The role of an inert shell in improving energy utilization in lanthanide-doped upconversion nanoparticles

Coating with an inert shell is considered an efficient approach for enhancing the emission and improving the luminescence quantum yield of lanthanide-doped upconversion nanoparticles. In this work, the role of an inert shell in the photon energy collection and energy transfer behavior is carefully studied. Our experimental results indicate that, owing to the valid energy trapping by the shell, an energy clustering effect is formed among the Yb³⁺ ions in the core. Such an effect reduces the total absorption and enhances the upconversion emission, especially the emission from the higher lying level of lanthanide emitters. Moreover, the morphology of the inert shell, which could be controlled during our synthesis, is deemed to be critical to the formation of energy clustering.