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Facile Patterning of Ag Nanowires Network by Micro-Contact Printing of Siloxane Sung-Soo Yoon, and Dahl-Young Khang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05909 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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ACS Applied Materials & Interfaces

Facile Patterning of Ag Nanowires Network by Micro-Contact Printing of Siloxane Sung-Soo Yoon, Dahl-Young Khang* Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea

ABSTRACT

A simple, low-cost, scalable patterning method has been demonstrated for chemically welded Ag nanowires (AgNWs) network. The chemically welded network of AgNWs on substrates has been patterned by modified micro-contact printing (µCP). As an ink for the µCP, uncured highviscosity siloxane polymer has been applied. Using elastomeric polydimethylsiloxane (PDMS) stamp that has been replicated from micro-machined Si master mold by metal-assisted chemical etching, the printed siloxane ink materials have been cured by simple UV/ozone exposure for 3min, which acts as an etch barrier in ensuing wet-removal of exposed AgNWs network. The proposed patterning technique has no limitation in the choice of substrates and pattern shape, in addition to high resolution. The patterned AgNWs network electrodes have shown excellent optical, electrical, and mechanical performances, such as high flexibility (up to ~10%) and stretchability (up to 40%). Finally, the patterned AgNWs network electrodes have been applied as a transparent heater, which can be used for rapid raindrop removal or de-icing of car windows

ACS Paragon Plus Environment

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and outside mirrors. This can be a valuable help for driving safety under harsh weather conditions.

KEYWORDS

Ag

nanowire,

Chemical

welding,

Micro

contact

prining,

Transparent

electrodes,

Flexible/Stretchable electrodes, Transparent heater.

*To whom correspondence should be addressed.


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1. INTRODUCTION Ag nanowires (AgNWs) network has been actively studied as a possible candidate for transparent electrodes, which can play the role of electrical conductor while maintaining excellent optical transparency. AgNWs network has thus been applied for various applications, such as in light emitting diode (LED),1-5 heater,6-8 field effect transistor (FET),9 pressure sensor,10 and energy harvester.11 To further enhance the electrical conductivity of AgNWs network, various approaches have been demonstrated to join individual Ag nanowire together. The joining or welding of AgNWs in a network can greatly increase the electrical conductivity by reducing contact resistance at junctions. Thermal annealing,12,13 laser or flash-light sintering,3,14-16 electron-beam exposure,17,18 have been suggested as methods for nano-joining or welding of junctions among AgNWs. These methods, however, are suffering from various shortcomings inherent to each process, such as limitation in the choice of substrates that can tolerate high temperature and the use of complex and expensive equipment. Regarding the enhancement of optical transparency, increase in opening ratio by proper patterning of the AgNWs network is essential. If one uses densely populated 3D network of AgNWs, the formed electrode will not be optically transparent unless it is patterned to increase the opening ratio. In addition, almost all practical applications using AgNWs network needs a patterned electrode, instead of blanket one. Thus there have been proposed various patterning techniques for the AgNWs network. Solution printing onto a substrate, such as ink-jet,19,20 needle,21 or e-jet,22,23 has advantages of drop-on-demand capability and arbitrariness of patterns to be generated. But these processes are basically serial and thus may take long time for the printing. The complex chemistry and fine tuning of ink formulation is another hurdle in these methods. Mask-based printings of AgNWs solution, such as printed wax or paraffin,2 stencil


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screen,1,24-27 and photoresist,28,29 and following lift-off of the mask materials with unnecessary AgNWs on them are generally multi-step processes and/or suffers from limited pattern resolution. Photolithographic patterning of the wettability of a surface30 for patterned and aligned deposition of AgNWs has been suggested, which needs complex control of surface/solution properties but with high resolution down to 8µm. It should be noted that wafer-scale high resolution patterning using photolithography and etch-back has recently been demonstrated.31 Therefore, a simple, low-cost and scalable to large area patterning method, as well as without limitation in the choice of substrates or pattern size and shape, is essential for practical application of AgNWs network. In this work, facile welding of AgNWs network by exposure to chemical vapor of H2O2 at room temperature and its patterning by a soft lithographic technique has been demonstrated. The room temperature chemical welding32 of AgNWs network enhances its electrical conductivity. Further, the connectedness of the whole network has led to enhanced flexibility by reducing stress localization upon mechanical deformation such as bending or stretching. The chemically welded network of AgNWs has been patterned by modified micro-contact printing (µCP). Contrary to conventional µCP, we have used high-viscosity siloxane polymer as an ink. Considering the rather

large

linewidths

of

patterns,

ranging

from

500µm

down

to

25µm,

the

polydimethylsiloxane (PDMS) stamp should have enough pattern height to mitigate the unwanted sagging or roof-collapse.33 For this, Si master molds that have been micro-machined by the metal-assisted chemical etching (MaCE)34,35 have been employed. The printed siloxane ink materials have been solidified by simple UV/ozone exposure for 3 min, which acts as an etch barrier in the following wet-removal of exposed portions of the AgNWs. The proposed patterning technique has no limitation in the choice of substrates and pattern shape, in addition to high resolution down to 5µm. Finally, the patterned AgNWs network electrodes array has been


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applied as a transparent heater, which can be used for rapid raindrop removal or de-icing of car windows and outside mirrors. This can be a valuable help for driving safety under harsh weather.

