Substrateless Welding of Self-Assembled Silver Nanowires at Air

Jul 20, 2016 - Integrating connected silver nanowire networks with flexible polymers has ... silver nanowires at the air/water interface using plasmon...
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Substrateless Welding of Self-Assembled Silver Nanowires at Air/ Water Interface Hang Hu,†,‡ Zhongyong Wang,†,‡ Qinxian Ye,‡ Jiaqing He,‡ Xiao Nie,‡ Gufeng He,§ Chengyi Song,‡ Wen Shang,‡ Jianbo Wu,‡ Peng Tao,*,‡ and Tao Deng*,‡ ‡

State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China § School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China S Supporting Information *

ABSTRACT: Integrating connected silver nanowire networks with flexible polymers has appeared as a popular way to prepare flexible electronics. To reduce the contact resistance and enhance the connectivity between silver nanowires, various welding techniques have been developed. Herein, rather than welding on solid supporting substrates, which often requires complicated transferring operations and also may pose damage to heat-sensitive substrates, we report an alternative approach to prepare easily transferrable conductive networks through welding of self-assembled silver nanowires at the air/ water interface using plasmonic heating. The intriguing welding behavior of partially aligned silver nanowires was analyzed with combined experimental observation and theoretical modeling. The underlying water not only physically supports the assembled silver nanowires but also buffers potential overheating during the welding process, thereby enabling effective welding within a broad range of illumination power density and illumination duration. The welded networks could be directly integrated with PDMS substrates to prepare high-performance stable flexible heaters that are stretchable, bendable, and can be easily patterned to explore selective heating applications. KEYWORDS: welding, silver nanowire, interfacial self-assembly, plasmonic heating, stretchable heater

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INTRODUCTION

resistances and poor mechanical integrity especially under stretching. Several techniques such as thermal annealing,20 plasma treatment,21 pressure welding,22 soldering,7,23 electrochemical deposition,24 plasmonic welding,6,25−28 and joule heating29,30 have been utilized to help remove the PVP layer and strengthen the connection between Ag NWs. Among them, the welding driven by local plasmonic heating effect is a fast and simple method to enhance the connectivity at NW junctions.25−28 To instantly generate sufficient plasmonic heat, this welding technique utilizes tungsten halogen lamps or xenon flash lamps that have both high power densities and a broad irradiation spectrum as the light source.25−28 Such direct intense irradiation would limit the large-scale fabrication of conductive networks due to safety and cost concerns,27−30 and may also pose possible damage to the underlying substrates that are sensitive to heat and strong light exposure.32,33 Previously, surface coating with metal oxides,30 in situ growth of Ag nanoparticles at NW junctions29 and electrical heating

Stretchable conductors comprising highly conductive metallic networks have been considered as a critical component for flexible and wearable optoelectronics.1−3 Among them, silver nanowires (Ag NWs) are one of the most promising candidate materials in part because their high aspect ratio favors the easy formation of connected network.3−5 In addition to their widely pursued electrode applications in optical displays,6 LEDs,7 solar cells,8,9 sensors,10,11 and actuators,12 flexible heaters, another emerging application of stretchable conductors, recently have gained increasing attention because of their important roles in enabling smart windows defogging,13,14 thermal-based sensors,15 thermochromics,16 and thermal therapy.17,18 Integrating Ag NW thin films with flexible polymeric substrates has appeared as a more favorite route to prepare stretchable conductors,10,11,13,17 as it can avoid the incompatibility issue between aqueous solution of Ag NWs and organic-phase polymers in the direct mixing method.18 However, in the physically blended thin films, Ag NWs only have loose contact with each other because of the lack of direct bonding and the existence of insulating polyvinylpyrolidone (PVP) synthetic ligands on Ag NW surfaces,19 resulting in high contact © 2016 American Chemical Society

