Capillary-Force-Induced Cold Welding in Silver-Nanowire-Based

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Capillary force induced cold welding in silvernanowire-based flexible transparent electrodes Yuan Liu, Jianming Zhang, Heng Gao, Yan Wang, Qingxian Liu, Siya Huang, Chuanfei Guo, and Zhifeng Ren Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04613 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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Capillary force induced cold welding in silver-nanowire-based flexible transparent electrodes

Yuan Liu1, Jianming Zhang2, Heng Gao2, Yan Wang2, Qingxian Liu2, Siya Huang1, Chuan Fei Guo2,*, and Zhifeng Ren1,* 1

Department of Physics and TcSUH, University of Houston, Houston, TX 77204, USA

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Department of Materials Science & Engineering, South University of Science & Technology of China, Shenzhen, Guangdong 518055, China

*To whom correspondence should be addressed. Email: [email protected] (C.F.G.), [email protected] (Z.R.)

ABSTRACT: Silver nanowire (AgNW) films have been studied as the most promising flexible transparent electrodes for flexible photoelectronics. The wire-wire junction resistance in the AgNW film is a critical parameter to the electrical performance, and several techniques of nanowelding or soldering have been reported to reduce the wire-wire junction resistance. However, these methods require either specific facilities, or additional materials as the “solder”, and often have adverse effects to the AgNW film or substrate. In this study, we show that at the nanoscale, capillary force is a powerful driving force that can effectively cause self-limited cold-welding of the wire-wire junction for AgNWs. The capillary-force-induced welding can be simply achieved by applying moisture on the AgNW film, without any 1

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technical support like the addition of materials or the use of specific facilities. The moisture-treated AgNW films exhibit a significant decrease in sheet resistance, but negligible changes in transparency. We have also demonstrated that this method is effective to heal damaged AgNW films of wearable electronics, and can be conveniently performed not only indoors but also outdoors where technical support is often unavailable. The capillary force based method may also be useful in the welding of other metal NWs, the fabrication of nanostructures, and smart assemblies for versatile flexible optoelectronic applications.

Keywords: silver nanowire; flexible transparent electrode; cold welding; capillary force

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MAINTEXT The emerging field of flexible electronics has recently been gaining increasing attention and interests because it offers smarter, lighter weight and more flexible designs to optoelectronic devices, such as flexible displays, flexible solar cells and e-skins.1-6 As one of the key elements in optoelectronic devices, transparent conducting films are facing a new challenge, that is, being flexible and/or stretchable. Standard transparent electrodes are typically made of sputtered indium tin oxide (ITO) thin films, which possess excellent electrical conductivity and high optical transmittance (10-20 Ω sq-1 at a transmittance of 90%). However, ITO is brittle, which largely limits its application in flexible devices, therefore, alternatives are desired.7 In the past decade, researchers have proposed numerous candidates to replace ITO films, such as carbon materials (e.g., graphene8-11, carbon nanotubes3,8,12-13), conducting polymers (e.g., PEDOT:PSS5,14) and metallic materials8,15-20. Among them, one-dimensional silver nanowires (AgNWs) exhibit the most promising performance with a sheet resistance of ~10 Ω sq-1 and a transmittance of 90%, whereas carbon materials and conducting polymers feature much higher sheet resistance (>100 Ω sq-1) at the same transmittance.8 In addition, AgNWs can be dispersed as an ink and printed or coated on different substrates at room temperatures, enabling low-cost and high throughput production of AgNW based flexible transparent electrodes (FTEs). The electrical percolation of a AgNW network depends strongly on the effective point contact at nanowire-nanowire junctions. However, many as-prepared nanowire films suffer from high contact resistance due to the nanogaps or weak contact at the 3


