The Yvo <sub> 4 </ Sub>: Eu Of <sup> 3 + </ Sup> Core / Shell Structure In Nano Phosphor Preparation And Performance

Due to the potential applications in biological fluorescence labeling, higher-resolution displays and illumination, rare earth nanophosphors have received considerable research attention. As a important red phosphor, the nanosize Eu-doped yttrium orthovanadate (YVO₄:Eu³⁺) is supposed to be a significant promise for excellent luminescent characteristic and thermal stability. Owing to serious surface recombination and higher surface defects density, luminescent efficiency of nanophosphors is lower than bulk YVO₄:Eu³⁺. Thereby, it is extremely significant to increase the luminescent efficiency for nanophosphors.In this dissertation, we pay attention to present the synthesis, structure and photoluminescence properties of YVO₄:Eu³⁺ core/shell structures. Formation of core/shell structure of YVO₄:Eu³⁺ nanophosphors can decrease the surface defects, thus increase the photoluminescence intensity.We presented core/shell structures of YVO₄:Eu³⁺@YVO₄,YVO₄:Eu³⁺@ YVO₄:Bi3+ and YPO₄@YVO₄:Eu³⁺. Characterizations by means of XRD,TEM and photoluminescence reveal that the as-prepared core/shell structures yield a much stronger photoluminescence intensity, the most significant results attained in this paper are given as follow:Firstly, YVO₄:Eu³⁺@YVO₄ core/shell structures were proposed by a two-step hydrothermal method which takes YVO₄:Eu³⁺ nanophosphor as core and YVO₄ as shell. The structures exhibit much stronger photoluminescence than the YVO₄:Eu³⁺ nanophosphor, which is due to the great decrease of surface recombination, surface defects density and surface state density of YVO₄:Eu³⁺ core. Meanwhile, a small fraction of Eu³⁺ available on the surface of the core in the core/shell structures can migrate to the YVO₄ shell leading to the increase in Eu³⁺- Eu³⁺distance , reducing the extent of quenching and hence increasing the luminescent efficiency of nanophosphors. The optimal molar ratio of YVO₄/YVO₄:Eu³⁺( defined as R ) is 1:12, and the structures yield a photoluminescence intensity 113% higher than that of the YVO₄:Eu³⁺ nanocrystals. Secondly, YVO₄:Eu³⁺@YVO₄:Bi³⁺ core/shell structures were also prepared by a two-step hydrothermal method which takes YVO₄:Eu³⁺ nanophosphor as core and YVO₄:Bi³⁺ as shell. Because for core/shell structures, surface recombination and surface defects around the Eu³⁺ present on the surface of the core are removed effectively by the YVO₄:Bi³⁺ shell. It results in improved luminescence .Moreover, Bi³⁺ acts as a sensitizer for Eu³⁺,leading to increase the energy transfer of Eu³⁺-Eu³⁺ and the photoluminescence intensity . The optimal molar ratio of YVO₄:Bi³⁺/YVO₄:Eu³⁺ (defined as R) is 1:12, and the structures yield photoluminescence intensity 142% higher than that of the YVO₄:Eu³⁺ nanocrystals.Thirdly, YPO₄@YVO₄:Eu³⁺ core/shell structures were presented by a immersion-hydrothermal process. They were readily formed by hydrothermal epitaxial growth of YVO₄:Eu³⁺ onto YPO₄ because YPO₄ and YVO₄ have the same crystal structure and similar lattice parameters. YPO₄ is useful to energy transfer process from VO₄3- to Eu³⁺, and absorption of energy. The core/shell structures (R = 2:1, R was defined as molar ratio of YVO₄:Eu³⁺/YPO₄) exhibit much stronger photoluminescence and yield a photoluminescence intensity 71% higher than that of the YVO₄:Eu³⁺ nanocrystals under the same conditions. The photoluminescence intensity is nearly a constant value even if R increases.In conclusion, the YVO₄:Eu³⁺@YVO₄,YVO₄:Eu³⁺@YVO₄:Bi³⁺ and YPO₄@YVO₄:Eu³⁺ core/shell structures are presented, and PL measurements show that the structures can enhance not only the PL intensity but also the thermal stability of the nanophosphors.