同步輻射奈米顯微術在發光礦物的譜學研究

Abstract

發光礦物及其光學現象長久以來備受關注。傳統上對礦物色彩與發光機制的研究大多著眼於晶體場理論中的電子躍遷、電荷轉移及晶格缺陷等。隨著奈米材料科技的發展，新的機制例如: 金屬奈米顆粒引發的局域表面電漿共振(Localized Surface Plasmon Resonance, LSPR)效應，以及稀土元素摻雜發光礦物的長磷光現象，為研究發光礦物提供新一個的方向。本論文利用先進的同步輻射奈米顯微技術，結合多種奈米尺度的分析方法與理論模擬，探討發光礦物中複雜的譜學特徵與機制。

本研究首先著眼於局域表面電漿共振效應對礦物色彩的影響。藉由歐傑電子能譜(Auger Electron Spectroscopy, AES)、球差校正穿透式電子顯微鏡(Aberration-Corrected Transmission Electron Microscope, AC-TEM)並結合聚焦離子束(Focused Ion Beam, FIB)、電子能量損失譜(Electron Energy Loss Spectroscopy, EELS)以及能量色散X光譜(Energy-Dispersive X-ray Spectroscopy, EDS)等技術，對金奈米顆粒(Gold Nanoparticles, AuNPs)塗層的合成莫桑石進行了微觀結構與成分分析。研究發現金屬奈米顆粒的尺寸、形狀、分佈及其周圍介電環境(包括基底的結晶性)會顯著影響局域表面電漿共振效應的共振波長與強度，進而決定礦物所呈現的外觀色彩。此外，本研究利用時域有限差分法(Finite-Difference Time-Domain, FDTD)進行了數值模擬，驗證局域表面電漿共振效應與奈米顆粒參數間的關係，模擬結果與實驗觀測相符。

本論文進一步利用同步輻射光源(台灣光子源-TPS 23A)的X光奈米探針技術，深入研究稀土元素(Rare-Earth Elements, REEs)摻雜的合成發光礦物(主成分為SrAl2O4:Eu, Dy)的長效磷光(Afterglow)特性與機制。結合X光螢光光譜(X-ray Fluorescence Spectroscopy, XRF)、X光螢光光譜成像(XRF mapping)、X光吸收光譜(X-ray absorption spectroscopy, XAS)、X光激發發光光譜(X-ray Excited Optical Luminescence, XEOL)及X光激發發光光譜成像(XEOL mapping)等多種技術，分辨出多個發光中心及其價態。主要發光貢獻來自Eu2+的4f65d1→4f7躍遷，呈現強烈的藍綠色螢光與磷光。X光吸收光譜分析確認了樣品中存在Eu2+與Eu3+兩種價態，且Eu2+濃度相對較高。同時確定了Dy3+的存在。Eu3+與Dy3+則貢獻源自4f-4f躍遷的多個窄線發射峰。研究也發現由氧空位缺陷形成的F中心(F-Center)以及微量雜質Cr3+(R線發射)和Mn2+離子(藉由X光吸收光譜確認存在)等其他潛在的發光中心。本研究整合並修改目前對於稀土摻雜鋁酸鹽磷光體的解釋與模型，特別是Dy3+作為陷阱中心在長效磷光機制中的作用，藉由先進的同步輻射分析技術提供更直接、更全面的譜學證據，釐清各發光中心的貢獻與相互作用。

此外，也將所建立的先進分析方法拓展至天然礦物。以產自墨西哥、具強烈綠色螢光的玻璃蛋白石(Hyalite Opal)為例，結合同步輻射真空紫外光致發光光譜(Vacuum Ultraviolet Photoluminescence and Photoluminescence Excitation Spectroscopy, VUV-PL/PLE)與X光吸收光譜技術，證實其發光中心為鈾醯離子(UO2)2+，並精確判定樣本中的鈾價態為U6+。透過變溫光致發光光譜分析進一步解釋其複雜的光物理過程，包括存在孤立的鈾醯離子(UO2)2+與鈾-鈾聚集體(U–U clusters)兩種發光群體，以及兩者之間隨溫度變化的能量轉移與熱淬滅競爭機制。本論文針對天然玻璃蛋白石（Hyalite Opal）提出了一個完整且創新的光物理模型。

