Polymer Composite

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Heterogeneous Configuration of a Ag Nanowire/Polymer Composite Structure for Selectively Stretchable Transparent Electrodes Youngmin Kim,† Sungwoo Jun,†,‡ Byeong-Kwon Ju,‡ and Jong-Woong Kim*,† †

Display Materials & Components Research Center, Korea Electronics Technology Institute, 68 Yatap-dong, Bundang-gu, Seongnam 463-816, South Korea ‡ Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 136-713, Korea S Supporting Information *

ABSTRACT: One of the most important aspects that we need to consider in the design of intrinsically stretchable electrodes is that most electronic devices that can be formed on them are not stretchable themselves. This discrepancy can induce severe stress singularities at the interfaces between stiff devices and stretchable electrodes, leading to catastrophic device delamination when the substrate is stretched. Here, we suggest a novel solution to this challenge which involves introducing a photolithography-based rigid-island approach to fabricate the heterogeneous configuration of a silver nanowire (AgNW)/polymer composite structure. For this, we designed two new transparent polymers: a photopatternable polymer that is rigid yet flexible, and a stretchable polymer, both of which have identical acrylate functional groups. Patterning of the rigid polymer and subsequent overcoating of the soft polymer formed rigid island disks embedded in the soft polymer, resulting in a selectively stretchable transparent film. Strong covalent bonds instead of weak physical interactions between the polymers strengthened the cohesive force at the interface of the rigid/soft polymers. Inverted-layer processing with a percolated AgNW network was used to form a heterogeneous AgNW/polymer composite structure that can be used as a selectively stretchable transparent electrode. An optimized structural configuration prevented the resistance of the rigid electrode from varying up to a lateral strain of 70%. A repeated stretch/release test with 60% strain for 5000 cycles did not cause any severe damage to the structure, revealing that the fabricated structure was mechanically stable and reliable. KEYWORDS: Ag nanowire, transparent electrode, stretchable electronics, rigid island, heterogeneous composite structure

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INTRODUCTION Mechanically robust, stretchable electrodes with high transparency are considered one of the most important components for achieving various promising stretchable devices such as activematrix light-emitting devices for in vivo applications, solar energy systems, touch pressure sensors, heaters, and biomedical epidermal devices, all of which have their own mechanical stretchability requirements.1−14 This class of electronics can be stretched, compressed, twisted, and deformed into complex, nonplanar shapes while maintaining good performance, reliability, and integration,15,16 which makes it possible for them to be embedded into garments, skin, and even various organs. Such stretchable electrodes have been developed by two major strategies: (1) fabricating intrinsically stretchable conductive materials and (2) engineering novel structural constructs using © XXXX American Chemical Society

conventionally established materials. The intrinsic stretchability of conducting materials has been generally achieved by mixing or embedding functionalized new conductive materials such as carbon nanotubes (CNTs), graphene, glassy metals, and metal nanowires into stretchable polymers or by deposition of a few layers of graphene or other nanomaterials onto the prestretched elastomers. For example, silver nanowires (AgNWs) or CuZrbased metallic glass nanotroughs embedded in the surface of poly(dimethylsiloxane) (PDMS) have been reported to be highly stretchable so that they can be used for fabricating stretchable light-emitting diodes and transparent heaters.17,18 Regarding the Received: September 18, 2016 Accepted: February 1, 2017 Published: February 1, 2017 A

DOI: 10.1021/acsami.6b11853 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic showing the fabrication of the heterogeneous AgNWs/polymer composite structure.

reliable, and stretchable transparent electrode incorporating rigid islands could be successfully fabricated. Through optimization of the structural architecture of the electrodes, around 70% strain could be applied to the electrode without inducing failure of the rigid areas.

