Epitaxial-Growth-Induced Junction Welding of Silver Nanowire

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Epitaxial-Growth-Induced Junction Welding of Silver Nanowire Network Electrodes Hyungseok Kang, Sol-Ji Song, Young Eun Sul, Byeong-Seon An, Zhenxing Yin, Yongsuk Choi, Lyongsun Pu, Cheol-Woong Yang, Youn Sang Kim, Sung Min Cho, Jung Gu Kim, and Jeong Ho Cho ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01900 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Epitaxial-Growth-Induced Junction Welding of Silver Nanowire Network Electrodes Hyungseok Kang,1† Sol-Ji Song,2† Young Eun Sul,3 Byeong-Seon An,2 Zhenxing Yin,6 Yongsuk Choi,1 Lyongsun Pu,5 Cheol-Woong Yang,2 Youn Sang Kim,6 Sung Min Cho,1,3 Jung Gu Kim,2* Jeong Ho Cho1,3,4* 1

SKKU Advanced Institute of Nanotechnology (SAINT), 2School of Materials Science and Engineering, 3 School of Chemical Engineering, 4Department of Nano Engineering, 5Research & Business Foundation, Sungkyunkwan University, Suwon 440-746, Republic of Korea. 6 Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, Republic of Korea. E-mail: [email protected] and [email protected] †

H. Kang and S.-J. Song contributed equally to this work.

Abstract In this study, we developed a roll-to-roll Ag electroplating process for metallic nanowire electrodes using a Galvanostatic mode. Electroplating is a low-cost and facile method for deposition of metal onto a target surface with precise control of both the composition and the thickness. Metallic nanowire networks [silver nanowires (AgNWs) and copper nanowires (CuNWs)] coated onto a polyethylene terephthalate (PET) film were immersed directly in an electroplating bath containing AgNO3. Solvated silver ions (Ag+ ions) were deposited onto the nanowire surface through application of a constant current via an external circuit between the nanowire networks (cathode) and a Ag plate (anode). The amount of electroplated Ag was systematically controlled by changing both the applied current density and the electroplating time, which enabled precise control of the sheet resistance and optical transmittance of the metallic nanowire networks. The optimized Ag-electroplated AgNW (Ag-AgNW) films exhibited a sheet resistance of ~19 Ω/sq at an optical transmittance of 90% (550 nm). A transmission electron microscopy (TEM) study confirmed that Ag grew epitaxially on the AgNW surface but a polycrystalline Ag structure was formed on the CuNW surface. The Ag-electroplated metallic nanowire electrodes were successfully applied to various electronic devices such as organic light-emitting diodes, triboelectric nanogenerators, and a resistive touch panel. The proposed roll-to-roll Ag electroplating process provides a simple, low-cost, and scalable method for the fabrication of enhanced transparent conductive electrode materials for nextgeneration electronic devices.

Keywords: silver nanowire, electroplating, epitaxial growth, transparent electrode, roll-to-roll

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Transparent conductive electrodes (TCEs) are essential components of various flexible optoelectronic devices such as organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and touch screens.1-6 The use of commercially available indium tin oxide (ITO) in flexible electronics is limited by its high cost and brittle nature; hence, the percolation network of metallic nanowires can serve as a mechanically stable TCE with sheet resistance and optical transmittance comparable to those of ITO.7-12 Moreover, nanowire networks can be readily prepared by simple solution deposition methods such as the Meyer-rod coating, spray-coating, and spin-coating methods.13-17 (3) However, several crucial issues need to be addressed before nanowire networks can be applied commercially, such as their high surface roughness, poor adhesion with the substrate, naturally formed surface oxide layer, and high contact resistance at the wire-to-wire junction. Recently, various post-welding processes have been intensively studied to reduce the high junction resistance of nanowire networks. For example, the thermal annealing process was reported to dramatically reduce the junction resistance; however, this annealing process may cause substantial damage to a plastic substrate with a low glass transition temperature.18, 19 Alternative post treatments that do not damage the plastic substrate have been reported, such as plasmonic welding and Joule heating; however, the limitation of mass production capability and the necessity of a high optical power density of the lamp would inhibit the commercial application of large-scale flexible TCEs.20-26 Recently, the chemical reduction method27-34 and electrochemical metal deposition method35-37 have been proposed to minimize the junction resistance and produce large-scale TCEs. However, the partial damage to or delamination of nanowires induced by severe chemical reduction cannot be excluded. Irregular deposition of metal ions by electrochemical reactions such as electroless plating, electrodeposition, and Galvanostatic displacement yielded nanowires with extremely rough surfaces. Herein, we demonstrate a roll-to-roll Ag electroplating process for metallic nanowire electrodes using the Galvanostatic mode. Electroplating is a low-cost and facile method for deposition of metallic materials onto a target surface with precise control of the composition and thickness. This electrochemical reduction process enables accurate deposition of metallic materials with minimal metal waste. It should be noted that electroplating is currently used as the roll-to-roll process at the industrial level, which is beneficial for efficient fabrication of large-scale electronic devices. In our roll-to-roll system, metallic nanowire films, i.e., silver nanowire (AgNW) and copper nanowire (CuNW) films, coated onto a polyethylene terephthalate (PET) film were directly immersed in an electroplating bath containing AgNO3. Silver ions (Ag+ ions) solvated in the electroplating bath were deposited onto the nanowire surface by applied current via an external circuit between the metallic nanowire networks (cathode) and a Ag plate (anode). The amount of electroplated Ag was systematically controlled by varying both the applied current density and the electroplating time, which enabled precise control of both the sheet resistance and the optical transmittance of the electrodes. A transmission electron microscopy (TEM) study revealed that Ag grew epitaxially on the AgNW surface but a polycrystalline Ag structure was formed on the CuNW surface. The resulting Ag2 ACS Paragon Plus Environment

