Nano Patterning

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ARTICLESPUBLISHED ONLINE: 26 SEPTEMBER 2010 | DOI: 10.1038/NNANO.2010.175

Direct nanoprinting by liquid-bridge-mediated nanotransfer mouldingJae K. Hwang1, Sangho Cho1, Jeong M. Dang1, Eun B. Kwak1, Keunkyu Song2, Jooho Moon2 and Myung M. Sung1 *Several techniques for the direct printing of functional materials have been developed to fabricate micro- and nanoscale structures and devices. We report a new direct patterning method, liquid-bridge-mediated nanotransfer moulding, for the formation of two- or three-dimensional structures with feature sizes as small as tens of nanometres over large areas up to 4 inches across. Liquid-bridge-mediated nanotransfer moulding is based on the direct transfer of various materials from a mould to a substrate through a liquid bridge between them. We demonstrate its usefulness by fabricating nanowire eldeffect transistors and arrays of pentacene thin-lm transistors.

T

he fabrication of micro- and nanoscale structures is essential for electronics1, micro/nanoelectromechanical systems24, biological and chemical sensors58, microuidics912, display units, and optoelectronic devices13. Of existing patterning methods, the direct printing of functional materials is the most efcient method for the fabrication of new types of structures and devices at low cost and low environmental impact. Direct printing includes a number of non-photolithographic techniques that directly transfer the functional materials to the substrates: ink-jet printing14, screen printing15, exographic printing16, gravure printing17,18, offset printing1921, and microtransfer moulding2227. Microtransfer moulding is the most versatile and cost-effective method for the fabrication of functional microstructures over a large area, but it suffers from problems such as poor edge resolution (due to the lateral diffusion of the liquid inks), residues between patterns, and difculty in multi-alignment. Several alternative residue-free direct printing methods have been developed for patterning at the nanoscale, such as nanoimprint lithography2832, capillary force lithography33,34, and nanotransfer printing28,3539. Recently, nanoimprint lithography and capillary force lithography have been used with selective dewetting to fabricate residue-free patterns of functional polymers. However, imprinting methods suffer from residues and difculty in multi-alignment. Nanotransfer printing is based on the adhesive transfer of a patterned metal thin lm from a stamp to a substrate with tailored surface chemistries3537, but it also suffers from problems. For instance, it only works with a limited number of materials (mainly metals), it only works in a small range of processing conditions, and continuous operation can be difcult because vacuum conditions are required. We have developed a direct printing technique that is based on a liquid-bridge-mediated transfer moulding process. The polar liquid layer serves as an adhesion layer that provides good conformal contact between the functional materials and the substrate38,39. Unlike microtransfer moulding, our technique is not subject to surface diffusion and can generate complex nanostructures with minimum feature sizes below 60 nm with an edge resolution of 26 nm. The new technique allows two- or three-dimensional complex nanostructures to be directly fabricated over a large area using many types of inks.1

Liquid-bridge-mediated transfer mouldingFigure 1 illustrates the procedure for patterning functional materials using liquid-bridge-mediated nanotransfer moulding (LB-nTM). In a rst step, patterned hard and soft moulds were fabricated by using polyurethane acrylate (PUA) and polydimethylsiloxane (PDMS), respectively. These two materials have very low surface free energies (PUA, 25 mJ m22, PDMS, 20 mJ m22). The patterned mould was then lled with an ink solution using selective inking. Discontinuous dewetting40 was used to ll only the recessed areas of the mould with the ink solution. By dragging a deposited ink solution over the patterned mould with a glass stick or a needle, the meniscus of the ink solution moves over the surface of the mould to ll inside the features without leaving any residues on the raised surface (Fig. 1b). Discontinuous dewetting takes advantage of the interfacial free energy between the mould and the ink solution, and the ink solution must have a surface free energy (between 30 mJ m22 and 70 mJ m22) appropriate to the PDMS and PUA moulds. The rate of dragging the solution, the aspect ratio of the features in the mould (depth/width 1/20), and the viscosity of the ink solution (,500 cP) also determine the success of the discontinuous dewetting process40. The lled ink is next solidied by drying it at mild temperatures (,100 8C). Almost no residue remains on the protruding surfaces of the mould as a result of the selective inking (Fig. 1c). The very small amount of excess residue can be removed by application of a brisk stream of nitrogen, because the mould has a very low surface free energy. The absence of residue was conrmed by analysis of the patterns using energy dispersive X-ray analysis and a cross-sectional view obtained by means of scanning electron microscopy (SEM; Supplementary Fig. S1). Because of the solidication of the ink solution, LB-nTM does not suffer from surface diffusion and can generate nanostructures on a scale well below 100 nm. The mould with the solidied ink was then brought into contact with a substrate surface covered by a thin polar liquid layer. A substrate of area 1 1 cm2 can be uniformly covered with a 100-mmthick liquid layer by using 10 ml of a polar liquid. The polar liquid layer on the substrate forms a liquid bridge (a capillary bridge) between the substrate and a mould containing recessed patterns (Fig. 1d). The liquid bridge allows good conformal contact between the solidied ink and the substrate38. The substrate must

