上转换激发方式下稀土离子非常规能级辐射跃迁的研究

Rare earth ions are the main research subjects of solid state luminescence because of their possess rich electronic states with various energies and their excellent luminescent characteristics (large Stoke’s shift, sharp emission peaks, and long fluorescence lifetime, etc). Although dramatic efforts have been dedicated to the elaboration of rare earth ions, luminescence emission of their considerable energy levels can not be observed due to extremely low radiative transition rates of these levels, as well as their property can not be understood completely. Recently, the discovery of unusual radiative transitions of rare earth ions in upconversion scheme provides us a new approach for investigating the characteristics of these levels. This project plan to study unusual radiative transitions of rare earth ions in upconversion scheme experimentally and theoretically on the basis of previous work. Objectives can be realized through the multi-photon upconversion processes while can not be realized through downconversion processes under high-energy excitation. Through the research process such as designing and fabricating materials with dual- (or multi-) sensitizers ions, selecting of single or dual excitation mode, measurement of upconversion spectra, measurement of high resolution spectra, analysis of temporal evolution of luminescence, we can reveal the physical essentials of unusual radiative transitions of rare earth ions, define the characteristics of unusual levels of rare earth ions, build the theoretical modal of unusual radiative transitions of rare earth ions in upconversion scheme. The theoretic and experimental foundation can be established for inaugurating such a new research interest.

稀土离子能级丰富、荧光谱线特征性强，是固体发光学研究的一个主要对象。虽然人们对稀土离子能级进行了广泛而深入的研究，但依然有相当多的能级由于极低的辐射跃迁速率而观察不到荧光发射，导致它们的属性不为人们所完全了解，我们称之为非常规能级。上转换激发下非常规能级辐射跃迁现象的发现为研究这些能级提供了新的方法。本项目拟在前期工作的基础上开展上转换激发方式下稀土离子非常规能级辐射跃迁的实验与理论研究；通过多光子激发下的上转换荧光发射手段，实现高能光子在下转换激发方式下不能达到的研究目的。通过双（多）敏化材料设计与制备、单/双激发方式的选择、上转换发光光谱测量、高分辨光谱测量、荧光动力学分析等研究过程，揭示导致非常规能级辐射跃迁的物理本质、标定非常规能级的发光学属性、建立上转换激发方式下非常规能级辐射跃迁的理论模型，为更全面地阐明某些稀土离子的能级属性和设计新型发光材料奠定实验与理论的基础。

稀土离子掺杂材料的上转换发光特性由于极低的效率使其在应用上受到了极大的限制。我们探索了一系列新型的发光基质材料：Sc2O3，K2NaScF6，CaF2@NaYF4，Y2O3核壳结构，Gd2O3@ Gd2O3纳米晶，LiLuF4:Yb,Tm@ LiGdF4等，以及通过单掺杂或者共掺杂惰性离子Ca2+，Zn2+，Cu2+或Li+，来大幅度提高上转换材料的发光效率。此外，我们还对基质材料的晶格演变，上、下转换发光机理进行了详细的研究。.为了便于纳米材料在生物上的应用，我们以乙二醇作为溶剂，水溶性的PVP（聚乙烯吡咯烷酮）作为螯合剂合成了水溶性的立方相K2NaScF6以及利用CaF2核成功的诱导了六角相NaYF4壳层的生长，区别于油溶性的材料，它们不仅发光效率更高，而且更容易应用在生物学方面。.接下来的工作我们将在已有的成果基础上继续探索稀土离子非常规能级辐射跃迁的特性。