Transparent Electrodes Fabricated via the Self-Assembly of Silver

Article pubs.acs.org/Langmuir

Transparent Electrodes Fabricated via the Self-Assembly of Silver Nanowires Using a Bubble Template Takehiro Tokuno, Masaya Nogi,* Jinting Jiu, Tohru Sugahara, and Katsuaki Suganuma* The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka 8−1, Ibaraki, Osaka, 567−0047, Japan ABSTRACT: To shore up the demand of transparent electrodes for wide applications such as organic light emitting diodes and solar cells, transparent electrodes are required as an alternative for indium tin oxide electrodes. Herein the selfassembly method with a bubble template paves the way for cost-effective fabrication of transparent electrodes with high conductivity and transparency using self-assembly of silver nanowires (AgNWs) in a bubble template. AgNWs were first dispersed in water that was bubbled with a surfactant and a thickening agent. Furthermore, these AgNWs were assembled by lining along the bubble ridges. When the bubbles containing the AgNWs were sandwiched between two glass substrates, the bubble ridges including the AgNWs formed continuous polygonal structures. Mesh structures were formed on both glass substrates after air-drying. The mesh structures evolved into mesh transparent electrodes following heat-treatment. The AgNW mesh structure exhibited a low sheet resistance of 6.2 Ω/square with a transparency of 84% after heat treatment at 200 °C for 20 min. The performance is higher than that of transparent electrodes with random networks of AgNWs. Furthermore, the conductivity and transparency of the mesh transparent electrodes can be adjusted by changing the amount of the AgNW suspension and the space between the two glass substrates.

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assembled metallic nanoparticles have been investigated.9−13 The mesh structures can be made thick with wide spacing to drastically increase the conductivity without loss of transparency. Moreover, self-assembly methods are cost-effective processes because transparent electrodes are easily formed by solution processes. Layani et al. fabricated transparent electrodes with micrometer-sized rings composed of silver nanoparticles using a self-assembly method called the coffee ring effect.9 The silver nanoparticles were automatically arranged into these rings. Arrays of the rings became conductive following chemical treatment to form mesh transparent electrodes. Moreover, Higashitani et al. fabricated mesh transparent electrodes via the self-assembly of a gold nanoparticle suspension using a stainless steel mesh template.12 These gold nanoparticles were assembled in mesh structures along the stainless steel mesh. The mesh self-assembled gold nanoparticles were formed after simple evaporation. After heat treatment, the mesh structures became transparent electrodes with high conductivity and transparency. All of these fabrication methods have produced transparent electrodes by assembling metal nanoparticles into mesh structures using templates or patterns that have resulted in high conductivity and transparency. Herein, we report the use of a bubble template method to assemble AgNWs and fabricate mesh-structured self-assembled AgNWs. Mesh structures with bundles of AgNWs were

INTRODUCTION Transparent electrodes are critical for many electronic devices such as liquid crystal displays, touch screens, plasma displays, organic light emitting diodes, and solar cells.1−3 Doped metal oxides such as tin-doped indium oxide (ITO) have been leading candidates for these technological applications.1 The high cost and limited supply of indium for ITO electrodes has stimulated the search for alternative transparent electrodes,2,3 including carbon nanotubes,4 graphenes,5 random networks of metal nanowires,6−8 and mesh structures self-assembled metallic nanoparticles.9−13 Transparent electrodes with random networks of silver nanowires (AgNWs) exhibit high electrical conductivity and transparency, because silver has the lowest resistivity (as low as 1.59 × 10−6 Ω·cm) among metals. Silver is stable in pure air and water and, therefore, has a high corrosion resistance. Moreover, since the AgNW electrodes are coated by costeffective manufacturing, AgNW electrodes are expected to be applied to various electric devices. However, the conductivity and transparency of AgNW electrodes reach a limiting value. For example, if the transparency of AgNW electrodes is 80%, then the sheet resistance is limited to a range of 8−10 Ω/ square or more.6−8 The main reason for the limited performance is the small diameter of the AgNWs (several tens of nanometers). This sheet resistance is higher than best value of ITO transparent electrodes (5−8 Ω/square).14 Transparent electrodes with higher conductivity are required for current driven devices, such as solar cells.14 To achieve transparent electrodes with a combination of high conductivity and transparency, mesh structures of self© 2012 American Chemical Society

