稀土氟化物螢光體量子剪裁與發光特性之研究

Abstract

本研究利用量子剪裁效應的原理，設計BaGdF5:R3+及Ca22Gd3F53:R3+(R=Eu, Er, Tb)兩系列氟化物螢光體，並利用Gd3+-Eu3+、Gd3+-Er3+及Gd3+-Tb3+稀土離子組合之下轉換機制，成功地開發可見光量子效率大於100%的真空紫外螢光材料。在含Gd3+-Eu3+組合的BaGdF5:Eu與Ca22Gd3F53:Eu中，藉由交叉鬆弛及直接能量轉移兩步驟的能量轉移，分別可得到最高可見光區整體量子效率值137%及144%；而在含Gd3+-Er3+組合的螢光體中，僅在BaGdF5:Er3+中觀察到量子剪裁的現象，其最高可見光區整體量子效率為115%，在此氟化物中Er3+為主要的量子剪裁離子，利用交叉鬆弛所產生額外的光子放射，達到量子效率提升的目的。 又在含Gd3+-Tb3+組合的BaGdF5:Tb與Ca22Gd3F53:Tb中，其量子剪裁及光子放射均發生在Tb3+離子上，當Tb3+離子受激發而躍升至4f75d能態時，可藉交叉鬆弛能量轉移激發並提升另一鄰近Tb3+離子，使其放射出一可見光光子，而本身則緩解至4f8激發態，隨後放射出第二個光子，故其可見光區整體量子效率超過100%。在波長215 nm與187 nm激發下，BaGdF5:Tb3+(15%)之最高可見光區整體量子效率可分別達168%及180%；而在波長212 nm與186 nm激發下，Ca22Gd3F53:Tb3+(9%)之最高可見光區整體量子效率則為154%及129%。

The advancement of luminescent materials for plasma display panel (PDP) and mercury-free lighting devices is highly dependent on the development of vacuum ultraviolet (VUV) excited phosphors. The research is attempted to investigate the VUV luminescence spectroscopy, visible quantum cutting and design of phosphors based on the energy levels for lanthanide ions in the VUV range. The occurrence of visible quantum cutting (QC) via downconversion has been observed and investigated in two series of newly discovered R3+ (R = Eu,Tb,Er)-doped gadolinium fluorides of Ca22(Gd1-xRx)3F53 and Ba(Gd1-yRy)F5 synthesized by sealed-tube solid-state reactions at 1,000℃ and 800℃, respectively. In the QC process, one UV photon absorbed by quantum cutter Gd3+ was found to split into more than one visible photon emitted via a two-step energy transfer (cross-relaxation and direct energy transfer) from the Gd3+ 6GJ level to Eu3+ ions and results in increasing quantum efficiency. Our results indicate that a visible quantum efficiency of 144% (□ex = 195 nm) and 137% (□ex = 195 nm) has been achieved in Ca22Gd3F53:Eu3+(7%) and BaGdF5:Eu3+(7%), respectively. On the other hand, Tb3+ acts as a quantum cutter in the green-emitting Ca22(Gd1-xTbx)3F53 and Ba(Gd1-yTby)F5 with Gd3+-Tb3+ QC couple. Upon VUV excitation in the Tb3+ 4f75d levels, energy can be transferred to another Tb3+ by cross relaxation and only feeds the 5D4 level, which makes only the relative intensity of 5D4→7FJ emission increase. The calculated visible quantum efficiency has been found to be 168% (□ex = 215 nm) and 180% (□ex = 187 nm) for BaGdF5:Eu3+(15%) and 154% (□ex = 212 nm) and 129% (□ex = 186 nm) for Ca22Gd3F53:Eu3+(9%), respectively. Finally, in the Er3+-activated Ca22Gd3F53 and BaGdF5 phosphors, only BaGdF5:Er3+, but not Ca22Gd3F53:Er3+, was found to exhibit apparent QC giving a visible quantum efficiency of 115% observed in BaGdF5:Eu3+(9%). The occurrence of the cross relaxation causes the population of Er3+ 4S3/2 state and it subsequently emits an extra photon.