Red emission of Praseodymium ions (Pr ³⁺)
Noto, Luyanda Lunga
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English: Red glowing phosphors were prepared by adding Pr3+ ions as activators to several oxide host matrixes; CaTiO3, LaTaO4, YTaO4, and GdTaO4. The perovskite CaTiO3:Pr3+ compound is a phosphor that glows with a single red emission around 613 nm at room temperature upon irradiation with UV light of 230 – 360 nm wavelengths or an electron beam. The source of the single red emission is the intervalence charge transfer between Pr3+ and Ti4+ ions, which opens up a channel to completely depopulate the 3P0 state, by populating the 1D2 state. This leads to a dominant emission coming from the 1D2 → 3H4 transition. The dynamics of Pr3+ in YTaO4, LaTaO4, and GdTaO4 have not been explored excessively, and the resulting emission of these compounds doped with Pr3+ comes from both 3P0 and 1D2 states of Pr3+. The compounds were prepared by solid state reaction at 1200 oC and CaTiO3 was prepared by directly firing TiO2 (Anatase phase) and CaCO3 for 4 hours. The compound was doped with several mol% concentrations of Pr3+ from PrCl3 compound to optimize the output emission intensity. The rare-earth tantalate phosphors were prepared by directly firing Ta2O5 with Y2O3, La2O3, or Gd2O3 for 4h to obtain LaTaO4, YTaO4, and GdTaO4 respectively. The tantalates were doped with 0.5 mol% concentration of Pr3+ from PrCl3 and the synthesis was carried through in the presence of 30 wt% Li2SO4 flux agent. The role of the flux agent in this instant was to increase the reaction rate by acting as an intermediate that converts the reagents to reactive species, lower the reaction temperature required for the final compound to form and to facilitate crystallinity and to control particle sizes. The phase of the phosphor compounds was identified by using X-ray diffraction (XRD, Bruker AXS D8 Advance). The XRD patterns of CaTiO3 with different Pr3+ concentrations match that of the standard orthorhombic CaTiO3 (JCPDS card no. 22-0153). The XRD patterns of LaTaO4, YTaO4, and GdTaO4 with 0.5 mol % of Pr3+ suggest the presence of the reagent ions in the final product. The surface morphology of the compounds was traced using Scanning Electron Microscopy (SEM) and that of CaTiO3 showed particles of different shapes and sizes. The SEM shows the surface morphology of GdTaO4 and LaTaO4 to be of particles with different shapes and also to have sharp edges. The luminescence properties of CaTiO3:Pr3+, LaTaO4:Pr3+, YTaO4:Pr3+, and GdTaO4:Pr3+ were monitored using a PerkinElmer Lambda 950 UV/VIS spectrometer, for diffuse reflectance measurements to identity the absorbing centers in the phosphors. Photoluminescence (PL) and phosphorescence lifetime measurements of CaTiO3:Pr3+ were done using Varian Carry-Eclipse fluorescence spectrometer. PL of LaTaO4:Pr3+,YTaO4:Pr3+, and GdTaO4:Pr3+ was measured with DESY synchrotron working with photons from 50 to 330 nm wavelengths. Phosphorescence lifetime measurements and the energy distribution of localized trap levels of LaTaO4:Pr3+, YTaO4:Pr3+, and GdTaO4:Pr3+ were measured using Thermoluminescence (TL) 10091, NUCLEONIX spectrometer. CaTiO3:Pr3+ phosphor with a single red emission peak around 613 nm is co-doped with In3+ to charge compensate the local sites where a trivalent ion Pr3+ substitutes for a divalent ion Ca2+. It is found that In3+ charge compensation from 0.05 to 0.1 mol% has an effect of enhancing the red emission intensity and afterglow decay time of CaTiO3:Pr3+. The lifetime measurements were carried out using Varian Carry-Eclipse for CaTiO3:Pr3+ co-doped with different In3+ concentrations and using (TL) spectroscopy at 30 oC for LaTaO4:Pr3+, YTaO4:Pr3+, and GdTaO4:Pr3+. The phosphorescence lifetime (τ) observed for different In3+ co-doped in CaTiO3:Pr3+ was 7.6 s for 0.05 mol% In3+, 11.2 s for 0.1 mol% In3+, 6.3 s for mol% In3+ and 2.03 s for mol% In3+. For the orthotantalates it was approximated 620 s for GdTaO4:Pr3+, 655 s for YTaO4:Pr3+ and 663 s for LaTaO4:Pr3+. The depth of the trap levels was investigated using TL and were found to be residing at 0.71, 0.83, 1.02 and 1.48 eV depths for GdTaO4:Pr3+, at 0.68, 1.02, 1.43, and 1.60 eV depths for YTaO4:Pr3+ and at 0.46, 0.55 and 0.75 eV depths for LaTaO4:Pr3+. Surface chemical stability is an important parameter for phosphors that are projected for industrial purposes, such as the manufacturing of field emission displays (FED) screens and others. The surface chemical stability and its effects on CL intensity under prolonged electron beam irradiation were investigated, for CaTiO3:Pr3+, LaTaO4:Pr3+, YTaO4:Pr3+ and GdTaO4:Pr3+ in-situ using AES (PHI 549) at 1×10-8 Torr and 1×10-6 Torr O2 . The resulting surface chemical state changes were traced using PHI 5000 versa-probe XPS. The XPS revealed that on the surface of CaTiO3:Pr3+ new species such as CaO and CaOx suboxide non luminescent layers had formed on the surface during the electron beam irradiation process as per the ESSCR mechanism. On the surfaces of the tantalate phosphors there was also a formation of sub oxides due to the electron stimulated surface chemical reaction (ESSCR) that is stimulated by the prolonged electron beam irradiation. These showed stability under the electron beam irradiation.