Optoelectronics
High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode Han-Jung Kim, Su-Han Lee, Jihye Lee, Eung-Sug Lee, Jun-Hyuk Choi, Jun-Ho Jung, Joo-Yun Jung, and Dae-Geun Choi*
Uniform
metal nanomesh structures are promising candidates that may replace of indium-tin oxide (ITO) in transparent conducting electrodes (TCEs). However, the durability of the uniform metal mesh has not yet been studied. For this reason, a comparative analysis of the durability of TCEs based on pure Ag and AgNi nanomesh, which are fabricated by using simple transfer printing, is performed. The AgNi nanomesh shows high long-term stability to oxidation, heat, and chemicals compared with that of pure Ag nanomesh. This is because of nickel in the AgNi nanomesh. Furthermore, the AgNi nanomesh shows strong adhesion to a transparent substrate and good stability after repeated bending.
1. Introduction Transparent conducting electrodes (TCEs) are key materials in the information technology industry. Smartphones, touchscreens, flat-display panels, and solar cells all require a TCE layer that allows charge transport without blocking the transmission of light.[1–4] Indium-tin oxide (ITO) has been widely used as the TCE in various applications as it possesses excellent physical properties, such as high optical transmittance and low sheet resistance. However, indium is scarce. Furthermore, ITO on a flexible substrate is brittle and cracks easily, and its production requires high-temperature processing.[1–10] Therefore, there is a strong need to find alternative materials for high-performance TCEs. Research has been done to explore the potential of conductive polymers[11–13] and carbon nanomaterials such as nanotubes[14–16] and graphenes[17–20] to replace ITO; however, TCEs based on such materials are unable to compete with those based on ITO in terms of their optical and electrical properties.[21]
Dr. H.-J. Kim, S.-H. Lee, Dr. J. Lee, Dr. E.-S. Lee, Dr. J.-H. Choi, Dr. J.-H. Jung, Dr. J.-Y. Jung, Dr. D.-G. Choi Nano-Mechanical Systems Research Division Korea Institute of Machinery & Materials (KIMM) 171 Jang-dong, Yuseong-gu Daejeon 305–343, Republic of Korea E-mail: [email protected] DOI: 10.1002/smll.201400911 small 2014, DOI: 10.1002/smll.201400911
Metal-based TCEs such as random networks of silver nanowires (AgNWs)[21–27] and uniform metal meshes[6,7,28–31] are considered the most promising replacement of ITO because of their excellent optical and electrical properties, good flexibility, and compatibility with low-cost solution processes.[22] Several issues, however, still need to be addressed before AgNW networks can be widely applied in electronic devices. First, strong adhesion between the AgNW network and substrate needs to be achieved, as the AgNW network is easily detached from the substrate.[9,22] Second, the AgNW network cannot be used as-prepared, but heat treatment at around 200 °C is first required in order to obtain the high electrical conductivity of AgNW-based TCEs. This limits the application of AgNWs in heat-sensitive substrates.[9,21,22] Lastly, they still have a long-term stability issues, which makes them difficult for practical use.[10,22] Specifically, when AgNW networks are exposed to air and water, they easily undergo oxidation, leading to a sharp increase in the sheet resistance of the AgNW-based TCEs. To address these problems, several methods have been investigated, such as embedding of the AgNW network in a polymeric matrix; application of additional protective layers on the AgNW network using polymers,[8,22] graphene,[32] and metal oxide;[1,33] and the use of metal nanowires with a nickel coating.[10] However, all of these methods have resulted in complication of the TCE manufacturing process. Recently, uniform metal-mesh structures have also appeared promising because their transmittance and electrical conductivity can be easily tuned simply by changing
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the line width, line spacing, or thickness of the metal pattern. In contrast to AgNWbased TCEs, the work function of metalmesh for different applications can be easily controlled by simply changing the metallic material.[6,7,29,34] However, several conventional metal-patterning methods necessitate the use of complex and expensive technologies such as electron (E) beam lithography and lift-off processes.[29,34] Although a facile method of fabricating metal-mesh TCEs using nano-imprint lithography (NIL) and transfer printing has been recently proposed,[6,7] longterm stability remains an important issue that poses difficulties in their practical applications. It is well-known that metals such as Ag and copper can be easily oxidized when they are exposed to air.[8,10]Nevertheless, the durability of metal-mesh TCEs has never been studied. Therefore, further research on highly robust and stable metal-mesh TCEs must be done for their development for practical use. We demonstrate a high-durable AgNi nanomesh for the TCE, which exhibits strong adhesion to a transparent substrate. Our nanomesh had excellent long-term stability to oxidation, high temperature, and chemicals compared with pure Ag nanomesh. It also showed high stability in terms of bending and folding. The nanomesh could be easily prepared by transfer printing. The transmittance and sheet resistance of the nanomesh TCE could be controlled by manipulating process conditions such as evaporation materials, metal thickness, and pattern dimensions. The TCE based on AgNi nanomesh may be an alternative to ITO in high-performance optoelectronic devices such as flexible solar cells and displays.