2. RESULTS AND DISCUSSION

Figure 1. Overall process scheme and corresponding SEM images of sample surface. Figure 1 shows the overview on the patterning of AgNWs network by µCP with corresponding scanning electron microscopy (SEM) images of sample surface. AgNWs network is prepared by the spin-coating of AgNWs solution (0.17 wt% in ethanol) on a cleaned Si substrate. The AgNWs network is then exposed to H2O2 vapor for 20 min for chemical welding by capillary condensation, which greatly enhances the electrical conductivity.32 Briefly, H2O2 vapor condenses predominantly at/around nanoscale gaps or junctions between AgNWs compared to other surfaces. The condensed H2O2 dissolves Ag at junctions (or, H2O2 is chemically reduced into H2O by taking electron from Ag, leaving Ag+ ions in water), and weld AgNWs together after complete drying (inset SEM image of the second left panel, where the overlying nanowires seem to be sunk down into bottom–lying ones). A PDMS stamp that has pattern features to be


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transferred is inked with siloxane that is prepared on a cleaned glass substrate by spin coating of diluted siloxane/methylethylketone (MEK) solution. Note here that the size of pattern features to be printed is typically in the range of few tens to few hundreds micrometers. Thus, the pattern height on the PDMS stamp should be in the similar range; otherwise, clean and well-resolved printing would not be possible due to stamp deformation such as roof-collapse.33 To meet such stringent requirement, Si master mold has been machined by metal-assisted chemical etching (MaCE).34,35 Transferred siloxane ink material onto the chemically-welded AgNWs network has been solidified by simple UV/ozone treatment. Although detailed mechanism is not known yet for the curing of siloxane by UV /ozone treatment, high energy UV irradiation combined with high chemical reactivity of ozone seem to be enough for the curing of rather thin (500nm or less) siloxane layer. It should be noted that the siloxane-covered regions appear dark (right two SEM images in the bottom panel of Figure 1) under low-magnification SEM observation, due to insulating nature of the cured siloxane. Final wet-etch removal of unprotected regions of the AgNWs network has been performed by dipping the sample into H2O2/ethanol (1:1 v/v) solution (Supporting Information, Figure S1 for ghost patterns on substrates and their elemental analysis after the wet-removal of exposed AgNWs network). Here, the H2O2 acts as the role of etchant for Ag, while ethanol is a wetting agent.32, 34-36 Etchant based on H2O2/water has not been found to be successful, due to limited wetting of etching solution onto the exposed AgNWs network between the siloxane-covered hydrophobic line patterns. Note here that the printed siloxane layer by µCP is not uniform, as shown in Figure 2a. A surface height profile, measured by surface profiler across the printed lines, is overlaid in the Figure (more details on the height profiles can be found in Figure S2). The dark stripes at both


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ends of printed lines denote rather thick (~200nm) siloxane layer, while the whitish central regions do AgNWs network that is not completely covered with printed siloxane (thickness of ~50nm). The same phenomenon has been found to occur for other substrates such as glass, polyethyleneterephthalate (PET), and PDMS. Therefore, there exist spikes at both ends of printed lines after the transfer printing of siloxane material. This non-uniformity has resulted in catastrophic failure in patterning during the following wet-etching step, as shown in Figure 2b. The AgNWs network has been attacked during the wet etching step, due to the incomplete coverage/protection. Considering the fact that the nominal diameter of AgNW used in this work to be 30nm and the thickness of the printed siloxane layer is ~50nm, the chemical attack has occurred predominantly at junctions between AgNWs, as denoted by red circles in Figure 2b.

Figure 2. SEM images of sample surface before/after the final etch-removal of unprotected regions of AgNWs network by H2O2/ethanol. (a) Patterning result without thermal reflow of printed siloxane layer, and (b) high-magnification image of the attacked AgNWs network due to incomplete coverage of siloxane layer. Red circles indicate the place where severe attack has occurred, that is, the junctions between AgNWs. (c) Patterning result with thermal reflow of


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printed siloxane layer, and (d) well-preserved AgNWs (inset: highly magnified image of the AgNWs junctions). This adverse phenomenon can easily be overcome by thermal reflow after the µCP. Upon thermal reflow at 70oC for 10min in a convection oven, the profile has been smoothened and now the siloxane layer covers the AgNWs completely, as shown in Figure 2c. The AgNWs network that is covered with siloxane layer could survive the following wet-etching step, preserving the mutually-connected network structure intact (Figure 2d). No noticeable distortion of printed line features, especially its width, has been found to occur by this thermal reflow. AgNWs network is well preserved and is found to be fully embedded into the cured siloxane layer of ~200nm (Figure S3).

