Spontaneous and Selective Nanowelding of Silver Nanowires by

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Spontaneous and Selective Nano-Welding of Silver Nanowires by Electrochemical Ostwald Ripening and High Electrostatic Potential at the Junctions for High-Performance Stretchable Transparent Electrodes Hyo-Ju Lee, Semi Oh, Ki-Yeop Cho, Woo-Lim Jeong, Dong-Seon Lee, and Seong-Ju Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Spontaneous and Selective Nano-Welding of Silver Nanowires by Electrochemical Ostwald Ripening and High Electrostatic Potential at the Junctions for High-Performance Stretchable Transparent Electrodes Hyo-Ju Lee†, Semi Oh†, Ki-Yeop Cho†, Woo-Lim Jeong‡, Dong-Seon Lee‡, and SeongJu Park*,†

†

School of Materials Science and Engineering, Gwangju Institute of Science and

Technology, Gwangju 500-712, Republic of Korea ‡

School of Electrical Engineering and Computer Science, Gwangju Institute of Science

and Technology, Gwangju 500-712, Republic of Korea

ABSTRACT Metal nanowires have been gaining increasing attention as the most promising stretchable transparent electrodes for emerging field of stretchable optoelectronic devices. Nano-welding technology is a major challenge in the fabrication of metal nanowire networks since the optoelectronic performances of metal nanowire networks are mostly limited by the high junction resistance between nanowires. We demonstrate the spontaneous and selective welding of Ag nanowires (AgNWs) by Ag solders via an *

Corresponding author. E-mail: [email protected] 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrochemical Ostwald ripening process and high electrostatic potential at the junctions of AgNWs. The AgNWs were welded by depositing Ag nanoparticles (AgNPs) on the conducting substrate and then exposing them to water at room temperature. The AgNPs were spontaneously dissolved in water to form Ag+ ions, which were then reduced to single crystal Ag solders selectively at the junctions of the AgNWs. Hence, the welded AgNWs showed higher optoelectronic and stretchable performance compared to that of as-formed AgNWs. These results indicate that electrochemical Ostwald ripening-based welding can be used as a promising method for high performance metal nanowire electrodes in various next-generation devices such as stretchable solar cells, stretchable displays, organic light-emitting diodes, and skin sensors.

KEYWORDS Electrochemical Ostwald ripening, welding, metal nanowire, metal nanoparticle, stretchable transparent electrode

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Introduction Metal nanowire networks have attracted considerable attention as stretchable transparent conducting electrodes (TCEs) in next-generation devices such as stretchable solar cells, stretchable displays, organic light-emitting diodes, transparent heaters, and skin sensors due to their high electrical conductivity, high flexibility, and lower processing temperature than those of commonly used indium tin oxide (ITO).1-4 However, as-formed nanowire networks exhibit low conductivity owing to the high junction resistance between nanowires. To reduce the junction resistance, various nanowelding technologies including thermal annealing, mechanical pressure, electrical current, light, and soldering methods have been investigated by many research groups.521

It was reported that soldering the AgNWs by using the soldering materials such as

poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate)

(PEDOT:PSS),

graphene

oxide (GO), and metal could improve the conductivity and stretchability of AgNWs.16-21 However, PEDOT:PSS and GO could not efficiently reduce the junction resistance because the high acidity and water absorption of PEDOT:PSS induced the corrosion of AgNWs and electrical conductivity of GO is too low compared with metal.22 Previously reported metal soldering technologies, which commonly used electrochemical galvanic process, involves soldering AgNWs with metal solders by immersing AgNWs in a boiling aqueous solution that contains metal salts and reducing agents. However, this 3



