Surface effect of nanocrystals doped with rare-earth ions enriched on surface and its application in upconversion luminescence

By employing a certain proportion of hydrogen peroxide, ammonia, ammonium fluoride, and ethylene diamine tetraacetic acid （EDTA） as precipitator, well-crystallized LaOF：Eu3＋ and LaOF：Yba＋, Era＋ nanocrystals are synthe- sized by using the chemical co-precipitation method. The structural properties of these samples are characterized by X-ray diffraction （XRD）, transmission electron microscopy （TEM）, and Fourier transform infrared spectroscopy （FTIR） spectra. The results show that all the samples have an average size below 70 nm, which decreases gradually with the increase of the EDTA content, and a certain number of EDTA molecules are coupled with doped ions on the surfaces of nanocrystals. Most of the doped ions are proved to be enriched on the surfaces of nanocrystals and surrounded by the high energy vibration groups and bonds in EDTA molecules. The observed differences in upconversion emission spectrum among the different LaOF：Yba＋, Er3＋ nanocrystals are explained by the different two-photon upconversion mechanisms. Especially, in the LaOF：Yba＋, Era＋ nanocrystals with EDTA, the two-photon processes that contain several special cross-relaxation processes are introduced to analyse the corresponding upconversion mechanisms.

Optimum fluorescence emission around 1.8μm for LiYF_4 single crystals of various Tm^(3+)-doping concentrations

In this paper, optical spectra of LiYF4 single crystals doped with Tm3＋ ions of various concentrations are reported. The emission intensity at 1.8 ktm first increases with increasing Tm3＋ concentration, and reaches a maximum value when the concentration of Tm3＋ is about 1.28 mol%, then it decreases rapidly as the concentration of Tm3＋ further increases to 3.49 mol%. The emission lifetime at 1.8 p.m also shows a similar tendency to the emission intensity. The maximum lifetime of 1.8 μm is measured to be 17.68 ms for the sample doped with Tm3＋ of 1.28 mol%. The emission cross section of 3F4 level is calculated. The maximum reaches 3.76 × 10 -21 cm2 at 1909 nm. The cross relaxation （3H6, 3H4 →3 F4, 3F4） between Tm3＋ ions and the concentration quenching effect are mainly attributed to the change of emission with Tm3＋ concentration. The largest quantum efficiency between Tm3＋ ions is estimated to be ,-147% from the measured lifetime and calculated radiative lifetime. All the results suggest that the Tm3＋/LiYF4 single crystal may have potential applications in 2 μm mid-infrared lasers.

Luminescent properties of dysprosium(III) ions in LaAlO_3 nanocrystallites

The absorption and emission spectra as well as decay time profile of Dy^3＋ ions in LaAlO3 nanocrystals were analyzed. The crystal structure of LaAlO3 was confirmed from XRD measurement. The emission peaks from blue to red came from main emitting level of dysprosium ^4F9/2 to the ground and other excited levels of Dy^3＋ ions. Cross relaxation process led to non-radiative quenching of luminescence, so that the lifetime of the ^4F9/2 energy level ions decreased with increasing amount of doped Dy^3＋ ions. The cross relaxation transfer rates were experimentally determined as a fimction of Dy^3＋ concentration.