Preparation and Photoluminescent Characteristics of Strontium Orthosilicate and Strontium Aluminate Phosphors

螢光材料是目前國內極力推展的重大方向，白光LED使用之螢光粉以及長餘暉螢光粉包含在其內，以下是其個別的實驗及結果摘要： 矽酸鍶摻雜銪離子 (Sr2SiO4: Eu2+) 粉體為LED所需新型且重要之螢光粉體，在UV/ Blue區間寬廣的吸收帶。本研究以固相法，固定不同成分之計量比，添加不同顆粒大小SiO2為起始物，進行高溫還原反應，發現隨著添加粒徑較小之起始物SiO2，螢光強度增加，且發光波長往長波長方向移動。當掺雜相同大小粒徑SiO2，發現鍛燒時間縮短時，造成放光波長亦會往長波長方向移動。推測是由於Eu2+離子間距離較短，造成晶場強度改變，4f65d1外層的5d電子軌域分裂，引發非輻射能量傳遞增加，輻射能量減少，造成的放射峰的紅移。 鋁酸鍶摻雜銪，鏑離子之長餘暉螢光粉體 (SrAl2O4: Eu2+, Dy3+ ) 為新一代長餘暉夜光材料，在UV/ Blue區間有寬廣的吸收帶，放射出綠光。本研究利用溶膠-凝膠法，成功以在1300oC煆燒四小時，合成出單相鋁酸鍶粉體，放射光譜呈現一寬廣的峰，最高峰位置落在524 nm，這是由於Eu2+置換Sr2+位置，經由f-d能階轉換 (4f65d1→ 4f7) 所造成。由SEM照片得知，合成出的粉體為粒徑約0.6μm，分散均勻的粉體。本研究改變溶液pH值，發現pH值在7.0的時候，粉體有最強的螢光強度，且餘暉時間亦隨著pH值增加而增長。推測是pH值改變檸檬酸與金屬離子螯合的程度所造成的差異，由平衡常數計算出的模型，可以支持這項推測。The field of luminescence material is a promising industry, and the government pays much attention on it. LED and afterglow phosphors are widely applied and were discussed in this thesis. Strontium orthosilicate (Sr2SiO4: Eu2+) is a new type phosphor. Sr2SiO4: Eu2+ provides the broadband absorption in UV/ Blue region and results in a strong yellow emission. In this study, the phosphors were synthesized via solid-state reaction by using silica with different particle sizes. It was found that the luminescent intensity increased and the red shift occurred in wavelength from 570 nm to 590 nm with using small silica. Additionally, when silica with the same particle size was used, the blue shift of emission happened with increasing reaction time. The above behavior is because the decrease of distance between Eu2+ ions leads to the changes of the crystal field around Eu2+ ions. The 4f electrons of Eu2+ are insensitive to a lattice environment due to the shielding function of the electrons in the outer shell, the 5d electrons split by the crystal field, which lead to the increase of non-radiative energy transferring between Eu2+ ions. Therefore, the radiative energy decreases and the wavelength of emission has a red shift. Strontium aluminates doped europium and dysprosium ions (SrAl2O4: Eu2+, Dy3+) phosphor is a new type of afterglow phosphor. This phosphor provides a broad band absorption in UV/ blue region, and emits green light. A sol-gel process employing citric acid and ethylene glycol as polymerizing agent was developed for synthesizing strontium aluminates phosphors. From SEM analysis, it was found the particles were dispersed well and the grain was around 0.6μm. A broad band was obtained on the emission spectrum and the peak at about 524 nm is attributed to the f-d transition of Eu2+ ions, substituting Sr2+ ions in the host. In the research, the pH value of the solution was adjusted in sol-gel method, and it was found that the highest intensity of emission was obtained at the pH value was equal to 7.0. Additionally, the lifetime increased with increasing pH values. It was proved that the extent of chelation between citrate ions and metal ions influenced by pH value through the calculation from critical stability constants.摘要 Abstract Contents……………………………………………………...………………..Ⅰ List of Figures………………………………………………………...……….Ⅳ List of Tables………………………………………………………..…………Ⅷ Chapter 1 Introduction…………………………………………………………1 1.1 Preface……………………………………………………………………1 1.2 Luminescent Materials……………………………………………………2 1.2.1 Types of Luminescence………………………………………………3 1.2.2 Mechanisms of Luminescence……………………………………….3 1.2.3 Application of Phosphors…………………………………………...5 1.2.4 White Light LED Phosphor…………………………………………5 1.2.5 Afterglow Phosphor…………………………………………………6 1.3 Luminescence Theory…………………………………………………….9 1.3.1 Symbols of Quantum Numbers…………………………………….10 1.3.2 Spin-Orbit Coupling and j-j Coupling………………….