Received: May 27, 2016 Accepted: July 20, 2016 Published: July 20, 2016 20483

DOI: 10.1021/acsami.6b06334 ACS Appl. Mater. Interfaces 2016, 8, 20483−20490

Research Article

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Figure 1. (a) Schematic illustration of plasmonic interactions between self-assembled Ag NWs floating on top of water. (b) Schematic of detailed preparation process. (c) SEM image of as-synthesized Ag NWs. The inset shows a photograph of Ag NW dispersion within DI water. (d) TEM image of Ag NW showing a PVP layer thickness of 2 nm. (e) SEM image of assembled Ag NW film. The inset shows a photograph of assembled Ag NW film floating on top of water.

methods27 have been proposed to facilitate the joining between Ag NWs. To avoid damage to the heat-sensitive substrates, other approaches such as directly blending Ag NWs with nanopapers have been used to prepare conductive composites for foldable solar cell and flexible transistor applications.34−36 To date, however, almost all the reported treatments were carried out with Ag NWs being drop-casted, spin-coated, sprayed, or filtrated onto a solid substrate.7,20−30 When targeting for device applications, the welded networks often need to be separated from the supporting substrate first before being transferred to the final device substrate. This process frequently involves time-consuming complicated substrate removal and transferring operations, and causes performance deterioration of Ag networks.16,21,27,31 Facile welding of Ag NWs into easily transferable conductive networks within a broad range of operational parameters is highly desired for versatile applications of the connected networks in stretchable electronics. Compared with randomly distributed NW networks, recently it has been shown that ordered Ag NWs have exhibited superior properties and significantly improved performance for functional devices.37−39 Several techniques including selfassembly,40 Langmuir−Blodgett,41electro-spinning,42 suspension evaporation,43 and capillary printing37 have been developed to fabricate aligned NWs. However, the welding behavior of the partially aligned NWs has not been investigated although the connected ordered networks can offer unique properties and advantages. In this work, by combing the spontaneous self-assembly process and plasmonic heating effect we propose a new approach to prepare interconnected Ag NW networks via welding of self-assembled Ag NW thin films at the air−water interface, without the requirement of solid supporting substrates. Under solar illumination, the assembled NWs have demonstrated interesting welding behavior and could be easily welded into easily transferrable conductive networks. By directly transferring the welded Ag NW networks onto PDMS substrates we fabricated stretchable conductor composites and explored their application as flexible heaters that have demonstrated stable heating performance under

repeated stretching and bending owing to their excellent combination of high electrical conductivity and mechanical flexibility. The flexible heaters prepared from such route not only have a fast thermal response but also enable selective heating through patterning of the welded NW films.

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EXPERIMENTAL SECTION

Ag NW Synthesis. First, 20 mL of ethylene glycol (EG) was refluxed in a three-necked flask at 160 °C for 3 h. Next, 10 mL of 0.1 M AgNO3 EG solution and 10 mL of 0.2 M PVP (Mw = 55 000 g/ mol) solution were simultaneously injected into the refluxed EG solvent at a rate of 0.2 mL/min, without stirring. After injection, the mixture was kept at 160 °C for 1 h. To remove excess PVP and EG, the obtained mixture was precipitated with 200 mL of acetone at 4 °C for 12 h, before washing with ethanol and centrifugation for 3 times. After washing, Ag NWs were redispersed in water (0.5 mg/mL). Ag NW Self-Assembly. Typically, 10 mL of chloroform was added to a 25 mL glass beaker with a diameter of 2.5 cm, followed by addition of 8 mL of Ag NW aqueous solution on the surface to form an annulus-like oil−water−air triple interface. After sitting for 1 h, an integrated film was formed covering the whole water surface. The underneath chloroform layer was extracted with a syringe and refilled with DI water before subject to solar illumination. Welding at the Air/Water Interface. The glass beaker (25 mL) containing floating assembled Ag NW thin films was placed within a larger beaker (100 mL) filled with DI water. The whole setup was placed under a solar simulator with tunable power densities and illumination duration. An aluminum foil with a defined aperture (2 cm in diameter) was used to cover the setup to confine the illumination area. A thermocouple was placed underneath the Ag NW film to monitor the temperature change of the water during the welding process. Fabrication of Flexible Heater. A PDMS film was first prepared by thoroughly mixing two components (Sylgard 184, Dow Corning) at a ratio of 10:1, degassing, and curing at 70 °C for 12 h. The cured PDMS film was then surface treated using a plasma cleaner (Harrick, US) for 5 min before it was used to pick up welded Ag networks from air−water interface. The composites were placed within an oven at 50 °C for 30 min to remove residual water. To realize uniaxial stretching, we fixed the Ag NW PDMS composites on two stages, one of which can be moved by tuning a motion controller. To prepare patterned heaters, prepatterned adhesive tapes were first attached onto PDMS substrates following by plasma treatment and transferring of welded 20484