Department of Chemistry, Hanyang University, Seoul 133-791, Korea, 2 Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea. *e-mail: [email protected] NANOTECHNOLOGY | VOL 5 | OCTOBER 2010 | www.nature.com/naturenanotechnology

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range of materials, and we have made various functional structures using many types of inks (liquid prepolymers, metal particle solutions, molecular precursors, and so on). It can also be used to fabricate nanometre-sized structures without leaving any residue on the regions of the substrates not to be coated. In contrast to other direct patterning methods using liquid inks, such as microtransfer moulding and gravure printing, here, the lled inks are solidied before transfer onto the substrate to prevent lateral diffusion. The nanometre-sized patterns can be made on diverse substrates as long as their surface free energies are high enough to exhibit strong capillary action with a polar liquid layer. In fact, by using LB-nTM with UV activation of the substrates, complex structures can be patterned on various substrates including silicon, TiO2 , polyethersulphone (PES) and gold (Supplementary Fig. S2). LB-nTM can be used to create complex two- or three-dimensional nanostructures over a large area in a repetitive, continuous process. The mould can be aligned easily on complex structures because, before the polar liquid layer is dried, it acts as an adhesive lubricant, enabling the mould to be moved over the substrate. Furthermore, deformation and distortion of the polymer mould can result in errors in the replicated patterns, as well as misalignment of the patterns. Such problems are difcult to correct in direct printing methods because the pattern transfer occurs immediately at the time of contact. In the LB-nTM method, however, the position of the mould can be adjusted even after contact with the substrate, because the pattern is not transferred to the substrate before drying of the liquid layer.

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Nanoscale patternsNanometre-scale patterns of various materials were made on silicon substrates using the LB-nTM method with hard moulds (PUA). The masters used for fabrication of the moulds were silicon wafers with dense nanoscale patterns, which were made by laser interference lithography and subsequent dry etching steps, as described previously43. The moulds were fabricated by casting PUA on them. After UV curing, the PUA moulds were peeled away from the masters. To pattern an array of zinctin oxide (ZTO) on a nanometre scale, the recessed spaces of the patterned PUA moulds were lled with a 2-methoxyethanol solution of ZTO ink. The ZTO ink solution in the mould was solidied at 80 8C for 10 min. The mould was then placed in contact with an oxidized Si(100) substrate covered by a thin ethanol layer. Following drying of the ethanol layer between the mould and the substrate at 70 8C for 10 min, the mould was peeled away, leaving the ZTO nanopatterns on the substrate. Scanning electron microscopy (SEM) images of the representative structures formed in this manner are shown in Fig. 1, including the PUA mould (Fig. 1a), the mould lled with ZTO ink (Fig. 1c) and the ZTO patterns fabricated on the substrate (Fig. 1e). SEM images of the ZTO patterns fabricated using the PUA mould (140-nm-wide parallel lines, 60-nm-wide spaces) clearly show that the ZTO patterns retain the x and y dimensions of the mould, as shown in Fig. 2a. The height of each ZTO pattern, however, is 54 nm, which means that it is reduced by 46% in the z-direction when compared to the depth of the mould (100 nm). Figure 2b shows an SEM image of ZTO dots with widths of 165 nm and depths of 100 nm that were fabricated using the PUA mould. The height of the ZTO dot is reduced to 50 nm, but it retains a width of 165 nm. Nanoscale lines and dots of silver and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) were also made using LB-nTM with PUA moulds (silver lines: line, 95 nm, space, 105 nm, height, 200 nm; silver dots: width, 245 nm, depth, 200 nm; TIPS-PEN lines: lines, 105 nm, space, 95 nm, height, 200 nm; TIPS-PEN dots: width, 150 nm, depth, 200 nm). The silver and TIPS-PEN ink solutions lling the moulds were solidied at 70 8C and 90 8C, respectively, for 10 min. The moulds with the solidied inks were then placed in contact with the silicon substrates covered by thin ethanol layers, which were dried at 70 8C. The silver and743

