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系統識別號 U0002-1301201720303100
中文論文名稱 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
英文論文名稱 Defect Induced Magnetism in Nano-materials Studied by X-ray-based Spectroscopic and Microscopic Techniques
校院名稱 淡江大學
系所名稱(中) 物理學系博士班
系所名稱(英) Department of Physics
學年度 105
學期 1
出版年 106
研究生中文姓名 王玉富
研究生英文姓名 Yu-Fu Wang
學號 801210013
學位類別 博士
語文別 英文
口試日期 2017-01-13
論文頁數 200頁
口試委員 指導教授-彭維鋒
委員-薛宏中
委員-陳家浩
委員-董崇禮
委員-邱昭文
中文關鍵字 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 
英文關鍵字 STXM  XMCD  XANES  EXAFS  XPS  XEOL 
學科別分類
中文摘要 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。
英文摘要 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.
論文目次 Acknowledgement ................................................................................... i
Abstract ................................................................................................... ii
Table of Contents .................................................................................... v
List of Figures ....................................................................................... vii
Introduction
Defect Induced Magnetism ........................................................ 1
Need for X-ray Microscopic and Spectroscopic Techniques .................................................................................. 2
Study the Mechanism of Room Temperature Ferromagnetism in Non-magnetic Materials ............................................................. 4
Experimental Techniques
Synchrotron Radiation ................................................................ 7
X-ray Absorption Spectroscopy (XAS) ..................................... 9
X-ray Photoelectron Spectroscopy (XPS) ................................ 12
X-ray Magnetic Circular Dichroism (XMCD) ......................... 14
X-ray Excited Optical Luminescence (XEOL) ........................ 16
Scanning Transmission X-ray Microscopy (STXM) ............... 19
DIM in Graphene Oxides
Introduction .............................................................................. 22
Experimental ............................................................................ 25
Results and Discussion ............................................................. 27
Conclusion ................................................................................ 61
DIM in Zinc Oxides
Introduction .............................................................................. 62
Experimental ............................................................................ 65
Results and Discussion ............................................................. 67
Conclusion .............................................................................. 101
DIM in Carbon implanted Zinc Oxide
Introduction ............................................................................ 102
Experimental .......................................................................... 106
Results and Discussion ........................................................... 109
Conclusion .............................................................................. 155
Summary ......................................................................... 157
Reference ............................................................................................. 159
Appendix ............................................................................................. 193
List of Figures
Figure 2-1 Transitions of X-ray absorption edges. .......................... 10
Figure 2-2 Regions of XAS data. .................................................... 11
Figure 2-3 Schematic of XPS process. ............................................ 14
Figure 2-4 (a) Conventional L-edge XANES, and (b,c) XMCD, illustrated in a one-electron model. The transitions occur from the spin-orbit split 2p core shell to empty conduction band states. In conventional x-ray absorption the total transition intensity of the two peaks is proportional to the number of d holes (first sum rule). By use of circularly polarized x-rays the (b) spin moment and (c) orbital moment can be determined from linear combinations of the dichroic difference intensities A and B, according to other sum rules. ........................... 17
Figure 2-5 Schematic of the K- and L3,2-edge XEOL processes in matter. ............................................................................................... 18
Figure 2-6 Schematic diagram of STXM. ....................................... 21