2. Results and Discussion Metal-based nanomesh TCEs on a transparent substrate were prepared by using a combination of NIL and the transfer printing method.[6,7,35,36] Figure 1a shows the fabrication process of uniform metal-based nanomesh TCE on transparent substrate. Figure 1. a) Fabrication process of uniform metal nanomesh TCEs on a transparent First of all, a polyurethane acrylate (PUA) substrate. b) Transmittance spectra and FE-SEM images of 60 nm Ag nanomesh patterns (150 nm width and various line spacing) on a glass. c) Transmittance and sheet resistance mold was replicated from a silicon master of pure Ag and AgNi nanomesh TCEs on PES film with 60, 80, and 100 nm thickness, and through the replica molding method (see Sup- same line-spacing (3.2 µm) and line width (150 nm). d) Comparison of the FoM σ / σ ( dc opt ) porting Information, Figure S1a). To increase values for TCEs of PEDOT:PSS,[11,12] graphene,[39–42] metal nanowires,[3,43,44] and metal the transfer ratio of the metal nanopat- mesh.[6,28,34] The red stars indicate the experimental results obtained in this work. tern from the mold, the surface of the PUA mold was treated with an antisticking layer (1H,1H,2H,2H- evaporator (see Supporting Information, Figure S1b). At this perfluorooctyl-trichlorosilane) before metal evaporation. point, the main material of the metal nanomesh could be Next, a thin metal layer with a thickness of 60–100 nm was easily changed by changing the evaporation materials. Furdeposited on the PUA mold by using an electron-beam thermore, the thickness of the nanomesh could be controlled
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High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode
by changing the deposition thickness. The evaporation rate was about 0.1 nm s−1. To transfer a metal layer from the PUA mold, a layer of transparent ultraviolet (UV) -curable adhesive (Norland Optical Adhesive 61, NOA 61) with thickness of ≈800 nm was spin-coated on a transparent substrate. To cure partially the adhesive, the coated substrate was exposed to UV for 20 s by using a 50 W mercury lamp. The metalcoated PUA mold was then placed in contact with the top of the partially cured substrate, and the assembly was completely UV-cured with 0.15 MPa static pressure for 3 min. Finally, the uniform metal nanomesh that was formed on the transparent substrate was obtained after peeling the PUA mold (see Supporting Information, Figure S1c). Figure 1(b) shows the optical transmittance spectra and the field emission scanning electron microscopy (FE-SEM) images of the fabricated 60 nm Ag nanomesh patterns on a glass substrate, which had a width of 150 nm and line spacing of 1.0 to 3.2 µm. The transmittance spectrum of the fabricated Ag nanomesh ranged from 400 to 800 nm, where the black line represents a commercial high-transmittance ITO glass for comparison. All transmittance spectral measurements were referenced to air. Commercial ITO glass had a transmittance of 88% at 550 nm and an average transmittance of 84% over the entire visible range. Average transmittances of the Ag nanomesh on glass with 1.0, 1.6, and 3.2 µm line spacing at 550 nm were 62.5, 76.1, and 87.5%, respectively. This verifies that the transmittance increased with the increase in line spacing. The sheet resistance, another important parameter of TCEs, was measured by using the four-point probe-type sheet resistance meter. The sheet resistances of the Ag nanomesh on glass with 1.0, 1.6, and 3.2 µm line spacing were 7.9, 25.1, and 49.2 Ω sq−1, respectively. This verifies that the sheet resistance increased with the increase in line spacing. These results indicate that the transmittance and sheet resistance depend on the line spacing of the nanomesh pattern. They are identical to those for the AgNi nanomesh on the flexible substrate. Indeed, we fabricated a AgNi nanomesh on a flexible polyethersulfone (PES) film and measured its transmittance and sheet resistance (see Supporting Information, Figure S2a). To confirm the presence of nickel in the AgNi nanomesh, we carried out energy-dispersive spectroscopy (EDS) measurements on the nanomesh. EDS analysis confirmed the presence of nickel in the AgNi nanomesh film (see Supporting Information, Figure S2b). The transmittance and sheet resistance of the fabricated nanomesh samples of pure Ag and AgNi of each line spacing size and thickness were measured. Figure 1c shows their transmittance and sheet resistance on PES film with 60, 80, and 100 nm thickness and line spacing of 3.2 µm. As shown in Figure 1c, the sheet resistance and transmittance increased with decreasing mesh thickness. This means that the sheet resistance and transmittance depended on the thickness of the nanomesh pattern. These experimental results are in agreement with previous studies.[7,29] Furthermore, we found that the transmittance of the AgNi nanomesh was similar to that of Ag nanomesh TCEs, but the sheet resistance of the AgNi nanomesh was slightly larger than that of Ag nanomesh TCEs. The high sheet resistance small 2014, DOI: 10.1002/smll.201400911
of the AgNi nanomesh is due to the electrical conductivity of pure Ag, which is higher than that of AgNi.[37] As shown in Figure 1b,c, there is a trade off between transmittance and sheet resistance in the fabricated metal-based nanomesh. In other words, the transmittance increased with decreasing mesh thickness or with increasing line spacing sizes of the nanomesh patterns, both of which could cause an increase in sheet resistance. Decreased transmittance could be easily compensated for by selecting mesh patterns with wide line spacing. Theses experimental results indicate that the transmittance and sheet resistance of metal-based nanomeshes prepared through the polymer-based transfer method was comparable to that of commercial ITO. To gain insight into the effect of nanomesh thickness on the overall performance of the TCE, we determined the figure of merit (FoM), which is the ratio of the electrical conductance to the optical conductance (σ dc / σ opt ) as a function of nanomesh thickness (inset, Figure 1c). The expression for σ dc / σ opt is as follows:[11,21,34] ⎛ 188.5 σ opt ⎞ T = ⎜1 + RS σ dc ⎟⎠ ⎝
−2
(1)
where RS and T are the measured sheet resistance and transmittance at 550 nm, respectively. The dashed black horizontal line at FoM value of 35 (RS < 100 Ωsq−1 at T > 90%) indicates the minimum value needed for most TCE applications.[11,38] However, some applications such those for liquid crystal displays require FoM values upward of 50.[11,38] As shown in the inset of Figure 1c, the FoM of our metal-based mesh TCEs was higher than the minimum FoM value required for most TCE applications. Figure 1d shows some of the best FoM values for TCEs of poly-(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) (37–50),[11,12] graphene (10–120),[39–42] metal nanowires (70–160),[3,43,44] and metal meshes (55–160).[6,28,34] Here, red stars indicate the experimental results obtained in our study. Figure 2a shows the AgNi nanomesh on the PES film fabricated through the polymer-assisted transfer method. As shown in Figure 2a, a large area 100 mm × 100 mm of AgNi nanomesh was successfully fabricated. To evaluate the adhesion between metal nanomesh film and substrate, the fabricated metal-based nanomesh was subjected to a repeated adhesive tape test. Figure 2b shows the changes in sheet resistance of the AgNW-based and metalbased nanomesh TCEs during the test. As shown in Figure 2b, the sheet resistance of the AgNW-based TCE increased significantly during this test. After four consecutive tests, the sheet resistance of the AgNW-based TCE could no longer be measured. However, the sheet resistance of the metalbased nanomesh remained unchanged after 100 cycles. This suggests that adhesion between the metal-based nanomesh film and substrate was very strong even without any additional treatments (see also Supporting Information Figure S3 for adhesive test result of AgNWs based TCE prepared by using our transfer printing method). The reason was that the AgNi nanomesh film was partially embedded inside the UVadhesive layer, as shown in the FIB analysis (see Supporting Information, Figure S4).