DOI: 10.1021/acsami.6b06334 ACS Appl. Mater. Interfaces 2016, 8, 20483−20490

Research Article

ACS Applied Materials & Interfaces Ag NW films. The patterned adhesive tapes were then peeled off to expose the uncoated PDMS substrate. Characterization and Property Measurement. A high-resolution field-emission SEM (FEI, Sirion 200) was utilized to characterize the morphology of as-synthesized and assembled Ag NWs. Surface PVP thickness was determined by a TEM (Tecnai G2 Spirit Biotwin, FEI) operating at 120 kV. Power density of solar illumination was measured by a power meter. The temperature at the interface between Ag NW film and water was measured by a thermocouple that is placed below the floating NW film during the welding process. The sheet resistance of Ag NW networks was measured by a standard four-probe method. The electrical resistance of Ag NW PDMS composites was measured by a multimeter. A DC power source (TPR3005T, Atten) was used to supply constant voltage input to generate joule heating at the Ag NW PDMS composite heater. Temperature evolution of the heater was monitored in realtime by an IR camera (T620 FLIR) operated at the video mode. Simulation. FDTD (finite-difference time-domain) simulations were performed by using FDTD Solutions software to evaluate plasmonic interaction between neighboring NWs with different configurations by using water as the surrounding dielectric medium. In our model, Ag NWs on the air−water surface are illuminated by polarized plane wave light perpendicular to the long axis of NWs (Figure S1). The NWs were modeled to have a cylinder shape with a diameter of 100 nm based on experimental observation. In our model, the length of NWs were modeled to be 2 μm rather than the experimentally observed 10−50 μm by considering that the dominant localized plasmonic heat generation at the NW junctions is from the LSPR (localized surface plasmonic resonance) in the transverse direction.25 The maximum local electric filed distribution of excited Ag NWs was calculated by scanning the incident light wavelength from 300 to 700 nm. The scanning range was selected by considering the fact that the effective plasmonic absorption of Ag NWs is within visible-light wavelength range and the intensive illumination spectrum range of the solar simulator (Figure S2).

solution or at the air−water interface. The welded conductive NW films could be easily transferred to different target substrates. This approach avoids the potential damage to the substrates, especially for heat, chemical, or light-sensitive substrates. Because of the facile interfacial transfer process, it could also alleviate possible disturbance on the structural integrity and electrical conductivity of welded networks during conventional transferring processes between solid supporting substrates. Figure 2a shows that under a high-magnification sharp and distinct interfaces between NWs are clearly identified as the

Figure 2. (a) SEM image of Ag NWs within assembled films. (b−d) SEM images of perpendicularly overlapped, crossed, and parallel welding junctions, respectively.

assembled NWs are only weakly connected with PVP ligands lying between. After illumination by a solar simulator with a power density of 15 W/cm2 for 5 min, different welded structures are observed. Figure 2b presents that at the crossed junctions the top Ag NW is fused into the bottom NW. In the meanwhile, the welding of crossed junctions also promotes the fusion of directly contacted aligned NW pairs. Obvious fusion between crossed contacts (Figure 2c) and two parallel NWs (Figure 2d) were also observed. Interestingly, although in the assembled film most NWs are closely packed and aligned parallel with each other, the most frequently observed welding happens at the crossed junctions rather than between aligned NWs. As shown by Figure 2d, the observed fusion between parallel NWs occurs in discrete single NW pairs instead of multiple NWs assembly. Figure S4a presents that two parallel Ag NWs were fully fused together into a single NW. More SEM images of the welded structure at crossed junctions can be found in Figure S4b−d. Although previous theoretical analyses of welding behavior were mostly focused on perpendicularly crossed junctions,25−28 in the assembled film most of the NWs are aligned parallel with each other. To understand the observed various welded structures, we carried out FDTD simulation to evaluate plasmonic interaction between Ag NWs under different configurations. Considering the fact that plasmonic heat generation is proportional to the square of electric field and the plasmonic photothermal conversion process is ultrafast,44,45 the electric field distribution of Ag NWs with different configurations was calculated and compared. Figure 3a shows the intensely enhanced electric field in the vicinity of Ag NW junctions that are perpendicular with each other. Similar to crossed junctions, the local plasmonic interaction intensity between a single pair of aligned NWs is also extremely sensitive to the interwire gap size (Figure 3b). With the same interwire

