Synthesis and upconversion luminescence properties of YF3:Yb3+/Tm3+ octahedral nanocrystals

YF3:Yb3+(20%)/Tm3+(2%) octahedral nanocrystals were synthesized by a microemulsion method with NH4HF2. Pumped with a 980-nm diode laser, teh nanocrystals emitted weak blue and intense ultraviolet light. Especially, unusual P-3(2) -> H-3(6) (similar to 265 nm) and P-3(2) -> F-3(4) (similar to 309 nm) emissions, coming from a five-photon excitation process, were observed. The emissions fro D-1(2) and I-1(6) were much stronger than those from (1)G(4) and H-3(4). The upconversion mechanism was discussed in detail.