Synthesis And Optical Property Of White Luminescence Of Dy³⁺ Ions Doped 12CaO·7Al₂O₃ Nanopowders Under UV Light Excitation

The objective of this work was to develop a kind of phosphors, which is trivalent dysprosium (Dy³⁺)-doped 12CaO·7Al₂O₃ phosphors for white luminescence under ultraviolet light excitation. In this thesis, analysis had been made about the relationships of composition, structure and optical properties of Dy³⁺-doped 12CaO·7Al₂O₃ (12CaO·7Al₂O₃:Dy³⁺) phosphors, which were synthesized by the chemical co-precipitation method. The microstructure of 12CaO·7Al₂O₃:Dy³⁺ phosphors was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared spectra (FTIR), while the optical properties of 12CaO·7Al₂O₃:Dy³⁺ were characterized by photoluminescence, photoluminescence excitation, the temperature dependent luminescence, and so on.The XRD study revealed that the cage structural polycrystalline 12CaO·7Al₂O₃ powders of single cubic phase were obtained. It demonstrated that Dy³⁺ had been incorporated into 12CaO·7Al₂O₃ lattice site without destroying the lattice structure. The FE-SEM micrographs show that the surface topography of 12CaO·7Al₂O₃:Dy³⁺ annealed in vacuum ambient was spherical shape and covered a narrow distribution of particle sizes.The emission spectra show two dominant emission bands in the blue region and yellow region, one was attributed to the magnetic dipole transition of 4F9/2â†'6H15/2 centered at 484 nm, the other was forced electric dipole transition of 4F9/2â†'6H13/2 centered at 575 nm, respectively, were observed. There is also a weak red emission band at 659 nm from 4F9/2â†'6H11/2 transition. The 4F9/2â†'6H13/2 is a hypersensitive transition, which is strongly influenced by the surrounding environment around the Dy³⁺ ion. In general, 4F9/2â†'6H13/2 transition is prominent when Dy³⁺ ions are located at low-symmetry sites with no inversion center, whereas the 4F9/2â†'6H15/2 transition is stronger when Dy³⁺ is located at high symmetry sites with an inversion center. The blue emission (4F9/2â†'6H15/2) is stronger than the yellow emission, which indicates that the Dy³⁺ ions occupy a high-symmetry site with inversion symmetry in this matrix.The spectrum consists of many excitation bands of f-f transitions, which are originated from different transitions from the ground state 6H15/2 to the various excited states of 4f9 electronic configurations of the Dy³⁺ ions. The abundant excitation peaks in the 300–420 nm range implies that the Dy³⁺ ions can be efficiently excited by the near-UV (350–400 nm) LEDs. This is of promise for the white-light emissions in practical application.According to the field emission scanning electron microscopy micrographs and the FT-IR spectra, the enhanced emission intensities of 12CaO·7Al₂O₃:Dy³⁺ annealed in vacuum ambient could be ascribed to the decrease of OH- groups and the change in the surface topography.All the results indicate that Dy³⁺-doped 12CaO·7Al₂O₃ phosphor may be a good candidate for emissive display technology, the solid state lighting phosphor, white LEDs, and so on.