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Halide Welding for Silver Nanowires Network Electrode Hyungseok Kang, Yeon Tae Kim, Siuk Cheon, Gi-Ra Yi, and Jeong Ho Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09839 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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ACS Applied Materials & Interfaces

Halide Welding for Silver Nanowires Network Electrode Hyungseok Kang,1 Yeon Tae Kim,1 Siuk Cheon,1 Gi-Ra Yi,2 Jeong Ho Cho1,2* 1

SKKU Advanced Institute of Nanotechnology (SAINT), 2School of Chemical Engineering, Sungkyunkwan University Suwon 440-746, Republic of Korea. E-mail: [email protected] Abstract We developed a method of chemically welding silver nanowires (AgNWs) using an aqueous solution containing sodium halide salts (NaF, NaCl, NaBr, or NaI). The halide welding was performed simply by immersing the as-coated AgNW film into the sodium halide solution, and the resulting material was compared with that obtained using two typical thermal and plasmonic welding techniques. The halide welding dramatically reduced the sheet resistance of the AgNW electrode due to the strong fusion among nanowires at each junction while preserving the optical transmittance. The dramatic decrease in the sheet resistance was attributed to the auto-catalytic addition of dissolved silver ions to the nanowire junction. Unlike thermal and plasmonic welding methods, the halide welding could be applied to AgNW films with a variety of deposition densities because the halide ions uniformly contacted the surface or junction regions. The optimized AgNW electrodes exhibited a sheet resistance of 9.3 Ω/sq at an optical transmittance of 92%. The halide welding significantly enhanced the mechanical flexibility of the electrode compared with the ascoated AgNWs. The halide-welded AgNWs were successfully used as source–drain electrodes in a transparent and flexible organic field-effect transistor (OFET). This simple, low-cost, and low-power consumption halide welding technique provides an innovative approach to preparing transparent electrodes for use in next-generation flexible optoelectronic devices. Keywords: silver nanowire, sodium halide, welding, transparent conductive electrode, sheet resistance

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INTRODUCTION 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

Transparent conductive electrodes (TCEs) have attracted significant attention as crucial components in various flexible optoelectronic devices, such as organic light emitting diodes (OLEDs), organic solar cells (OSCs), touch screens, and smart windows.1-10 Indium tin oxide (ITO) is the most commonly used commercial TCE, with a high electrical conductivity and optical transparency; however, ITO is of limited utility in flexible electronic applications because it is brittle and requires high-temperature fabrication processes, in addition to the cost problems associated with the scarcity of indium. Several alternative TCEs have been proposed, including carbon nanomaterials (carbon nanotube, graphene),11-14 conductive polymers,15-18 and metallic nanostructures. 19-27 Among these candidate electrode materials, silver nanowires (AgNWs) show promise, with a low sheet resistance < 50 Ω/sq, high optical transmittance of > 90%, and high mechanical flexibility. Unlike conventional electrodes, which form by thermal evaporation, AgNWs may be dispersed in a variety of organic solvents and readily deposited onto arbitrary substrates using simple solution-coating techniques, such as spray-coating, spin-coating, or Meyer-rod coating.28-33 Therefore, solution-processed AgNWs have been considered as a practical alternative TCE material amenable to mass production on the industrial level. The conductivity of AgNW electrodes relies on solid inter-connections at the nanowire junctions. The contact resistance at the junctions strongly affects the total sheet resistance of the AgNW electrodes. Many research groups have intensively explored post-welding processes in an effort to reduce the junction resistance. Physical welding methods have been proposed, including thermal welding,34-35 plasmonic welding,36-38 and compression welding.39-40 These welding methods have successfully reduced the sheet resistance of the electrodes; however, both thermal and plasmonic welding require significant heat energy to obtain sufficiently high optoelectronic properties, and such heating can damage the underlying plastic substrates. During compression welding, a reduction in the sheet resistance is insufficient for practical optoelectronic devices. Recently, several solution welding methods have been introduced, utilizing oxidation, reduction, and electrochemical reactions.41-47 Unlike other welding techniques, the chemical welding method does not require any external energy because this process proceeds under a solution environment. In this manuscript, we demonstrated a chemical AgNW welding method using an aqueous solution containing a variety of sodium halide salts (NaF, NaCl, NaBr, and NaI). The halide welding was conducted simply by immersing the as-coated AgNW films into a sodium halide solution for only 20 s. This halide welding dramatically reduced the AgNW electrode sheet resistance while preserving the optical transmittance. The NaF-welded AgNW electrodes exhibited a sheet resistance of 9.3 Ω/sq at an optical transmittance of 92%. The halide welding efficiency was compared with that of other typical thermal- and plasmonic welding techniques. Unlike the thermal and plasmonic welding methods, halide welding may be applied to AgNW films with a variety of deposition densities, because the halide ions can contact the 2