Received: March 6, 2012 Revised: May 24, 2012 Published: May 29, 2012 9298

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much as possible. To stabilize the bubbles, a solution with a low surface tension and a high viscosity is necessary.19 An SDS solution was selected as the surfactant to provide the low surface tension and an LDAO solution was selected as the thickening agent to provide the high viscosity. To achieve transparent electrodes with high conductivity, the dispersed materials are important. AgNWs were selected as the dispersed material because silver possesses the lowest resistivity among the metals and also has high corrosion resistance. Moreover, AgNWs are likely to make a conductive path because of their unique shape and high aspect ratio.20,21 AgNWs were synthesized using a polyol method and were dispersed into water that was bubbled with the surfactant SDS and the thickening agent LDAO. Figure 1a shows the bubbles

fabricated via a self-assembly method using bubble templates. The mesh transparent electrodes exhibited high conductivity and transparency after heat treatment. The self-assembly method with the bubble template paves the way for costeffective fabrication of transparent electrodes with high conductivity and transparency.

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EXPERIMENTAL SECTION

Synthesis of the AgNWs. AgNWs with an average diameter of approximately 70 nm and an average length of approximately 8 μm were synthesized by reducing silver nitrate in the presence of polyvinylpyrrolidone (PVP) in ethylene glycol.8 A 16 g portion of FeCl3 solution (6 × 10−4 mol/L, in ethylene glycol), 1.08 g of silver nitrate, and 0.98 g of PVP (average molecular mass 360 k in terms of monomeric units) were added to 125 g of ethylene glycol. After the mixture was heated at 150 °C for 1.5 h, 30 mL of the reacted suspension was filtered through a cellulose acetate membrane filter (Y100A047A, Toyo Roshi Kaisha, Ltd.). The filtered AgNWs were then dispersed in 2 mL of water to form the AgNW suspension. The concentration of the AgNWs was approximately 50 mg/mL. Fabrication of the Transparent Electrodes with AgNW Mesh Structures. One milliliter of sodium dodecyl sulfate (SDS) solution (3 wt %, in water), 0.01 mL of lauryl dimethylamine oxide (LDAO) solution (32 wt %−34 wt % in water, Aromox DM12D-WC, Lion Akzo Co. Ltd.), and 2 mL of the AgNW suspension (50 mg/mL, in water) were mixed and stirred using a vortex mixer (Maxi Mix Plus, Thermoline Corp.) to form bubbles. These bubbles containing the AgNWs were sandwiched between two glass substrates with spacers of 0.3 or 1.0 mm in thickness. The size of the glass substrate was 76 × 26 mm. After the water in the bubbles was air-dried for approximately 2 h, the glass substrates were carefully separated. AgNW mesh structures were formed on both glass substrates. The AgNW mesh structures on the glass substrates were heated at 200 °C for 20 min to achieve their conductivity. Characterization of Bubble Containing the AgNWs and the AgNW Mesh Structures. The bubbles containing the AgNWs and AgNW mesh structures were observed using a digital microscope. The mesh structures were observed using a field emission scanning electron microscope (FE-SEM) (JSM-6700F, JEOL Ltd.) at an accelerating voltage of 5.0 kV. The optical transparency at a wavelength of 550 nm was measured using a spectrophotometer (V670, JASCO Corp.) with a glass substrate as a reference. The sheet resistance of mesh transparent electrodes was measured by using the four-probe method (Loresta GP T610, Mitsubishi Chemical Analytech Co. Ltd.).

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Figure 1. (a) Photograph of bubbles containing the AgNWs in a case. The AgNW suspension was bubbled using a vortex mixer. (b) The bubbles containing the AgNWs were placed on a glass substrate. (c) Optical image of bubbles containing the AgNWs. The AgNWs were assembled at the bubble ridges.