Figure 3-1 Field Emission SEM images of (a) GO, (b) M-rGO and (c) H-rGO. TEM images of (d) GO, (e) M-rGO and (f) H-rGO. (g) Raman spectra of GO, M-rGO and H-rGO; inset magnifies Raman spectrum of H-rGO. .............................................................................................. 28
Figure 3-2 (a) PL spectra of GO, M-rGO and H-rGO. (b) Room-temperature [M(H)/M(1T)]-H curves of GO, M-rGO and H-rGO after subtraction of diamagnetic background that arises from silicon substrate. Inset in Fig. (b) plots M-H curves (without background subtraction) of GO, M-rGO and H-rGO. .......................................... 30
Figure 3-3 OD images and corresponding stack mapping from STXM images of GO, M-rGO and H-rGO are shown in panels I and II. Panels III-VI present stack mappings from C K-edge STXM images of GO, M-rGO and H-rGO, which are decomposed into blue, yellow, red and green regions that are associated with the different thicknesses of samples. Spectra of all samples typically present background (blue), flat (yellow), medium (red) and wrinkle (green) regions. ...................................... 34
Figure 3-4 C K-edge STXM-XANES spectra of GO, M-rGO and H-rGO at (a) flat and (b) wrinkle region respectively. These are the sums of XANES spectra of yellow and green regions of flat and wrinkle regions in panels IV and VI of Fig. 2. Insets magnify 284–290 eV region of STXM-XANES spectra of flat and wrinkle regions. ................... 37
Figure 3-5 VB-PES spectra of GO, M-rGO and H-rGO with HOPG as reference. Inset magnifies the rising edges of VB-PES spectra. Lower panel displays difference between VB-PES of M-rGO and H-rGO and that of GO. ........................................................................................ 44
Figure 3-6 C K-edge XANES spectra of GO with photo-helicity of incident X-rays parallel (μ+) and anti-parallel (μ−) to direction of magnetization, respectively. Inset magnifies π-σ (σ) region of C K-edge XANES spectra with incident X-rays μ+ and μ− to direction of magnetization. Bottom panel presents C K-edge XMCD spectrum of GO. ................................................................................................... 47
Figure 3-7 O K-edge XANES spectra of GO with photo-helicity of incident X-rays parallel (μ+) and anti-parallel (μ−) to direction of magnetization, respectively. Inset magnifies π-σ (σ) region of O K-edge XANES spectra with incident X-rays μ+ and μ− to direction of magnetization. Bottom panel presents C K-edge XMCD spectrum of GO. ................................................................................................... 50
Figure 3-8 The local (a) symmetric structure and (b) J-T defect structure around a single vacancy in graphene with corresponding spin-density projections are shown. Local distortions and corresponding C-C distances (in Å) of carbon triangles that surround vacancy centers are highlighted. ....................................................................................... 55
Figure 3-9 Spin-polarized electronic band structures of (a) symmetric and (b) J-T defect structures are also illustrated. Majority and minority spins are indicated by blue and red curves, respectively. Band structures of pristine graphene are denoted as black curves for reference. Fermi level (EF), indicated as a dashed line, is set to 0 eV for alignment. ......................................................................................... 57
Figure 3-10 Total DOS, spin-polarized PDOS, and spin density of symmetric structure. Contributions from π and σ electrons in defect (pristine) structure are represented in blue (black) and red (green) curves, respectively. Fermi level (EF) is set to 0 eV for alignment. ............. 59
Figure 3-11 Total DOS, spin-polarized PDOS, and spin density of J-T defect structure. Contributions from π and σ electrons in defect (pristine) structure are represented in blue (black) and red (green) curves, respectively. Fermi level (EF) is set to 0 eV for alignment. ............. 60
Figure 4-1 Fourier-transformed k3χ data of Zn K-edge EXAFS measurements from k = 3.0 to 13.0 Å (not shown here). The insets show top-view and cross-sectional SEM images and the XRD patterns of ZnO NC, NW and powder samples. ......................................................... 68
Figure 4-2 PL spectra and M-H plots of ZnO NCs and NWs. Insets I and II present the magnetic field applied parallel to the growth direction (c axis) and the magnified M-H loops of ZnO NCs and NWs, respectively, at 300 K. ........................................................................................... 71
