| Summary: | 博士 === 國立交通大學 === 應用化學系所 === 96 === 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.
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