Structure And Fluorescence Properties Of Single-substrate White Fluorescent Materials Excited By Near-ultraviolet LEDs

The main approach to fabricate WLED devices is to combine a blue LED chip with the yellow-emitting phosphor Y3Al5O12:Ce3+, which has already been commercialized. However, the white light generated by this kind of LED device generally shows a poor color rendering index (CRI) and a high correlated color temperature (CCT) due to the deficiency of red component in the spectral region. In addition, An alternative method to combine near-ultraviolet (NUV) LED chips with trichromatic red, green and blue light-emitting phosphors has been proposed. The phosphor mixture produces an inevitable problem of fluorescence re-absorption between different components and a non-uniformity of the luminescence properties, resulting in a loss of luminous efficiency and time dependent shift of the color point. A series of Srs(PO4)3F:0.02Eu2+,xMn2+ phosphors were synthesized by solid-state reactions. The luminescence property, and the crystal and electronic structures of the fluorophosphates were studied by photoluminescence analysis, XRD Rietveld refinement and density functional theory calculation (DFT), respectively. A series of single-phased white-light-emitting phosphor were prepared by solid-state reaction in this paper. In addition, the luminescent property, crystal and electronic structure of the fluorophosphates were studied by photoluminescence, XRD Rietveld refinement and density functional theory calculation (DFT), respectively. A series of fluxes were investigated to improve the photoluminescence intensity. The main results are summarized as follows:In the second chapter, a series of single-phased broadband white-light-emitting Sr5(PO4)3F:Eu2+, Mn2+ were prepared by solid state reaction. Phosphates with the hexagonal apatite structure (space group P63/m) were confirmed by XRD, When the molar ratio of Eu2+ ions fixed at 0.02, increasing Mn2+ ions up to x=0.06 does not cause significant change of structure. The samples Sr5(PO4)3F:0.02Eu2+, xMn2+(x=0.02-0.06) are refined by the MAUD suite program of Rietveld method using Sr5(PO4)3F as initial model (ICSD# 95737), Mn2+ and Eu2+ prefer occupy the Sr1 sites due to the reliability parameters of refinement is optimum. The electronic structure of the fluorophosphates were studied by density functional theory calculation (DFT), which showed hexagonal Sr5(PO4)3F host is broad band gap materials with direct band gap energy 5.099 eV. By contrast, with Eu2+/Mn2+ co-doped, the band gap energy of Sr5(PO4)3F:0.02Eu2+, 0.05Mn2+ decreased to 3.035 eV. This phenomenon was in agreement with the absorption spectrum. For the excitation and emission spectra of Sr5(PO4)3F:0.02Eu2+,xMn2+ (x=0-0.06), The excitation spectrum of the obtained phosphors exhibits a broad excitation band in the wavelength range of 250-430 nm, which is matchable with NUV LED chips. The emission band of Sr5(PO4)3F:0.02Eu2+,xMn2+(x=0.02-0.06)centered at 440 nm and 554nm, the emission color can be trimmed by adjusting the ratio between Mn2+ and Eu2+ ions according to the energy transfer from the sensitizer Eu2+ to activator Mn2+ in the host. In addition, in order to improve the emission intensity of Sr5(PO4)3F: 0.02Eu2+, xMn2+, different fluxes (KCl,NaCl,HBO3)with double molar amount of the final fluorophosphate compound were added in synthesis of phosphors, it can be found that the photoluminescence intensity of phosphor Srs(PO4)3F:0.02Eu2+,0.05Mn2+are prominently enhanced by KCl, NaCl, and H3BO3, among which KCl presents the optimum promotion effect. The PL intensity of Sr5(PO4)3F:0.02Eu2+,0.05Mn2+ is drastically enhanced when the phosphor is prepared with twice molar KC1 as flux. The PL intensity increases by 85% compared with the sample synthesized without flux added. Those luminescent properties indicate it was a prospective for application in white light emitting diodes (LEDs).In the third chapter, we also investigate a series of single-phased white-light-emitting Sr7Zr(PO4)6:Eu3+,Tb3+. Based on XRD patterns and XRD Rietveld refinement of the phosphor, no diffraction peak of impurity is discerned, suggesting that the obtained samples are single phase. For the excitation and emission spectra of Sr5(PO4)3F:0.02Eu2+,xMn2+(x=0-0.06),The PLE of the obtained phosphors exhibits a broad excitation band in the wavelength range of 300-400 nm and centered at 374, which made it meet the demand of NUV LED chips. The emission band of Sr7Zr(PO4)6: 0.01Tb3+,xEu3+(x=0-0.02)centered at 544 nm and 613nm, the emission color can be changed from blue to white by adjusting the ratio between Tb3+ and Eu3+ ions according to the energy transfer from the sensitizer Tb3+ to activator Eu3+ in the host. We also studied the the possible energy transfer mechanism of the sample, it suggested that the energy transfer from Eu2+ to Mn2+ occurs following a dipole-dipole interaction and the critical distance of energy transfer from Tb3+ to Eu3+is calculated to be 11.72 A. Those optical properties showed the obtained phosphors meet the demand of Near-UV White LEDs, which made it have significant valued in the practical and theory study.