Xiang Guotao, Zhang Jiahua, Hao Zhendong, Zhang Xia, Pan Guo-Hui, Luo Yongshi, Lü Wei, Zhao Haifeng
†State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China.
‡Graduate School of Chinese Academy of Sciences, Beijing 100039, China.
Inorg Chem. 2015 Apr 20;54(8):3921-8. doi: 10.1021/acs.inorgchem.5b00109. Epub 2015 Apr 7.
Nanosized Yb(3+) and Er(3+) co-doped β-NaYF4 cores coated with multiple β-NaYF4 shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb(3+) ions. We also explored the population of the Er(3+4)F9/2 level. The results reveal that energy transfer from the lower intermediate Er(3+4)I13/2 level is dominant for populating the Er(3+4)F9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er(3+4)S3/2 level for populating the Er(3+4)F9/2 level, which mainly comes from the cross relaxation energy transfer from Er(3+) ions to Yb(3+) ions followed by energy back transfer within the same Er(3+)-Yb(3+) pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er(3+)-Yb(3+) pairs. This population route is in good agreement with that in nanomaterials and bulk materials.
通过溶剂热法合成了包覆有多个β-NaYF4壳层的纳米尺寸Yb(3+)和Er(3+)共掺杂β-NaYF4核。利用X射线衍射和扫描电子显微镜对材料的晶体结构和形貌进行了表征。还测量了可见和近红外光谱以及衰减曲线。当核包覆三层壳层时，观察到绿色上转换强度增加40倍，红色上转换强度增加34倍。基于光致发光光谱和衰减时间研究了上转换增强的起源。我们的结果表明，夹心结构中的上转换增强主要源于对Yb(3+)离子去激发的抑制。我们还研究了Er(3+4)F9/2能级的粒子数。结果表明，当纳米晶体尺寸相对较小时，从较低的中间Er(3+4)I13/2能级的能量转移是填充Er(3+4)F9/2能级的主要方式；然而，随着纳米晶体尺寸的增加，绿色发射的Er(3+4)S3/2能级对填充Er(3+4)F9/2能级（主要来自Er(+)离子到Yb(3+)离子的交叉弛豫能量转移，随后在同一Er(3+)-Yb(3+)对中进行能量反向转移）的贡献变得越来越重要。此外，这种机制仅在最近的Er(3+)-Yb(3+)对中发生。这种粒子数填充途径与纳米材料和块状材料中的情况非常一致。