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electroplated metallic nanowire electrodes were successfully applied to various devices such as OLEDs, triboelectric nanogenerators (TENGs), and a resistive touch panel. The proposed roll-to-roll electroplating process provides a simple, low-cost, and large-scale approach to enhancing the electrical properties of TCE materials for next-generation electronic devices.

RESULTS AND DISCUSSION Figure 1a shows a schematic illustration of the roll-to-roll electroplating process of the AgNW network electrodes. The AgNWs were deposited onto the PET film by the Meyer-rod coating method. The as-coated AgNW film was then immersed in a Ag electroplating bath. The Ag electroplating solution38 was composed of five compounds: AgNO3, K2S2O5, Na2S2O3, CH3COONH4, and CH3N3S. AgNO3 provides Ag+ ions for deposition on the metallic nanowire surface. K2S2O5 is an auxiliary complex agent, which prevents precipitation of sulfur and thus stabilizes the electroplating solution. Na2S2O3 improves the conductivity of the solution by forming a complexing agent [Ag(S2O3)]3-, which facilitates acceptance of electrons by Ag ions. The pH of the solution is buffered via addition of CH3COONH4 to maintain a value between 5.5 and 6. If the pH value is higher than 7 (base), the Ag+ ions will react with OH- and then form AgOH. On the other hand, if the pH value is lower than 4 (acid), the complexing agent will easily decompose into a black precipitate such as Ag2S or S. Finally, CH3N3S prevents immersion plating of Ag ions. As described in the magnified scheme in Figure 1a, the metal roller (cathode) and Ag plate (anode) were connected via an external circuit outside of the bath. Because the metal-roller cathode was brought into contact with the AgNWs coated onto the PET film, the current could be applied over the entire nanowire network. The Ag+ ions in the solution were then electroplated on the nanowire surfaces via the Galvanostatic mode. The distance between the Ag plate anode and the AgNW films was kept constant to ensure application of a uniform current density. Both the applied current density and the electroplating time were systematically controlled to optimize the electrical properties of the AgNW transparent electrode. First, variations in both the junction conductance and the wire conductance of the AgNWs were investigated after Ag electroplating.39 Figure 1b shows a scanning electron microscopy (SEM) image of the single-nanowire junction with four Au contact pads prepared by conventional e-beam lithography. The distance between adjacent contact pads was fixed at ~30 µm, and the electrical conductance was measured between two electrodes. The current–voltage (I–V) curves of both the wire and the junction (Figure 1c) revealed that the junction conductance (measured between electrodes 1 and 3) was much lower than the wire conductance (measured between electrodes 1 and 2). This huge difference in the conductance of the ascoated nanowire originated from the electrical instability of the junctions. It should be noted that the junction conductance of the as-coated AgNWs showed a large distribution (Figure S1): some junctions were almost nonconductive whereas the conductance of some junctions was only one order of magnitude lower than the wire conductance, which resulted in poor electrical properties of the as-coated AgNW electrode. 3 ACS Paragon Plus Environment

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Variations in the wire conductance and junction conductance with an increase in the electroplating time were monitored at the constant current density of 25 µA/cm2 (Figure 1d). The wire conductance increased gradually with the electroplating time, whereas the junction conductance jumped up after 10 s of electroplating and then increased gradually in the same manner as the wire conductance. Figure 1e shows representative SEM images of the junctions at different electroplating times. As the electroplating time increased from 0 s to 300 s, the diameter of the nanowire increased from 97 nm to 1084 nm (Figure S2a). The cross-sectional area of the nanowire had a linear relationship with the electroplating time (Figure 1f). According to Faraday’s law of electrolysis, the amount of substances undergoing oxidation or reduction at an electrode in a cell during electrolysis is directly proportional to the amount of electricity that passes through the cell. As the electroplating time increased at the constant current density, the amount of electroplated Ag increased uniformly on the nanowire surface, resulting in a linear increase in the crosssectional area.40 In addition to thickening of the nanowire, the incomplete contact region and air gap at the junction were filled up with electroplated Ag during the electroplating process. Importantly, the wire and junction exhibited identical conductances for a nanowire with a cross-sectional area of 0.2 µm2 (after 50 s of electroplating), as shown in Figure 1g and Figure S2b. The Ag+ ions would first be electroplated near the junction area owing to its higher electrostatic potential (within 50 s),31 and thus, the quality of the junction could be of the same level as that of the wire through complete interlocking of the junction by the electroplated Ag. The roll-to-roll electroplating process was applied to the AgNW film prepared by the Meyer-rod coating method. First, the AgNWs were coated onto a PET substrate by using Meyer-rod No. 7 and Ag was electroplated on the as-coated AgNW films in a AgNO3 solution bath as described in Figure 1a. In order to optimize the electroplating conditions, both the current density and the electroplating time were controlled systematically. Figure 2a shows the sheet resistance of the AgNW films as a function of the electroplating time at four different current densities (2.5, 25, 75, and 125 µA/cm2). As the electroplating time increased, the sheet resistance of the AgNW film decreased because of the increasing thickness of the nanowires and interlocking of the junction. At higher current densities, the sheet resistance decreased much faster, and thus, a much shorter time was required for the films to attain the same sheet resistance as that at lower current densities. The variation in the sheet resistance during the electroplating process was plotted as a function of the charge density (Figure 2b). The charge density was calculated by multiplying the current density with time. Interestingly, the plotted sheet resistance values of the AgNW films as a function of the charge density showed the same curve at different current densities. Since the amount of electroplated Ag is proportional to the charge density according to Faraday’s law, the sheet resistance of the AgNW film would be almost constant at a specific charge density. A reduction in the optical transmittance with an increase in the charge density was also observed (Figure 2c) because the transmittance was linearly related to the opening area of the nanowire electrode. The optical transmittance-sheet resistance plot was also shown in Figure S3. The 4 ACS Paragon Plus Environment