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Figure 1 | Liquid-bridge-mediated nanotransfer moulding. a, Schematic illustration of LB-nTM. b, SEM image of the PUA mould. c, SEM image of the mould lled with ZTO ink. d, Schematic illustration of a liquid bridge formed by a polar liquid layer between a solidied ink and a substrate. e, SEM image of ZTO patterns on a silicon substrate.

have a high surface free energy ( 40 mJ m22) to exhibit strong capillary action as the two surfaces come into contact with the polar liquid layer. As the liquid evaporates, the attractive capillary force gradually increases, pulling the two surfaces into contact, providing good conformal contact between them with no additional pressure to the mould. The majority of the liquid layer initially evaporates through the open sides between the mould and the substrate; the remainder, which is conned in the features, is absorbed or evaporates and permeates through the mould41,42. According to our experiment, the PUA mould can absorb 5.6% of its weight in ethanol at 70 8C. About 99 wt% of the absorbed ethanol diffuses into the air in 10 min at 70 8C (Supplementary Table S1). After drying, the separation of the mould from the substrate then results in the formation of the patterns. There are several reasons why LB-nTM is well suited for use in automated printing machines. First, it can be applied to a wide

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Figure 2 | SEM images of nanoscale patterns (white) of different materials fabricated by LB-nTM on silicon substrates (black). a, ZTO line pattern (width, W, 60 nm; spacing between features, S, 140 nm; height, H, 54 nm). b, ZTO dot pattern (W, 165 nm; H, 54 nm). c, Silver line pattern (W, 105 nm; S, 95 nm; H, 123 nm). d, Silver dot pattern (W, 245 nm; H, 130 nm). e, TIPS-PEN line pattern (W, 95 nm; S, 105 nm; H, 145 nm). f, TIP-PEN dot pattern (W, 150 nm; H, 140 nm).

TIPS-PEN patterns prepared in this manner are shown in Fig. 2cf, which clearly shows that the patterns retain the x and y dimensions of the moulds. When compared to the sizes of the moulds, the z dimensions of the silver and TIPS-PEN patterns are reduced by 35% and 30%, respectively. All these results indicate that the patterns fabricated using LB-nTM retain the x and y dimensions of the moulds and are only reduced in the z-direction (height) because the ink solutions locked inside the features are solidied. The shrinkage of the pattern height mainly depends on the concentration and composition of the ink solutions. The ZTO and silver patterns exhibit strong adhesion to the substrate surface and, thus, easily pass Scotch tape adhesion tests. TIPS-PEN, which has low surface free energy, the adhesion of the patterns is not as strong as those of ZTO and silver.