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RESULTS AND DISCUSSION Figure 1a schematically illustrates the concept of preparing interconnected conductive Ag NW networks via welding of selfassembled Ag NWs at air−water interface through plasmonic heating. Figure 1b presents the detailed fabrication process. Ag NWs with a diameter around 100 nm and an average length of 10−50 μm (Figure 1c) were synthesized by a seed-mediated polyol method using PVP as the capping agent.19 Excessive PVP ligands were removed to such extent that it improves direct contact between NWs while maintaining the stable dispersion of NWs within deionized water. The highmagnification transmission electron microscopy (TEM) observation indicates a final PVP layer thickness of ∼2 nm (Figure 1d). The obtained Ag aqueous dispersion was added on top of chloroform along the inner wall of the beaker to form choloroform-water−air triple interface.35,40 Under the evaporation driving force of the chloroform solvent, the Ag NWs that were dispersed in water started to self-assemble and finally formed a densely packed donut-shaped film that can float at the air−water interface (Figure S3). The SEM image in Figure 1e indicates that in the assembled film while most of the Ag NWs were driven to side-by-side alignment by both the lateral capillary force and the binding between surface capping PVP ligands on adjacent NWs, some disordered and overcrossed NWs were also observed. Excited by simulated solar illumination, a large amount of plasmonic heat could be instantly and locally generated at the junctions of crossed Ag NWs and at the gaps between aligned NWs, resulting in effective welding between Ag NWs. Without the presence of any solid substrate, the whole process was carried out within 20485

DOI: 10.1021/acsami.6b06334 ACS Appl. Mater. Interfaces 2016, 8, 20483−20490

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gap size of 2 nm, the simulation results indicated that the maximum local electric field intensity at the perpendicularly crossed single pair Ag NW junction is about twice as that at the parallel interstitial. Figure 3c shows that in the parallel NW array the locally enhanced electric field is distributed among many interstitials leading to further reduced enhancement effect (Figure S5). During simulation, it was found that for both crossed and paralleled junctions only when the polarization of incident light is perpendicular to the NW the LSPR (localized surface plasmonic resonance) mode that is majorly responsible for the local plasmonic heating effect can be excited.45 By comparison, with paralleled polarization the localized electric field enhancement is very weak (Figure S6). For different structured Ag NW junctions we found that their maximum local electric field intensity varied with the wavelength of excited light, indicating that wide-spectrum light such as the solar light in this work is effective to simultaneously excited welding of different structures in the assembled NW films. The local electric filed enhancement effect rapidly decays when the interwire gap size increases from 2 to 4 nm (Figure 3d). Figure 3e presents that the maximum local electric field intensity for 50 aligned NWs is only half of that in the single pair of aligned Ag NWs. This is consistent with our observation that the fusion of two aligned Ag NWs was only identified when they were discrete and not well-organized. These simulation results in part explain the more frequent observation of welded NW pair with crossed arrangement than parallel arrangement. Another limiting factor for effective welding between Ag NWs is the surface capping PVP layer, which has a significant influence on both the assembly process and the interwire gap size. In the assembly process, part of the driving force for selfassembly comes from the attraction between PVP layers on neighboring NW surfaces. Thus, NWs capped with uniform thick layer of PVP would preferentially form the organized parallel assembled structure.40 Those NWs with thinner PVP

Figure 3. (a) Simulated cross-sectional electric field distribution in the vicinity of two perpendicularly crossed Ag NWs. (b) Simulated electric field distribution between a pair of parallel Ag NWs with increasing interwire gap size. (c) Snapshot of the simulated electric field distribution across 50 aligned Ag NWs. (d) Plot of maximum electric field intensity of two parallel Ag NWs as a function of interwire gap size. (e) Comparison of local electric field enhancement between a single pair of aligned Ag NWs and 50 aligned Ag NWs.

