In this work, colloidal hexagonal-phase core/shell nanoparticles with intense visible upconversion emissions under 808-nm laser excitation were prepared. Compared with the core-only nanoparticles, a maximum 990-fold overall enhancement in the emission intensity of ions was achieved with the help of active-shell coating design, due to the significant increase in the near-infrared absorption and efficient energy transfer from primary-sensitizers to activators via bridging sensitizers. The luminescence-enhancement effect exhibited a strong dependence on the doping concentrations of active-shell. The optimal concentrations of and ions in the active-shell layer were found to be 30 and 5 mol. %, respectively. Moreover, the upconversion emission intensity of -coated nanoparticles was about 2.5 times higher than the one coated with a active-shell.
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