氧化鎳奈米線的合成與特性應用之研究

The synthesis, characteristics and applications of NiO-based nanowires

黃彥鈞(Huang, Yen Chun)

指導教授 ： 賴志煌

國立清華大學/工學院/材料科學工程學系/博士(2012年)

摘要

近年來，奈米技術研發以及材料的開發逐漸在高科技產業中成為不可或缺的一部分。由於奈米線的形狀特性，並且可結合不同的元素而得到有別於塊材的獨特性質。如此多變的物理特性使得奈米線的應用研究變得廣泛而多樣化。本篇論文主要在研究不同結構的氧化鎳奈米線和其特性應用，當中包括探討核殼結構中鐵磁層(鎳)和反鐵磁層(氧化鎳)之間磁交互耦合力和形狀異向性如何相互影響造成磁矩結構改變以及多層膜式鉑-氧化鎳奈米線在電阻式記憶體上的應用。
　首先我們探討一維核殼奈米結構中鐵磁-反鐵磁的界面耦合現象，觀察核殼奈米線中鐵磁層(鎳)和反鐵磁層(氧化鎳)之間磁交互耦合力和形狀異向性如何相互影響造成磁矩結構改變。鎳奈米線由於形狀異向性的影響，其易軸會平行於長軸方向。然而，我們發現在核殼奈米線中會產生額外的水平異向性和形狀異向性相互競爭造成易軸的轉向。此一水平異向性的來源可能是由於鎳以及氧化鎳界面的磁交互耦合力造成界面上鐵磁磁矩的重排。利用微磁學模擬所得到的結果，我們發現當改變奈米線的粗細度，可以改變兩種異向性的競爭情況，而讓奈米線的磁矩結構產生渦漩性、螺旋性以及同調性逕而影響到磁阻的改變。
另一方面，在電性上探討多層膜式鉑-氧化鎳奈米線在電阻式記憶體上的應用。藉由鉑插入層，可以改變氧化鎳電阻式記憶體元件的長度，而達到縮小操作電壓，改善電壓分布的情形。此外，我們利用施加脈衝電壓的方式，得到多重阻態的結果。從實驗結果推測其傳導機制主要為電洞跳躍傳導。氧化鎳的電動傳導特性主要是因為鎳原子和氧原子之間呈現非化學計量比的現象。因此當我們施加電壓於氧化鎳中，氧離子會重新分布使得電洞濃度的改變，造成多重電阻態變化。
本論文藉由改變奈米線結構，討論奈米線元件的應用可能性。在磁性應用方面可藉由調變磁性交互耦合力控制磁矩結構；在電性應用方面，除了奈米線元件展現出優異的阻值轉換特性外，還能幫助了解阻值轉換背後的傳導機制。相信此研究結果將有助於奈米元件的應用與發展。

關鍵字

氧化鎳 ； 奈米線 ； 陽極氧化鋁 ； 鐵磁反鐵磁交互耦合 ； 阻值轉換 ； 電阻式記憶體

並列摘要

Recently, nanotechnology plays an important role in lots of high-technical industries. Nanowires possess degrees of freedom associated with their intrinsic shape effect and their ability of incorporation with different components leading to unique properties different from those of bulk materials. The diverse range of applications has resulted in interest in nanowires with a wide range of physical properties. This dissertation focuses on the applications of NiO-based nanowires with different properties including interfacial coupling between ferromagnets and antiferromagnets in Ni/NiO core/shell nanowires and resistive switching characteristics of NiO/Pt multilayered nanowires.
Ni/NiO core/shell nanowire arrays composed of a ferromagnetic Ni core and an antiferromagnetic NiO shell were fabricated to investigate the interfacial coupling 1D system. The magnetic behaviors of nanowire arrays are studied by varying the diameter of nanowires ranging from 30 to 100nm. Unusual anisotropy changes were observed in Ni/NiO core/shell nanowires which can be ascribed to the competition between shape anisotropy and exchange anisotropy. The OOMMF simulated results depict the modulated spin structures in NiO/Ni nanowires adjusting the magnitude of both shape anisotropy and in-plane exchange anisotropy by varying thickness ratio of shell and core.
The resistive switching characteristics are also discussed in NiO/Pt multilayered nanowires, which can be considered as millions of NiO-based cells collected together. Non-polar resistive switching prevails reproducibly in millions of cells with significantly reduced switching voltages, narrow distributions in switching voltages, and a robust multilevel memory effect. A high resistance ratio (~105) between high and low resistance states in nano-scale cells enables stable multilevels induced easily by a pulsed voltage of various numbers.
This dissertation provides possibilities of the applications with nanowire-based devices. For the magnetic application, the spin structures can be modulated by controlling the exchange anisotropy between ferromagnetism and antiferromagnetism. In addition, we demonstrate that nanowire-based devices not only have good performance of resistive switching but also help to clarify the conduction mechanism. Our findings are important for the development of nanowire-based device.

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