19-12-2012, 04:22 PM
Silver Nano wires
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ABSTRACT
Transparent conductive thin film electrodes are widely used for liquid crystal displays (LCDs), touch screens, solar cells, and flexible displays. 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 . Among these, carbon nanotubes are extensively explored in the past few years because of their theoretically ballistic conductivity, good electromechanical properties, and chemical inertness. 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.
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. Since then, Ag NW films have been fabricated using techniques, such as vacuum filtration, transfer printing onto poly(ethylene terephthalate) (PET) substrates , drop casting, 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.