Abstract The green and red upconversion (UC) emissions of 35 mol% Ca2+ doped β-NaYF4:Yb3+/Er3+ microcrystals were improved to 102.8 and 43.1 times higher than those of Ca2+-free counterparts. X-ray powder diffraction spectra, scanning electron microscopy, Fourier transform infrared spectroscopy, upconversion luminescence (UCL) spectroscopy and Judd-Ofelt analysis were performed to investigate the mechanism of UCL intensity enhancement caused by Ca2+ doping. The hexagonal-to-cubic phase transition was observed when Ca2+ doping concentration exceeded 35 mol%. The incorporation of Ca2+ into hexagonal microcrystals lattice induced unit cell volume enhancement and microcrystals surface to volume ratio variation, which affected the UCL intensity by hindering the non-radiative energy loss process via Yb3+/Er3+ ions and changing the amount of surface quenchers adsorbed on the microcrystals surface. The hypersensitive transition related local crystal field asymmetry around Er3+ in the grown hexagonal microcrystals was also investigated by Judd-Ofelt analysis but unexpectedly found to be the less dominant factor for the UCL intensity variation. This study helps to realize the mechanism of impurity ion doping resulted UCL intensity enhancement, which is instructive for high-performance UC microcrystals synthesis.

中文翻译：

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Ca2+掺杂对β-NaYF4:Yb3+/Er3+微晶上转换发光强度和晶场不对称性的影响

摘要 35 mol% Ca2+ 掺杂的β-NaYF4:Yb3+/Er3+ 微晶的绿光和红光上转换(UC) 发射比不含Ca2+ 的微晶提高到102.8 和43.1 倍。进行X射线粉末衍射光谱、扫描电子显微镜、傅里叶变换红外光谱、上转换发光(UCL)光谱和Judd-Ofelt分析以研究Ca2+掺杂引起的UCL强度增强的机制。当 Ca2+ 掺杂浓度超过 35 mol% 时，观察到六方到立方的相变。Ca2+ 掺入六方微晶格引起晶胞体积增强和微晶表面体积比变化，它通过阻碍 Yb3+/Er3+ 离子的非辐射能量损失过程并改变吸附在微晶表面上的表面淬灭剂的数量来影响 UCL 强度。还通过 Judd-Ofelt 分析研究了生长的六方微晶中与 Er3+ 周围的超敏跃迁相关的局部晶体场不对称性，但出人意料地发现它是 UCL 强度变化的次要因素。该研究有助于了解杂质离子掺杂导致UCL强度增强的机理，对高性能UC微晶的合成具有指导意义。还通过 Judd-Ofelt 分析研究了生长的六方微晶中与 Er3+ 周围的超敏跃迁相关的局部晶体场不对称性，但出人意料地发现它是 UCL 强度变化的次要因素。该研究有助于了解杂质离子掺杂导致UCL强度增强的机理，对高性能UC微晶的合成具有指导意义。还通过 Judd-Ofelt 分析研究了生长的六方微晶中与 Er3+ 周围的超敏跃迁相关的局部晶体场不对称性，但出人意料地发现它是 UCL 强度变化的次要因素。该研究有助于了解杂质离子掺杂导致UCL强度增强的机理，对高性能UC微晶的合成具有指导意义。