Luminescence studies and stability of bismuth doped lanthanum oxide and oxysulphide
Jabraldar, Babiker Mohammed Jaffar
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Different concentrations of bismuth doped lanthanum oxide (La2-xO3:Bix) phosphor powder (x = 0.001, 0.002, 0.003, 0.004, 0.006, 0.008 and 0.01) were synthesised by means of the sol-gel combustion method at 250 °C using citric acid as the fuel. The product was annealed at different temperatures and the luminescence properties were investigated. The maximum photoluminescence (PL) emission was obtained for the sample which was doped with x = 0.002 and annealed at 1200 °C in air. The X-ray diffraction (XRD) analysis confirmed that all samples crystallized in the La2O3 hexagonal phase. The scanning electron microscopy (SEM) data showed that the grain size increased with increasing annealing temperature and the shape of the grains changed from rectangular to more round, but faceted, after annealing at 1200 °C in air. Energy dispersive X-ray spectroscopy (EDS) confirmed the chemical composition, while diffuse reflectance spectroscopy was used to study the absorption of the La2-xO3:Bix samples. All the samples were absorbing in the ultraviolet range between 220 to 320 nm. The band gap of the La2O3 pure host sample was obtained from the reflectance data as 5.1 eV. Excitation at a wavelength of 308 nm resulted in a single broad blue luminescence emission band centred at 462 nm. The excitation and emission bands were attributed to transitions between the 1S0 ground state and the 3P1 excited state of Bi3+ ions, with a Stokes shift of 1.35 eV. It was found that the samples no longer exhibited PL after storage of several weeks. Further XRD measurements revealed that the La2O3 had changed to La(OH)3. This is consistent with reports that La2O3 can absorb moisture from the air and transform to La(OH)3, which was observed to occur completely in about a week. Unlike for La2O3:Eu and La2O3:Ho phosphors for which the transformation reduced the luminescence, but did not quench it completely, the luminescence of the degraded La2O3:Bi was negligible so that simply the presence of luminescence can be used to indicate whether the transformation is not yet complete. This may be useful to use PL to monitor the transformation of La2O3 for other applications, e.g. ceramics and catalysis. If the transformed samples were re-annealed in air at 800 °C for 2 h, XRD results showed that the structure reverted completely to La2O3 and the blue PL emission was once again observed, however at only about one third of the intensity as for freshly prepared samples. For samples stored in a vacuum desiccator for one week, no change for XRD and PL were observed. Therefore La2O3:Bi phosphor may have application as a moisture sensor, because while the luminescence remains high it is evidence that it has not been exposed to the atmosphere. Degradation was effectively slowed, but not eliminated, by encapsulation of the phosphor in poly(methyl methacrylate) polymer. Therefore Bi3+ doped lanthanum oxysulphide (La2O2S) phosphor was synthesised to compare its stability and suitability as a blue emitting phosphor material. Synthesis was performed via the ethanol-assisted solution combustion method, followed by annealing for 2 h at 900 °C in a reducing atmosphere (5% H2 in Ar gas). XRD data confirmed that all samples crystallized in the La2O2S hexagonal lattice. SEM data showed that the particles aggregated and had irregular shapes. EDS confirmed the chemical composition, although the Bi dopant could not be identified since its expected peak position overlapped that of S. The samples, measured by diffuse reflectance spectroscopy, were absorbing in the ultraviolet range between 220 to 350 nm. The band gap of the pure host La2O2S was found to be 4.90 eV. Excitation at a wavelength of 260 nm and 344 nm resulted in a single broad blue luminescence emission band centred at 456 nm, which was attributed to transitions between the 1S0 ground state and the 3P1 excited state of Bi3+ ions. La2O2S:Bi phosphor was found to have a similar emission colour as La2O3:Bi, although less pure and closer to the centre of the Commission International Eclairge (CIE) diagram. Although the emission intensity of La2O2S:Bi phosphor was initially less than the La2O2:Bi phosphor, it was found to be stable and therefore superior for applications where the phosphor will be exposed to the atmosphere. La2O2S:Bi phosphor also exhibited persistent luminescence, which was attributed to the Bi3+ ions acting as hole traps and host defects acting as electron traps. The cathodoluminescence (CL) of the La2-xO3:Bix=0.002 and La2-xO2S:Bix=0.002 phosphors was compared and they were assessed for possible application in field emission displays (FEDs). Since the phosphor is not exposed to the atmosphere such an application, bulk hydroxylation of the La2O3:Bi cannot occur. However, electron-stimulated surface chemical reactions caused by the electron beam are known to induce changes on the surface of phosphors that can lead to CL degradation. Simultaneous CL and Auger electron spectroscopy (AES) measurements were performed during long term exposure of the samples to an electron beam to assess the CL degradation and chemical changes on the surface. X-ray photoelectron spectroscopy (XPS) measurements were also made on the samples before and after CL degradation. It was found that after a small amount of initial CL degradation, associated with removal of contamination from the surface, the La2O3:Bi sample remained stable under the electron beam and it may be suitable for use in FEDs. However, the La2O2S:Bi showed continuous and severe CL degradation and is not suitable for CL applications. During degradation AES measurements showed that there was a decrease in the surface concentration of S, suggesting the formation of a non-luminescent La2O3 surface layer which was responsible for degradation. However, some S remained on the surface and XPS spectra showed that a sulphate, possibly La2O2SO4, was present on the surface, which may have contributed to the degradation.