固態陽離子交換反應形成多重金屬氧化物異質結構奈米線之研究

Abstract

金屬氧化物奈米結構具有相當廣泛的物理性質，其中，氧化鋅是一種極具潛力應用於半導體及光電元件的材料，除了本身具有的優異性質與可變性高的形貌，與其他材料的結合可改變及提升特性，更利於製作成奈米元件。 本實驗利用固態陽離子交換反應，讓氧化鋅奈米線與純金屬鋁在超高真空的環境下加熱，當鋁離子擴散進氧化鋅晶格時，會置換掉鋅離子與氧結合，形成塊狀的氧化鋁晶體以軸向排列、鑲嵌在單晶的氧化鋅奈米線上。陽離子交換法相當利於合成奈米晶體材料，由於反應過程僅有陽離子的轉換，因此可維持母晶體結構與形貌，很適合製備奈米異質結構。 然而陽離子交換法的機制尚不明確，異質介面對離子交換行為的影響更難以評估，本實驗在臨場超高真空穿透式電子顯微鏡下進行加熱反應，能夠直接觀察反應中的現象，並利用球面像差校正掃描穿透式電子顯微鏡分析氧化鋅/氧化鋁異質介面，繪製原子模型模擬磊晶介面之陰離子次晶格框架，及運用電子顯微鏡搭載之能量散佈光譜儀分析異質結構的成分組成。綜合以上實驗結果，推論出陽離子交換之離子行進路徑與反應機制，並分析反應中所造成的結構缺陷。

Metal oxide nanostructure has been investigated extensively due to its wide range of physical properties; among them, zinc oxide is one of the most promising materials. It exhibits fascinating functional properties and various kinds of morphology. ZnO heterostructure especially has attracted great attention since its performance can be varied readily and further improved by combining with other materials. The mechanism of cation exchange is remains elusive, especially in metal oxide heterostructures; it is unpredictable how the confined interface and the effects of ions diffusion will affect the transformation. In this work, the experiment was carried out in in situ UHV-TEM, which equipped with video recorder; in this way, we could observe the phenomena of the transforming directly. Moreover, we analyze the structure and composition of the epitaxial interface by Cs-corrected STEM equipped with EDS, and simulate the anion sublattice of epitaxial interface by atomic model. In this study, we deposited few nanometer of alumina onto ZnO nanowire by thermal evaporation, and successfully transformed ZnO nanowires into multiple Al2O3/ZnO heterostructure through solid state cation exchange reaction. During heating, alumina ions diffused into ZnO lattice, exchange with zinc ions, and combined with oxygen to form Al2O3 crystals mosaicked into the nanowire. In the process, the anion sublattice remains the basics of the parent crystal; therefore, it is a unique method to form new crystals with desired shape and size in nano-heterostructure. Based on these experiment results, we infer a model of the ion path in cation exchange reactions. Additionally, the defects appeared in cation reaction were investigated, which resulted in the remaining of zinc ions.