………….11 1.3.3 Radiative and Non-radiative Transitions……………….………….12 1.3.4 Hund’s Rules and Selection Rules………………………………….13 1.3.5 Crystal-Field Theory and Stark Splits……………………………..15 1.3.6 The Rare Earth Ions………………………………………………..16 1.3.7 4fn-15d1states and Charge Transfer States………………………….16 1.3.8 Emission Characteristics of Eu2+ Ions-the f-d Transition…………17 1.3.9 Emission Characteristics of Eu3+ Ions-the f-f transition……….….18 1.4 Introduction to Strontium Orthosilicate…………………………………19 1.4.1 Structure of Strontium Orthothilicate……………………………...19 1.4.2 Spectroscopy of Sr2SiO4:Eu2+……………………………………...20 1.5 Introduction of Strontium Aluminates……………………………………..21 1.5.1 Structure of Strontium Aluminates…………………..……………..21 1.5.2 Dopant and Codopant Sites…………………………………..……22 1.5.3 Probable Vacancies………………………………………….…….23 1.5.4 Luminescence Characteristics………………………………..……23 1.6 Research Objective………………………………………………………..26 Reference…………………………………………………………...…………38 Chapter 2 Investigation on Fluorescence properties of Sr2SiO4:Eu2+ phosphor………………………………………………………...….42 2.1 Introduction…………………………………………………………..…42 2.2 Experiment…………………………………………………………..…..43 2.2.1 Synthesis of Sr2SiO4:Eu2+ via the Solid-State Method………..……43 2.2.2 Analysis Technique…………………………………………...……44 2.3 Investigation of the luminescence properties of Sr2SiO4:Eu2+……...…..44 2.4 Results and Discussion………………………………………………….45 2.5 Conclusions………………………………………………………….…..48 Reference…………………………………………………………...…………57 Chapter 3 Investigation on phosphorescence properties of SrAl2O4 afterglow phosphor……………………………………………………………60 3.1 Introduction……………………………………………………………..60 3.2 Luminescence studies of SrAl2O4:Eu2+, Dy3+ synthesized at different pH value………………………………………………………........…..62 3.3 Experimental………………………………………………………...…..65 3.3.1 Synthesis of SrAl2O4:Eu2+, Dy3+ via the Sol-Gel Method…………...65 3.3.2 Analysis Technique……………………………………...…………..66 3.4 Results and Discussion………………………………………………….66 3.5 Conclusions………………………………………………………...……69 Reference……………………………………………………….……………..81 Chapter 4 Conclusions………………………………………………………..83 List of Figures Figure 1.1 Schematic diagram of radiative transitions between the conduction band (Ec), the valence band (Ev) and excition (EE), donor (ED) and acceptor (EA) levels in a semiconductor…………………………....30 Figure 1.2 Applications of phosphors under different excitation sources….…30 Figure 1.3 The structure of GaN-based white LED……………………….…..31 Figure 1.4 The schematic of optical centers in the host lattice with corresponding energy level scheme……………………………….31 Figure 1.5 Radiative and non-radiative transitions explained by the configurational coordinate model…………………...……………..32 Figure 1.6 The splitting of the energy levels due to coulomb repulsion, spin-orbit coupling and crystal field splitting. Crystal field numbers are for Y2O3: Eu3+………………….……………………………….32 Figure 1.7 Energy levels of 4f n configurations of trivalent lanthanide ions….33 Figure 1.8 Energies for 4f→5d and CTS transitions of trivalent rare-earth ions………………………………………………………………….33 Figure 1.9 4fn energy levels scheme of the free divalent lanthanides. Solid curve (a) connects the locations of the lowest energy fd-level given by EAfree(n, 2+). Dashed curve (b) represents for n>7 the values Esa (n, 2+)…………………………………………………………….……34 Figure 1.10 Schematic diagram of the energies of 4f 7 and 4f 65d1 levels in Eu2+ influenced by crystal field Δ…………………………………….…..34 Figure 1.11 Emission spectrum of Eu3+ in NaLuO2 and NaGdO2. While 5D0-7F1 transition dominates in the previous one, and 5D0-7F2 transition dominates in the later…………………………………………..…..35 Figure 1.12 Phase diagram for system SrO-SiO2…………………………..