Intense infrared upconversion luminescence of NaGdF4:Yb/Tm with controlled intensity.

Nanoparticles with different core@shell structures (NaGdF4:Yb/Tm@NaLnF4 with Ln=lanthanide) were successfully synthesized for luminescence upconversion from 980 nm to 800 nm. The upconversion luminescence of the core@shell nanoparticles (NaGdF4:Yb/Tm@NaGdF4) has a much stronger infrared emission centered at 800 nm than that of the core-only nanoparticles (NaGdF4:Yb/Tm). This is not only due to the removal of surface defects of the core nanoparticles by the shell, which results in the decrease of the luminescence quenching, but also due to the breaking of the crystal field symmetry around the Tm3+ ion. By replacing the NaGdF4 shell with an NaYF4, the emission could further be enhanced. We suggest that the breaking of the crystal field symmetry plays also in this case a key role for further enhancement of upconversion emission. The importance of this asymmetry was further revealed by investigating the energy transfer between Er3+ ion in the shell and the Tm3+ ion of the core nanoparticle. [ABSTRACT FROM AUTHOR]