Studies On UV Excited Phosphors For White LEDs

UV-excited single-phase white light emitting or the mix of the NUV-excitedhigh brightness blue, yellow and red phosphors are the two commenly ways toachieve white light emitting. Aimed at the study of UV or NUV-excited white lightemitting LED, We discussed the Ce³⁺,Tb³⁺and Sm³⁺doped Ca₂SiO₄phosphors, theluminescence porperties of the illumination ions and the energy transfer properties ofCe³⁺-Tb³⁺, Ce³⁺-Sm³⁺and Tb³⁺-Sm³⁺. We also discussed the excitation, emission andtemperature properties of the NUV-excited red emitting phosphorsSr₂NaMg₂V₃O₁₂:Eu³⁺,Bi³⁺. The mainly study results are as follows:（1） Based on high temperature solid state reaction method, we synthesized theCe³⁺, Tb³⁺, and Sm³⁺, single-doped, co-doped and tri-doped Ca₂SiO₄samples. Theemission spectrum of the single-doped samples showed that all the three kinds ofions can efficiently illuminate in Ca₂SiO₄. The excitation spectrum of the Ce³⁺ionscentred at360nm and the emission spectrum is centred at430nm and the spectrumrange is380-550nm. The fitting result of the emission spectrum shows that theemission spectrum is assembled by two Gauss spectrums, which centred at417and453nm, seperately. The excitation spectrum of the Tb³⁺ion is combined by two wideexcitation spectrums, which located at252and286nm and a series of sharpexcitation lines, which are in the range of300-450nm. The emission spectrum ofTb³⁺is composed by four emission peaks,489,545,590and624nm. The excitationspectrum of Sm³⁺ion is assembled by a series of sharp excitation lines, located at therange of300-500nm. The main emission peak is located at601nm. To theco-doping samples, we focused our attaintion in Ca₂SiO₄:Ce³⁺,Tb³⁺and Ca₂SiO₄:Ce³⁺,Sm³⁺. When we monitor the542nm emission of Tb³⁺inCa₂SiO₄:Ce³⁺,Tb³⁺, both the excitation bands of Ce³⁺and Tb³⁺could be observed.When we monitor the601nm emission of Sm³⁺in Ca₂SiO₄:Ce³⁺,Sm³⁺, both theexcitation bands of Ce³⁺and Sm³⁺could be observed. Those facts show that there areenergy transfer between Ce³⁺-Tb³⁺and Ce³⁺-Sm³⁺in Ca₂SiO₄. Under the360nmexcitation the emission intensity of the Ce³⁺decreased constantly and the emissionintensity of Tb³⁺first increase and then decrease the most suitable Tb³⁺concentrationis15%. The emission trand of the Ce³⁺and Sm³⁺codoping samples are the same asthe Ce³⁺and Tb³⁺codoped samples, but the most suitable Sm³⁺concentration is3%.According the emission spectrum, we can acculate the CIE coordinates of theemission spectrums of our samples. The CIE range of our Ce³⁺,Tb³⁺-codopedsamples is （0.17,0.11）-（0.28,0.35）. The CIE range of the Ce³⁺,Sm³⁺-codoped samplesis （0.20,0.10）-（0.24,0.12）.（2）Based on the Perrin model and the Inokuti–Hirayama formula, we studied theenergy transfer properties of Ce³⁺-Tb³⁺and Ce³⁺-Sm³⁺in Ca₂SiO₄. We discoveredthat in the Ce³⁺and Tb³⁺codoped samples, the difference is significant between theacculated energy transfer efficiency based on the Ce³⁺illumination intensity and theenergy transfer efficiency based on the Ce³⁺lifetime data. The reason of thediffirence is caused by the exchange interaction. Therefore, the study of the energytransfer properties of the Ce³⁺-Tb³⁺should based on Perrin model and uses theillumination intensity data of the Ce³⁺. The energy transfer efficiency of theCe³⁺-Tb³⁺can as high as75%, when the Tb³⁺concentration is25%. To the energytransfer between Ce³⁺and Sm³⁺, the caculated results show that the energy transfertype is belongs to dipole-diople interaction.（3） Under the360nm excitation the emission spectrums of the tri-dopedCa₂SiO₄:0.05Ce³⁺,xTb³⁺,ySm³⁺（x=0.10-0.20, y=0.001-0.030） samples show that theintroduce of the Sm³⁺into the Ce³⁺and Tb³⁺codoped samples, it can not only add thered-emitting component in the spectrum but also can adjust the emission intensityrate of Ce³⁺and Tb³⁺. When monitor the650nm emission of the Sm³⁺ions, the excitation peaks of the Ce³⁺, Tb³⁺and Sm³⁺were all can be observed, however whenthe monitor emission wavelength is542nm, only the excitation peaks of Ce³⁺andTb³⁺can observed and no Sm³⁺excitation peaks, which means there exist energytransfer from Tb³⁺to Sm³⁺and this energy transfer path is single direction. When theconcentration rate of the Ce³⁺/Tb³⁺/Sm³⁺is0.05/0.15/0.003, the CIE coordinates ofthis sample is （0.31,0.32）, which is very close to the CIE coordinates of the standardwhite light,（0.33,0.33）.（4） We synthesized pure-phase vanadate garnet phosphors, Sr₂NaMg₂V₃O₁₂：Eu³⁺,Bi³⁺. We studied the crystal structure properties and the excitation and emissionproperties of the phosphors. Even though the introduce of the Bi³⁺ions in the crystalwill decrease the excitation intensity, it can also increase the f-f transition of theEu³⁺.Under the excitation of365and394nm, we compared the luminescentintensity with the commercial used red light emitting phosphors,. The comparisonproved that our phosphors can act as potential candidates for the NUV-excited redlight emitting phosphors.