Efficient Tailoring of Upconversion Selectivity by Engineering Local Structure of Lanthanides in NaxREF3+x Nanocrystals

Efficient tailoring of upconversion emissions in lanthanide-doped nanocrystals is of great significance for extended optical applications. Here, we present a facile and highly effective method to tailor the upconversion selectivity by engineering the local structure of lanthanides in NaxREF3+x nanocrystals. The local structure engineering was achieved through precisely tuning the composition of nanocrystals, with different [Na]/[RE] ([F]/[RE]) ratio. It was found that the lattice parameter as well as the coordination number and local symmetry of lanthanides changed with the composition. A significant difference in the red to green emission ratio, which varied from 1.9 to 71 and 1.6 to 116, was observed for NaxYF3+x:Yb,Er and NaxGdF3+x:Yb,Er nanocrystals, respectively. Moreover, the local structure-dependent upconversion selectivity has been verified for NaxYF3+x:Yb,Tm nanocrystals. In addition, the local structure induced upconversion emission from Er3+ enhanced 9 times, and the CaF2 shell grown epitaxially over the nanocrystals further promoted the red emission by 450 times, which makes it superior as biomarkers for in vivo bioimaging. These exciting findings in the local structure-dependent upconversion selectivity not only offer a general approach to tailoring lanthanide related upconversion emissions but also benefit multicolor displays and imaging.