Figure 4-3 (a and b) O K-edge XEOL spectra of ZnO NCs and NWs, respectively, under excitation at various energies in the range of 520-580 eV. (c and d) Zn L3,2-edge XEOL spectra of ZnO NCs and NWs, respectively, under excitation at various energies in the range of 1010–1050 eV. ................................................................................. 73
Figure 4-4 Schematic of the O K-edge and Zn L3,2-edge XEOL processes in ZnO nanostructures. ..................................................................... 75
Figure 4-5 (a and b) O K-edge STXM and the corresponding XANES spectra of ZnO NCs and NWs, respectively. The top left panels in (a) and (b) present stacked images of the target region with different thickness indices (blue, green and red), which correspond to the maps of the thick, thin and impurity/edge regions in the left-hand sides of the lower panels, respectively. The right-hand sides of (a) and (b) present the sums of the XANES spectra of the differently colored (blue, green and red) regions. ............................................................................................. 78
Figure 4-6 Comparison of the O K-edge STXM-XANES spectra of the thick, thin and impurity/edge regions of ZnO NCs with those of the NWs. ................................................................................................. 82
Figure 4-7 (a)-(d) O K-edge STXM images of two selected regions in ZnO NCs (NC-1 and NC-2) and NWs (NW-1 and NW-2), respectively. ...................................................................................... 85
Figure 4-8 O K-edge STXM-XANES spectra of regions bordered by yellow dashed lines in ZnO NCs and NWs. The insets show the corresponding SEM images. ............................................................. 86
Figure 4-9 Spatially resolved valence-band SPEM spectra of the ZnO NC and NW, obtained from three selected areas (A, B and C). The solid lines that smoothly fit the SPEM spectra of the ZnO NC were guided by eye. Inset shows Zn 3d SPEM images of cross-sectional views of ZnO NC and NW, respectively. ............................................................................. 88
Figure 4-10 Normalized O K-edge XANES spectra with the photon helicity of the incident X-rays parallel (μ+) and anti-parallel (μ-) to the direction of magnetization for ZnO NCs and ZnO NWs. The insets display the magnified O K-edge XANES spectra and the magnified O K-edge XMCD spectra of ZnO NCs and ZnO NWs, respectively. ...................................................................................... 91
Figure 4-11 Normalized Zn L3,2-edge XANES spectra with the photon helicity of the incident X-rays parallel (μ+) and anti-parallel (μ-) to the direction of magnetization for ZnO NCs and ZnO NWs. The insets display the magnified Zn L3,2-edge XMCD spectra of ZnO NCs and ZnO NWs, respectively. ............................................................................ 92
Figure 4-12 Total electronic density of states (TDOS, in top panel) and partial density of states (PDOS) of Zn 3d, Zn 3p and Zn 4s (middle panel) and O 2p (bottom panel) of bulk wurtzite ZnO with oxygen anion vacancies (VO) (Zn36O35, indicated by solid lines). TDOS of defect-free bulk ZnO (Zn36O36, indicated by dotted lines) is also presented in top panel for comparison. EF (denoted as a dashed line) is aligned to 0 eV. Notably, VO in Zn36O35 does not contribute to any local net spin moment. ............................................................................................ 94
Figure 4-13 TDOS (top panel) and PDOSs of Zn 3d (middle panel) and O 2p (bottom panel) of wurtzite ZnO with VZn (Zn35O36). The TDOS of defect-free bulk ZnO (Zn36O36, indicated by dotted lines) is also shown in the top panel for comparison. EF, denoted by the dashed line, is aligned to 0 eV. ................................................................................................. 95
Figure 4-14 Jahn–Teller distortion of the native defect VZn in a 72-atom supercell of wurtzite ZnO. The net spin density (defined as the difference between the majority spin density and the minority spin density) is indicated by yellow isosurfaces near the defect center. Zn and O atoms are depicted as blue and red spheres, respectively. .......................... 97
Figure 4-15 TDOS obtained from nonmagnetic (LDA + U, black curve) and ferromagnetic (LSDA + U, blue and red curves) calculations, respectively. EF denoted as the dashed line is aligned to 0 eV. ........ 99
Figure 5-1 PL spectra and cross-sectional SEM images of ZnO-C:NW and ZnO-NW. The insets show the magnified PL spectra of ZnO-C:NW and ZnO NW in defects transition region. ...................................... 110
