【摘要】 采用水热法制备了六方相NaYF₄:Yb³⁺/Er³⁺(β-NaYF₄:Yb³⁺/Er³⁺)微米晶体颗粒,并基于银岛膜（Ag film）衬底构建了一种Ag film/β-NaYF₄:Yb³⁺/Er³⁺微纳复合体系.通过扫描电子显微镜、场发射电子显微镜及X射线衍射仪对样品的形貌及晶体结构进行表征.实验结果表明,NaYF₄:Yb³⁺/Er³⁺微米晶体均为六方相晶体结构的六角盘.采用共焦显微测试系统,在980 nm激光激发下,系统研究了Ag film/β-NaYF₄:Yb³⁺/Er³⁺微纳复合体系中单个微米晶体上转换荧光强度与银岛膜的厚度依赖关系.结果发现:单颗粒β-NaYF₄:Yb³⁺/Er³⁺微米晶体的上转换荧光强度随着银岛膜的厚度增加而明显增大.根据理论推导及相应数值模拟得出,影响其荧光增强主要因素是由于银岛膜的反射效应和其表面附近的局域电磁场增强效应所导致.由此可见,构建这种新型结构可为增强单个微米颗粒的上转换荧光发射提供新的途径,为进一步拓展稀土微纳晶体的实际应用提供实验依据.更多还原

【Abstract】 Rare-earth-doped upconversion（UC） is a nonlinear optical phenomenon in which the successive absorption of two or more low-energy photons via intermediate energy states leads to high-energy light emission（anti-Stokes emission）. It endows their unique optical properties including low auto-fluorescence, high signal to noise ratio, a large anti-Stokes shift, and low toxicity. Therefore, rare-earth-doped UC materials have been widely used in photovoltaics, color displays,optical storages, biological imaging, and solar cells. However, the UC luminescence efficiency is much lower and largely limited the practical applications. The noble metal nanostructures as promised enhanced substrates could efficiently improve the intensity of UC luminescence and expand its applications. In this work, Yb³⁺and Er³⁺co-doped β-NaYF₄ microcrystals are synthesized via a simple and facile hydrothermal method with assistance of sodium citrate to control the crystal growth. Then, a Ag films/β-NaYF₄:Yb³⁺/Er³⁺hybrid structure that the β-NaYF₄:Yb³⁺/Er³⁺microcrystals were deposited on silver films with different thickness is constructed. The crystal phase and the morphology of the β-NaYF₄:Yb³⁺/Er³⁺microcrystals are analyzed with X-ray power diffraction（XRD）, scanning electron microscope（SEM）, and transmission electron microscopy（TEM）. According to the results of XRD, SEM, and TEM, the pure hexagonal phased NaYF₄:Yb³⁺/Er³⁺are arc-shaped hexagonal plate microcrystals with smooth surface. The average overall dimension of each microcrystal is about 5.0 μm in diameter. The UC luminescence property of Ag film/β-NaYF₄:Yb³⁺/Er³⁺hybrid structure is investigated carefully by a confocal setup equipped with an optical microscope. When 980 nm continuous wave（CW） laser is focused on the central of a single microcrystal particle deposited on silver films with different thickness, the intensity of blue(2^H_9/2→4^I_15/2), green(4^S_3/2→4^I_15/2 and2^H_11/2→4^<sub>I15/2), and red(4^F_9/2→4^I_15/2) emission bands increases with increasing the Ag film thickness. The maximum enhancement factor（EF） for each emission band occurs at the Ag film thickness of 100 nm. But we realize that the intensity EF of each emission band is different, the EF for the green emission is greater than the blue and red one. A comprehensive analysis, based on the rate equations and the numerical calculation |E|², reveals that the optical reflection and local electric field enhancement effect of the Ag film play a role in the UC luminescence enhancement for the single β-NaYF₄:Yb³⁺/Er³⁺microcrystal. This finding breaks the limitation of conventional core-shell system, and it also provides a unique platform for micro-optoelectronic devices,solar energy conversion, optical sensors, and biological applications.更多还原