利用同步輻射光源探討非氟化物量子剪裁螢光體之發光特性與機制

Abstract

近年來由於環保意識高漲，日常生活中所需的燈管及顯示器皆趨向省電及無汞之方向發展，綠色照明與平面顯示器通常以鈍氣取代傳統汞蒸氣作為激發光源，但效率不高，故以真空紫外波段激發螢光體以達高效率放光為目前螢光體研究之趨勢，因此，若能利用量子剪裁原理，即一個光子剪裁成兩低能光子之原理，則其量子效率可達100%以上，即可以克服此效率不高之問題。本論文主要利用同步輻射中心之真空紫外光源設備，探討包含Gd4O3F6:Eu3+、Gd4O3F6:Tb3+、GdOF:Tb3+與Ba3Gd(PO4)3:Tb3+四種氟氧化物與磷酸鹽等化合物，量子剪裁現象及其發光機制，文獻顯示大部分量子剪裁現象皆發生於氟化物，但其合成不方便且易吸水潮解，為了改善此現象故本論文之主要以氟氧化物及氧化物為主，探討組合包含Gd3+-Eu3+與Gd3+-Tb3+之發光機制，以下為本論文所估計理論量子效率：以247 nm激發Gd4O3F6:Eu3+ (7%)量子效率可達177%，發射橘紅光；以226 nm激發Gd4O3F6:Tb3+ (15%)發射綠光，量子效率約為170%；以237 nm激發GdOF: Tb3+ (5%)之量子效率約為168%；而以193 nm及225 nm分別激發Ba3Gd(PO4)3:Tb3+ (13%)時，兩者最佳量子效率皆約為132%。

Recently, the world is paying more and more attention about environmental consciousness and the research and development of luminescent materials have been directed toward green and environmental friendly products. Hg-free fluorescence lamp can be made by replacing Hg by noble gas, such as Xe, which has also been used for plasma display panel (PDP). These applications initiate a challenge in the field of luminescence of lanthanide ions excitable in the vacuum ultraviolet (VUV) (λ<200 nm) spectral region. The problem is lower excitation efficiency when mercury vapor is replaced by noble gases as the excitation source. In order to solve this problem, we have investigated luminescent materials with quantum-cutting (QC) properties. The problem of low efficiency can be effectively circumvented by QC phosphors with luminescence efficiency higher then 100% through multi-photon and downconversion processes. The occurrence of QC processes via downconversion has been investigated in two series of newly discovered gadolinium oxyfluorides of Gd4O3F6:Eu3+ (or Tb3+) and GdOF:Tb3+ and one green-emitting QC phosphate of Ba3Gd(PO4)3:Tb3+ that were synthesized by solid state reactions. In the QC process of Gd4O3F6:Eu3+ containing the Gd3+-Eu3+ couple, one UV photon absorbed by Eu3+ and split into more than one visible photons emitted via a two-step energy transfer (cross-relaxation and direct energy transfer). The theoretical quantum efficiency (QE) was calculated to be 177% for Gd4O3F6:Eu3+ (7%) when excited at 247 nm. Furthermore, in the QC process of phosphors containing the Gd3+-Tb3+ couple, one UV or VUV photon absorbed by Tb3+ can be split into more than one green-emitting photons by downconversion mechanism. The theoretical QE was found to be higher then 100% during cross-relaxation and direct energy transfer. These phosphors give visible quantum efficiency of 170% in Gd4O3F6:Tb3+ (15%) (λex =226 nm), 168% in GdOF: Tb3+ (5%) (λex =237 nm), and 132% in Ba3Gd(PO4)3:Tb3+ (13%) (λex =193 or 225 nm).