features large optical losses because light scattering is increased by the roughened surface of the AgNWs because the metal ions become reduced both on the surface of the AgNWs and at their junctions.16-19 The electrochemical Ostwald ripening of metal nanoparticles is a process whereby the size distribution of metal nanoparticles tends to increase on a conducting layer in an aqueous solution.23 This is an interesting feature of nanoscale metallic materials and plays an important role in the synthesis of nanocomposite materials.24-28 Previous studies have reported that electrochemical reaction-based welding processes require an aqueous solution that contains metal salts and reducing agents together with additional inputs such as electrical current and thermal annealing.16-19 However, in this work, we employed the electrochemical Ostwald ripening process to spontaneously and selectively weld AgNWs in water at room temperature without any reducing agents or metal salts. In our experiment, Ag+ ions were produced from separately-supplied AgNPs and moved to the junctions of AgNWs in water. In this process, the electrons which were transferred from the AgNPs to the junctions of AgNWs through the conducting layer on the substrate induced high electrostatic potential at the junctions of AgNW and the electrons could reduce the Ag+ ions to Ag solders selectively at the junctions of AgNWs. The spontaneous and selective deposition of Ag solders at the

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junctions of the AgNWs progressed until all AgNPs were consumed. The Ag solders at the junctions of the AgNWs improved both the electrical conductivity and the mechanical stretchability of the AgNW networks; this study demonstrates and proposes a method of spontaneous and selective growth of Ag solder at the junctions of AgNWs for high-performance stretchable conductive electrodes.

Results and Discussion Figure 1a, b, and c show SEM images of Ag-deposited AgNW/ITO films before exposure to water. The SEM images show that the Ag layer transformed to AgNPs on the AgNWs and the ITO conducting layer. As the Ag layer thickness was increased from 1 to 5 nm, the average size of AgNPs increased from 17 to 24.5 nm. After exposure to water for 10 min, the AgNWs became welded at their crossing points and transformed into interconnected AgNW networks. The AgNPs dissolved in water and disappeared from the substrate, as shown in Figure 1d, e, and f. These figures also show that the volume of Ag deposited selectively at the junctions of AgNWs increased as the thickness of the deposited Ag layer was increased.

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Figure 1. SEM images of (a) 1 nm, (b) 3 nm, and (c) 5 nm-thick Ag layers deposited on AgNW/ITO films prior to exposure to water. (d) 1 nm, (e) 3 nm, and (f) 5 nm-thick Ag layers deposited on AgNW/ITO films after exposure to water for 10 min.

Figure S1a shows the absorbance spectra of Ag (5 nm)/Ag NW with different immersion time in water. Absorbance spectra of Ag (5 nm)/Ag NW before exposure to water shows the strong absorption in the visible wavelength region due to surface plasmon resonance (SPR) by AgNPs.29 The absorption peak which correspond to the SPR wavelength of AgNPs is shifted from 575 nm to 422 nm and the absorbance is

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decreased with increasing immersion time in water as shown in Figure S1a. The blueshift of SPR peak and the decrease of absorbance indicate that the size and density of Ag NPs on AgNW/ITO glass are decreased as the AgNPs are dissolved in water. Figure S1b show the absorbance spectra of Ag (x nm)/Ag NW before and after exposure to water for 10 min. The SPR wavelength of Ag (x nm)/Ag NW shifts from 448 nm to 575 nm and the absorbance increases as the thickness of Ag layer increases because the size and density of Ag NP are increased. After exposure to water for 10 min, the SPR peak of Ag NPs is not observed on the spectra because the Ag NPs are completely dissolved in water. Figure 2 shows the optical transmittance spectra and sheet resistances of Ag/AgNW/ITO glasses before and after exposure to water; the solid lines in Figure 2a show that the transmittance of the Ag/AgNW/ITO glass remarkably decreased in the visible range as the Ag thickness was increased from 1 to 5 nm prior to exposure to water. The decrease in the transmittance was attributed to the greater surface scattering of light by the larger and denser AgNPs. However, the optical transmittance of the Ag/AgNW/ITO glass increased after exposure to water. This result could be attributed to a decrease in the surface scattering of incident light by the dissolution of AgNPs on the substrate in water, as indicated by the dotted lines in Figure 2a. The transmittance of