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Ag-electroplated AgNW (Ag-AgNW) films showed a sheet resistance of ~19 Ω/sq at an optical transmittance of 90% (550 nm). Next, Ag was electroplated on CuNW films41 coated onto a PET substrate in the same way as done on the AgNW films (Figure S4). Figures 2d and 2e show the variations in the sheet resistance and optical transmittance, respectively, of the Ag-electroplated CuNWs (Ag-CuNWs) as functions of the charge density and wire diameter. As the charge density increased, the diameter of the Ag-CuNWs increased, and thus, both the sheet resistance and the optical transmittance decreased accordingly. However, the reduction of the sheet resistance was not as small as that in the case of the Ag-AgNWs even though the same amount of charge density was provided to the CuNWs. However, the environmental stability of the CuNWs improved dramatically through electroplating of Ag on the CuNW surface. When the CuNWs and Ag-CuNWs were exposed to harsh conditions of a relative humidity (RH) of 70% and temperature of 70 °C, the sheet resistance and optical transmittance of the as-coated CuNW film increased dramatically owing to the oxidation of Cu whereas the Ag-CuNWs exhibited excellent environmental stability (Figure 2f). The improved environmental stability of the Ag-CuNWs could be explained by the higher standard reduction potential of Ag (0.799 V) than that of Cu (0.342 V) according to the electromotive force (EMF) series.42 In addition, the junction interlocking caused by Ag electroplating significantly enhanced the mechanical stability of the metallic nanowires (Figure S5). In order to investigate the growth mechanism of the electroplated Ag on the AgNW surface, transmission electron microscopy (TEM) analysis was performed on both pristine AgNWs and Ag-AgNWs, as shown in Figures 3a and 3b. The Ag-AgNWs exhibited a larger diameter than the pristine AgNWs (charge density: 750 µC/cm2) because Ag was electroplated on the nanowire surface. Interestingly, the highresolution TEM image and its Fast Fourier Transform (FFT) analysis (right panel of Figure 3b) indicated that Ag grew epitaxially on the AgNW surface by electrodeposition. The Ag atoms at the electroplated boundary—indicated by the white dotted line—were orderly aligned and no atomic reconstruction was observed. The cross-sectional high-resolution (HR)-TEM image of the Ag-AgNWs (Figure 3c) confirmed the epitaxial electrodeposition with complete pentagonal shapes and twin boundaries. FFT analysis was performed on all pentagonal twin segments from (i) to (v) in three different regions: the inside region (red), outside region (green), and overall region (blue) of each twin segment. The inside region of a twin segment represents the pristine AgNW part, whereas the outside region represents the electroplated Ag part. Notably, each crystal plane identified from these FFT images was identical in the inside, outside, and overall indexed regions in each of the five different twin segments because the three regions had the same atomic lattice structure. Figure 3d shows SEM images of both pristine CuNWs and Ag-CuNWs electroplated with a increasing charge density. However, the electroplated Ag film on the CuNWs contained numerous grains in a polycrystalline, which was a strong contrast to the Ag film electroplated on the AgNWs. The native oxide 5 ACS Paragon Plus Environment