second strategy, wavy layouts of metal circuits or buckled structures of thin metal layers are used to interconnect the devices. In these cases, unfortunately, the design rule is quite limited, and the fabricated circuits are mostly opaque. An important aspect that we need to consider in the design of intrinsically stretchable electrodes is that most electronic devices that can be formed on them are not stretchable. This discrepancy induces severe stress singularities at the interfaces between rigid devices and elastomeric electrodes and could cause catastrophic delamination of devices when the substrate is stretched significantly. In an attempt to resolve this problem, one possible solution was proposed in which functional islands of stiff materials are adhered to or embedded into the elastomeric polymer. Because the rigid islands are much stiffer than the stretchable polymer, when such a heterogeneous configuration is stretched, the soft substrate carries most of the deformation so that the islands experience little strain.19 To achieve this, Someya et al. fabricated PDMS with a stretchability gradient by varying the contents of the curing agent to control the crosslinking density of the PDMS.20 The devices formed on rigid polyimide were embedded in the stiffer parts, and a newly developed elastic conductor was used to interconnect them. A large stretchability was achieved by this approach, but full rigidity without accepting a strain of larger than 1% could not be realized by increasing the amount of curing agent, and the stretchable conductor was not at all transparent. Studart et al. successfully fabricated stretchable heterogeneous composites with an extreme mechanical gradient by using a hierarchical reinforcement approach, but the composite was also opaque.21 Lacour et al. embedded polyimide disks into PDMS to attain a mechanical gradient. Even without consideration of the issue of the opaque polyimide, fabrication of complicated architectures and large-area devices could be significantly limited by the timeconsuming pick-and-place procedures required for the aforementioned approaches.22 Herein, we propose a novel solution to this challenge and demonstrate a heterogeneous configuration of a AgNWs/polymer composite structure by employing a well-established photolithography-based approach. To attain this goal, we designed and synthesized two transparent polymers: a UV-patternable transparent polymer which is rigid yet flexible, and a stretchable UV-curable polymer, both of which have identical acrylate functional groups. Using just a photolithographic procedure, well-defined rigid islands could be formed that were solidly embedded in and adhered to a soft polymer overcoat, resulting in a rigid-island-based stretchable transparent substrate. We also employed inverted layer processing to embed AgNWs at the surface of the heterogeneous film so that a mechanically robust,

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EXPERIMENTAL SECTION

Materials for Synthesis of Oligomers. The urethane acrylate (UA) compound was prepared by a previously reported method.23 Bisphenol A glycerolate diacrylate (BPA diacrylate), 4,4-biphthalic anhydride (BiA), triphenylphosphine, PGMEA, acetone-d6, and 1,2,3,6-tetrahydrophthalic anhydride were purchased from SigmaAldrich, United States. Dipentaerythritol hexaacrylate (DPHA) was purchased from ENTIS, South Korea. Irgacure 754 was purchased from Shinyoung Rad. Chem. Ltd., South Korea. The developer (DPD-200) was purchased from Dongjin Semichem Co., Ltd., South Korea. All chemicals were used as received without purification. Synthesis of EA and Preparation of Photoresist Resin. A round-bottom flask was charged with BPA diacrylate (7.65 g, 16.3 mmol) and PGMEA (15.8 g). The mixture was stirred until it became clear. To this mixture were added 4,4-biphthalic anhydride (2.4 g, 8.15 mmol) and triphenylphosphine (cat.). This solution was stirred at 100 °C. After 4 h, cis-1,2,3,6-tetrahydrophthalic anhydride (1.24 g, 8.15 mmol) was added to this solution, and it was stirred at 100 °C for a further 3 h. After being cooled to room temperature, this solution was used for the next step without further purification. For the NMR measurements, the EA compound that was precipitated from the PGMEA/Et2O solution was used. The PGMEA solution of EA was mixed with DPHA and Irgacure 754 in the ratio of 10:2.1:0.38 to prepare the negative-type photoresist (PR) resin. Fabrication of Heterogeneous AgNWs/Polymer Composite Structure. The procedure used for fabrication of the heterogeneous AgNWs/polymer composite structure is schematically illustrated in Figure 1. A detailed description of the procedure can be found in our previous work.24 Here, we used the AgNWs with the average diameter and length of 25 nm and 15 μm (Nanopyxis Ltd., Korea), respectively. Intense-pulsed-light (IPL) exposure was repeated two times, each with a pulse duration of 600 μs, to enhance the adhesion between AgNWs and polyimide (Kapton, DuPont, United States). Onto the patterned AgNW electrodes, the synthesized EA-based photoresist resin was spin-coated, followed by a typical photolithographic procedure involving UV exposure and development to produce cured EA (PEA) patterns. A solution containing the synthesized UA compound was then spin-coated and cured by UV exposure to form the stretchable polymer (PUA). The resulting structure was subsequently immersed into cold water (25 °C) to induce hygroscopic swelling of the polymers, allowing it to be peeled off safely from the Kapton film. Electroless plating of Cu onto the embedded electrode was used to investigate whether the AgNWs are exposed or fully buried. Electroless plating was used to induce Cu metallization according to our previous study.25 During the plating process, the solutions were vigorously stirred. B