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junctions. Poor wire-wire contact also affects the mechanical flexibility of the AgNW film, because the loosely stacked nanowires would easily move under deformation, leading to deteriorated conductivity. A series of work, including thermal heating,18,21 plasmonic welding,22-23 mechanical pressing,24 electrochemical coating,25 addition of materials as soldering agents,26-27 etc., has been developed to improve the wire-wire contact. However, those methods often have many drawbacks. For example, thermal heating requires an accurate control over the heating temperature and time to avoid the oxidation of the metal nanowires and damage to heat-sensitive substrates;18,23 mechanical pressing may not be applied to some devices as the high pressure may destroy some useful structures or the active layer. Moreover, all these methods need either chemical reagent (such as HAuCl4 for electrochemical coating,15 silver– ammonia and glucose for electroless welding,25 graphene oxide for soldering28) or specific equipment (e.g., laser for plasmonic welding23). It is of great interest to find a simple but effective way to improve the contact of the wire-wire junctions. Capillarity has recently attracted the attention of some researchers as a driving force to form closely packed self-assembly from nanoparticles floating or immersed in liquid.29-31 On nanoscale, the pressure between two contacting particles induced by capillary force can achieve 10 MPa to GPa level, which is comparable to the pressure (25 MPa) of mechanical pressing for the welding of Ag nanowires.24 We therefore apply capillary force as the driving force for cold welding of Ag nanowires to improve the contact of junctions. In this paper, we report on a simple, fast, room temperature, and self-limited 4


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nanowelding method based on capillary force that is free of any additional materials or specific facilities. This method can be simply performed by applying moisture (e.g., breathing water vapor, or applying mist) to the Ag nanowire junctions. By controlled application of moisture, a thin layer of water or distributed water droplets are introduced into the nanostructures of the AgNW film to induce capillary force upon drying. Without any further treatment, we are able to significantly reduce the sheet resistance as well as improve the mechanical flexibility of AgNW films, while without observable changes of the optical transmittance. It is shown that this treatment is effective to AgNWs of different diameters and lengths. We have also demonstrated that the same treatment could be applied in effective healing of damaged AgNW films. This low-cost, room temperature, technical-support-free method can be easily applied to large scale treatment of AgNW and other metal nanowire films, and can be effective not only in laboratories or factories, but also in any general outdoor environments. This method may open a new path for durable and wearable electronics. Since capillary force is a universal phenomenon, it is expected to have foreseeable applications to other metal nanowires or nanostructures as well. Figure 1a shows the schematic of our experimental process of moisture treatment, which basically includes two steps: applying moisture and drying. Moisture is applied onto the AgNW film electrode using a humidifier or simply breathing for 1-3 s. It has been found that liquid prefers to condense near nanowire junctions where the narrow gaps and spaces act as certain kind of capillaries.26-27 When highly humid air reaches the AgNW film, small water drops accumulate near the junctions and fill up the gaps 5


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between nanowires. The sample is then left to dry in air for 30 to 40 s (assisted by gentle fanning). As the water evaporates to some extent during drying, a meniscus shaped capillary bridge would form between the nanowires, yielding an attractive force to join the separate nanowires into contact, as shown in Figure 1a. We assume the model of two spherical surfaces connected by a liquid bridge for simplicity (Figure 1b). The value of the capillary force between two identical spheres connected by a liquid bridge may be estimated analytically by the following equation:32

F =−

2π Rγ cos θ 1 + ( H / 2d )

(1),

where γ is the surface tension of liquid, R is the radius of the sphere, θ is the contact angle, and H is the separation distance between the two spheres, d is the immersion length given by d = ( H / 2) × [ −1 + 1 + 2V / (π RH 2 )] , where V is the liquid volume. The surface tension of water is 71.97 mN m-1 and we assume that the diameter of the particles R = 50 nm. For a typical case of the liquid volume 1×103 nm3 and θ = 60°, at a separation distance of 10 nm, the capillary force is calculated to be 0.66 nN. The capillary force becomes larger as the two particles are driven closer to each other. When the two spheres contact as the drying process continues, the capillary force remarkably increases to 10 nN. Theoretically, the corresponding compressive pressure between the two spheres is superlarge (GPa level) at the very beginning (since the contact area is close to 0), and decreases to ~10 MPa level as the two spheres joint together. At these pressure levels, the contacting nanoparticles can be sufficiently welded. Therefore, capillary force induced welding is expected to be an efficient way to improve the electrical contact of metal nanostructures. 6


