新穎三價鈰與二價銪離子活化螢光體發光特性、微結構與能量轉移之研究

Abstract

本論文主要依據螢光體設計通則，探討供螢光粉轉換白光發光二極體所使用的三價鈰與二價銪活化新穎氧化物與硼酸鹽螢光材料合成、能量轉移機制與發光特性，在磷酸鹽螢光體系統中，除上述特性，我們試圖建立所使用助熔劑之計量與微結構之關係，以期能準確掌握該螢光體最適化製程參數。 本研究利用固態合成法，於氫氣還原氣氛下，成功合成了Ce3+或Eu2+分別摻雜含有下列主體Ca5Al6O14、A、Ca2BaSi3O9、La5Si2BO13、La3Si2BO10、Ba3La2(BO3)4、Ba3Y2(BO3)4、LaCa4O(BO3)3與YCa4O(BO3)3等九種螢光材料，並利用X光粉末繞射、螢光光譜、漫反射光譜與場發射電子顯微鏡進行上述材料之晶體結構、發光特性、色度學與微結構之鑑定。 本論文第一部分探討取代格位配位數為六或八之Eu2+活化Ca5Al6O14:Eu2+、A、Ca2BaSi3O9:Eu2+等螢光體，其5d軌域易受晶場作用而產生分裂，本研究發現配位數越高，發光中心5d軌域分裂越大，以致造成上述螢光體發光波長紅位移的現象。 第二部份探討同為反應物與助熔劑的碳酸鈉之添加，如何有效提升 A發光強度與改善其微結構，以期能準確掌握該螢光體最適化之製程參數。 第三部份則探討了Ce3+及Tb3+共摻La5Si2BO13與La3Si2BO10螢光體之能量轉移機制，其中Ce3+為敏化劑而Tb3+為活化劑，經由能量轉移，Tb3+在長波長放光，以彌補單摻Ce3+所導致的波段過短、激發範圍過窄之缺點，本研究發現在La3Si2BO10基質中，Ce3+→Tb3+能量轉移以偶極—偶極交互作用為主，而在La5Si2BO13中Ce3+→Tb3+能量轉移則為偶極—四極交互作用所主宰。 第四部份為RCa4O(BO3)3:Ce3+與Ba3R2(BO3)4:Ce3+(R=La，Y)等具有相異Ce3+配位環境兩系列螢光體發光特性之研究。本研究發現當主體結構相似，取代格位種類越多時，其激發與放射特徵越豐富，最佳激發與最佳放光波長亦呈現多樣化，YCa4O(BO3)3:Ce3+激發波長較LaCa4O(BO3)3:Ce3+位移大約30 nm，前者最佳發光波長由414 nm位移至後者的456 nm;另一方面，本研究發現Ba3Y2(BO3)4:Ce3+激發波長則較Ba3La2(BO3)4:Ce3+紅位移了30 nm，且波形亦有所差異。

The thesis mainly investigates the design principle, synthesis, energy transfer mechanism and the luminescence of novel Ce3+-and Eu2+-activated oxide and borate phosphors for white light emitting diodes (WLED). For the Eu2+-activated borates, other than the luminescence properties our investigations have been extended to the flux-assisted synthesis and we attempted to establish a correlation between the amount of flux used and the microstructure to obtain the optimization parameters of A. Ce3+ or Eu2+-activated new hosts with compositions of Ca5Al6O14, A, Ca2BaSi3O9, La5Si2BO13, La3Si2BO10, Ba3La2(BO3)4, Ba3Y2(BO3)4, LaCa4O(BO3)3, and YCa4O(BO3)3 were synthesized by solid state method under hydrogen reduction conditions. The structure, luminescence and chromaticity properties, and microstructure of these phosphors were characterized by X-ray power diffraction, fluorescence spectroscopy, diffuse reflectance spectroscopy, and field-emission scanning electron microscope. With emphasis and discussions on different subjects, this research is divided in four parts. Firstly, with the unique features of the inherent d-f electronic transition in Ce3+ and Eu2+, Eu2+-activated Ca5Al6O14, A, and Ca2BaSi3O9 phosphors with six or seven-coordinated lattice sites for doping were investigated. The degree of crystal field splitting for 5d orbital was found to increase with increasing number of coordination number of the doped site and the emission wavelength of Eu2+ in the above-described phosphors were found to undergo red shifting. The second part investigates the addition of Na2CO3, acting as a reactant as well as a flux, in the synthesis of A phosphors to obtain the optimized processing parameters for the preparation of A whose luminescence and grain homogeneity and morphology were found to improve appreciably. The third part investigates the mechanism of energy transfer between Ce3+ and Tb3+ in host matrices of La5Si2BO13 and La3Si2BO10. The Ce3+ acts as a sensitizer and Tb3+ as an activator. Codoping of Tb3+ was attempted to circumvent the problems such as short excitation wavelength or narrow excitation range. Based on the analysis on the experimental results, the energy transfer mechanism has been dominated to be a dipole-dipole type interaction in La3Si2BO10, and a dipole-quadrupole type in La5Si2BO13, respectively. The fourth part investigates the relationship between luminescence properties and structural diversity of the RCa4O(BO3)3: Ce3+ and Ba3R2(BO3)4:Ce3+ (R = La, Y) phosphors. The luminescence behaviors become diversified when the corresponding coordination environment of the activator becomes complicated. With similar crystal structures of hosts, we have found that the complexity of spectral features of these Ce3+-activated borate phosphors increases with increasing variety of dopant lattice sites. The □em of YCa4O(BO3)3 (□em of 414 nm) is longer than that of LaCa4O(BO3)3 (□em of 456 nm) for 40 nm and the □ex of Ba3Y2(BO3)4 is also longer than that of Ba3La2(BO3)4 for 30 nm.