Exploratory Research Of The Rare-earth Phosphate-based K₂LnZr(PO₄)₃(Ln=Gd,Y) Quantum Cutting Phosphors

Recently, for the development of non-mercury fluorescent tubes and plasma display panels (PDP) technology, new quantum cutting (two-photon luminescent) phosphors are required for the investigation of high efficient luminescent materials under VUV excitation. Rare-earth orthophosphates have stronger absorption in the range of the VUV and are more easily implemented. Besides, they have many advantages, such as high physical-chemical stability, low synthesis temperature and solubility, transparent, etc. Therefore, rare-earth orthophosphates could be ideal hosts for high luminescence efficiency VUV phosphors. Rare-earth (RE) ions are proper activators due to their abundant energy levels and physical-chemical properties. In the present work, the phenomena of energy transfer between different ion pairs and visible quantum cutting in rare-earth orthophosphate K2LnZr(PO4)3 (Ln= Gd, Y) doped with different rare-earth ions have been investigated. The content of this paper includes:(1) K2Gd1-xZr(PO4)3:Eux3+(0.02≤x≤0.1) were prepared by solid-state reaction method and their photoluminescence properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The phenomenon of visible quantum cutting through downconversion was observed for the Gd3+-Eu3+ couple in this Eu3+-doped K2GdZr(PO4)3 system. Visible quantum cutting, the emission of two visible light photons per absorbed VUV photon, occured upon the 186 nm excitation of Gd3+ at the 6GJ level via two-step energy transfer from, Gd3+ to Eu3+ by cross relaxation and sequential transfer of the remaining excitation energy. The results revealed that the efficiency of the energy transfer process from Gd3+ to Eu3+ in the Eu3+-doped K2GdZr(PO4)3 system could reach to 155% and K2GdZr(PO4)3:Eu3+ was effective quantum cutting material.(2) In present work, K2Y1-xZr(PO4)3:Prx3+(1 mol.%≤x=≤5 mol.%) samples were prepared by solid-state reaction method and their photoluminescence properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The results indicate that, in Pr3+-doped K2YZr(PO4)3 phosphor, even if the 1S0 state is above the lowest 4f5d energy level, the photon cascade emission (PCE) process for Pr3+ still could occur under 147 nm and 234 nm (4f15d1 state) excitation.(3) K2YZr(PO4)3:Pr3+, Mn2+ single-phase powder samples were prepared by solid-state reaction method and their photoluminescence (PL) properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The results indicated that in Pr3+ singly doped K2YZr(PO4)3 sample, the first-step transition(1So→I6,3PJ around 405 nm) of Pr3+ is near the ultraviolet (UV) range, not useful for practical application. When Mn2+ was doped as a co-activator ion, the energy of 1So→I6,3PJ transition can be transferred synchronously from Pr3+ to Mn2+ and then emit a visible photon. The optimal quantum efficiency (QE) of this co-doped system K2YZr(PO4)3:Pr3+, Mn2+ reached to 126.3%, suggesting a novel type of practical visible quantum cutting phosphor in promising application.(4) Tb3+, Er3+ co-doped K2GdZr(PO4)3 were prepared by solid-state reaction method and their photoluminescence (PL) properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The results indicate that the energy transfers from Er3+ to Gd3+ and from Gd3+ to Tb3+ required for occurring of visible quantum cutting through downconversion were efficient. In this cooperative energy transfer process, Gd3+ was used as an intermediate of energy transfer between Er3+ and Tb3+. The optimal quantum efficiency (QE) of K2GdZr(PO4)3:Er3+, Tb3+ phosphor was estimated to be 110%.(5) In this work, K2GdZr(PO4)3:Tb3+, Dy3+ samples were synthesized by high-temperature solid-state reaction method and their photoluminescence (PL) properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The results indicated that the 5D4→7F5 emission of Tb3+ was enhanced observably by the efficient energy transfer from Dy3+. In this phosphor, Tb3+ emitted two green photons through a two-step energy transfer process. In addition, the energy transfer of Gd3+→Tb3+ and Gd3+→Dy3+ was also been observed, suggesting that the existence of Gd3+ is helpful for improving the luminescence intensity of this novel efficient green emitting phosphor.In addition, from the aspect of practical application, it is necessary to seek for new rare-earth ions or ion pairs which can implement the high efficient quantum cutting process. It is unfortunately that there was no ideal result. However, these works still provide reference for the further researches.