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We expect that capillary force induced welding of metal nanoparticles would present several advantages. First, this method is free of reagent and expensive facilities. It can be performed by applying mist or simply breathing on the sample. Second, the welding is self-limited. It occurs only at the contacting areas of the particles, and does not affect the other areas or the underlying layer. By contrast, methods like mechanical pressing and thermal heating affect the whole area of the material and cannot be localized. Third, theoretically this welding effect is extremely strong when two particles begin to form contact, and the effect is especially strong on nanoscales. The capillary force induced cold welding is observed in AgNW films. Because the water droplets at the junctions dry quickly, and the mist condenses randomly on the sample surface, this process needs to be repeated for a few cycles to ensure a sufficient welding effect for the whole sample. Figure 1c-f show two sets of scanning electron microscopy (SEM) images of two wire-wire junctions before and after moisture treatment for a few cycles. Here the sample was initially made by freeze-drying. It is shown that in the as-prepared AgNW film the nanowires are typically loosely stacked (Figure 1c and e, and Figure S1) and there is a gap (several hundred nanometers in length and tens of nanometers in height, Figure 1c and e, and Figure S1) between the top nanowire and the substrate. After the treatment, however, the gap disappears and the nanowires are jointed. Our SEM and atomic force microscopy (AFM) images also exhibit a height reduction as illustrated in Figure 1c, d, and Figure S2, being a proof that the nanowires are welded at the junction. A large 7


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area SEM image (Figure 1g) showing tens of junctions demonstrates that all junctions are well welded and the film is relatively smooth after moisture treatment. The capillary force also causes movement of some nanowires when the adhesion is poor (e.g., a silicon wafer or a glass substrate, see Figure S3). However, such a movement and aggregation of nanowires can seldom happen when an adhesive substrate such as polydimethylsiloxane (PDMS) is used. Figure 2a shows that the resistance of a AgNW film obtained by Meyer-rod coating decreases as we apply mist on the film followed by drying it in air. The decreasing of resistance can be observed every cycle we apply moisture, though it is not as effective as the previous cycles. During each cycle of moisture application, an increase in resistance at the very beginning is observed, which might be due to the fact that a growing water droplet can push some loosely connected AgNWs apart. Instantly after the moisture application stops, the sample begins to dry, and a rapid drop in resistance is observed, indicating that the attractive capillary force takes effect and the AgNWs joints together. As the sample is completely dried and the capillary force is no longer effective, the resistance stops decreasing and finally maintains a relatively stable value. It should be pointed out that the as-prepared samples have already undergone capillary force induced welding during the Meyer-rod coating process. The capillary force induced by using isopropyl alcohol (IPA) is a few times smaller than that induced by water moisture, due to the smaller surface tension of IPA (γ = 21.7 for IPA, and γ = 71.97 mN·m-1 for water). As a result, the welding effect of the AgNW film 8


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with IPA solvent is relatively weak and the typical initial sheet resistance is in the range from 103 to 106 Ω sq-1.

We are also able to completely avoid capillarity during

film coating by using freeze drying: a film of water suspension of AgNWs is made and quickly frozen by using liquid nitrogen. After that, the sample is placed in a lyophilizer to let ice sublimate. The sample shown in Figure 1c and e is made by freeze drying, showing that the nanowires are loosely stacked before applying moisture. According to Wiley et al.,33 the junction resistance is critical to the sheet resistance of metal nanowire films. We have observed exceptionally large changes in resistance (or sheet resistance) of the AgNW films after a few cycles of moisture treatment, but negligible change in transmittance. Figure 2b shows the sheet resistance and optical transmittance at 550 nm of a AgNW film with a 90 nm diameter (AgNW-90) before and after moisture treatment. At a 89.4% transmittance, an as-prepared AgNW-90 film exhibits a sheet resistance ~2.25×105 Ω sq-1, but significantly reduces to 179 Ω sq-1 after moisture treatment, while the change in optical transmittance is smaller than 1% (supplementary Figure S4). Another sample also shows no change in transmittance but the sheet resistance decreases from over 6.3×104 Ω sq-1 to 37 Ω sq-1. All AgNW films with different nanowire diameters and/or lengths (AgNW-50L and AgNW-50S, Figure 2b, supplementary Figure S5) show significant decrease in sheet resistance but negligible changes in transmittance after moisture treatment. The change in resistance of films for our method is quite close to that by the commonly used heating method (Figure S6). The stable transmittance lies in the fact that the attractive capillary force 9


