Er3+,Yb3+共掺杂SrTiO3超细粉末的可见上转换发光

钛酸锶(SrTiO₃)具有高介电常数、良好的绝缘性质、优良的物理化学稳定性和在可见光范围内的优异的透明度等优点,是一种重要的无机功能材料。近年来,Al³⁺,Ga³⁺等离子共掺杂的SrTiO₃:Pr³⁺做为一种优异的红色场发射显示(FED)荧光粉引起了人们的兴趣。另外,SrTiO₃具有相对较小的声子频率,因而它可以作为一种有利于上转换发光的基质材料。以NaCl为助熔剂制备了Er³⁺,Yb³⁺共掺杂的SrTiO₃超细粉末。在980nm激发下,样品发出很强的来自于Er³⁺离子的²H_11/2→⁴I_15/2,⁴S_3/2→⁴I_15/2(绿光)和⁴F_9/2→⁴I_15/2(红光)跃迁的上转换发光。Yb³⁺离子的共掺杂对Er³⁺离子的上转换发光起明显的增强作用。研究了上转换发光强度与稀土离子浓度以及激发光强度之间的依赖关系,表明在Er³⁺单掺杂和Er³⁺,Yb³⁺共掺杂的样品中,绿光和红光都是被双光子激发过程激发的。还对上转换发光的机理做了初步分析。

Visible Upconversion Luminescence in Er3+/Yb3+-codoped SrTiO3 Ultra-fine Powders

There has been an intense interest in the investigation of ultra-fine and nanocrystalline oxide upconversion materials these years for potential applications in upconversion phosphors, detectors for infrared radiation, fluorescent labels for sensitive detection of biomolecules and two-photon confocal-microscope imaging. Numerous host materials, such as Y₂O₃, Gd₂O₃, ZrO₂, TiO₂ and BaTiO₃, have been investigated as host material for upconversion phosphors due to their low phonon-vibration frequencies. Er³⁺ doped materials are well suitable for upconversion because the metastable levels ⁴I_9/2 and ⁴I_11/2 of Er³⁺ can be conveniently popu-lated by low-cost high-power 800 nm and 980 nm laser diodes, respectively. The sensitization of Er³⁺ doped materials by Yb³⁺ ions is a well-known method for increasing the optical pump efficiency because of the high absorption cross section of Yb³⁺ ions around 980 nm and the efficient energy transfer from Yb³⁺ to Er³⁺ ions. Strontium titanate (SrTiO₃) is a well-known material because of its good properties, such as its high dielectric constant, high charge storage capacity, good insulating property, its chemical and physical stability and its excellent optical transparency in the visible range. In addition, similar to BaTiO₃, its phonon-vibration frequency is quite low, which makes it suitable for host material as upconversion phosphors. But to the best of our knowledge, there is no reports on upconversion of Er³⁺ doped, Er³⁺/Yb³⁺-codoped SrTiO₃ powders. Er³⁺-doped and Er³⁺ /Yb³⁺-codoped SrTiO₃ ultra-fine powders have been prepared in a molten NaCl flux. The structure properties of the samples were examined by X-ray diffraction (XRD);Fourier transform infrared spectroscopy (FT-IR) and field emission scan electron microscopy (FE-SEM). FT-IR spectra showed that SrTiO₃ powders may have higher upconversion luminescence efficiency because of its low phonon-vibration frequency. The intense green and red emissions around 528, 550 and 663 nm corresponding to the ²H_11/2→⁴I_15/2,⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ions were observed under excitation by 980 nm. Laser power dependence and rare earths ions concentration dependence of the upconverted emissions were investigated to understand the upconversion mechanisms. Excited state absorption and energy transfer process were discussed as the possible mechanisms for the visible emissions.