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Figure 2. a) Large-area, flexible AgNi nanomesh. b) Changes in the sheet resistance of the AgNW-based, Ag nanomesh, and AgNi nanomesh TCE samples during the repeated adhesive tape test.
High-performance TCEs should be able to withstand high temperature in the post-processing stages in order to ensure excellent device performance. For this reason, the thermo-mechanical stabilities of the Ag and AgNi nanomeshes on PES film were also evaluated by directly measuring sheet resistance when the samples were placed on a hot plate. Figure 3 shows the changes in the sheet resistance of both samples during this test. The inset of Figure 3 shows the FE-SEM image of the Ag and AgNi nanomesh TCEs based on PES film after the test. For comparison, a sample of spin-coated AgNWs on a PES film was also tested under the same conditions (see Supporting Information, Figure S5). The sheet resistance of all samples was measured during heat treatment for 60 min at 150, 200, and 250 °C. It should be noted that the test temperature was deliberately not raised above 250 °C, as the PES film could not withstand such temperatures. The PES film with AgNWs showed stable electrical performance at 150 and 200 °C for 60 min; however, its sheet resistance increased significantly upon heating to 250 °C (see Supporting Information, Figure S5). As shown in Figure 3a, the PES film with Ag nanomesh showed stable electrical performance at 150 °C for 60min. However, its sheet resistance increased significantly when it was heated to 200–250 °C.
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Figure 3. Changes in the sheet resistance and morphology of a) pure Ag and b) AgNi nanomesh TCE samples during the thermo-mechanical stability test.
High-temperature (200–250 °C) annealing caused the Ag nanomesh to disintegrate, as shown in the inset of Figure 3a. In contrast, the PES film with AgNi nanomesh showed stable electrical performance at high temperature, reflecting its excellent thermo-mechanical stability (Figure 3b). As shown in the inset of Figure 3b, the AgNi nanomesh on PES film showed a practically negligible change in its morphology after annealing at 250 °C for 60 min. This observation may be due to the higher melting point of AgNi compared with that of pure Ag, as seen in the Ag-Ni phase diagram.[45] The change in morphology of pure Ag and AgNi nanomeshes after the thermo-mechanical stability test was confirmed by FE-SEM analysis (see Supporting Information, Figure S6). We also performed the chemical stability test on the Ag and AgNi nanomeshes. Figure 4 shows the changes in sheet resistance of both samples during the test; its inset shows the FE-SEM image of the Ag and AgNi nanomesh on PES films after the test. For comparison, a sample of spin-coated AgNWs on PES film was also tested under the same conditions (see Supporting Information, Figure S7). The sheet resistance of all TCE samples was measured after they were dipped in deionized (DI) water, isopropyl alcohol (IPA), or
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High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode
(85–90% relative humidity, RH) at 85–90 °C for 20 days. For comparison, a sample of spin-coated AgNWs on PES film was also tested under the same conditions. Figure 5 shows the changes in sheet resistance and morphology of all samples for the test. As shown in Figure 5a, the sheet resistance of AgNWs increased significantly after they were exposed for only 4 days. After 7 days, the sheet resistance of AgNWs could no longer be measured. The sheet resistance of the Ag nanomesh increased more than a hundredfold (from 10.54 to 1042.2 Ω sq−1) after 13 days, after which time it was no longer functioning. It is well-known that Ag is easily oxidized when it is exposed to air and water.[8,10] In contrast, the sheet resistance of the AgNi nanomesh only slightly increased from 25.5 to 99.4 Ω sq−1, even after exposure for 20 days. This vast difference clearly confirms that nickel was a good antioxidant for the AgNi nanomesh, as it slowed the rate of oxidation.[46–48] FE-SEM and EDS analyses of pure Ag and AgNi nanomeshes were performed before and after the long-term stability test to investigate the cause of the enhanced stability of AgNi nanomesh toward oxidation. The results reveal that silver oxides formed on the surface of the Ag nanomesh (see Figure 5b and Supporting Information Figure S8). In contrast, the AgNi nanomesh had scarcely any silver oxides on its surface. Thus, the AgNi nanomesh TCE showed little change in sheet resistance compared with that of TCEs based on AgNWs and on Ag nanomesh. These experimental results clearly indicate that nickel could protect the metal-based nanomesh from oxidation, and could thereby greatly improve the long-term stability of the AgNi nanomesh.
Figure 4. Changes in the sheet resistance and morphology of a) pure Ag and b) AgNi nanomesh TCE samples during the chemical stability test.
acidic PEDOT:PSS solution for 72 h. The sheet resistance of AgNWs and Ag nanomesh increased significantly when they were dipped in IPA and PEDOT:PSS solution (see Figure 4a and Supporting Information Figure S7). In particular, the sheet resistance of the AgNWs and Ag nanomesh increased more than tenfold when they were dipped in acidic PEDOT:PSS solution. This change was due to corrosion of some parts of the AgNWs and Ag nanomesh by the acidic solution. The inset of Figure 4a shows the FE-SEM image of the Ag nanomesh partially etched by the acidic PEDOT:PSS solution. In contrast, the sheet resistance of the AgNi nanomesh slightly increased upon dipping in each of the solutions, as shown in the Figure 4b. In particular, the AgNi nanomesh showed high resistance to acidic corrosion by the PEDOT:PSS solution, confirming that nickel conferred chemical inertness of the AgNi nanomesh toward acidic conditions. As shown in the inset of Figure 4b, the AgNi nanomesh on PES film was maintained without serve corrosion damage or structure breakdown after the chemical stability test, even though it has slightly morphological change such as roughness increase in edge profiles. To evaluate the long-term stability of the metalbased nanomesh to oxidation, TCEs of pure Ag and AgNi nanomesh were exposed to high humidity conditions small 2014, DOI: 10.1002/smll.201400911
Figure 5. Changes in the a) sheet resistance and b) morphology of pure Ag and AgNi nanomesh TCE samples during the oxidation-resistance test.