RESULTS AND DISCUSSION

The fabrication of mesh structures via the self-assembly of AgNWs using a bubble template was attempted. Recently, it has been reported that porous structures can be fabricated on metal electrodes using electrochemically generated hydrogen bubbles as templates.15−18 Bubble templates are useful because bubble templates do not need processes to remove the templates. The authors have investigated methods of preparing mesh transparent electrodes using air bubbles instead of hydrogen bubbles. It is known that a bubble is a thin film of water enclosing air that forms a hollow sphere. Many bubbles were aggregated to form a bubble template. Bubble templates include a hollow sphere part, a film part, and a ridge part. The hollow sphere part consists of air and cannot include AgNWs. The film part consists of a single film between hollow spheres is called the bubble film. The ridge part is the connection region among the hollow spheres and is called a bubble ridge. A key factor for using a bubble template is to maintain the stability of the bubbles, because bubbles are delicate and can be easily ruptured. In the present study, components of the AgNW suspension were selected to inhibit rupture of the bubbles as

that are formed by continuously stirring the AgNW suspension. The bubbles containing the AgNWs, which were 100−400 μm were carefully placed on a glass substrate (Figure 1b). These bubbles containing the AgNWs were observed using an optical microscope (Figure 1c). The AgNWs were observed to move into the connection parts of the bubbles, i.e., the bubble ridges. Furthermore, the AgNWs were self-assembled into mesh structures with a line of 100−200 μm in diameter. It is clear that the bubble ridges tend to attract the AgNWs. Few AgNWs were contained in the bubble films. AgNWs are assembled at the bubble ridges for two reasons: First, it has been reported that hydrophilic particles in bubbles are assembled at the bubble ridges by drainage flow.22 The AgNWs used in the study were covered with the capping agent PVP, which is highly hydrophilic. Therefore, the AgNWs were easily assembled at the bubble ridges. Second, AgNWs that are too large (length of 8 μm and diameter of 70 nm) were repelled from the bubble 9299

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films and moved to the bubble ridges. The bubble film cannot retain the big AgNWs because the thickness of the bubble films reaches only several tens of nanometers when the films rupture.23 To fabricate AgNW mesh structures, the bubbles on the glass substrate were sandwiched by another glass substrate as reported in refs 24−26. The space between the two glass substrates was set as 0.3 mm. The bubbles formed continuous polygonal shapes with bubble ridges (the white part) and a bubble film (the transparent part), as shown in Figure 2a.

Figure 2. (a) Photograph of bubbles sandwiched between two glass substrates. Bubbles formed continuous polygonal structures. (b) Optical image of bubbles containing the AgNWs.

Figure 2b shows a typical image of the bubbles containing the AgNWs. The bubble ridges included a large number of AgNWs. The diameter of the bubble ridges was 100−200 μm. All of the bubbles were ruptured, hence resulting in continuous polygonal mesh structures due to the slow evaporation of the water. After air-drying, self-assembled AgNW mesh structures were formed on both of the glass substrates. The AgNW mesh structures fabricated on the lower glass substrate were nearly identical to that on the upper glass substrate. Figure 3a presents an optical image of the AgNW mesh structures on the upper glass substrate. The aperture size was 0.2−3 mm. Parts b and c of Figure 3 show a top view and an off angle view of the AgNW mesh structures as determined by FE-SEM observation. It is observed that the line width and line height varied depending on the location. The line width and line height were 20−100 μm and 2−20 μm, respectively. Figure 3d shows an FE-SEM image of a single line. The line was a bundle of AgNWs. The line height was not uniform and had a high central region and a low edge region. Moreover, self-standing structures were obtained at the intersection regions of the lines shown in Figure 3e. The unique shape of the lines and the intersection region was similar to the results obtained for the simulation of bubble ridges.25,26 The self-standing structures were bent due to the gravitation attraction of the AgNWs, but they can be stabilized by entanglement of the longer length AgNWs with mechanical rigidity. Alargova et al. reported that long polymer microrods in bubble films remarkably stabilized bubbles.22 Thus, the AgNWs assembled at the bubble ridges became entangled with each other and constructed the self-standing structures. The sheet resistance of the mesh structures was over a measurable region (107 Ω/square) with a transparency of 95%. This is because PVP surrounded AgNWs, preventing high electric conductivities of the AgNW electrodes.6,8 To enhance the conductivity of the mesh structures, they were heat-treated at 200 °C for 20 min. It is reported the heattreatment fused the AgNWs into tight connections leading to high conductivity on random networks of AgNWs.8 Figure 4 shows the transparent electrodes with the heated AgNW mesh structures on a glass substrate. The transparent electrodes

