Synthesis and upconversion luminescence properties of NaYF4:Yb3+/Er3+ microspheres

Cubic NaYF4:Yb3+(20%) /Er3+(1%) microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980 nm excitation,ultraviolet(4G11/2→4I15/2) ,violet(2H9/2→4I15/2) ,green(4F7/2→4I15/2,2H11/2→4I15/2,and 4S3/2→4I15/2) ,and red(4F9/2→4I15/2) upconversion fluorescence were observed. The number of laser photons absorbed in one upconversion excitation process,n,was determined to be 3.89,1.61,2.55,and 1.09 for the ultraviolet,violet,green,and red emissions,respectively. Obviously,n=3.89 indicated that a four-photon process was involved in populating the 4G11/2 state,and n=2.55 indicated that a three-photon process was involved in populating the 4F7/2/2H11/2/4S3/2 levels. For the violet and red emissions,the population of the states 2H9/2 and 4F9/2 separately came from three-photon and two-photon processes. The decrease of n was well explained by the mechanism of competition between linear decay and upconversion processes for the depletion of the intermediate excited states.