Synthesis And Luminescence Properties Of Rare Earths Doped Molybdate, Phosphate Phosphors For LED

As a new kind of green light source in the 21 st century, white LED (WLED) has attracted much attention and has been widely applied in lighting market. For the preparation of commerical WLED, it is important to explore some novel promising phosphors, which are the key for the development of the semiconductor lighting technology. Up to now, although more than 1000 kinds of phosphors have been developed, but they suffer from many serious problems, such as thermal stability, chemical stability, corrosion resistance, difficult synthesis conditions (high temperature and high pressure), high color temperature and poor color rendering index (CRI) because of the color deficiency in the red spectrum region. This thesis aimed at developing novel superior stability, easy synthesis, strong absorption in near ultraviolet, matching blue chip and high luminescence efficiency red phosphors used for WLEDs. Furthermore, an novel warm white light emtting single matrix phosphors was synthesized and studied.Novel rare-earth ions doped molybdate and phosphate phosphors were synthesized by a standard solid-state reaction under an ambient atmosphere. Their structural and optical properties were investigated. X-ray diffraction (XRD) patterns and diffuse reflection spectra (DRS) were used to explore structural properties. The photo luminescence (PL) and photoluminescence excitation (PLE) were used to study their photoluminescent properties. They have excellent luminescent performance, providing some new candidates for WLED. The main work was as follows:1. Novel LiLa1-x-y(MoO4)2:xSm3+,yEu3+(LL1-x-yM:xSm3+,yEu3+) double molybdates red phosphor was synthesized by solid-state reaction under 610℃. Pure phase phosphor was obtained by low temperature sintering. Nano-scale materials and longtime grinding method were used to reduce synthesis temperature. Compared with the previous phosphor synthesis technology, our method has the advantages of simpler process, lower synthesis temperature and lower equipment requirements. Optimal doping concentration of Sm3+ in LiLa1-x(MoO4)2:xSm3+(LL1-xM:xSm3+)phosphor was just about 0.05 mol(x= 0.05) and corresponding quenching behavior was ascribed to be electric dipole-dipole exchange interaction according to Dexter’s theory. Efficiency energy transfer from Sm3+ to Eu3+ was found in further study. By the co-doping of Eu3+ ions, the absorption spectrum became wider and stronger from ultraviolet to visible spectrum region than that of the single doped samples. Under the near ultraviolet (n-UV) excitation of 403 nm, the chromaticity coordinates of LL0.95-yM:0.05Sm3+,yEu3+ approach regularly to the NTSC standard values (0.670,0.330) with the increasing of Eu3+ doping concentration. Chromaticity coordinate of LL0.83M:0.05Sm3+,0.12Eu3+ was (0.629,0.370), extremely close to the commercial phosphor Y2O2S:Eu3+(0.631,0.631), and the phosphor has a pure color and high emission intensity. It overcomes the difficulty that red commercial phosphor cannot effectively excited by n-UV LED. The strong absorption at wavelength around 400 nm can match well with commercial n-UV LED. The results indicate that LL1-x-yM:xSm3+,yEu3+ could be a promising WLED red phosphor.2. Orthophosphate with stable physical and chemical properties was chosen for matrix. Novel Ba3Y1-x-y(PO4)3:xSm3+,yEu3+(BY1-x-yP:xSm3+,yEu3+) red phosphors were synthesized by a standard solid-state reaction under an ambient atmosphere. The study of luminescent spectra and fluorescence lifetime indicate that there is efficient energy transfer from Sm3+to Eu3+, Sm3+ can be not only used as sensitizer but also luminescence center, it was not reported in previous studies of this kind matrix. The work also shows that the phosphor has a strong excitation band around ultraviolet region and higher red emission intensity from Eu3+ under 393 nm light excitation. The excitation around 393 nm and 465 nm become broader and were prominently enhanced by the co-doping of Sm3+-Eu3+, solving the problem that commercial red phosphor (Y2O3:Eu3+, Y2O2S:Eu3+) cannot match well with LED chip and low luminescent efficiency. The concentration quenching appeared when Sm3+ molar concentration was higher than 0.04 mol of Ba3Y1-x(PO4)3:xSm3+(BY1-xP:xSm3+). The corresponding quenching behavior was ascribed to be electric dipole-dipole exchange interaction. The chromaticity coordinate of BY0.96P:0.04Sm3+ and BY0.82P:0.04Sm3+,014Eu3+were (0.585,0.585) and (0.617, 0.617), respectively, under the excitation of 402 nm. With the co-doping of Eu3+. BY1-xP: xSm3+ shows great improvement in both color rendering index and emission strength. The controllable optical characteristic can satisfy the requirements of different occasions. The results indicate that BY0.82P:0.04Sm3+,014Eu3+ is a promising red-emitting phosphor for n-UV based white LEDs or backlight of LCD.3. Novel Sr3Y1-x-y(PO4)3:xDy3+, yEu3+(SY1-x-yP:xDy3+,yEu3+) phosphors were synthesized by a standard solid-state reaction under an ambient atmosphere. Their structural and optical properties were investigated. X-ray diffraction (XRD) patterns and diffuse reflection spectra (DRS) were used to explore structural properties. The former showed that single phase phosphors were obtained and that the rare earth ions entered into the cubic host by substituting the smaller Y3+ ions and thereby enlarging the unit cell. The diffuse reflection spectra indicated the SYP host has a direct bandgap of 4.5 eV, similar to those of the doped and co-doped materials. A strong and stable warm white light emission with high color purity was achieved by adjusting the doping concentration of the Eu3+ and Dy3+ ions in the SYP host. Under 393 nm excitation, the SY0.92P: 0.06Dy3+,0.02Eu3+ phosphor exhibited a lower correlated color temperature (CCT) of 5484 K, compared with the commerical phosphors, and the CIE coordinates was (0.3325, 0.3110), closely to white light standard point (0.333,0.333). Previous reports on single phase white phosphors show higher color temperature arid lower color purity. The energy transfer from Dy3+ to Eu3+ ions was investigated and the related mechanism was discussed based on the optical spectra and emission decay curves. The thermal quenching of emission is similar to that of YAG:0.06Ce3+. The results indicated that single phase SY1-x-yP:xDy3+,yEu3+ phosphor has a promising potential for economical application in warm white LED.