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Review on the Synthesis and Anti-oxidation of Copper Nanowires for Transparent Conductive Electrodes

May 06, 2019 09:53 AM EDT
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Cu NWs are synthesized from Cu salts, capping agents and reductants. However, Cu is easily oxidized in air to form oxide layer, and there will be unreacted organic residues, which will greatly increase the resistance of NWs and therefore need to be removed. Annealing, organic acid cleaning and light treatment can remove the oxide layer and residual organic matter on the surface of Cu NWs to improve the conductivity of Cu NWs. Organic coatings, metal coatings and reduced graphene oxide coatings can block the contact between Cu NWs and air, thus significantly improving the oxidation resistance of Cu NWs and expanding the application scope of Cu NWs.
(Photo : Jia Feng Chao et al.)

In a paper to be published in the forthcoming issue in NANO, a team of researchers has reviewed the methods of synthesizing copper nanowires (Cu NWs) and techniques to improve its oxidation resistance. With excellent electrical, optical, and thermal properties, Cu NWs are an attractive alternative to indium tin oxide (ITO) as a traditional electrode material. However, it is still a challenge to improve the aspect ratio and long-term chemical stability of Cu NWs.

To explore the effect of capping agents on Cu NWs, the type of capping agent is usually changed to vary the length and diameter of NWs, because different capping agents have different effects on the aspect ratio of NWs. In order to explore how to improve the conductivity and oxidation resistance of Cu NWs, various post-processing methods are carried out to find the best treatment method.

During the synthesis of Cu NWs, ethylenediamine (EDA) as a capping agent requires a simple operation, but the aspect ratio of NWs is very low. The NWs synthesized by nickel acetylacetonate as a capping agent have larger aspect ratio. However, Ni will be introduced and Cu will be polluted. Oleamine (OLA) and alkylamine synthesized NWs have the largest aspect ratio, which exceeds 3000, and requires a simple operation, making this the best method.

In the aspect of improving the conductivity of Cu NWs, high temperature annealing can reduce the oxide layer on the surface of Cu NWs and improve conductivity, but high temperature can also cause the breakage of NWs. High intensity pulsed light technology is the best way to improve the conductivity of NWs, which can remove the oxide layer and weld NWs together, but the equipment is too expensive. Organic acid cleaning can remove the oxide layer on the surface of NWs without causing any damage to the NWs, and the operation is simple, making it a viable alternative (e.g., 55 Ω/sq. at 94% transparency).

For Cu NWs, improving their long-term chemical stability is the most important, as it will directly affect their applications. Cu NWs are now being used to make low-cost, transparent electrodes in touch screens, OLEDs and solar cells. With the development of science and technology, flexible transparent conductors remain the central focus of many researchers towards achieving flexible electronic displays and durable solar cells. Other emerging applications such as stretchable conductors, electronic skins and sensor devices would continue to expand the horizon of Cu NWs.

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