Microscale patternsMicrometre-scale patterns of various materials were made on silicon substrates using the LB-nTM method but with soft moulds (PDMS). The masters used for mould fabrication were silicon wafers with patterned resists on scales from 2.5 to 200 mm. The moulds were fabricated by casting PDMS on the masters. After curing, the PDMS moulds were peeled away from the masters. The PDMS moulds were then lled with the ZTO, silver and TIPS-PEN ink solutions. The solidied inks in the moulds were transferred to the silicon substrate surface by the liquid-bridge-mediated transfer process. Figure 3 presents various patterns with micrometre-scale744

features formed by applying LB-nTM with PDMS moulds. Figure 3a,b shows SEM images of ZTO patterns fabricated using masters having 9-mm-wide parallel lines with 11-mm-wide spaces and complex features (3150 mm). These images clearly show that the transferred patterns retain the features of the masters. Micrometre-scale line and complex patterns with silver and TIPSPEN are also shown in Fig. 3cf, demonstrating that they are also fabricated with high pattern delity and structural integrity. Multilayer structures formed from silver patterns were formed by consecutive printing of silver line patterns on pre-patterned substrates using LB-nTM (Fig. 3g). The PDMS mould with silver ink was placed in contact with the silicon substrate, which had already been patterned with one layer of silver line patterns. The mould was rotated 908 with respect to the direction of the rst line patterns. A thin ethanol layer on the pre-patterned silicon substrate is able to produce strong capillary action as the mould comes into contact with it. As the ethanol layer evaporates, the attractive capillary force strongly pulls the solidied silver ink into contact with the substrate and provides good conformal contact between them (Fig. 3h). Through additional printing steps, many more layered structures can be added (Supplementary Fig. S3).

Nanowire eld-effect and TIPS-PEN thin-lm transistorsNanowire eld-effect transistors were fabricated on silicon substrates using LB-nTM, as described in Fig. 4a. Nanometre-scale line patterns of ZTO where the lines have a width of 60 nm and

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Figure 3 | SEM images of microscale patterns (white) of different materials fabricated by LB-nTM on silicon substrates (black). a, ZTO line pattern (W, 9 mm; S, 11 mm; H, 430 nm). b, ZTO complex pattern. c, Silver line pattern (W, 11 mm; S, 9 mm; H, 550 nm). d, Silver isolated pattern (10200 mm). e, TIPS-PEN line pattern (W, 9.5 mm; S, 10.5 mm; H, 500 nm). f, TIPS-PEN complex pattern. g,h, Image of a two-layer silver nanopattern (g) with and expanded view (h).

length of 10 mm were created on heavily doped silicon substrates comprising 200-nm-thick SiO2 by applying LB-nTM with PUA moulds. The ZTO nanowire arrays were then annealed at 500 8C in air to achieve complete thermal decomposition of any organic residues and metal salts44,45. Finally, source and drain electrodes composed of 200-nm-thick silver were dened on the substrate by LB-nTM using PDMS moulds. ZTO-nanowire eld-effect transistors were thus obtained, with metal contacts functioning as source and drain electrodes and the silicon substrate as a back-gate. Figure 4b shows SEM images of eld-effect transistors with from 1 to 100 ZTO nanowires. The width of the ZTO nanowire narrowed to 55 nm, indicating 23% shrinkage in volume, which was due to the annealing at 500 8C in air. A post-annealing step at 200 8C under an atmosphere of hydrogen and nitrogen was performed to improve the electrical performance of the transistors before carrying

out measurements44,45. Figure 4c,d presents the typical drain currentgate voltage (IDVG) transfer curves and drain current drain voltage (IDVD) output curves from the eld-effect transistors with 10 ZTO nanowires. The ZTO-nanowire eld-effect transistors were well modulated, depending on the gate voltage, and exhibited clear saturation behaviour with a eld-effect mobility of 0.4 cm2 V21 s21, an on/off current ratio of 1 106 and a threshold voltage of 5 V. This performance is comparable to ZTO thin-lm transistors fabricated by spin-coating with the same ZTO solution44 (Supplementary Fig. S4). Arrays of TIPS-PEN thin-lm transistors were fabricated on 4-inch PES substrates using LB-nTM using PDMS moulds (Fig. 5a). An inverted staggered structure was used in the fabrication of the thin-lm transistor device. A 150-nm-thick indium-tin oxide (ITO) gate electrode and a 200-nm-thick SiO2 dielectric layer were745

