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

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 Cr-3+,Cr- 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 E-2(e(g)) and E-2(t(g)) 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 (R-3) and 689 nm (R-2) 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 wi(t)hout Mn2+, indicating its good application prospect in optical thermometry. (C) 2021 Elsevier B.V. All rights reserved.