Silver nanowire-based transparent, flexible, and conductive thin film
© Liu and Yu; licensee Springer. 2011
Received: 21 July 2010
Accepted: 12 January 2011
Published: 12 January 2011
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© Liu and Yu; licensee Springer. 2011
Received: 21 July 2010
Accepted: 12 January 2011
Published: 12 January 2011
The fabrication of transparent, conductive, and uniform silver nanowire films using the scalable rod-coating technique is described in this study. Properties of the transparent conductive thin films are investigated, as well as the approaches to improve the performance of transparent silver nanowire electrodes. It is found that silver nanowires are oxidized during the coating process. Incubation in hydrogen chloride (HCl) vapor can eliminate oxidized surface, and consequently, reduce largely the resistivity of silver nanowire thin films. After HCl treatment, 175 Ω/sq and approximately 75% transmittance are achieved. The sheet resistivity drops remarkably with the rise of the film thickness or with the decrease of transparency. The thin film electrodes also demonstrated excellent flexible stability, showing < 2% resistance change after over 100 bending cycles.
Transparent conductive thin film electrodes are widely used for liquid crystal displays (LCDs), touch screens, solar cells, and flexible displays [1, 2]. Among these applications, the most commonly used materials are doped metallic oxides, mainly indium tin oxide (ITO) because of their high electrical conductivity and high optical transparency. However, there are many drawbacks in the case of ITO transparent electrodes. They are prone to cracking on flexible substrates. In addition, the ITOs are costly and require high temperature during thin film fabrication processes . However, the future display and other optic-electronic devices will require suitable methods for flexible transparent electrodes to be produced at low cost and in a large scale, such as roll-to-roll coating or ink-jet printing method. The substitutes for ITO are required for the developing electronic industry, and the search for such materials is mainly focused on conductive polymers and conductive nano-structures with high aspect ratio, whereas the low conductivity of polymer transparent electrode (approximately 1 S/cm)  restricts their applications. In recent years, 1D nano-structures have been actively researched as candidates for future transparent electrode materials, including nanowires, nanotubes, and nanorods [2, 5–8]. Among these, carbon nanotubes are extensively explored in the past few years because of their theoretically ballistic conductivity, good electromechanical properties, and chemical inertness [8, 9]. The ID nature of these nanostructures leads to increased optical transparency compared to a continuous, 3D material. However, nanotube films have yet to match the properties of ITO continuous films (i.e., transmittance = 90%, and sheet resistance < 100 Ω/sq), because nanotube film performances are hampered by the inevitable defects on the tubes, bundling between tubes, and the mixture of metallic and semiconducting carbon nanotubes . On the other hand, silver (Ag) nanowire (NW) is another promising alternative, and it has been reported to have the potential to surpass and replace ITO [10, 11].
Silver nanowires have been attracting more and more attention because of their intriguing electrical, thermal, and optical properties . Silver has the highest electrical conductivity (6.3 × 107 S/m) among all the metals, by virtue of which Ag NWs are considered as very promising candidates in flexible electronics. Lee et al.  have pioneered this material and shown that the cast Ag NW thin film used as transparent electrode showed equal merit or better than that as compared with sputter-coated ITO in solar cells. The gold/silver alloy NWs were also synthesized and studied as transparent electrode material by Azulai et al. . Since then, Ag NW films have been fabricated using techniques, such as vacuum filtration, transfer printing onto poly(ethylene terephthalate) (PET) substrates , drop casting [11, 14], and air-spraying  from NW suspension. The vacuum filtration, the so-called transfer method, produces highly transparent films with excellent conductivity , but the films possess irregular morphologies and significant roughness . Moreover, the process is not scalable. Using drop casting method always shows coffee rings and discontinuous film on the substrates. The film obtained from air-spraying coating is much better, but still forms sparse and non-uniform networks. In brief, most of the processes proposed so far cannot be ported easily to large scale production. Moreover, the researches on the film properties and effect factors have been very limited. In this article, we demonstrate uniform and transparent Ag NW film on plastic substrate via a scalable, simple, and low-cost process (i.e., rod coating), and means to improve the performance of flexible electrode, i.e., HCl treatment and protection coating.