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surfaces or junction regions uniformly. Moreover, the halide welding significantly enhanced the mechanical 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

flexibility of the AgNW film. The AgNWs subjected to halide welding were successfully applied as source and drain electrodes in a transparent organic field-effect transistor (OFET).48 This simple, safe, and lowpower consumption halide welding method provides an innovative approach to preparing electrode materials for use in next-generation flexible optoelectronics.

RESULTS AND DISCUSSION AgNW films with five different nanowire densities were prepared on polyethylene naphthalate (PEN) substrates using the spray-coating method. The nanowire densities were deterministically controlled by varying the spray time. The AgNWs with highest and lowest densities corresponded to sample #1 and #5, respectively. Figure 1a shows the optical transmittances of the AgNWs prepared with five different nanowire densities as a function of the wavelength. The optical transmittance at the wavelength 550 nm decreased from 92 to 0.1% as the nanowire density increased (from sample #1 to #5). The scanning electron microscopy (SEM) images shown in Figure 1b confirmed the different areal densities of AgNWs deposited on the PEN substrate. Samples #1 and #2 featured large areas of exposed regions, yielding a relatively higher optical transmittance compared to the other samples. On the other hand, almost the entire surface area was covered by nanowires in sample #5, and an extremely low optical transmittance was obtained. Figure 1c plots the optical transmittance as a function of the sheet resistance for each sample. The AgNWs prepared with a higher nanowire density yielded a lower optical transmittance and a lower sheet resistance, as expected. The as-prepared AgNW films were immersed in the aqueous solution containing the sodium halide salts (Figure 1d). Four sodium halide salts, including NaF, NaCl, NaBr, and NaI, were utilized. The salt concentration was fixed at 0.5 M, and the treatment time was varied from 10 to 60 s. After halide treatment, the films were washed thoroughly in distilled water to remove residual salts from the AgNW surface. The results obtained from this halide welding method were compared with those obtained from previously reported thermal and plasmonic welding processes (Figures 1e and 1f). Thermal welding was performed on a hot plate under inert conditions at various temperatures ranging from 90 to 160°C for 10 to 30 minutes. The plasmonic welding using a tungsten-halogen lamp with a power density of ~12 Wcm–2 was performed over various exposure times ranging from 2 to 30 minutes. The tungsten-halogen lamp was positioned on the upper side of the as-coated AgNW film. The distance between the halogen lamp and the AgNW film was fixed at 2 cm. Firstly, the thermal welding was performed. The as-coated AgNW film (sample #1) was placed on the hot plate, and the substrate was heated at a certain temperature under inert conditions. Figure 2a shows the sheet resistance of the AgNW film as a function of the treatment temperature. The treatment time was 3



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fixed at 10 minutes. As the temperature was increased from 30 to 160°C, the sheet resistance of AgNW film 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