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layer formed on the CuNW surface may disturb the epitaxial growth of Ag (see the black arrows in the left panel of Figure 3e). The HR-TEM image and FFT analysis of the pristine CuNWs confirmed that a copper oxide layer was formed over the nanowire surface, as shown in Figure 3f. The TEM image of the AgCuNWs (Figure 3g) and the FFT analysis of the cross-sectional image of the Ag-CuNWs (Figures 3h and S6) indicated that the electroplated Ag on copper nanowire surface was polycrystalline. Numerous grain boundaries of polycrystalline Ag could act as resistors, which interrupted the current flow (Figure 3i). Therefore, the resistance reduction of the CuNWs induced by Ag electrodeposition was not as effective as that of the Ag-AgNWs. The Ag-electroplated nanowire electrodes were successfully applied to various flexible electronic devices such as OLEDs, TENGs, and resistive touch panels. First, blue-emitting fluorescent OLEDs were fabricated via thermal deposition of seven different organic layers on the nanowire-based anode electrodes (AgNWs, Ag-AgNWs, CuNWs, and Ag-CuNWs).43 All the metallic nanowires were embedded in a UVcurable polymer matrix to form a smooth surface of the electrode. The surface roughness decreased from 18.1 to 0.98 nm after the embedding process (Figure S7). The seven organic layers were as follows: 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) as a hole injection layer (HIL); N,N′-Di(1naphyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as a hole transport layer (HTL); 4,4′,4″-tri(Ncarbazolyl)triphenylamine (TCTA) as an electron-blocking layer (EBL); 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and 1,6-bis(N-phenyl-p-CN-phenylamino)-pyrenes (Pyrene-CN) as the host and dopant materials, respectively, in the emissive layer (EML); LG201 doped with lithium quinolate (Liq) as an electron-transporting layer (ETL); and Liq as an electron-injecting layer (HIL). Finally, an Al cathode electrode was thermally deposited on the stacked organic layers. The lower panel of Figure 4a shows a photographic image of the OLEDs with Ag-AgNWs. The mechanical robustness of the Ag-electroplated nanowire electrode enabled the fabrication of flexible OLEDs. Figure 4b shows the current density– voltage–luminance characteristics of the OLEDs with pristine AgNW and Ag-AgNW anode electrodes. The Ag-AgNW OLEDs showed a significant performance enhancement (42.6 mA/cm2 and 5349.3 cd/m2 at 4.7 V) in comparison to the device with pristine AgNWs (18.0 mA/cm2 and 2173.2 cd/m2 at 4.7 V). The lower sheet resistance (~19 Ω/sq) of the Ag-AgNWs caused an increase in both the current density and the luminance of the Ag-AgNW OLEDs despite the slightly lower optical transmittance of the nanowires (90% at 550 nm). Notably, these electrical properties of the Ag-AgNW OLEDs were comparable to those of OLEDs with commercial ITO (62.3 mA/cm2 and 5161.2 cd/m2 at 4.7 V). Similarly, the Ag-CuNW OLEDs showed a higher current density and luminance (5.5 mA/cm2 and 374.2 cd/m2, respectively, at 6 V) than the devices with pristine CuNWs (0.2 mA/cm2 and 33.0 cd/m2, respectively, at 6 V). The natural oxide layer formed on the CuNW surface (see Figure 3e) may impede the hole injection from Cu to the HAT-CN layer. The enhanced performance of the Ag-CuNW OLEDs could be explained in terms of both the lower sheet resistance and the low hole injection barrier. 6 ACS Paragon Plus Environment

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Second, the metallic nanowires were applied to the electrodes of TENGs (Figure 4d). CuNWs or Ag-CuNWs embedded in the UV-curable polymer matrix were utilized as the top triboelectric layer, and perfluoroalkoxy alkane (PFA) with a strongly negative triboelectric potential was utilized as the bottom triboelectric layer. Figure 4e shows the output open-circuit voltage and the short-circuit current of the TENGs with CuNWs and of those with Ag-CuNWs. The TENG performance is governed by both the triboelectric potential difference and the effective contact area between the two triboelectric layers.44 The Ag-CuNW TENGs exhibited higher output voltage and current than the CuNW TENGs despite the similar tribopotentials of Ag and Cu. The formation of polycrystalline grains on the CuNW surface increased the effective contact area and thus enhanced the mechanical contact between the two triboelectric layers; this, in turn, caused the generation of a higher number of induced triboelectric charges. Figure 4f shows the variations in the output voltage and current of the Ag-CuNW TENGs as a function of load resistance in the range of 0.2–100 MΩ. As the load resistance increased, the output voltage increased from 14 V to 160 V but the output current decreased from 36 µA to 4.5 µA. The maximum power of 2.1 mW was obtained at a load resistance of 5 MΩ, corresponding to a power density of 3.4 W/m2. Finally, a resistive touch panel with the Ag-CuNW electrodes was fabricated and its performance was examined, as shown in Figure 4g. A AgCuNW electrode embedded on a PET film was separated from another Ag-CuNW electrode by an insulating spacer. An electrical connection established between the two electrodes via direct writing on the top surface of the resistive touch panel successfully led to the display of the written letters (SKKU) on an externally connected monitor, as shown in Figure 4h.

CONCLUSION In conclusion, we reported a roll-to-roll welding process of Ag electroplating for metallic nanowire electrodes using the Galvanostatic mode. This electrochemical reduction process enabled sophisticated deposition of Ag onto both the surface region and the junction region of metallic nanowires, which caused a significant decrease in the sheet resistance of the nanowire electrode. The resulting Ag-electroplated metallic nanowires electrodes were successfully applied to various electronic devices such as OLEDs, TENGs, and resistive touch panels. This simple, low-cost, and scalable Ag electroplating technique provides an innovative approach for the preparation of next-generation flexible TCEs for future optoelectronic devices.