To improve the conductivity of Ag NW film, concentrated HCl was employed to destroy the oxidation layer on the NW surface. Ag NW thin films on PET substrate was incubated in the HCl vapor (20 to 60°C) volatilized from concentrated HCl for 5-10 min. The prepared NW films are easy to scratch and damage due to the weak adhesion to PET substrate. For protecting the prepared transparent electrodes and prolonging their lifetime, a protective layer is required. Several drops of colorless nail polish liquid (commercial products) were cast on the film surface. Then, a slim glass vial or Meyer rod was rolled to the other end of the electrode to form a uniform nail polish layer. After being placed in the air for 1 min at room temperature, the nail polish formed a solid and thin protective layer on the Ag NW film. No observable scratch or change on Ag NW layer was found during the coating of nail polish layer.
All the SEM images were taken using a JEOL JSM-6490LV SEM. The Energy Dispersive Spectrometer (EDS) was utilized to examine the approximated chemical compounds for the Ag NWs before and after the HCl acid vapor treatment/incubation. Optical transmission spectra of the Ag NW films were recorded using a UV-Vis-NIR spectrometer (Jasco V-670) without using integrating sphere, with a sheet of PET being used as the reference. Sheet resistance measurements were made using the four-probe technique with a Keithley 2701 source meter. Bending experiments were performed by the two-point bending test in which a piece of Ag NW film on PET was bent into nearly a circle which was constrained manually. Then, the sheet resistance was measured using a digital multimeter.
Figure 1b shows a Ag NW film on a PET substrate fabricated via wire-wound rod coating. Draw-down rod coating is a well-known coating technique widely used by industrial laboratories for making thin films in a continuous and controlled manner. The liquid that can be coated effectively by the Meyer rod method can then be readily adapted to more controllable, higher throughput methods, such as slot, slide, and roll-to-roll coating . Different from the carbon nanotubes film previously obtained from rod-coating technique [18, 19], Ag NW suspension can be directly and uniformly coated onto PET at room temperature without any hydrophilic or hydrophobic pre-treatment of PET or coating materials, and no surfactants are required. However, the solvents are found to be very important to the uniformity of films. We have dispersed Ag NW in distilled water, surfactants aqueous, ethylene glycol (EG), methanol, and isopropanol. After 1-h sonication, all of these suspensions agglomerated in 5 h. On the other hand, Ag NWs show much better dispersibility, and are easier to re-dispersion in methanol and isopropyl alcohol than in aqueous solution. It is found that isopropyl alcohol leads to the most uniform coatings. The samples illustrated in this article were all from Ag NW isopropyl alcohol suspensions. Dan et al.  have illustrated that the uniformity of the films obtained from rod coating were determined by the surface tension and viscosity of liquid, which means that the groove size in wire-wound rod, solvents evaporation, concentration, and dispersibility of suspension, interactions between nanomaterial and substrates all impact the film homogeneity. In this case, because of the good evaporation of isopropanol, the films show line recession when the concentration of Ag NWs reaches above 5 mg/ml. For the same NW suspension, 10# rod is fabricated with better coating than 20# rod.
The HCl treatment destroys the oxidation layer on the nanowire surfaces and improves the contacts between NWs, thus increasing the film conductivity and transparency. To detect the element distribution variation during these processes, the EDS with a large spot size (approximately 1 μm) was performed (Figure 3d, e, f). In these EDS spectra, we can see a clearly intensity change of oxygen element peak (at around 0.5 keV). In Figure 3d, there is no appearance of "O" peak in the EDS spectra of the drop-cast sample with very good conductivity, from a very concentrated Ag NW suspension. However, after the rod-coating procedure, the content of oxygen distinctively increased (Figure 3e). Then, the oxidation layer was effectively removed by the HCl treatment, as shown in Figure 3f. During the process, the silver oxide (e.g., AgO, Ag2O, Ag3O2, or Ag4O3) reacted with HCl to form AgCl, and then the AgCl was partially photodecomposited under light illumination. However, Cl species is one of the main impurity during synthetic process , and nearly all the samples show very small amount of chlorine (ClK at approximately 2.65 keV). There is no obvious increase on the amount of Cl in sample after HCl treatment, implying an efficient photodecomposition.