Figure 4. (a) Sheet resistance and temperature of underlying water as a function of illumination power density with the same illumination duration of 5 min. (b) Sheet resistance and temperature of underlying water as a function of illumination duration under a constant solar illumination power density of 5 W/cm2. (c) Photograph of plasmonic welding on heat-sensitive substrates of airlaid paper, PE and PVC films under an illumination power density of 0.5 W/cm2. 20486

DOI: 10.1021/acsami.6b06334 ACS Appl. Mater. Interfaces 2016, 8, 20483−20490

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Figure 5. (a) Strain-dependent electrical resistance of welded Ag NW/PDMS composites. (b) IR images of the composite heater under different strains after heating for 3 min under applied 2 V. (c) Average temperature profiles of the composite heater under different applied strains with applied 2 V before and after 50 cycles of stretching and releasing. (d) On/off response of composite heater under applied strain of 40% and voltage of 2 V.

obtained. Here, water provides the physical support for the Ag NW thin films thus eliminating any possible damage to supporting substrates. Owing to the large heat capacity of water and the welding design equipped with external cooling sources, the generated excessive heat was timely removed. Such interfacial welding design minimizes potential overheating generated from strong solar illumination and helps stabilizing the welded structure and sheet resistances. Moreover, the slightly heated water would also help dissolve the PVP ligands benefiting direct contact and effective welding between Ag NWs. To verify the observed sheet resistance decrease is dominantly resulted from the localized plasmonic welding effect rather than the heating effect from the water, we placed the assembled NW films on the warm water (45 °C) surface for different durations (10, 20, 30 min) and measured the change of sheet resistance. As shown in (Figure S8), the sheet resistance shows negligible change compared to the resistance of the as-assembled NW films. In a control experiment, we found that direct illumination of assembled Ag NW films on air-laid paper, PET (polyethylene) and PVC (polyvinyl chloride) thin-film substrates (∼30 μm in thickness), even with a low illumination power density of 0.5 W/cm2 for 5 s, resulted in serious shrinkage and even burning of the substrates (Figure 4c). Similar severe damage of the heat-sensitive substrates was observed during direct plasmonic welding processes of the drop-casted Ag NWs (Figure S9). In addition to enhanced electrical conductivity, the interfacial welding also significantly improved the mechanical integrity of the assembled Ag NW networks. The welded films can be easily transferred to both rigid and soft substrates, or even directly picked up with a tweezer as a freestanding film. The excellent transferability of the welded NW networks allows them to be easily integrated with flexible PDMS substrates to fabricate stretchable conductors to explore their applications as flexible heaters. Figure 5a shows that the electric resistance of the stretchable conductor increases with applied uniaxial tensile strain (up to 80%). The composite conductor could be

coating layers might lately form disordered crossed connections with the assembled NWs. Therefore, it is highly possible that the interwire gap size of the parallel aligned NWs is larger than the crossed NWs. The thick PVP layer would become the limiting factor inhibiting the atomic diffusion and fusion between adjacent aligned NWs. Additionally, compared with parallel aligned NWs that have a large contact area restricting the free movement of surface PVP ligands, in the crossed junctions PVP ligands have much more freedom to rearrange themselves,46−49 thus facilitating closer contact between overlapped NWs. Figure 4a presents that with an illumination power density of 5 W/cm2, the sheet resistance decreases from ∼1100 to ∼20 Ω/sq after illumination for 5 min. When the illumination power density reached 15 W/cm2, sheet resistances of the welded Ag NW films drop to ∼4 Ω/sq, which is much lower than the reported values in other transparent Ag conductive films25,27,29 due to both the dense microstructure of assembled films and effective welding of them into interconnected networks. Further increasing the power density, the sheet resistance remained almost unchanged. Figure 4b shows that illumination under a power density of 5 W/cm2 for 1 min could quickly decrease the sheet resistance to ∼25 Ω/sq and prolonging illumination duration evenly brings down the sheet resistance. It can be seen that higher illumination power density and longer illumination time could induce stronger plasmonic heating of the Ag NW networks, thus favoring diffusion of Ag atoms to further decrease the resistance. It is worth noting that in contrast to the aligned microstructure of the assembled NWs the measured sheet resistances are isotropic. This could be attributed to the fact that although assembled NWs are locally aligned, at the macro-scale these bundles are randomly distributed as shown by the optical image at a low magnification (Figure S7). The unique welding at the air/water interface offers broader operational welding power density and duration range within which consistently conductive welded networks could be easily 20487

