Bright green upconversion fluorescence of Yb3+, Er(3+)-codoped NaYF4 nanocrystals.

Uniform hexagonal NaYF4: Yb3+, Er3+ nanocrystals (NCs) with bright green upconversion fluorescence were prepared. The nanocrystals were highly hydrophobic and dispersed in cyclohexane forming a transparent colloidal solution due to the use of oleic acid as a capping ligand. Temperature dependence for emission of Er3+ ions in this sample was also studied. The green emission intensity of Er3+ ions had a maximum at 160 K under 980 nm excitation, which was attributed to the thermally activated distribution of electrons and the thermal quenching effect. The transparent colloidal solution is expected to be a good candidate for research in biological imaging or display applications.