Figure 3. AgNW mesh structures on a glass substrate before heattreatment. The aperture size was 0.2−3 mm. (a) Photograph of the AgNW mesh structures. (b−e) FE-SEM images of the AgNW mesh structures. (b, c) A top view and an off angle view of the AgNW mesh structures. (d) The line was a bundle of AgNWs and had high central region and low edge areas. (e) Some intersections of these lines formed self-standing structures.

Figure 4. Photograph of a transparent electrode with AgNW mesh structures on a glass substrate. The transparent electrode exhibited a transparency of 95% and a sheet resistance of 320 Ω/square.

consisting of the heated mesh structures exhibited a sheet resistance of 320 Ω/square with a transparency of 95%. Furthermore, when the space between the two glass substrates is increased, the bubble template can contain much more AgNW suspension between the two glass substrates. When the space between the two glass substrates was set as 1 mm and a larger amount of AgNW suspension was used, a mesh transparent electrode with the line width of 40−140 μm, line height of 4−30 μm, and aperture size of 0.5−3 mm was 9300

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(4) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F; Rinzler, A. G. Transparent, conductive carbon nanotube films. Science 2004, 305, 1273−1276. (5) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y. J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J. H.; Hong, B. H.; Iijima, S. Roll-to-roll production of 30-in. graphene films for transparent electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (6) Lee, J. Y.; Connor, S. T.; Cui, Y.; Peumans, P. Solution-processed metal nanowire mesh transparent electrodes. Nano Lett. 2008, 8, 689− 692. (7) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. Silver nanowire networks as flexible, transparent, conducting films: Extremely high DC to optical conductivity ratios. ACS Nano 2009, 3, 1767−1774. (8) Tokuno, T.; Nogi, M.; Karakawa, M.; Jiu, J.; Nge, T. T.; Aso, Y; Suganuma, K. Fabrication of silver nanowire transparent electrodes at room temperature. Nano Res. 2011, 4, 1215−1222. (9) Layani, M.; Gruchko, M.; Milo, O.; Balberg, I.; Azulay, D.; Magdassi, S. Transparent conductive coatings by printing coffee ring arrays obtained at room temperature. ACS nano 2009, 11, 3537−3542. (10) Vakarelski, I. U.; Chan, D. Y. C.; Nonoguchi, T.; Shinto, H.; Higashitani, K. Assembly of gold nanoparticles into microwire networks induced by drying liquid bridges. Phys. Rev. Lett. 2009, 102, 058303. (11) Layani, M.; Magdassi, S. Flexible transparent conductive coatings by combining self-assembly with sintering of silver nanoparticles performed at room temperature. J. Mater. Chem 2011, 21, 15378. (12) Higashitani, K; McNamee, C. E.; Nakayama, M. Formation of large-scale flexible transparent conductive films using evaporative migration characteristics of Au nanoparticles. Langmuir 2011, 27, 2080−2083. (13) Prezo, B. G.; Kuncicky, D. M.; Velev, O. D. Engineered deposition of coatings from nano- and micro-particles: A brief review of convective assembly at high volume fraction. Colloids Surf. A 2007, 311, 2−10. (14) Krebs, F. C. Roll-to-roll fabrication of monolithic large-area polymer solar cells free from indium-tin-oxide. Sol. Energy Mater. Sol. Cells 2009, 93, 1636−1641. (15) Jia, W.-Z.; Wang, K.; Zhu, Z.-J.; Song, H.-T.; Xia, X.-H. Onestep immobilization of glucose oxidase in a silica matrix on a Pt electrode by an electrochemically induced sol−gel process. Langmuir 2007, 23, 11896−11900. (16) Li, Y.; Song, Y.-Y.; Yang, C.; Xia, X.-H. Hydrogen bubble dynamic template synthesis of porous gold for nonenzymatic electrochemical detection of glucose. Electrochem. Commun. 2007, 9, 981−988. (17) Li, Y.; Jia, W.-Z.; Song, Y.-Y.; Xia, X.-H. Superhydrophobicity of 3D porous copper films prepared using the hydrogen bubble dynamic template. Chem. Mater. 2007, 19, 5758−5764. (18) Yang, S.; Jia, W.-Z.; Qian, Q.-Y.; Zhou, Y.-G.; Xia, X.-H. Simple approach for efficient encapsulation of enzyme in silica matrix with retained bioactivity. Anal. Chem. 2009, 81, 3478−3484. (19) Bhakta, A.; Ruckenstein, E. Decay of standing foams: Drainage, coalescence and collapse. Adv. Colloid Interface Sci. 1997, 70, 1−124. (20) Wu, H. P.; Liu, J. F.; Wu, X. J.; Ge, M. Y.; Wang, Y. W.; Zhang, G. Q.; Jiang, J. Z. High conductivity of isotropic conductive adhesives filled with silver nanowires. Int. J. Adhesion Adhesives 2006, 26, 617− 621. (21) Chen, D.; Qiao, X.; Qiu, X.; Tan, F.; Chen, J.; Jiang, R. Effect of silver nanostructures on the resistivity of electrically conductive adhesives composed of silver flakes. J. Mater. Sci.: Mater. Electron. 2010, 21, 486−490. (22) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Foam superstabilization by polymer microrods. Langmuir 2004, 20, 10371−10374.