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only induces subtle movements of AgNWs on nano-/microscale (Figure S3) but does not change the area ratio of the nanowires, and there is no addition of other materials. The amount of water condensed on the AgNW film is important to the welding effect. Increasing moisture application time will significantly increase the amount of wire-wire junctions that are immersed in water at the very beginning. However, when most of the junctions are covered with water (~3 s of moisture application in our experiment), further addition of water no longer increase the amount of water bridges and thus does not remarkably contribute to the welding (Figure S7a). In addition, water drying rate does not have a significant impact on the reduction of resistance (Figure S7b). The drying rate can vary using natural drying, gentle fanning, or by using an electric drier. However, our experimental results show that the reduction in resistance of the AgNW films for different drying methods is quite close (Figure S7c). Figure 2c presents a demonstration of the changing luminance of an LED light connected in series with a AgNW-90 film. The LED light is dim when connected to an as-prepared AgNW-90 film and the luminance significantly increases when moisture is intermittently applied on the AgNW film and dried. The result indicates that our moisture treatment can significantly improve the electrical conductivity of AgNW films due to the welding effect caused by capillarity. In literature, the improved performance of AgNW films is often achieved by using a “solder”, or by using mechanical/optical/thermal welding.22-28 By contrast, our method is efficient, but much simpler and free of expensive reagents or facilities. As mentioned above, the ability to sustain mechanical deformation is one of the 10


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key criteria for an FTE material. Stretchability is the most demanding mechanical mode of flexibility. A material that can withstand stretching can also survive under bending or folding. In our work, the flexibility of the AgNW films is evaluated by the stability in resistance under stretching and bending, and the results are shown in Figure 3 and Figure S6. The AgNW films are first transferred to PDMS substrate and then treated by moisture before performing the flexibility tests. AgNW films without treatment are also tested as control samples. The relative resistance of samples under different tensile strains is plotted in Figure 3a. The moisture-treated AgNW films show much larger stretchability: the treated AgNW-90 film is able to endure a 100% strain with a small increase in resistance (by ~1.5 times), while for the as-prepared AgNW-90 film resistance increases by over 24 times at the same strain. We have also observed much larger stretchability in moisture-treated sample than the as-prepared sample with different nanowire diameters (e.g., AgNW-50L). The latter even fails at a strain larger than 75%. By contrast, the moisture-treated sample is still conducting at a strain of 90%. Our tests also show that the moisture-treated AgNW films are more stable under cyclic bending (Figure S8). The results indicate that the treated samples are more stretchable and bendable than the as-prepared samples. We have used SEM to study the microstructures of the AgNW-50L films relaxed from stretching. As shown in Figure 3b, long and wide cracks are observed in the stretched AgNW film without moisture treatment as a result of delamination (white parts marked by arrows). Such delamination has a serious adverse effect on the mechanical strength and thereby electrical conductivity of the AgNW film. By contrast, in the moisture-treated AgNW 11


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film, although cracks are also formed due to distributed ruptures, the film does not locally delaminate (Figure 3c). This is largely attributed to the improved interaction between the nanowires and the substrate, because capillary force can also pull the nanowires to the substrate and achieve stronger adhesion, as can be seen in Figure 1d, f, and g, where the gap between the nanowire and the substrate disappears after moisture treatment. The enhanced adhesion has also been evidenced in our SEM observation of the taping test (Figure S9). Capillary force can strengthen the loosely contacting nanowires, so that it not only improves the performance of as-prepared metal nanowire films, but can also be used for the recovery of damaged AgNW films. As liquid fills up the ruptures and failed junctions caused by strain, the attractive capillary force is also effective in pulling the non-connected nanowires back together. Figure 4a-d is a demonstration of this healing effect. A AgNW-90 film on PDMS was connected in series with a green LED light in a circuit and the LED is on at first. The AgNW-90 FTE is then deliberately damaged by stretching/releasing for a few times, and the LED is not shining anymore due to the failure of the AgNW FTE. Next, we apply moisture by breathing upon the damaged sample and immediately the LED is on again. After a few cycles of treatment, the brightness of the LED has almost recovered to its initial state. The self-healing effect of the damaged AgNW film lies in the attractive capillary force. On an elastic substrate, wire-wire junctions break and nanowires delaminate upon stretching, leading to the dramatic increase of film resistance and finally the failure of the conducting film. Since the substrate is elastic, most of the AgNWs will 12


