security pattern, it would be unforgeable. In addition, our proposed method has a considerable advantage in that it can be used to fabricate security patterns more directly than conventional methods. Thus, the nanofabrication method and resulting optical properties provide important support for security identification and plasmonic sensor applications.

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3. Conclusion In summary, we proposed a novel method for fabricating nanopatterns embedded on the micropillar-structured surfaces as security patterns through nanowelding. An ultra-thin metal film was deposited instead of the conventional chemical adhesive layer on the target surface, allowing pre-designed nanopatterns to be embedded onto the target surface by nanowelding. Various types of substrates and nanopatterns were evaluated, proving the wide applicability of the proposed method. In addition, different ultra-thin metal films were employed as adhesive layers. The characteristics of the fabricated nanostructures were evaluated by SEM, FIB, AFM, and TEM. The experimental results demonstrated that good nanotransfer can be achieved on arbitrary surfaces without the need for an additional chemical adhesive layer. The proposed method was also used to fabricate nanostructures on micropillar-structured surfaces, displaying its potential application to security pattern fabrication. The security patterns of the fabricated nanostructures were evaluated via spectrometry. An enhanced security pattern was easily identified by a shift in resonance peaks. We believe that the proposed method can make significant contributions to various applications, including those in optics, metamaterials, biosensors, and security identification.

4. Experimental Section A Si stamp 100 nm wide, with 200 nm pitch, and 150 nm deep was chosen to fabricate the metal nanowires. The polymer patterns were fabricated using nanoimprint RM-311 resin (Minuta Technology Co., Ltd. Korea, polyurethane). The fabrication process is shown in Figure 1a-e. The fabrication methods of the metal nanowires and nanoparticles were described in our previous research.41 Using the same method, the nanoprint resin was coated onto the prepared Si stamp and a PET film was covered on the coated resin and rolled to 19



ensure thorough permeation of the resin into the stamp (Figure 1a). Ultraviolet (UV) curing was conducted to form the polymer patterns (Figure 1b), and the polymerized resin was detached from the Si stamp (Figure 1c). To fabricate metal nanowires, an e-beam evaporator was used to deposit metals onto the fabricated polymer patterns (Figure 1d). To improve the transfer of the fabricated metal nanowires onto the curved surface, a thin metal film used as a nanowelding layer was deposited onto the prepared surface (Figure 1e). A Cr layer less than 5 nm thick was first deposited onto the curved surface as an adhesive layer (Figure 1e). Then, the fabricated metal nanostructures were transferred onto the curved surface under a vacuum, at a pressure of 500 KPa, heating temperature of 90 °C, and duration of 10 min using the nanowelding method (Figure 1f). The PET film with polymer patterns was separated from the prepared curved surface, and the fabricated nanostructure was then formed on the curved surface (Figures 1g-h). To prove the applicability of the proposed method, PET (flexible), Si, and glass substrates commonly used in nanofabrication were employed in the experiments. In addition, Ag and Au dot patterns were fabricated using the proposed method. The sizes of the fabricated nanostructures were adjusted to analyze the relationship between the nanowelding layer and nanostructures. The metal film and nanopatterns were deposited on the polymer mold through e-beam evaporation ((DAEKI HIOTECH Co, Ltd. Korea). The nanowelding process was implemented via thermal nanoimprinting ((Hutem Co, Korea). The morphologies and crosssectional images of the fabricated nanostructures were analyzed via SEM (FE-SEM; Sirion, FEI Netherlands) and an FIB (Helios Nanolab, FEI Netherlands), respectively. AFM (XE100, Park Systems) was performed to confirm the roughness of the nanostructures and nanofilms. To observe the metallic bond of atoms more effectively, TEM (JEM-ARM200F,

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JEOL Japan) was conducted. A spectrometer (QE Pro 6000, Ocean Optics, USA) was used to measure the optical properties of the fabricated nanostructures.

ASSOCIATED CONTENT Supporting Information The roughness values of the Ag and Au films are shown in Figure S1. Figure S2, S3, and S4 show the morphologies of the various samples. AFM images are shown in Figure S5. Figure S7 shows the dual micro-nano pattern images after transfer. Figure S6 and S8 show the TEM and EDS-mapping images to confirm the good junction and composition.

ACKNOWLEDGMENT This work was supported by the Center for Advanced Meta-Materials (CAMM), which is funded by the Ministry of Science, ICT and Future Planning, Korea, through the Global Frontier Project (CAMM-No. 2014M3A6B3063707). AUTHOR INFORMATION Corresponding Author *Corresponding author: Dr. Jun-Ho Jeong Tel.: +82-42-868-7604; Fax: +82-42-868-7123 E-mail: [email protected]

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ToC

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