Letter pubs.acs.org/NanoLett
High Throughput Ultralong (20 cm) Nanowire Fabrication Using a Wafer-Scale Nanograting Template Jeongho Yeon,† Young Jae Lee,‡ Dong Eun Yoo,§ Kyoung Jong Yoo,‡ Jin Su Kim,‡ Jun Lee,‡ Jeong Oen Lee,† Seon-Jin Choi,† Gun-Wook Yoon,† Dong Wook Lee,§ Gi Seong Lee,§ Hae Chul Hwang,§ and Jun-Bo Yoon*,† †
Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea ‡ LG Innotek Components & Materials R&D Center, 55 Hanyang Daehak-ro, Ansan-si, Gyeonggi-do, 426-791, Republic of Korea § Korea National NanoFab Center (NNFC), 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea S Supporting Information *
ABSTRACT: Nanowires are being actively explored as promising nanostructured materials for high performance flexible electronics, biochemical sensors, photonic applications, solar cells, and secondary batteries. In particular, ultralong (centimeterlong) nanowires are highly attractive from the perspective of electronic performance, device throughput (or productivity), and the possibility of novel applications. However, most previous works on ultralong nanowires have issues related to limited length, productivity, difficult alignment, and deploying onto the planar substrate complying with well-matured device fabrication technologies. Here, we demonstrate a highly ordered ultralong (up to 20 cm) nanowire array, with a diameter of 50 nm (aspect ratio of up to 4 000 000:1), in an unprecedented large (8 in.) scale (2 000 000 strands on a wafer). We first devised a perfectly connected ultralong nanograting master template on the whole area of an 8 in. substrate using a top-down approach, with a density equivalent to that achieved with e-beam lithography (100 nm). Using this large-area, ultralong, high-density nanograting template, we developed a fast and effective method for fabricating up to 20 cm long nanowire arrays on a plastic substrate, composed of metal, dielectric, oxide, and ferroelectric materials. As a suggestion of practical application, a prototype of a large-area aluminum wire grid polarizer was demonstrated. KEYWORDS: Nanomaterial, nanowire, nanopatterning, wire grid polarizer, top-down, spacer lithography
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researchers23−27 have proposed novel methods and have achieved millimeters and centimeters long nanowires with great crystal uniformity, high electrical conductivity, and material versatility. In spite of their excellent advantages, their productivity and predictable alignment for reliable device fabrication have still been challenging issues. Although various previous works have been reported on interesting methods for nanowire alignment, the degree of ordering nanowires with these methods has still been coarse and not perfectly predictable. Recently, fiber drawing technology28−31 for fabricating an indefinitely long nanowire array with high productivity and with good alignment have been sought by previous researchers. Although they have achieved aligned ultralong nanowires with good throughput, it is still a challenging issue deploying them to a planar substrate (i.e., silicon wafer), for combining with currently well matured planar-substrate-based device fabrication technology.
anowires have recently attracted widespread attention because of their unique properties, which have not been observed in the relevant bulk materials, and show considerable potential for applications in a wide range of scientific and engineering fields. Researchers have thus far applied onedimensional nanostructures to sensors,1,2 electronics devices,3,4 biochemical applications,5,6 photonics,7,8 and so on.9−11 Accordingly, various methods for nanowire fabrication have been reported. Chemically based synthesis methods form the majority of nanowire fabrication techniques. Solution-based silver (Ag) or zinc oxide (ZnO) nanowire growth,12−14 ZnO nanowire generation in vapor-state reactions,15,16 and electrogrowth of metal nanowires through porous membrane templates17,18 can be classified among these “bottom-up” approaches. Top-down-based fabrication methods, usually based on lithographic patterning technologies, have also been used to generate nanowires.19−21 In recent days, ultralong nanowires with their length of more than centimeter level are especially desired on the basis of their potential advantages including decreased junction resistance,13 convenient manipulation,22 fabrication yield, and greater possibility to realize new applications. So far, many previous © 2013 American Chemical Society
Received: January 17, 2013 Revised: July 9, 2013 Published: July 30, 2013 3978
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Figure 1. Overall nanograting-based nanowire fabrication process. (a) Fabrication of a master nanograting template for nanowire fabrication which holds a nanograting pattern with 100 nm pitch, uniformly formed over a whole 8 in. Si wafer, with no discontinuity in lines. (b) Fabrication of multiple disposable plastic templates by transferring nanograting patterns on the master template to multiple plastic substrates. (c) Fabrication of highly ordered nanowire arrays composed of diverse materials, including single metal, dielectric, and ferroelectric materials. (d) Extraction of a diverse material nanowire network from the plastic disposable template.