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largely return to the original positions when the strain is released, but they are either in loose contact or not in contact at the breaking sites (Figure S10a), such that the film exhibits a high resistance or becomes non-conducting. By applying moisture, the AgNWs at the delaminated or broken sites can be dragged back to the substrate and healed due to capillarity (Figures S10b), and the film becomes highly conducting again. Therefore, the moisture treatment is a quite useful approach for the recovery of damaged metal nanowire films. To demonstrate the effect of moisture treatment for the healing of damaged AgNW FTEs in outdoor wearable electronics, we show in Figure 4e that in the outdoors the red LED light mounted on a finger can recover by simply breathing upon the AgNW FTE which has been damaged by stretching. This method does not require technical support such as special facilities or reagents, and can be conveniently carried out by a simple breathing or applying mist to the electrode. Our method may offer an alternative for healing wearable electronics in outdoor environments where technical support is unavailable. In conclusion, at the nanoscale, capillary force can generate high pressure which is effective for the cold-welding of metallic nanostructures. We have used a moisture-treatment to introduce capillarity for improving the performance of AgNW networks, including a remarkable increase in electrical conductance and mechanical stretchability, but negligible change in optical transmittance. The capillary force induced cold-welding is also effective to recover damaged metal nanowire networks, and we have demonstrated that it is possible to recover damaged AgNW electrodes in a wearable electronic device in the outdoors by simply breathing water vapor on the 13


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AgNW electrode, without the use of any additional reagent or facility. This method may open a simple and convenient way for making high performance stretchable transparent electrodes, and for the healing of damaged metal nanowire electrodes. We also demonstrate that capillary force can be used for the recovery of damaged metal-nanowire-based electrodes in outdoor wearable electronics. The capillary force based method may also be useful in the welding of other metal NWs, nanoscale interconnection, smart assemblies, and so forth.

Experimental Section Preparation of AgNWs: Three types of AgNWs were used. Purchased AgNWs dispersed in IPA were from Blue Nano (diameter = 90 nm, length=20 µm), and ACS materials (diameter = 50 nm, length=150 µm). Synthesized AgNWs (diameter ~50 nm, length=40 µm) were synthesized using a reported polyol method controlled by bromine concentration.34 The synthesized AgNWs were purified using a reported selective precipitation method34 and dispersed in deionized water for further use. Experimental conditions: All treatments and measurements mentioned below were performed in an open space at room temperatures, except for the AgNW film prepared in the lyophilizer. Moisture application on the AgNW film was either performed by breathing water vapor or by blowing water mist from a humidifier (Crane, ultrasonic cool mist, USA). When performed by the humidifier, a saturated flow of mist was directed with a velocity of ~30 cm s-1 to the sample using a tube at a height of ~3-5 cm from the sample surface. The typical moisture application time was ~3 s. The 14


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amount of moisture in water vapor from breathing and water mist from humidifier was estimated to be ~0.5-1 mg s-1 cm-2 and ~1-2 mg s-1 cm-2, respectively (estimated based on a 10 s application duration on a glass surface). The sample was then left to dry for 30-40 s in ambient environment (temperature 20-25 °C, humidity ~30-60% RH). The drying was assisted by gentle fanning.