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(AgNWs, pure Ag, and AgNi nanomesh) were rolled around a bending radius of 5 mm, and then unrolled at a speed of 0.5 cycle/s, after which the sheet resistance of each TCE was compared to against its initial value. Figure 6a shows the changes in sheet resistance of pure Ag nanomesh, AgNi nanomesh, and AgNWs during this test. The inset of Figure 6a shows an image of the bending-fatigue test. After 50,000 bending cycles, the change in sheet resistance of AgNWs, pure Ag nanomesh, and AgNi nanomesh was within 4% of their respective original values, as shown in Figure 6a. The resistance change of all flexible TCE films as a function of the bending radius (3–15 mm) was also shown in Figure 6b. This indicates that the metal nanomesh possessed high mechanical flexibility, comparable to that of AgNW-based TCE.[49] Figure 6c shows an image of a lightemitting diode lit by using an electrically connected flexible AgNi nanomesh that is in the bending state. The electrical performance of the metal-based nanomesh TCEs was also found to be stable even after severe folding and unfolding in the test (see Supporting Information, Figure S9). In summary, the metal-based nanomesh TCE proved to be highly stable in terms of bending and folding.
3. Conclusion In conclusion, we demonstrated the high-durable AgNi nanomesh TCE that exhibit strong adhesion. The AgNi nanomesh TCE was prepared by using a combination of NIL and transfer printing. The flexible AgNi nanomesh TCE exhibited optical and electrical properties comparable to those of ITO, long-term stability to oxidation, as well as thermal and chemical stability. The optical and electrical performance of the AgNi nanomesh TCE was found to be robust and largely unaffected even during high-temperature annealing at 250 °C for 1 h. The chemical stability of the AgNi nanomesh TCE was further confirmed by testing in acidic PEDOT:PSS solution corrosion for extended period. The resistance of the AgNi nanomesh TCE slightly increased during the longterm stability test (85–90 °C and 85–90% RH condition for 20 days) relative to that of pure Ag nanomesh TCE. The results indicate that nickel could protect the AgNi nanomesh from heat, chemical attack, and oxidation. We expect that the AgNi nanomesh TCE could be used as an alternative to ITO for use in high-performance optoelectronic devices. Figure 6. a) Changes in the sheet resistance of the AgNW-based, Ag nanomesh, and AgNi nanomesh TCEs during the mechanical flexibility test. b) Resistance change as function of the bending radius. c) Photograph of the lighting of a red light-emitting diode by using an electrically connected, flexible AgNi nanomesh TCE in the bended state.
The mechanical flexibility of TCE is of prime importance for its use in flexible optoelectronic devices. The bending flexibility of the metal-based nanomesh was assessed by using bending-fatigue test equipment. For comparison, a sample of spin-coated AgNWs on PES film was also tested. In general, the high flexibility of AgNWs is widely known.[8,21] For the bending-fatigue test, all TCE samples
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4. Experimental Section Materials: NOA 61 was purchased from Norland Products, Inc. UV-curable PUA resin (YNIL-M2) was purchased from Young Chang Chemical Co., Ltd. PES film with thickness of 200 µm was purchased from i-components Co., Ltd. Ag (99.99%) and AgNi (80:20 wt%) for e-beam deposition were purchased from Kojundo Chemical Laboratory Co., Ltd. and Shenzhen JYK Metal Materials Co., Ltd., respectively. Preparation of AgNW-based TCEs: AgNW-based TCEs were prepared through the spin-coating process and were formed on precleaned PES films. An as-received dispersion containing AgNWs
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High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode
(ClearOhm Ink, Cambrios) was spin coated for 60 s at 1500 rpm. The dispersion was sonicated for 60 s and shaken well before spin-coating. The formed AgNW networks were annealed on a hot plate for 25 min at 230 °C. Optical, Electrical, and Microscopic Characterization: Transmittance spectra were obtained on a Neosys-2000 UV-Vis Spectrophotometer (SCINCO). Air was used reference. The sheet resistances of the TCEs were measured through the four-probe method using a sheet resistivity meter (FPP-2400, DASOL ENG) that was directly in contact with the center of the TCE. FE-SEM images were taken and EDS was performed on a S-4800 (Hitach) at an accelerating voltage of 15.0 kV. Focused ion beam (FIB) images were obtained on a Helios NanoLab at an accelerating voltage of 2.0 kV. Adhesion Test: The adhesion tape test was done as follows: An adhesive tape (810, 3M) was firmly pressed onto the surface of the TCEs and slowly peeled off. The process was repeated several times to evaluate the adhesion property of the TCEs. The sheet resistance of all TCE samples was measured during this test. Thermo-Mechanical Stability Test: The thermo-mechanical stabilities of all TCEs based on PES film were evaluated by directly measuring the sheet resistance while the samples were placed on a hot plate. The sheet resistance of all samples was measured during heat treatment for 60min at 150, 200, and 250 °C. Chemical Stability Test: All TCE samples were immersed in a DI water, IPA, or acidic PEDOT:PSS (CLEVIOS PH 1000, Heraeus) solution for 72 h. The sheet resistance of all samples was measured during this test. Long-Term Stability: All TCEs were exposed to high temperature (85–90 °C) and high humidity (85–90% RH) for 20 days. The sheet resistance of all TCE samples was measured during this test. Bending Test: The bending test was performed as follows: TCEs on PES films were fixed on a cylinder with a curvature radius of 5 mm with the TCE film facing the convex side. All TCE samples were then repeatedly bent by an automatic bending apparatus. The sheet resistance of all samples was measured during this test.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This research was supported by the Korea Institute of Machinery & Materials (KIMM) Research Funds (KM3460) and National Research Foundation (NRF) grants of Korea (Nos. 2011–0028585 and 2009–0082527).