Figure 3. Patterning results for various linewidths (a to d), and the change of electrical resistance and the number of junctions as a function of linewidths (e). (f), (g) Expanded view of patterned


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AgNWs networks under the cured siloxane line pattern that were prepared using 0.17wt% AgNWs solution, showing the drastic difference in the number of mutual junctions. Figures 3a to 3d show representative patterning results by the µCP of siloxane layer. As shown, lines/spaces patterns from 500µm down to 25µm could be successfully patterned by the method developed in this work. As shown in Figure S4, the method can be used to make patterns down to 5µm, and other shapes such as isolated square. Because the main purpose of the patterning of AgNWs network is to use it as conducting electrodes, the electrical property of such patterned electrode is utmost importance. Thus, we have measured the change in electrical resistance of chemically-welded and patterned AgNWs network (on glass) as a function of linewidth, as shown in Figure 3e (electrical conductance along an individual line, and electrical isolation between neighboring lines on a PDMS substrate can be found in Figure S5). Firstly, the effect of chemical welding of AgNWs network can clearly be seen: the electrical resistance of non-welded AgNWs network prepared from rather diluted AgNWs/ethanol solution of 0.17wt% (denoted as ‘T-90% non-welded’ in the figure, where the T-90% means the optical transmittance on a cleaned glass substrate; for this rather dilute solution, the transmittance remains constant at ~90% regardless of patterning) is ~5X times higher than that of chemically-welded counterpart (denoted as ‘T-90% welded’). The drastic reduction in resistance is due to the chemical welding of nanowire junctions by the capillary condensation of H2O2 vapor. Secondly, the electrical resistance remains almost constant until the linewidth is decreased down to 25µm. At the linewidth of 25µm, the resistances for both welded and non-welded samples increase abruptly. This is due to reduction in the number of junctions (counted manually with enlarged hardcopy of SEM images) contained in the patterned linewidth, which is directly related to the electrical conduction through the percolating network: >1500 for 100µm and ~600


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for 50µm linewidths, respectively, while ~200 for 25µm-wide line. The difference in the number of junctions can be judged qualitatively from SEM images shown in Figure 3f (50µm linewidth) and 3e (25µm linewidth). Therefore, there seems to be certain threshold number of junctions (somewhere between 300~500 in the present case) above which the electrical resistance remains almost the same independent of pattern width. The change in electrical property due to difference in the number percolating junctions can easily be circumvented by applying more materials.37 Indeed, the electrical resistance remains constant even at 25µm-linewidth when higher concentration of AgNWs/ethanol solution (0.5wt%) is used for the patterning, as denoted by ‘T85% welded’ in Figure 3e (the optical transmittance in this case, 85%, was measured with lines/spaces patterned samples). The number of junctions has been found to be ~600 for the 25µm-wide pattern prepared from the 0.5wt% solution. Therefore, the change of electrical resistance depending upon the pattern linewidth can easily be overcome by increasing the number of nanowires in the network, although a slight reduction in optical transmittance is inevitably accompanied.


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Figure 4. Transparent, flexible or stretchable electrodes by the patterned AgNWs network. (a) Optical transmittance of patterned AgNWs network on a glass prepared from high concentration (0.5 wt%) solution. (b) Change in the electrical resistance of patterned AgNWs network on PET under bending deformation. (c) SEM image of patterned AgNWs network on PDMS after stretching to 70%, with inset photograph showing sample during measurement. (d) Change in the electrical resistance of patterned AgNWs network on PDMS under stretching deformation.

The patterning process could have been successfully performed on other substrates, such as glass, PET and PDMS (Figure S6 for the patterning results on various substrates and the resulting line edge resolution). And the inevitable reduction in optical transparency discussed above, which may be important in some applications that require transparency as well as


