Er3+/Yb3+共掺NaYF4/LiYF4微米晶体的上转换荧光特性

采用水热法成功制备了Er³⁺/Yb³⁺共掺杂的NaYF₄和LiYF₄微米晶体. 通过X射线衍射仪和环境扫描电子显微镜对样品的晶体结构及形貌进行表征. 实验结果表明: 六方相NaYF₄微米晶体为棒状结构, 而四方相LiYF₄微米晶体则为八面体结构. 在近红外光980 nm激发下, NaYF₄:Yb³⁺/Er³⁺和LiYF₄:Yb³⁺/Er³⁺ 微米晶体均展现出很强上转换荧光发射. 且NaYF₄:Yb³⁺/Er³⁺微米晶体的荧光发射强度大约是LiYF₄:Yb³⁺/Er³⁺微米晶体的2倍, 但红绿比明显较低. 根据荧光光谱, 并借助激光光谱学及发光动力学深入探讨基质变化及表面修饰剂乙二胺四乙二酸(EDTA)对荧光特性的影响. 实验结果发现: 影响荧光强度的主要因素是基质环境的局域对称性, 而导致不同红绿比则是由于样品表面较多的EDTA分子所引起. Er³⁺掺杂的NaYF₄和LiYF₄ 微米晶体呈现出很强的绿光发射可被应用于全色显示, 荧光粉和微光电子器件中.

Upconversion flourescence characteristics of Er3+/Yb3+ codoped NaYF4 and LiYF4 microcrystals

Lanthanide-doped upconverting fluoride nano-and micro-materials have aroused much research interest due to their potential applications in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging. In order to synthesize Ln³⁺ doped crystals with favorable optical properties, such as high upconversion (UC) efficiency and controllable emission profile, the two major parameters that affect luminescence processes including host materials and lanthanide activator ions should be selected appropriately in the synthesis process. Majority of scientists deem that lanthanide doped fluoride nano-and micro-materials with low phonon energy are currently the efficient UC host materials. In this work, Yb³⁺ and Er³⁺ ions codoped NaYF₄ and LiYF₄ microcrystals are synthesized by a facile hydrothermal method with ethylene diamine tetraacetic acid (EDTA) as a chelator. The NaYF₄:Yb³⁺/Er³⁺ and LiYF₄:Yb³⁺/Er³⁺ microcrystals are characterized by X-ray diffraction (XRD), scanning electron microscope(SEM), and the photo-luminescence spectra method. The influences of EDTA on the crystal phase, shape and upconversion luminescence are explored in detail. According to the results of XRD and SEM, the pure hexagonal phased NaYF₄:Yb³⁺/Er³⁺ rod-like microcrystals each with smooth surface are all around 12 μm in the length. While the pure tetragonal phased LiYF₄: Yb³⁺/Er³⁺ microcrystals each with smooth surface are octahedral in shape, and their average size is around 12 μm. Under near infrared (NIR) 980 nm excitation, the two dominant emission peaks of Er³⁺ ions at 544 nm and at 650 nm are observed in NaYF₄ and LiYF₄ microcrystals, which can be assigned to the transitions of (²H_11/2/⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2, respectively. It is found that the upconversion luminescence intensity of NaYF₄:Yb³⁺/Er³⁺ microcrystals is about two times that of LiYF₄:Yb³⁺/Er³⁺ microcrystals under the same excitation conditions. The ratio of red-to-green emission of Er³⁺ ions in LiYF₄ microcrystals is higher than that of the NaYF₄:Yb³⁺/Er³⁺microcrystals. The changes of the spectra in the different hosts could stem from two sources: one is that the nonradiation relaxation probability relative to phonon energy of matrix, the other is that the radiative transition probability relative to the site symmetry of the crystal field acting on the ion. The ratios between ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions in NaYF₄:Yb³⁺/Eu³⁺ and LiYF₄:Yb³⁺/Eu³⁺ microcrystals are employed to compare and elucidate the site symmetry of the crystal field for Ln³⁺ ions. Note that the ratio of ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions in NaYF₄:Yb³⁺/Eu³⁺ microcrystals is smaller than that of the LiYF₄:Yb³⁺/Eu³⁺ microcrystals, which indicates a much higher radiative relaxation rate in NaYF₄ microcrystals than in LiYF₄ microcrystals. The organic ligands of EDTA on the surface of microparticles affect the properties of luminescence through changing the nonradiative relaxation rate, resulting in the different R/G ratios in NaYF₄ and LiYF₄ microcrystals. This result can be further supported by the comparison between NaYF₄ and LiYF₄ microcrystals without EDTA added in the preparation process. The micro-sized luminescence materials usually present stronger upconversion luminescence because of their higher degree of crystallinity and less surface quenching centers. Thus, the Er³⁺ codoped NaYF₄ and LiYF₄ microcrystals exhibit strong green upconversion emission, which has potential applications in full-color displays and microelectronic devices.