| Er3+/Yb3+doped materials with up-converted luminescence exhibit no spontaneousfluorescence effect and excitation and emission wavelength having high transmittancein vivo application. On the other hand, for oxide materials with high thermal andchemical stability, it is flexible to fabricate thin films and high quality devices withlarge area by using magnetron sputtering, molecular beam epitaxy and pulsed laserdeposition methods, thus Er3+/Yb3+doped oxides materials with up-convertedluminescence can be widely used in the applications of solar cells, anti-counterfeiting,laser and bio-imaging, etc.12CaO·7Al2O3(C12A7) with good thermal and chemical stability has unique cagestructure, encaged O2-ions in C12A7can be replaced by OH-, H-, e-, O-and Cl-ions.For Er3+doped C12A7with encaged OH-, the existence of OH-will weakenup-converted luminescence of Er3+. With increasing the Er3+/Yb3+/Gd3+dopingconcentration, the converted luminescence intensity was obviously enhanced. However,the mixed-phase samples (C12A7/CaGdAlO4) were obtained. Under980nm excitationthe converted luminescence of CaGdAlO4:Er,Yb is stronger than that of C12A7: Er,Yb.Under the conditions of the optimal doping concentration, the converted emissionintensity of CaGd0.90AlO4:Er0.01,Yb0.09is reached to6%of commercial NaYF4:Er,Yb.The experimental results show that the existence of Gd3+in the matrix has played animportant role in enhancing green emission of Er3+in CaGdAlO4: Er, Yb through theenergy transfer process.CaGdAlO4:Er3+,Yb3+samples with different concentrations of high-energyvibration group were prepared by self-propagating combustion method. Theexperimental results show that the ratio of red to green emission intensity graduallyincreases with increasing the high-energy vibration group numbers. It can be attributedto the existence of the high-energy vibration group has accelerated the4I11/2→4I13/2and4S3/2→4F9/2non-radiation transitions of Er3+. At the same time, with increasing thenumber of high-energy vibration group, the green and red emission intensity decreasesimultaneously due to the the existence of the high-energy vibration group and theacceleration of nonradiation transition of electrons above4F9/2Er3+levels of Er3+.With the increase of Yb3+concentration in CaGdAlO4:Er3+,Yb3+, the up-converted emission intensity firstly increases and then decreases, and the ratio of red to greenemission intensity increases. Combined with the down-converted luminescent spectrumwith488nm excitation and the lifetime measurements of the4S3/2and4F9/2levels, theseresults show that, as Yb3+concentration is lower than the critical concentration, theincrease of the converted luminescence intensity of Er3+can be attributed to the energytransfer from Yb3+to Er3+and it makes Er3+intermediate level easier to be populated tohigher2H11/2,4S3/2and4F9/2levels. When the Yb3+concentration exceeds the criticalconcentration, the decrease of the up-converted luminescence intensity can be due to theenergy transfer from4S3/2and4F9/2of Er3+to4F7/2level of Yb3+. It also leads to the ratioof red to green emission intensity increases. With increasing the Er3+concentration, theup-converted luminescence intensity exhibits first increase and then decrease and theratio of red to green emission intensity increases. It can be explained by the crossingrelaxation between Er3+-Er3+.Based on the influence of excitation power density on the population mechanismof red and green up-converted emissions, we put forward a new method to investigatethe thermal effect of980nm laser excitation by using the ratio of red to green emissionintensity and (2H11/2-4I15/2) to (4S3/2-4I15/2) luminescence intensity ratio (I528/I549) inEr3+/Yb3+co-doped CaGdAlO4system. The results suggest that the existence of theYb3+is helpful to improve the critical laser power density. According to the rateequations, the critical quenching concentration of activator concentration as a functionof laser power density was discussed.From the temperature-dependent CaGd0.90AlO4:Er0.01,Yb0.09up-convertedluminescent spectra, the thermal activation energy of ΔE=595cm-1was obtained byfitting the relationship between R(I528/I549) and T. Compared with NaYF4:Er,Yb,up-converted luminescence intensity of CaGdAlO4:Er,Yb shows better thermal stability.Under X-ray irradiation, the up-converted luminescence intensity ofCaGdAlO4:Er,Yb became lower compared as-prepared sample due to the formation ofthe ionization damage. For oxides, since irradiation-resistance ability is high, the lessthe ionization was damaged, the more the recoverable defects were generated. Therefore,after X-ray removal, the up-converted luminescence intensity of CaGdAlO4: Er,Yb wasimproved and recovered quickly. For NaYF4: Er, Yb fluoride matrix, because it is ioniccrystal, its ionization damage is relatively serious. Under X-ray irradiation, theup-converted luminescence intensity of NaYF4: Er, Yb decreases rapidly to an order ofmagnitude. After X-ray removal, the up-converted luminescence intensity is difficult toreturn to the initial intensity.Under the irradiation of14MeV high-energy neutrons, neutrons will collide with heavier elements in material and cause relatively obvious displacement effect. Theexperimental results show that the up-converted luminescence intensity ofCaGdAlO4:Er,Yb oxide matrix under neutron irradiation exhibits less change and that ofNaYF4: Er, Yb fluoride matrix presents obvious decrease and an irreversible behavior. |
