Studies Of Luminescence Characteristics Of Eu³⁺-doped Rare Earth Borates REBaB₉O₁₆ (RE=rare Earth Ions And Y)

Borates are exceptional materials because of their excellent chemical and thermal stability. Rare earth doped borates are especially attractive, because of their high luminescence efficiency and low firing temperatures. As one of the most important activator ions, Eu³⁺ is widely applied in red-emitting materials. Being the well known probe-ion, the luminescence of Eu³⁺ is highly affected by the surrounding environments in a lattice. The luminescence from 5D0â†'7F0 transitions of Eu³⁺ ions can present different emission spectra and decay profiles when their crystallographic surroundings have even small changes. The surrounding environment of Eu³⁺ ions doped in a host can be elucidated by applying the site-selective excitation and emission spectra technique. And this also can provide the symmetry of different luminescence centers in a matrix and then give details of the host structure.In this work, the Eu³⁺-doped REBaB₉O₁₆ (RE=La, Gd, Sm and Y) was prepared by high temperature solid-state reaction and their crystal structures and morphologies were investigated. The photoluminescence excitation and emission spectra, the luminescence decay curves, the thermal stability of luminescence and their potential applications as white LEDs phosphors were studied. The crystallographic sites of Eu³⁺ doped in REBaB₉O₁₆ were investigated by the laser site-selective excitation and emission spectroscopy together with the luminescence decay curves.In Chapter 3, the detailed structures of REBaB₉O₁₆ (RE=La, Gd, Sm and Y), including space groups and structural features, were investigated by XRD and FT-IR. The results indicate that REBaB₉O₁₆ has a hexagonal structure. The changes of cell volume follow the rule of shrinkage of lanthanide series, which is to say, as the decreasing radius of RE³⁺ ions, the relative cell volumes decrease. Otherwise, the vibration modes of B-O in the lattice were compared and analyzed by FT-IR spectra. In Chapter 4, REBaB₉O₁₆:Eu³⁺ (RE=La, Gd, Sm and Y) phosphors were synthesized and the photoluminescence properties were investigated. The excitation spectra showe very wide absorption bands in the region of 220-330 nm and 300-550 nm, which is due to the charge transfer (CT) transition of O^2-â†'Eu³⁺ and f–f transition of Eu³⁺ ions, respectively. The excitation bands match well with the radiation of near UV-emitting InGaN based LED chips. The phosphor can be efficiently excited to realize an intense red luminescence (614 nm) corresponding to the electric dipole transition 5D0â†'7F2 of Eu³⁺ ions. The luminescence intensity of REBaB₉O₁₆:Eu³⁺ phosphor was higher and higher with the elevated the doping concentration of Eu³⁺.? And the REBaB₉O₁₆:Eu³⁺ phosphors also show the stable color purity with the increasing concentration.In Chapter 5, the luminescence performances of EuBaB₉O₁₆ were systematically studied. The crystallite phase of the phosphor was observed by XRD patterns. Scanning electron micrographs were obtained to study the morphologic features and the particle size of the phosphor. It is clearly seen that the grains have hexagonal morphology. The particle size of the phosphor is about 3 to 5μm. The phosphor, when excited with near-UV wavelength radiation, exhibited an intense red luminescence. The characteristic emission of Eu³⁺ ions corresponding to the electric dipole transition 5D0â†'7F2 was located at 613 nm under 365 nm excitation. Its room temperature fluorescent decay profile exhibits a single exponent shape with a decay time of 2.03 ms. EuBaB₉O₁₆ phosphor also showed high quenching temperature and stable color purity with the elevated temperature. In addition, the luminescence intensity of EuBaB₉O₁₆ was higher than those of Eu³⁺ doped LaBaB₉O₁₆ and GdBaB₉O₁₆. So EuBaB₉O₁₆ could be a potential red-emitting phosphor in application on near UV chip-based white light-emitting diodes.In Chapter 6, the local structure and luminescent properties of EuBaB₉O₁₆ were discussed on the base of the spectroscopic probe of Eu³⁺ by the site-selective excitation and emission spectroscopy. The luminescence decay curves of Eu³⁺ were investigated. The site-selective excitation and emission spectra, the luminescence decay curves have been investigated by using a pulsed, tunable, narrowband dye laser in the 5D0â†'7F0 region in EuBaB₉O₁₆ crystals at 10 and 300 K. The excitation spectra corresponding to the 7F0â†'5D0 transition show that there are at least three different inequavalent crystallographic sites distributions in EuBaB₉O₁₆ lattices. The emission intensity ratios of the 5D0â†'7F1 and 5D0â†'7F2 transitions in each spectrum are also almost the same. The luminescence from Eu³⁺(A) (575.2 nm), Eu³⁺(B) (578.7 nm) and Eu³⁺(C) (579.2 nm) ?exhibits nearly the same lifetime. The selectively excited luminescence spectra and decay curves give the conclusion that three Eu³⁺ sites are arranged in EuBaB₉O₁₆ lattices with similar environment. The decay curve of the 5D0 emission of Eu³⁺(C) shows fast decay at the initial stage, while that of Eu³⁺(A) has rise time. This indicates that the population of the 5D0 state of Eu³⁺(A) takes place in part via energy transfer from Eu³⁺(C). The emission spectra under the excitations at 575.2 nm (A) and 579.2 nm (C) are nearly the same at 15 K. This means that energy transfer occurs between sites A and C. With the analysis by site-selective spectroscopy it can be suggested that the rare earth ions substitute for the crystal field sites with the hexagonal C3 or C3v symmetry in the EuBaB₉O₁₆ lattices. It is discussed that the influence of local structures of the Eu³⁺ sites is in relation with the excellent thermal stability of luminescence of EuBaB₉O₁₆.The novelties of this dissertation are as follows: the structure characteristics, photoluminescence performances and luminescence decays of Eu³⁺ doped borates REBaB₉O₁₆ (RE = La, Gd, Sm and Y) were systematically studied. The features about the microstructure of REBaB₉O₁₆ and the crystallographic site-occupations of RE ions were firstly investigated by the site-selective excitation and emission spectra. It provided a useful reference for the further development and application of rare earth doped REBaB₉O₁₆ phosphors.