Influence Of Surface Coating On Upconversion Luminescence Of Rare Earth Ions Doped Y₂O₃ Nanomaterials

Upconversion luminescence materials capable of converting infrared radiation into visible light in an efficient way are potential candidates for photonic applications in several areas such as detection of infrared radiation, color displays, sensors, upconversion laser and biomedicine. However, due to the larger specific surface area of nanoparticles and the isolated luminescence centers of rare earth ions, numerous surface effects, such as lattice distortion, broken bond, surface adsorbate and other surface defect inevitably degenerate the fluorescent properties of rare earth ions doped nanoparticles, and even quench the fluorescence. A convenient strategy of growing an undoped shell around the nanoparticle surfaces, i.e. the so-called core-shell structure, can resolve this problem. In this paper, in order to improve the photoluminescence properties of rare earth ions doped nanoparticles, we reported the synthesis, structure and upconversion luminescence properties of core-shell type Y2O3:Tm3+-Yb3+ nano-particles.To verify the influence of surface effects on upconversion luminescence of Y2O3:Tm3+-Yb3+ nanoparticles, nanoparticles with average sizes of 3.6～35 nm were synthesized using the Pechini type sol-gel method. Their microstructures show that both prolonging the sinterring time and increasing the sinterring temperature can result in the increase of nanoparticle sizes. The optical spectral results indicate that the upconversion luminescence intensities of Y2O3: Tm3+-Yb3+ nanoparticles are closely related to the sizes of nanoparticles. Considering the luminescence mechanism of rare earth ions doped nanoparticles, the molar concentration ratio of rare earth ions in outside spherical shell to the entire sphere suggests that the increase of nanoparticle sizes can increase the number of rare earth ions participating in upconversion processes, directly resulting to the enhancement of upconversion luminescence intensities. From the model above, we can predict that the limit size of rare earth ions doped nanoparticles for upconversion luminescence is about 4 nm.To decrease the influence of surface effects on upconversion luminescence of Y2O3:Tm3+-Yb3+ nanoparticles, SiO2 or TiO2 coated Y2O3:Tm3+-Yb3+ nano- particles were prepared using the St?ber method. Their microstructures show that the shell thicknesses depend on the coating time and the number of the coated-layer. The optical spectral results indicate that the upconversion luminescence intensities of SiO2 or TiO2 coated Y2O3:Tm3+-Yb3+ nanoparticles are higher than those of non-coated nanoparticles, while the radiative quantum efficiencies of Eu3+ ions doped Y2O3 nanoparticles also increase with prolonging the coating time. The improved mechanism in upconversion luminescence intensities is that the cooperation ligand fields between shell and core structures can activate the"dormant"rare earth ions near or on the surfaces of nanoparticles; and new isolated luminescence centers of rare earth ions on the surfaces of nanoparticles will be formed; a lot of new luminescence centers result in the increase of upconversion luminescence intensities of the coated Y2O3:Tm3+-Yb3+ nano-particles. A competition process between two mechanisms was proposed to explain the effects of different thickness shells on upconversion luminescence intensities. One mechanism is the role conversion of rare earth ions on the nanoparticles'surfaces, which is from the"dormant"state to the"activated"state due to the cooperation ligand fields. The other is the absorption effects of shells on incident pump light and the reabsorption effects of shells on upconversion luminescence. The differences of the absorption coefficients for different shell materials can result in different upconversion luminescence intensities when they have similar shell thicknesses.To weaken the agglomeration of upconversion nanomaterials and bio- functionalize the upconversion nanomaterials, SiO2 coated Y2O3:Tm3+-Yb3+ nanoparticles were modified using the ligand-exchanging method. The microstructures show that amine, carboxyl and aldehyde functional groups have been modified on the surfaces of nanoparticles. The optical spectral results indicate that the upconversion luminescence intensities of surface-modified nanoparticles are higher than those of Y2O3:Tm3+-Yb3+ nanoparticles, while the upconversion luminescence intensities of surface-modified nanoparticles are slightly lower than those of SiO2 coated Y2O3:Tm3+-Yb3+ nanoparticles. Organic functional groups become the limiting factors for enhancing upconversion luminescence intensities, because the organic ligands with high energy C-H and C-C vibrational oscillators on the nanoparticle surfaces can absorb certain photon energies to decrease pump light and upconversion luminescence intensities. The results of Zeta potential (ζ) and plain sedimentation suggest that the stabilities of surface-modified nanoparticles in polarity solutions are better than those of non-modified nanoparticles.In summary, a simple model is utilized to demonstrate the influence of quantum size effect on the upconversion luminescence intensities of nano-particles,and then predict the limit size of rare earth ions doped nanoparticles for upconversion luminescence. The mechanism of the enhancement of upcon-version luminescence for core-shell type nanoparticles is presented, and two relative deductions are confirmed by spectral experiments. A competition process between two mechanisms is proposed to explain the effects of different thickness shells and different shell materials on upconversion luminescence intensities: the role conversion of rare earth ions on the nanoparticles'surfaces from the"dormant"state to the"activated"state, and the absorption effects of shells on incident pump light and the reabsorption effects of shells on upconversion luminescence. A two-photon simultaneous absorption upconversion lumine-scence of SiO2-coated Y2O3 nanoparticles doped with Eu3+ ions is obtained, and a Judd–Ofelt theory is used to quantificationally probe the influence of shell thicknesses on upconversion luminescence intensities. The influence of different shell materials on upconversion luminescence intensities is analyzed. After the surface modification with amine, carboxyl and aldehyde functional groups, the upconversion nanomaterials with better stabilities in polarity solutions and higher luminescence intensities are obtained.