Speaker
Mr
Babiker Jaffar
(ufs)
Description
Bismuth (Bi) ions exhibit interesting optical properties, yet have been less intensively studied compared to rare earth ions. Bi doped lanthanum oxide (La2O3:Bi3+) phosphor was synthesized via the sol-gel combustion method at 250 °C using citric acid as the fuel. The product powder was annealed at different temperatures between 800 °C to 1400 °C in air for 2 h. X-ray diffraction (XRD) data confirmed that the La2O3 host as well as doped phosphor crystallized in a hexagonal lattice. For photoluminescence spectroscopy (PL) an excitation wavelength of 308 nm, the phosphor powder produced a blue emission band centered at 462 nm. The maximum intensity was obtained for the sample doped with 0.2 mol% Bi which was annealed at 1200 °C. It was observed that the luminescence from the samples decreased after storage of several weeks. XRD measurements revealed that the La2O3 had changed to La(OH)3, which does not give luminescence when doped with Bi. It has been reported that La2O3 can absorb moisture from the air and transform to La(OH)3. This was observed to occur completely in about 6 days. The poorly emitting samples were re-annealed between 500 °C and 800 °C for 2 h. For the highest temperature it was found that the structure reverted completely to La2O3 and the blue PL emission was once again observed at about the same intensity as for freshly prepared samples. Although La2O3:Bi3+ can be used as a blue emitting phosphor, it is only suitable for applications where it will not be exposed to moisture in the atmosphere. For samples stored in a vacuum desiccator for one week, no change for XRD and PL were observed. Therefore, it may have an application as a moisture sensor, because while the luminescence remains high it is evidence that it has not been exposed to the atmosphere.
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Prof Kroon, UFS, KroonRE@ufs.ac.za
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Primary authors
Dr
Abdelrhman Yousif
(UofK)
Mr
Babiker Jaffar
(ufs)
Dr
Hassan Seed Ahmed
(assistant Professor)
Prof.
Hendrik Swart
(University of the Free State)
Prof.
R. E. Kroon
(University of the Free State)
