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:Yb3+, Er3+ nanocrystals are synthesized 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:Yb3+, Er3+ nanocrystals are explained by the different two-photon upconversion mechanisms. Especially, in the LaOF:Yb3+, Er3+ nanocrystals with EDTA, the two-photon processes that contain several special cross-relaxation processes are introduced to analyse the corresponding upconversion mechanisms.
Transformation from Lu-based nanocrystals in hexagonal and cubic mixed phases to pure cubic phase was observed through adjusting the doping concentration of Mn2+. The mechanism for the phase transformation was discussed in detail. Studies on the time and frequency domain spectra indicated that the semi-pure red emissions in cubic Na5Lu9F32: 40% Mn2+, 20% Yb3+, 2% Ln3+ (Ln=Er3+, Ho3+) nanocrystals were caused by a two-step energy transfer between Mn2+ and Ln3+ ions. After incorporating of Mn2+ ions into the host lattices, the local symmetry around the luminescent ion was reduced, which induced the increase of radiative rates for transitions that were mainly contributed by electric dipole radiations. Considerable enhancements in upconversion and downconversion luminescence were accompanied. The result of the current study has great application potential in bioimaging and solar cells.
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