Influence of Sensitizer Concentration on the Upconversion Luminescence of NaYF4: Yb3+, Er3+ Nanoparticles

A series of NaYF4: Yb3+, Er3+ nanoparticles with different doping concentration were successfully prepared by the combination of coprecipitation and hydrothermal methods using sodium citrate as chelator. Upconversion luminescence can be observed in all of the samples at room temperature. Fixed the Er3+ concentration with 1 mol% unchanged, the effect of the Yb3+ concentration on the upconversion luminescence of NaYF4: Yb3+, Er3+ samples were studied when the doped Yb3+ concentrations were 0 mol%, 1 mol%, 2 mol%, 5 mol%, 10 mol% and 20 mol%, respectively. As increasing the Yb3+ concentration, the total intensity of the upconversion luminescence of the samples is increasing while the ratio of red to green emission increases firstly and then decreases. The maximum ratio value is 22.1 when the Yb3+ concentration is 10 mol%. It is found that the cross relaxation and cooperative sensitization effect are the reasons for the phenomenon above.