fabricated. The AgNW mesh structure exhibited a low sheet resistance of 6.2 Ω/square with a transparency of 84% after heat-treatment at 200 °C for 20 min. The sheet resistance is lower than that of transparent electrodes with random networks of AgNWs (approximately 15−20 Ω/square at transparency of 84%).6,7 This difference results from the fact that the line height and line width of the mesh electrodes were much larger than the small diameter of the AgNWs (several tens of nanometers). Considering that the mesh structures contained a lot of impurities, the sheet resistance could be reduced by removing the impurities. Moreover, the sheet resistance and a transparency of the transparent electrodes can be easily adjusted by changing the amount of AgNW suspension and the space between the two glass substrates. One problem for the AgNW transparent electrodes is their adhesion to substrates. The Scotch tape test was carried out on the mesh transparent electrodes on glass substrates. The electrodes failed the test, being removed from the glass substrates. However, the adhesion can be improved by overcoat layers or embedding processes. We believe this report is the first example of the production of transparent electrodes using the self-assembly of AgNWs in a bubble template. The self-assembly method with the bubble template might be applied to not only metal nanowires but also to other materials, such as spherical metal nanoparticles, carbon nanotubes, and polymer fibers. Among these examples, materials having long length are likely to be applied to mesh structures owing to the mechanism of selfassembly and entanglement. This self-assembly method paves the way for cost-effective fabrication of transparent electrodes with high conductivity and transparency.

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CONCLUSION AgNW mesh structures were fabricated via the self-assembly of AgNWs in a bubble template. AgNW mesh structures with a transparency of 84% and a sheet resistance of 6.2 Ω/square were successfully fabricated after heating the dried mesh structures at 200 °C for 20 min. The properties of the AgNW mesh transparent electrodes are better than those of random networks of AgNWs. The transparency and sheet resistance can be adjusted by changing the amount of AgNW suspension and the space between the glass substrates. The success of transparent electrodes fabricated using the self-assembly of AgNWs in a bubble template gives rise to the possibility of a cost-effective process for creation of transparent electrodes with high conductivity and transparency.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-6-6879-8521. Fax:+81-6-6879-8522. E-mail: [email protected] (K.S.). [email protected]. osaka-u.ac.jp (M.N.). Notes

The authors declare no competing financial interest.

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REFERENCES

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