METHODS Preparation of Ag electroplating bath: A cyanide-free Ag electroplating bath was prepared using five compounds. In brief, 0.4 g/L of silver nitrate (AgNO3) was dissolved in deionized water to a quarter of the bath volume; 4 g/L of potassium metabisulfite (K2S2O5) was dissolved in deionized water to a quarter of the bath volume; and 22.5 g/L of sodium hyposulfite (Na2S2O3) was dissolved in deionized water to a third of the bath volume. First, the dissolved K2S2O5 solution was added to the AgNO3 solution under stirring to 7 ACS Paragon Plus Environment

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produce silver metabisulfite (Ag2S2O5) solution. The Ag2S2O5 solution was then slowly poured into the preprepared Na2S2O3 solution. Subsequently, 2.5 g/L of ammonium acetate (CH3COONH4) and 0.07 g/L of aminothiourea (CH3N3S) were sequentially added to the mixed solution at time intervals. Finally, deionized water was poured to fill the remaining bath for completing the preparation of the electrolyte for Ag electroplating. The prepared electrolyte was stirred for one day in an indoor shaded environment at room temperature for stabilization. Roll-to-roll Ag electroplating process: A 0.5 wt% AgNW suspension in isopropyl alcohol was purchased from Nanopyxis Co. (diameter of 32 ± 4 nm (Figure S8a) and length of 25 ± 5 µm), and 0.25 wt% CuNWs were synthesized by a previously reported method (diameter of 81 ± 10 nm (Figure S8b) and length of 33 ± 9 µm). The AgNWs or CuNWs were deposited onto the PET film by the Meyer-rod coating method (#7). The as-coated nanowire films were immersed in the pre-prepared Ag electroplating bath at constant current densities (2.5, 25, 75, and 125 µA/cm2) for various times. The surfaces of the pristine and electroplated nanowire films were visualized by scanning electron microscopy (SEM, JSM-7600F, JEOL, Ltd.). The sheet resistance was measured by a four-point probe technique (Keithley 2182A and 6221), and the optical transmittance was measured using a UV-vis spectrophotometer (Agilent 8453). In order to fabricate the single-wire junction of the AgNWs, the AgNW suspensions (Aldrich Co., diameter ~97 nm and length ~50 µm) were diluted with isopropyl alcohol and deposited onto a silicon wafer. Further, 100-nm-thick Au contact pads were patterned by e-beam lithography. The electrical characteristics of both the wire and the junction of the single nanowire were measured using a Keithley 4200 probe system. Device fabrication: For fabrication of blue-emitting fluorescent OLEDs, seven organic layers were deposited

via

the

thermal

evaporation

method:

35-nm-thick

1,4,5,8,9,11-hexaazatriphenylene-

hexacarbonitrile (HAT-CN) as a hole injection layer; 80-nm-thick N,N′-bis-(1-naphyl)-N,N′-diphenyl-1,1′biphenyl-4,4′-diamine (NPB) as a hole transport layer; 20-nm-thick 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) as an electron-blocking layer; 20-nm-thick 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and 5% 1,6-bis(N-phenyl-p-CN-phenylamino)-pyrenes (Pyrene-CN) as the host and dopant materials, respectively, for the blue fluorescent emission layer; 40-nm-thick LG201 doped with 50% lithium quinolate (Liq) as an electron-transporting layer; and 1.5-nm-thick Liq as an electron-injecting layer. Finally, 100-nmthick Al was deposited thermally as a cathode electrode. The emission area was 1 × 1 cm2. The optoelectrical properties were characterized using a Keithley 236 source meter and a Minolta CS-2000 spectroradiometer. For fabrication of the TENGs, CuNWs were deposited onto a hydrophobic octadecyltrichlorosilane (ODTS)-treated glass substrate by the Meyer-rod coating method. The prepared CuNW-coated glass was electroplated for 200 s at 25 µA/cm2. A PET film coated with UV-curable prepolymer was laminated onto the pristine-CuNW-coated or the Ag-CuNW-coated substrate. After UVV exposure (365 nm and 25 mW/cm2), the PET film with the CuNWs or Ag-CuNWs embedded in the UVcurable resin was delaminated from the glass. The embedded CuNW electrodes or Ag-CuNW electrodes 8 ACS Paragon Plus Environment

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were utilized as the top triboelectric layer in the TENGs. For fabrication of the touch screen, the embedded Ag-CuNWs were deposited onto the PET films by the same method as that used for the TENG fabrication. Two electrodes were positioned facing each other and separated by spacers.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Current-voltage curves of the pristine junction measured between 1 and 3 electrodes; Variations of the AgNW diameter as increasing Ag electroplating time, Resistance of the wire and junction as a function of the cross-sectional area of AgNWs; Optical transmittance-sheet resistance plot of the AgNWs at four different current densities (2.5, 25, 75, and 125 µA/cm2); Variations in the sheet resistance of the Ag-CuNWs as a function of the electroplating time at three different current densities (2.5, 25, and 125 µA/cm2); (RR0)/R0 of the metallic nanowires electrodes as a function of the number of fatigue cycle; Three different polycrystalline Ag FFT patterns of the cross-sectional Ag-CuNW; Atomic force microscopy (AFM) images of the AgNWs before and after embedding process; Diameter distribution and SEM images of pristine AgNWs and CuNWs.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.H.C) and [email protected] (J.G.K)

ACKNOWLEDGMENTS This work was supported by the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (NRF-2013M3A6A5073177) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A2B2005790).

