Mn2+ ions substitution inducing improvement of optical performances in ZnAl2O4: Cr3+ phosphors: Energy transfer and ratiometric optical thermometry

Abstract

Zn1-xMnxAl2O4:0.1 mol% Cr3+ (0.04≤x≤0.16) phosphors with single spinel phase were synthesized by using sol–gel method and the structure, optical and temperature sensing performances were reported herein. The results of X-ray photoelectron spectra indicate that the inversion defects related to octahedral Zn are reduced and the crystal field surrounding Al changes with Mn2+ doping in ZnAl2O4 lattices. Mn2+/Cr3+ co-doped ZnAl2O4 nanophosphors reveal a green emission band assigned to Mn2+ and a series of red emission peaks assigned to Cr3+, respectively. With the concentration of Mn2+ increasing, the intensity of zero phonon line (R line) assigned to Cr3+ increases, reaching the maximum at the optimal Mn2+ concentration of x=0.14. The energy transfer from Mn2+ to Cr3+ is confirmed with the energy transfer efficiency of 83%. The separation between 2E(eg) and 2E(tg) of Cr3+ is enlarged due to Mn2+ dopants giving rise to a change of crystal field. The luminous intensity ratio between two separated emission peaks at 685 nm (R3) and 689 nm (R2) reveals an obvious temperature dependence. The relative sensitivity changes from 3.7 %K−1 to 0.25 %K−1 with the temperature increasing from 80 K to 310 K, which is much larger than that of ZnAl2O4:Cr3+ nanophosphors without Mn2+, indicating its good application prospect in optical thermometry.