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electrical conductance simultaneously, can be solved by patterning. As shown in Figure 4a, the non-patterned blanket layer from 0.5wt% AgNWs solution has optical transmittance of ~70%. The transparency can be increased to >85% by patterning into line patterns. The patterning, i.e., selective removal of AgNWs on areas that are not masked by cured siloxane, increases the opening ratio, leading to increase in optical transparency (Figure S7 for optical transmittance data of samples prepared from 0.17wt% solution, which shows T~90% independent of patterning). The chemically welded and patterned AgNWs network can be applied for flexible electrode when prepared on flexible substrates such as PET. As shown in Figure 4b, the change in electrical resistance upon bending the welded and patterned AgNWs/PET samples remains almost constant up to ~8% bending strain,32 for samples having linewidths of >50µm prepared from 0.17wt% solution (i.e., T-90%). Patterned sample having 25µm linewidth shows constant electrical resistance up to ~6% bending strain, although the absolute value of electrical resistance is much higher than those from larger pattern widths as discussed in Figure 3. On the other hand, the patterned samples prepared from 0.5wt% solution (or, T-85%) show constant resistance even at ~10% bending strain, in addition to very low value of electrical resistance compared to T-90% samples. Also, the resistance does not depend on the pattern width. The observed high flexibility, constant resistance up to 6~10% of bending strain, is due partially to the network configuration of the electrode material. The interconnected network structure of AgNWs, especially strongly held together by the chemical welding, can distribute the externally applied strain evenly throughout the whole network, instead of concentrating the strain at certain spots. Furthermore, the encapsulation of the patterned electrode lines by the cured siloxane layer prevents premature


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delamination of the AgNWs network off the substrate. Altogether, these have enabled enhanced bendability of the patterned AgNWs network. When the patterning process by the µCP is performed on a cured slab of PDMS, the sample can be mechanically stretched while maintaining the electrical conductivity and optical transparency, as shown in Figures 4c and 4d. Due to complete embedding into cured siloxane layer, the AgNWs network does not show any change in morphology such as buckling as observed in nonembedded cases32 after stretching up to 70%, as shown in Figure 4c. Upon stretching, there observed slight hysteresis in electrical resistance between the very first and later stretching cycles, as shown in Figure 4d. After the sample has been experienced the first stretching cycle, the samples have then shown negligible hysteresis, i.e., reproducible electrical behavior upon repeated stretching. For the patterned sample prepared from low solution concentration of 0.17 wt%, the resistance remains almost constant up to ~20% stretching, while that from high solution concentration of 0.5 wt% have maintained its initial resistance up to ~40% tensile strain. Higher stretchability, higher than ~40%, could be obtained if one use more concentrated solution than 0.5wt%. At the same time, the reduction in optical transparency with the use of concentrated solution can be compromised by patterning the sample with smaller linewidth.


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Figure 5. Transparent heater for the rapid removal of water drops on car window and mirror. (a) Temperature rise of the heaters as a function of time, under different bias voltage and pattern widths. (b) Cartoonist’s drawing of the situation where the driver’s sight through the car window and mirror. (c) Photograph of car window/mirror that have sprayed water drops (green circles) on them. The red circles show the clean removal of water droplets by transparent heaters attached on the window and mirror. (d) and (e) show car window/mirror before (d) and after biasing the heaters for 30sec. Both the window and mirror ensure clear vision to driver, after removal of water drops by transparent heaters. The unique combination of optical transparency and electrical conductance of the patterned AgNWs network has enabled us to implement high-performance transparent heater,6-8,30 as shown in Figure 5. The transparent heater samples have been patterned on PDMS slabs with


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different linewidths (Figure S8 for the uniformity of AgNWs network on PDMS). Upon applying rather low voltage of 3V or 4.5V, the maximum temperature of samples rapidly increases and then saturates. The saturation time has been found to be shorter for higher applied voltage, ~60sec for 3V and ~20sec for 4.5V, respectively. Also, the higher the applied voltage, the higher the saturation temperature, ~100oC for 4.5V and ~75oC for 3V, respectively, as shown in Figure 5a (Figure S9 for IR camera images). As a practical application of the fabricated heaters, Figure 5b shows the cartoonist drawing of the situation on how a driver can recognize other vehicles through car window and mirror while driving. For driving safety, the clear view of other cars or any other traffic obstacles around the car is very important. Under harsh weather conditions such as rainy or snowy days, the windows/mirrors may not be clear enough. The situation gets worse at night due to added darkness, especially at the start of driving. Rain drops, snow pileup, or frozen ice on car windows/mirrors hinder the clear sight of driver, which should be cleared off manually before driving. A transparent heater can be a very helpful aid for this. The heater can rapidly remove those hurdles and secure clear sight for the driver. Figure 5c is a photograph showing the application of transparent heaters on car window and mirror. The sprayed water drops on window/mirror can clearly be seen and denoted as green circles. By applying 4.5V to transparent heaters that were attached onto window and mirror, the water droplets were removed in ~30sec., as marked with red circles. Due to the limited size of our heater samples, more magnified photographs have been shown in Figure 5d and 5e. These photos show the car window in focus, though another heater is attached on the mirror outside the car as shown in Figure 5c. Note the photos were taken inside the car, sitting on the driver’s seat. Without the application of voltage, it is almost impossible to see through the window and mirror due to severe light scattering by water droplets, as shown in Figure 4d. Upon applying voltage to


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the heaters, as shown in Figure 5e, now the driver can secure clear sight through window and reflecting outside mirror. Similarly, snow or ice on window/mirror during winter can easily be removed by transparent heaters on surfaces in the line of driver’s sight, which may be a very convenient and helpful aid for driving safety.

