Downconversion Luminescence Properties And Energy Transfer Mechanisms In Rare Earth Ions Doped Luminescent Materials

In the field of luminescent materials,how to improve the efficiency has been an attention-attracting topic for materials scientists.In recent years,near-infrared?NIR?quantum cutting?QC?luminescent materials with quantum yield more than 100%are gaining attention,due to the characteristic of multi-photon emission behaviors which have immense application prospects for highly effective luminescent materials and devices.With the going deep of the research of NIR QC,more and more novel high-efficiency NIR QC materials have been discovered and synthesized.However,there are still some fundamental scientific problems to be solved:what is the essential relation between NIR QC phenomenon and the energy transfer between ions?how to quantitatively analyse the complicated non-linear relations between the concentration of dopants and quantum yield of NIR QC,and so on.Because these problems restrain the development of NIR QC materials,it is necessary to make a deep investigation on NIR QC phenomenon.Based on the related-research in our lab,this dissertation is to extend and promote the development of NIR QC,and to solve the fundamental problems.This thesis is composed of six chapters.Chapter 1 briefly introduces the fundamental theory of energy transfer,advances in NIR QC materials as well.On the basis of the introduction,the research topic is proposed.Then,the synthesis and characterizations of samples are presented in Chapter 2.Chapter 3-6provide a detail investigation on the energy transfer mechanism of broadband-sensitized NIR QC and the influence of the concentration of rare earth ions dopant on the NIR QC mechanism.The main research results of this dissertation are:?1?Bi³⁺/Yb³⁺ions pair is considered for efficient downconversion combining strong broadband absorption of Bi³⁺with photon splitting by cooperative energy transfer from Bi³⁺to two Yb³⁺neighbors.However,evidence for photon splitting is lacking.Here,we investigate the Bi³⁺-to-Yb³⁺energy-transfer mechanism in Y₂O₃ and LuVO₄ hosts by means of static and dynamic photoemission spectroscopies.In Y₂O₃:Bi³⁺,Yb³⁺system,analysis of Yb³⁺-concentration and temperature-dependent decay curves however demonstrates that the energy transfer mechanism is not cooperative but single step,probably through a Bi⁴⁺-Yb²⁺charge-transfer state.Besides,the temperature dependence of the Bi³⁺-to-Yb³⁺energy-transfer efficiency is unusual as it decreases with temperature,unlike commonly observed thermally activated energy transfer.This is a signature of energy transfer via exchange interaction.In LuVO₄:Bi³⁺,Yb³⁺system,the dependence of Yb³⁺concentration on the integrated intensity of Bi³⁺(VO₄^3-)visible emission were analyzed using the shell model for energy transfer in crystalline environment.The results show energy transfer may occur between Bi³⁺ions and neighboring Yb³⁺ions.Based on temperature dependent spectral measurements,the unusual deactivated energy transfer is a convincing demonstration for exchange interaction mediated energy transfer.?2?Energy transfer process from the vanadate group to Yb³⁺involves the conversion of ultraviolet to NIR and has been reported in a variety of host lattices.However,the energy transfer mechanism is still under debate.First downconversion via cooperative energy transfer was proposed,resulting in emission of two NIR photons for every absorbed UV photon.Later single-step energy transfer was proposed involving a one-to-one photon downshifting process.Upon excitation by UV light,it can be observed that the VO₄^3--to-Yb³⁺energy transfer occurs in Y_1-x-x Yb_x(P_1-yV_y)O₄ solid solutions.By means of quantitatively analyzing the Yb³⁺-concentration-dependent VO₄^3-decay curves,we find strong evidence for single-step energy transfer between the vanadate group and nearest neighboring as well as next nearest neighboring Yb³⁺ions via exchang interaction.Temperature dependent measurements and the characteristic behavior of the luminescence decay curves reveal that the energy migration over VO₄^3-groups plays an important role on increasing the occurrence of possibility for the VO₄^3--to-Yb³⁺energy transfer.?3?NIR QC from the Tm³⁺:¹G₄ state essentially occurs via a three-step cascade radiation?case-1?and two-step cross-relaxations?case-2?by varying the Tm³⁺concentration?x?from0.0005 to 0.05 in La_2-x-x Tm_xBaZnO₅.In case 1,with the Tm³⁺concentration x<0.005,the energy of the ¹G₄ excited state is down-converted to three NIR photons emitted at approximately 1200,1480,and 1800 nm,with ³H₄ and ³F₄ acting as intermediate levels,while in case 2,with the Tm³⁺concentration x?0.005,the energy will be triply cut into approximately 1800 nm photon emissions.The three-photon NIR QC phenomena are investigated in terms of the static and dynamic photoluminescence.Based on the dependence of cross-relaxation and concentration quenching on Tm³⁺density,a rate-equation model was built to describe the energy transfer dynamics of Tm³⁺.The calculation of internal quantum yield was developed by considering nonradiative processes,and the maximum quantum yield for photon emission at 1800 was 198%in La_1.99Tm_0.01BaZnO₅.?4?A single-band NIR QC is efficiently achieved for 1190 nm fluorescence by structuring Ho³⁺-Yb³⁺couple as energy clustering in KLu₂F₇ host under excitation of ultraviolet-to-green photons.On the basis of experimental data and theoretical analysis,the formation of Yb³⁺(fractional Ho³⁺)energy clustering at sub-lattice level is demonstrated to account for resonant energy transfer of Ho³⁺?Yb³⁺,back energy transfer of Yb³⁺:²F_5/2?Ho³⁺:⁵I₆,and the crucial energy preserving by confining Yb³⁺energy migration.By means of steady and dynamic spectroscopies,an internal quantum yield for the single-band 1190 nm emission is appropriately evaluated to be more than 190%for K(Lu_0.24Ho_0.01Yb_0.75)₂F₇without any obvious concentration quenching.