Li Yingying, Liu Chunyan, Zhang Peisen, Huang Jiayi, Ning Haoran, Xiao Peng, Hou Yi, Jing Lihong, Gao Mingyuan
Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China.
School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing, China.
Front Chem. 2020 Sep 23;8:832. doi: 10.3389/fchem.2020.00832. eCollection 2020.
The rare-earth nanocrystals containing Er emitters offer very promising tools for imaging applications, as they can not only exhibit up-conversion luminescence but also down-conversion luminescence in the second near-infrared window (NIR II). Doping non-lanthanide cations into host matrix was demonstrated to be an effective measure for improving the luminescence efficiency of Er ions, while still awaiting in-depth investigations on the effects of dopants especially those with high valence states on the optical properties of lanthanide nanocrystals. To address this issue, tetravalent Zr doped hexagonal NaGdF:Yb,Er nanocrystals were prepared, and the enhancement effects of the Zr doping level on both up-conversion luminescence in the visible window and down-conversion luminescence in NIR II window were investigated, with steady-state and transient luminescence spectroscopies. The key role of the local crystal field distortions around Er emitters was elucidated in combination with the results based on both of Zr and its lower valence counterparts, e.g., Sc, Mg, Mn. Univalent ions such as Li was utilized to substitute Na ion rather than Gd, and the synergistic effects of Zr and Li ions by co-doping them into NaGdF:Yb,Er nanocrystals were investigated toward optimal enhancement. Upon optimization, the up-conversion emission of co-doped NaGdF:Yb,Er nanocrystals was enhanced by more than one order of magnitude compared with undoped nanocrystals. The current studies thus demonstrate that the local crystal field surrounding emitters is an effective parameter for manipulating the luminescence of lanthanide emitters.
含有铒发光体的稀土纳米晶体为成像应用提供了非常有前景的工具，因为它们不仅可以表现出上转换发光，还能在第二近红外窗口（NIR II）表现出下转换发光。将非镧系阳离子掺杂到主体基质中被证明是提高铒离子发光效率的有效措施，然而，对于掺杂剂尤其是高价态掺杂剂对镧系纳米晶体光学性质的影响仍有待深入研究。为了解决这个问题，制备了四价锆掺杂的六方相NaGdF:Yb,Er纳米晶体，并利用稳态和瞬态发光光谱研究了锆掺杂水平对可见窗口上转换发光和NIR II窗口下转换发光的增强作用。结合基于锆及其低价态对应物（如钪、镁、锰）的结果，阐明了铒发光体周围局部晶体场畸变的关键作用。利用单价离子如锂替代钠离子而非钆离子，并研究了将锆和锂离子共掺杂到NaGdF:Yb,Er纳米晶体中的协同效应以实现最佳增强。经过优化，共掺杂的NaGdF:Yb,Er纳米晶体的上转换发射比未掺杂的纳米晶体增强了一个多数量级。因此，目前的研究表明，发光体周围的局部晶体场是操纵镧系发光体发光的有效参数。