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NATURE NANOTECHNOLOGYc1 105 1 106 1 07 1 108 ID (A) 1 109 1 10 Source Ag metal Drain10

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Figure 4 | ZTO nanowire eld-effect transistors. a, Schematic diagram of the procedure for fabricating ZTO nanowire FETs using LB-nTM. b, SEM images of ZTO nanowire FETs. c, Transfer curve for a ZTO nanowire FET. Drain current ID is plotted as a function of gate voltage VG on a linear scale (red, right axis) and a logarithmic scale (blue, left axis). Drain voltage VD is 40 V. d, Output curves for a ZTO nanowire FET. Drain current is plotted as a function of drain voltage for different values of the gate voltage.

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Figure 5 | TIPS-PEN thin-lm transistors. a, Photograph and SEM images of TIPS-PEN thin-lm transistors. b, Transfer curve for a TIPS-PEN thin-lm transistor on a linear scale (red, right axis) and a logarithmic scale (blue, left axis). VD 250 V. c, Output curves for a TIPS-PEN thin-lm transistor.746NATURE NANOTECHNOLOGY | VOL 5 | OCTOBER 2010 | www.nature.com/naturenanotechnology

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formed on a PES substrate by sputter deposition. An array of TIPSPEN patterns (thickness, 60 nm) acting as active channel layers was fabricated on the substrate using LB-nTM. The nominal channel length of the TIPS-PEN thin-lm transistor was 10 mm, and the channel width was 135 mm. Finally, the source and drain electrodes composed of 200-nm-thick silver were dened on the substrate using LB-nTM. Figure 5b,c displays the typical drain currentgate voltage (IDVG) transfer curves for VD 50 V and drain currentdrain voltage (IDVD) output curves for several gate voltages from our TIPS-PEN thin-lm transistors prepared on exible substrates. The maximum ID level was approximately 2 mA under a gate bias of 50 V. According to the transfer characteristics (IDVG) of Fig. 5b, a eld-effect mobility of 0.02 cm2 V21 s21 was achieved in the saturation regime of VD 50 V together with an on/off ratio of 1 105 and a threshold voltage of 13 V. In comparison, thin-lm transistors fabricated using the spin-coated TIPS-PEN46 showed a saturation mobility of 0.03 cm2 V21 s21 and an on/off ratio of 1 105 (Supplementary Fig. S5). The TIPS-PEN thinlm transistors can endure strenuous bending and are also transparent in the visible range (Fig. 5a), and therefore potentially useful for exible and invisible electronics.