DOI: 10.1021/acsami.6b06334 ACS Appl. Mater. Interfaces 2016, 8, 20483−20490

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ACS Applied Materials & Interfaces stretched up to 100% without failure and further increasing the strain led to fracture of the PDMS substrate. After releasing the strain, the resistance can recover to initial values. Intimate contact and effective welding between Ag NWs are critical for full recovery of electric resistance.10,12 During the stretching, Ag NWs might slide between each other leading to reduced contact area therefore larger electrical resistance. Without welding, the assembled NWs only physically contact with each other and stretching the film results in permanent detachment of contacted NWs (Figure S10). After the welding, the welded Ag NW networks only undergo elastic deformation within the applied strain range. Upon releasing the strain, the deformed NW networks can easily recover to the initial state. This low, stable, and recoverable electrical resistance of the welded Ag NW films is highly desired for the stretchable heater application.50 According to joule heating equation (P = U2/R), where P is the heating power, U is the applied voltage and R is the electrical resistance of the stretchable heater, low R means that the heater can generate enough amount of heat with a low voltage (U) input, i.e., a small portable, and possibly integrated battery could drive the heater. As shown by the thermal infrared (IR) images in Figure 5b, the stretched heater has exhibited stable and uniform heating under an applied voltage of 2 V. The low resistance is also beneficial for quick heating response of the fabricated stretchable heater. Figure 5c shows that after applying a DC voltage of 2 V the average temperature of the heater delineated by the dotted lines in the center quickly increased from 15 to 60 °C, then reached a steady plateau that is governed by the balance between joule heating input and heat dissipation. Under applied 20 and 40% strain, the stretchable heater worked stably at temperatures of 40 and 30 °C, respectively. The slightly lower heating temperature of the strained heaters can be attributed to the fact that the applied strain increased the electrical resistance, leading to lower heating power under the constant voltage input. The fabricated heater has demonstrated consistent heating performance evidenced by the overlapped heating curves after repeating the heating tests for 50 cycles. The fast and stable heating performance of the stretched heater under 40% of strain was also confirmed by the repeated on−off heating/cooling test with an interval of 60 s (Figure 5d). Under a higher applied DC voltage of 5 V, the repeatable swiping of heating and cooling of the flexible heater within larger temperature range was also demonstrated (Figure S11). Because of the excellent flexibility of the PDMS substrate the composite heater is also bendable. Figure 6a presents that with a 5 V DC voltage the heater temperature reached more than 100 °C, and at a bending radius of 5 mm the bended heater exhibited stable performance. In addition to the robust structure and stable heating behavior under mechanical stretching and bending test conditions, Figure 6b displays that the heater can also perform well even under water immersion, which would expand its application in possible wet conditions.51 Besides uniform heating, the composite heater also allows for precise patterning of the welded thin films on the PDMS substrate to tune the heating area and heating spot size, thereby exploring specific control over local heating temperature that is critical for point-of-care medical therapy applications.17,18 Figure 6c presents that through a simple tape adhesion and peeling process the designed pattern can be easily transferred onto the heater. The patterned flexible heater has demonstrated obvious temperature contrasts. Under the same applied voltage of 5 V, the area with deposition of welded Ag

Figure 6. (a) IR image of bended composite heater under applied 5 V. The inset shows the corresponding photograph. (b) IR image of heater placed within water under applied 2 V. (c) Selective heating through patterning Ag NW films on the flexible heater. Through (I) a simple tape adhesion and peeling-off process, (II) a periodic pattern, (III) Sshaped pattern, and (IV) T-shaped pattern are inscribed on the flexible composite heater for selective heating. The top right insets are the photographs of patterned heater.

