Enhancing red upconversion emission in NaErF4@NaYF4 core-shell nanoparticles by introducing Yb3+ ions as energy trapping centers

Rare-earth-doped red upconversion nanocrystals (UCNPs) have shown excellent prospects for application in biological imaging, labeling and other fields. In this work, NaErF4@NaYF4 core-shell (CS) UCNPs were successfully constructed to enhance red emission by introducing Yb3+ ions as energy trapping centers under different near-infrared excitation wavelengths. The experimental results showed that the red upconversion emission intensity of NaErF4@ NaYF4 CS UCNPs doped with 15%Yb3+ ions in the core is nearly 202 times higher than that of NaErF4 UCNPs under 980 nm excitation wavelength, with a red/green emission intensity ratio of 18.7. When Yb3+ ions were introduced into the core and shell of NaErF4@NaYF4 CS UCNPs, NaErF4:15%Yb3+@NaYF4:10%Yb3+ CS UCNPs showed the best red emission due to the energy network characteristics of the Yb3+ ions. Furthermore, the enhancement mechanism of red emission in NaErF4@NaYF4 CS UCNPs with different Yb3+ ion doping concentrations were discussed, based on the spectral characteristics and lifetime. It was found that doping of Yb3+ ions in NaErF4@ NaYF4 CS UCNPs can significantly improve the red emission intensity of Er3+ ions through controlling energy migration (EM) and energy back transfer (EBT) processes, which has considerable potential for applications in bioimaging and anti-counterfeiting.