利用反射式兆赫光譜技術研究銀奈米線 薄膜的電性轉變及熱穩定性

Abstract

透明導電電極在軟性元件上的應用蓬勃發展，透明導電膜在可饒式基板上的發展更是日漸重要，金屬奈米線是取代常見硬質材料ITO的其中一個可行方案。金屬奈米結構所形成的薄膜電極，其電性表現取決於金屬線之間的連接、奈米結構所構成的網絡。其中金屬奈米線薄膜特殊的相變態過程將可用來決定最佳參數 (既透明且導電) 之奈米金屬網絡。我們成功的使用反射式兆赫光譜技術來研究銀奈米線 (AgNW) 的相變態過程。 在此同時，我們也發現銀奈米線的特殊結構將會導致其熱穩定性不佳。因此提出了一個可以幫助熱穩定性的材料–石墨烯 (Graphene)。當石墨烯轉印在銀奈米線薄膜之上，我們發現不但在不導電狀態下的銀奈米線之電性被大大的提升，成功使得奈米結構從絕緣態躍升為導電態，而且也有效的將銀奈米線在加熱過程的斷裂、崩解時間往後延長，石墨烯成功扮演幫助熱穩定性的材料，使得銀奈米線可以應用的範圍更加廣泛。

Realization of transparent conductive electrode films on flexible plastic substrates is critically important for the emerging applications in printed electronic devices. Metal nanowires (NWs) are one of the most suitable candidates as an alternative to brittle indium tin oxide (ITO). The performance of metal nanostructures as thin electrodes is ultimately governed by the connectivity of nanostructures through the ohmic contact. Percolation threshold of metal NW films corresponds to the minimum density of NWs to form the transparent, yet conductive metal NW networks. Here, we determined the percolation threshold density of silver NW (AgNW) networks by using the terahertz (THz) spectroscopy as well as morphology analysis. Typically individual AgNW is covered by thin polymer layer to avoid fast oxidation. Thermal annealing at the temperature of 200 oC can effectively remove the polymer layer and the connectivity of NWs can be improved, but dramatic oxidation of the surface of NW degrades the performance of metallic electrodes. Our THz analysis clearly and optically observed the change of electrical properties of bare AgNWs after thermal annealing. We also investigated the thermal stability of AgNW films covered by a single layer graphene, which can serve as a thin heat-proof material. With the protective layer of graphene on the surface of AgNWs, percolational transition from the insulating to conducting state occurs at a lower critical NW density. Obviously, graphene prolongs the turning point from connective to broken down network of AgNW film.