NW films has a temperature of 43 °C, while the exposed PDMS substrate has a temperature of 24 °C (Figure S12). Here the temperature was measured locally with thermocouples. The IR images in Figure 6c shows brighter color of PDMS substrate due to the large emissivity effect of the PDMS substrate compared to Ag NW film. We also demonstrated that the designed S-shaped and T-shaped patterns could be inscribed into our flexible heater for selective heating. The facile patterning of identity features may expand the use of flexible conductors from conventional heating application into other areas such as IR anticounterfeiting in the future. In the meanwhile, our interfacial welding approach could enable easy transfer of welded networks to a broad range of substrates, in particular to prepare heaters on curved surfaces (Figure S13), which is challenging to fabricate with conventional preparation methods. Combining the excellent features of high electrical conductivity, mechanical flexibility, transferability, patternability, and biocompatibility, it is anticipated that the heater prepared from the route reported in this work would have important applications in future wearable electronics.

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CONCLUSIONS In summary, a new approach was proposed to prepare mechanically robust and electrically conductive networks through welding of assembled Ag NW thin films floating at the air−water interface. This interfacial welding eliminates the requirement of solid supporting substrates, thereby prevents any potential substrate damage and facilities easy transfer of the welded networks to targeted device surfaces. Because of the timely removal of excessive heat with underlying water, the delicate interfacial welding design also broadens the operational range of illumination power densities and illumination duration, 20488

