Preparation And Luminescence Properties Of Ce~3+ Activated Germanate Phosphors

A series of Ce3+doped Mg3Y2Ge3O12 garnet phosphors were synthesized by solid-state reaction method and their luminescence properties were studied. Mg3Y2Ge3O12:Ce3+ absorbs in the wavelength range of 380-520 nm. Under 460 nm excitation, Mg3Y2Ge3O12:Ce3+shows a yellow emission band in the wavelength range of 480-750 nm with the maximum intensity appearing at 575 nm. When a part of Ge in the Mg3Y2Ge3O12 host is replaced by Si, the crystal field splitting between the two lowest 5d level of Ce3+is enhanced which could be due to a smaller Ce3+-O2-bond length, as result, the emission peak of the Mg3Y2Ge3O12:Ce3+is redshifted from 575 nm to 604 nm when 70%of Ge is replaced by Si. Further more, Doping Si into the host lattice remarkably broaden the band-gap of the Mg3Y2Ge3O12, which makes the Ce3+photoionization difficult, as a result, the quantum efficiency and the emission intensity of the Mg3Y2Ge3O12:Ce3+phosphor is greatly enhanced. The enhanced yellow-to-orange emission of Si-doped Mg3Y2Ge3O12:Ce3+makes it a potential conversion phosphor for warm white light-emitting diodes.Ce3+activated alkaline earth germanates Ca2GeO4, CaMgGeO4, Sr2Ge04 and Ba2GeO4 were synthesized by solid-state reaction method and their luminscence properties were studied. The electronic structure of Ca2GeO4 and CaMgGeO4 were calculated using the CASTEP code. Ca2GeO4 and CaMgGeO4 are olivine structure mineral, they both show indirect optical band-gap with band-gap energy of about 4.0 eV and 4.16 eV respectively. The top of their valence band are dominated by O 2p states, while the bottom of their conduction band are mainly composed of Ca 3d and Ge 4s states. The excitation band of Ca2GeO4:Ce3+ranges from 380 to 520 nm with a maximum intensity appearing at about 465 nm. Under 465 nm excitation, Ca2GeO4:Ce3+exhibits a broad yellow emission band in the wavelength range of 480-750 nm with the maximum intensity appearing at 565 nm. The emission of Ca2GeO4:Ce3+shows an obvious redshift when compared to the YAG:Ce phosphor, which makes it an potential white LED conversion phosphor. CaMgGeO4:Ce3+shows excellent absorption in the blue region as well, however, no visible emission is observed when the phosphor is excited under the blue light. Doping Al3+to replace a part of Ge4+in the Ca2GeO4:Ce3+could significantly enhance its emission intensity, which could be due to the charge compensation effect. When Mn2+is codoped into Ca2Ge04:Ce3+, the emission peak of Ca2Ge04:Ce3+, Mn2+shifts from 565 nm to 600 nm, and the bandwidth is reduced from 140 nm to 75 nm when the concentration of Mn2+increase. The excitation peak of Sr2GeO4:Ce3+and Ba2GeO4:Ce3+are 367 nm and 344 nm respectively. Under 350 nm excitation, they both exhibit an emission band in the wavelength range of 370-520 nm. Compared with commercial blue LED phosphor BAM, their chromaticity coordinates get close to the purple region. Moreover, the quantum efficiency of Sr2GeO4:Ce3+and Ba2GeO4:Ce3+are measured to be 35.2% and 28.3% respectively. Thus, although Sr2Ge04:Ce3+and Ba2GeO4:Ce3+have efficient absorption in the near-UV region, they are not proper to serve as blue LED phosphors.Single phase of Sr2Al2Ge07 and Ce3+, Eu2+, Pr3+, Tb3+doped as well as Ce3+/Pr3+, Ce3+/Tb3+codoped Sr2Al2Ge07 were synthesized by solid-state reaction method. The Rietveld refinement of the Sr2Al2GeO7 powder sample was carried out using GSAS program. Sr2Al2Ge07 crystallizes as a tetragonal structure with space group P42(?)m and lattice constants of a=7.947(1) A and c=5.361(4) A。Its electronic structure was calculated using the CASTEP code. S2Al2GeO7 shows an indirect optical band-gap with band-gap energy of about 4.38 eV. The top of the valence band is dominated by O 2p states, while the bottom of the conduction band is mainly composed of Sr 5p states. The efficient excitation band of Sr2Al2Ge07:Ce3+is in the wavelength range of 330-400 nm. Under 363 nm excitation, it shows an emission band in the wavelength range of 380-510 nm with a maximum at 430 nm. Sr2Al2GeO7:Eu2+shows efficient absorption in the wavelength range of 240-480 nm, but there is no visible emission is observed when the phosphor is excited under the related wavelength range. The emission of Sr2Al2GeO7:Pr3+is composed of several emission lines peaking around 485,530,563,616 and 647 nm respectively. The emission spectrum of Sr2Al2GeO7:Tb3+is composed of a group of emission peaking at 489,542,589 and 619 nm. For the Ce3+/Tb3+codoped Sr2Al2Ge07 phosphors, because of the energy transfer process from Ce3+to Tb3+, both the blue emission of Ce3+and the yellowish green emission of Tb3+are obtained. Combining the two emissions whose intensity is adjusted by changing the doping level of the coactivator, an optimized white light with chromaticity coordinates of (0.25,0.29) is generated. The phosphor could be a potential white phosphor for near-UV LEDs.