Browsing by Author "Dejene, F. B."

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    Characterization of cerium doped yttrium gadolinium aluminate garnet (Y-Gd)3Al5O12:Ce3+ phosphor thin films fabricated by pulsed laser deposition
    (2015) Wako, Ali Halake; Dejene, F. B.; Swart, H.C.
    Thin films of cerium doped yttrium gadolinium aluminate garnet (Y-Gd)3Al5O12: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, namely; substrate temperature, substrate – target distance, number of laser pulses and the working atmosphere during the film deposition process. The effect of annealing temperatures on the structure and luminescence properties of the as-deposited (YGAG:Ce) thin films were analysed. Photoluminescence (PL) data were collected in air at room temperature using an F-7000 FL Spectrophotometer. 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. The PL intensity of the samples increased as the annealing temperature was increased from 400 oC to around 700 oC and then decreased with continued increase in the annealing temperature.
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    Combustion synthesis, characterization and luminescence properties of barium aluminate phosphor
    (Elsevier, 2014-09) Wako, Ali Halake; Dejene, F. B.; Swart, H.C.
    The blue-green emitting Eu2+ and Nd3+ doped polycrystalline barium aluminate (BaAl2O4:Eu2+,Nd3+) phosphor, was prepared by a solution-combustion method at 500 °C without a post-annealing process. The characteristic variation in the structural and luminescence properties of the as-prepared samples was evaluated with regards to a change in the Ba/Al molar ratio from 0.1:1 to 1.4:1. The morphologies and the phase structures of the products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), while the optical properties were investigated using ultra-violet (UV) and photoluminescence (PL) spectroscopy, respectively. The XRD and TEM results revealed that the average crystallite size of the BaAl2O4:Eu2+,Nd3+ phosphor was about 70 nm. The broad-band UV-excited luminescence of the phosphors was observed at λmax=500 nm due to transitions from the 4f65d1 to the 4f7 configuration of the Eu2+ ion. The PL results indicated that the main peaks in the emission and excitation spectrum of phosphor particles slightly shifted to the short wavelength due to the changes in the crystal field due to the structure changes caused by the variation in the quantity of the Ba ions in the host lattice.
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    Effect of Ga3+ and Gd3+ ions substitution on the structural and optical properties of Ce3+-doped yttrium aluminium garnet phosphor nanopowders
    (John Wiley & Sons, 2016-01) Wako, Ali Halake; Dejene, F. B.; Swart, H.C.
    ABSTRACT: The structural and optical properties of commercially obtained Y3Al5O12:Ce3+ phosphor were investigated by replacing Al3+ with Ga3+ and Y3+ with Gd3+ in the Y3Al5O12:Ce3+ structure to form Y3(Al,Ga)5O12:Ce3+ and (Y,Gd)3Al5O12:Ce3+. X-Ray diffraction (XRD) results showed slight 2-theta peak shifts to lower angles when Ga3+ was used and to higher angles when Gd3+ was used, with respect to peaks from Y3Al5O12:Ce3+ and JCPDS card no. 73–1370. This could be attributed to induced crystal-field effects due to the different ionic sizes of Ga3+ and Gd3+ compared with Al3+ and Y3+. The photoluminescence (PL) spectra showed broad excitation from 350 to 550 nm with a maximum at 472 nm, and broad emission bands from 500 to 650 nm, centred at 578 nmfor Y3Al5O12:Ce3+ arising fromthe 5d→4f transition of Ce3+. PL revealed a blue shift for Ga3+ substitution and a red shift for Gd3+ substitution. UV–Vis showed two absorption peaks at 357 and 457 nm for Y3Al5O12:Ce3+, with peaks shifting to 432 nm for Ga3+ and 460 nm for Gd3+ substitutions. Changes in the trap levels or in the depth and number of traps due to Ce3+ were analysed using thermoluminescence (TL) spectroscopy. This revealed the existence of shallow and deep traps. It was observed that Ga3+ substitution contributes to the shallowest traps at 74 °C and fewer deep traps at 163 °C, followed by Gd3+ with shallow traps at 87 °C and deep traps at 146 °C. Copyright © 2016 John Wiley & Sons, Ltd.