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ACS Applied Materials & Interfaces Scheme 1. Synthetic Route to EA

Figure 2. 1H NMR spectrum of EA in acetone-d6. Structural Evaluation. The 1H NMR and COSY NMR spectra were measured at 400 MHz on a Bruker AscendTM 400 spectrometer. A field-emission scanning electron microscope (FESEM; JSM6700F, JEOL Ltd., Japan) was used to investigate the microstructure of the AgNW networks. The surface morphology was measured by atomic force microscopy (AFM; XE-100TM, Park Systems, United States). X-ray photoelectron spectroscopy (XPS; VG Multilab 2000, Thermo Fisher Scientific, U.K.) was employed to analyze the surface of the AgNWs with a Mg Kα source (1253.6 eV). The optical transmission was measured using a UV−vis spectrophotometer (V-560, Jasco, Japan). The sheet resistance Rs was recorded with a noncontact measurement system (EC-80P, Napson Corporation, Japan). An automatic stretchtesting machine (Stretching Tester, Jaeil Optical Systems, Korea) was used to measure the resistance variations of the electrodes during

stretching sequences. The electrodes were stretched at a rate of 0.5 mm/s to various strains, and the local resistance during testing was measured using a probe station (MS Tech, 5500B, Korea) equipped with an inductance−capacitance−resistance (LCR) meter (Hewlett-Packard, 4284A, United States). More than 10 samples were fabricated and measured to determine most of the parameters.

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RESULTS AND DISCUSSION To achieve stretchable films in which patterned rigid islands were embedded, the oligomers of UA and EA were first synthesized. As the soft polymer (PUA) was overlapped on the rigid polymer patterns (PEA), there were interfaces between soft and rigid polymers. Due to the significant difference in C


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Figure 3. SEM micrographs of various parts of the samples: (a) AgNWs coated on a Kapton film, (b) patterned PEA in panel a, and (c) magnified view of the area indicated by the dashed square in panel b.

facilitated smooth peeling of the composite structure from its surface and enhanced the efficiency of transferring the AgNWs to the surfaces of the PUA and PEA. One issue still remaining is that the AgNWs could be detached from the Kapton during subsequent photolithography, Ag etching, or PR stripping, which leads to deterioration in electrical conductivity of the electrodes. To resolve this, we treated the AgNWs/Kapton (Figure 3a) with IPL and optimized the control parameters to deal with the following conflicting factors: (1) AgNWs networks should not be lost or lose their conductivity during the follow-up procedures, which means that the adhesion between the nanowires and the Kapton should be sufficiently high, and (2) the AgNWs need to be perfectly transferred to the surfaces of the overcoated polymers. To fulfill requirement 2, mild adhesion between them is preferred. We noticed that IPL irradiation to the AgNWs deposited on a polymer substrate can enhance their adhesion by partial melting of the polymer areas adjacent to AgNWs in our previous study.24 The resistance of the IPL treated electrode was about 10% lower than that of the pristine electrode originated from a photoenhanced sintering effect with a negligible microstructural change, as shown in Figure S5. The EA-based PR was then spin-coated onto the AgNWs/Kapton, followed by full photolithography to create a specific cured pattern, as shown in Figure 3b. Very ordered, circular patterns with high, sharp edges are shown in the figure, and the AgNWs on the Kapton were not detached by the patterning procedure, as shown in Figure 3c. This is attributed to the enhanced adhesion between the nanowires and the Kapton. A solution containing a UA-based compound was subsequently coated and exposed to UV irradiation to make the PUA. Figures 4a−c show the surface of the peeled composite structure, revealing that the PEA pattern was successfully embedded in the surface of PUA and safely peeled from the Kapton without forming any significant defects. An example of the completed sample is also shown in Figure S6. As mentioned above, the PUA and PEA were designed to be connected via strong covalent bonds. During the formation of the rigid islands by UV irradiation, the mobility of the cured polymer was reduced, leaving unreacted acrylate groups in the polymer.28 These unsaturated groups partook in the subsequent photopolymerization of the UA. Presumably, this strengthened the cohesive force at the interface between the rigid and soft polymers. Figures 5a and b show magnified scanning electron microscopy (SEM) images of the electrodes in rigid islands and the soft polymer, respectively. We confirmed that the nanowires were perfectly transferred to the surface of both polymers, leaving no residues on the Kapton film. The Rs values of the unpatterned AgNWs (25.7 ohm/sq) on the Kapton decreased by about 13% on the PEA area (22.4 ohm/sq) and increased by about 4% on the PUA area (26.7 ohm/sq) after transfer.

