| As a new concept of new solid-state light source, white light emitting diodes (LEDs) are opening a new revolution in human lighting field, because of their long life, high energy saving, environmental protection and other significant characteristics. At present, the mainstream white LED in market is phosphor convert LED (abbreviated pc-LED), wherein the luminescent material as an important part, and its performance directly affects the performance of white LED devices, which has received extensive attention. Generally, two ways can be employed in obtaining white light:one is using yellow phosphor YAG:Ce3+ to convert blue light to obtain white light; the other is using three phosphors named as blue, green and red, to convert ultraviolet light into white light. Aiming at the existed problems of phosphors for (near) UV LEDs, in this thesis, through designing and exploring the synthesis of materials, crystal structure refinement and spectral characterization, we systematically investigated the synthesis and luminescence properties of several new green, red and white light emission phosphors. The main research contents are as follows:1. In view of the darwbacks of the current green phosphors for (near) UV LEDs, we successfully developed three green emission phosphors Sr4La2Ca4(PO4)6O2:Ce3+, NaBaScSi2O7:Eu2+ and CaZr4(PO4)6:Eu2+ by high temperature solid slate reaction. Through the crystal structure refinement and spectroscopic characterization, we studied the crystal structure, luminescent properties and the effect of crystal field on the luminescence properties. Upon UV light excitation, Sr4La2Ca4(PO4)6O2:Ce3+ showed green light with dominate emission centered at 506 nm, and its emission intensity was measured to be 78.6% of the commercial green phosphor LMS520B, but the thermal stability need to be improved. White LED devices with excellent performance could be obtained based on the fabrication of Sr4La2Ca4(PO4)6O2:Ce3+ with commercial red phosphor and ultraviolet chips. NaBaScSi2O7:Eu2+ showed efficient green emission at 506 nm when excited by UV light and the emission intensity was measured as high as 89% of LMS520B, with good thermal stability. The bandgap value was calculated by local-density approximation. Through selective excitation spectra and fluorescence lifetime testing of the trace Eu2+ doped samples, we detailedly studied the site occupancy situation of Eu2+ in NaBaScSi2O7. In the research of redox mechanism of Eu, the self-reduction phenomenon was。 observed in Eu3+ doped CaZr4(PO4)6 sample prepared in air atmosphere. Under UV light excitation, the sample exhibited green emission band centered at 496 nm. Through analyzing the emission spectra, fluorescence lifetimes, electron paramagnetic resonance and X-ray photoelectron spectroscopy, it was proved that Eu2+ and Eu3+ coexistence in CaZr4(PO4)6. Then the self-reduction mechanism of Eu3+ in CaZr4(PO4)6 was proposed and verified by means of charge compensation.2. In view of the darwbacks of the current red phosphors for (near) UV LEDs, we successfully developed a novel efficient red emission phosphor Ca19Mg2(PO4)14:Eu3+. Through the crystal structure refinement, we investigated crystal structure and the crystal field situation around the cations in detail. Eu3+ would randomly occupy the asymmetric Ca2+ sites when it entered into the lattice. Diffuse reflectance spectra showed that the luminescent material had strong absorption in the ultraviolet region. Upon 393 nm excitation, the sample showed red emission with dominate peak at 612 nm, and the emission intensity was 1.92 times of the commercial Y2O3:Eu3+, with good thermal stability. Based on the crystal structure information by the structure refinement, the relationship between the characteristic emission spectra and crystal structure was systematically discussed. Through the Arrhenius activation energy mode, we explained the thermal quenching process and based on the configuration coordinates diagram, the abnormal temperature dependent emission of high energy transitions of Eu3+ was discussed and the mechanism was investigated.3. In view of the drawbacks of the current white LEDs based on three color phosphors, we successfully developed two novel white emission phosphors Ca5La5(SiO4)3(PO4)3O2:Ce3+, Mn2+ and Sr10[(PO4)5.5(BO4)0.5](BO2):Eu2+, Tb3+, Mn2+. For Ca5La5(SiO4)(PO4)3:Ce3+, Mn2+, we investigated the luminescence properties, site occupying situation and the energy transfer process among Ce3+-Mn2+ ion pairs through the luminescence spectra and fluorescence lifetimes. Based on the efficient energy transfer, color tunable white light emission could be obtained in Ca5La5(SiO7)3(PO4)3O2:Ce3+, Mn2+ samples. For Sr10[(P04)5.5(BO4)0.5](BO2):Eu2+, Tb3+, Mn2+ phosphor, we systematically studied the site occupancy situation and luminescent properties of Eu2+ and Mn2+ through the refinement of crystal structure and luminescence spectroscopy characterization. Based on the fluorescence lifetimes measurement and the luminescence spectra characterization, it was proved that energy transfer process existed among Eu2+-Mn2+ ion pairs and the energy transfer efficiency was calculated. After co-doping Tb3+ ion, the warm white emission with CIE (0.37,0.30) and CCT (3512K) could be obtained by adjusting the doping contents of Tb3+ and Mn2+ ions. |
PDF Full Text Request