Impact of High Ytterbium(III) Concentration in the Shell on Upconversion Luminescence of Core-Shell Nanocrystals

After coating 20Yb/2Er:NaGdF4 core nanocrystals with a NaYbF4 shell, upconversion emission of the rare earth ions weakens. So far, the exact reason for this phenomenon is still unclear due to lack of the direct evidence. In this report, a core@shell@shell sandwich-like structure is designed and fabricated to investigate this phenomenon. We find that high Yb3+ concentration in the shell has mainly two adverse impacts: it promotes not only the deleterious back energy transfer from Er3+ in the core to Yb3+ in the shell but also the energy transfer from Yb3+ in the core to Yb3+ in the shell. To obtain nanocrystals with high upconversion efficency, appropriate Yb3+ concentration should be introduced into the shell or the transition layer.