Received 2 June 2010; accepted 21 July 2010; published online 26 September 2010

References1. Wallraff, G. M. & Hinsberg, W. D. Lithographic imaging techniques for the formation of nanoscopic features. Chem. Rev. 99, 18011822 (1999). 2. Yao, J. J. RF MEMS from a device perspective. J. Micromech. Microeng. 10, R9R38 (2000). 3. Walker, J. A. The future of MEMS in telecommunications networks. J. Micromech. Microeng. 10, R1R7 (2000). 4. Spearing, S. M. Materials issue in microelectromechanical systems (MEMS). Acta Mater. 48, 179196 (2000). 5. Dong, Y. & Shannon, C. Heterogeneous immunosensing using antigen and antibody monolayers on gold surfaces with electrochemical and scanning probe detection. Anal. Chem. 72, 23712376 (2000). 6. Lahiri, J., Isaacs, L., Tien, J. & Whitesides, G. M. A strategy for the generation of surfaces presenting ligands for studies of binding based on an active ester as a common reactive intermediate: a surface plasmon resonance study. Anal. Chem. 71, 777790 (1999). 7. Sirkar, K., Revzin, A. & Pishko, M. V. Glucose and lactate biosensors based on redox polymer/oxidoreductase nanocomposite thin lms. Anal. Chem. 72, 29302936 (2000). 8. Wells, M. & Crooks, R. M. Interactions between organized, surface-conned monolayers and vapor-phase probe molecules. 10. Preparation and properties of chemically sensitive dendrimer surfaces. J. Am. Chem. Soc. 118, 39883989 (1996). 9. Beebe, D. J. et al. Microuidic tectonics: a comprehensive construction platform for microuidic systems. Proc. Natl Acad. Sci. USA 97, 1348813493 (2000). 10. Beebe, D. J., Mensing, G. A. & Walker, G. M. Physics and applications of microuidics in biology. Annu. Rev. Biomed. Eng. 4, 261286 (2002). 11. Rossier, J., Reymond, F. & Michel, P. E. Polymer microuidic chips for electrochemical and biochemical analyses. Electrophoresis 23, 858867 (2002). 12. Becker, H. & Gartner, C. Polymer microfabrication methods for microuidic analytical applications. Electrophoresis 21, 1226 (2000). 13. Maes, H. E. et al. Trends in microelectronics, optical detectors, and biosensors. Adv. Eng. Mater. 3, 781787 (2001). 14. Sirringhaus, H. et al. High-resolution inkjet printing of all-polymer transistor circuits. Science 290, 21232126 (2000). 15. Pardo, D. A., Jabbour, G. E. & Peyghambarian, N. Application of screen printing in the fabrication of organic light-emitting devices. Adv. Mater. 12, 12491252 (2000). 16. Harri, L. Microscopic studies of the inuence of main exposure time on parameters of exographic printing plate produced by digital thermal method. Microsc. Res. Tech. 72, 707716 (2009). 17. Kopola, P., Tuomikoski, M., Suhonen, R. & Maaninen, A. Gravure printed organic light emitting diodes for lighting applications. Thin Solid Films 517, 57575762 (2009). 18. Kittila, M., Hagberg, J., Jakku, E. & Leppavuori, S. Direct gravure printing (DGP) method for printing ne-line electrical circuits on ceramics. IEEE Trans. Electron. Packag. Manuf. 27, 109114 (2004). 19. Pudas, M., Hagberg, J. & Leppavuori, S. Printing parameters and ink components affecting ultra-ne-line gravure-offset printing for electronics applications. J. Eur. Ceram. Soc. 24, 29432950 (2004). 20. Zielke, D. et al. Polymer-based organic eld-effect transistor using offset printed source/drain structures. Appl. Phys. Lett. 87, 123508 (2005). 21. Pudas, M., Hagberg, J. & Leppavuori, S. Roller-type gravure offset printing of conductive inks for high-resolution printing on ceramic substrates. Int. J. Electron. 92, 251269 (2005). 22. Zhao, X.-M., Xia, Y. & Whitesides, G. M. Fabrication of three-dimensional micro-structures: microtransfer molding. Adv. Mater. 8, 837840 (1996). 23. Yang, H., Deschatelets, P., Brittain, S. T. & Whitesides, G. M. Fabrication of high performance ceramic microstructures from a polymeric precursor using soft lithography. Adv. Mater. 13, 5458 (2001). 24. Leung, W. Y. et al. Fabrication of photonic band gap crystal using microtransfer molded templates. J. Appl. Phys. 93, 58665870 (2003). 25. Thibault, C., Severac, C., Trevisiol, E. & Vieu, C. Microtransfer molding of hydrophobic dentrimer. Microelectron. Eng. 83, 15131516 (2006). 26. Kim, M. J., Song, S. & Lee, H. H. A two-step dewetting method for large-scale patterning. J. Micromech. Microeng. 16, 17001704 (2006). 27. Kraus, T. et al. Nanoparticle patterning with single-particle resolution. Nature Nanotech. 2, 570576 (2007). 28. Gates, B. D. et al. New approaches to nanofabrication: molding, printing, and other techniques. Chem. Rev. 105, 11711196 (2005). 29. Guo, L. J. Nanoimprint lithography: methods and material requirements. Adv. Mater. 19, 495513 (2007). 30. Rolland, J. P. Direct fabrication and harvesting of monodisperse, shape-specic nanobiomaterials. J. Am. Chem. Soc. 127, 1009610100 (2005). 31. Yang, K.-Y., Yoon, K.-M., Choi, K.-W. & Lee, H. The direct nano-patterning of ZnO using nanoimprint lithography with ZnO-sol and thermal annealing. Microelectron. Eng. 86, 22282231 (2009).747