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REFERENCES (1) Park, J. H.; Lee, D. Y.; Kim, Y.-H.; Kim, J. K.; Lee, J. H.; Park, J. H.; Lee, T.-W.; Cho, J. H. Flexible and Transparent Metallic Grid Electrodes Prepared by Evaporative Assembly. ACS Appl. Mater. Interf. 2014, 6, 12380-12387 (2) Li, N.; Oida, S.; Tulevski, G. S.; Han, S.-J.; Hannon, J. B.; Sadana, D. K.; Chen, T.-C. Efficient and Bright Organic Light-Emitting Diodes on Single-Layer Graphene Electrodes. Nat. Commun. 2013, 4, 2294. (3) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, 1446 (4) Chen, P.-Y.; Li, C.-T.; Lee, C.-P.; Vittal, R.; Ho, K.-C. PEDOT-Decorated Nitrogen-Doped Graphene as the Transparent Composite Film for the Counter Electrode of a Dye-Sensitized Solar Cell. Nano Energy 2015, 12, 374-385 (5) Han, B.; Pei, K.; Huang, Y.; Zhang, X.; Rong, Q.; Lin, Q.; Guo, Y.; Sun, T.; Guo, C.; Carnahan, D. Uniform Self-Forming Metallic Network as a High-Performance Transparent Conductive Electrode. Adv. Mater. 2014, 26, 873-877 (6) Liang, J.; Li, L.; Niu, X.; Yu, Z.; Pei, Q. Elastomeric Polymer Light-Emitting Devices and Displays, .Nat. Photon. 2013, 7, 817-824 (7) Ye, S.; Rathmell, A. R.; Chen, Z.; Stewart, I. E.; Wiley, B. J. Metal Nanowire Networks: the Next Generation of Transparent Conductors. Adv. Mater. 2014, 26, 6670-6687 (8) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios. ACS Nano 2009, 3, 1767-1774 (9) Chen, D.; Zhao, F.; Tong, K.; Saldanha, G.; Liu, C.; Pei, Q. Mitigation of Electrical Failure of Silver Nanowires under Current Flow and the Application for Long Lifetime Organic Light-Emitting Diodes. Adv. Electron. Mater. 2016, 2, 1600167. (10) Park, J. H.; Han, S.; Kim, D.; You, B. K.; Joe, D. J.; Hong, S.; Seo, J.; Kwon, J.; Jeong, C. K.; Park, H. J. Plasmonic-Tuned Flash Cu Nanowelding with Ultrafast Photochemical-Reducing and Interlocking on Flexible Plastics. Adv. Funct. Mater. 2017, 27, 1701138 (11) Im, H.-G.; Jung, S.-H.; Jin, J.; Lee, D.; Lee, J.; Lee, D.; Lee, J.-Y.; Kim, I.-D.; Bae, B.-S. Flexible Transparent Conducting Hybrid Film Using a Surface-Embedded Copper Nanowire Network: a Highly Oxidation-Resistant Copper Nanowire Electrode for Flexible Optoelectronics. ACS Nano 2014, 8, 1097310979 (12) Lee, P.; Lee, J.; Lee, H.; Yeo, J.; Hong, S.; Nam, K. H.; Lee, D.; Lee, S. S.; Ko, S. H. Highly Stretchable and Highly Conductive Metal Electrode by Very Long Metal Nanowire Percolation Network. Adv. Mater. 2012, 24, 3326-3332