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(7) Liang, J. J.; Li, L.; Tong, K.; Ren, Z.; Hu, W.; Niu, X. F.; Chen, Y. S.; Pei, Q. B. Silver Nanowire Percolation Network Soldered with Graphene Oxide at Room Temperature and Its Application for Fully Stretchable Polymer Light-Emitting Diodes. ACS Nano 2014, 8, 1590−1600. (8) Gaynor, W.; Lee, J. Y.; Peumans, P. Fully Solution-Processed Inverted Polymer Solar Cells with Laminated Nanowire Electrodes. ACS Nano 2010, 4, 30−34. (9) Leem, D. S.; Edwards, A.; Faist, M.; Nelson, J.; Bradley, D. D. C.; de Mello, J. C. Efficient Organic Solar Cells with Solution-Processed Silver Nanowire Electrodes. Adv. Mater. 2011, 23, 4371−4375. (10) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24, 5117−5122. (11) Amjadi, M.; Pichitpajongkit, A.; Lee, S.; Ryu, S.; Park, I. Highly Stretchable and Sensitive Strain Sensor Based on Silver NanowireElastomer Nanocomposite. ACS Nano 2014, 8, 5154−5163. (12) Wu, J. H.; Zang, J. F.; Rathmell, A. R.; Zhao, X. H.; Wiley, B. J. Reversible Sliding in Networks of Nanowires. Nano Lett. 2013, 13, 2381−2386. (13) Kim, T.; Kim, Y. W.; Lee, H. S.; Kim, H.; Yang, W. S.; Suh, K. S. Uniformly Interconnected Silver-Nanowire Networks for Transparent Film Heaters. Adv. Funct. Mater. 2013, 23, 1250−1255. (14) Kiruthika, S.; Gupta, R.; Kulkarni, G. U. Large Area Defrosting Windows Based on Electrothermal Heating of Highly Conducting and Transmitting Ag Wire Mesh. RSC Adv. 2014, 4, 49745−49751. (15) Kim, D.; Zhu, L. J.; Jeong, D. J.; Chun, K.; Bang, Y. Y.; Kim, S. R.; Kim, J. H.; Oh, S. K. Transparent Flexible Heater Based on Hybrid of Carbon Nanotubes and Silver Nanowires. Carbon 2013, 63, 530− 536. (16) Huang, Q. J.; Shen, W. F.; Fang, X. Z.; Chen, G. F.; Guo, J. C.; Xu, W.; Tan, R. Q.; Song, W. J. Highly Flexible and Transparent Film Heaters Based on Polyimide Films Embedded with Silver Nanowires. RSC Adv. 2015, 5, 45836−45842. (17) Hong, S.; Lee, H.; Lee, J.; Kwon, J.; Han, S.; Suh, Y. D.; Cho, H.; Shin, J.; Yeo, J.; Ko, S. H. Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications. Adv. Mater. 2015, 27, 4744−4751. (18) Choi, S.; Park, J.; Hyun, W.; Kim, J.; Kim, J.; Lee, Y. B.; Song, C.; Hwang, H. J.; Kim, J. H.; Hyeon, T.; Kim, D. H. Stretchable Heater Using Ligand-Exchanged Silver Nanowire Nanocomposite for Wearable Articular Thermotherapy. ACS Nano 2015, 9, 6626−6633. (19) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Uniform Silver Nanowires Synthesis by Reducing AgNO3 with Ethylene Glycol in the Presence of Seeds and Poly(vinyl pyrrolidone). Chem. Mater. 2002, 14, 4736−4745. (20) Coskun, S.; Ates, E. S.; Unalan, H. E. Optimization of Silver Nanowire Networks for Polymer Light Emitting Diode Electrodes. Nanotechnology 2013, 24, 125202. (21) Zhu, S. W.; Gao, Y.; Hu, B.; Li, J.; Su, J.; Fan, Z. Y.; Zhou, J. Transferable Self-Welding Silver Nanowire Network as High Performance Transparent Flexible Electrode. Nanotechnology 2013, 24, 335202. (22) Tokuno, T.; Nogi, M.; Karakawa, M.; Jiu, J. T.; Nge, T. T.; Aso, Y.; Suganuma, K. Fabrication of Silver Nanowire Transparent Electrodes at Room Temperature. Nano Res. 2011, 4, 1215−1222. (23) Chang, J. H.; Chiang, K. M.; Kang, H. W.; Chi, W. J.; Chang, J. H.; Wu, C. I.; Lin, H. W. A Solution-Processed Molybdenum Oxide Treated Silver Nanowire Network: a Highly Conductive Transparent Conducting Electrode with Superior Mechanical and Hole injection Properties. Nanoscale 2015, 7, 4572−4579. (24) Hu, L. B.; 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. (25) Garnett, E. C.; Cai, W. S.; 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. (26) Jiu, J.; Nogi, M.; Sugahara, T.; Tokuno, T.; Araki, T.; Komoda, N.; Suganuma, K.; Uchida, H.; Shinozaki, K. Strongly Adhesive and

which would significantly facilitate the utilization of plasmonic heating to effectively weld metallic NWs into conductive networks, thus to explore their practical important applications. We demonstrated that the welded networks could be directly transferred onto the flexible PDMS substrate to prepare stably performing flexible heaters. Considering that the synthesis, assembly and welding process of Ag NWs are all performed within solution or at the air/water interface, this approach would offer a new route for large-scale, efficient, continuous fabrication of easily transferrable conductive networks and flexible conductors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06334. SEM images of assembled Ag NW films; SEM images of interfacial welding structures; FDTD simulation of electric field distribution among 50 aligned Ag NWs; optical image of assembled NW film; electrical resistance measurement after repeated stretching; temperature measurement of patterned flexible composite heater (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

H.H. and Z.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (Grant No: 51420105009, 51521004, 91333115, 51403127, 21401129), Natural Science Foundation of Shanghai (Grant No: 13ZR1421500, 14ZR1423300), “Chen Guang” project from Shanghai Municipal Education Commission and Shanghai Education Development Foundation under Grant No. 15CG06, and the Zhi-Yuan Endowed fund from Shanghai Jiao Tong University. The authors also thank Instrumental Analysis Center of Shanghai Jiao Tong University.

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DOI: 10.1021/acsami.6b06334 ACS Appl. Mater. Interfaces 2016, 8, 20483−20490