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Ag (1 nm)/AgNW/ITO glass after spontaneous welding was similar to that of the asformed AgNW/ITO glass. However, the optical loss of the welded AgNW/ITO glass was increased due to the greater volume of Ag solder at the junctions as the thickness of deposited Ag film was increased. Figure 2b shows that the sheet resistance of the Ag/AgNW/ITO glass before exposure to water was slightly decreased as the thickness of the Ag film increased. However, the sheet resistance of the Ag/AgNW/ITO glasses after exposure to water remarkably decreased as the thickness of the Ag increased. This result could be attributed to the increased size of the Ag solder at the junction regions of the crossed AgNWs, which can remarkably reduce junction resistance. We also investigated the electrochemical Ostwald ripening welding process for AgNWs on NiO/glass to examine what effect a conducting layer has on the welding process, as shown in Figure S2. The sheet resistance of NiO (> 10 MΩ/sq) was much higher than that of ITO (80 Ω/sq), whereas the optical and electrical properties of the AgNWs welded on NiO/glass were considerably improved after exposure to water, as shown in Figures S2a and b. This result indicates that the spontaneous welding of AgNWs can occur, even on the semi-conducting layer of NiO with its higher resistance than ITO. However, the spontaneous welding process of the AgNWs was not observed on the insulating glass substrate, as shown in Figure S3, indicating that the electrons generated

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from the AgNPs by the ionization of Ag were not transferred to the AgNWs on the insulating glass substrate. Furthermore, the electrical double layer that was formed at the interface between the AgNPs and the solution to maintain the solution’s electroneutrality could also inhibit the transfer of electrons from the AgNPs to the junction area of the AgNWs through the solution.30 These results indicate that the electrons that were generated from the AgNPs through the ionization of Ag moved to the junction area of the AgNWs through the fully conducting ITO or semi-conducting NiO layer on the glass substrate.

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Figure 2. (a) Optical transmittance spectra and (b) sheet resistance of Ag (x nm)/AgNW/ITO film before and after exposure to water.

Figure 3 shows SEM images of the Ag (5 nm)/AgNW/ITO films after exposure to water. After 10 min of exposure, the Ag/AgNW film appeared to be interconnected with a few residual AgNPs on the ITO layer, as shown in Figure 3a. As the exposure time was increased from 5 to 90 min, the size and density of the AgNPs rapidly decreased until all the particles disappeared and the AgNWs were selectively welded at their junctions. The AgNWs, however, showed no dissolution in water, as indicated by Figure 3b-d. Plieth reported that the standard electrode potential of metal nanocrystals is negatively shifted from that of the bulk metal and that the potential shift is inversely proportional the metal nanocrystal size.31,32 We calculated the standard electrode potential of the AgNPs using Plieth’s equation32: E = E −

2γv zFr

Here, E and E are the standard electrode potentials of AgNP and bulk Ag, γ

is surface tension, v is molar volume, z is the lowest valence state, F is the Faraday constant, and r is the radius of the AgNP. The standard electrode potential of AgNPs (r = 12 nm) was negatively shifted by 0.03 V from that of the AgNWs owing to the standard electrode potential of the AgNWs being similar to that of bulk Ag 10


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(E = + 0.8 V).33 This indicates that a galvanic reaction process can occur in the