AgNW film fabrication and characterization: For sheet resistance and optical transmittance tests, the AgNW film was coated onto a glass substrate by using a Meyer rod. Silver paste was deposited on the two sides of the AgNW film as contact electrodes. The sheet resistance of all samples was measured by a two-probe method. Resistance change during treatments was monitored by a Keithley 2100 6 1/2 Digit Multimeter. The optical transmittance was measured by a Cary 5000 UV-Vis-NIR Spectrophotometer from Agilent Technologies. For SEM characterizations, the AgNW film was first coated onto silicon substrate or PDMS by using a Meyer rod, and frozen with liquid nitrogen. Next, it was dried in a lyophilizer performed at -20 °C for 10 h. For AFM characterizations, the AgNW film was coated onto silicon susbtrates with a Meyer rod. Mechanical flexibility tests: AgNWs were coated onto PDMS using a PTFE filter (pore size 2 µm, Whatman Nuclepore) transfer method and the resistance was adjusted to appropriate initial values by changing the dispersion concentration. Silver paste was deposited on two sides of AgNW film as electrodes. Bending and stretching 15


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of the PDMS were performed by using a home-made apparatus. Self-healing demonstration: AgNWs were coated onto PDMS with a PTFE filter transfer method. Ag electrodes of 50 nm thickness were deposited on two sides of AgNW film by sputtering and copper tapes were attached onto the electrodes. Alligator clips were fixed at the other sides of the copper tapes to connect the AgNW FTE to the circuit in series with a green LED light.

Acknowledgement The work performed at UH was funded by DOE under a contract DE-SC0010831 (part of the sample fabrication and testing). The work performed at SUSTC was supported by the funding of “The Recruitment Program of Global Youth Experts of China” (No. K16251101), the advance funding of the “Peacock Plan” (No. Y01256120), and the National Natural Science Foundation of China (No. U1613204).

Author contributions C.F.G. and Y.L. conceived the idea. Y.L. and C.F.G. planned and carried out the majority of the experiment. H.G. and J.Z. carried out freeze drying and SEM observation of the effect of capillary force on a single junction. Y.W. and Q.L. assisted in electrode fabrication. Y.L. and S.H. recorded the video for the demonstrations. Y.L., C.F.G, and Z.R. wrote the manuscript. C.F.G and Z.R. directed the work. All authors proofread the paper, made comments and approved the final manuscript.

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Associated Content Supporting Information. Supporting Information is Available: Supplemental SEM and AFM analysis of cold welding effect and healing effect of moisture treatment on AgNWs; supplemental comparison of sheet resistance, optical transmittance and flexibility of AgNW films before and after moisture treatment; comparison of welding effect between thermal heating and moisture treatment; welding effect under different moisture application time and evaporation rate; details of taping tests of moisture treated AgNW films. This material is available free of charge via the Internet at http://pubs.acs.org.

Conflict of Interest Disclosure: The authors declare no competing financial interests.

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G.; Kim, J. K.; Park, J.; Kang, Y. C.; Heo, J.; Jin, S. H.; Park, J. H.; Kang, J. W. Adv. Funct. Mater. 2013, 23, 4177-4184. 20. Zhang, D.; Wang, R.; Wen, M.; Weng, D.; Cui, X.; Sun, J.; Li, H.; Lu, Y. J. Am. Chem. Soc. 2012, 134, 14283-14286. 21. Madaria, A. R.; Kumar, A.; Ishikawa, F. N.; Zhou, C. Nano Res. 2010, 3, 564-573. 22. Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Nat. Mater. 2012, 11, 241-249. 23. Han, S.; Hong, S.; Ham, J.; Yeo, J.; Lee, J.; Kang, B.; Lee, P.; Kwon, J.; Lee, S. S.; Yang, M. Y.; Ko, S. H. Adv. Mater. 2014, 26, 5808-5814. 24. Tokuno, T.; Nogi, M.; Karakawa, M.; Liu, J.; Nge, T. T.; Aso, Y.; Suganuma, K. Nano Res. 2011, 4, 1215-1222. 25. Xiong, W.; Liu, H.; Chen, Y.; Zheng, M.; Zhao, Y.; Kong, X.; Wang, Y.; Zhang, X.; Kong, X.; Wang, P.; Jiang, L. Adv. Mater. 2016, 28, 7167-7172.. 26. Lee, J.; Lee, P.; Lee, H. B.; Hong, S.; Lee, I.; Yeo, J.; Lee, S. S.; Kim, T. S.; Lee, D.; Ko, S. H. Adv. Funct. Mater. 2013, 23, 4171-4176. 27. Lu, H.; Zhang, D.; Cheng, J.; Liu, J.; Mao, J.; Choy, W. C. H. Adv. Funct. Mater. 2015, 25, 4211-4218. 28. Liang, J.; Li, L.; Tong, K.; Ren, Z.; Hu, W.; Niu, X.; Chen, Y.; Pei, Q. ACS Nano 2014, 8, 1590-1600. 29. Duan, H.; Berggren, K. K. Nano Lett. 2010, 10, 3710-3716. 30. Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 19


