利用反射式兆赫光譜技術研究銀奈米線薄膜的光電特性

Abstract

我們利用反射式兆赫光譜技術來研究分布不均勻的銀奈米線薄膜在兆赫波段的響應，為了準確分析樣品的複數導電率，我們利用有效介質理論（EMT）從銀奈米線和空氣混合的材料中取得純銀奈米線的導電率。對濃度較濃的銀奈米線薄膜而言，我們可以使用簡單Drude模型成功地擬合其電導率；而對於奈米線密度較稀疏的薄膜，則是利用Drude-Smith模型來得到良好的擬合結果。銀奈米線之間的接面有助於形成一個電子傳輸的導電網路，但隨著銀奈米線表面的自然氧化，接面之間逐漸形成非歐姆接觸造成電阻率增加，我們的量測反映出氧化的結果，其銀奈米線的導電率從原Drude模型可擬合的導電網路變成用Drude-Smith模型擬合的非導電性。我們也從X光繞射分析圖上觀察到銀奈米線氧化濃度增加。此外，對於濃度很低的銀奈米線薄膜，我們鋪一層石墨烯在上面可大幅提升導電率，或許石墨烯與銀奈米線之間的表面電漿耦合是造成導電率提升的因素，但未來仍需要再進一步的研究。我們的研究對未來發展石墨烯與銀奈米線結合之間載子動力學的透明電極有應用的潛力。

We successfully demonstrated the reflection type terahertz time-domain spectroscopy (THz-TDS) for accurate measurement of complex conductivity of inhomogeneous silver nanowire (NWs) films. The effective medium theory (EMT) is used to extract the conductivity from the NW-air mixed composites. The conductivity can be well described by the Drude model for Ag NW films with higher NWs densities, whereas the conductivity is fitted with Drude-Smith model for lower density one. Interfacial junctions of NWs help to form the conducting network of Ag-NWs, but natural oxidation of NW surfaces gradually leads to the non-ohmic contact and increases the resistivity near the junctions of NWs. As a result, initially Drude-like Ag NW networks become non-conductive, which is following non-Drude behavior. X-ray diffraction measurement shows the increase of oxygen concentration as the oxidation layer increases. Deposition of a monolayer graphene on top of Ag NWs can significantly enhance the conductivity of sparser NWs film. Surface plasmon coupling between graphene and NWs is suggested to be responsible for this enhancement, but it still needs further comprehensive study. Our results provide the potential for further explorations of carrier dynamics in graphene/NW-based transparent electrode applications.