electrochemical solution in which the AgNPs behave as a sacrificial anode, the AgNW as a cathode, and the ITO as a carrier transport layer due to the difference between the standard electrode potentials of the AgNWs and AgNPs. Therefore, the AgNPs spontaneously dissolved as Ag+ ions in water and the Ag+ ions were reduced and deposited as Ag on the AgNWs due to their acceptance of electrons from the AgNPs through the ITO conducting layer. Figure 3e shows the optical transmittance and sheet resistance of the Ag (5 nm)/AgNW/ITO film after it was exposed to water. Before exposure to water this film showed a low optical transmittance in the visible wavelength region because of surface scattering of light by the AgNPs. However, after 1 min of exposure to water, the transmittance of the film markedly increased from 49% to 79% because scattering of incident light was reduced by dissolution of the AgNPs in water. After an exposure time of 5 min, the optical transmittance saturated because the AgNPs were completely dissolved in water. Figure 3e also shows that the sheet resistance of the Ag (5 nm)/AgNW/ITO film markedly decreased after 1 min of exposure. The improved electrical conductivity of the AgNWs was attributed to a decrease of the junction resistance through the interconnection of AgNWs by the welding process. The sheet resistance was not further decreases with increasing

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exposure time as most of Ag NPs were rapidly consumed as solders at the junctions of Ag NWs within 1 min. These results indicated that the AgNWs were efficiently and rapidly welded by the AgNPs in water at room temperature.

Figure 3. SEM images of Ag (5 nm)/AgNW/ITO film obtained (a) 1 min, (b) 5 min, (c) 15 min, and (d) 90 min after exposure to water, and (e) optical transmittance and sheet resistance of Ag (5 nm)/AgNW/ITO film after various exposure times.

Previous reports have indicated that the surface of AgNWs becomes rough after

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electrochemical reaction-based welding processes. This effect is attributed to metal ions from the metal salts in solution being deposited not only at the junctions of the AgNWs but also on their surfaces.16-19 However, in this study, we found no increase of the surface roughness of the welded AgNWs as the thickness of the Ag layer was increased, as shown in Figure 1. Transmission electron microscope (TEM) images and fast Fourier transform (FFT) patterns were measured to characterize the structure of the Ag/AgNW films before and after exposed to water, as shown in Figure 4. The TEM images of the Ag/AgNW film before exposure to water featured AgNPs and AgNWs, as shown in Figure 4a and b. The FFT patterns of a AgNW and AgNP, shown in Figure 4b, clearly indicate the different crystal structures of the two nanomaterials. The FFT pattern of the AgNW featured weak double diffraction spots between the strong primary spots along one direction indicating that the AgNW had a single crystal structure.34 However, the FFT pattern of the AgNPs featured ring patterns, which indicated that the AgNPs had a polycrystalline structure owing to the random deposition and localization of Ag atoms on the substrate as AgNPs at room temperature. After exposure of the Ag/AgNW films to water, the AgNWs became welded and the AgNPs disappeared from the substrate, as shown in Figure 4c. The FFT pattern and surface roughness of the AgNWs were unchanged compared with those before exposure to water. The Ag+ ions from the

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AgNPs on the substrate were reduced to an Ag solder, which accumulated selectively at the junctions of the AgNWs, as shown in Figure 4b and d. Figure 4e shows a high resolution TEM image of a junction of welded AgNWs. The Ag solder formed at the junction area between a lower AgNW and upper AgNW. The FFT patterns of the upper and lower AgNWs showed two different crystal directions with roughly equal intensity, as shown in Figure 4f and h. The diffraction patterns of the upper AgNW and lower AgNW represent AgNWs tilted around the growth direction of [110].34,35 However, the FFT pattern of the Ag solder, shown in Figure 4g, indicated that the Ag solder formed between the two AgNWs featured a single face-centred cubic structure along the [110] zone axis direction. Previously reported Ag solders, formed by reduction of Ag+ with reducing agents in solution typically show polycrystalline structures36; however, the Ag solder in this study showed a single crystal structure. The Ag+ ions that were produced from the polycrystalline AgNPs, were reduced and then grew on the surface of the AgNWs as a solder only in the junction areas.

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Figure 4. TEM images (a) and (b) of Ag/AgNW films prior to exposure to water. The inset shows the FFT patterns of a AgNW and AgNP. TEM images (c) and (d) of Ag/AgNW film after exposure to water. The insets show the FFT patterns of an AgNW. TEM image (e) of a AgNW junction; FFT patterns of (f) the lower AgNW, (g) Ag solder, and (h) upper AgNW.

