Effect of host anion or cation substitution on the luminescence and stability of lanthanum oxide based phosphors doped with bismuth
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Date
2015
Authors
Jabraldar, Babiker Mohammed Jaffar
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
LaOCl:Bi3+ phosphor powders were successfully prepared via the solid-state reaction method and compared to LaOBr:Bi3+. Photoluminescence measurements revealed that the phosphors displayed ultraviolet emission around 344 nm for LaOCl:Bi and 358 nm for LaOBr:Bi under excitations of 266 nm and 273 nm, respectively. The optimum Bi doping concentration and annealing temperature were established as 0.7 mol% and 900 °C for both hosts. The scanning electron microscopy data showed that the particles are randomly oriented flat thin plates with diameters close to or larger than 100 nm. The chemical composition of the phosphor was studied by using energy dispersive X-ray spectroscopy, while diffuse reflectance spectroscopy was employed to study the absorption. The ultraviolet emission peaks were assigned to the 3P1 → 1S0 transition of the Bi3+ ions, while additional relatively weak emissions in the visible range were attributed to Bi clustering. LaOCl:Bi was found to undergo a little degradation while exposed to the atmosphere for several months, while LaOBr:Bi was significantly less stable. The LaOBr:Bi phosphor was also found to degrade rapidly under 254 nm ultraviolet excitation, while LaOCl:Bi did not degrade under the same conditions. X-ray photoelectron spectroscopy measurements indicated that similar surface changes occurred for both samples during UV-exposure, suggesting that the cause of degradation is not primarily related to surface changes, but may be due to the greater probability of forming non-radiative defects in the bulk of LaOBr having the smaller bandgap. Luminescence properties of bismuth (Bi) doped lanthanum oxyfluoride (LaOF) were investigated experimentally with samples prepared via the solid-state reaction method using ammonium fluoride, lanthanum oxide and bismuth oxide as precursors. Energy dispersive X-ray spectroscopy data showed that the samples were not completely stoichiometric and that the F/O ratio decreased with increasing synthesis temperature. The X-ray diffraction data confirmed that the phosphor powder crystallized in the tetragonal structure for lower synthesis temperatures up to 1000 C. When prepared at 1100 C, the LaOF took the rhombohedral form which showed almost no luminescence when doped with Bi. Therefore the investigation focussed on the tetragonal phase of LaOF for which scanning electron microscopy data showed that the shape of the particles did not change when it was doped with Bi. A single broad white luminescence emission band centred at 518 nm was observed by exciting the phosphor at 263 nm. The optimum Bi doping concentration was found to be La1-xOF:Bix=0.005. This sample was stable when exposed to the atmosphere for up to six months, although its luminescence degraded slowly under a 254 nm ultraviolet lamp. XPS data confirmed that the annealing at 1100 °C and exposure of the sample to UV radiation result in segregation of the Bi3+ on the surface of the sample. Bi3+ doped LaOCl and LaOF phosphor thin films were successfully prepared via the pulsed laser deposition technique in vacuum and different Argon (Ar) pressures. All peaks of the XRD patterns of the films were consistent with the tetragonal structure of the LaOCl and LaOF, but in the case of LaOF the signal was weaker and not all peaks were present, suggesting some preferred orientation. The elemental composition of the films were studied by using energy dispersive X-ray spectroscopy, while plan-view and cross-sectional scanning electron microscopy was used to study the morphology of the films. Photoluminescence measurements revealed that the films exhibited emission around 344 nm for LaOCl:Bi and 518 nm for LaOF:Bi under excitations of 266 nm and 263 nm, respectively. The emission peaks were attributed to the 3P1 → 1S0 transition of the Bi3+ ions. The luminescence from the LaOF:Bi sample was less intense compared to the LaOCl:Bi sample prepared under the same conditions, which was also the case for powder samples. The amount of ablated material present on the substrate was much less for LaOF:Bi compared to LaOCl:Bi, which is attributed to the greater bandgap and hence weaker absorption of the laser pulses for LaOF:Bi. La2-yYyO3 phosphor powders were prepared via the citric acid sol-gel combustion method and their stability was checked after four months of storage in the atmosphere using X-ray diffraction. The material, like La2O3, was prone to hydroxylation unless a high proportion of Y was added, which also caused changes in the phases present. Based on its stability and single phase (cubic), the composition La0.5Y1.5O3 was selected for further study with Bi doping. Under an excitation of 333 nm the phosphor presented two emission peaks located at 424 and 529 nm. This corresponded to excitation of Bi3+ ions in the non-centrosymmetric C2 symmetry sites. Blue (424 nm) emission could be excited from the same samples when excited at 371 nm from Bi3+ ions on the centrosymmetric S6 sites, which was similar in colour to the emission of Bi-doped La2O3. The optimum doping concentration for La0.5-xY1.5O3:Bi powder was found to be x = 0.005. The luminescence from this material was not as intense as that from Y2-xO3:Bix=0.005, but there was a significant shift in colour as a result of the added La which meant that emission from the Bi3+ ions at the S6 sites of the alloy was close to that of unstable La2O3 on the colour chart. Bismuth (Bi3+) doped lanthanum gallate (LaGaO3) phosphor powder and thin films were successfully synthesised via the citric acid sol-gel combustion synthesis and pulsed laser deposition (PLD), respectively. Firstly the stability of La2-yGayO3 powders under ambient conditions was assessed. The optical properties of La1-xGaO3:Bix phosphor powders were then investigated. X-ray diffraction data confirmed that the structure of LaGaO3 belong to the orthorhombic perovskite structure with Pbnm space group, while diffuse reflectance spectroscopy data showed that the band gap energy of the LaGaO3 host was 4.65 eV. Scanning electron microscopy data showed that the grain size increased with increasing annealing temperature. There was no change in the particle size and morphology when dopant was added to the host. Photoluminescence (PL) and cathodoluminescence (CL) measurements indicated that the phosphor presented efficient ultraviolet emission around 370 nm, which was attributed to transitions between the 3P1 excited state and 1S0 ground state of the Bi3+ ions. The optimum Bi doping concentration and annealing conditions for photoluminescence were found to be for La1-xGaO3:Bix=0.003 and 1200 °C. Thin films of the optimized powder were prepared via PLD. The PL and CL of the LaGaO3:Bi3+ thin films were similar to the powder.
Description
Thesis (Ph.D.(Physics))--University of the Free State, 2015
Keywords
LaOCl, LaOBr, LaOF, LaGaO3, LaYO3, Bi3+ ions, Photoluminescence, Stability, Cathodoluminescence, PL Degradation