Figure 2. Initial patterning for silicon master grating fabrication. (a) Conceptual illustration of a conventional and stitchless stepper lithography process. (b) Top-view SEM image of a poly-Si pattern made by the proposed method at shot-to-shot borders of the stepper lithography system. Shot 1 and shot 2 show how the left and right shots are placed and no pattern overlap or abnormal separation was observed. Shot 1 and shot 3 show how well the up and down shots are connected without discontinuity or being piled up. Shot colors are added on purpose. Shot 4 is empty (no actual shot) on purpose. The scale bar indicates 1 μm.
Here, we demonstrate an ultralong (up to 20 cm) nanowire array with a diameter of 50 nm (aspect ratio of up to 4 000 000:1), fabricated on an unprecedented large (8 in.) flexible substrate with perfect alignment. This is allowed by a top-down approach based on innovation of nanopatterning technology based on two key techniques: stitchless stepper lithography and repetitive pattern downscaling technology. On the basis of these two key techniques, we first achieved a very long (perfectly connected to 20 cm) nanograting pattern on a large area (8 in.) with resolution equivalent to electron beam lithography (100 nm pitch), as a master template. Using this high-density, ultralong, and large-area nanograting template, we developed a fast and effective fabrication method for achieving highly ordered nanowire arrays composed of diverse materials, and demonstrated a wire grid polarizer as a practical application in optical perspectives.
An overview of the nanograting-template-based nanowire fabrication process is given in Figure 1. We first prepare a nanograting pattern over an 8 in. silicon wafer as a master template based on stitchless stepper lithography and repetitive pitch downscaling technology (see below), as shown in Figure 1a. As noted above, 100 nm pitch nanograting patterns are formed over the surface area of the entire 8 in. wafer (the longest line reaches 20 cm in length without disconnection). Nanograting patterns on the Si master template are then transferred to multiple plastic substrates to fabricate multiple identical disposable templates (Figure 1b). Finally, nanowires made of diverse materials are fabricated on the plastic disposable template by simply depositing a target material using a well-known physical vapor deposition (PVD) process and an additional wet etching process, if necessary. As parts c and d of Figure 1 show, both randomly networked and 3979
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Figure 3. Pattern downscaling for silicon master grating. (a) Conceptual illustration of one cycle of pattern downscaling and recovery. (b) SEM images at each step of a single pattern scaling process cycle. Each panel in part b corresponds to that in part a. (c) Top-view SEM photographs of the nanograting pattern, where pitch is gradually reduced as pattern scaling cycle is repeated. From the top, the pattern pitches are 400, 200, and 100 nm, respectively. (d) Optical photograph of the 8 in. Si nanograting master template with 100 nm pitch fabricated by the proposed method. No stitch line was observed. The cross section image of the fabricated master template is shown in the inset. All scale bars in SEM photographs indicate 500 nm.
