利用溶剂热法制备了β-NaYF4∶20%Yb3+/2%Er3+核颗粒和β-NaYF4∶20%Yb3+/2%Er3+@β-NaYF4∶x%Yb3+(x=0,20,50,70,100)核壳结构纳米颗粒。在未包覆β-NaYF4前, 核纳米颗粒的尺寸约为30 nm; 在包覆β-NaYF4壳层后, 纳米颗粒的尺寸增加至40 nm左右, 并且上转换绿光和红光分别提高了14倍和25倍。上转换发光强度能够增强如此之多是因为包覆的壳层有效地抑制了处于激发态的Yb3+与纳米颗粒表面缺陷之间的能量传递过程。随着壳层中Yb3+掺杂浓度的提高, 纳米颗粒的尺寸并未发生明显变化, 一直保持在40 nm左右。但是, 纳米颗粒的上转换发光强度却随着Yb3+浓度的提高而明显减弱。由于在980 nm波长的激光辐照时, 大部分980 nm的光子会被纳米颗粒壳层中的Yb3+所吸收, 能够被核中的Yb3+所吸收的980 nm光子数目非常少。然而, 由于壳层中的Yb3+距离核颗粒中的Er3+较远, 使得二者之间的能量传递效率非常低, 从而大大降低了纳米颗粒的上转换发光强度。

In this work, a solvothermal process was used to synthesize the β-NaYF4∶20%Yb3+/2%Er3+ core nanoparticles(NPs) and β-NaYF4∶20%Yb3+/2%Er3+@β-NaYF4∶x%Yb3+(x=0, 20, 50, 70, 100) core-shell NPs. The size of the core NPs and core-shell NPs is about 30 nm and 40 nm respectively, implying that the thickness of the layer is 5 nm. After coating a β-NaYF4 shell without Yb3+ doping, the upconversion(UC) intensity is increased with a factor of 14 and 25 for green emission and red emission respectively, resulting from the suppression of deexcitation of Yb3+ ions by the core-shell structure. However, the UC intensity is decreased dramatically with the increasing Yb3+ ions concentration in the shell, due to the inefficient energy transfer process between the Yb3+ ions in the shell and the Er3+ ions in the core caused by the large distance between them. As the β-NaYF4 shell completely converts to β-NaYbF4, the UC intensity decreased 98.8% and 99.4% for green and red emission, respectively.