单颗粒稀土微/纳晶体上转换荧光行为的光谱学探究

利用激光共聚焦显微镜系统研究了系列单颗粒NaYF_4:Yb/Er微晶的上转换荧光强度、空间分布和动力学过程.结果表明:荧光强度和动力学过程不但依赖于样品的长径比,而且依赖于样品的具体制备途径.荧光强度和红色荧光寿命随样品长径比的增大而增大,在具有相同长径比的NaYF_4:Yb/Er微米棒中,相比于调柠檬酸的量,调控pH制备的样品展示了更优异的上转换荧光特性.更有趣的是:不同样品的荧光图案展示了异向空间分布,暗示了其在编码和显示等领域的应用优势.荧光特性依赖于样品长径比和制备过程的物理机理被进一步研究和揭示:在微米晶体内,荧光强度主要依赖于样品的晶格内Na~+缺陷的数量.该研究为高效上转换荧光材料的合成积累了数据,而NaYF_4:Yb/Er微晶中红色荧光寿命对晶格缺陷更加敏感的特性也可能使其成为晶格结晶度的探针.

Spectroscopic exploration of upconversion luminescence behavior of rare earth-doped single-particle micro/nanocrystals

In recent years, rare earth-doped upconversion (UC) micro/nanocrystals are useful for many applications, especially in biology because of their unique luminescent properties and specific geometry. The luminescence efficiency of lanthanide-doped UC nanoparticles is of particular importance for their applications. However, the unsatisfactory UC efficiency is still one of the main hurdles. In the present article, a series of Yb³⁺/Er³⁺ doped NaYF₄ micro/nanoparticles with different ratios of length to diameter are successfully synthesized by a facile hydrothermal route. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX) analyses, photoluminescence spectra, and the dynamic process of the luminescence are used to characterize the samples. The intrinsic structural feature of fluoride, the solution pH value, and organic additive Cit^3- account for the ultimate shape evolution of the final products. The ratio of length to diameter of NaYF₄ microrod can be tuned only by varying the value of pH or the amount of an organic additive (Cit^3-). The UC characteristics of a single NaYF₄:Yb³⁺/Er³⁺ microrod obtained by tuning the value of pH or the amount of Cit^3- are investigated by laser confocal microscopy with a 980 nm laser. The two series of codoped fluoride crystals both exhibit the characteristic UC luminescence from Er³⁺ ions and display the rich luminescence patterns in space. The UC luminescence from a single NaYF₄:Yb³⁺/Er³⁺ microrod obtained by tuning the value of pH is brighter than that from a single NaYF₄:Yb³⁺/Er³⁺ microrod with the same size obtained by tuning the amount of Cit^3-. The EDX analysis indicates that the number of Na⁺ defects depends on the specific synthesis conditions of the sample. The Na⁺ defects of samples obtained by tuning the values of pH are lower than those of samples with the same size obtained by tuning the amount of Cit^3-. It conduces to reducing Na⁺ defects at lower pH value. The parameters of the luminescence kinetics are found to be unambiguously dependent on the size of sample, which relates to higher energy phonon of surface and Na⁺ defects. The mechanism of luminescence enhancement by pH controlling is explored, and a mechanism based on the reduced intrinsic defects of Na⁺ is proposed. The investigation not only enriches the controllable synthesis approach of fluoride micro/nanomaterials, but also indicates the potential applications of rare earth materials with a rich luminescence pattern in the photonic devices and anti-counterfeiting devices.