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(13) Hu, L.; Kim, H. S.; Lee, J.-Y.; Peumans, P.; Cui, Y. Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano 2010, 4, 2955-2963 (14) Kang, H.; Kang, I.; Han, J.; Kim, J. B.; Lee, D. Y.; Cho, S. M.; Cho, J. H. Flexible and Mechanically Robust Organic Light-Emitting Diodes Based on Photopatternable Silver Nanowire Electrodes. J. Phys. Chem. C 2016, 120, 22012-22018 (15) Choi, D. Y.; Kang, H. W.; Sung, H. J.; Kim, S. S. Annealing-Free, Flexible Silver Nanowire–Polymer Composite Electrodes via a Continuous Two-Step Spray-Coating Method. Nanoscale 2013, 5, 977-983 (16) Krantz, J.; Stubhan, T.; Richter, M.; Spallek, S.; Litzov, I.; Matt, G. J.; Spiecker, E.; Brabec, C. J. Spray-Coated Silver Nanowires as Top Electrode Layer in Semitransparent P3HT:PCBM-Based Organic Solar Cell Devices. Adv. Funct. Mater. 2013, 23, 1711-1717 (17) Lee, H.; Lee, D.; Ahn, Y.; Lee, E.-W.; Park, L. S.; Lee, Y. Highly Efficient and Low Voltage Silver Nanowire-Based OLEDs Employing a n-Type Hole Injection Layer. Nanoscale 2014, 6, 8565-8570 (18) Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Solution-Processed Metal Nanowire Mesh Transparent Electrodes. Nano Lett. 2008, 8, 689-692 (19) Hauger, T. C.; Al-Rafia, S. I.; Buriak, J. M. Rolling Silver Nanowire Electrodes: Simultaneously Addressing Adhesion, Roughness, and Conductivity. ACS Appl. Mater. Interf. 2013, 5, 12663-12671 (20) Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Self-Limited Plasmonic Welding of Silver Nanowire Junctions. Nat.Mater. 2012, 11, 241-249 (21) Hu, H.; Wang, Z.; Ye, Q.; He, J.; Nie, X.; He, G.; Song, C.; Shang, W.; Wu, J.; Tao, P. Substrateless Welding of Self-Assembled Silver Nanowires at Air/Water Interface. ACS Appl. Mater. Interf. 2016, 8, 20483-20490 (22) Kim, J.; Nam, Y. S.; Song, M. H.; Park, H. W. Large Pulsed Electron Beam Welded Percolation Networks of Silver Nanowires for Transparent and Flexible Electrodes. ACS Appl. Mater. Interf. 2016, 8, 20938-20945 (23) Park, J. H.; Hwang, G. T.; Kim, S.; Seo, J.; Park, H. J.; Yu, K.; Kim, T. S.; Lee, K. J. Flash-Induced Self-Limited Plasmonic Welding of Silver Nanowire Network for Transparent Flexible Energy Harvester. Adv. Mater. 2016, 29, 1603473. (24) Song, T.-B.; Chen, Y.; Chung, C.-H.; Yang, Y.; Bob, B.; Duan, H.-S.; Li, G.; Tu, K.-N.; Huang, Y.; Yang, Y. Nanoscale Joule Heating and Electromigration Enhanced Ripening of Silver Nanowire Contacts. ACS Nano 2014, 8, 2804-2811 (25) Spechler, J. A.; Arnold, C. B. Direct-Write Pulsed Laser Processed Silver Nanowire Networks for Transparent Conducting Electrodes. Appl. Phys. A 2012, 108, 25-28 (26) Wang, R.; Zhai, H.; Wang, T.; Wang, X.; Cheng, Y.; Shi, L.; Sun, J. TiO2 Nanotube Arrays Based Flexible Perovskite Solar Cells with Transparent Carbon Nanotube Electrode, Nano Res. 2016, 9, 2138-2148 11 ACS Paragon Plus Environment

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(27) Kang, H.; Kim, Y. T.; Cheon, S.; Yi, G.-R.; Cho, J. H. Halide Welding for Silver Nanowires Network Electrode. ACS Appl. Mater. Interf. 2017, 9, 30779-30785. (28) Ahn, J.; Seo, J.-W.; Kim, J. Y.; Lee, J.; Cho, C.; Kang, J.; Choi, S.-Y.; Lee, J.-Y. Self-Supplied NanoFusing and Transferring Metal Nanostructures via Surface Oxide Reduction. ACS Appl. Mater. Interf. 2016, 8, 1112-1119 (29) Chang, Y.-M.; Yeh, W.-Y.; Chen, P.-C. Highly Foldable Transparent Conductive Films Composed of Silver Nanowire Junctions Prepared by Chemical Metal Reduction. Nanotechnology 2014, 25, 285601 (30) Lee, J.; Lee, I.; Kim, T. S.; Lee, J. Y. Efficient Welding of Silver Nanowire Networks without PostProcessing. Small 2013, 9, 2887-2894 (31) Lee, S. J.; Kim, Y.-H.; Kim, J. K.; Baik, H.; Park, J. H.; Lee, J.; Nam, J.; Park, J. H.; Lee, T.-W.; Yi, G.-R.; Cho, J. H. A Roll-to-Roll Welding Process for Planarized Silver Nanowire Electrodes. Nanoscale 2014, 6, 11828-11834 (32) Lu, H.; Zhang, D.; Cheng, J.; Liu, J.; Mao, J.; Choy, W. C. Locally Welded Silver Nano-Network Transparent Electrodes with High Operational Stability by a Simple Alcohol-Based Chemical Approach. Adv. Funct. Mater. 2015, 25, 4211-4218 (33) Yin, Z.; Song, S. K.; You, D. J.; Ko, Y.; Cho, S.; Yoo, J.; Park, S. Y.; Piao, Y.; Chang, S. T.; Kim, Y. S. Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes. Small 2015, 11, 4576-4583 (34) Yoon, S.-S.; Khang, D.-Y. Room-Temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation. Nano Lett. 2016, 16, 3550-3556 (35) Chen, Z.; Ye, S.; Wilson, A. R.; Ha, Y.-C.; Wiley, B. J. Optically Transparent Hydrogen Evolution Catalysts Made from Networks of Copper-Platinum Core-Shell Nanowires. Energy Environ. Sci. 2014, 7, 1461-1467 (36) Lee, H.; Hong, S.; Lee, J.; Suh, Y. D.; Kwon, J.; Moon, H.; Kim, H.; Yeo, J.; Ko, S. H. Highly Stretchable and Transparent Supercapacitor by Ag–Au Core–Shell Nanowire Network with High Electrochemical Stability. ACS Appl. Mater. Interf. 2016, 8, 15449-15458 (37) Wang, H.; Wu, C.; Huang, Y.; Sun, F.; Lin, N.; Soomro, A. M.; Zhong, Z.; Yang, X.; Chen, X.; Kang, J. One-Pot Synthesis of Superfine Core–Shell Cu@metal Nanowires for Highly Tenacious Transparent LED Dimmer. ACS Appl. Mater. Interf. 2016, 8, 28709-28717 (38) Ren, F.-Z.; Yin, L.-T.; Wang, S.-S.; Volinsky, A.A.; Tian, B.-H. Cyanide-Free Silver Electroplating Process in Thiosulfate Bath and Microstructure Analysis of Ag Coatings. Trans. Nonferrous Met. Soc. China 2013, 23, 3822-3828 (39) Bellew, A. T.; Manning, H. G.; Gomes da Rocha, C.; Ferreira, M. S.; Boland, J. J. Resistance of Single Ag Nanowire Junctions and Their Role in the Conductivity of Nanowire Networks. ACS Nano 2015, 9, 11422-11429 12 ACS Paragon Plus Environment

