Synthesis And Properties Of Phosphors For White-light-emitting Diodes

Recently, white-light-emitting diode（WLED） as the fourth generation of lighting source has drawn a lot of interests in the scientific and business areas because of its low energy consumption, high efficiency, long lifetimes, and so on. Currently, much attention has been focused on WLED generated by combining UV chip with phosphors（UV-LED）. Therefore, many researchers have put great efforts to explore novel phosphors for UV-LED. Unfortunately, the white light phosphors, especially single-phase white-light-emitting phosphors for UV-LED are very limited. Besides, the current single component white-light-emitting phosphors usually show low luminescent efficiency and poor stability. In order to overcome above problems, this paper focus on developing novel efficient single-phase white-light-emitting phosphors excited by UV light for UV-LED. The detai LED research contents are shown as follows:（1） GdNbO₄:Ln³⁺(Ln³⁺ = Eu³⁺/Tb³⁺/Tm³⁺) phosphors: A series of rare earth ions doped GdNbO₄ nanocrystalline phosphors were synthesized by a Pechini-type sol-gel process. The results of XRD and FE-SEM indicate that the pure phase of GdNbO₄ phosphor with a grain diameter of around 20 nm 40 nm appeared after annealing at 1000 oC. Under the excitation of UV light and low-voltage electron beams, the GdNbO₄ shows novel blue broad band emission, which is different from previous reports. The emission spectra of GdNbO₄:Ln³⁺(Ln³⁺ = Eu³⁺/Tb³⁺/Tm³⁺) consist of the blue broad band emission of GdNbO₄ host lattice and the characteristic emission lines of Ln³⁺ ions under the excitation of UV light. There is an efficient energy transfer from GdNbO₄ host lattice to Eu³⁺ and Tb³⁺ ions in GdNbO₄:Ln³⁺ phosphors. However, the energy transfer in GdNbO₄:Tm³⁺ is inefficient. The emission color of Eu³⁺/Tb³⁺/Tm³⁺ single and double doped GdNbO₄ phosphors can be tuned from blue, bluish-green, green, pink, orange, yellow, etc., respectively, by changing doping concentrations, relative ratios, as well as species in GdNbO₄:Ln³⁺(Ln³⁺ = Eu³⁺/Tb³⁺/Tm³⁺) phosphors. White-light-emitting has been realized by changing Ln³⁺ ions doping contents and relative ratios in GdNbO₄:Eu³⁺,Tb³⁺,Tm³⁺ phosphors.（2） Ca₁₉Mg₂（PO₄）₁₄:Ln³⁺(Ln³⁺ = Eu³⁺/Tb³⁺) phosphors: Ca₁₉Mg₂（PO₄）₁₄:Ln³⁺ nanocrystalline phosphors were synthesized by a Pechini-type sol-gel process. Under the excitation of UV light, Ca₁₉Mg₂（PO₄）₁₄ sample gives blue broad band emission. The emission spectra of Ca₁₉Mg₂（PO₄）₁₄:Ln³⁺(Ln³⁺ = Eu³⁺/Tb³⁺) phosphors show not only the blue broad band emission of Ca₁₉Mg₂（PO₄）₁₄, but also the characteristic emission lines of Ln³⁺ ions. The results show that there exists an energy transfer from Ca₁₉Mg₂（PO₄）₁₄ to Eu³⁺, and Tb³⁺ ions in Ca₁₉Mg₂（PO₄）₁₄:Ln³⁺ phosphors. Under the irradiation of UV light, the emission color of Ca₁₉Mg₂（PO₄）₁₄:Ln³⁺(Ln³⁺ = Eu³⁺/Tb³⁺) phosphors can be tuned from blue, bluish-green, green, pink, orange, etc., respectively, by adjusting the concentrations, relative ratios, and species of Ln³⁺ ions. White-light-emitting can be obtained in Ca₁₉Mg₂（PO₄）₁₄:Eu³⁺,Tb³⁺ phosphors.（3） Sr₄Al₁₄O₂₅:Eu phosphors: A series of Eu²⁺ and Eu³⁺ co-activated Sr₄Al₁₄O₂₅:Eu phosphors were synthesized via a Pechini-type sol-gel process. The study indicates that the pure Sr₄Al₁₄O₂₅ can only be obtained when boric acid is added as flux with sintering temperature beyond 1000 oC. The Eu³⁺ ions can be initiatively reduced to Eu²⁺ ions in Sr₄Al₁₄O₂₅:Eu phosphors sintered in air with boric acid as flux. Under the excitation at 395 nm, the emission spectra of Sr₄Al₁₄O₂₅:Eu consist of the blue broad band emission of divalent europium(Eu²⁺) and the characteristic orange-red emissions of trivalent europium(Eu³⁺). The emission color of Sr₄Al₁₄O₂₅:Eu²⁺/3+ phosphors can be tuned from bluish-green region, through white region, to orange-red region by changing doping route and concentration of boric acid, and Eu³⁺ content.