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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-05172013-151117 |
| Date | 17 May 2013 |
| Creators | Ahmed, Hassan Abdelhalim Abdallah Seed |
| Contributors | Prof OM Ntwaeaborwa, Dr RE Kroon |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-05172013-151117/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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