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Jan 16, 2017 - Department of Materials Sciences and Engineering, California NanoSystems Institute, Henry Samuli School of Engineering and. Applied ...

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Removable Large-Area Ultrasmooth Silver Nanowire Transparent Composite Electrode Yunxia Jin,†,‡ Kaiqing Wang,† Yuanrong Cheng,† Qibing Pei,§ Yuxi Xu,*,‡ and Fei Xiao*,† †

Department of Materials Science, ‡Department of Macromolecular Science, Fudan University, 220 Handan Road, Shanghai 200433, People’s Republic of China § Department of Materials Sciences and Engineering, California NanoSystems Institute, Henry Samuli School of Engineering and Applied Science, University of California, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: In this work, a composite silver nanowire (AgNW) transparent electrode that is large-area ultrasmooth without conductivity or transmittance scarifice, removable but with good resistance to both water and organic solvent, is reported. Via a simple low-temperature solution process without complicated transfer steps or additional pressure pressing, a new kind of AgNWs composite with biocompatible and patternable chitosan polymer complex demonstrates a quite low root-mean-square roughness ∼7 nm at a largest reported scan size of 50 μm × 50 μm, which is among the best flat surface. After long-term exposure to both water and organic solvent, it still shows strong adhesion, unchanged transparency, and no obvious conductivity reduction, suggesting a good stability staying on the substrate. Meanwhile, the polymer and silver nanowire in the composite electrode can be damaged via the same process through concentrated acid or base etching to leave off the substrate, allowing a simple patterning technology. Besides, the imported insulating polymer does not lower down the opto-electrical performance, and a high figure of merit close to 300 is obtained for the composite electrode, significantly outperforming the optoelectronic performance of indium-tin oxide (ITO) coated plastics (∼100) and comparable to ITO-coated glass. It shows great advantage to replace ITO as a promising transparent electrode. KEYWORDS: Silver nanowire, chitosan, transparent composite electrode, surface roughness, stability, sensor

1. INTRODUCTION Transparent electrode concurrently possessing high conductivity, high optical transparency, good stability, and strong adhesion to substrate is a key challenge to materials science. The typical transparent electrodes used in touch screens, displays, light-emitting diodes, solar cells, and so forth are made from doped metal oxides, especially indium tin oxide (ITO).1,2 Although on glass ITO has an excellent conductivity, on plastic the electrical performance is significantly reduced because of the lower processing temperature due to the heat-sensitive substrate. Because of the waste of ITO untargeted and relatively low sputtering speed, especially for thick ITO layer, the traditional ITO sputtering manufacturing is high in cost. Regarding incorporation to flexible electronics the brittle nature of ITO makes it a great challenge.3 Because of these inherent drawbacks, alternative materials replacing ITO are highly expected. Silver nanowire (AgNW) networks exhibit the desired electrical, optical, and mechanical performances, and additionally easy deposition on various types of substrates via a low-cost, high-speed, and large-scale roll-to-roll solutionprocess, allowing it to be a promising candidate.4−15 Despite the significant benefits of AgNW networks, there are still limitations. The rough surface topology results in nonuniform top coating layer leading to shorts and device failure. Several strategies have been reported to address the © 2017 American Chemical Society

problems. Adoption of inorganic nanoparticles (NPs), such as TiO2, SiO2, ZnO, and so forth, flattened the surface of AgNW networks via filling the voids among networks, but no specific root-mean-square (RMS) roughness has been released and transparency of AgNW networks reduced with NPs addition.16−18 AgNWs composite with polymer addition generally enables improved surface roughness. For example, polymer encapsulation involving the cured polymer peeling off from AgNWs coated glass to transfer AgNW networks to polymer surface allows low surface roughness and good adhesion simultaneously.19−27 But only the polymer sticking to metal well but releasing from glass easily can survive as a substrate. However, polymer overcoating provides a quite simple onestep solution-process and versatile option of substrate including rigid or flexible, inorganic or organic, commercial or cured without complicated AgNWs transfer. Three main kinds of polymers have been introduced. First, poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS), which is among the most popular overcoating polymer,28−30 improved the surface roughness and conductivity as well due to its electrical nature and enhanced crossed Ag nanowires. Received: November 22, 2016 Accepted: January 16, 2017 Published: January 16, 2017 4733

