Preparation And Properties Of Metal Ion Doped Phosphate Phosphors For LEDs

Phosphate phosphors have been widely used in the lighting field due to the characters of thermal and chemical stability, low synthesis temperature and high absorption efficiency under vacuum ultraviolet excitation. Nowdays, the light emitting diode (LED) is regarded as the most promising lighting technology in the 21th century, because of high efficiency, energy saving, environmental friendliness, long life and good color rendering index. And traditional red LED phosphors suffer a setback in low luminous efficiency and other serious problems. Besides, how to realize white emission in LED has become a severe challenge in recent sdudies. In this paper, the properties and synthesis method of phosphate phosphors as well as the basic theories of photoluminescence were reviewed. Rare earth ions and transition metal ions doped, near ultraviolet activated phosphate phosphors were prepared by traditional high-temperature solid-state method. The phase, morphology and luminescent properties were investigated by XRD, SEM (TEM), excitation and emission analysis, fluorescent thermal stability, and fluorescence lifetime. The main work of the paper were as follow:(1) A series of high brightness orange-red phosphors β-Ca2-xP2O7:xEu3+(x= 0.02,0.04,0.06,0.08,0.10,0.12) were synthesized via solid state reaction in air atmosphere. The behavior of concentration induced lattice distortion, anti-thermal quenching ability and site symmetry of Eu3+ in β-Ca2-xP2O7 was systematically investagated. The intense emissions at 585,591 nm (5D0â†'7Fi), 610 nm (5D0â†'7F2) were observed under NUV excitation. The ratio of the emission of 5D0â†'7F2 transition to that of 5D0â†'7F1 transition changed dramatically with varied Eu3+ concentration (1-6%), indicating the existence of the site symmetry transformation of Eu3+ luminescence center in the matrix. XRD analysis showed that, in the case of low concentration, the lattice distortion tends to shrink normally; while in high concentration, crystal defects increased, the ability of substitution weekened, and the lattice distortion presented to be expansion. The chromaticity coordinates of the samples with 1% and 4% concentration were calculated and located in the orange-red region. The phosphors performed excellent properties at 423 K with temperature quenching at 473-573 K, and may be potential orange-red phosphor in lighting field.(2) Synthesis of a charge compensated phosphor Sr2P2O7:Eu3+,Na+ by high-temperature solid-state method was reported. Under the excitation of 393 nm, the phosphor shows intense red emission centered at 610 nm, which is due to the 5D0â†'7F2 transition of Eu3+ ions. The influence on the luminescent properties of different compensators was investigated. And it is proved that Na+ is the best compensator among Li+, Na+, K+ ions by XRD and emission analysis. The luminescent properties of Sr2P2O7:Eu3+,Na+ were dramatically improved in the aspect of the emission intensities and chromaticity. And the mechanism of which was discussed in detail. Furthermore, the decay time, concentration quenching and temperature quenching effect of the samples were also investigated. The results showed that Sr2P2O7:Eu3+,Na+ could be an efficient red phosphor for light emitting diodes (LEDs).(3) A series of phosphors Sr2-xP2O7:xEu2+(x= 0,0.005,0.01,0.02,0.025) and Sr1.99-xP2O7:0.01Eu2+,xMn2+(x= 0.05,0.1,0.15,0.2) were synthesized through solid stare reaction. The phase, morphology and luminescent properties were characterized by XRD, SEM and photoluminescence analysis. It showed that these samples crystallize in pure orthorhombic with good crystallinity and uniform grains when sintered at 1050℃. Under the excitation of 355 nm, a broad emission band ranging from 370-480 nm with a peak at 418 nm was obtained in Sr2P2O7:Eu2+, which is attributed to the 5d-4f allowed transitions of Eu2+. The concentration quenching effect and its mechanism were also discussed. The white emitting can be realized by co-doping with Mn2+ adjusting the chromaticity. It is suggested that Eu2+ plays a role of sensitizer and transfers energy to Mn2+. The d-d transitions of Mn2+ can be improved by energy feeding from Eu2+, which makes it suitable for the application of white light emitting diodes (WLEDs).