elastic moduli between the two polymers, cohesive fracture at the interfaces was expected. To overcome this problem, the two polymers with the acrylate functionalities were designed to be connected through strong covalent bonds instead of weak physical interactions. The MEK solution of UA was prepared via a two-step process according to our previous study.23 The IPDI was allowed to react with a polyol in the presence of a tin catalyst and then with hydroxylated acrylate to afford the UA. Owing to the soft segment of poly(1,4-butylene-adipate), the UA compound produced a soft and stretchable film after being cured by UV irradiation. For UV patterning of the rigid islands, a negative-type photoresist resin comprising an alkaline-soluble EA binder, a multiacrylate monomer, and a photoinitiator was prepared.26 An alkaline-soluble EA compound was prepared by reacting BPA diacrylate with 4,4′-biphthalic anhydride (BiA) followed by cis-1,2,3,6-tetrahydrophthalic anhydride, as shown in Scheme 1. In the first step, the reaction between diol and dianhydride yielded the EA-1 oligomer with carboxylic acids and alcohols. In the second step, some of the alcohol groups were allowed to react with anhydride to afford the EA oligomer with more carboxylic acids. The amount of carboxylic acid in EA played a key role in patterning the rigid polymer.27 For patterning, the area that was exposed to UV light remained a polymer, and the area under the shadow mask was washed away during the developing process. As the developer was an aqueous alkaline solution, the solubility of the EA oligomer with more carboxylic acid groups was increased. When the solubility of EA oligomer was low in the aqueous alkaline solution, the residual polymer under the mask remained after the developing process. The identity of the EA oligomer in acetone-d6 was characterized by 1H NMR spectroscopy, as shown in Figure 2. First, the broad peaks in the spectrum indicate that polymerization occurred. As the alcohols of BPA diacrylate reacted with the anhydrides in BiAs to produce ester groups, a chemical shift of the protons adjacent to the ester groups is expected. The proton resonances from 5.4 to 5.8 ppm correspond to the CH protons next to the formed ester groups and the HCCH protons of cyclohexene moieties. Similar peaks were observed in the 1H NMR spectrum of EA-1 in which cyclohexene groups were not incorporated (Figure S1). Furthermore, these peaks were absent in the 1 H NMR spectrum of the BPA diacrylate, indicating that these protons were shifted downfield via the esterification reaction between BPA diacryate and BiA (Figure S2). The connectivity of the protons in the oligomer backbone was investigated by COSY NMR spectroscopy. The appearance of cross peaks (5.4−5.8 ppm) implied that the protons adjacent to the formed ester groups were correlated with the protons of the alkyl chains in BPA diacrylate (Figures S3 and S4). As a temporary substrate to form the composite electrodes, we employed a Kapton film due to its low surface energy. The nonpolar surface of the Kapton without any treatment D


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Figure 4. SEM micrographs of various parts of the fabricated electrode: (a) surface of the heterogeneous AgNWs/polymer composite structure after being peeled from the Kapton film, (b) magnified tilt view of the area indicated by the dashed square in panel a, and (c) magnified top view of the interface in panel a.