DOI: 10.1021/acsami.6b15025 ACS Appl. Mater. Interfaces 2017, 9, 4733−4741

Research Article

ACS Applied Materials & Interfaces

Figure 1. AFM height images, section profiles, and corresponding 3D images of pristine AgNW (a,d,g), AgNW/1% Chi (b,e,h) and AgNW/1.5% Chi (c,f,i). (j,k) SEM images (top) and AFM height images (bottom) of AgNW/1%Chi and AgNW/1.5%Chi with partial cutoff. (l) Step profiles obtained from AFM height images in panels j and k.

Unfortunately, the surface roughness of AgNW/PEDOT−PSS is still pretty high, ranging from 19.6 to 51.8 nm reported, and PEDOT−PSS is highly water-soluble demonstrating weak resistance to solvent exposure and failure of thorough wash before device fabrication.28,31,32 Second, photoresist coated on AgNW networks can form a quite flat composite surface, but result in serious transmittance reduction due to the light absorption nature of photoresist. Third, traditional insulating polymer including epoxy, Teflon, and polyether sulfone7,33,34 help improved the surface roughness or adhesion of AgNWs networks in some cases but usually suffered from unsuccessful simultaneous improvement of conductivity and surface roughness. Specifically, the thicker polymer layer benefits lower surface roughness but when it comes to >150 nm thick the negative influence to conductivity of AgNW networks is considerable. However, most monomer or polymer has a high enough viscosity to form a relatively thick coating to significantly lower down the conductivity, whereas the diluted monomer or polymer usually causes a poor nanofilm with limited improved surface roughness, and the solvent usually affects monomer curing to lower the mechanical performance. Besides, the common problem in almost all the reported insulating polymers is that they are usually hard to be removed from

substrate via a fast and productive way, failing or complicating the further processing of AgNW composite. Moreover, to be applicable the polymer/AgNWs is desired to be solutionprocessed but considering stability and washing requirement the composite electrode is expected to resist water and organic solvent exposure. It is paradox for the build-in of polymer/ AgNWs composite. Therefore, developing a smooth AgNW/ polymer composite with RMS below 10 nm via a simple process without bringing new problems or dropping any other performance is still in great challenge. In this work, we report a new kind of AgNWs composite transparent electrode with biocompatible chitosan (Chi) complex as overcoating layer via a quite simple annealing-free one-step solution process, which can enable a quite flat surface with enhanced conductivity and transmittance, and good resistance to inorganic and organic solvent with easy ablation. Chitosan is a nature biopolymer obtained from deacetylation of chitin abundant in invertebrates and lower forms of plant life. Thanks to its good film formability in nanoscale even at a low concentration of ∼0.5 wt %, resultant AgNWs composite achieved a quite low RMS roughness below 10 nm at large scan size of 50 μm × 50 μm with strong adhesion to substrate and comparable opto-electrical performance with ITO coated glass. Unlike tranditional polymer, the AgNWs/Chi composite can 4734

DOI: 10.1021/acsami.6b15025 ACS Appl. Mater. Interfaces 2017, 9, 4733−4741

Research Article

ACS Applied Materials & Interfaces

Figure 2. AFM height and 3D images of AgNW/Chi on glass (a) and PEN (b) with scan size of 50 μm × 50 μm.

