Preparation And Luminescence Of Nanophosphors For White Light-emitting Diodes

In this thesis,the YAG:Ce³⁺nanophosphors as well as the pure and doped Sr₂CeO₄ nanophosphors were synthesized by combustion method.The phases,morphologies and luminescence properties of the nanophosphors were studied by employing the techniques of XRD,TEM,absorption,excitation and photoluminescence spectra.Firstly,the YAG:Ce³⁺nanophosphors with the mean particle sizes in the range of 16～42 nm were prepared through a combustion method,using glycine as a fuel.The pure-phase YAG:Ce³⁺nanophosphor was obtained at the annealing temperature of 1000℃which is lower than the sintering temperature for the bulk material.Blue shifts of the absorption edge and the position of the emission peak were observed for the YAG:Ce³⁺ nanophosphor annealed at 900℃with respect to the bulk material.Besides,compared with that of the bulk material,the lifetime of Ce³⁺in the YAG nanophosphor decreased from 51.5 ns to 10.4 ns.Furthermore,KC1 was added to the combustion process in order to improve the luminescence properties of the YAG:Ce³⁺nanophosphor.The results showed that the presence of KC1 could lower the annealing temperature for the pure-phase YAG:Ce³⁺nanophosphor to 900℃and increase the luminescence intensity by 1.6 times.Secondly,the Sr₂CeO₄ nanophosphors with the mean particle sizes in the range of 12～50 nrn were prepared through a combustion method,using glycine as a fuel.The results showed that the pure orthorhombic phase Sr₂CeO₄ nanophosphor was obtained at the annealing temperature of 1000℃when the G/N ratio was 1.5 times of the stoichiometric amount and the ignition temperature was 400℃.Compared with the bulk material,the absorption edge of the Sr₂CeO₄ nanophosphor had a blue shift. Besides,with the increasing of the annealing temperature,the absorption edge of the Sr₂CeO₄ nanophosphor red-shifted.The excitation spectrum of the Sr₂CeO₄ nanophosphor was a broadband bimodal structure with two peaks at 300nm and 340 nm,respectively,which belonged to terminal Ce⁴⁺-O^2--Sr²⁺bond and surface Ce⁴⁺-O^2--Ce⁴⁺bond.The intensity ratio of the two peaks changed significantly under different conditions.The annealing at the temperature of 1200℃was more conducive to the excitation of 340 nm.The emission spectrum of the Sr₂CeO₄ nanophosphor was also a broad band centered at 470 nm,which was corresponding to the transitions of Ce⁴⁺-O^2-charge transfer(CT)states in Sr₂CeO₄. Thirdly,we studied the luminescence properties of the Sr₂CeO₄:Re³⁺(Re³⁺=Eu³⁺, Sm³⁺or Dy³⁺)nanophosphors.The results showed that there existed energy transfer from the Sr₂CeO₄ host to the rare earths.The emission spectra of the Sr₂CeO₄:Re³⁺phosphors exhibited the corresponding characteristic peaks of the rare earth ions.The emission colors can be tailored by adjusting the concentrations of the rare earth ions.White light was achieved in the Sr₂CeO₄:Eu³⁺phosphor.When Eu³⁺concentration was 2 mol%,the chromatic coordinates of the sample were x=0.3312 and y=0.3178.When Sm³⁺ concentration was 1 mol%,the chromatic coordinates were x=0.3062 and y=0.2674. When Dy³⁺concentration was 1 mol%,the chromatic coordinates were x=0.2558 and y=0.2739.Finally,we also studied the luminescence properties of the codoped Sr₂CeO₄ nanophosphors.For the Sr₂CeO₄:Mn²⁺,Sm³⁺(0.25 mol%)or Sr₂CeO₄:Dy³⁺,Sm³⁺(0.25 mol%)nanophosphors,their chromatic coordinates changed with Mn²⁺or Dy³⁺ concentration and got closer to those of the standard white light.The chromatic coordinates of the Sr₂CeO₄:Sm³⁺(0.25 mol%)were x=0.2149 and y=0.2352.When doped with 5 mol%Mn²⁺,its chromatic coordinates changed to x= 0.2350 and y= 0.2920.When doped with 1 mol%Dy³⁺,its chromatic coordinates were x=0.2681 and y=0.2792.