After coating NaYbF₄ shell on the 20Yb/2Er: NaGdF₄core nanocrystals, upconversion emission intensity decreased. Through acore@shell@shell sandwich like structure, we find that high Yb³⁺concentration in the shell has mainly two adverse impacts: it promotes not only the deleterious back-energy-transferfrom Er³⁺ in the core to Yb³⁺ in the shell, but also theenergy transfer from Yb³⁺ in the core to Yb³⁺ in theshell. These reuslts are important for designing appropriate core@shellstructure to obtain high UC emission efficency NCs.

Asian Chem. J. 全文链接：http://onlinelibrary.wiley.com/doi/10.1002/asia.201402610/abstract