Figure 5-2 The room temperature M-H curves of the ZnO-C:NW and ZnO-NW. Insets present the magnetic field applied parallel to the growth direction (c axis) and the magnified M-H loops of ZnO-C:NW and ZnO-NW at 300 K. ......................................................................... 112
Figure 5-3 RRBS spectra recorded at ~4.27 MeV energy (α-particles) for verifying the elemental ratio depth profile in ZnO-C:NW and ZnO-NW. The elements include in the spectra are C (~200 keV), O (~350 keV), Zn (~750 keV) and Sn (~850 keV) atoms. ........................................... 114
Figure 5-4 The elemental ratio depth profile of ZnO-C:NW and ZnO-NW. ........................................................................................ 116
Figure 5-5 The FT spectra of Zn K-edge EXAFS of ZnO-C:NW and ZnO-NW, and their corresponding k3χ data (lower insets). The upper insets show the angles θ between surface normal and the direction of incident X-ray are selected to obtain the local atomic structure in specific regions, θ= 800 and 00 for primarily probing surface- and bulk-depth regions, respectively. ...................................................................... 117
Figure 5-6 (a)-(d) Inverse FT spectra and (e)-(h) phase-derived analysis of NN Zn-O and NN Zn-Zn bonds in near-surface (θ= 80o) and bulk-depth (θ= 0o) region of ZnO-NW and ZnO-C:NW. ............... 119
Figure 5-7 Two-dimensional atomic structure of surface and bulk regions of ZnO-NW and ZnO-C:NW. ........................................................ 123
Figure 5-8 The core-level XPS spectra of C, O 1s and Zn 3d states along with their de-convoluted bonding states of ZnO-C:NW and ZnO-NW, for obtaining the binding states on implanted C atoms in ZnO-C:NW. .................................................................................... 124
Figure 5-9 The (a) O K-edge and (b) Zn L3,2-edge XANES and XMCD spectra of ZnO-C:NW and ZnO-NW. ............................................ 128
Figure 5-10 The valence-band DOSs at/below the EVBM or EF of ZnO-C:NW and ZnO-NW. ............................................................. 131
Figure 5-11 The C K-edge XANES and XMCD spectra of ZnO-C:NW. .................................................................................... 134
Figure 5-12 Optical density images (panel I), O K-edge STXM stack mapping (panel II) and decomposed STXM mapping (panels III-V) of randomly selected regions of ZnO-C:NW and ZnO-NW. The stack mappings (panel II) are decomposed into impurity (panel III), thin (panel IV) and thick (panel V) regions. ..................................................... 138
Figure 5-13 The O K-edge STXM-XANES spectrum of impurities/surface-depth and thick/bulk-depth regions of ZnO-C:NW and ZnO-NW. ........................................................................................ 140
Figure 5-14 Wurtzite ZnO crystal structure in the configuration of (a) bulk and (b) surface with a [112 ̅0] surface orientation. Zn and O atoms are represented as large-grey and small-red, respectively. ............. 144
Figure 5-15 Spin-resolved total DOS (TDOS) of (a) vacancy-free bulk ZnOB, (b) VZnB and (c) CZnB. .......................................................... 145
Figure 5-16 The spin density of interstitial-vacancy complex (Ci +VZn) in (a) bulk and (b) surface regions. The net spin-up/spin-down density is indicated by yellow/cyan isosurfaces near defect centers. Zn, O and C atoms are depicted as large-grey, small-red, and middle-blue spheres, respectively. .................................................................................... 148
Figure 5-17 Spin-resolved total DOS (TDOS) of (a) defect complex CiB+VZnB, (b) individual CiB(2p) and (c) ONNB(2p) of individual VZnB. ............................................................................................... 149
Figure 5-18 Spin-resolved total DOS (TDOS) of (a) vacancy-free ZnOS, (b) VZnS and (c) CZnS in the [112 ̅0] surface. ................................. 151
Figure 5-19 Spin-resolved total DOS (TDOS) of (a) defect complex CiS+VZnS, (b) individual CiS(2p) and (c) ONNS(2p) of individual VZnS in the [112 ̅0] surface. .............................................................................. 153
Fig. 5-20 (color online) Calculated formation energy of vacancy (VZn), interstitial-vacancy complex (Ci+VZn), C-substitution (CZn) in the bulk (solid lines) and surface (dashed lines) at different experimental growth conditions (Zn-rich and Zn-poor). The formation of C interstitial (Ci) defect in bulk region is denoted as a green dot-dashed line for reference. ........................................................................................ 154

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