The concentration of Ag+ ions around the AgNPs is higher than that near the AgNWs at equilibrium due to the difference of standard electrode potential. The high concentration of Ag+ ions around Ag NPs due to the different standard electrode potential of 0.03 V between AgNP and AgNW is 3 times higher than that of AgNWs.23 The concentration gradient between AgNPs and AgNWs allows diffusion of Ag+ ions from the AgNPs to AgNW. However, in the AgNW welding process (Figure 5), Ag+

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ions diffused from the AgNPs were reduced to Ag solders selectively at the junctions of crossing AgNWs rather than over the whole Ag NW surface, as shown in Figure 4c and d. This selective AgNW welding could be attributed to the high electrostatic potential at the junctions of AgNWs. AgNWs are negatively charged by electrons that are generated from the AgNPs by the formation of Ag+ ions. The electrons are redistributed in AgNWs and a high electrostatic potential is generated at the junctions of AgNWs because electrons tend to concentrate at the junctions of AgNWs.37 Furthermore, the Ostwald ripening process occurs via the migration of atoms from surfaces with high surface curvature to one with low surface curvature.38,39 If the surfaces of the AgNWs and the junction area of the AgNWs have a round shape with respective radii of r and r

!"#$ % ,

their Gibbs free energies are given by:

G = 4πr#) γ, (r# = r and r

!"#$ % , γ

= surface tension)

If a sphere contains a number of atoms ni:

n# =

678:9 ;Ω

, (Ω = atomic volume),

then, its chemical potential is defined as: @A

μ# = @ = 9

)ΩB 89

.

Here, the AgNWs have a convex surface (i.e., r# is positive) and the junctions of the AgNWs have a concave surface (i.e., r# is negative); this analysis indicates that the

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surface of AgNWs and the junctions between AgNWs have different chemical potentials depending on their surface curvatures.38 Therefore, the Ag+ ions near the surface of AgNW, which have high chemical potential, are also expected to selectively move towards the junctions of AgNWs, where the chemical potential is lower than those of surface of Ag NWs. Hence, the selective deposition and reduction of Ag+ ions occur preferentially at the junctions of AgNWs rather than at their whole surfaces. Therefore, the Ag+ ions produced from the AgNP are reduced to Ag solder at the junctions of AgNWs by accepting the electrons transferred through the conducting substrate until the Ag NPs are completely dissolved on the conducting substrate.

Figure 5. Schematic illustration of the welding process (red and blue indicate electrons and Ag+ ions).

To understand the dependence of the solubility of Ag in aqueous solution on the welding process, the AgNW/ITO film deposited with 5 nm–thick Ag was immersed in

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water with various pH values, as shown in Figure S4. After exposure to solutions of pH 4 and 10 for 1 min, we found that the AgNW welding process was accelerated in a solution of pH 4. However, most of the small AgNPs remained on the substrate exposed to pH 10 solution, as shown in Figure S4a and c. After 10 min of exposure to the pH 4 and 10 solutions, the welded AgNWs partially dissolved in pH 4 solution, while the density of AgNPs on the substrate slightly decreased in a pH 10 solution, as shown in Figure S4b and d. The dissolution process of the Ag in aqueous solution is explained by the oxidation of Ag to Ag+ induced by protons and dissolved oxygen in the solution.40,41

1 I 2Ag (%) + O)(FG) + 2H(FG) → 2Ag I (FG) + H) O(K) 2 This reaction explains the results shown in Figure S4; i.e., the low pH solution enhanced AgNW welding and the high pH solution suppressed welding because of the pH dependence of the Ag solubility. It is also interesting to notice that the dissolution of Ag in water was favored by the slightly oxidized Ag surface because the dissolution process of Ag in water required initial oxidation of Ag surface.23,40 Finally, we transferred the AgNWs welded by the electrochemical Ostwald ripening process on ITO glass into polydimethylsiloxane (PDMS) to fabricate stretchable TCEs. Figure 6a shows the optical transmittance spectra of the AgNWs and welded AgNWs (Ag (x nm)/AgNW after exposure to water) embedded in PDMS substrates. The

