摘要/Abstract

四方LiYF₄是一种与六方NaYF₄相当的稀土上转换发光基质材料，由于相对优异的短波上转换发光特性，近年受到业界的相当关注，但对产生这一优异特性的原因还未见报道.经研究发现，四方LiYF₄具有六边环状亚晶格结构，每5个间距小于0.4 nm的近邻三价阳离子形成一个团簇，稀土离子间的能量传递容易在团簇内或六边形环内环绕传递.基于它的这一亚晶格结构特征，本工作通过引入不同量敏化剂Yb³⁺和掺杂离子Sc³⁺和Hf⁴⁺，考察不同激活离子Er³⁺、Ho³⁺和Tm³⁺与敏化离子Yb³⁺构建亚晶格能量团簇以及异质离子Sc³⁺和Hf⁴⁺掺杂晶场调控对稀土离子上转换发光性能的影响机制.发现不同稀土离子与Yb³⁺存在不同的能级匹配，导致不同概率的无辐射交叉弛豫行为，当引入适量的Yb³⁺时，可分别构建1Er-2Yb、1Ho-2Yb和1Tm-4Yb亚晶格能量团簇，实现最佳的上转换发光性能；当6 mol% Sc³⁺和4 mol% Hf⁴⁺引入时，可有效调控晶场的不对称性；掺杂后，5个近邻三价离子的团簇结构中，只有3个近邻Yb³⁺离子，无法同时实现Er³⁺离子的⁴F_5/2和Sc³⁺的²G_7/2或Hf⁴⁺的⁴F_5/2^o激发态的双光子合作上转换电子布居，导致Sc³⁺或Hf⁴⁺成为荧光猝灭中心，Sc³⁺掺杂晶场调控最大仅提升50%的荧光强度，Hf⁴⁺掺杂没有提升反而降低Er³⁺离子的上转换发光强度，不同于它们掺杂六方NaYF₄：Er/Yb那样扮演储能离子角色，显著提升Er³⁺离子的短波上转换发光强度.本研究揭示了在六边环状亚晶格基质结构中不同稀土离子与敏化剂Yb³⁺的敏化上转换发光机制，以及基质结构特征对掺杂晶场调控行为的影响机制，可为设计和制备高性能上转换发光材料提供借鉴.
关键词: LiYF₄, 稀土, 亚晶格结构, 晶场调控, 上转换发光
Lanthanide ions doped tetragonal LiYF₄ has became an investigative focus of upconversion luminescence (UCL) materials for its well properties of multi-photon UCL and as a comparable matrix material with hexagonal NaYF₄. While the cause for its well performance on short bands emission is still unrevealed. After the exploration of crystal structure characteristic of tetragonal LiYF₄, a hexagonal circle sublattice structure of Y³⁺ with 0.3710 nm interval between adjacent Y³⁺ ions and larger than 0.5 nm interval between meta-position and para-position Y³⁺ ions were revealed. The energy transfer of rare earth ions are easy take place around the hexagonal circles or among the cluster of five adjacent trivalent ions. Base on the sublattice structure characteristic of tetragonal LiYF₄, we have an idea to study UCL mechanism systematically of tetragonal LiYF₄:RE by the construction of sublattice energy cluster 1M-xYb (M=Er, Ho, Tm) and the manipulation of crystal field symmetry by introducing different amount Yb³⁺ ions and Sc³⁺ or Hf⁴⁺ ions, respectively. Hydrothermal method was employed to prepare LiY_0.98-xYb_xEr_0.02F₄, LiY_0.98-xYb_xHo_0.02F₄, LiY_0.995-xYb_xTm_0.005F₄, LiY_0.68-xYb_0.3Er_0.02Sc_xF₄ and LiY_0.68-xYb_0.3Er_0.02Hf_xF₄ series samples. A typical preparation process demonstrate as follows, at first, (1-x) mmol Y(NO₃)₃ (0.2 mol/L), x mmol (x=0.2, 0.5, 0.7 and 0.9) Yb(NO₃)₃ (0.20 mol/L) and Er(NO₃)₃ (0.02 mmol) solution was dropwise added into 20 mL deionized (DI) water with 1 mmol EDTA to form a solution under vigorous stirring for 30 min. Secondly, 3.0 mL LiOH (1.0 mol/L) and 4.0 mL NH₄HF₂(1.0 mol/L) aqueous solution were dropwise added to the solution under thorough stirring for 30 min until the solution completely became a white emulsion, the pH value of the emulsion is 3~4. Finally, the white emulsion was slowly transferred into a 50 mL Teflon-lined autoclave, sealed and heated at 190℃ for 18 h. The final products were collected by centrifugation, and then washed with DI water several times. The collected samples were dried at 60℃ over night. X-ray powder diffraction (XRD) and Rietveld refinement method were employed to reveal the variation of crystal structure, field emission scanning electron microscopy (FESEM) and field emission transmission electron microscopy (FETEM) were employed to the analysis of crystal morphology and crystal structure. UCL performance was analyzed by Edinburgh fluorescence spectrophotometer FSP920. After investigation, we found excited energy levels distribution of different RE ions is diverse, and the level matching with Yb³⁺ are different too, it result in different luminescence quenching of energy cross relaxation, so the different sublattice energy clusters 1Er-2Yb, 1Ho-2Yb and 1Tm-4Yb of different active rare earth ions can be constructed for the best UCL performance. The cystal field symmetry of tetragonal LiYF₄:Yb/Er were manipulated successfully by 6 mol% Sc³⁺ or 4 mol% Hf⁴⁺ doping, and UCL intensity were enhanced about 50% with 6 mol% Sc³⁺, while the UCL intensity were weaken after Hf⁴⁺ doping. After Sc³⁺ or Hf⁴⁺ doping, there are only three Yb³⁺ ions in the five trivalence ions cluster that can't realize two-photon cooperation upconversion synchronous electron population of ⁴F_5/2 excited state level of Er³⁺ ions and ²G_7/2 or ⁴F_5/2^o excited state level of Sc³⁺ or Hf⁴⁺ respevticely, and then Sc³⁺ and Hf⁴⁺ ions become a quenching center in the asymmetric crystal field that is conversed with them doped hexagonal NaYF₄:Yb/Er that Sc³⁺ and Hf⁴⁺ ions were taken as energy storage ions and dramatically enhanced UCL performance. In this work, the UCL mechanism of sublattice energy cluster construction and crystal field manipulation were revealed that may be an inspiration for high efficient UC luminescence materials design and preparation.
Key words: LiYF₄, rare earth, sublattice structure, crystal field manipulation, upconversion luminescence


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