Luminescence from lanthanide ions and the effect of co-doping in silica and other hosts
Ahmed, Hassan Abdelhalim Abdallah Seed
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Amorphous silica powders doped with lanthanide ions were synthesised by the sol-gel method and their cathodoluminescence (CL) and photoluminescence (PL) emissions were compared. Interesting differences depending on the type of excitation were observed for Tb and Ce-doped samples. For Tb-doped samples blue 5D3®7FJ emission was measured during CL in samples for which PL results showed this emission to be concentration quenched due to cross-relaxation, while for Ce-doped samples luminescence occurred for CL but not during PL measurements. Unlike the other lanthanides, Tb and Ce ions are sometimes found in the tetravalent rather than the trivalent state, and these differences were attributed to the possibility of electron capture of tetravalent ions possible during CL but not PL. A scheme for the energy levels of divalent and trivalent lanthanide ions relative to the conduction and valence bands in silica was proposed, making use of experimental data and the known relative positions of the energy levels for the lanthanides. Although the location of the divalent europium ion f-level above the valence band can be located by using the charge transfer energy of trivalent europium, this process cannot be generalized to find the location of the trivalent cerium ion f-level above the valence band using the charge transfer energy of tetravalent cerium as has been suggested. Initial investigations of the luminescence properties of Ce doped silica were complicated by overlapping luminescence from oxygen deficiency defects from the host itself and the fact that Ce took the tetravalent state which is nonluminescent for PL measurements. Spectra obtained using a wide variety of excitation methods, including synchrotron radiation, were compared and evaluated in the light of previously published data. Radically improved results were obtained by annealing in a reducing atmosphere instead of air. X-ray photoelectron spectroscopy as well as ultraviolet reflectance spectroscopy provided evidence of the conversion of Ce from the tetravalent to trivalent state and this was accompanied by strong luminescence of these sample during PL measurements. Ce,Tb co-doped silica was used to study the energy transfer from Ce to Tb ions. Initial results were disappointing when measurements showed that adding Ce quenched the Tb emission intensity instead of increasing it. However, after annealing the samples in a reducing atmosphere, a quantum efficiency of 97% for energy transfer from Ce to Tb was achieved. The mechanism for energy transfer was investigated by comparing experimental measurements of the changes in donor (Ce) emission intensity and lifetime as a function of the amount of acceptor (Tb) with numerical simulations of various models. Measurements correlated well to models for dipole-dipole and exchange interactions, but the critical transfer distance obtained was not appropriate for the exchange interaction, hence dipole-dipole interaction was identified as the interaction mechanism. Nanocrystalline LaF3 powders were synthesized by the hydrothermal method and strong luminescence was obtained from samples doped with Ce and Tb. Photoluminescence spectra from co-doped samples revealed that the emission from Ce was quenched and the emission from Tb was enhanced, showing that energy transfer from Ce to Tb occurred. The energy transfer mechanism was investigated in a similar way as for the silica samples, but in this case the experimental results fitted models for the quadrupolequadrupole and exchange interactions. Much higher concentrations of Tb were required to significantly affect the Ce luminescence properties than in the case for silica, and the critical transfer distance obtained was appropriate for the exchange interaction. Either or both of these interaction mechanisms are therefore possible. The results show that the interaction mechanism for energy transfer between lanthanide ions depends not only on the ions themselves, but also on the lattice hosting them.