Effect of Ce3+ Concentration on the Luminescence Properties of Ce3+/Er3+Nb3+ Tri-Doped NaYF4 Nanocrystals.

In order to investigate the effect of Ce3+ concentration on the emission properties of ErS+/y3+ codoped NaYF4 nanocrystals, Ce3+/Er3+/Yb3+ tri-doped NaYF4 nanocrystals were prepared through a facile EDTA-assisted hydrothermal method. The upconversion .(UC) and the near infrared (NIR) emission properties of Er3+ ions were systematically investigated in the NaYF4:Ce3+/Er3+/Yb3+ nanocrystals. Under 980 nm excitation, with the increasing of Ce3+ concentration, the emission intensity of Er3+ at 1550 nm (4I13/2 --> 4I15/2) band increases initially and then decreases. The increase of the fluorescence intensity of 1550 nm is due to the energy transfer between Er+ and Ce3+ ions: Er3+:4(11/2)+Ce3+:2F5/2 --> Er3+:4I13/2 +Ce3+:2F7/2. But when Ce3+ doping concentration is 2.0%, the cross relaxation:Er3+:4I13/2+ Ce3+:2F5/2 --> Er3+:4I15/2+Ce3+:2F7/2 happens, which depopulates the 4113/2 level of Er3+ and results in the decrease of the emission intensity of Er3+ at 1550 nm band. Meanwhile, incorporation of Ce3+ dramatically decreases the visible UC emission intensity. A possible emission mechanism was proposed.