Upconversion emission tuning from green to red in Yb3+/Ho3+-codoped NaYF4 nanocrystals by tridoping with Ce3+ ions

Upconversion (UC) emission tuning from green to red in monodisperse NaYF4:Yb3+/Ho3+ nanocrystals was successfully achieved by tridoping with Ce3+ ions under diode laser excitation of 970 nm. It is proposed that two efficient cross-relaxation processes, 5S2/5F4(Ho)+2F5/2(Ce)→5F5(Ho)+2F7/2(Ce) and 5I6(Ho)+2F5/2(Ce)→5I7(Ho)+2F7/2(Ce) between Ho3+ and Ce3+ ions, have been employed to select UC pathways to tune the UC radiation. Theoretical investigations based on steady-state equations demonstrate the proposed UC mechanisms and explain well the observed linear increase of the UC red-to-green intensity ratio with the increment of Ce3+ ion concentrations.