The two-photon excitation of SiO2-coated Y2O3:Eu3+ nanoparticles by a near-infrared femtosecond laser

In order to improve the photoluminescence property of Eu3+-doped nanoparticles, Y2O3: Eu3+ nanoparticles were synthesized using the Pechini-type sol-gel method, then coated with SiO2 shells by using the Stober method for different coating times. The SiO2-coated nanoparticles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and Raman spectroscopy, and their photoluminescence spectra were recorded under 800 nm femtosecond laser excitation. The results indicate that a two-photon simultaneous absorption upconversion luminescence is obtained, and their upconversion luminescence intensities are further enhanced after the surfaces of the nanoparticles are coated with different thickness SiO2 shells. Compared to the upconversion luminescence intensity of non-coated nanoparticles at 611 nm, the upconversion luminescence intensities of SiO2-coated Y2O3: Eu3+ nanoparticles with coating times of 60, 90 and 120 min were enhanced by 3.30, 3.96 and 4.13 times, respectively. This can be attributed to the contributions of the increased amounts of Eu3+ ions populated at the D-5(0) level on the surfaces of the nanoparticles because the cooperative ligand fields between the Y2O3 core and non-crystalline SiO2 shell interfaces activate the 'dormant' Eu3+ ions near or on the surfaces of the nanoparticles. From a Judd-Ofelt (J-O) theory analysis, the coated shell structures can improve the radiative quantum efficiencies of Eu3+- doped nanoparticles. It is therefore concluded that more intense red upconversion luminescence with high radiative quantum efficiencies can enable the SiO2- coated Y2O3: Eu3+ nanoparticles to have the great potential to be used as a fine resolution phosphor.