Silica-/titania-coated Y_2O_3:Tm~(3+), Yb~(3+) nanoparticles with improvement in upconversion luminescence induced by different thickness shells

摘要

In order to improve the upconversion (UC) luminescence of lanthanide-doped nanoparticles (NPs), different sized Y_2O_3: Tm~(3+), Yb~(3+) NPs were synthesized using the Pechini type sol-gel method, and their surfaces were coated with different thickness of SiO_2 or TiO_2 shells using the Stoeber method. The results indicate that large-sized NPs have more intense UC luminescence intensities than small-sized NPs. The core-shell structures can enhance the UC luminescence intensities. Comparing with the UC luminescence intensity of noncoated NPs, the UC luminescence intensities of SiO_2-coated NPs for the sintering time of 60 min and the coating time of 30, 60, 90, and 120 min are enhanced by 1.53, 1.54, 1.40, and 1.16 times, respectively. According to the relative variable ratios of the UC luminescence intensities, a competition process between two mechanisms was proposed to explain the effects of different thickness shells and different shell materials on the UC luminescence intensities. One mechanism is the role conversion of lanthanide ions on the NPs' surfaces, which is from the "dormant" state to the "activated" state due to the complementary ligand fields from noncrystalline SiO_2 shells. The other is the absorption effects of the shells on the incident pump light and the reabsorption effects of the shells on the UC luminescence. Therefore, it can be concluded that more intense UC luminescence can be achieved in doped core-shell NPs by selecting the appropriate shell materials and their thickness.