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Received: April 2, 2014 Revised: April 29, 2014 Published online:
small 2014, DOI: 10.1002/smll.201400911
High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode Han-Jung Kim, Su-Han Lee, Jihye Lee, Eung-Sug Lee, Jun-Hyuk Choi, Jun-Ho Jung, Joo-Yun Jung, and Dae-Geun Choi*
Uniform
metal nanomesh structures are promising candidates that may replace of indium-tin oxide (ITO) in transparent conducting electrodes (TCEs). However, the durability of the uniform metal mesh has not yet been studied. For this reason, a comparative analysis of the durability of TCEs based on pure Ag and AgNi nanomesh, which are fabricated by using simple transfer printing, is performed. The AgNi nanomesh shows high long-term stability to oxidation, heat, and chemicals compared with that of pure Ag nanomesh. This is because of nickel in the AgNi nanomesh. Furthermore, the AgNi nanomesh shows strong adhesion to a transparent substrate and good stability after repeated bending.
1. Introduction Transparent conducting electrodes (TCEs) are key materials in the information technology industry. Smartphones, touchscreens, flat-display panels, and solar cells all require a TCE layer that allows charge transport without blocking the transmission of light.[1–4] Indium-tin oxide (ITO) has been widely used as the TCE in various applications as it possesses excellent physical properties, such as high optical transmittance and low sheet resistance. However, indium is scarce. Furthermore, ITO on a flexible substrate is brittle and cracks easily, and its production requires high-temperature processing.[1–10] Therefore, there is a strong need to find alternative materials for high-performance TCEs. Research has been done to explore the potential of conductive polymers[11–13] and carbon nanomaterials such as nanotubes[14–16] and graphenes[17–20] to replace ITO; however, TCEs based on such materials are unable to compete with those based on ITO in terms of their optical and electrical properties.[21]
Dr. H.-J. Kim, S.-H. Lee, Dr. J. Lee, Dr. E.-S. Lee, Dr. J.-H. Choi, Dr. J.-H. Jung, Dr. J.-Y. Jung, Dr. D.-G. Choi Nano-Mechanical Systems Research Division Korea Institute of Machinery & Materials (KIMM) 171 Jang-dong, Yuseong-gu Daejeon 305–343, Republic of Korea E-mail: [email protected] DOI: 10.1002/smll.201400911 small 2014, DOI: 10.1002/smll.201400911
Metal-based TCEs such as random networks of silver nanowires (AgNWs)[21–27] and uniform metal meshes[6,7,28–31] are considered the most promising replacement of ITO because of their excellent optical and electrical properties, good flexibility, and compatibility with low-cost solution processes.[22] Several issues, however, still need to be addressed before AgNW networks can be widely applied in electronic devices. First, strong adhesion between the AgNW network and substrate needs to be achieved, as the AgNW network is easily detached from the substrate.[9,22] Second, the AgNW network cannot be used as-prepared, but heat treatment at around 200 °C is first required in order to obtain the high electrical conductivity of AgNW-based TCEs. This limits the application of AgNWs in heat-sensitive substrates.[9,21,22] Lastly, they still have a long-term stability issues, which makes them difficult for practical use.[10,22] Specifically, when AgNW networks are exposed to air and water, they easily undergo oxidation, leading to a sharp increase in the sheet resistance of the AgNW-based TCEs. To address these problems, several methods have been investigated, such as embedding of the AgNW network in a polymeric matrix; application of additional protective layers on the AgNW network using polymers,[8,22] graphene,[32] and metal oxide;[1,33] and the use of metal nanowires with a nickel coating.[10] However, all of these methods have resulted in complication of the TCE manufacturing process. Recently, uniform metal-mesh structures have also appeared promising because their transmittance and electrical conductivity can be easily tuned simply by changing
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the line width, line spacing, or thickness of the metal pattern. In contrast to AgNWbased TCEs, the work function of metalmesh for different applications can be easily controlled by simply changing the metallic material.[6,7,29,34] However, several conventional metal-patterning methods necessitate the use of complex and expensive technologies such as electron (E) beam lithography and lift-off processes.[29,34] Although a facile method of fabricating metal-mesh TCEs using nano-imprint lithography (NIL) and transfer printing has been recently proposed,[6,7] longterm stability remains an important issue that poses difficulties in their practical applications. It is well-known that metals such as Ag and copper can be easily oxidized when they are exposed to air.[8,10]Nevertheless, the durability of metal-mesh TCEs has never been studied. Therefore, further research on highly robust and stable metal-mesh TCEs must be done for their development for practical use. We demonstrate a high-durable AgNi nanomesh for the TCE, which exhibits strong adhesion to a transparent substrate. Our nanomesh had excellent long-term stability to oxidation, high temperature, and chemicals compared with pure Ag nanomesh. It also showed high stability in terms of bending and folding. The nanomesh could be easily prepared by transfer printing. The transmittance and sheet resistance of the nanomesh TCE could be controlled by manipulating process conditions such as evaporation materials, metal thickness, and pattern dimensions. The TCE based on AgNi nanomesh may be an alternative to ITO in high-performance optoelectronic devices such as flexible solar cells and displays.