was reduced from 24 to 16 Ω/sq. The right panel of Figure 2a shows SEM images of pristine AgNWs and the 160°C-treated AgNWs, indicating that the nanowire junctions were fused by thermal welding. The minimum temperature required for self-diffusion of Ag atoms at the grain boundaries was reported to be 150°C.49 Ag self-diffusion allowed the nanowires to make contact and fuse together. This connecting process under heat treatment improved the electrical conductivity of the AgNW network. Further increase in the treatment temperature above 160°C partially disconnected the nanowires and they coalesced into droplets (Figure S1),34, 50 which increased dramatically the sheet resistance. The sheet resistance of the thermallywelded AgNWs under the optimal condition (at 160°C for 10 minutes) was as low as 15.8 Ω/sq; applying the treatment time exceeding 10 minutes (up to 60 minutes) at 160°C did not further decrease the sheet resistance (see the inset of Figure 2a). The optical transmittance of the AgNW film remained at 92% during the thermal welding process (Figure S2). Next, plasmonic welding was performed using a tungsten-halogen lamp. Figure 2b plots the sheet resistance of an AgNW film (sample #1) as a function of the tungsten-halogen lamp exposure time. As the exposure time increased from 2 to 10 minutes, the sheet resistance decreased gradually from 24 to 17 Ω/sq. The upper right panel of Figure 2b shows a SEM image of the plasmonic-welded junctions of the AgNWs after 10 minutes exposure. Fusion among the nanowire junctions was observed, similar to that observed during thermal welding. The plasmonic light could be converted to local heating to temperatures exceeding 150°C at the nanowire junction. At this temperature, the self-diffusion of Ag atoms at grain boundaries allowed Ag recrystallization at the nanowire junction, which minimized the junction resistance.36 Exposures beyond 10 minutes further reduced the sheet resistance to 11 Ω/sq (at 30 minutes); however, as indicated in the bottom right panel of Figure 2b, the transparent PEN substrate was distorted and exhibited discoloration as a result of excessive plasmonic welding, thereby dramatically decreasing the optical transmittance of the AgNW film by 13% (inset of Figure 2b). After a 30 minute exposure time, the AgNWs on the PEN substrate became disconnected, and the film was no longer conductive (Figure S3). The plasmonic-welded AgNWs obtained at an optimal lamp exposure time of 10 minutes exhibited a sheet resistance of 16.3 Ω/sq at an optical transmittance of 93%. The halide welding was conducted using four different sodium halide salts (NaF, NaCl, NaBr, or NaI). The as-coated AgNW film (sample #1) was immersed in each aqueous solution containing sodium halide salt. The concentration and the temperature of sodium halide solution were fixed to be 0.5 M and 25 °C, respectively (Figure S4). The sodium halide treatment decreased the sheet resistance of the AgNW film as the treatment time increased as shown in Figure 2c and S5. In the presence of the halide ions and dissolved oxygen molecules present in the aqueous solution, silver ions (Ag+) dissolved from the AgNW surface via the redox reaction 4Ag + O2 + 2H2O ↔ 4Ag+ + 4OH-. The halide ions acted as a catalyst for the 4


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redox reaction. The dissolved Ag+ ions could be re-deposited via an autocatalytic reduction onto both the 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