3. CONCLUSION Demonstrated in this work are transparent electrodes that have excellent electrical, optical and mechanical properties, based on chemically welded and patterned AgNWs network. The chemically welded network of AgNWs has been patterned by modified µCP. Uncured, highviscosity liquid siloxane polymer has been used as an ink for the µCP. Using PDMS stamp that has been replicated from bulk micro-machined Si master mold by metal-assisted chemical etching, the printed siloxane ink materials have been cured by simple UV/ozone exposure for 3min, which acts as an etch barrier in ensuing etch-removal of exposed AgNWs network. The proposed patterning technique has no limitation in the choice of substrates and pattern shape, in addition to high resolution. The chemically-welded and patterned AgNWs network has shown excellent optical and electrical performances, such as high flexibility (up to ~10%) and stretchability (up to 40%). Finally, the patterned AgNWs network electrodes array has been applied as a transparent heater, which can be used for rapid raindrop removal or de-icing of car windows and outside mirrors. This can be a valuable help for driving safety under harsh weather conditions.

4. EXPERIMENTAL


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Preparation of chemically welded AgNWs network on various substrates. Si(100) wafer was cut into small pieces (~2cm*3cm) and cleaned by acetone, ethanol and deionized water (DI) with sonication. PET (200µm) substrate was cut and cleaned by ethanol and DI with ultrasonication. PDMS slab was prepared by mixing base resin and curing agent in 10:1 ratio (by weight; Sylgard 184, Dow) and cured at 70oC for 3hrs after de-airing, and cut into proper size. The surface of PET and cured PDMS slab were treated by UV/ozone for 30min. to facilitate the spin-coating of AgNWs/ethanol solution. Two different solutions of AgNWs (30nm and 30µm in diameter and length, respectively; Nanopyxis) in ethanol, 0.17 wt% and 0.5 wt%, were spun (3000rpm, 60sec.) on substrates mentioned above. For chemical welding, the substrate with AgNWs network was simply faced down on top of a beaker containing H2O2 (28%, Duksan) for 20min. at room temperature. The low concentration AgNWs solution (0.17wt%) has high transmittance but reduced conductivity. The transmittance does not depend upon patterning. Also, the conductivity of patterned AgNWs network from this solution varies as a function of pattern width. On the other hand, the high concentration solution (0.5wt%) has high conductivity but reduced transmittance. The transmittance can be enhanced a lot by patterning, thanks to the increase in opening ratio. Further, the conductivity of patterned AgNWs network from this solution does not change upon patterning down to 25µm of linewidth. Overall, the two different solutions (0.17wt.% vs. 0.5wt.%) highlight the importance of patterning (in terms of transmittance) and pattern width dependent electrical conductivity. This is the reason that we have used these two different solution concentrations in the present work. Metal-assisted chemical etching of Si. Si(100) wafer was cut into small pieces of 3cm*4cm and was cleaned by piranha solution (H2SO4:H2O2 = 3:1) for 15 min. Photoresist (AZ-5214E)


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was spin-coated on the cleaned Si substrate (3000 rpm, 50 sec) and baked at 110 °C for 3 min. The sample was then exposed by contact aligner (MJB-6, Karl Suss) and developed by AZ300MIF. The photomask has various lines having different linewidths, ranging from 5µm up to 500µm. De-scum was performed to completely remove any residual photoresist by oxygen reactive ion etching (100 W, 30 sec). Then, Ag(3nm)/Au(6nm) was deposited on the photoresistpatterned Si by e-beam evaporation, followed by lift-off in acetone and ethanol with sonication sequentially. The MaCE was carried out in etching solution of H2O2 (0.46 M), HF (5.28 M), DI and ethanol (7:2:12:19 in volume). After the MaCE process, the Ag/Au metal bilayer was removed by Au etchant (GOLD ETCH-Type TFA, Transcene). The etched Si was used as a master mold for the replication of PDMS stamp. The edge roughness of etched structure onto Si by the MaCE depends on the pattern size: the larger the pattern size, the larger the edge roughness (Figure S10). µCP of the chemically welded AgNWs network. As an ink, the base resin of Sylgard 184 kit was used, thanks to its complete wettability on the patterned PDMS stamp as well as easy availability. Other dissolvable polymers such as polystyrene (PS) or polymethylmethacrylate (PMMA) have not been successful for the contact-printing due to dewetting on the PDMS stamp. After diluting the highly viscous base resin in MEK to 30wt%, the solution was spun on a cleaned glass substrate at 2000 rpm for 30 sec. Lines/spaces patterned PDMS stamp was then made conformal contact onto the siloxane/glass substrate. The siloxane-inked PDMS stamp was contact-printed onto the chemically welded AgNWs network on substrates. To smoothen the printed siloxane layer, thermal reflow has been performed at 70oC for 10min. in a convection oven. Finally, the printed siloxane layer has been cured by exposure to UV/ozone for 3 min. The