nanowires to each other and to the substrate, which benefited the surface roughness improvement and conductivity of AgNW networks as well. This may be due to the strong binding force of chitosan to substrate. Besides, the increased chitosan layer from 45.3 to 125.6 nm kept reducing AgNW rising to 8 nm, indicating the chitosan layer on the Ag nanowire top was thinner than that deposited on the substrate among Ag nanowires voids. To provide more insight of the uniformity of AgNWs composite, we have scanned the surface at a largest area reported, 50 μm × 50 μm, for AFM measurement. As shown in Figure 2, roughness of AgNW networks on two kinds of substrates, rigid glass and flexible PEN was studied. The RMS of AgNW/Chi/glass at 50 μm × 50 μm scan size was 6.9 nm, and it was only slightly increased to 7.6 nm using PEN as the substrate instead, which was as small as that at 10 μm × 10 μm scan size in Figure 1 although scan area has been extended 5 times. Moreover, we cut a 4 cm2 sample to four pieces and tested each piece. There was no obvious difference in surface roughness among these pieces. It suggests a uniformly smooth surface of AgNW networks and an effective decoration of the thin chitosan layer. The surface roughness of our AgNW composite represents a significant improvement in contrast to that of the most reported polymer-overcoated AgNW networks, even with additional pressure, whose surface roughness was almost at dozens of nanometers and was comparable to or better than that of AgNW embedded in the polymer surface as well as just a little bit larger than that of the ITO surface of ∼3 nm (Figure S1).25,26,28,31,35,36 As depicted in Figure S2, the substrate glass has a RMS of 1 nm, whereas the thin chitosan layer on the glass slide has a half RMS of 0.5 nm with average chitosan rising to ∼1 nm, demonstrating a pretty good film formability of chitosan in nanoscale thick. Thus, it can be concluded that the reduced surface roughness of AgNWs

resist both water and organic solvent exposure without obvious transmittance or conductivity change. Meanwhile, chitosan can be removed via concentrated base or acid, an identical process as Ag etching, allowing feasible further processing. It is also noted that as a highly biocompatible polymer, chitosan composite with AgNWs is possible interest for wearable and smart electronics and biosensors.

2. RESULTS AND DISCUSSION The morphologies of pristine AgNWs networks and its composites were studied via atomic force microscopy (AFM). As shown in Figure 1a−f, the tapping mode AFM images revealed considerable change of morphologies between pristine and composite AgNW film with different thick chitosan layer. The RMS roughness of pristine AgNW networks was 23.4 nm with average Ag nanowires rising ∼75 nm above substrate obtained from multiple section scan profiles (Figure 1a). However, the RMS of AgNW/1% Chi composite was significantly reduced to 6.3 nm, one-fourth of that of pristine AgNW networks, and Ag nanowire rising sharply dropped to ∼21 nm (Figure 1b), demonstrating a much flatter surface, which can be further smoothed to 3.6 nm of RMS and 8 nm of rising up as thicker chitosan layer coating (Figure 1c). The 3D AFM images (Figure 1g−i) corresponding to the height images further visually confirmed the reduced Ag nanowire rising and surface roughness of AgNWs composite. The thickness of spincoated chitosan layer with 1 wt % of chitosan solution at 3000 rpm was about 45.3 nm and 1.5 wt % at 2000 rpm was 125.6 nm (Figure 1j−l). Therefore, the average Ag nanowire rising of AgNW/Chi is supposed to be larger than 29.7 nm (75−45.3 nm) if the Ag−Ag junctions of AgNWs composite stay identically loose as pristine AgNWs. Actually, the Ag nanowire rising of AgNW/Chi (∼21 nm) was much lower than the supposed one, suggesting more tightened stack of crossed Ag 4735

DOI: 10.1021/acsami.6b15025 ACS Appl. Mater. Interfaces 2017, 9, 4733−4741

Research Article

ACS Applied Materials & Interfaces

Figure 3. UV−vis spectra (a) and haze (b) of pristine AgNW and AgNW/Chi composite film spin-coated by 1 wt % of chitosan solution at 3000 rpm, and 1.5 wt % at 2000 rpm, respectively. (c,d) A 30° tilt view of SEM images of pristine AgNW networks and AgNW/1% Chi composite film. Scale bar is 100 nm. (e) Sheet resistance changes of pristine AgNW networks with varied starting sheet resistance after chitosan coating. (f) Comparison of the opto-electrical performance of AgNW/Chi electrode with reported various kinds of TCEs in literature (all transmittance are exclusive of substrate).

