Studies On Energy Transfer And Luminescence Characteristics Of Rare Earth Ions Doped CaO And CaSc₂O₄

Rare earth doped luminescent materials have been widely application in lighting,biology,energy,and so on for their high conversion rate and various of colors.The energy transfer between doped ions is one of the important factors in determing the luminescence characteristics of the luminescent materials.Studying the process of energy transfer has the great academic significance and valuable applicability for exploring new materials and extending the function of materials.According to the solid-state lighting and biomarkers demand,the luminescence characteristics of CaO: Ce³⁺/Mn²⁺,CaSc₂O₄: Er³⁺/Yb³⁺ and CaSc₂O₄: Tb³⁺/Yb³⁺ were studied in this paper and the following innovations are achieved:?1?CaO: Ce³⁺,Mn²⁺ phosphors with various Mn²⁺ concentrations were synthesized by a solid state reaction method.Efficient energy transfer from Ce³⁺ to Mn²⁺ was observed and it allows the emission color of CaO: Ce³⁺,Mn²⁺ to be continuously tuned from yellow(contributed by Ce³⁺)to red(by Mn²⁺)with an increase in Mn²⁺ concentration and upon blue light excitation.CaO: Ce³⁺ exhibits a yellow emission band peaking at around 560 nm and an excitation band peaking at 460 nm which suitable for excitation by InGaN-based blue LEDs.CaO: Mn²⁺ exhibits a red emission band peaking at 600 nm and an excitation band peaking at 550 nm,which efficiency is very low when direct excitation.The excitation of Mn²⁺ overlaps very well with the emission band of CaO: Ce³⁺,implying the possibility of an efficient energy transfer from Ce³⁺ to Mn²⁺.When excitation of Ce³⁺,efficient energy transfer from Ce³⁺ to Mn²⁺ was observed and it allows the emission color of CaO: Ce³⁺,Mn²⁺ to be continuously tuned from yellow to red with an increase in Mn²⁺ concentration.The red emission becomes dominant when the Mn²⁺ concentration is ?0.014 with an energy transfer efficiency higher than 87% which can reach as high as 94% for a Mn²⁺ concentration of only 0.02.A warm white LED was fabricated through integrating an InGa N blue LED chip and a blend of two phosphors(YAG: Ce³⁺ yellow phosphor and CaO: 0.007Ce³⁺,0.014Mn²⁺ red phosphor)into a single package,which has CIE chromaticity coordinates of?x = 0.37,y = 0.35?,a correlated color temperature of 3973 K and a color rendering index of 83.1.The results indicate that CaO: Ce³⁺,Mn²⁺ may serve as a potential red phosphor for blue LED based warm white LEDs.?2?The energy transfer efficiency from Ce³⁺ to Mn²⁺ was enhanced along with the increase of the concentration of Mn²⁺.So,the emission intensity and lifetime of Ce³⁺ should be decreased.The emission intensities reduce much faster than the lifetime values with increasing Mn²⁺.So,it is proposed that the energy transfer from Ce³⁺ to Mn²⁺ in CaO: Ce³⁺,Mn²⁺ matched the Perrin model.We may consider an ‘active sphere' centered at a Ce³⁺ ion and the emission of Ce³⁺ can be completely quenched by a Mn²⁺ ion located within the sphere through the energy transfer.If the number of effective cation sites within the sphere is n,the fluorescence intensity and lifetime of Ce³⁺ exist the relationship of I ??1-x?nt.The value of n is 70 according to fitting experimental results.Combining the crystal structure of CaO,the conclusion of Ce³⁺ excitation energy can be completely transferred to Mn²⁺ if the Ce³⁺-Mn²⁺ distance is shorter than 7.6 ? is obtained.?3?CaSc₂O₄: 0.2%Er³⁺/x Yb³⁺ phosphors are prepared by high temperature solid-state reaction.The emission characteristics of two Er³⁺ luminescence centers in CaSc₂O₄ are investigated for the first time.It is found the emission shape of Er³⁺: 4F9/2 changed with the varies Yb³⁺ concentration,that demonstrating two Er³⁺ luminescence centers are existed in CaSc₂O₄: Er³⁺/Yb³⁺.These two luminescence centers can be named Er³⁺?I?centers and Er³⁺?II?centers respectively.So,the spectrum can be decomposed into Er³⁺?I?luminescent center emission and Er³⁺?II?luminescent center emission.It can be concluded that the number of Er³⁺?I?luminescence centers decreases and the number of Er³⁺?II?luminescent centers increases with the increase of Yb³⁺ concentration according to the results of spectral decomposition.Combining the lattice character of Ca2+ and Sc3+,Er³⁺?I?are assigned to substitution for Ca2+ site and Er³⁺?II?are assigned to substitution for Sc3+ site.Meanwhile,intense upconversion luminescence with color tuning from green to red is observed upon 980 nm excitation.The decomposition of the upconversion luminescence spectra exhibits the main contribution from the Er³⁺?I?centers in the range of Yb³⁺ concentration of this work.This result is attributed to the long lifetimes of Er³⁺?I?both on the green and red emitting levels.Furthermore,we have observed the upconversion luminescence intensity of CaSc₂O₄: 0.2%Er³⁺,15%Yb³⁺ is 4 times as strong as that in Y2O3: 0.2%Er³⁺,15%Yb³⁺,that shows a better upconversion luminescence efficiency of CaSc₂O₄: Er³⁺,Yb³⁺.?4?CaSc₂O₄: Tb³⁺,Yb³⁺ phosphors with varied Yb³⁺ concentrations were synthesized by a solid-state reaction method and the upconversion energy transfer of Yb³⁺ to Tb³⁺ in CaSc₂O₄ was observed for the first time.Upon excitation at 980 nm,all studied samples yielded relatively strong Tb³⁺ green luminescence corresponding to the ⁵D₄ ? 7FJ transition with the dominant component around 544 nm,indicating the existence of energy transfer from Yb³⁺ to Tb³⁺.Yb³⁺-Yb³⁺ cooperative luminescence are obtained,which can explain cooperative sensitization process for the population of the Tb³⁺: ⁵D₄ level.The energy transfer efficiency increased with the increasing Tb³⁺ concentration and reach 40% for Tb³⁺ concentration of 7%.