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un-protected areas of AgNWs network has been etched away by dipping the sample into H2O2/ethanol (1:1 v/v) solution for 3 min and rinsed with pure ethanol. Transparent heater device. Chemically welded AgNWs network have been prepared on a cured PDMS slab. Then, Ti/Au(5nm/80nm) was shadow evaporated, as electrodes, on both ends of patterned lines by e-beam evaporation with spacing of 1cm. The network was patterned into 250µm lines/spaces by the µCP process described above. Therefore the transparent heater is basically stretchable, and thus conformable onto non-flat surfaces. Although the car window is curved, the small sample size of our stretchable heater does not allow showing its conformability. Characterization. Scanning electron microscope (JSM6700F, Jeol) were used for imaging the sample at various stages of the process. Optical transmittance was measured by spectrophotometer (V-630, Jasco). The electrical resistance of patterned AgNWs network (~1 cm long lines with varying linewidths) was measured by semiconductor parameter analyzer (HP4156A). Because the patterned AgNWs network gets fully embedded into the cured siloxane layer, the electrical contact for probe was deposited (Ti(5nm)/Au(100nm)) at both ends of the sample before the patterning. As shown in our previous work,32 the resistance by simple 2-points I-V measurements does not differ significantly from the sheet resistance measured by 4 points probe (4PP) technique. Further, we are interested in the relative change in electrical resistance, depending on pattern size or upon mechanical deformation such as bending or stretching, which makes the simple 2 points I-V measurements good enough for this purpose. For bending test, AgNWs were spun on flexible PET substrate. For a good electrical contact with probe tips, patterned electrodes of Ti(10nm)/Au(100nm) at both ends of sample were pre-


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deposited on sample surface by e-beam evaporation using shadow mask, before the spin of AgNWs solution. After treating the sample with hydrogen peroxide vapor for chemical welding and patterning by the µCP of siloxane, the bending tests were carried out on a home-made stage with 2-points I-V measurements. The electrodes were in 3mm*20mm (L*W) in size and 10mm spaced apart. For the fabrication of stretchable AgNWs network, the AgNWs layer was spun on UV/ozone treated cured slab of PDMS, which has pre-deposited electrodes at both ends. After the roomtemperature chemical welding and patterning, the sample was loaded onto home-built stretching stage. For the 2-points I-V measurements under stretching, eutectic GaIn (Aldrich) droplets were applied onto the pre-fabricated electrode surfaces (refer to Figure 3c). Elemental analysis was performed by an energy dispersive X-ray spectroscopy (EDX) that is integrated into the SEM equipment. Heater temperature was measured by infra-red camera (R300W2 R17, Nec-Avio). Thickness of printed and cured siloxane layer was measured by surface profiler (Dektak XT Stylus Profiler, Bruker) and cross-sectional SEM.

ASSOCIATED CONTENT Supporting Information. SEM images and EDX elemental analysis of sample surface after etch-removal of AgNWS network. Height profile by surface profiler. AgNWs network embedded into cured siloxanes. High-resolution patterning and patterning of arbitrary shape. Electrical resistance as a function of the number of patterned lines, and complete electrical isolation between neighboring patterned


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lines. Zoomed SEM images of patterned line edges. Optical transmittance of sample prepared from 0.17wt% solution. AgNWs uniformity on a PDMS substrate. Temperature measurements of heater sample and water drop removal on a mirror. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *D.-Y. Khang. E-Mail: [email protected] Present Addresses Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors clearly declare no conflict of interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation Grant funded by the Korean Government (MEST) (NRF2010-C1AAA001-0029061).