composite is highly related to the excellent film formability of chitosan solution especially at low concentration and the tighter stack of Ag nanowires to lower down the top-to-bottom height. The aforementioned opto-electrical performance is one of the most important requirements for a transparent conductor. Unlike the reported polymer in other AgNWs composite, such as PEDOT−PSS, photoresist, and so forth,26,30 our chitosan layer in nanometer thickness shows almost no absorption over the entire visible light region (Figure S3), facilitating the fabrication of highly transparent AgNWs composite. Interestingly, the optical transmittances of AgNW networks after chitosan coating were all improved at the light region >550 nm instead of the regular reduction. As shown in Figure 3a, average transmittance of AgNWs composite over 550−850 nm was increased by 2.4% as 1 wt % of chitosan solution spin-coated on the pristine AgNW networks, and it was more obviously increased by 3.6% (from 89.4 to 93.0%) as 1.5 wt % of chitosan used. The AgNW composites also exhibited reduced haze that was calculated from diffusion transmittance divided by total

transmittance. As shown in Figure 3b, the average haze over 550−850 nm of AgNW networks was reduced from 1.67 and 2.43 to 1.47 and 1.91 for AgNW/1%Chi and AgNW/1.5%Chi, respectively. In general, the haze value is related to the light scattering of Ag nanowires that directly impacts the optical performance. Light scattering can occur inside AgNW networks when the site with comparable size to incident light wavelength along with refractive index mismatch. In this case, the size, especially the nanoscaled diameter of AgNW, subwavelengthdimensioned gap among AgNW networks, and nanostructured air-surface of AgNW networks, all affect the haze, and narrow Ag nanowires and low surface roughness are preferred. As determined in Figure 1, the surface roughness of AgNW composite was significantly reduced as chitosan overcoating layer thickness, which is in accordance with the change of haze as a function of chitosan thickness. It indicates the overcoating single thin layer of chitosan enhanced the optical performance of AgNW networks resulting from the improved surface roughness.28 4736

DOI: 10.1021/acsami.6b15025 ACS Appl. Mater. Interfaces 2017, 9, 4733−4741

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a,b) Sheet resistance and transmittance at 550 nm of AgNW/Chi as a function of tape test cycle. (c,d) Resistances of pristine AgNW and AgNW/Chi composite film annealing at 180 °C and upon temperature increasing on hot plate. Insets in c are the SEM images corresponding to AgNW and AgNW/Chi after annealing. (e) Sheet resistance changes of AgNW/Chi composite film immersed in water, acetone, and IPA followed by tape test. (f) Resistances of AgNW/Chi to ammonia−water and HCl solution.

It is also noticed that a featured deep and broad dip in transmittance around 347 nm and an additional weak dip near 372 nm were observed for pristine AgNW networks. These dips are caused by the excitation of localized surface plasmon resonance (LSPR) attributing to the oscillations of free electrons in the transverse direction to the individual Ag nanowire as the frequency of incident photons matches the nature frequency of surface electrons oscillating. As we know, the extremely intense localized electromagnetic fields induced by LSPR make NWs highly sensitive to the small change in the local refractive index (RI). Thus, organic molecules with a relatively higher RI compared to air, binding to NWs, result in LSPR redshift.37,38 As shown in Figure 3a, the main resonance is red-shifted from 347 to 378.5 nm when a relatively thick chitosan layer was coated, indicating a strong binding of chitosan to NW surface, which benefits the adhesion and conductivity enhancement of AgNW networks. The enhanced intensity of the red-shifted dip is due to the enhanced extinction cross-section of NWs that are related to the dielectric constant of surrounding medium according to Mie’s solution of Maxwell’s equation.38

