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  • 學位論文

計算能帶工程:以氧化物與矽奈米線為例

Computational Band Structure Engineering: The Case of Metal Oxides and Si nanowires

指導教授 : 郭光宇

摘要


計算能帶工程是一種理論方法,它基於材料能帶的計算,用來探討如何藉由控制能帶來改變材料的性質。它為理論材料設計提供了一個新的角度。在這篇論文中,本人總結近來密度泛函計算的結果,包含一系列材料的物性研究: 一、五氧化二釩系統。這裡利用投影綴加波(PAW)與GGA方法,來研究五氧化二釩在出現氧空缺缺陷之下的幾何結構、電子結構與磁性性質。結果發現氧空缺容易在O1位置出現,並導致層間鍵結的產生。氧空缺提供了還原電子,這還原電子會占據dxy能帶,並產生自發磁化的現象。此外,還原電子的濃度會影響到材料的磁性組態。 二、銪摻雜的鉀鍶磷酸鹽系統。這裡利用PAW與GGA方法,來研究鉀鍶磷酸鹽在銪摻雜之下的電子結構。結果發現氧在導帶底部有很強的貢獻,這有助於傳遞光激發的電子,並促進光致發光的效率。而銪摻雜在導帶底部提供5d-4f能階,可作為光致發光的發光中心。 三、矽奈米線系統。這裡利用線性原子軌道組合(LCAO)與局部密度近似(LDA),來研究矽[110]奈米線的電子結構。研究時同時考慮不同種類的奈米線表面處理,包含氫或氟。結果發現量子侷限效應會增加能隙,而氟的表面處理卻會降低能隙。另外,在矽[110]奈米線中的電子與電洞有效質量,各對應到橫向電子質量與輕電洞質量。這可以用量子侷限所引發的能帶投影來解釋。 基於以上的研究,本人提出一些能帶工程的想法: 一、半導體或絕緣體,與金屬的轉換,可利用調整價電子的方式來控制。 二、能隙的大小,可利用量子侷限效應或奈米尺度下的表面處理來控制。 三、磁性-非磁性的轉換,可利用調整費米級上的電子態密度來控制。 四、磁矩的大小,可利用調整價電子的方式來控制。

關鍵字

密度泛函理論 能帶 奈米

並列摘要


Computational band structure engineering is a theoretical method with the energy band calculations to investigate the treatments of the materials properties via the control of the band structures. It provides a new vision to theoretical material design. In this thesis, I summarize my recent results of density functional calculations about the physical properties for a series of materials, which include: 1. The V2O5−x system. The projector-augmented wave (PAW) method and GGA are used to study the geometrical, electronic and magnetic properties of V2O5−x. The results show that the oxygen vacancy prefers to appear on the O1 site, and the interlayer bonding is formed. The oxygen vacancy provides reduction electrons to occupy on the dxy band, which induces the spontaneous magnetization. The magnetic phase is altered with different concentration of reduction electrons. 2. The KSrPO4:Eu system. The PAW and GGA method are used to study the electronic structure of KSrPO4 with the Eu dopant. The results show that high oxygen contribution of the conduction band minimum (CBM) plays an important rule on the charger transfer of the photon-excited electrons, and it enhances the efficiency of the photo-luminescence (PL). The Eu dopant provides 5d-4f levels near the CBM as the recombination center to emission PL. 3. The Si [110] nanowire system. The linear combination of atomic orbitals (LCAO) and local density approximation (LDA) are used to study the electronic structure of Si [110] nanowire. Different types of surface passivation (H or F) are discussed. The results show that the quantum confinement enlarges the band gap, but the fluorine passivation lowers the band gap. It is found that in Si [110] nanowires the effective masses of the electron and hole are related to the transverse electron and light hole masses of bulk Si, which can be explained by the projection of the bands by quantum confinement. According to the results of the above researches, I conclude some ideas for band structure engineering: 1. The transition between semiconductor (insulator) and metal state can be switched by the valence electron control. 2. The band gap can be adjusted by the quantum confinement effect or surface passivation in the nano scale. 3. The magnetic-nonmagnetic transition can be switched by the adjustment of the electron density of states at the Fermi level. 4. The modification of the magnetic moments can be adjusted by the valence electron control.

參考文獻


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