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transmittances at 550 nm of AgNW/PDMS and welded AgNW/PDMS are 93.7, 92.1, 91.3, and 86.8% with respective sheet resistances of 49.5, 23, 19.5, and 13.7 Ω/sq. The welded AgNWs/PDMS (Ag (3 nm)/AgNW after exposure to water) shows a much lower sheet resistance of 19.5 Ω/sq and a higher optical transmittance of 91.3% compared with these of 80 Ω/sq and 90% for our ITO glass. Figure 6b shows the resistance variation ((R − R )/R , where R is the resistance under different strains and R0 is the initial resistance prior to stretching) of AgNWs and welded AgNWs (Ag (3 nm)/AgNW after exposure to water) embedded in PDMS substrates as a function of the tensile strain (∆L/L , where L0 is the initial length and ∆L is the extended length). The initial resistances of AgNW/PDMS and welded AgNW/PDMS are 190 and 84 Ω, respectively. As shown in Figure 6b, the resistance variation of AgNW/PDMS is slowly increased before reaching the strain of 10%, but the resistance variation is rapidly increased and reaches 51.6 under 20% strain while the resistance variation of welded AgNW/PDMS is 0.58 under 20% strain and reaches 29.9 under 80% strain. The electrical resistance of welded AgNW/PDMS dramatically increased under 80% strain. However, the resistance of welded AgNW/PDMS pre-strained under 80% strain (R = 88 Ω) is similar to the initial resistance of welded AgNW/PDMS (R0 = 84 Ω). We compared the SEM images of welded AgNW/PDMS before and after stretching the

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samples to confirm the surface morphology of welded AgNWs as shown in Figure S5. Figure S5a shows the welded AgNWs are embedded in PDMS and Figure S5b shows Ag solders are not separated from the junctions of AgNWs under 80% strain. The difference in stretchability between AgNW/PDMS and welded AgNW/PMDS is mainly due to the contact strength between AgNWs because the welded AgNWs, which are strongly connected at the junctions of AgNWs by Ag solders, have a lower frequency for the detachment of AgNWs. We evaluated the electrical connection between AgNWs under various strains by connecting the as-formed AgNW/PDMS and the welded AgNW/PDMS to an LED and a power supply and passing a direct current through the stretchable TCE to LED, as shown in Figure 6c. As the resistance of as-formed AgNW/PDMS was increased to 10 KΩ under 20% strain, the LED did not operate as shown in Figure 6d, while the LED connected to the welded AgNW/PDMS operated under the 80% strain, as shown in Figure 6e. We compare the electro-optical performance and stretchability of Ag-soldered AgNW with those of AgNW-based stretchable TCEs published in the literatures as shown in Table 1. It shows that the Agsoldered AgNWs exhibit high electro-optical performance compared with other AgNWbased stretchable TCEs. The change of resistance and stretchability of Ag-soldered AgNWs under strain are comparable to or even better than those of other welded AgNW

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stretchable TCEs while our TCEs maintain the highest transmittance.

Figure 6. (a) Optical transmittance spectra of AgNWs and welded AgNWs (Ag (x nm)/AgNW after exposure to water) after being transferred to PDMS. (b) Resistance variation of AgNWs and welded AgNWs as a function of the tensile strain. (c) Photograph of LED connected to welded AgNW/PDMS and schematic illustration of the LED circuit (insert). (d) and (e) photographs of LED bias at 3 V under various tensile strains using AgNWs and welded AgNWs.

