金屬氧化物之奈米級磊晶核殼結構的成長與磁性性質

Abstract

本論文研究領域為過渡金屬氧化物之磊晶薄膜與奈米結構，希望藉由簡單的製程手法與材料基礎觀念，成長出具有傳統核殼式結構之磊晶薄膜奈米結構，進而創造出同時擁有磊晶性質與核殼結構的奈米複合物，有助於該領域研究的突破。我們選用了兩種含鉍(Bi)的三元元素氧化物-鉍鐵氧(BiFeO3)、鉍鈷氧(BiCoO3)和鍶鈦氧(SrTiO3)基板做為研究的材料，藉由脈衝雷射沉積法(PLD)與高能量反射式電子繞射儀(RHEED)的觀測之下，利用含鉍氧化物在不同的熱力學條件之下，會相分離成氧化鉍與另一個二元元素的金屬氧化物之特性，再加上氧化鉍容易揮發與材料傾向擴散至最低整體自由能處的基本現象，成功地創造出第一個有基板支持的氧化物奈米級磊晶核殼結構。此研究過程中，我們藉由XRD、SEM、AFM、TEM等分析儀器來分析與證明此新結構的存在；此外，由於最後的產物為氧化鈷(CoO)與氧化鐵(Fe3O4)皆屬於磁性材料的範疇，因此，我們進一步研究此系統之磁性質，發現到在此新結構之下，可以藉由製程上的改變與材料的選擇來輕易地控制材料的磁性質，如磁的異向性、矯頑磁場、飽和磁化量與交換偏置等等。此項研究不但創造出新的材料結構，並在磁性領域上有所收穫，對於金屬氧化物奈米結構材料之基礎物理性質研究與功能性材料在元件設計的發展上有很重要的影響。

Transition metal oxides have been one of the target of primary research due to their tremendous potential for practical applications such as non-volatile memories, magnetic recording media, solar cells, chemical catalyst and so on. Recently, oxide nanocrystals have gradually caught significant attention as results of their fascinating physical properties. Since functional oxide nanocrystals possess interesting magnetic, electric, and optical properties than other scales, a combination of two or more different nanocrystals would deliver a new pathway to design the material systems in nanoscale. In this study, monodispersed epitaxial oxide nanocrystals with one covering the other have been successfully created by utilizing the instable characteristics of bismuth-based complex ternary oxides. This method takes advantage of both the virtues of traditional core-shell nanostructures and the epitaxial supported nanostructures, showing the control of facet and interface of core-shell nanocrystals. Here, we fabricated nanocrystals combined with rock-salt structure of antiferromagnetic CoO and spinel structure of ferrimagnetic Fe3O4, where we could manipulate one through the other easily. Our results show that the magnetic properties such as the magnetic anisotropy, the coercivity, and the magnetization are tunable and can be precisely controlled by the size, thickness, orientation, interface and the role of core or shell at room temperature. In addition, a large exchange bias has been observed due to the strong magnetic interaction of the core and shell magnetic nanocrystals. This approach can be expanded into all sorts of bismuth-containing oxide and then demonstrates different epitaxial core-shell metal oxide nanocomposite easily. Based on the novel structure and their possibility of convenient control of physical characteristics, this study provides us a new opportunity to understand the fundamental properties of nanoscopic oxides and the potential to design more functional devices in the future.