…35 Figure 1.13 Crystal structure of (a) β-Sr2SiO4 and (b) α'-Sr2SiO4, projected along [001]……………………………………………………...….36 Figure 1.14 Unit cell of the α'-Sr2SiO4 compound…………………………….36 Figure 1.15 Schematic views of the monoclinic phase of SrAl2O4 along the a- and c-directions………………………………………………...…..37 Figure 1.16 Deconvolution of the 520 and 450 nm fluorescence peaks of Sr0.98Al2O4: Eu2+0.02, measured at the 310 nm excitation light at 100 K using a Fluorolog-3 spectrofluorometer……………………..…..37 Figure 2.1 Photoluminescence spectra excited at 460 nm of Sr1.95Eu0.05SiO4 prepared from silica with different sizes via solid-state process at 1250oC for 2 h…………………………………………………..…49 Figure 2.2 X-ray diffraction patterns of Sr2SiO4: Eu2+ phosphors prepared from silica with different sizes via solid-state route at 1250oC for 4 h….50 Figure 2.3 A microscopic reaction model of Sr2SiO4: Eu2+ for the beginning of the reaction………………............................................................…51 Figure 2.4 A microscopic reaction model of Sr2SiO4: Eu2+ shows the relation between reaction time, particle size and concentration of Eu2+…...52 Figure 2.5 Photoluminescence spectra excited at 460 nm of Sr1.95Eu0.05SiO4 prepared with 90 nm silica via solid-state process at 1250oC for various heating times…………………………………………….....53 Figure 2.6 Relation between luminescence wavelength of Sr1.95Eu0.05SiO4 phosphors and heating time at 1250oC………………………….….54 Figure 2.7 Relation between luminescence intensity of Sr1.95Eu0.05SiO4 phosphors and heating time at 1250oC……………………………..55 Figure 2.8 SEM images of Sr1.95Eu0.05SiO4 specimens prepared by solid-state route from silica with different sizes at 1250oC for 10h. (a) 10 nm, (b) 90 nm and (c) 120 nm…………………………………………...…..56 Figure 2.9 SEM images of Sr1.95Eu0.05SiO4 specimens prepared by solid-state route with 10 nm silica at 1250oC for (a) 6 h and (b) 10 h………....57 Figure3.1 The evaluated concentration of strontium species in the solution. The molar ratio of citric acid to the total moles of cations is 8:1……....70 Figure3.2 The evaluated concentration of Aluminum species in the solution. The molar ratio of citric acid to the total moles of cations is 8:1…..71 Figure3.3 The evaluated concentration of europium species in the solution. The molar ratio of citric acid to the total moles of cations is 8:1………72 Figure3.4 The evaluated concentration of dysprosium species in the solution. The molar ratio of citric acid to the total moles of cations is 8:1….73 Figure 3.5 Flowchart of synthesis procedure of the sol-gel method employing citric acid and ethylene glycol…………………………………..….74 Figure 3.6 The mechanism of the sol-gel route……………………….……….75 Figure3.7 X-ray diffraction patterns of SrAl2O4: Eu2+, Dy3+ prepared at different pH values………………………………………………….76 Figure 3.8 Photoluminescence spectra excited at 325nm of Sr0.97Eu0.01Dy0.02Al2O4 prepared by sol-gel process with different pH value…………………………………………………………..…….77 Figure 3.9 Decay curves of Sr0.97Eu0.01Dy0.02Al2O4 prepared by sol-gel process with different pH values………………………………………...……78 Figure 3.10 The relation between time constants and pH values of Sr0.97Eu0.01Dy0.02Al2O4 prepared by sol-gel process……………...…79 Figure 3.11 SEM images of Sr0.97Eu0.01Dy0.02Al2O4 prepared by sol-gel method with different pH value of the solution (a) 1.0, (b) 3.0, (c) 4.0, (d) 5.0, (e) 7.0 and (f) 9.0…………………………………………………...80 List of Tables Table 1. Emission Colors of the Phosphors with Phosphorescence Lasting Longer than 1 h………………………………………………..……27 Table 1.2 Spectroscopic terms and calculation methods………………..…….28 Table 1.3 Crystallographic information of α'-Sr2SiO4 and β-Sr2SiO4……..….2