表面对纳米微粒中稀土离子光谱性质的影响

Influence of Surface on Spectroscopic Properties of Rare Earth Ions in Nanocrystals

对于稀土和过渡金属这样的离子中心,由于电子轨道的半径小,纳米尺度的限域对能级位置和跃迁速率的影响并不显著,表面效应成为这类材料与相应体材料光谱性质出现差异的主要因素。把使稀土离子光谱性质产生可测量变化的环境用一个以其为中心、半径为D的作用球表示,可以定义纳米材料厚度为D的表面层。用完整晶格的晶体场和一个补偿电荷的场近似描述表面层中稀土离子的晶体场。本文用这个模型分析了纳米微粒中稀土离子光谱的非均匀宽化,激光选择激发下发射光谱随激发波长的变化,非选择激发下发射光谱随温度的变化以及跃迁分支比的变化等实验现象。

For their small orbital radius, quantum confinement effect would not be expected in the spectroscopic properties of 4f and 3d electrons in nanocrystals. Instead surface effect would play the most important role. The environment of a rare earth ion, whose change causes detectable variation in the spectroscopic properties, was described by an influence sphere with radius D. Thus a surface layer with thickness D in a nanocrystal could be defined. Crystal field acted on a rare earth ion in the surface layer was approximated by the combination of a crystal field in bulk material and a field of a compensatory point charge. With this model, some experimental results in rare earth-doped nanocrystals were analysed, including enlarged inhomogeneous broadening, variation of emission spectrum with excitation wavelength in laser selective excitation, temperature dependence of emission spectrum and change of the branching ratio.