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REFERENCES (1) Liu, G. –S.; Liu, C.; Chen, H. –J.; Cao, W.; Qiu, J. –S.; D. Shieh, H. –P.; Yang, B. –R. Electrically Robust Silver Nanowire Patterns Transferrable onto Various Substrates. Nanoscale 2016, 8, 5507-5515. (2) Tybrandt, K.; Voros, J. Fast and Efficient Fabrication of Intrinsically Stretchable Multilayer Circuit Boards by Wax Pattern Assisted Filtration. Small 2016, 12, 180-184. (3) Woo, J. Y.; Kim, K. K.; Lee, J.; Kim, J. T.; Han, C. –S. Highly Conductive and Stretchable Ag Nanowire/Carbon Nanotube Hybrid Conductors. Nanotechnology 2014, 25, 285203. (4) Liang, J.; Li, L.; Tong, K.; Ren, Z.; Hu, W.; Niu, X.; Chen, Y.; Pei, Q. Silver Nanowire Percolation Network Soldered with Graphene Oxide at Room Temperature and Its Application for Fully Stretchable Polymer Light Emitting Diodes. ACS Nano 2014, 8, 1590-1600. (5) Kim, J. –H.; Park, J. –W. Foldable Transparent Substrates with Embedded Electrodes for Flexible Electronics. ACS Appl. Mater. Interfaces 2015, 7, 18574-18580. (6) Choi, S.; Park, J.; Hyun, W.; Kim, J.; Kim, J.; Lee, Y. B.; Song, C.; Hwang, H. J.; Kim, J. H.; Hyeon, T.; Kim, D. –H. Stretchable Heater Using Ligand Exchanged Silver Nanowire Nanocomposite for Wearable Articular Thermotherapy. ACS Nano 2015, 9, 6626-6633. (7) Kim, T.; Kim, Y. W.; Lee, H. S.; Kim, H.; Yang, W. S.; Suh, K. S. Uniformly Interconnected Silver-Nanowire Networks for Transparent Film Heaters. Adv. Funct. Mater. 2013, 23, 1250-1255. (8) Hong, S.; Lee, H.; Lee, J.; Kwon, J.; Han, S.; Suh, Y. D.; Cho, H.; Shin, J.; Yeo, J.; Ko, S. H. Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications. Adv. Mater. 2015, 27, 4744-4751.


22

Page 23 of 27

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


(9) Liang, J.; Li, L.; Chen, D.; Hajagos, T.; Ren, Z.; Chou, S. –Y.; Hu, W.; Pei, Q. Intrinsically Stretchable and Transparent Thin-Film Transistors Based on Printable Silver Nanowires, Carbon Nanotubes and an Elastomeric Dielectric. Nat. Commun. 2015, 6, 8647-8656. (10) Wang, J.; Jiu, J.; Nogi, M.; Sugahara, T.; Nagao. S.; Koga. H.; He, P.; Suganuma, K. A Highly Sensitive and Flexible Pressure Sensor with Electrodes and Elastomeric Interlayer Containing Silver Nanowires. Nanoscale 2015, 7, 2926-2932. (11) Jeong, C. K.; Lee, J.; Han, S.; Ryu, J.; Hwang, G. –T.; Park, D. Y.; Park, J. H.; Lee, S. S.; Byun, M.; Ko, S. H.; Lee, K. J. A Hyper-Stretchable Elastic-Composite Energy Harvester. Adv. Mater. 2015, 27, 2866-2875. (12) Langley, D. P.; Lagrange, M.; Giusti, G.; Jimenez, C.; Brechet, Y.; Nguyen, N. D.; Bellet, D. Metallic Nanowire Networks: Effects of Thermal Annealing on Electrical Resistance. Nanoscale 2014, 6, 13535-13543. (13) Hwang B.; Shin, H. –A. –S.; Kim, T.; Joo, Y. –C.; Han, S. M. Highly Reliable Ag Nanowire Flexible Transparent Electrode with Mechanically Welded Junctions. Small 2014, 10, 3397-3404. (14) Nian, Q.; Saei, M.; Xu, Y.; Sebyasachi, G.; Deng, B.; Chen, Y. P.; Cheng, G. J. Crystalline Nanojoining Silver Nanowire Percolated Networks on Flexible Substrate. ACS Nano 2015, 9, 10018-10031. (15) Lee, J.; Lee, P.; Lee, H.; Lee, D.; Lee, S. S.; Ko, S. H. Very Long Ag Nanowire Synthesis and Its Application in a Highly Transparent Conductive and Flexible Metal Electrode Touch Panel. Nanoscale 2012, 4, 6408-6414.


23


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Page 24 of 27

(16) Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Self-Limited Plasmonic Welding of Silver Nanowire Junctions. Nat. Mater. 2012, 11, 241-249. (17) Hong, C. –H.; Oh, S. K.; Kim, T. K.; Cha, Y. –J.; Kwak, J. S.; Shin, J. –H.; Ju, B. –K.; Cheong, W. –S. Electron Beam Irradiated Silver Nanowires for a Highly Transparent Heater. Sci. Rep. 2015, 5, 17716-17723. (18) Xu, S.; Tian, M.; Wang, J.; Wu, J.; Redwing, J. M.; Chan, M. H. W. Nanometer-Scale Modification and Welding of Silicon and Metallic Nanowires with a High-Intensity Electron Beam. Small 2005, 1, 1221-1229. (19) Chen, S. –P.; Retamal, J. R. D.; Lien, D. –H.; He, J. –H.; Liao, Y. –C. Inkjet-Printed Transparent Nanowire Thin Film Features for UV Photodetectors. RSC Adv. 2015, 5, 70707-70712. (20) Komoda, N.; Nogi, M.; Suganuma, K.; Otsuka, K. Highly Sensitive Antenna Using Inkjet Overprinting with Particle-Free Conductive Inks. ACS Appl. Mater. Interfaces 2012, 4, 5732-5736. (21) Li, R.–Z.; Hu, A.; Zhang, T.; Oakes, K. D. Direct Writing on Paper of Foldable Capacitive Touch Pads with Silver Nanowire Inks. ACS Appl. Mater. Interfaces 2014, 6, 21721-21729. (22) Lee, H.; Seong, B.; Kim, J.; Jang, Y.; Byun, D. Direct Alignment and Patterning of Silver Nanowires by Electrohydrodynamic Jet Printing. Small 2014, 10, 3918-3922. (23) Park, S.–E.; Kim, S.; Lee, D.–Y.; Kim, E.; Hwang, J. Fabrication of Silver Nanowire Transparent Electrodes Using Electrohydrodynamic Spray Deposition for Flexible Organic Solar Cells. J. Mater. Chem. A 2013, 1, 14286-14293.