ConclusionsWe have reported a direct printing method that is based on the transfer of various materials from a mould to a substrate via a liquid bridge between them. Ink solution in the mould is solidied and transferred onto a substrate via a liquid bridge between the mould and the substrate. The mould can be aligned easily on complex structures, because it is movable on the substrate before the polar liquid layer is dried, which acts as an adhesive lubricant. This procedure is well suited for use in automated direct printing machines and is capable of generating patterns of various functional materials with a wide range of feature sizes on diverse substrates.

MethodsMaterials. Unless otherwise noted, all commercial materials were obtained from Aldrich Chemical Co. and used without further purication. TIPS-PEN was synthesized following the procedure reported by Anthony et al.47,48, and the resulting crude product was puried using chromatography on silica gel, rst eluting the excess (triisopropylsily)acetylene with hexane and then eluting a deep blue band with 90% hexane and 10% dichloromethane. The TIPS-PEN ink solution was prepared by dissolving 2 wt% TIPS-PEN in tetralin solvent. The ink solution for printing the ZTO semiconductor was prepared by dissolving zinc acetate dehydrate [Zn(CH3COO)2.2H2O] and tin acetate [Sn(CH3COO)2] in 2-methoxyethanol44,45. The silver nanoparticle ink (DGP 40LT-15C) was purchased from Advanced Nano Products. The ink contained 20 wt% silver nanoparticles, with particle diameters of 4050 nm, dispersed in methanol solvent. Polyurethane acrylate (MINS-ERM, Minuta Tech.) was used to prepare the UV-curable hard moulds. Polydimethylsiloxane (Sylgard 184) was ordered from Dow Corning. Preparation of substrates. The silicon substrates used in this research were cut from n-type (100) wafers with resistivity in the range 15 V.cm. The silicon substrates were initially treated by a chemical cleaning process, which involved degreasing, HNO3 boiling, NH4OH boiling (alkali treatment), HCl boiling (acid treatment), rinsing in deionized water and blow-drying with nitrogen, as proposed by Ishizaka and Shiraki, to remove contaminants49. A thin oxide layer was grown by placing the silicon substrate in a piranha solution (4:1 mixture of H2SO4:H2O2) for 1015 min. The substrate was rinsed several times in deionized water (resistivity 18 MV.cm), then dried with a stream of nitrogen. The exible substrates used in this study were cut from Glastic PES lms (i-components Inc.). The PES substrates were cleaned with methanol and deionized water, and nally blow-dried with nitrogen to remove the contaminants. Analysis techniques. The samples were analysed using a Hitachi S4800 SEM. Water contact angles of the samples were determined on a model A-100 Rame-Hart NRL goniometer in ambient air by using the sessile drop method. All currentvoltage (IV ) properties of the eld-effect transistors and thin-lm transistors were measured with a semiconductor parameter analyser (HP 4155C, Agilent Technologies), and CV measurements were made using a capacitance meter (HP 4284 LCR meter, Agilent Technologies, 1 MHz) in the dark and in air ambient (relative humidity, 45%) at 20 8C.

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DOI: 10.1038/NNANO.2010.175

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AcknowledgementsThis work was supported by the National Research Foundation of Korea (2009-0092807; 2010-0019125; 2009-0086302), the Seoul R&BD programme (ST090839), the IT R&D program of MKE/KEIT (10030559) and the Korea Research Foundation (KRF-2007-313-C00383).

Author contributionsM.M.S. conceived and designed the experiments. J.K.H., E.B.K., S.C. and J.M.D. performed the experiments. K.S. and J.M. contributed to materials and analysis. S.C. and M.M.S. co-wrote the paper.

Additional informationThe authors declare no competing nancial interests. Supplementary information accompanies this paper at www.nature.com/naturenanotechnology. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressed to M.M.S.

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