Synthesis and Upconversion Luminescence of YF3:Yb3+,Tm3+ and TiO2-Coated YF3:Yb3+,Tm3+ Microcrystals

YF3:Yb3+,Tm3+ microcrystals were prepared by a microemulsion method. The microcrystals were coated with TiO2 by hydrolysis of titanium n-butoxide (TBOT). Transform electron microscopy and X-ray diffraction were used to characterize the core/shell materials. The results indicated that the low TBOT/YF3 molar ratio was favorable to preparing the uniform TiO2 coatings. The upconversion luminescence properties of YF3:Yb3+,Tm3+ and TiO2-coated YF3:Yb3+,Tm3+ microcrystals were studied under 980-nm excitation. The I-1(6) -> 3H(6), I-1(6) -> F-3(4), D-1(2) -> H-3(6), D-1(2) -> F-3(4), (1)G(4) -> H-3(6) and (1)G(4) -> F-3(4) emissions were observed. The upconversion mechanisms were discussed in detail.