2. Results and Discussion Metal-based nanomesh TCEs on a transparent substrate were prepared by using a combination of NIL and the transfer printing method.[6,7,35,36] Figure 1a shows the fabrication process of uniform metal-based nanomesh TCE on transparent substrate. Figure 1. a) Fabrication process of uniform metal nanomesh TCEs on a transparent First of all, a polyurethane acrylate (PUA) substrate. b) Transmittance spectra and FE-SEM images of 60 nm Ag nanomesh patterns (150 nm width and various line spacing) on a glass. c) Transmittance and sheet resistance mold was replicated from a silicon master of pure Ag and AgNi nanomesh TCEs on PES film with 60, 80, and 100 nm thickness, and through the replica molding method (see Sup- same line-spacing (3.2 µm) and line width (150 nm). d) Comparison of the FoM σ / σ ( dc opt ) porting Information, Figure S1a). To increase values for TCEs of PEDOT:PSS,[11,12] graphene,[39–42] metal nanowires,[3,43,44] and metal the transfer ratio of the metal nanopat- mesh.[6,28,34] The red stars indicate the experimental results obtained in this work. tern from the mold, the surface of the PUA mold was treated with an antisticking layer (1H,1H,2H,2H- evaporator (see Supporting Information, Figure S1b). At this perfluorooctyl-trichlorosilane) before metal evaporation. point, the main material of the metal nanomesh could be Next, a thin metal layer with a thickness of 60–100 nm was easily changed by changing the evaporation materials. Furdeposited on the PUA mold by using an electron-beam thermore, the thickness of the nanomesh could be controlled
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small 2014, DOI: 10.1002/smll.201400911
High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode
by changing the deposition thickness. The evaporation rate was about 0.1 nm s−1. To transfer a metal layer from the PUA mold, a layer of transparent ultraviolet (UV) -curable adhesive (Norland Optical Adhesive 61, NOA 61) with thickness of ≈800 nm was spin-coated on a transparent substrate. To cure partially the adhesive, the coated substrate was exposed to UV for 20 s by using a 50 W mercury lamp. The metalcoated PUA mold was then placed in contact with the top of the partially cured substrate, and the assembly was completely UV-cured with 0.15 MPa static pressure for 3 min. Finally, the uniform metal nanomesh that was formed on the transparent substrate was obtained after peeling the PUA mold (see Supporting Information, Figure S1c). Figure 1(b) shows the optical transmittance spectra and the field emission scanning electron microscopy (FE-SEM) images of the fabricated 60 nm Ag nanomesh patterns on a glass substrate, which had a width of 150 nm and line spacing of 1.0 to 3.2 µm. The transmittance spectrum of the fabricated Ag nanomesh ranged from 400 to 800 nm, where the black line represents a commercial high-transmittance ITO glass for comparison. All transmittance spectral measurements were referenced to air. Commercial ITO glass had a transmittance of 88% at 550 nm and an average transmittance of 84% over the entire visible range. Average transmittances of the Ag nanomesh on glass with 1.0, 1.6, and 3.2 µm line spacing at 550 nm were 62.5, 76.1, and 87.5%, respectively. This verifies that the transmittance increased with the increase in line spacing. The sheet resistance, another important parameter of TCEs, was measured by using the four-point probe-type sheet resistance meter. The sheet resistances of the Ag nanomesh on glass with 1.0, 1.6, and 3.2 µm line spacing were 7.9, 25.1, and 49.2 Ω sq−1, respectively. This verifies that the sheet resistance increased with the increase in line spacing. These results indicate that the transmittance and sheet resistance depend on the line spacing of the nanomesh pattern. They are identical to those for the AgNi nanomesh on the flexible substrate. Indeed, we fabricated a AgNi nanomesh on a flexible polyethersulfone (PES) film and measured its transmittance and sheet resistance (see Supporting Information, Figure S2a). To confirm the presence of nickel in the AgNi nanomesh, we carried out energy-dispersive spectroscopy (EDS) measurements on the nanomesh. EDS analysis confirmed the presence of nickel in the AgNi nanomesh film (see Supporting Information, Figure S2b). The transmittance and sheet resistance of the fabricated nanomesh samples of pure Ag and AgNi of each line spacing size and thickness were measured. Figure 1c shows their transmittance and sheet resistance on PES film with 60, 80, and 100 nm thickness and line spacing of 3.2 µm. As shown in Figure 1c, the sheet resistance and transmittance increased with decreasing mesh thickness. This means that the sheet resistance and transmittance depended on the thickness of the nanomesh pattern. These experimental results are in agreement with previous studies.[7,29] Furthermore, we found that the transmittance of the AgNi nanomesh was similar to that of Ag nanomesh TCEs, but the sheet resistance of the AgNi nanomesh was slightly larger than that of Ag nanomesh TCEs. The high sheet resistance small 2014, DOI: 10.1002/smll.201400911
of the AgNi nanomesh is due to the electrical conductivity of pure Ag, which is higher than that of AgNi.[37] As shown in Figure 1b,c, there is a trade off between transmittance and sheet resistance in the fabricated metal-based nanomesh. In other words, the transmittance increased with decreasing mesh thickness or with increasing line spacing sizes of the nanomesh patterns, both of which could cause an increase in sheet resistance. Decreased transmittance could be easily compensated for by selecting mesh patterns with wide line spacing. Theses experimental results indicate that the transmittance and sheet resistance of metal-based nanomeshes prepared through the polymer-based transfer method was comparable to that of commercial ITO. To gain insight into the effect of nanomesh thickness on the overall performance of the TCE, we determined the figure of merit (FoM), which is the ratio of the electrical conductance to the optical conductance (σ dc / σ opt ) as a function of nanomesh thickness (inset, Figure 1c). The expression for σ dc / σ opt is as follows:[11,21,34] ⎛ 188.5 σ opt ⎞ T = ⎜1 + RS σ dc ⎟⎠ ⎝
−2
(1)
where RS and T are the measured sheet resistance and transmittance at 550 nm, respectively. The dashed black horizontal line at FoM value of 35 (RS < 100 Ωsq−1 at T > 90%) indicates the minimum value needed for most TCE applications.[11,38] However, some applications such those for liquid crystal displays require FoM values upward of 50.[11,38] As shown in the inset of Figure 1c, the FoM of our metal-based mesh TCEs was higher than the minimum FoM value required for most TCE applications. Figure 1d shows some of the best FoM values for TCEs of poly-(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) (37–50),[11,12] graphene (10–120),[39–42] metal nanowires (70–160),[3,43,44] and metal meshes (55–160).[6,28,34] Here, red stars indicate the experimental results obtained in our study. Figure 2a shows the AgNi nanomesh on the PES film fabricated through the polymer-assisted transfer method. As shown in Figure 2a, a large area 100 mm × 100 mm of AgNi nanomesh was successfully fabricated. To evaluate the adhesion between metal nanomesh film and substrate, the fabricated metal-based nanomesh was subjected to a repeated adhesive tape test. Figure 2b shows the changes in sheet resistance of the AgNW-based and metalbased nanomesh TCEs during the test. As shown in Figure 2b, the sheet resistance of the AgNW-based TCE increased significantly during this test. After four consecutive tests, the sheet resistance of the AgNW-based TCE could no longer be measured. However, the sheet resistance of the metalbased nanomesh remained unchanged after 100 cycles. This suggests that adhesion between the metal-based nanomesh film and substrate was very strong even without any additional treatments (see also Supporting Information Figure S3 for adhesive test result of AgNWs based TCE prepared by using our transfer printing method). The reason was that the AgNi nanomesh film was partially embedded inside the UVadhesive layer, as shown in the FIB analysis (see Supporting Information, Figure S4).
