Preparation and characterization of powders and pulsed laser deposited thin films of rare-earths doped oxyorthosilicates
Ogugua, Simon Nnalue
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Today, phosphor materials have found many technological applications such as in electronic information display, advertising, solid-state lighting, solar cells, theft prevention, medicine, data storage, quality control, optical amplifiers, optical laser, scintillation, etc. Crystal field determines the chemical nature of the optical center of a phosphor material. Engineering the crystal field of the host matrix can lead to shifting of the wavelength ranges of optical transitions, increasing the rates of radiative transitions and minimizing loss by nonradiative decay and excited state absorption. These objectives can be accomplished by manipulating the unit cell containing the optical center. The unit cell can be manipulated by changing its chemical composition. By varying the molar ratio of La, Gd and Y in mixed La2-xGdxSiO5 or La2-xYxSiO5 hosts, the unit cells of the host can be manipulated and hence tuning the crystal field strength. These, to large extent, can influence the luminescence properties of Pr3+ and Dy3+ in these host matrices and can lead to the generation of tunable colour and white light for LED application. Phosphors are commercially available in powder form. However, for practical purposes, they must be in thin film forms. Hence, we have prepared a series of powder and thin film phosphors. The powder phosphors were prepared using solution combustion synthesis (SCS), before they were deposited on Si(111) substrates using the pulsed laser deposition (PLD) technique. In our previous report , we prepared La2-xGdxSiO5:Dy3+ (x = 0, 0.5, 1, 1.5 and 2) powder phosphors in which the International Commission of Illumination or CIE colour coordinates calculated from the photoluminescence data showed tunable colour, from blue to yellow through white. A near white emission was obtained from the sample when x = 0.5 (i.e. La1.5Gd0.5SiO5:Dy3+), therefore, we chose to prepare the thin films using this sample. The thin films were deposited in different atmospheres (vacuum, argon and oxygen), different deposition time and different deposition temperatures. Effect of post-annealing on the properties of the films were studied. We also prepared five sets of La2-xYxSiO5:Pr3+ (x = 0, 0.5, 1, 1.5 and 2) powder phosphors. Here we investigated how the molar ratio of the hosts (La2SiO5 and Y2SiO5) affects the photoluminescence properties of Pr3+. The luminescence of phosphors based on Pr3+ doped matrices showed a strong dependence on the crystal field of the host. When incorporated into hosts with strong crystal field (e.g. La2SiO5 or Y2SiO5), Pr3+ gave green and red emission from 1P0 and 1D2 levels of the 4f state respectively. The intensity of these emissions and the band gaps of the phosphors varied with the molar ratio of La and Y. When Pr3+ was co-doped with Dy3+ in La2-xYxSiO5:Pr3+ (x = 0, 0.5, 1, 1.5 and 2) hosts, it was observed that energy transfer from Pr3+ to Dy3+ was dependent on the molar ratio of La to Y and post-preparation annealing atmospheres (Ar-H2 and air). We also demonstrated that the emitted colour from La2-xYxSiO5:Pr3+ (x = 0, 0.5, 1, 1.5 and 2) could be tuned by exciting the phosphors at different wavelengths. Finally, we used integrating sphere method to determine the quantum yield of some of the phosphors. The structure of the phosphors was analyzed using X-ray diffractometer (XRD). The morphologies and chemical composition were analyzed using field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) respectively. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical and electronic states and the elemental composition of the phosphors. The distribution of molecular and atomic ionic species on the surface region of the phosphors was analyzed using time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The diffuse reflectance, photoluminescence and cathodoluminescence of the phosphors were also measured. Additional characterization techniques such as, atomic force microscopy (AFM) and The Rutherford back scattering (RBS) were used to analyze the topography and the composition and thickness of the thin film samples respectively.