the electrode from the Kapton film, a sufficient force exceeding the initial adhesion between the polymer and the Kapton should be applied.29 This induces local stresses on the surface of the electrodes and eventually forms ripples to relieve the stresses, particularly in the case of the soft polymer due mainly to its lower stiffness. Here, whether or not the AgNWs were still exposed to air warranted investigation because many applications require sufficiently exposed electrodes for interconnection or carrier transfer.29 To examine this, we carried out an electroless plating of Cu according to our previous study.25 Here, we did not employ the reducing agent, meaning that the plating should be activated only onto the exposed AgNWs. The results in Figures 5c and d revealed that Cu was plated onto the exposed AgNWs irrespective of the polymer used without inducing any chemical or physical damages to the polymers. For further verification, we analyzed the surface composition of the electrode by the XPS. Figure S7b and S7d show the XPS spectra in the region of Ag 3d of the AgNWs, which can be verified by two peaks occurring at 367.5 and 373.1 eV, corresponding to Ag 3d5/2 and 3d3/2 binding energy, respectively.30 These analyses revealed that the AgNWs were evidently exposed to air. The roughness of the electrode surfaces was measured by AFM, as shown in Figure 6, because a network structure of AgNWs generally forms a highly porous morphology which is not appropriate for thin-film devices where a highly smooth surface is needed.31 Considering that the peak-to-valley roughness (Rpv) and root-mean-square roughness (RRMS) for the AgNWs/glass or AgNWs/Kapton are generally higher than 200 and 30 nm, respectively, the roughness of both electrodes was significantly reduced. This reduction is originated from the processing method used, in which the AgNW network was originally formed on a smooth Kapton surface. The more important factor is attributed to the scarce formation of nanoholes or steps, enabled by the fact that the liquids containing the EA and UA compounds perfectly infiltrated the nanopores or nanogaps between the AgNWs and the Kapton.31 The Rpv of the rigid electrode is 52.6 nm,

Figure 5. SEM micrographs of the electrodes: (a) AgNWs embedded in PEA, (b) AgNWs embedded in PUA, (c) Cu plated on the exposed AgNWs in a rigid island, and (d) Cu plated on the exposed AgNWs in the soft polymer.

The decrease in Rs for the PEA was possibly due to the decrease in contact resistance between the AgNWs that is created by shrinkage of the polymer during curing. This nearly perfect transference suggests that the IPL treatment is a very useful tool for deliberately controlling the metal nanowire adhesion to the underlying polymer. We conducted tape testing up to 20 times to evaluate the stability of the electrodes, but the resistance did not vary at all, revealing that the nanowires were well adhered to and embedded in the surface of the polymers. In the SEM micrographs, the AgNWs are clearly identifiable even though they were embedded. The morphologies are somewhat different between the two cases: the surface of the rigid electrode is more planar and smooth, whereas some ripples are formed on the surface of the soft electrode. To properly peel

Figure 6. AFM surface topography of (a) the rigid electrode and (b) the soft electrode. E


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Figure 7. (a) Transmittance and (b) haziness of the synthesized polymers and fabricated electrodes.

Figure 8. Resistance change of composite structures with increasing applied strain: a/b = (a) 0.7, (b) 0.35, (c) 0.2, and (d) 0.15. “a” is the thickness of the rigid electrode, and “b” is that of the soft electrode.

which is similar to that of the ITO film (46.1 nm)31 and implies that various thin-film-based rigid devices such as oxide thin-film transistors, inorganic sensors, and light-emitting diodes can be formed on its surface. The optical properties of the bare polymers and fabricated electrodes were measured, as shown in Figure 7. In our previous studies, we found that the colorless polyimide (cPI) is very highly compatible with the inverted layer processing of AgNWs, resulting in a mechanically stable and transparent electrode.31−33 Interestingly, the transmittances of PEA and PUA are higher than that of cPI, and the fabricated electrodes were more transparent, especially in the wavelength range 350−450 nm. The low transmittance of the cPI originates from the large light absorption of the aromatic compounds and the

formation of charge transfer complexes in their highly conjugated molecular structures.32,34 This absorption results in their films having a slight yellowish color. In contrast, the PEA and PUA we designed here contain no aromatic compounds in their complexes, which means that they are more transparent over the whole visible range.32 Consequently, this resulted in improved color neutrality and clarity of the films, as shown in Figure 7. The mechanical performance of the fabricated heterogeneous composite structure was evaluated by independent stretching tests for rigid and soft electrodes. The resistance measurements are schematically described in Figure S8, and the resistance changes of the “only rigid electrode” and “rigid + soft electrodes” with applied strain are shown in Figure S9. To optimize the structural configuration of the electrode, we varied the thickness F