connected. Shot 1 and shot 2 show how the left and right shots are placed, and no pattern overlap or abnormal separation was observed. Shot 1 and shot 3 display how well the up and down shots are connected without discontinuity or being piled up. Shot 4 intentionally contains no pattern. By repeating these stitchless shots on the entire 8 in. wafer, all line patterns in the whole 8 in. wafer can be perfectly connected. Next, to achieve a 100 nm resolution grating pattern from the KrF stepper lithography system, we had to further reduce the pattern pitch. Spacer lithography,32−34 a well-known pattern downscaling technology, can provide a solution to the problem of limited resolution of the KrF stepper lithography system. Using spacer lithography, we can reduce the pattern pitch by half from the initial pattern with larger dimensions. In conventional spacer lithography, however, pattern shape distortion after pitch reduction limits the pattern reduction process to being conducted only once (it is very difficult to perform this process twice to reduce the pattern pitch further; see Figure S1 in the Supporting Information). To solve this problem, we developed a special technique to recover the distorted spacer sidewall shape, enabling multiple pattern reduction by one-quarter or more (see section S2 in the Supporting Information). By inserting multiple dielectric protection layers between each polycrystalline Si layer on which the pattern is to be formed, we could recover the distorted pattern shape and continue the pitch downscaling task. Parts a and b of Figure 3 show the conceptual illustrations of the pattern downscaling and recovery process during one cycle, and SEM images corresponding to each of the steps
regularly ordered nanowire arrays can be fabricated through this final step. To fabricate the ultralong high-density nanograting master template, which is the heart of this work, we used a krypton fluoride (KrF) stepper lithography system, a conventional optical lithography tool capable of forming as small as 400 nm pitch patterns in a 2 cm by 3 cm field area. To achieve a disconnection-free 100 nm pitch grating pattern over the whole 8 in. wafer area using this KrF lithography system, two challenging issues had to be overcome. First, the limited field area (2 cm by 3 cm) of the stepper lithography system obstructs the formation of a seamless grating pattern over the whole wafer area. Also, although the KrF stepper lithography system provides the highest resolution (nearly 400 nm) among the existing high throughput top-down approaches, it still cannot directly form patterns with 100 nm grade resolution patterns due to diffraction limits. Because of the limited field area size of the conventional stepper lithography system (Figure 2a, inset), seamless field alignment, known as “stitching”, is required in the initial patterning step (line pitch is 400 nm) to generate a grating pattern that is continuously connected between each shot (Figure 2a). We controlled the stitching error to less than 30 nm during the initial patterning process, and thereby achieved almost perfect pattern connection between shots. Owing to this perfect field-to-field alignment between shots, seamless connection of the initial pattern was achieved, as shown in Figure 2b. Shot 1 through shot 4 in Figure 2b illustrate how the patterns in the adjacent shots are perfectly aligned and 3980
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nanowires, as shown in Figure 5a. As shown in Figure 5b, the metal (Al) nanowire array was uniformly formed on the whole area of the 8 in. nanograting disposable template. We fabricated various regularly ordered metal, semiconductor oxide, and ternary ferroelectric material nanowire arrays through simple sputtering or evaporation on the grating-patterned disposable template, as shown in Figure 5c. The greatest advantage of our method is that, once the target material is prepared, any kind of material can be formed into an ultralong nanowire array through a simple PVD process. In contrast with conventional chemical-synthesis-based bottom-up approaches, our method provides a generic way to fabricate diverse material nanowires through this simple thin film deposition process. Another advantage is that high-density nanowire arrays can be duplicated repeatedly without additional lithographic processing, after fabrication of the master nanograting template once. Although most metal nanowires show smooth morphologies, Al and some non-metals show coarse morphologies. These coarse morphologies seem to be attributed to the shadowing effect during the deposition process.41 The Al nanowire array with such a coarse morphology shows somewhat greater electrical resistivity (1.5 × 10−7 Ω·m) than that of the bulk Al (2.82 × 10−8 Ω·m). Nevertheless, we observed Al nanowire strings are well connected along their length without disconnection. Highly ordered metallic nanowire arrays are of particular interest for light polarizing devices. A periodic metal wire array on a transparent substrate polarizes incident light,42−44 transmitting the light wave with the electrical field oscillating perpendicular to the wires, while reflecting that with the electrical field oscillating parallel to the wire direction. Especially, Al is very well-known as the most suitable material because of its high reflectance and transmittance in TE and TM modes, respectively. We demonstrated a prototype of a wire grid polarizer (WGP) by depositing an Al thin film on a transparent plastic template (Figure 5d and e). Although the performance was not optimized for actual display devices, the obvious difference in the transmittance between TM and TE modes indicates that the fabricated WGP successfully polarized