(660f) Structural Effect to Rare Earth Luminescence Properties

Rare earth (RE) ions have the ability to absorb photon energy and emit visible light due to their unique electronic structures, but the efficiency of this process is affected by many factors, such as RE ion concentrations, local environments, oxidation states, and co-dopant effect. For example, a highly symmetric local environment surrounding the luminescence center increases the possibility of vibrational relaxation, which reduces the intensity emission transition. Meanwhile, the luminescence intensity of the most RE ions can be sensitized with other dopants, such as Ce³⁺ and Yb³⁺ due to their large absorption cross-sections. Since the electronic structure is dependent upon the oxidation state, not all ions of the same element can absorb energy and emit visible light. As such, it is vital to understand the relationship between dopant local and electronic structures for their application in phosphors, oxide ion conductors, and photocatalysts.

Herein, we studied the co-dopant effect to the local environment and oxidation states of rare earth dopants in complex yttrium oxide nanoparticles (NPs). To achieve homogeneous mixtures of these complex oxides, a two-step co-precipitation/molten salt process was performed. Specifically, Y_xB_yO_7-δ (B= Zr, Sc) compounds were investigated to understand the importance of cation size on RE dopant distribution, oxidation state, and luminescence. Photoluminescence (PL) spectroscopy and lifetime were applied to determine the luminescence efficiency, while X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopic (XPS) were used to measure the oxidation states of RE ions and oxygen vacancy change in the lattice. First, Tb/Ce codoped Y₂Zr₂O₇ (YZO) NPs were synthesized for enhanced green emission. However, PL results showed the incorporation of Ce quenches Tb emission due to the formation of undesirable non-luminescent Tb⁴⁺ species. Meanwhile, the incorporation of Ce increases the local symmetry level due to the incorporation of higher lattice oxygen concentrations. To overcome unwanted oxidation and control dopant positions, Y_2-xSc_xO₃hosts weresynthesized to force the RE dopants into the lower symmetric C₂ sites. PL results show that the Y_2-xSc_xO₃ NPs with x=0.5 shows an intense upconversion response, comparable to the state-of-the-art NaYF₄:Yb, Er standard. Additionally, Judd-Ofelt calculations were performed on this system to extract the theoretical lifetimes and luminescent efficiency of Er³⁺ emission. Ultimately, these results allow for the engineering of phosphor compositions to control local environment and dopant oxidation states of RE dopants to optimize the luminescence performance and develop new, high-efficiency phosphors.