"利用掃描穿透式電子顯微鏡結合電子能量損失能譜於RNiO3/SrTiO3(R=La,Pr,Nd)氧化物異質介面之研究"

Abstract

隨著奈米分析的需求增加，掃描穿透式電子顯微鏡 (Scanning Transmission Electron Microscope，STEM) 在奈米材料的研究中扮演重要地位。而電子能量損失能譜(Electron Energy-loss Spectroscopy，EELS)反映了未被佔用的狀態密度(density of states)的電子特徵。結合 STEM-EELS 技術，我們得以在原子級尺度下同時處理結構訊息與電子特徵。本論文將應用搭配球面像差修正器(Cs-Corrector) 的 STEM 與EELS，揭示RNiO3 / SrTiO3 異質界面的微觀物理學(R=La,Pr,Nd)，其中三組RNiO3 異質結構的厚度皆為 10 單位晶胞。鎳酸鹽(Nickelates) 本身存在金屬-絕緣體轉變(Metal-to-insulator transition) 現象，三組樣品在塊材(bulk)時在室溫皆為金屬相。雖然 LaNiO3 / SrTiO3 在室溫下保留了原本的金屬性，但 PrNiO3 / SrTiO3 和 NdNiO3 / SrTiO3 在室溫下卻呈現絕緣性。在結構分析中顯示 PrNiO3 和 NdNiO3 薄膜的結構變形相較於塊材時更大。有趣的是在本實驗中，所有三個異質界面都表現出正電荷密度（~1014 cm-2），這與傳統觀點不一致，即絕緣性界面不應顯示電荷密度。在這項研究中，我們討論了電荷密度與對應的結構性質的相關性。

With the increasing demand in structural and electronic characterizations at high spatial resolution, atomically-resolved scanning transmission electron microscope (STEM) has become an indispensable tool in modern materials research. When used in combination with electron energy-loss spectroscopy (EELS) that reflects the electronic features of unoccupied density of states, a simultaneous tackling of the structural and electronic characters at atomic resolution had been proven possible and this conjunct STEM-EELS technique is most suitable for addressing the physics at a reduced dimension. In this thesis, we report on such a STEM-EELS application in unveiling the microscopic physics across the RNiO3/SrTiO3 heterointerfaces (R = La, Pr, and Nd; RNiO3 thickness, 10 unit cells for all three heterostructural systems). The nickelates are well-known to display a metal-to-insulator transition, with the characteristic transition temperature being an intricate function of the given structural distortion, and all the three materials are metallic at room temperature in the bulk state. While the LaNiO3/SrTiO3 preserves the signature metallicity at room temperature, the PrNiO3/SrTiO3 and NdNiO3/SrTiO3 unexpectedly turn out to be insulating. A thorough structural characterization revealed that the structural distortion in the PrNiO3 and NdNiO3 films are noticeably large compared to the characteristic magnitude in the respective bulks. More surprisingly, all three heterointerfaces manifest a positive charge density (~1014/cm2), at odds with the conventional wisdom that an insulating interface is not supposed to display a residual charge density. The corresponding structure-property correlations were discussed in this work.