Enhanced blue and green upconversion in hydrothermally synthesized hexagonal NaY1-xYbxF4:Ln3+ (Ln3+ = Er3+ or Tm3+)

Hexagonal NaY1-xYbxF4:Ln(3+) (X = 0.05-1.00, Ln(3+) = Er3+ or Tm3+) have been prepared through hydrothermal synthesis. Powder X-ray diffraction (XRD) patterns have been presented to characterize the synthesized samples. The concentration of doped rare earth and pumping power on the upconversion emissions have been extensively investigated under 980 nm excitation. NaY1-xYbxF4:Er3+ emit green (H-2(11/2), S-4(3/2) --> I-4(15/2)), red (F-4(9/2) --> I-4/(15/2)), weak violet (H-2(9/2) --> I-4(15/2)) and weak UV ((4)G(11/2) --> I-4(15/2)) upconversion emission. NaY1-xYbxF4:Tm3+ exhibit intense blue ((1)G(4) --> H-3(6), D-1(2) --> F-3(4)), weak red ((1)G(4) --> F-3(4)) and UV (D-1(2) --> H-3(6)) upconversion emission. A two-photon process account for the green and red emission of an Er3+ ion with the energy transfer from Yb3+ ion. One blue emission ((1)G(4) --> H-3(6)) of Tm3+ results from a three-photon excitation, and the other (D-1(2) --> F-3(4)) from a four-photon process. The measured data about emission intensity suggest that hexagonal NaYF4:Yb3+/Er3+ is a more efficient upconverting phosphor than cubic one. Comparison of the measured properties in our work with values from the reported literature suggests that hydrothermally synthesized NaYF4:Yb3+/Er3+ (or Tm3+) samples emit more pure green or blue emission than solid-state synthesized samples with certain concentration of doping rare earth ions. (C) 2003 Elsevier B.V. All rights reserved.