本論文整合運用同步輻射奈米顯微術、先進電子顯微學以及光學模擬計算等多種方式，系統性地研究了兩類發光礦物體系: (1)由金屬奈米顆粒的局域表面電漿共振效應引發的顯色機制，同時探討基底結晶性的影響；(2)由稀土元素及缺陷中心主導的發光機制，並藉由同步輻射技術精確解析了多重發光中心及其價態。本研究結果對這些複雜光學現象背後物理、化學機制提出解釋，同時提供對於發光材料的鑑別、設計與應用的重要依據以及分析方法的基礎。

Luminescent minerals and their associated optical phenomena have long attracted significant scientific interest. Traditional studies on mineral coloration and luminescence mechanisms have largely focused on crystal field theory, including electronic transitions, charge transfer processes, and lattice defects. However, the advent of nanomaterials has introduced new paradigms, such as localized surface plasmon resonance (LSPR) induced by metallic nanoparticles and the persistent luminescence observed in rare-earth-doped minerals. This dissertation employs advanced synchrotron-based nanoscale microscopy, combined with a suite of nanoscopic characterization techniques and theoretical simulations, to investigate the complex spectroscopic behaviors and underlying mechanisms in luminescent minerals.

The first part of this work examines the influence of LSPR on the coloration of minerals. Using Auger Electron Spectroscopy (AES), Aberration-Corrected Transmission Electron Microscopy (AC-TEM), Focused Ion Beam (FIB) milling, Electron Energy Loss Spectroscopy (EELS), and Energy-Dispersive X-ray Spectroscopy (EDS), the microstructure and composition of gold nanoparticle (AuNP) coatings on synthetic moissanite were thoroughly analyzed. The results demonstrate that the LSPR wavelength and intensity are highly sensitive to nanoparticle parameters such as size, shape, spatial distribution, and surrounding dielectric environment, including the crystallinity of the substrate. Finite-Difference Time-Domain (FDTD) simulations were conducted to further validate the relationship between these parameters and LSPR behavior, showing strong agreement with experimental observations.

The second part focuses on the persistent luminescence (afterglow) of rare-earth-doped luminescent ceramics, specifically SrAl₂O₄:Eu, Dy. Utilizing the X-ray nanoprobe capabilities of the Taiwan Photon Source (TPS 23A), in conjunction with X-ray Fluorescence Spectroscopy (XRF), XRF mapping, X-ray Absorption Spectroscopy (XAS), X-ray Excited Optical Luminescence (XEOL), and XEOL mapping, multiple luminescent centers and their valence states were spatially and spectroscopically resolved. The dominant emission arises from Eu²⁺ 4f⁶5d¹ → 4f⁷ transitions, resulting in strong blue-green fluorescence and afterglow. XAS data confirm the coexistence of Eu²⁺ and Eu³⁺, with Eu²⁺ being predominant. The presence of Dy³⁺ was also confirmed, contributing several narrow emission lines via 4f–4f transitions. Additional emission was attributed to F-centers formed by oxygen vacancies, Cr³⁺ (R-line emission), and Mn²⁺ ions, as identified by XAS. An updated luminescence model is proposed, clarifying the role of Dy³⁺ as a trap center and offering direct spectroscopic evidence for the interplay among multiple emission centers.

Finally, the developed analytical approach was extended to natural minerals. Using a green-luminescent hyalite opal from Mexico as a case study, a combination of vacuum ultraviolet photoluminescence and photoluminescence excitation spectroscopy (VUV-PL/PLE) and XAS confirmed the presence of uranyl ions (UO₂)²⁺ as the principal luminescent species, with uranium predominantly in the hexavalent oxidation state (U⁶⁺). Temperature-dependent PL spectra revealed two distinct emissive species: isolated uranyl ions and U–U clusters. Their interactions, governed by thermally activated energy transfer and competitive thermal quenching, were elucidated through a comprehensive photophysical model proposed for this material.

This dissertation integrates synchrotron-based nanoscale imaging, advanced electron microscopy, and optical simulations to systematically explore two major classes of luminescent minerals: (1) coloration mechanisms governed by LSPR from metallic nanoparticles, including the influence of substrate crystallinity, and (2) luminescence processes dominated by rare-earth elements and defect centers. The findings offer mechanistic insights into these complex optical phenomena and establish a methodological framework for the identification, design, and functionalization of luminescent materials.