Tailoring luminescence properties of rare earth doped aluminate and darnet phosphors

This work covers several aspects about different phosphors activated with rare earth (RE) metal ions which were synthesised and characterised using different techniques with the sole aim of tailoring their chemical and structural properties so as to optimize their output in order to meet the requirements for industrial and commercial applications. Eu²⁺ doped and Dy³⁺ co-doped strontium aluminate (SrA1₂O₄:Eu²⁺, Dy³⁺) phosphors were prepared by solution - combustion synthesis technique at 500 °C using urea as a reducer and fuel, a widely known method for preparing nano sized phosphors. The SrA1₂O₄:Eu²⁺, Dy³⁺ was irradiated with beta (β) rays and thermoluminescence (TL) properties studied with regards to the electron-trapping properties in terms of TL glow curves. The results of the peak shape method showed that the kinetics of the obtained glow curves were of first order. Blue-green emitting Eu²⁺ and Nd³⁺ doped polycrystalline barium aluminate (BaA1₂O₄:Eu²⁺, Nd³⁺) phosphor was prepared by a solution - combustion method at 500 °C without a post­annealing process. The characteristic variation in the structural and luminescence properties of the as-prepared samples have been evaluated with regards to when the Ba/Al mole ratio changed from 0.1: 1 to 1.4: 1. The XRD and TEM results reveal that the average particle size of the BaA1₂O₄:Eu²⁺, Nd³⁺ phosphor is about 60 nm. The broad-band UV-excited luminescence of the phosphors was observed at 𝜆ₘₐₓ = 500 nm due to transitions from the 4:f⁶5ct¹ to the 4f⁷ configuration of the Eu²⁺ ion. The results indicated that the main peaks in the emission and excitation spectrum of the nanophosphors slightly shifted to the short wavelength because of changes in the crystal field effect due to structural changes caused by the variation of quantity of Ba ion in the host lattice. Thin films of cerium doped yttrium gadolinium aluminium garnet (Y,Gd)₃A1₅O₁₂:Ce³⁺ (YGAG:Ce) were grown on Si (100) substrates by a pulsed laser deposition (PLD) technique using a 266 nm Nd:YAG pulsed laser under varying deposition conditions. The effect of annealing temperatures on the structure and luminescence properties of the as-deposited (YGAG:Ce) thin films were analysed. A slight shift in the wavelength of the PL spectra was observed from the thin films when compared to the PL spectra of the phosphor in powder form, which is probably due to a change in the crystal field. Thin films of Strontium Aluminate (SrA1₂O₄:Eu²⁺, Dy³⁺, Nd³⁺) phosphors were grown on Si (100) substrates using a 266 nm Nd-YAG laser. XRD patterns showed that with increasing substrate temperature and argon partial pressure the peaks in the direction (220) shifted to the lower 2-theta angles. Photoluminescence (PL) data revealed a slight shift in the peak wavelength which is probably due to crystal field effects. The structural and optical properties of commercially obtained Y₃A1₅O₁₂ :Ce³⁺ phosphor with respect to substitution of AI³⁺ with Ga³⁺ and y³⁺ with Gd³⁺ in the Y₃A1₅O₁₂ :Ce³⁺ garnet structure resulting in Y₃(A1,Ga)₅O₁₂ :Ce³⁺ and (Y,Gd)₃ A1₅O₁₂:Ce³⁺ systems respectively were investigated. X ray diffraction (XR.D) results show slight shifts in the 2theta angles; to the lower angles when Ga³⁺ was used and to the higher angles when Gd³⁺ was used with respect to the XRD peaks from Y₃A1₅O₁₂ :Ce³⁺ and JCPDS card No. 73-1370. This could be attributed to the induced crystal field effects due to different ionic sizes of Ga³⁺ and Gd³⁺ d3+ as compared to the AI³⁺ and Y³⁺ respectively. Trap characteristics of commercially obtained Y₃(A1,Ga)₅O₁₂ :Ce³⁺ phosphor powder is also reported. The effects of UV irradiation time and different heating rates have been discussed. The TL glow curves showed two TL peaks, a maximum centered around 74 °C and a shoulder at around 163 °C when excited with UV radiation. Varying the UV irradiation (exposure) time or hence the absorbed dose and heating rate significantly affected the peak intensity (Iₘₐₓ) and peak temperature (Tₘₐₓ) of the glow curve. The intensity of the two TL peaks moved to the higher side with an increase in the heating rate.