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8635-8640. 31. Hu, Y.; Lao, Z.; Cumming, B. P.; Wu, D.; Li, J.; Liang, H.; Chu, J.; Huang, W.; Gu, M. Proc. Natl. Acad. Sci. USA 2015, 112, 6876-6881. 32. Rabinovich, Y. I.; Esayanur, M. S.; Moudgil, B. M. Langmuir 2005, 21, 10992-10997. 33. Mutiso, R. M.; Sherrott, M. C.; Rathmell, A. R.; Wiley, B. J.; Winey, K. I. ACS Nano 2013, 7, 7654-7663. 34. Li, B.; Ye, S.; Stewart, I. E.; Alvarez, S.; Wiley, B. J. Nano Lett. 2015, 15, 6722-6726.

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Figures Moisture

Evaporation

AgNW film

Water Welding

Apply moisture

Apply moisture

Figure 1. Capillary-force-induced cold-welding of Ag nanowires. (a) Schematic of moisture treatment for capillary-force-induced cold welding of Ag nanowires. (b) Schematic of the mechanism of capillary interaction between two particles connected with a liquid bridge. (c,d) and (e,f) Two sets of SEM images of Ag wire-wire junctions before and after moisture treatment. The thickness (t) of the top nanowire is significantly smaller than the original diameter (d) due to welding. Scale bar: 200 nm. (g) SEM image of a relatively large area of AgNWs showing well welded wire-wire junctions and relatively smooth surface caused by the moisture treatment. Scale bar: 1 µm.

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100

500

a

400

Transmittance (%)

Resistance (Ohm)

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300 200 Apply moisture

100 0

50

100

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b 80

60

As-prepared Moisture treated

40 10

200

1

10

2

10

3

10

4

5

10

6

10

Sheet Resistance (Ohm/sq)

Time (s)

c

Initial

After 5 times

After 10 times

Figure 2. The effect of moisture treatment on resistance/sheet resistance and transmittance. (a) Resistance of a Ag nanowire film significantly decreases after moisture-treated for a few cycles. (b) Sheet resistance and optical transmittance of as-prepared and moisture treated AgNW-90 FTEs. (c) The brightness of an LED light connected in series with a AgNW-90 FTE significantly increases after moisture-treatments for a few times.

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a

AgNW-90 As-prepared AgNW-90 Moisture treated AgNW-50L As-prepared AgNW-50L Moisture treated

100 80

R/R0

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b

X

Disconnected

60 40

c

20 0 0

20

40

60

80

100

Strain (%) Figure 3. The effect of moisture treatment on stretchability and surface morphology. (a) Stretchability tests of as-prepared and moisture treated AgNW-90 and AgNW-50L films, showing that moisture-treated samples are more stretched than as-prepared samples. (b) and (c) SEM images of as-prepared and moisture-treated AgNW-50L films on PDMS after the stretching test. The former shows serious delamination and wide cracks.

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a

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Stretch to 150-200%

Before damage

b

Stretched

c

d

Applying moisture

Healed

Stretched

Initial

Released

e

Healed

Breathing

Figure 4. Self-healing effect of moisture treatment of damaged AgNW FTEs. (a) Before damage. (b) After damage by stretching to 150%-200% for a few cycles and the LED turns off, indicating that the electrode becomes non-conducting. (c) Moisture healing by breathing upon the damaged sample, for which the LED is on again. (d) Healed sample, the brightness of the LED light has largely recovered to the initial state. (e) A demonstration of breathe-recoverable wearable electronics with AgNW electrode in the outdoors. The LED light turns off after stretching/releasing, but turns on again after breathing water vapor on the damaged electrode. The insets show enlarged images of the LED light.

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ToC figure 29x10mm (300 x 300 DPI)


Capillary-Force-Induced Cold Welding in Silver-Nanowire-Based

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