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ACS Applied Materials & Interfaces ratio (a/b) of the rigid and soft electrodes by controlling the spin-coating speed used for EA deposition. The total thickness of the fabricated film was 45 μm. As shown in the figure, the resistance of the whole structure was smoothly elevated by an increase in the applied strain regardless of the thickness ratio employed. A decrease in the thickness of the rigid electrode results in a faster increase in resistance, possibly due to the diminishing strengthening effect of the embedded rigid islands. An interesting point can be found in Figure S9b in that the resistance for the rigid electrode did not vary at all with an increase in strain but abruptly increased at specific strains for each case. The high stiffness of the PEA made the AgNW electrode resist a specific amount of lateral strain, but a larger strain finally led to an increase in resistance, followed by a steep elevation. The thinner the PEA, the earlier the increase in resistance was observed due to the decrease in PEA rigidity with decreasing thickness. Resistance changes for the whole structure and for only the rigid electrode are compared in Figure 8. It is noteworthy that the difference in the increment tendencies between cases is most significant for the case of the thickest PEA, whereas the tendency is similar for the case of the thinnest PEA. This means that if rigid islands are too thin, they may not be efficient in enhancing the strength of the completed heterogeneous structure or enlarging the applicable strain without causing damage to the rigid islands. For the condition shown in Figure 8a (a/b = 0.7), we measured the resistance change with repeating stretch/releasing cycles, as shown in Figure 9. The

Figure 10. Interfacial micrographs between soft and rigid electrodes: (a and b) at released state and (c and d) at stretched state (50%).

When the soft electrode is elongated, the contacts between AgNWs could be weakened or even detached, possibly due to sliding between each other, which can lead to a decrease in the percolation density, as can be found in Figures 10c and d. When the elongated film is released, the AgNWs slide back a certain degree but cannot slide back to their initial state because of the frictional force between the AgNWs and the PUA matrix;35 rather, the soft electrode buckles out of plane as a whole, as can be seen in Figures 11a and b. The resistance variation of the whole structure (a/b: 0.7) during repeated stretch/release testing employing a maximum strain of 60% is shown in Figure S10. The figure shows that the resistance increased with increasing strain and decreased upon releasing, but the resistance variation continuously increased as more stretch/release cycles were completed, mainly due to the residual deformation issue. However, even after 5000 cycles of stretch/release testing with 60% strain, we could not find any severe damage to the rigid electrode or at the interfaces between the rigid and soft polymers. Only application of 80% strain to this sample caused catastrophic cracking to the rigid electrode, as shown in Figure 11c, implying that the heterogeneous composite structure fabricated here is mechanically stable and reliable up to very large strains. To the best of our knowledge, this is the most severe stretching condition that has been reported for such a material, demonstrating that stiff devices formed on the rigid islands could survive even under these significant stretching conditions. To evaluate the selective stretchability of the fabricated electrode, the microstructural deformation of each area was separately investigated. For this, ZnS microparticles dispersed in a stretchable adhesive, polyurethane urea (PUU), were deposited onto the surface of the electrode (a/b: 0.7) via a spin-coating process. The synthesis of PUU and its properties were explained in detail in our previous study.29 The PUU was strongly adhered to both the PEA and PUA so that the ZnS particles could be firmly fixed at specific locations. We investigated specific locations on the rigid (Figures 12a−d) and soft (Figures 12e−h) electrodes by inducing strains of up to 60%. We observed that the distance between two specific particles on the soft electrode could be precisely determined by the applied strain, whereas the distance did not vary at all even at a strain of 60% in the case of the rigid electrode. This is consistent with the results of the resistance measurements during stretching.

Figure 9. Resistance change of the electrodes with stretch/releasing (up to 60%) cycles.

resistance of the rigid electrode was still very stable, while that of the soft electrode reproducibly increased with stretching and recovered with releasing even after 100 cycles of the stretch/ releasing (up to 60%) cycles with only a negligible hysteresis. Considering that the resistance of the rigid electrode did not vary with increasing strain up to a specific strain, the increased resistance of the whole structure was resulted from the deformation of (1) the soft electrode and (2) the interface between the rigid and soft electrode. Here, we needed to investigate the interfacial micrographs to confirm that the interface was stable even at a largely stretched state. In Figure 10, the soft and rigid electrodes are very stably bonded without exhibiting severe defects (such as delamination or cracking) at the released and stretched states (50% strain induced), implying that the increased resistance was dominantly affected by the deformation of the soft electrode. The increase in resistance of the soft electrode is mainly attributed to two aspects: a decrease in AgNW percolation density and buckling instability of the soft polymer. G


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Figure 11. Micrographs of electrodes after 100 cycles of 80% stretch/release testing: (a) an interface between a rigid electrode and a soft electrode, (b) a soft electrode, and (c) a rigid electrode.

