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Removable large-area ultra-smooth silver nanowire transparent composite electrode Yunxia Jin, Kaiqing Wang, Yuanrong Cheng, Qibing Pei, Yuxi Xu, and Fei Xiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15025 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017
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Removable large-area ultra-smooth silver nanowire transparent composite electrode Yunxia Jin, Kaiqing Wang, Yuanrong Cheng, Qibing Pei, Yuxi Xu*, Fei Xiao* Dr. Y. Jin, K. Wang, Prof. Y. Cheng, Prof. F. Xiao Department of Materials Science, Fudan University 220 Handan Road, Shanghai 200433, P. R. China E-mail:
[email protected] Dr. Y. Jin, Prof. Y. Xu Department of Macromolecular Science, Fudan University 220 Handan Road, Shanghai 200433, P. R. China E-mail:
[email protected] Prof. Q. Pei Department of Materials Sciences and Engineering, California NanoSystems Institute Henry Samuli School of Engineering and Applied Science, University of California Los Angeles, CA 90095, USA Keywords: silver nanowire, chitosan, transparent composite electrode, surface roughness, stability, sensor Abstract In this work, a composite silver nanowire (AgNW) transparent electrode that is largearea ultra-smooth 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 RMS (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 doesn’t lower down 1 ACS Paragon Plus Environment
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the opto-electrical performance, and a high figure of merit close to 300 is obtained for the composite electrode, significantly outperforming the opto-electronic performance of indiumtin oxide (ITO) coated plastics (~100), and comparable to ITO-coated glass. It shows great advantage to replace ITO as a promising transparent electrode. 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 etc. 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
significantly reduced because of the lower processing temperature due to the heat-sensitive substrate. Owing to the waste of ITO untargeted and relatively low sputtering speed especially for thick ITO layer the traditional ITO sputtering manufacturing is high-cost. Regarding incorporation to flexible electronics the brittle nature of ITO makes it a great challenge.3 Due to these inherent drawbacks, alternative materials replacing ITO are highly expected. Amongst silver nanowire (AgNW) networks exhibit 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 solution-process, allowing it a promising candidate.4-15 Despite the significant benefits of AgNW networks, there are still limitations. The rough surface topology results in un-uniform top coating layer leading to shorts and device failure. Several strategies have been reported to address the problems. Adoption of inorganic nanoparticles (NPs), such as TiO2, SiO2, ZnO etc., 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 2 ACS Paragon Plus Environment
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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 be survived as substrate. However, polymer overcoating provides a quite simple one-step 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. 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 through wash before device fabrication.
28, 31, 32
Second, photoresist coated on AgNWs networks can form a quite flat composite surface, but resulted 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 case, but usually suffer 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, while diluted monomer or polymer usually causes a poor nanofilm with limit improved surface roughness, and solvent usually affects monomer curing to lower down 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 3 ACS Paragon Plus Environment
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desired to be solution-processed, 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 an 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 abalation. 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 um*50 um with strong adhesion to substrate and comparable opto-electrical performance with ITO coated glass. Unlike tranditional polymer, the AgNWs/Chi composite can resit both water and orgainc 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 possibly interests 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 root mean square (RMS) roughness of pristine AgNW networks was 23.4 nm with average Ag nanowires rising ~75 nm above substrate obtained
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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 cut-off. (l) Step profiles obtained from AFM height images in g and h.
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 5 ACS Paragon Plus Environment
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roughness of AgNWs composite. The thickness of spin-coated 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 would be supposed to be larger than 29.7 nm (75-45.3 nm) if the Ag-Ag junctions of AgNWs composite stay identical 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 nanowires to each other and to 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 nm to 125.6 nm kept reducing AgNW rising to 8 nm, indicating the chitosan layer on the Ag nanowire top was thinner than that deposit on substrate among Ag nanowires voids. To provide more insight of the uniformity of AgNWs composite, we have scanned the surface at a largest area among report, 50 um*50 um, 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 um*50 um scan size was 6.9 nm, and it was only slightly increased to 7.6 nm using PEN as substrate instead, which was as small as that at 10 um*10 um scan size in Figure 1 although scan area has extended 5 times. Moreover, we cut a 4 cm2 sample to 4 pieces, and tested every single piece. There is 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 pressing whose surface roughness almost all up to dozens of nanometer, and comparable to or better than that of AgNW embedded in polymer surface, and just a little bit larger than that of 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, while the thin chitosan layer on glass slide has a half RMS of 0.5 6 ACS Paragon Plus Environment
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nm with average chitosan rising ~1 nm. , demonstrating pretty good film formability of chitosan in nanoscale thick. Thus, it can be concluded that the reduced surface roughness of AgNWs 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 topto-bottom height. a RMS 6.9 nm
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Figure 2. AFM height and 3D images of AgNW/Chi on glass (a) and PEN (b) with scan size of 50 um*50 um.
As aforementioned opto-electrical performance is one of the most important requirements for a transparent conductor. Unlike reported polymer in other AgNWs composite, such as PEDOT:PSS, photoresist etc.,[26, 30] our chitosan layer in nanometer thick show almost no absorption over the entire visible light region (Figure S3), facilitating the fabrication of highly transparent AgNWs composite. Interestingly, the optical transmittances 7 ACS Paragon Plus Environment
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of AgNW networks after chitosan coating were all improved at light region >550 nm instead of 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 ascended by 3.6% (from 89.4 to 93.0%) as 1.5 wt% of chitosan used. The AgNW composites also exhibited reduced haze which 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 which 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, subwavelength-dimensioned 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] It’s 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 known the extremely intense localized electromagnetic fields induced by LSPR makes 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 results in LSPR 8 ACS Paragon Plus Environment
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redshift.[37, 38] As shown in Figure 3a the main resonance is redshifted 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 redshifted dip is due to the enhanced extinction cross-section of NWs which is related to the dielectric constant of surrounding medium according to Mie’s solution of Maxwell’s equation.[38] Since 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 revealed from 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 and 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, and 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 etc.
[21, 26, 39-44]
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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, confirmed the effectiveness of our strategy in improving the performance of AgNWs electrode. 9 ACS Paragon Plus Environment
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Figure 3. UV-vis spectra (a) and haze (b) of pristine AgNW and AgNW/Chi composite film spin-coated by 0.1 wt%, 1 wt% of chitosan solution at 3000 rpm and 1.5 wt% at 2000 rpm, respectively. (c, d) 30° tilt view 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-
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electrical performance of AgNW/Chi electrode with reported various kinds of TCEs in literature (all transmittance are exclusive of substrate).
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 chemicals 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 and 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 and d displays the different thermal stability of pristine AgNW and AgNW/Chi. During 180 °C aging on hotplate 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 11 ACS Paragon Plus Environment
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networks before complete decomposition. It means actually only an extremely thin layer of chitosan can works well to prevent thermal attacks. It is noted that AgNWs composite kept transparent during high-termperature heating, but it became yellowish upon chitosan decomposition and further darker 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 polymer can be served as a protecting layer for AgNWs, many of them maybe 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 shown in Figure 4e. The sheet resistance only increased by 5-6% in water and acetone, and 23% in IPA, and