surfaces and junctions of the nanowires (Figure 2d), leading to strong fusion among the nanowires.51 Because the electrostatic potential near the junctions was higher than that in other regions, the Ag+ ions were preferentially re-deposited near the junctions, which decreased the junction resistance of the AgNW film.44 Importantly, the welding efficiency increased as the atomic number of the halide ion decreased. NaI and NaBr displayed only 23 and 29% reductions in the sheet resistances, respectively, and these welding processes took over 60 s. NaCl and NaF, on the other hand, exhibited a significant decrease in the sheet resistance: 11 Ω/sq for NaBr and 9.3 Ω/sq for NaF after 60 sec. Note that for the NaF salt, only 20 s were required to obtain a minimum sheet resistance. Overall, the NaF-welded AgNWs exhibited the highest welding efficiency in terms of both the sheet resistance value and the welding time. Two factors could plausibly explain the observation that NaF provided the highest welding efficiency. Firstly, we considered the solubility of silver halides that are readily formed during redox reaction of Ag in the aqueous solution.52 The Ag+ ions generated by the redox reaction could react with halide ions in the solution. AgCl, AgBr, and AgI have a low solubility product constant (Ksp = 1.8 × 10–10, 7.7 × 10–13, and 8.5 × 10–17, respectively); therefore, precipitation may have occurred due to their insolubility in an aqueous solution.53 These silver ionic compounds may have precipitated and adhered to the AgNW surface, which may have prohibited a redox reaction involving Ag. By contrast, AgF has a high Ksp (= 205) and may have readily re-dissolved due to its extremely good solubility, even if AgF had formed in solution. Thus, a higher number of fluoride ions were available to facilitate the welding process at the junctions. The second possible factor was the concentration of hydroxide (OH–) in sodium halide solution. The pH values of 0.5 M NaF solution at 25 °C was measured to be 9.5, which was higher than those of NaCl, NaBr and NaI solutions (~8.4). Therefore, the NaF treatment facilitated the re-deposition of Ag+ ions onto the Ag surface at high pH (at a higher concentration of OH– ions), according to Chatelier's principle in autocatalytic reduction reaction.54-56 Halide welding was applied to AgNW films with five different nanowire densities (see Figure 1c), and the resulting materials were compared with those obtained from thermal or plasmonic welding methods. The fractional resistance [(R0-R)/R0], where R is the sheet resistance after the welding process and R0 is the initial sheet resistance, was monitored, as shown in Figure 2e. As the nanowire density increased (from sample #1 to #5), the effect of thermal welding was suppressed from 0.37 to 0.03. Because the highnanowire-density samples comprised a thick layer of heavily stacked AgNWs, the heat from the hot plate was not effectively transferred to the surface of the AgNW film. Likewise, the plasmonic welding effect decreased from 0.25 to 0.01 as the nanowire density increased, because the top-side photo illumination could not penetrate the thick AgNW film to reach the bottom of the AgNW film. By contrast, the AgNWs prepared via halide welding exhibited a dramatic reduction in the fractional resistance (0.58), regardless of the 5



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nanowire density. The solution containing sodium halide salts uniformly and intimately contacted the 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

nanowire surfaces and junction regions when the AgNW film was immersed in the solution. In addition, the halide welding reduced significantly the surface roughness from 57 to 31 nm (Figure S6). Overall, the halide welding was the most efficient welding technique applicable to thick AgNW films, as compared with thermal and plasmonic welding. The mechanical flexibility of the AgNW films was investigated by monitoring the sheet resistance during bending and releasing cycles under tensile (or compressive) strains of 2.75% (Figure 3). The applied strain (εy) was caculated from from εy = h/2R, where h is the PEN thickness and R is the radius of curvature (Figure S7). The variation in the sheet resistance could be expressed as (R–R0)/R0, where R is the sheet resistance at a certain point and R0 is the initial sheet resistance. Figure 3a shows that the as-coated AgNW film exhibited a dramatic increase in the sheet resistance as the tensile strain increased up to 2.75%. A large hysteresis was observed during release of the tensile strain, and the sheet resistance did not fully recover to its initial value; however, NaBr treatment yielded less hysteresis, and the sheet resistance of the NaF-treated AgNW film recovered completely to its initial value. Figure 3b plots (R–R0)/R0 over long-term bending cycles at a tensile stain of 2.75%. The sheet resistances of the as-coated AgNW films increased gradually during 1000 cycles, but the halide-treated samples exhibited improved cyclic stabilities. Note that the NaF treatment yielded a negligible variation in the sheet resistance, even after 1000 cycles. The NaF welding induced strong fusion of the AgNWs junctions, which minimized the slipping and delamination of the AgNWs during bending. Compressive strains were applied to the AgNW film, as shown in the inset of Figure 3c, and the sheet resistances were monitored. Unlike the tensile strain, all AgNW films exhibited a decrease in the sheet resistance during bending, and the sheet resistance returned to a value higher than its initial value during release. Application of compressive strain to the AgNW junctions may bring the nanowires into closer contact. The fully contacted nanowire configuration during bending enhanced the electrical conductivity of the AgNW films. Neither the as-coated AgNWs nor the NaBr-treated AgNWs fully recovered their initial states; however, the NaF-treated AgNW film exhibited a complete recovery in the sheet resistance, with a low hysteresis. The long-term stability under compressive strain cycles was investigated, as shown in Figure 3d. The sheet resistances of all AgNWs decreased up to a certain number of compressive bending cycles (indicated by the arrow) due to compression-induced mechanical welding of the nanowires. Permanent detachment of the nanowire junctions occurred above a certain number of cycles due to the poor junction properties, which increased the sheet resistance. The NaF-treated AgNWs exhibited the smallest resistance change. Overall, NaF welding of the AgNW film not only reduced the sheet resistance, it improved the mechanical flexibility. Finally, the NaF-welded AgNWs were applied as source–drain electrodes in flexible and transparent 6