Because AgNW/1%Chi is much thinner than AgNW/1.5% Chi but with significant improved surface roughness, further studies were carried out on AgNW/1%Chi. Ag nanowires after chitosan coating not only became tightened, which was revealed from the above AFM and haze results, but also deformed to each other at the crossing junctions, especially Ag nanowires near black dashed line shown in Figure 3c,d, resulting in larger conductive area contact benefiting conductivity enhancement. As depicted in Figure 3e, the sheet resistance of AgNWs composite exhibited a reduction by ∼5% compared to that of pristine AgNW networks. Figure 3f compares the opto-electrical performance of our AgNW/Chi composite electrodes with featured transparent electrodes reported in literature and commercial ITO. It is shown that our AgNW/Chi composite electrodes prepared with a simple solution process exhibit a comparable electrical performance with commercial ITO-coated glass, which is significantly higher than ITO-coated PET and also superior or comparable to most reported AgNWs composite transparent electrode with graphene, GO, conducting polymer PEDOT−PSS, photoresist, poly(ionic) gel, liquid crystal, and so forth.21,26,39−44 It is also 4737

DOI: 10.1021/acsami.6b15025 ACS Appl. Mater. Interfaces 2017, 9, 4733−4741

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) Morphology changes of bare AgNWs/Chi and AgNW/Chi with PI mask during 36 wt % of HCl solution etching at 5 and 25 min. (b) Schematic diagram of touch sensor. The black square is silver paint. (c) Change in capacitance of one pixel during repeated finger touching.

much better than our previously reported AgNW/Aa-PDA and AgNW/CaAlg composites, attributing to the improved AgNWs deposition technology to obtain better pristine AgNW and lowered surface roughness to make higher transmittance. The characteristic sheet resistance of AgNW/Chi is 11.4 Ohm sq−1 for transmittance of 90.0%. The incorporation of chitosan layer into AgNW networks enhanced the optical transmittance and conductivity concurrently and confirmed the effectiveness of our strategy in improving the performance of AgNWs electrode. Presently, the thorough research of adhesion and stability of AgNW composite electrode is still limited. Here Figure 4 studies the resistance of AgNW/Chi to tape test, thermal annealing, and chemical exposure. The sheet resistance of AgNW/Chi almost maintained its initial value and only increased by 3.4% even after 100 times of tape test in which new 3M tape was used for every single tape test, suggesting a quite strong adhesion of AgNW networks to substrate (Figure 4a,b). In contrast, the pristine AgNW networks failed fast, showing about 10 times the original sheet resistance only after 3 cycles of tape tests. Figure 4c,d displays the different thermal stability of pristine AgNW and AgNW/Chi. During 180 °C aging on hot plate in air, AgNWs on glass showed great resistance increase to 1625 Ohm in 550 min. In stark contrast, AgNW/Chi on glass almost retained its original resistance, suggesting the great help of overcoating chitosan layer to the thermal stability improvement of AgNW networks. Insets in Figure 4c are SEM images of AgNW and AgNW/Chi after high-temperature aging. A clear disconnection of most Ag−Ag junctions can be observed in pristine AgNW, and some nanowires became droplets due to contact ripening and Rayleigh instability. But AgNW/Chi composite maintained its conductive path and showed no obvious morphology change accounting for the unchanged resistance. The pristine AgNW and AgNW/Chi were further

treated at upward temperature and kept for 1 min at every single temperature point. As shown in Figure 4d, the pristine AgNW networks tripled the resistance at 210 °C and almost failed at 240 °C. In contrast, the resistance of AgNW/Chi started to increase until 290 °C and spiked to thousands up to 380 °C. Interestingly, although chitosan started to decompose upon 200 °C it still enhanced the thermal stability of AgNW networks before complete decomposition. It means actually only an extremely thin layer of chitosan works well to prevent thermal attacks. It is noted that AgNWs composite remained transparent during high-termperature heating but became yellowish upon chitosan decomposition and then darkened along with more chitosan decomposition. The chemical stability of AgNW/Chi was examined through a solvent shock followed by tape test and a corrosion test. Although lots of polymers can be served as a protecting layer for AgNWs, many of them may dissolve or become opaque in organic or inorganic solvent, or some may be quite stable, but is hard to be removed for patterning or dissolved for solution process. In this case, deionized water, acetone, isopropanol (IPA), ammonia, and diluted acid were applied to AgNW/Chi composite. The AgNW/Chi composites were soaked in water, acetone, and IPA for 30 h. High chemical stability of AgNW composite was observed as shown in Figure 4e. The sheet resistance only increased by 5−6% in water and acetone, 23% in IPA,

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