the incident light. The low transmittance for the TM mode is attributed to the excessive thickness of the Al film deposited on the patterned plastic template. By adjusting the Al nanowire thickness, the TM mode transmittance of the WGP can be improved. Because the pitch of the fabricated WGP (=100 nm) was much smaller than the shortest wavelength of visible light (∼400 nm), we could achieve a clear polarization characteristic over the whole visible light range. Furthermore, due to the perfect seamless stitching during the master template fabrication process, we were able to achieve a large area WGP wherein the ordered Al nanowires were fully connected through the whole 8 in. template. To the best of our knowledge, such a large area WGP prototype with a 100 nm pitch has not been reported previously. We believe that the present results are a considerable achievement in the WGP fabrication technology field, and demonstrate that the suggested approach will help make it possible to realize WGP devices that are applicable to large area information display devices. Along with the perfectly aligned nanowire arrays, randomly networked nanowires are also widely used in electronic applications.13,45 To provide the versatility of our approach for nanowire random networks, we extracted nanowires from the disposable templates by depositing additional sacrificial layers prior to the target nanowire material (see Figure S5 in
during one cycle, respectively. As shown in Figure 3b, the distorted pattern sidewall in panel iv is totally recovered in panel v. The full process of repetitive pattern downscaling and recovery process is shown in Figure S2 in the Supporting Information. Scanning electron microscope (SEM) images in Figure 3c show the progress of pattern downscaling, from the 400 nm pitch initial KrF lithography pattern to the final 100 nm pitch grating pattern. The periodicity of the grating pattern was perfectly maintained during the pattern scaling process. We then obtained a 100 nm pitch nanograting pattern perfectly connected in the entire 8 in. Si master template. An optical photograph of the fabricated 8 in. master template is shown in Figure 3d. No shot-to-shot borders were observed over the entire area thanks to the precise shot-to-shot connection of the pattern. In Figure 3d, the longest nanograting line is 20 cm long with 50 nm width, corresponding to a 4 000 000:1 aspect ratio. There are 2 000 000 lines in this template. To the best of our knowledge, 100 nm pitch grating patterns over such a large area with no pattern break-off have not been demonstrated yet with any of the conventional nanopatterning technologies, including photolithography,35,36 e-beam lithography,37,38 and interference lithography.39,40 We believe that this large-area nanopatterning technology also provides considerable advantages for nanoimprinting stamp fabrication. Once the nanograting master template is made, the grating pattern on the Si master template can be transferred to multiple plastic substrates. This pattern transfer enables multiple uses of the Si master template, maximizing the productivity and economics of the fabrication process. Through a simple nanoimprinting method (see Figure S3 in the Supporting Information), we successfully transferred the nanograting pattern on the Si master template to the same-sized 8 in. plastic substrate (Figure 4a). The lack of light scattering on the
Figure 4. Plastic disposable template. (a) Optical photograph of the plastic disposable template. (b) Top-view SEM photograph of the transferred nanograting pattern on the disposable template. (c) Crosssectional view of the SEM image of the nanograting pattern in the plastic template. All scale bars indicate 500 nm.
surface of the disposable template indicates that the pitch of the transferred pattern, which is much shorter than the wavelength of visible light, is well maintained. As shown in Figure 4b and c, the nanograting pattern on the master template was successfully duplicated and transferred onto the plastic substrate. The wavy pattern degradation in Figure 4c is because of electron charging and local heating during the SEM observation. On the basis of the flexible disposable template, regularly ordered ultralong nanowire arrays were formed by simple PVD and wet etching processes. When target materials are deposited on the plastic template, the pattern valleys are shadowed by the peaks, leading to a shadowing effect (see Figure S4 in the Supporting Information). Consequently, the growth of the deposited materials in the valleys is suppressed relative to that at the peaks. During etching in a solution, the deposited films at the valleys are etched out much earlier than the films at the peaks, separating the thin films into regularly ordered individual 3981
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Figure 5. Fabrication of the highly ordered nanowire array and its wire grid polarizer (WGP) application. (a) Cross-sectional view of the SEM image of the Cu nanowire array through the simple PVD process (top) and after an additional wet etching process (bottom). (b) Optical photograph of the ultralong Al nanowire array uniformly formed on the 8 in. plastic disposable template. (c) Regular nanowire arrays made of diverse materials, including single metal, dielectric, and ternary-element ferroelectric materials. (d) Transmittance spectrum of the fabricated WGP for TM and TE modes. (e) Optical photograph of the wire grid polarizer transmitting TM mode (left) and blocking TE mode (right), respectively. The backside image displayed on an LCD monitor was shown through a circular open region in the plastic grating plate where there was a defect in the Al thin film. All scale bars indicate 500 nm.