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(40) Liu, J.-S.; Wang, X.-D.; Qin, F.-X.; Cao, F.-Y.; Xing, D.-W.; Peng, H.-X.; Xue, X.; Sun, J.-F. GMI Output Stability of Glass-coated Co-based Microwires for Sensor Application. PIERS Online 2011, 7, 661665 (41) Yin, Z.; Song, S. K.; Cho, S.; You, D.-J.; Yoo, J.; Chang, S. T.; Kim, Y. S. Curved Copper Nanowires-Based Robust Flexible Transparent Electrodes via All-Solution Approach. Nano Res. 2017, 10, 3077-3091 (42) Jones, D. A., Principles and Prevention of Corrosion; Macmillan, 1992. (43) Jung, E.; Kim, C.; Kim, M.; Chae, H.; Cho, J. H.; Cho, S. M. Roll-to-Roll Preparation of SilverNanowire Transparent Electrode and Its Application to Large-Area Organic Light-Emitting Diodes. Org. Electron. 2017, 41, 190-197 (44) Lin, L.; Xie, Y.; Wang, S.; Wu, W.; Niu, S.; Wen, X.; Wang, Z. L. Triboelectric Active Sensor Array for Self-Powered Static and Dynamic Pressure Detection and Tactile Imaging. ACS Nano 2013, 7, 82668274

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Figure 1. (a) Schematic illustration of continuous roll-to-roll Ag electroplating process for AgNW network electrodes and tabular data of constituents of electroplating bath. (b) SEM image of single-nanowire junction fabricated by e-beam lithography. (c) Current–voltage curves of both wire and junction of as-coated AgNWs measured between two electrodes (wire: electrodes 1 and 2 and junction: electrodes 1 and 3). (d) Current– voltage curves of both wire and junction measured during Ag electroplating at constant current density of 25 µA/cm2. (e) SEM images of AgNW junction after electroplating from 0 s to 300 s. (f) Variation in crosssectional area as a function of Ag electroplating time. (g) Conductances of wire and junction as a function of cross-sectional area of AgNWs.

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Figure 2. (a) Sheet resistance of Ag-AgNW film as a function of electroplating time at four different current densities (2.5, 25, 75, and 125 µA/cm2). (b) Sheet resistance and (c) optical transmittance of Ag-AgNW film as functions of charge density and nanowire diameter. (d) Sheet resistance and (e) optical transmittance of Ag-CuNW film as functions of charge density and nanowire diameter. (f) Variations in sheet resistance (R/R0) and optical transmittance (T/T0) of Ag-CuNWs as a function of time of exposure to harsh conditions of RH of 70% and temperature of 70 °C.

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Figure 3. TEM images of (a) as-coated AgNWs and (b) Ag-AgNWs. The right panel shows the FFT pattern. (c) Cross-sectional HR-TEM image of Ag-AgNW along with FFT analysis results for five twin segments. (d) SEM images of Ag-CuNWs. (e) TEM image of as-coated CuNW. (f) HR-TEM image and FFT pattern of oxide layer of as-coated CuNW. (g) TEM image of Ag-CuNW. (h) HR-TEM image of cross-sectional AgCuNW along with FFT pattern. (i) Schematic illustration of Ag-CuNWs.

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Figure 4. (a) Schematic device structure and photographic image of flexible OLEDs with Ag-AgNW anode. (b) Current density and luminance characteristics as functions of applied voltage (J–V–L characteristics) of OLEDs based on ITO, as-coated AgNW, and Ag-AgNW anodes. (c) J–V–L characteristics of OLEDs based on ITO, as-coated CuNW, and Ag-CuNW anodes. (d) Schematic device structure of TENGs based on ascoated CuNWs and Ag-CuNWs embedded in UV-curable polymer. (e) Output voltage and current of TENGs. (f) Output voltage, current, and power of TENGs with Ag-CuNWs as functions of load resistances. (g) Schematic device structure and (h) operation of resistive touch screen panel based on Ag-CuNW electrodes.

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