Figure 12. Micrographs of ZnS particles adhered to (a−d) a rigid electrode and (e−h) a soft electrode: panels a and e show the pristine state, b and f show the 20% strain stretched state, c and g show the 40% strain stretched state, and d and h show the 60% strain stretched state.

Figure 13. Photographs of light-emitting diodes connected through soft and rigid electrodes with increasing strains applied: (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50%.

These results clearly demonstrate that the approaches suggested here can be successfully used in mechanically stable and reliable rigid-island-based stretchable electronics.

Finally, the functionality of the fabricated heterogeneous structure (a/b: 0.7) was demonstrated by employing it as a stretchable electrode to turn on two light-emitting diodes (LEDs; one being connected through a rigid electrode and the other through a soft electrode), where the brightness change is related to the conductivity variation of the AgNW electrode. As shown in Figure 13, the LED lamp connected through the rigid electrode showed no noticeable degradation in brightness even after imposing 50% strain, whereas the emission intensity continuously decreased with increasing strain for the soft electrode. Luminance of the LEDs measured with stretching is summarized in Figure 14.

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CONCLUSION A mechanically stable, rigid-island-based stretchable transparent electrode was successfully achieved by employing a photolithography-based approach. To implement the approach, we designed and synthesized two new transparent polymers; the first is an epoxy-based rigid yet flexible polymer, and the second is a urethane-based soft polymer, both of which are UV-curable H



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ACKNOWLEDGMENTS

This work was supported by the Ministry of Trade, Industry and Energy (MOTIE)/the Korea Institute of Energy Technology Evaluation and Planning (KETEP) of the Republic of Korea (Grant 20153010140030 and 20155020301000). Support was also provided by the MOTIE/Korea Evaluation Institute of Industrial Technology (KEIT) (Grant 10051080), and the Ministry of Science, ICT and Future Planning/the National Research Foundation of Korea (NRF) of the Republic of Korea (Grant 2016M3A7B4910).

■ Figure 14. Luminance of the LEDs measured with stretching.

and possess acrylate functional groups. Combining a photolithographic procedure to make specific patterns of the rigid polymer with inverted-layer processing to embed the AgNWs in the surface of the polymers results in a reliable heterogeneous composite electrode with rigid islands on its surface. Presumably due to the strong covalent bonds formed between the rigid and soft polymers, the cohesive force at their interfaces was greatly strengthened. The percolated AgNW networks buried at the surface of the polymers led to a smooth, transparent, and mechanically stable electrode. An optimized structural configuration prevented the resistance of the rigid electrode from varying up to a lateral strain of 65%. Even after 5000 cycles of repeated stretch/release with 60% strain, no severe damage such as cracking of the rigid polymer or delamination of the two polymers was found, revealing that the fabricated structure was mechanically stable and reliable. A demonstration in which two LEDs were connected through the rigid and soft electrodes confirmed that the photolithographic method using the two polymers designed here can be successfully used in stretchable electronics requiring various stiff and fragile components to be interconnected. The materials and the fabrication approach we introduced in this study are expected to provide a practical manual for the fabrication of stretchable devices.

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REFERENCES

(1) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Stretchable Active-Matrix Organic Light-Emitting Diode Display Using Printable Elastic Conductors. Nat. Mater. 2009, 8 (6), 494−499. (2) Kim, R. H.; Bae, M. H.; Kim, D. G.; Cheng, H.; Kim, B. H.; Kim, D. H.; Li, M.; Wu, J.; Du, F.; Kim, H. S.; Kim, S.; Estrada, D.; Hong, S. W.; Huang, Y.; Pop, E.; Rogers, J. A. Stretchable, Transparent Graphene Interconnects for Arrays of Microscale Inorganic Light Emitting Diodes on Rubber Substrates. Nano Lett. 2011, 11 (9), 3881−3886. (3) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457 (7230), 706−710. (4) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ö zyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5 (8), 574−578. (5) Lee, M.-S.; Lee, K.; Kim, S.-Y.; Lee, H.; Park, J.; Choi, K.-H.; Kim, H.-K.; Kim, D.-G.; Lee, D.-Y.; Nam, S.; Park, J.-U. HighPerformance, Transparent, and Stretchable Electrodes Using Graphene-Metal Nanowire Hybrid Structures. Nano Lett. 2013, 13 (6), 2814−2821. (6) Lee, S. K.; Kim, B. J.; Jang, H.; Yoon, S. C.; Lee, C.; Hong, B. H.; Rogers, J. A.; Cho, J. H.; Ahn, J. H. Stretchable Graphene Transistors with Printed Dielectrics and Gate Electrodes. Nano Lett. 2011, 11 (11), 4642−4646. (7) Zang, J.; Ryu, S.; Pugno, N.; Wang, Q.; Tu, Q.; Buehler, M. J.; Zhao, X. Multifunctionality and Control of the Crumpling and Unfolding of Large-Area Graphene. Nat. Mater. 2013, 12 (4), 321− 325.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11853. Additional experimental data and a schematic description (PDF)

