Synthesis and upconversion luminescence properties of NaYF 4:Yb 3+/Er 3+ microspheres

Abstract

Cubic NaYF 4:Yb 3+(20%)/Er 3+(1%) microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980 nm excitation, ultraviolet ( 4G 11/2→ 4I 15/2), violet ( 2H 9/2→ 4I 15/2), green ( 4F 7/2→ 4I 15/2, 2H 11/2→ 4I 15/2, and 4S 3/2→ 4I 15/2), and red ( 4F 9/2→ 4I 15/2) 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 4G 11/2 state, and n=2.55 indicated that a three-photon process was involved in populating the 4F 7/2/ 2H 11/2/ 4S 3/2 levels. For the violet and red emissions, the population of the states 2H 9/2 and 4F 9/2 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.