量子剪裁螢光材料之設計、發光特性研究與應用

Abstract

近年來由於電漿平面顯示器與無汞照明的綠色產品需求與日俱增，因此其相關且可供真空紫外波段激發螢光體之需求亦漸趨殷切，以電漿態鈍氣取代傳統汞蒸氣為激發光源，前者通常效率低落。因此，若能利用多電子躍遷與光子能量轉換原理，發展效率高於100%的量子剪裁螢光材料則可以有效克服低激發效率的問題。 本論文主要利用同步輻射真空紫外光源探討K2GdF5:Tb3+、K0.265Gd0.735F2.47:Tb3+、K2GdF5:Eu3+,Pr3+等三種氟化物和Gd(PO3)3:Tb3+磷酸鹽之量子剪裁現象並探討其相關機制，本研究發現並證實Tb3+-Gd3+組合可藉由上述系統中能量傳遞而產生量子剪裁效應。我們並依據光譜數據，推論上述四種量子剪裁螢光體中可能的能量傳遞機制，並發現進一步經由量子剪裁的效應可以獲得較強的Tb3+ 5D4 → 7FJ綠光放射之成因，其中以172 nm波長激發K2GdF5 : Tb3+ (23%)時可獲得193%量子效率，以181 nm波長激發K0.265Gd0.735F2.47 : Tb3+ (50%)時可獲得192%量子效率，而以217 nm波長激發Gd(PO3)3 : Tb3+ (19%) 時，則可獲得193%量子效率。 此外，在氟化物K2GdF5 : Eu3+, Pr3+中，Gd3+-Eu3+-Pr3+組合改善了僅含Gd3+-Eu3+組合時，在真空紫外光譜中貧乏的吸收，而將原本由Gd3+的f → f電子遷移所對應之能量吸收，藉由Pr3+之共摻，將其餘4f→4f5d電子遷移所吸收能量傳遞至Eu3+，並發現除Gd3+-Eu3+系統中原有源自於Eu3+ 5D0能階的放射強度顯著增加之外，源自於5D1能階之發光亦隨之增強，導致藉由1% Pr3+的共摻，K2GdF5 : Eu3+ (5%)量子剪裁效率，可由107% 顯著提升至138%。

In recent years, the development trend of plasma display panel (PDP) and mercury-free lighting have been directed toward green and environmental friendly products. Accordingly, the demand for developing new phosphors suitable for vacuum ultraviolet (VUV) excitation is becoming increasing intense. In general, the excitation efficiency becomes lower when mercury vapor is replaced by noble gases as the excitation source. The problem of low excitation efficiency with VUV radiation can be effectively circumvented by developing quantum-cutting (QC) phosphors with luminescence efficiency higher than 100% through multiphoton and downconversion processes. In this research, by using synchrotron radiation we have investigated and demonstrated that K2GdF5:Tb3+, K0.265Gd0.735F2.47:Tb3+, and K2GdF5:Eu3+,Pr3+ and Gd(PO3)3:Tb3+ phosphors exhibit quantum cutting behavior through the Tb3+-Gd3+ couple via a downconversion mechanism. We have also discovered and demonstrated that the Tb3+-Gd3+couple can act as a quantum cutter in energy transfer and QC processes. Based on the analysis of experimental PL (photoluminescence) and PL excitation (PLE) spectra, we have proposed plausible mechanisms for the rationalization of excitation, emission, and energy transfer in QC processes. Furthermore, the significant enhancement of the green luminescence attributed to Tb3+5 D4→7FJ transition can be rationalized by using QC processes depicted in the proposed schematic QC energy level diagram. The theoretical quantum efficiency (QE) was calculated to be 193%, 192%, and 193% for K2GdF5:Tb3+(23%), K0.265Gd0.735F2.47:Tb3+(25%), and Gd(PO3)3:Tb3+(19%) when excited at 172, 181, and 217 nm, respectively. Moreover, the QC phosphor K2GdF5:Eu3+ shows poor optical absorption and a theoretical QE 107 % in the UVand VUV excitation spectral ranges. Codopant Pr3+ was demonstrated to act as a sensitizer in K2GdF5:Eu3+,Pr3+, thus increasing the absorption in the UV and VUV spectral regions; the theoretical QE of K2GdF5:Eu3+, Pr3+ was increased to 138 %. Our research indicates that the possible mechanisms of QC and energy transfer differ from those of phosphors containing the Gd3+-Eu3+ couple. Temporally resolved measurements of fluorescence decay confirm the proposed QC mechanism for the phosphor containing the Gd3+-Eu3+-Pr3+ system.