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organic field-effect transistors (OFETs). Figure 4a shows a schematic diagram of the device structure and a 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

photographic image of the OFETs prepared based on NaF-treated AgNWs source–drain electrodes. The OFETs were fabricated onto an AgNWs-coated PEN substrate, and AgNWs (prepared by Meyer rod coating) was used as the gate electrode. The cross-linked poly-4-vinylphenol (cPVP) film formed by spin-coating and subsequent thermal annealing was utilized as the gate dielectric layer. The AgNW source–drain electrodes were prepared by spray-coating an AgNW solution onto the cPVP surface through a shadow mask. The NaF welding was performed by the aforementioned method. P-type pentacene was thermally deposited onto the channel region. The average optical transparency of an OFET device array fabricated onto a PEN substrate was approximately 83% in the wavelength range 400 – 1000 nm. Figure 4b plots the typical transfer characteristics of OFETs based on the as-coated AgNWs and NaF-welded AgNWs electrodes. The as-coated AgNW device exhibited a hole mobility (µh) of 0.12 cm2V–1s–1. The NaF-welded AgNW electrodes improved the electrical properties of the OFETs: µh = 0.37 cm2V–1s–1. This value was comparable to that obtained from devices prepared with Au electrodes. Considering the spiked edges of the wire-type electrodes, this calculated carrier mobilities of AgNWs devices are overestimate values (The effective channel width (W) should be larger).44 The AgNWs electrodes with larger effective W/L ratio are beneficial for achieving higher current compared with those of the pad-type Au electrodes. Note that the enhanced mobility of the OFETs with NaF-welded AgNWs (compared with that with pristine AgNWs) could be understood in terms of a reduced sheet resistance and a reduced surface roughness in the NaF-welded AgNW electrodes, compared with the as-coated one. Figure 4c plots the output characteristics of the OFETs prepared based on the NaFwelded AgNWs electrodes. The curve indicated reasonable gate modulation of the drain current in both the linear and saturation regimes. Moreover, the OFETs prepared based on a NaF-welded AgNW electrode exhibited negligible variations in the hole mobility and the threshold voltage during 300 tensile bending cycles (Figure 4d).

CONCLUSION In conclusion, we developed a chemical welding method for AgNW electrodes using sodium halide salts (NaF, NaCl, NaBr, or NaI). The simple, low-cost, and effective halide welding process dramatically reduced the sheet resistance by inducing strong fusion at the nanowire junctions. Unlike typical thermal and plasmonic welding methods, halide welding was successfully applied to AgNW films with various nanowire densities because the halide ions moved freely and readily contacted all surfaces and junction areas. Halide welding dramatically enhanced the mechanical flexibility of the AgNW film. Moreover, the halide-welded AgNWs were successfully applied as source–drain electrodes in OFETs.

METHODS 7



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Halide welding of AgNWs film: A 0.5 wt% silver nanowires (AgNWs) dispersed in isopropyl alcohol (IPA) 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

