Defect induced magnetism in nano-materials studied by X-ray-based spectroscopic and microscopic techniques

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Title:	Defect induced magnetism in nano-materials studied by X-ray-based spectroscopic and microscopic techniques
Other Titles:	利用X光能譜及顯像技術探討缺陷對奈米材料磁性的誘發機制
Authors:	王玉富;Wang, Yu-Fu
Contributors:	淡江大學物理學系博士班彭維鋒
Keywords:	掃描式穿透X光顯像術;X光磁圓偏振二向性;X光吸收近邊結構;衍生X光吸收精細結構;X光光電子能譜術;X光激發螢光;STXM;XMCD;XANES;EXAFS;XPS;XEOL
Date:	2017
Issue Date:	2018-08-03 14:45:53 (UTC+8)
Abstract:	X光顯像及相關能譜實驗技術提供了具備高空間解析度及元素針對性的電子、原子結構資訊於基礎科學研究、材料研發及產業應用上，包括X光吸收近邊結構(X-ray Absorption Near Edge Structure, XANES)、X光光電子能譜術(X-ray Photoelectron Spectroscopy, XPS)、X光磁圓偏振二向性(X-ray Magnetic Circular Dichroism, XMCD)、X光激發螢光(X-ray Excited Optical Luminescence, XEOL)、衍生X光吸收精細結構(Extended X-ray Absorption Fine Structure, EXAFS)及掃描式穿透X光顯像術(Scanning Transmission X-ray Microscopy, STXM)等，這些新穎技術提供了研究非磁性材料之室溫鐵磁現象誘發機制更豐富多元的實驗證據及方法。藉由掃描式穿透X光顯像術在碳的K邊上所量測到之X光吸收近邊結構顯示，在利用光熱法還原氧化石墨烯後所發生的室溫鐵磁轉順磁現象，與碳原子缺陷或空缺相關的2p(σ)電子軌域有很強的關連性，而非與在石墨烯表面處所形成之官能基相關的2p(π)電子軌域有關。密度泛函理論計算結果更顯示，當石墨烯中碳原子空缺所造成的局域缺陷結構發生楊-泰勒扭曲現象時，將有助於磁矩的產生，如同在氧化石墨烯中所量測到的室溫鐵磁現象。結合掃描式穿透X光顯像術與X光磁圓偏振二向性等能譜術發現，在氧化鋅奈米線中所觀測到的室溫鐵磁現象，與奈米線近表面處的氧2p軌域磁矩有很強的關連性，而非與鋅3d軌域有關。局域密度近似理論計算結果也證實，鋅原子空缺會誘發最鄰近氧原子位置上的未鍵結2p電子產生局域化磁矩，導致室溫鐵磁現象的發生。另外，磁性量測結果顯示，氧化鋅奈米線的室溫飽和磁矩強度，在經過碳離子佈值後，被大幅度的增強。藉由掃描式穿透X光顯像術及X光相關能譜術的實驗結果推測，磁矩增強的主要原因，是來自於佔據在晶格間隙中的碳原子所貢獻。 X-ray microscopic and spectroscopic techniques provide highly spatial-resolved and element-specific information of electronic and atomic structures for fundamental researches, material studies and industrial applications. Techniques include X-ray absorption near edge structure (XANES), X-ray photoelectron spectroscopy (XPS), X-ray magnetic circular dichroism (XMCD), X-ray excited optical luminescence (XEOL), extended X-ray absorption fine structure (EXAFS) and scanning transmission X-ray microscopy (STXM) provide fruitful experimental evidences which are using to unravel the mechanism of room temperature ferromagnetism in non-magnetic materials. The results of C K-edge STXM-XANES provide clear evidence that the higher number of C 2p(σ)-derived defect/vacancies states, rather than of the C 2p(π) states that are bound with oxygen-containing and/or hydroxyl groups on the graphene oxide (GO) surface, is related to the change of magnetic behavior from that of ferromagnetic GO to that of paramagnetic photo-thermal reduced GO observed from experimentally. The spin-polarized density functional theory calculations of graphene with monovacancy further support the finding that the magnetism originates in defects/vacancies, and in particular that the J-T distortion of the local defect structure is responsible for magnetic moments in GO. Results of O K-edge STXM-XANES and XMCD confirm the argument that the magnetic moments are arising from the O 2p orbitals in the surface of ZnO nanostructures rather than from Zn d orbitals. Local density approximation calculations further support the conclusion that the presence of defects and/or vacancies or dangling/unpaired 2p bonds at O sites around cation vacancy centers is responsible for room temperature ferromagnetism in ZnO. Furthermore, the C-implanted ZnO nanowires shows strongly enhanced in saturation magnetization, results of X-ray-based spectroscopy and microscopy suggest interstitial C is responsible for the enhancement.
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