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Figure 2. a) Large-area, flexible AgNi nanomesh. b) Changes in the sheet resistance of the AgNW-based, Ag nanomesh, and AgNi nanomesh TCE samples during the repeated adhesive tape test.
High-performance TCEs should be able to withstand high temperature in the post-processing stages in order to ensure excellent device performance. For this reason, the thermo-mechanical stabilities of the Ag and AgNi nanomeshes on PES film were also evaluated by directly measuring sheet resistance when the samples were placed on a hot plate. Figure 3 shows the changes in the sheet resistance of both samples during this test. The inset of Figure 3 shows the FE-SEM image of the Ag and AgNi nanomesh TCEs based on PES film after the test. For comparison, a sample of spin-coated AgNWs on a PES film was also tested under the same conditions (see Supporting Information, Figure S5). The sheet resistance of all samples was measured during heat treatment for 60 min at 150, 200, and 250 °C. It should be noted that the test temperature was deliberately not raised above 250 °C, as the PES film could not withstand such temperatures. The PES film with AgNWs showed stable electrical performance at 150 and 200 °C for 60 min; however, its sheet resistance increased significantly upon heating to 250 °C (see Supporting Information, Figure S5). As shown in Figure 3a, the PES film with Ag nanomesh showed stable electrical performance at 150 °C for 60min. However, its sheet resistance increased significantly when it was heated to 200–250 °C.
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Figure 3. Changes in the sheet resistance and morphology of a) pure Ag and b) AgNi nanomesh TCE samples during the thermo-mechanical stability test.
High-temperature (200–250 °C) annealing caused the Ag nanomesh to disintegrate, as shown in the inset of Figure 3a. In contrast, the PES film with AgNi nanomesh showed stable electrical performance at high temperature, reflecting its excellent thermo-mechanical stability (Figure 3b). As shown in the inset of Figure 3b, the AgNi nanomesh on PES film showed a practically negligible change in its morphology after annealing at 250 °C for 60 min. This observation may be due to the higher melting point of AgNi compared with that of pure Ag, as seen in the Ag-Ni phase diagram.[45] The change in morphology of pure Ag and AgNi nanomeshes after the thermo-mechanical stability test was confirmed by FE-SEM analysis (see Supporting Information, Figure S6). We also performed the chemical stability test on the Ag and AgNi nanomeshes. Figure 4 shows the changes in sheet resistance of both samples during the test; its inset shows the FE-SEM image of the Ag and AgNi nanomesh on PES films after the test. For comparison, a sample of spin-coated AgNWs on PES film was also tested under the same conditions (see Supporting Information, Figure S7). The sheet resistance of all TCE samples was measured after they were dipped in deionized (DI) water, isopropyl alcohol (IPA), or
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High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode
(85–90% relative humidity, RH) at 85–90 °C for 20 days. For comparison, a sample of spin-coated AgNWs on PES film was also tested under the same conditions. Figure 5 shows the changes in sheet resistance and morphology of all samples for the test. As shown in Figure 5a, the sheet resistance of AgNWs increased significantly after they were exposed for only 4 days. After 7 days, the sheet resistance of AgNWs could no longer be measured. The sheet resistance of the Ag nanomesh increased more than a hundredfold (from 10.54 to 1042.2 Ω sq−1) after 13 days, after which time it was no longer functioning. It is well-known that Ag is easily oxidized when it is exposed to air and water.[8,10] In contrast, the sheet resistance of the AgNi nanomesh only slightly increased from 25.5 to 99.4 Ω sq−1, even after exposure for 20 days. This vast difference clearly confirms that nickel was a good antioxidant for the AgNi nanomesh, as it slowed the rate of oxidation.[46–48] FE-SEM and EDS analyses of pure Ag and AgNi nanomeshes were performed before and after the long-term stability test to investigate the cause of the enhanced stability of AgNi nanomesh toward oxidation. The results reveal that silver oxides formed on the surface of the Ag nanomesh (see Figure 5b and Supporting Information Figure S8). In contrast, the AgNi nanomesh had scarcely any silver oxides on its surface. Thus, the AgNi nanomesh TCE showed little change in sheet resistance compared with that of TCEs based on AgNWs and on Ag nanomesh. These experimental results clearly indicate that nickel could protect the metal-based nanomesh from oxidation, and could thereby greatly improve the long-term stability of the AgNi nanomesh.
Figure 4. Changes in the sheet resistance and morphology of a) pure Ag and b) AgNi nanomesh TCE samples during the chemical stability test.
acidic PEDOT:PSS solution for 72 h. The sheet resistance of AgNWs and Ag nanomesh increased significantly when they were dipped in IPA and PEDOT:PSS solution (see Figure 4a and Supporting Information Figure S7). In particular, the sheet resistance of the AgNWs and Ag nanomesh increased more than tenfold when they were dipped in acidic PEDOT:PSS solution. This change was due to corrosion of some parts of the AgNWs and Ag nanomesh by the acidic solution. The inset of Figure 4a shows the FE-SEM image of the Ag nanomesh partially etched by the acidic PEDOT:PSS solution. In contrast, the sheet resistance of the AgNi nanomesh slightly increased upon dipping in each of the solutions, as shown in the Figure 4b. In particular, the AgNi nanomesh showed high resistance to acidic corrosion by the PEDOT:PSS solution, confirming that nickel conferred chemical inertness of the AgNi nanomesh toward acidic conditions. As shown in the inset of Figure 4b, the AgNi nanomesh on PES film was maintained without serve corrosion damage or structure breakdown after the chemical stability test, even though it has slightly morphological change such as roughness increase in edge profiles. To evaluate the long-term stability of the metalbased nanomesh to oxidation, TCEs of pure Ag and AgNi nanomesh were exposed to high humidity conditions small 2014, DOI: 10.1002/smll.201400911
Figure 5. Changes in the a) sheet resistance and b) morphology of pure Ag and AgNi nanomesh TCE samples during the oxidation-resistance test.
