Author: JORGE ROBERTO OLIVA UC
JORGE ROBERTO OLIVA UC (2008)
"We present our research on visible upconversion luminescence in Yb3+ doped Barium Zirconate (BaZrO3, BZO) under NIR excitation. Barium Zirconate was synthesized by hydrothermal method. The visible luminescence is attributed to both, Yb3+ cooperative emission and rare earth impurity upconversion emissions. Since the presence of Tm3+ and Er3+ impurities was not detected by absorption spectra, it has to be very low concentration. The strong intensity of all peaks is interesting considering the very small concentration. This fact suggests very efficient upconversion processes involving traces of Tm3+ and Er3+ ions having the Yb3+ ions as sensitizers. The emission band centered at 504 nm is twice the energy of the normal luminescence of a single ion, suggesting that it is the result of the radiative relaxation of an excited Yb3+-Yb3+ pair. For all Yb3+ concentrations, the lifetime for the Yb3+ cooperative emission is nearly half of the lifetime of single Yb3+ ion emission, suggesting the upconversion mechanisms are two photon uponversion processes. FTIR Spectra was recorded to indentify hydroxil groups, residual H2O and ionic bonds for the cubic crystalline structure of BZO.
In addition, we present the thermodarkening effect in 0.5 at% Yb3+:BaZrO3 microcrystal (an increase of the UV-VIS edge band with annealing temperature). Both, the cooperative up-conversion emission under NIR excitation and fluorescence lifetimes of cooperative emission, increase with annealing temperature. Under UV and VIS excitation no emission of Yb2+ ions could be observed. This suggests that the broad band absorption in the UV-VIS region that increase with annealing temperature might be related to F color centers and oxygen vacancies which are responsible for the inhibition of the Yb2+ ion emission."
We demonstrated the synthesis of LaPO4:Er:Yb-doped nanoparticles/nanorods and LaPO4:Er@Yb core−shell nanoparticles/nanorods by a solution-based technique. The mechanism related to morphology control of LaPO4:Er:Yb nanorods/nanoparticles is proposed and discussed. Bright-green (550 nm) and red (670 nm) emission were observed due to the transitions 2H11/2 + 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. The experimental data for 550- and 670-nm emission bands of doped nanoparticle/nanorod and core−shell nanoparticles/nanorods have been fit with a straight line with a slope of ∼2, which confirms the two-photon absorption process. The enhancement of upconversion emission of LaPO4:Er:Yb-doped nanoparticles and LaPO4:Er@Yb core−shell nanoparticles/nanorods are mainly due to modifications of surface-related effects. It is found that the tensile strain increases from +1.0% to +1.9% with changing the shape from nanoparticle to nanorod and reversal of the lattice strain (compressive) is obtained for coated nanoparticle/ nanorod. It is worth mentioning that the lattice strain varies with changing the shape and surface coating on nanocrystals and the upconversion emission intensity increases with decreasing the tensile lattice strain and it increases with increasing compressive strain. Analysis suggests that the lattice strain plays an important role in modification of the upconversion properties of the rare-earth-doped nanocrystals.