Figure 6. Extracted random network nanowire extraction. (a) SEM image of the Au nanowire random network extracted from the plastic disposable template. The scale bar indicates 500 nm. (b) HRTEM image of the sputter-deposited Au nanowire segment. The d-spacing between (111) planes of 2.338 Å implies that the lattice parameter of the Au nanowire is 4.050 Å, which is similar to that of bulk Au. The scale bar indicates 2 nm. (c) Optical photograph of the fabricated Cu nanowire arrays being extracted from the 4 cm long disposable template.
the Supporting Information). The diameter of the randomly distributed nanowire can be controlled by adjusting the wet etching time for the target nanowire material (see Figure S6 in the Supporting Information). Parts a and b of Figure 6 show the SEM and high-resolution transmission electron microscope (HRTEM) analysis results of the randomly distributed Au nanowire network. From the HRTEM and its fast Fourier transform images shown in Figure 6b, we can see the polycrystalline structure of the Au nanowires, which is an inherent feature of sputter-deposited metal films. It would be a good further research to improve the crystalline structures so that the electrical conductivity would be comparable with that of many bottom-up grown nanowires. The d-spacing between the (111) planes (=2.338 Å) indicates that the lattice parameter of the Au nanowire is 4.050 Å, which is similar to that of bulk Au. An example image of centimeter-long metal (Cu) nanowires being extracted from the disposable template is provided in Figure 6c.
Transferring a nanowire array maintaining its perfect regularity is an important stream of research trend.46,47 From the nanograting template, nanowires can be peeled off and transferred to other substrates with its regularity maintained (see section S6 in the Supporting Information). This transferring technique would facilitate deploying such long nanowires regardless of the substrate. In summary, we have developed a fast and cost-effective methodology for fabrication of ultralong nanowires from a large-area nanograting master template prepared by a novel top-down approach. As a practical application, a large-area Al wire grid polarizer was suggested and demonstrated. With the proposed ultralong nanowire fabrication technology, length, material limitations, and difficulties in regular ordering of the nanowire arrays, which have been major issues in conventional bottom-up approaches, can now be overcome. The inherent problems of the conventional high-resolution top-down approaches, including low throughput, high cost, and complex patterning processes, can also be resolved. By providing 3982
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(6) Yan, R.; et al. Nanowire-based single-cell endoscopy. Nat. Nanotechnol. 2012, 7, 191−196. (7) Yan, R.; Gargas, D.; Yang, P. Nanowire photonics. Nat. Photonics 2009, 3, 569−576. (8) Johnson, J. C.; et al. Single gallium nitride nanowire lasers. Nat. Mater. 2002, 1, 106−110. (9) Qin, Y.; Wang, X.; Wang, Z. L. Microfibre−nanowire hybrid structure for energy scavenging. Nature 2008, 451, 809−813. (10) Xu, S.; et al. Self-powered nanowire devices. Nat. Nanotechnol. 2010, 5, 366−373. (11) Yuhas, B. D.; Yang, P. Nanowire-based all-oxide solar cells. J. Am. Chem. Soc. 2009, 131, 3756−3761. (12) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Crystalline silver nanowires by soft solution processing. Nano Lett. 2002, 2, 165−168. (13) Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Solutionprocessed metal nanowire mesh transparent electrodes. Nano Lett. 2008, 8, 689−692. (14) Greene, L. E.; et al. Low-temperature wafer-scale production of ZnO nanowire arrays. Angew. Chem., Int. Ed. 2003, 42, 3031−3034. (15) Xing, Y. J.; et al. Optical properties of the ZnO nanotubes synthesized via vapor phase growth. Appl. Phys. Lett. 2003, 83, 1689− 1691. (16) Wan, Q.; et al. Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors. Appl. Phys. Lett. 2004, 84, 3654−3656. (17) Thurn-Albrecht, T.; et al. Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates. Science 2000, 290, 2126−2129. (18) Yin, A. J.; Li, J.; Jian, W.; Bennett, A. J.; Xu, J. M. Fabrication of highly ordered metallic nanowire arrays by electrodeposition. Appl. Phys. Lett. 2001, 79, 1039−1041. (19) Singh, N.; et al. Si, SiGe nanowire devices by top-down technology and their applications. IEEE Trans. Electron Devices 2008, 55, 3107−3118. (20) Jeon, K. J.; Lee, J. M.; Lee, E.; Lee, W. Individual Pd nanowire hydrogen sensors fabricated by electron-beam lithography. Nanotechnology 2009, 20, 135502. (21) Im, Y.; et al. Investigation of a single Pd nanowire for use as a hydrogen sensor. Small 2006, 2, 356−358. (22) Chen, S.; et al. Dense and vertically-aligned centimetre-long ZnS nanowire arrays: ionic liquid assisted synthesis and their field emission properties. Nanoscale 2012, 4, 2658. (23) Park, W. I.; Zheng, G.; Jiang, X.; Tian, B.; Lieber, C. M. Controlled synthesis of millimeter-long silicon nanowires with uniform electronic properties. Nano Lett. 2008, 8, 3004. (24) Sun, J.-L.; Xu, J.; Zhu, J.-L. Oxidized macroscopic-long Cu nanowire bundle photoconductor. Appl. Phys. Lett. 2007, 90, 201119. (25) Lin, D.; Pan, W.; Wu, H. Morphological control of centimeter long aluminum-doped zinc oxide nanofibers prepared by electrospinning. J. Am. Ceram. Soc. 2007, 90, 71−76. (26) Yu, K.; et al. Growth and optical applications of centimeter-long ZnO nanocombs. Nano Res. 2008, 1, 221−228. (27) Zhai, T.; et al. Centimeter-long V2O5 nanowires: From synthesis to field-emission, electrochemical, electrical transport, and photoconductive properties. Adv. Mater. 2010, 22, 2547−2552. (28) Suryavanshi, A. P.; Hu, J.; Yu, M.-F. Meniscus-controlled continuous fabrication of arrays and rolls of extremely long micro- and nano-fibers. Adv. Mater. 2008, 20, 793−796. (29) Zhang, X.; Ma, Z.; Yuan, Z.-Y.; Su, M. Mass-productions of vertically aligned extremely long metallic micro/nanowires using fiber drawing nanomanufacturing. Adv. Mater. 2008, 20, 1310−1314. (30) Deng, D. S.; et al. Processing and properties of centimeter-long, in-fiber, crystalline-selenium filaments. Appl. Phys. Lett. 2010, 96, 023102. (31) Yaman, M.; et al. Arrays of indefinitely long uniform nanowires and nanotubes. Nat. Mater. 2011, 10, 494−501. (32) Choi, Y.-K.; Zhu, J.; Grunes, J.; Bokor, J.; Somorjai, G. A. Fabrication of sub-10-nm silicon nanowire arrays by size reduction lithography. J. Phys. Chem. B 2003, 107, 3340−3343.
extreme length, high throughput, and versatile material choice, the proposed approach will further extend the application fields of nanowires, from scientific research to engineering and industrial fields. On the basis of these advantages, we believe that our nanowire fabrication method can contribute to major advances in conventional application fields, including sensors, electronics, photonics, and bioscience. We also hope that our approach can accelerate the application of nanowires in commercial and industrial fields.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental process, SEM images, and nanowire diameter controlling plot. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
J.Y. and Y.J.L. contributed equally to this work. J.Y., Y.J.L., and J.-B.Y. conceived the fundamental idea, and J.Y. and Y.J.L. performed the experimental work. D.E.Y. designed the process parameters and coordinated the overall nanograting fabrication process. J.O.L. designed the fabrication process, and S.-J.C. contributed to the data analysis. G.-W.Y. participated in the nanowire extraction process. K.J.Y., J.S.K., and J.L. proposed the concept of nanowire fabrication from the nanograting template and solved critical problems during the process. D.W.L., G.S.L., and H.C.H. contributed to the nanograting fabrication process. J.-B.Y. inspired the research with guidance and participated in the data analysis. The manuscript was written by J.Y. and J.-B.Y. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the engineers of the National NanoFab Center, especially Jin-Su Kim and Jae-Sub Oh, for their helpful advice and comments on the fabrication process. We also thank the engineers of the LG Innotek co., Ltd., especially Yong-In Lee, Nam-Yang Lee, Kyung-Jun Lee, and KiChul Han, for giving fruitful comments and advice about the research directions. This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2011-0028781).
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