Upconversion fluorescence studies of sol-gel-derived Er-doped KNN ceramics

Er-doped K0.5Na0.5NbO3 luminescent ceramics have been prepared using a sol-gel method. The results of X-ray diffraction show that Er3+ diffuses into the K0.5Na0.5NbO3lattices, forming an orthorhombic-structured perovskite. Under an excitation of 980 nm, the ceramics exhibit luminescent emission bands at 527 nm (green), 548 nm (green), 660 nm (red) as well as 487 nm (blue), which are attributed to the transitions2H11/24I15/2,4S3/24I15/2,4F9/24I15/2and4F7/24I15/2, respectively. Our Raman results also reveal that the ceramics have low phonon energy (∼ 860 cm-1) and thus slow multi-phonon relaxation rate. The sol-gel process and Er3+-doping can produce ceramics with better compositional homogeneity as well as finer and uniform grains. All these are beneficial for improving both the upconversion and luminescent efficiencies of the ceramics, and thus leading to very strong green emissions even at a low quenching concentration of 2 mol%. These suggest that even without using sensitizers, the sol-gel-derived Er-doped KNN ceramics are promising candidates for multifunctional optoelectronic applications.