was purchased from Nanopyxis Co. The average diameter and length were 30 nm and 30 µm, respectively. The AgNWs solution was deposited on a polyethylene naphthalate (PEN) substrate by spray-coating method. The deposition density of AgNWs (from sample #1 to #5) was controlled by varying deposition times. For thermal-welding, the AgNWs films are heated on the hot-plate under inert condition at given temperature ranging from 90 to 160 °С. For plasmonic-welding, commercial tungsten-halogen lamp (Osram GmbH.) was used and the exposure time was varied from 2 to 30 minutes. For halide-welding, four different kinds of sodium halide salts (NaF, NaCl, NaBr, and NaI) were used. The as-coated AgNWs film was immersed in 0.5 M halide salt solution for various times and subsequently washed with distilled water for 30 s. The surface morphologies of the AgNWs films were characterized using scanning electron microscopy (SEM, JSM7600F, Jeol Ltd.). The sheet resistance of the AgNWs film was measured using the four-point probe technique (Keithley 2182A and 6221). The optical transmittance was measured using a UV−vis spectrophotometer (Agilent 8453). Fabrication of OFET: A polyethylene naphthalate (PEN) film coated with AgNWs was used as a substrate. A dimethylformamide (DMF) solution containing 10 wt% poly-4-vinylphenol (PVP, Mw = 20,000 gmol-1) and 5 wt% poly(melamine-co-formaldehyde) (PMF, Mw = 511 g mol-1) were spin-coated onto the AgNWscoated PEN substrate. The resulting film was thermally annealed at 80 °С for 12 hrs in a vacuum oven. The specific capacitance of the 500 nm-thick cross-linked PVP (cPVP) gate dielectric was 7.1 nFcm-2. The AgNWs source–drain electrodes were patterned onto the cPVP layer through a shadow mask by spraycoating. The halide-welding was performed by aforementioned method. The channel length and width of the OFETs were 100 and 1000 µm, respectively. Finally, 50 nm-thick pentacene (Aldrich Chemical Co.) was deposited onto the channel region at a rate of 0.2 Ås-1 using an organic molecular beam deposition (OMBD) system. The electrical properties of the OFETs were measured at room temperature under ambient conditions in a dark environment using Keithley 2400 and 236 source/measurement units.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental procedures, SEM images and optical transmittance of the AgNWs film.

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

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Notes 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

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (NRF-2013M3A6A5073177) and Basic Science Research Program (NRF2017R1A2B2005790 and NRF-2014M3A9B8023471) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning, Korea.

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Figure 1. (a) Optical transmittance curves of the AgNWs prepared with five different nanowire densities, as a function of the wavelength. (b) Scanning electron microscopy images of the AgNWs. (c) Optical transmittance of the AgNWs as a function of the sheet resistance. (d - f) Schematic diagram of the welding methods: (d) halide welding, (e) thermal welding, and (f) plasmonic welding.

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Figure 2. (a) Sheet resistance of the thermally welded AgNWs as a function of the temperature. The inset shows the sheet resistance as a function of time at 160°C. The right panel shows SEM images of the ascoated AgNWs and the thermally welded AgNWs. (b) The sheet resistances of the plasmonic-welded AgNWs as a function of the time. The inset shows the optical transmittance as a function of time. The right panel shows SEM images of the plasmonically welded AgNWs (upper) and a photographic image of an AgNW film thermally welded (30 minutes) onto the PEN substrate (lower). (c) Sheet resistance of the halide-welded AgNWs as a function of the treatment time. The right panel shows the SEM images of NaBrwelded and NaF-welded AgNWs. (d) Schematic diagram of the halide welding process. (e) (R0–R)/R0 for the AgNW films after thermal welding, plasmonic welding, or halide welding. R is the sheet resistance after the welding process, and R0 is the initial sheet resistance.

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Figure 3. (a) (R–R0)/R0 for the AgNW films as a function of the tensile strain. R is the sheet resistance at a certain point, and R0 is the initial sheet resistance. (b) (R–R0)/R0 of the AgNW films as a function of the tensile strain bending cycle. (c) (R–R0)/R0 for the AgNW films as a function of the compressive strain. (d) (R–R0)/R0 for the AgNW films as a function of the compressive strain bending cycle.

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Figure 4. (a) Schematic diagram showing the device structure, and photographic image of OFETs prepared based on NaF-welded AgNWs source–drain electrodes. (b) Transfer characteristics of the OFETs based on the as-coated AgNW or NaF-welded AgNW source–drain electrodes. (c) Output characteristics of the OFETs based on NaF-welded AgNW source–drain electrodes. (d) Hole mobility and threshold voltage of the OFETs as a function of the bending cycle.

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Halide Welding for Silver Nanowire Network ... - ACS Publications

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