白光發光二極體之新穎螢光材料製備及螢光特性分析

Abstract

為了解決UV-LED 螢光粉體低熱穩定性，激發位置波長太短(<350 nm)，以及發光顏色可調性差等缺點，本研究利用三項理論進行螢光粉體特性之改善，分別為電子雲擴散效應(共價效應)、晶場理論，以及能量轉移機制。此外由於熱穩定性與螢光粉主體之共價性具有高度相關性，論文中依照螢光粉主體共價性的不同，製備氧化物、氮氧化物、氮化物以及氮碳化物四大類。 論文首先選擇鈰酸鍶(Sr2CeO4)氧化物螢光粉體，激發光譜中顯示一寬廣峰，為Ce4+-O2-之價荷轉移所造成，由於最高峰落在296 nm，為了增加UV-LED 之使用性，添加錫離子(Sn4+)進行材料改質，最高的激發峰波長會由296 nm 移動至346 nm，且隨Sn4+添加量增加，346 nm 位置之吸收也隨之增強，大幅增加螢光粉體在UV-LED 之應用，在346 nm 激發下，粉體發射出483 nm 之藍綠光，添加Sn4+可增加39%之粉體放光強度。 為了調整螢光粉體色光，第二部份製備氧化物主體，MgY4Si3O13: Ce3+,Mn2+，文中探討Ce3+及Mn2+之間的能量轉移機制，隨著Mn2+濃度增加，Ce3+所產生的藍光強度下降而Mn2+產生的橘紅光強度增加，此能量轉移使得粉體的發光顏色由藍光轉移為白光，最後再轉移為橘紅光，因此利用能量轉移機制可大幅度調整粉體之發光顏色。 為了提升粉體的熱穩定性，第三部份選擇高共價性之氮氧化物 SrSi2O2N2 為主體材料，僅添加Ce3+之SrSi2O2N2 螢光粉體產生447 nm 之藍光，共添加Tb3+會造成能量轉移，隨著Tb3+濃度的增加，造成放光顏色的改變，由藍光經水藍光再移動至綠光，另一方面，本研究第一次報導此材料在真空紫外光(VUV)之光學特性，發現分別添加Ce3+及Tb3+之粉體在真空紫外光區皆有良好的吸收，本章節亦針對粉體的熱穩定性進行研究，隨著溫度增加，粉體螢光強度下降，此乃由於熱淬滅效應所致，但由於氮氧化物之高共價性，此螢光粉具有優異之熱穩定性。 然而添加Ce3+及Tb3+之SrSi2O2N2 螢光材料之吸收波長小於350 nm，不利UV-LED 之使用，因此論文第四部份選擇CaSi2O2N2 氮氧化物為主體，僅添加Ce3+及Eu2+之CaSi2O2N2 螢光材料分別具有470 nm 及550 nm之放光，Ce3+及Eu2+共添加之粉體隨著Eu2+添加量增加，激發光譜強度增加且最高峰位置由330 往352 nm 移動，增加UV-LED 適用性，且Ce3+之藍光強度下降而Eu2+之黃光增加，發光顏色可由藍光移動至黃綠光。 為了持續提高粉體之熱穩定性，第五部份選擇共價性更高之氮化物(CeSi3N5)及氮碳化物(Y2Si4N6C)作為主體材料，CeSi3N5 螢光粉體之激發光譜最高峰位置在353 nm，在UV 波段激發下可獲得453 nm 之藍光，當添加Tb3+進入主體晶格，產生能量轉移現象，發光顏色由藍光往藍綠光方向移動。Y2Si4N6C: Ce3+螢光粉體之激發光譜在384 和429 nm 有最佳之吸收，在UV 激發下產生538 nm 之黃綠光，添加La3+離子取代主體中Y3+離子位置可大幅改變發光中心(Ce3+)周圍之晶場，吸收位置及發光波長往短波長移動，放光顏色由黃綠光移動至藍光，因此可透過適當La3+的添加，調整粉體之發光顏色，此外由於碳氮化物之高共價性，Y2Si4N6C: Ce3+螢光粉體具有優異之熱穩定性。 論文針對四大類(氧化物、氮氧化物、氮化物、氮碳化物)六種螢光粉體進行結構及光學特性分析，利用電子雲擴散效應(共價效應)、晶場理論，以及能量轉移機制，提升粉體熱穩定性，調整粉體最佳激發波長使大於350 nm，以及增加粉體放光顏色之可調性，並進而建立粉體吸放光位置調整規則，增加螢光粉體在UV-LED 之實用性。

To overcome the drawbacks of the phosphors for UV-LEDs, such as low thermal stability, short excitation wavelength (< 350 nm) and poor color tunability, the nephelauxetic (covalency) theory, crystal-field theory and energy transfer mechanisms are applied to improve the luminescent properties. In addition, it is known that the thermal stability of the phosphors strongly depends on the covalency of the host materials. In this study, four types of phosphors (oxides, oxynitrides, nitrides and carbonitrides) with increased covalency were prepared, and the luminescent properties were investigated. Sr2CeO4 phosphors have broad excitation bands in the range of 200 to 450 nm. Sn4+ ions were doped in the host materials to shift the highest excitation peak from 296 to 346 nm. Upon the excitation at around 346 nm, the intensity of the blue emission peak at 483 nm was enhanced 39% as compared to that of undoped Sr2CeO4 phosphors. In the second section, the energy transfer mechanism of MgY4Si3O13:Ce3+, Mn2+ phosphors from Ce3+ to Mn2+ was revealed to be dipole-quadrupole interaction. As the Mn2+ concentration increased, the Mn2+ emission intensity increased and the Ce3+ emission intensity decreased. This resulted in shifting the chromaticity coordinates of the prepared phosphors from the blue, white to orange region. In the third section, the oxynitride-based SrSi2O2N2 phosphors with high covalency were selected as the host materials. As Ce3+ and Tb3+ ions were co-doped into SrSi2O2N2, the energy transfer process occurred. With increasing the Tb3+ concentration, the emitting colors of SrSi2O2N2: Ce3+, Tb3+ phosphors shifted from the blue towards green region. The increased temperatures caused the reduction of the emission intensity of the prepared phosphors due to the thermal quenching effects. It is found the prepared SrSi2O2N2-based phosphors have excellent thermal stability. In the fourth section, oxynitride-based phosphors CaSi2O2N2 were selected as the host materials. Increasing the Eu2+ concentration of the Ce3+ and Eu2+-codoped phosphors led the excitation wavelength to shift from 330 to 352 nm and the excitation intensity to increase. In addition, the Eu2+ emission (550 nm) intensity increased and Ce3+ emission (470 nm) intensity decreased, leading the emitting colors of the prepared phosphors to shift from the blue to yellowish green region. In the fifth section, the nitridosilicate (CeSi3N5) and carbonitride (Y2Si4N6C) phosphors with high covalency were selected as the host materials. For CeSi3N5 phosphors, the wavelength of the maximum excitation peak was at 353 nm. The emission spectrum exhibited an intense blue emission at 453 nm. When Tb3+ ions were doped, the emitting colors shifted from the blue to greenish blue. For Ce3+-doped Y2Si4N6C phosphors, the incorporation of La3+ ions led the emitting colors of (Y, La)2Si4N6C: Ce3+ phosphors to vary from the yellowish green to blue region. The blue shift in emission bands was due to the variation in the crystal-field strength around the activators. In addition, the excellent thermal stability of Y2Si4N6C-based phosphors was revealed.