Fuheng Ding

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

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

Enhanced ultraviolet upconversion in YF3:Yb3+/Tm3+ nanocrystals

Abstract Unusual intense infrared-to-ultraviolet upconversion luminescence was observed in YF 3 :Yb 3+ (20%)/Tm 3+ (1%) nanocrystals under 980 nm excitation. The intense ultraviolet emissions ( 1 I 6 → 3 H 6 , 1 I 6 → 3 F 4 , and 1 D 2 → 3 H 6 ) were affirmed arising from the excitation processes of five-photon and four-photon. In comparison with the bulk sample with the same chemical compositions, ultraviolet upconversion luminescence of the nanocrystals was markedly enhanced. Spectral analysis indicated that the enhancement was attributed to the decrease of Judd-Ofelt parameter Ω 2 , which precluded the transition rate from 3 F 2 to 3 F 4 , enhanced the energy transfer process and populated the 1 D 2 level: 3 F 2 → 3 H 6 (Tm 3+ ): 3 H 4 → 1 D 2 (Tm 3+ ).