Effects of inert shell on the upconversion intensity and color of Na(Er/Yb)F4 nanocrystals

Inert passivation can significantly enhance the upconversion intensity of lanthanide ion doped nanocrystals, especially in the heavily doped cases. In this paper, we revealed that, besides the well-known enhancement of intensity, the inert shell can also influence the upconversion color of Er3+/Yb3+ heavily doped nanocrystals in two opposite directions. The mechanisms of the effects of inert shell passivation are discussed based on the spectroscopic observations. The quantum yields of upconversion nanocrystals were estimated for the core-only and core–shell nanocrystals. This work provides guidelines for designing upconversion nanomaterials with bright emission and desired color and thus can further help them to be applied in multichannel imaging, display devices, and phototherapy.