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Abstract: YF3:Yb3+(20%)/Tm3+(2%) octahedral nanocrystals were synthesized by a microemulsion method with NH4HF2. Pumped with a 980-nm diode laser, the nanocrystals emitted weak blue and intense ultraviolet light. Especially, unusual 3P2 → 3H6 (∼265nm) and 3P2 → 3F4 (∼309nm) emissions, coming from a five-photon excitation process, were observed. The emissions from 1D2 and 1I6 were much stronger than those from 1G4 and 3H4. The upconversion mechanism was discussed in detail. [Copyright &y& Elsevier]

Abstract: The upconversion luminescent properties of YF3:Yb3+(20%)/Tm3+(1%) nanobundles with different sizes (240–500nm in length) were studied under 980-nm excitation. Ultraviolet (1I6 → 3F4/3H6 and 1D2 → 3H6), blue (1D2 → 3F4 and 1G4 → 3H6), red (1D2 → 3H4, 1G4 → 3F4, and 3F3 → 3H6), and near infrared (3H4 → 3H6) emissions were observed. The results indicated that the relative intensity of the ultraviolet to the blue as well as the blue to the near infrared increased with decreasing the size of nanobundles. Especially, the position of the dominant red emission peak varied with the size of nanobundles. As the length of nanobundles increased to 500nm, unusual 3F3 → 3H6 transition was observed, which was theoretically explained considering the decrease of the nonradiative transition rate of 3F3 → 3H4. [Copyright &y& Elsevier]