應用於奈米材料之掃描穿透式電子顯微鏡分析技術：化學成分分布與三維斷層顯像

隨著奈米科技的快速發展，對於奈米分析的需求與日俱增。近年來掃描穿透式電子顯微鏡(Scanning Transmission Electron Microscope，STEM)由於其非同調成像(incoherent imaging)，且影像強度與原子序的1.7次方成正比之特性，使其在奈米材料研究中開始嶄露頭角。更特別的是當STEM結合各式光譜儀後，將可同時獲得材料局部區域之晶體結構與化學組成的信息。本研究將應用兩項新的STEM技術，從新的視野來研究奈米材料的結構與性質。一是光譜成像（spectrum imaging）技術，利用具有球面像差校正器（Cs-Corrector）的STEM，結合X-ray能量散佈能譜(energy dispersive X-ray spectroscopy，EDS)與電子能量損失譜(electron energy-loss spectroscopy，EELS)，進行原子解析度化學成分分布分析，另一則是三維斷層影像(3D Tomography)，提供材料結構更全方面的觀察。前二章將先針對STEM與相關實驗技術進行介紹，第三、四章將利用STEM-EDS與STEM-EELS在原子解析度下對氧化物異質結構界面(oxide-heterostructure interfaces)進行研究，我們所選用的氧化物異質結構系統(Nd0.35Sr0.65)MnO3/SrTiO3，由於NSMO薄膜與STO基材間的晶格失配(lattice mismatch)，隨著薄膜厚度不同，其界面將呈現契合（coherent）與非契合（incoherent）兩種不同的型態。在20 nm NSMO薄膜與STO基材所形成平坦之契合界面中，藉由原子解析度化學成分分布分析觀察到界面擴散(interdiffusion)的形況，並於進一步的EELS定量分析中，發現於界面處梯度化極性不連續(graded polar discontinuity)與局域化二維電荷密度(localized two-dimensional electron density)。而在40 nm NSMO薄膜試片所形成的非契合界面中，界面出現錯合刃差排(misfit edge dislocation)，藉由Cs-STEM可清楚觀察差排核(dislocation core)的結構，並成功利用STEM-EELS技術在原子級尺度下證明了差排為電荷聚積中心。第五章中應用STEM三維斷層影像技術分別觀察兩種不同的奈米複合材料(奈米金觸媒與有機太陽能電池光活化層)，在2 nm的解析度下，清楚觀察奈米粒子尺寸、形狀與三度空間分佈。

關鍵字

掃描穿透式電子顯微鏡；球面像差校正器；光譜成像技術；三維斷層影像；氧化物異質結構；界面擴散；錯合刃差排；奈米複合材料

並列摘要

With the rapid advances in nanotechnology, the demand for spatially-resolved nano-characterizations is greatly increasing. Recently, scanning transmission electron microscope (STEM) has gained growing attentions in the investigations of nano-materials owing to its atomic-number sensitivity in the incoherent imaging regime. Most importantly, the capability to combine STEM with various spectroscopies further allows a direct unveiling of the structural and chemical information of a local area without ambiguity. This Ph. D. thesis has been dedicated to the developments and applications of the advanced STEM spectrum imaging techniques, STEM-EDS (EDS, energy dispersive spectroscopy) and STEM-EELS (EELS, electron energy-loss spectroscopy), and the three-dimensional (3D) STEM tomography. A general introduction of STEM is presented in Chapter 1 and an overall experimental elucidation is documented in Chapter 2. In Chapters 3 and 4, we show the STEM-EDS and STEM-EELS investigations of the oxide-heterostructure interfaces, (Nd0.35Sr0.65)MnO3/SrTiO3, at atomic resolution. In (Nd0.35Sr0.65)MnO3/SrTiO3, the interface can have two different morphologies, coherent (defect free) and incoherent (misfit dislocations), which were dependent on lattice mismatch of NSMO thin film to STO substrate and the film thickness. In the atomically abrupt NSMO(20 nm)/STO interface, the interdiffusion across the interface was demonstrated by atomic resolved chemical mapping. Further EELS quantification revealed the graded polar discontinuity and a localized two-dimensional electron density at interface. In thicker (40 nm) NSOM film, the edge misfit dislocations with the [100] Burgers vector and A-site deficiency accordingly existed at interface. The dislocation behaves as the charge segregation center, totally undocumented atomic resolution before, and the associated implications were also discussed. In Chapter 5, we demonstrate the 3D visualizations of silica-supported gold nanocatalyst and the TiO2-nanowires/polymer hybird solar cells using STEM tomography. The size, sharp, and spatial distribution of all these nano-materials can be nicely revealed in the corresponding tomography results with an estimated spatial resolution of slightly better than ~2 nm. Chapter 6 represents the general conclusion of this thesis with a particular focus on the future opportunities of the STEM spectrum imaging and electron tomography.

並列關鍵字

Scanning transmission electron microscope ； Cs-Corrector ； spectrum imaging ； 3D Tomography ； oxide-heterostructure ； interdiffusion ； misfit edge dislocation ； nanocomposites

參考文獻

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應用於奈米材料之掃描穿透式電子顯微鏡分析技術：化學成分分布與三維斷層顯像

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