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:Yb³⁺, Er³⁺ nanocrystals are synthesized with 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 and 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.