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ABBREVIATIONS CNT, carbon nanotube AgNW, silver nanowire PDMS, poly(dimethylsiloxane) UA, urethane acrylate EA, epoxy acrylate BPA diacrylate, bisphenol A glycerolate diacrylate BiA, 4,4-biphthalic anhydride DPHA, dipentaerythritol hexaacrylate IPL, intense-pulsed-light PUA, polyurethane acrylate PEA, polyepoxy acrylate FESEM, field-emission scanning electron microscope AFM, atomic force microscopy XPS, X-ray photoelectron spectroscopy UV, ultraviolet cPI, colorless polyimide LED, light-emitting diodes

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-31-789-7438; E-mail: [email protected]. ORCID

Jong-Woong Kim: 0000-0003-4010-056X Author Contributions

J.-W.K. designed and supervised the research and wrote the manuscript. Y.K. synthesized the polymers and wrote the synthetic part of the manuscript. S.J. and B.-K.J. participated in fabrication and evaluation of the stretchable transparent electrodes. All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest. I


Research Article

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(28) Kiatkamjornwong, S.; Tessiri, S. Synthesis and Characterization of Poly[(methyl Methacrylate)-Co-(Methacrylic Acid)] for a UVSensitive Aqueous Base Developable Lithographic Plate. J. Appl. Polym. Sci. 2002, 86 (8), 1829−1837. (29) Kim, D.-H.; Yu, K.-C.; Kim, Y.; Kim, J.-W. Highly Stretchable and Mechanically Stable Transparent Electrode Based on Composite of Silver Nanowires and Polyurethane−Urea. ACS Appl. Mater. Interfaces 2015, 7 (28), 15214−15222. (30) Kim, D. G.; Kim, J.; Jung, S. B.; Kim, Y. S.; Kim, J. W. Electrically and mechanically enhanced Ag nanowires-colorless polyimide composite electrode for flexible capacitive sensor. Appl. Surf. Sci. 2016, 380, 223−228. (31) Ok, K.-H.; Kim, J.; Park, S.-R.; Kim, Y.; Lee, C.-J.; Hong, S.-J.; Kwak, M.-G.; Kim, N.; Han, C. J.; Kim, J.-W. Ultra-Thin and Smooth Transparent Electrode for Flexible and Leakage-Free Organic LightEmitting Diodes. Sci. Rep. 2015, 5, 9464. (32) Kim, Y.; Kim, J. W. Silver Nanowire Networks Embedded in Urethane Acrylate for Flexible Capacitive Touch Sensor. Appl. Surf. Sci. 2016, 363, 1−6. (33) Kim, Y.; Ryu, T. I.; Ok, K.-H.; Kwak, M.-G.; Park, S.; Park, N.G.; Han, C. J.; Kim, B. S.; Ko, M. J.; Son, H. J.; Kim, J.-W. Inverted Layer-By-Layer Fabrication of an Ultraflexible and Transparent Ag Nanowire/Conductive Polymer Composite Electrode for Use in HighPerformance Organic Solar Cells. Adv. Funct. Mater. 2015, 25 (29), 4580−4589. (34) Ni, H.; Liu, J.; Wang, Z.; Yang, S. A Review on Colorless and Optically Transparent Polyimide Films: Chemistry, Process and Engineering Applications. J. Ind. Eng. Chem. 2015, 28, 16−27. (35) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24 (37), 5117−5122.

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Polymer Composite

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