利用固態擴散反應形成多孔性異質結構奈米線

Abstract

金屬氧化物材料擁有多樣且優異的物理與化學性質，因此被廣泛研究在眾多的過渡金屬氧化物中，氧化鋅與氧化鐵則在光電與磁性各自的應用領域裡扮演要角。氧化鋅本質為直接能隙N型半導體，具有寬能隙，在光學、電學及壓電特性上各方面都有優越的物理性質；而鐵是著名的磁性材料，有著許多種氧化態，包括氧化鐵、氧化亞鐵、四氧化三鐵。本實驗利用動態穿透式電子顯微鏡觀察氧化鋅奈米線及鐵金屬元素之擴散反應行為。在此我們將氧化鋅於超高真空環境下退火，透過固態擴散反應讓鐵與氧化鋅形成四氧化三鐵，及氧化鋅/四氧化三鐵同軸異質結構。 本實驗反應溫度約在800 K，反應時間只需10分鐘，即可透過動態影像觀察到鐵原子於氧化鋅奈米線內擴散。反應後得到的多孔性四氧化三鐵晶體依外型可分為兩種孔洞形式，分別是平板狀孔洞形(disc-like void)與鋸齒中空狀孔洞形(zigzag-like void)，其中以平板狀孔洞形較常出現。透過高解析度影像與FFT繞射圖形分析，我們發現平板狀孔洞會沿著四氧化三鐵晶體的最密堆積面來形成，且平板狀孔洞的四氧化三鐵晶體與氧化鋅晶體間有磊晶關係，兩者間的晶格不匹配(misfit)約為8.3%；而鋸齒中空狀孔洞形的四氧化三鐵晶體雖然與氧化鋅晶體之間有一平整介面，但兩者間並沒有磊晶關係。從擴散距離來看，同一試片上的兩種不同孔洞形貌之四氧化三鐵，鋸齒中空狀孔洞形的奈米線直徑雖為平板狀孔洞形的1.15倍，但其擴散距離卻為平板狀孔洞形的1.9倍，可知平板狀孔洞形四氧化三鐵晶體在與氧化鋅晶體形成磊晶時，必須消耗更多能量來適應晶格不匹配，故擴散距離會較鋸齒中空狀孔洞形還要來得短。

Metal oxide materials have been widely investigated owing to their various and excellent physical and chemical properties. Among these transitional metal oxides, zinc oxide and iron oxide each play an important role in applications of photoelectric and magnetic. Zinc oxide is naturally a direct band-gap, n-type semiconductor which has wide band-gap. As a result, it exhibits significant physical properties in optical, electrical and piezoelectric fields. Iron is known for its magnetic properties, which possesses various oxidation states, including wüstite(FeO), hematite(α-Fe2O3), maghemite(γ-Fe2O3) and magnetite(Fe3O4). In this study, we observe the diffusion behaviors between zinc oxide nanowire and iron metal by in-situ transmission electron microscope. Zinc oxide nanowires and iron metal are annealed under ultra-high-vacuum condition. By solid-state diffusion reaction, the Fe3O4 and ZnO/Fe3O4 heterostructure nanowires were formed. The diffusion of iron atoms in zinc oxide lattice can be observed at 800 K annealed for 10 minutes. As-formed porous Fe3O4 nanowire with voids can be divided into two types by their appearances : disc-like void and zigzag-like void. Disc-like voids are more common than zigzag-like voids. By HRTEM images and FFT diffraction pattern analysis, we found that disc-like voids formed along the close-packing plane of Fe3O4. Moreover, there is an epitaxial relationship between Fe3O4 crystal with disc-like voids and ZnO crystal. We also compare two voids type Fe3O4/ZnO heterostructure nanowires on the same sample. It indicated that the diffusion rate of disc-like Fe3O4 nanowire is much slower since it needs more time and energy to adjust the lattice mismatch and form epitaxial interface.