Enhancement Mechanism Of Photoluminescence Of Er³⁺-Doped Al₂O₃ By Y³⁺ Codoping

The Er³⁺-Y³⁺ codoped Al₂O₃ powders have been prepared by a non-aqueous sol-gel method.The effects of Y³⁺ codoping on the phase structure and the photoluminescence(PL) emission properties of the Er³⁺-doped Al₂O₃ powders are systematically investigated by using fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),transmission electron microscope(TEM),energy distribution spectrum(EDS) and PL measurement,to explore the enhancement mechanism of the stokes and the anti-stokes(up-conversion) emissions of the Er³⁺-doped Al₂O₃ powders due to Y³⁺ codoping.The Er³⁺-doped Al₂O₃ sols are prepared by using the aluminum isopropoxide [Al(OC₃H₇)₃]as precursor,and hydrated nitrate of Er(NO₃)₃·5H₂O as dopant in the isopropanol environment.Increasing the molar ratio of H₂O and Al³⁺(M_H2O/M_Al) from 0:1 to 5:1,the viscosity of the 1 mol%Er³⁺-doped Al₂O₃ sols recedes from 4.33 to 4.16 mm²/s and the size of the colloidal particles decreases from 40 to 3 nm.The Er³⁺-doped Al₂O₃ powders sintered at 900℃,derived from the sols with various M_H2O/M_Al,are all characteristic of the mixture ofγ-(Al,Er)₂O₃ andθ-(Al,Er)₂O₃ phases.With increasing the M_H2O/M_Al,the concentration of -OH in the powders increases,while the stokes emission intensity at the wavelength of 1530 nm,corresponding to the transition of ⁴I_{1 3/2}→⁴I_{1 5/2} of Er³⁺,first increases and then gradually decreases,and the maximum PL intensity of the Er³⁺-doped Al₂O₃ powders is achieved for the powders with the M_H2O/M_Al of 2:1.The 0.001-2 mol%Er³⁺- and 0-20 mol%Y³⁺ -codoped Al₂O₃ sols with the M_H2O/M_Al of 2:1 are prepared by using hydrated nitrate of Y(NO₃)₃·5H₂O as codopant.The viscosity of the 1 mol%Er³⁺- and 0-20 mol%Y³⁺ -codoped Al₂O₃ are all about 4.18 mm²/s.10 mol%Y³⁺ codoping increases the size of the sol particles from 3 nm to 10 nm.The Er³⁺-doped Al₂O₃ powders by 0-20 mol%Y³⁺ codoping at the sintering temperature of 900 and 1000℃are characteristic ofγ-(Al,Y,Er)₂O₃ andθ-(Al,Y,Er)₂O₃ phases,and the(Y,Er)₃Al₂(AlO₄)₃ phase precipitates for the 20 mol%Y³⁺ codoped powders sintered at 1000℃.Y³⁺ codoping suppresses the crystallization ofγ,andθphases.The content and the vibration frequency of -OH are not changed due to Y³⁺ codoping,however,the particles size increases to 40 nm due to 10-20 mol%Y³⁺ codoping.The inharmoniously-broadening stokes emission spectra centered at 1530 nm are observed for all the Er³⁺-Y³⁺ codoped Al₂O₃ powders composed ofγandθphases,and it is splitted for the(Y,Er)₃Al₂(AlO₄)₃ phase.The effect of Y³⁺ codoping on the PL intensity is related to the Er³⁺ concentration.For the super low Er³⁺ concentration of 0.001 mol%,Y³⁺ codoping has no affect on the PL intensity,while the PL intensities of the 0.01-2 mol% Er³⁺-doped Al₂O₃ powders increase with increasing the Y³⁺ codoping concentration.20 mol% Y³⁺ codoping intensifies the PL intensity by about 30 and 40 times for the 1 mol%Er³⁺ doped Al₂O₃ powders at the sintering temperature of 900℃and 1000℃,respectively.The emission-quenching concentration of the Er³⁺-doped Al₂O₃ powders is increased from 0.1 mol%to 1 mol%due to 20 mol%Y³⁺ codoping.Y³⁺ codoping increases the lifetimes of the ⁴I_13/2 level of Er³⁺ in Er³⁺-Y³⁺ codped Al₂O₃ powders,and the lifetimes increase with increasing the Y³⁺ codoping concentration.20 mol% Y³⁺ codoping increases the lifetime from 3.5 and 3.3 ms to 5.8 and 4.1 ms,for the 0.1 and 1 mol%Er³⁺-doped Al₂O₃ powders,respectively.The radiative lifetime and pump absorption cross section of Er³⁺ in Al₂O₃ powders is not changed due to Y³⁺ codoping.The optical activity of Er³⁺ in Al₂O₃ powders increases with increasing the Y³⁺ codoping codping conventration,and 20 mol%Y³⁺ codoping increases it by a factor of 10 and 20 times,for the 0.1 and 1 mol%Er³⁺-doped Al₂O₃ powders at the sintering temperature of 900℃, respectively.It is confirmed that the enhanced emission intensity at 1530 nm is mainly attributed to the improved optical activity of Er³⁺,whereas slightly to the increased litetime of Er³⁺ in Al₂O₃.The improved optical activity of Er³⁺ is attributed to the destroying of Er³⁺ cluster due to Y³⁺ codoping,which activates the inactive Er³⁺ in clustering state.The increase in the lifetimes is ascribed to the decrease in the Er³⁺-Er³⁺ energy transfer probability and that in the -OH quenching rate.The green and red up-conversion emissions centered at about 535,553 and 670 nm, corresponding to the ²H_{1 1/2},⁴S_3/2→⁴I_{1 5/2} and ⁴F_9/2→⁴I_{1 5/2} transitions of Er³⁺,ae detected for the Er³⁺-Y³⁺ codoped Al₂O₃ powders.The two-photon absorption up-conversion process is involved for the green and red up-conversion emissions of both the Er³⁺-doped and the Er³⁺-Y³⁺ codoped Al₂O₃ powders.Y³⁺ codoping greatly increases the up-conversion emission intensities of the Er³⁺ doped Al₂O₃ powders.The up-conversion emission intensities of the Er³⁺-Y³⁺ codoped Al₂O₃ first increase and then gradually decrease with increasing the Y³⁺ codoping concentration up to 20 mol%,because Y³⁺ codoping increases the optical activity of Er³⁺ and simultaneously limits the probability of cross relaxation between Er³⁺ ions.Y³⁺ codoping also tunes the color of the up-conversion emissions from red to green,and 20 mol% Y³⁺ codoping increases the intensity ratio of the green to red up-conversion emission(I_green/I_red) from 0.2 to 3 for the 1 mol%Er³⁺-doped Al₂O₃ powders sintered at 1000℃.The evolution of I_green/I_red has been analyzed with rate equations of Er³⁺-Y³⁺ codoped Al₂O₃ system.The limited ⁴I_{1 1/2}→⁴I_{1 3/2} nonradiative transition of Er³⁺ increases the concentration ratio of Er³⁺ in ⁴I_{1 1/2} and ⁴I_{1 3/2} level(N₃/N₂),leading to an increase in I_green/I_red.