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Uniform NaYF4∶18%Yb,0.5%Tm nanocrystals were synthesized via thermal decomposition method following high temperature,and then a change of the nanocrystals was made from oil-soluble to water-soluble by ligand-exchanged method.X-ray diffraction(XRD) analyses showed that the samples were β-NaYF4 nanocrystals.The TEM images exhibited that the nanocrystals had an average size of 35 nm.Under 980 nm near-infrared laser excitation,the samples showed intense ultraviolet upconversion emission.5-photon upconversion fluorescence of the Tm3+ ions in the nanocrystals was stronger than 4-photon and 3-photon upconversion fluorescence.Due to special optical properties and small size,the water-soluble nanocrystals would show potential applications in the biomedical field.

The ultraviolet upconversion luminescence properties of the Tm3+ ion sensitized by the Yb3+ ion were studied,in fluoride glass as excited by a 980 nm continuous wave diode laser.Red,blue,and even ultraviolet emissions were observed with ZrF4-BaF2-LaF3-A1F3-NaF glass,and it was also found that the peaks at 291,347,363,454,475,643,687 and 804 nm correspond to the transitions of 1I6→3H6,1I6→3F4,1D2→3H6,1D2→3F4,1G4→3H16,G4→3F4,3F3→3H6,and 3H4→3H6,respectively.According to the relationship between the pump power and the upconversion peaks intensity,347 nm and 363 nm ultraviolet emissions are five photon and four photon processes respectively.The research results show that the ZrF4-BaF2-LaF3-A1F3-NaF glass of the Tm3+ ion sensitized by the Yb3+ ion can effectively transform the near-infrared waveband laser to the ultraviolet.