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(AgNWs, pure Ag, and AgNi nanomesh) were rolled around a bending radius of 5 mm, and then unrolled at a speed of 0.5 cycle/s, after which the sheet resistance of each TCE was compared to against its initial value. Figure 6a shows the changes in sheet resistance of pure Ag nanomesh, AgNi nanomesh, and AgNWs during this test. The inset of Figure 6a shows an image of the bending-fatigue test. After 50,000 bending cycles, the change in sheet resistance of AgNWs, pure Ag nanomesh, and AgNi nanomesh was within 4% of their respective original values, as shown in Figure 6a. The resistance change of all flexible TCE films as a function of the bending radius (3–15 mm) was also shown in Figure 6b. This indicates that the metal nanomesh possessed high mechanical flexibility, comparable to that of AgNW-based TCE.[49] Figure 6c shows an image of a lightemitting diode lit by using an electrically connected flexible AgNi nanomesh that is in the bending state. The electrical performance of the metal-based nanomesh TCEs was also found to be stable even after severe folding and unfolding in the test (see Supporting Information, Figure S9). In summary, the metal-based nanomesh TCE proved to be highly stable in terms of bending and folding.
3. Conclusion In conclusion, we demonstrated the high-durable AgNi nanomesh TCE that exhibit strong adhesion. The AgNi nanomesh TCE was prepared by using a combination of NIL and transfer printing. The flexible AgNi nanomesh TCE exhibited optical and electrical properties comparable to those of ITO, long-term stability to oxidation, as well as thermal and chemical stability. The optical and electrical performance of the AgNi nanomesh TCE was found to be robust and largely unaffected even during high-temperature annealing at 250 °C for 1 h. The chemical stability of the AgNi nanomesh TCE was further confirmed by testing in acidic PEDOT:PSS solution corrosion for extended period. The resistance of the AgNi nanomesh TCE slightly increased during the longterm stability test (85–90 °C and 85–90% RH condition for 20 days) relative to that of pure Ag nanomesh TCE. The results indicate that nickel could protect the AgNi nanomesh from heat, chemical attack, and oxidation. We expect that the AgNi nanomesh TCE could be used as an alternative to ITO for use in high-performance optoelectronic devices. Figure 6. a) Changes in the sheet resistance of the AgNW-based, Ag nanomesh, and AgNi nanomesh TCEs during the mechanical flexibility test. b) Resistance change as function of the bending radius. c) Photograph of the lighting of a red light-emitting diode by using an electrically connected, flexible AgNi nanomesh TCE in the bended state.
The mechanical flexibility of TCE is of prime importance for its use in flexible optoelectronic devices. The bending flexibility of the metal-based nanomesh was assessed by using bending-fatigue test equipment. For comparison, a sample of spin-coated AgNWs on PES film was also tested. In general, the high flexibility of AgNWs is widely known.[8,21] For the bending-fatigue test, all TCE samples
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4. Experimental Section Materials: NOA 61 was purchased from Norland Products, Inc. UV-curable PUA resin (YNIL-M2) was purchased from Young Chang Chemical Co., Ltd. PES film with thickness of 200 µm was purchased from i-components Co., Ltd. Ag (99.99%) and AgNi (80:20 wt%) for e-beam deposition were purchased from Kojundo Chemical Laboratory Co., Ltd. and Shenzhen JYK Metal Materials Co., Ltd., respectively. Preparation of AgNW-based TCEs: AgNW-based TCEs were prepared through the spin-coating process and were formed on precleaned PES films. An as-received dispersion containing AgNWs
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201400911
High-Durable AgNi Nanomesh Film for a Transparent Conducting Electrode
(ClearOhm Ink, Cambrios) was spin coated for 60 s at 1500 rpm. The dispersion was sonicated for 60 s and shaken well before spin-coating. The formed AgNW networks were annealed on a hot plate for 25 min at 230 °C. Optical, Electrical, and Microscopic Characterization: Transmittance spectra were obtained on a Neosys-2000 UV-Vis Spectrophotometer (SCINCO). Air was used reference. The sheet resistances of the TCEs were measured through the four-probe method using a sheet resistivity meter (FPP-2400, DASOL ENG) that was directly in contact with the center of the TCE. FE-SEM images were taken and EDS was performed on a S-4800 (Hitach) at an accelerating voltage of 15.0 kV. Focused ion beam (FIB) images were obtained on a Helios NanoLab at an accelerating voltage of 2.0 kV. Adhesion Test: The adhesion tape test was done as follows: An adhesive tape (810, 3M) was firmly pressed onto the surface of the TCEs and slowly peeled off. The process was repeated several times to evaluate the adhesion property of the TCEs. The sheet resistance of all TCE samples was measured during this test. Thermo-Mechanical Stability Test: The thermo-mechanical stabilities of all TCEs based on PES film were evaluated by directly measuring the sheet resistance while the samples were placed on a hot plate. The sheet resistance of all samples was measured during heat treatment for 60min at 150, 200, and 250 °C. Chemical Stability Test: All TCE samples were immersed in a DI water, IPA, or acidic PEDOT:PSS (CLEVIOS PH 1000, Heraeus) solution for 72 h. The sheet resistance of all samples was measured during this test. Long-Term Stability: All TCEs were exposed to high temperature (85–90 °C) and high humidity (85–90% RH) for 20 days. The sheet resistance of all TCE samples was measured during this test. Bending Test: The bending test was performed as follows: TCEs on PES films were fixed on a cylinder with a curvature radius of 5 mm with the TCE film facing the convex side. All TCE samples were then repeatedly bent by an automatic bending apparatus. The sheet resistance of all samples was measured during this test.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This research was supported by the Korea Institute of Machinery & Materials (KIMM) Research Funds (KM3460) and National Research Foundation (NRF) grants of Korea (Nos. 2011–0028585 and 2009–0082527).
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Received: April 2, 2014 Revised: April 29, 2014 Published online:
small 2014, DOI: 10.1002/smll.201400911