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Sheet Transmittance

Resistance Strain

resistance (%)

change (%)

(Ω/sq)

((R- R0)/R0)

AgNW/dopamine42

80

35

20

∞

Long AgNW43

62

2

35

6

80

36.2

50

∞

85

25

20

10

82.5

14

80

9.6

91.3

19.5

60

10.7

Flash-induced welded AgNW44 PEDOT:PSS-soldered AgNW20 GO-soldered AgNW21 Ag-soldered AgNW (In this study)

Table 1. Comparison of the AgNW-based stretchable transparent electrodes properties.

CONCLUSION In summary, we have successfully demonstrated a highly conductive and stretchable TCE by employing the electrochemical Ostwald ripening and high electrostatic potential at the junctions of AgNWs in water. The difference in the standard electrode potentials between AgNWs and AgNPs induced the spontaneous welding of AgNWs.

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The Ag+ ions that were produced from the AgNPs were reduced to Ag solders selectively at the junctions of AgNWs due to the high electrostatic potential at the junctions of the AgNWs. As such, the sheet resistance of AgNWs after the welding process was reduced and the stretchability was remarkably improved, while the initial transmittance of the AgNWs film was maintained. We expect that this spontaneous and selective welding process can provide an alternative route to the efficient welding of metal nanowires for many transparent and stretchable device applications in the area of transparent displays, solar cells, and stretchable sensors.

Methods Materials. AgNW dispersion with a mean diameter of 32 nm and mean length of 25 µm, purchased from NANOPYXIS, was diluted in DI water to a concentration of 0.5 mg/ml. ITO glass was purchased from Sigma-Aldrich. 20 nm-thick NiO was deposited on glass substrate at room temperature by electron beam evaporation. Sodium hydroxide and nitric acid purchased from Sigma-Aldrich were added to DI water (σ > 17 MΩ/cm, pH 7) to control the pH value of water. PDMS was prepared by mixing the base and curing agent (Sylgard 184, Dow Corning) at a 10:1 weight ratio. Fabrication of stretchable AgNWs electrodes. The AgNW dispersion was sprayed at

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0.3 MPa onto ITO glass substrates. An Ag layer was additionally deposited on the AgNWs/ITO glass via electron beam evaporation (deposition rate = 0.2 Å/s). The electron beam evaporation time for 1 nm, 3 nm, and 5 nm-thick Ag layers was 50 s, 150 s, and 250 s and these Ag layers were transformed to AgNPs. These AgNPs were used as a solder for welding the AgNWs in water. The AgNW films with Ag NPs were immersed in water for the AgNW welding process by dropping water onto the AgNW films with AgNPs. After the AgNW was welded in water, the samples were dried with N2 gas to remove water from the welded AgNWs samples. The liquid mixture of PDMS was spin-coated at 500 rpm for 1 min onto the welded AgNW on an ITO glass substrate and cured at 100 °C for 30 min. The welded AgNWs that were embedded in PDMS substrate were peeled off from the ITO glass substrate. Characterization. The surface morphology of the welded AgNWs on conducting layers was observed with a scanning electron microscope (SEM) and transmission electron microscope (TEM). Sheet resistances of the films were measured by the four-point probe method. Specular transmittance was measured on a UV-VIS spectrophotometer (Agilent 8453) in the spectral range of 300–800 nm.

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ASSOCIATED CONTENT Supporting Information Available: SEM images and photographs of Ag (5 nm)/AgNW/ITO and Ag (5 nm)/AgNW/glass films after exposure to water and SEM images of the Ag (5 nm)/AgNW/ITO films after exposure to acidic and basic solution, as described in the supplementary materials. AUTHOR INFORMATION * Corresponding Author Prof. Seong-Ju Park School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea. E-mail: [email protected]

ACKNOWLEDGMENT This research was supported by the GIST (Gwangju Institute of Science and Technology), Korea, under the Practical Research and Development support program supervised by the GTI (GIST Technology Institute) and the “Nobel Research Project” grant funded by the GIST in 2018.

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Table of content (TOC)

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Spontaneous and Selective Nanowelding of Silver Nanowires by

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