Energy up-conversion in Tb3+/Yb3+ co-doped colloidal α-NaYF4 nanocrystals

- Colloidal Tb3+ and Yb3+ codoped NaYF4 NPs were synthesized.
- Green UC emission was strongly dependent on the terbium ions concentration.
- Tm3+ co-doping allowed five-fold enhancing of the UPC emission intensity from the Tb3+: 5D4 level.

The Tb3+ up-conversion phosphors are promising for biological applications due to their long luminescence lifetime and their spectral fingerprint being different from commonly known materials doped with Yb3+/Er3+ and Yb3+/Tm3+. The concentration dependent up-conversion emission of terbium ions has been studied in cubic NaYF4 colloidal nanocrystals doped with terbium activators and ytterbium sensitizer. Further thulium ions co-doping allowed five-fold enhancing of the up-conversion emission intensity from the 5D4 terbium level. A versatile optimization of Tb3+, Yb3+ and Tm3+ concentrations has been performed by means of luminescence spectra and lifetimes as well as power dependence studies. The up-conversion mechanisms have been proposed-cooperative energy transfer (CET) and mixed CET and energy transfer up-conversion (APTE) for Yb3+/Tb3+ and Yb3+/Tb3+/Tm3+ doped materials respectively.