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Page 25 of 27

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


(24) Chou, N.; Kim, Y.; Kim, S. A Method to Pattern Silver Nanowires Directly on WaferScale PDMS Substrate and Its Applications. ACS Appl. Mater. Interfaces 2016, 8, 62696276. (25) Hu, W.; Niu, X.; Zhao, R.; Pei, Q. Elastomeric Transparent Capacitive Sensors Based on an Interpenetrating Composite of Silver Nanowires and Polyurethane. Appl, Phys. Lett. 2013, 102, 083303 (26) Ahn, T.; Kim, H. –J.; Lee, J.; Choi, D. –G.; Jung, J. –Y.; Choi, J. –H.; Jeon, S.; Kim, J. – D.; Jeong, J. –H. A Facile Patterning of Silver Nanowires Using a Magnetic Printing Method. Nanotechnology 2015, 26, 345301-345306. (27) Song, C. –H.; Han, C. J.; Ju, B. –K.; Kim, J. –W. Photoenhanced Patterning of Metal Nanowire Networks for Fabrication of Ultraflexible Transparent Devices. ACS Appl. Mater. Interfaces 2016, 8, 480-489. (28) Kim, M.-S.; Lee, D.–H.; Kim, K.–B.; Jung, S.–H.; Lee, J.–K.; O, B.–H.; Lee, S.–G.; Park, S.-G. Lift-Off Patterning of Ag Nanowire/PEDOT:PSS Composite Films for Transparent Electrodes Using a Fluoropolymer Structure. Thin Solid Films 2015, 587, 100-105. (29) Kim, S.; Kim, S. Y.; Kim, J.; Kim, J. H. Highly Reliable AgNW/PEDOT:PSS Hybrid Films: Efficient Methods for Enhancing Transparency and Lowering Resistance and Haziness. J. Mater. Chem. C 2014, 2, 5636-5643. (30) Yang, B.-R.; Cao, W.; Liu, G.-S.; Chen, H.-J.; Noh, Y.-Y.; Minari, T.; Hsiao, H.-C.; Lee, C.-Y.; Shieh, H.-P. D.; Liu, C. Microchannel Wetting for Controllable Patterning and Alignment of Silver Nanowire with High Resolution. ACS Appl. Mater. Interfaces 2015, 7, 21433-21441. (31) Martinez, V.; Stauffer, F.; Adagunodo, M.O.; Forro, C.; Voros, J.; Larmagnac, A.


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Page 26 of 27

Stretchable Silver Nanowire−Elastomer Composite Microelectrodes with Tailored Electrical Properties. ACS Appl. Mater. Interfaces 2015, 7, 13467-13475. (32) Yoon, S.–S.; Khang, D.–Y. Room-Temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation. Nano Lett. 2016, 16, 3550-3556. (33) Delamarche, E.; Schmid, H.; Michel, B.; Birbuyck, H. Stability of Molded Polydimethylsiloxane Microstructures. Adv. Mater. 1997, 9, 741-746. (34) Yoon, S. –S.; Lee, Y. B.; Khang, D. –Y. Etchant Wettability in Bulk Micromachining of Si by Metal-Assisted Chemical Etching. Appl. Surf. Sci. 2016, 370, 117-125. (35) Kim, S. –M.; Khang, D. –Y. Bulk Micromachining of Si by Metal-assisted Chemical Etching. Small 2014, 10, 3761-3766. (36) Sigg, L.; Lindauer, U. Silver Nanoparticle Dissolution in the Presence of Ligands and of Hydrogen Peroxide. Environ. Pollut. 2015, 206, 582-587 (37) Du, F.; Fischer, J. E.; Winey, K. I. Effect of Nanotube Alignment on Percolation Conductivity in Carbon Nanotube/Polymer Composites. Phys. Rev. B 2005, 72, 121404.


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