Cui et al.  reported that the junction resistance between the Ag NWs is larger than 1 GΩ. If the surfaces of Ag NWs are oxidized, then both the single NW and the NW-NW junction resistances would increase because of the semiconducting property of silver oxide, especially the junction resistance. From the EDS spectra and film resistance test, the HCl treatment effectively reduces the "O" abundance in the film and the contact resistance between Ag NWs. Tests show that there is nearly no change in the resistance of HCl-treated thin film staying in air. This is most likely because the inner NW layer and NW contact points are not re-oxidized. Further optimization on HCl process will allow for better performance as a result of transparent, flexible electrode. The HCl treatment is an easier approach for improving the thin film conductivity than the reported method in the recent literature which employed Ag-Au core-shell NWs to replace Ag NWs . Other simple procedures also may help to improve the conductivity of film fabricated by rod-coating methods, such as H-plasma treatment and incubating/annealing in reducing atmosphere, etc.
It is known that the vacuum filtration method always produces transparent films with outstanding conductivity. It should be noted that the reported Ag NW transparent conductors prepared by vacuum filtration show better transparency and conductivity than the samples used in this study, which give a Rs = 13 Ω/sq for T = 85% as the best result in relation to the similar Ag NWs. The main reasons for the better conductivity are probably the better-dispersed Ag NWs as individual wires in the solid film and no oxidation in the contact area between the junction and the inner layer of NWs. However, vacuum-filtrated Ag NWs have weak adhesion to most substrates (e.g., PET, glass, Si, etc.) , making the transfer process much more challenging. Moreover, the vacuum filtration approach cannot meet the requirement of the scalable production. The excellent result from thin film produced via vacuum filtration method also implies that there is plenty of room for improving the performance of the transparent conductors fabricated by the scalable rod-coating.
Resistance of Ag NW film before and after protective coating
R after protective coating (Ω)
This research demonstrated Ag NW-based transparent and conductive thin films with excellent mechanical flexibility. The uniform and transparent thin films were fabricated via scalable, low-cost rod coating. With respect to the film uniformity, Meyer rod size, solvents, and concentration of NW suspension are the important factors. It was found that the simple HCl treatment can prominently decrease the thin film sheet resistance. SEM images and EDS spectra confirmed that the HCl treatment effectively eliminated the oxidation of nanowires and slight thinning of the wires, both of which could benefit the performance of NW film electrodes. The film transmittance and the sheet resistance as a function of film thickness were also studied. Electromechanical testing demonstrated that the film is extremely stable under bending with variation of sheet resistance being less than 2% over 100 bending cycles. Further treatment of thin film electrodes involved a performance-enhanced coating, i.e., a protective layer in this research. Using nail polish, NW films were successfully protected, and showed much better stability under both scratching and bending. Although the electronic and optical properties of the Ag NW thin film have not caught up with ITO yet, this proposed fabrication method is simple and easy to be scaled up for large films, which is one of the key issues for applying this technology to commercial applications. Moreover, this fabrication process also has the advantages of being low cost and with no need for expensive equipment. Therefore, it is believed that Ag NW will be a very promising candidate for transparent, flexible electrodes, especially in organic electronic devices and solar cells.
energy dispersive spectrometer
indium tin oxide
liquid crystal displays
The authors are grateful for the funding support received from the National Institutes of Health (NIH, Grant# R03DC009673-01A1).
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.