Synthesis and characterization of MgAl₂O₄ and (MgxZn₁-x)Al₂O₄) mixed spinel phospors
Tabaza, Wael Abed Ibrahim
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Magnesium aluminate (MgAl2O4) has received special attention as a technologically important material because of its attractive properties, such as mechanical strength, chemical inertness, a wideband gap, relatively low density, high melting point, high thermal shock resistance, low thermal expansion coefficient, resistance to neutron irradiation and low dielectric loss. It has also been used as a phosphor host activated by a variety of transition metal and lanthanide ions. Doped and undoped MgAl2O4 nanocrystalline powders were successfully prepared by a simple combustion method. The structure of the powders was analyzed with x-ray diffraction (XRD). The XRD data showed that all the samples had the spinel structure and the average particle size of the as-prepared samples was about 25 nm. The morphology of the samples was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) was used to obtain more information on the conversion of Ce ions from the non-luminescent Ce4+ ion to the luminescent Ce3+ oxidation state. The optical properties of the material were measured using photoluminescence (PL) spectroscopy and ultraviolet-visible (UV-Vis) spectroscopy at room temperature. At a 350 nm excitation wavelength the PL spectra of Ce3+ doped MgAl2O4 showed a broad green emission band centred at 490 nm. The maximum green emission was obtained for the sample doped with 0.75 mol% Ce. However the Ce3+ doped MgAl2O4 requires reducing at a high temperature (1400 °C) to convert the non-luminescent Ce4+ ions to the luminescent Ce3+ ions. For this reason Bi3+ doped MgAl2O4 was also investigated. For an excitation wavelength of 335 nm the Bi3+ doped MgAl2O4 produced a broad blue emission band centred at around 400 nm. The maximum blue emission was obtained for the sample doped with 0.5 mol% Bi. The results indicate that doping MgAl2O4 with Bi ions may be more attractive than doping with Ce ions. The blue emission from Bi doped MgAl2O4 was found to occur in the wavelength region corresponding to the well known host independent weak f-f excitation peaks of Tb. Therefore the possibility of using Bi3+ ions as a sensitizer for Tb3+ ions emitting green light was investigated. Although Tb can generally be excited efficiently using its strong f-d transitions, the possibility of exciting it through energy transfer from Bi to the normally weak f-f transitions is important, since the excitation wavelength of Bi (335 nm) is longer and more accessible than the f-d transition wavelength of Tb in MgAl2O4 (227 nm). The results show that Bi is a good sensitizer for Tb in the MgAl2O4 host and leads to significant enhancement of Tb emissions from the 5D4 level for excitation wavelengths between 300 nm and 380 nm (with a maximum enhancement of almost 100 times near 328 nm), thereby extending the useable excitation band of Tb to much longer wavelengths compared to Tb single doped samples. Zinc aluminate (ZnAl2O4) also has the spinel structure and has been used as the phosphor host. Therefore it is interesting to consider the mixed spinel magnesium zinc aluminate (MgxZn1-xAl2O4) as a host for Tb. The maximum PL intensity of the green emission at 544 nm occurred for Mg0.75Zn0.25Al2O4 and the optimum Tb concentration was found to be 0.5 mol%. The blue emission at 416 nm was almost absent for ZnAl2O4, but increased with the Mg content and was a maximum for MgAl2O4. The absence of blue emission peaks is usually attributed to the concentration quenching, but since the same Tb concentration was used for the MgAl2O4:Tb where blue emissions did occur, it is rather suggested that because of the smaller bandgap of ZnAl2O4, the 5D3 level lies close to or inside the conduction band and this prevents transitions from this level. An important application for phosphors is to make thin films for devices such as plasma displays and light emitting diodes. Tb doped Mg0.75Zn0.25Al2O4 thin films were fabricated on Si (100) substrates by two different techniques, namely sol-gel spin-coating and pulsed laser deposition (PLD) methods. The atomic force microscopy (AFM) and SEM data showed that the film deposited using a spin-coating method was more uniform and smooth compared to the thin film prepared by PLD. The thin film prepared by PLD has particles on the surface while the spincoated film was uniform. For the spin-coated samples annealing was necessary, but the morphology of these films was changed after annealing at 1200 °C and the surface formed ridges. The depth profile results obtained by Auger electron spectroscopy (AES) show that there was already some diffusion of the Si substrate into the material of the thin film for the samples annealed at 1000 °C. The emission spectra were similar for both kind of thin films, but there was a variation in the intensity of luminescence of the thin films. The spin coating process is the more simple method for growing the thin films, while PLD is a good technique for synthesizing of the thin films at lower substrate temperatures which might be important for the fabrication of the devices using glass with a melting point around 700 °C.