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    Influence of alkaline earth metal cations; Ca2+, Sr2+ and Ba2+ on the structural and optical properties of MAl2O4: Eu2+, Nd3+ phosphors.
    (2014) Wako, Ali Halake; Dejene, F. B.; Swart, H.C.
    Eu2+ doped and Nd3+ co-doped MAl2O4:Eu2+, Nd3+ (M = Ca, Sr, and Ba) phosphors were prepared by a solution-combustion method. The obtained powders were investigated in terms of their phase composition, particle morphology and photoluminescence (PL) by X-Ray diffraction (XRD), Scanning Electron Microscope (SEM) and Ultraviolet-Visible Spectroscopy (UV-VIS) techniques respectively. XRD analysis depicts a monoclinic phase for CaAl2O4:Eu2+, Nd3+ and SrAl2O4:Eu2+, Nd3+ and a hexagonal structure for BaAl2O4:Eu2+, Nd3+ phosphor. SEM results showed generally agglomerated particles with non-uniform shapes and sizes with irregular network structures having lots of voids and pores. PL excitation revealed broadband spectra with peaks corresponding to the crystal field splitting of the Eu2+ d-orbital. The emission spectra were also broadband with peaks at 447 nm for CaAl2O4:Eu2+, Nd3+, at 507 nm for SrAl2O4:Eu2+, Nd3+ and at 497 nm for BaAl2O4:Eu2+, Nd3+ due to the 4f65d1-4f7 emission of Eu2+. Sharp emission lines were observed arising from the f-f transitions of the Eu3+ ions. The differences in emissions from the three phosphors arise from the crystal field splitting of the 5d electron shell due to the changes in the crystalline environment of the Eu2+ ions caused by the substitution of the divalent alkaline earth metal cations with different ionic sizes in the MAl2O4 host lattice. UV-VIS spectra showed absorption edges at 330, 342 and 340 nm in agreement with the observed PL excitation peaks. The luminescence decay characteristics showed that these materials possess persistent luminescence whereby BaAl2O4: Eu2+; Nd3+ gave a longer afterglow as compared to the other two phosphors.
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    Structural and luminescence properties of SrAl2O4:Eu2+,Dy3+,Nd3+ phosphor thin films grown by pulsed laser deposition
    (Elsevier, 2015-09) Wako, Ali Halake; Dejene, F. B.; Swart, H.C.
    Thin films of Eu2þ doped and Dy3þ,Nd3þ co-doped Strontium Aluminate (SrAl2O4:Eu2þ,Dy3þ,Nd3þ) phosphors were grown on Si(100) substrates by a pulsed laser deposition (PLD) technique using a 266 nm Nd:YAG pulsed laser under varying substrate temperature and the working atmosphere during the film deposition process. The effect of substrate temperatures and argon partial pressure on the structure and luminescence properties of the as-deposited SrAl2O4:Eu2þ,Dy3þ,Nd3þ phosphor thin films were analysed. 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 collected in air at room temperature revealed a slight shift in the peak wavelength of the PL spectra 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. The PL intensity of the samples was highest for 100 °C substrate temperature and 20 mTorr argon partial pressure. Due to this, the effect of argon partial pressure was studied at a constant substrate temperature of 100 °C while the effect of Substrate temperatures recorded at 20 mTorr argon pressure respectively.
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    Structure,morphologyandopticalpropertiesofundopedand MN-doped ZnO(1 x)Sx nano-powderspreparedbyprecipitation method
    (Elsevier, 2015-08) Wako, Ali Halake; Dejene, F. B.; Onani, M. O .; Koao, L. F.; Motloung, S. V.; Yihunie, M. T.
    The undopedandMn-dopedZnO(1 x)Sx nano-powders weresuccessfullysynthesizedbyprecipitation method withoutusinganycappingagent.Itsstructure,morphology,elementalanalysis,opticaland luminescence propertiesweredeterminedbyscanningelectronmicroscopy(SEM),energy-dispersive X-rayspectroscopy(EDS),UV–vis spectroscopy(UV)andphotoluminescencespectroscopy(PL).Atypical SEM imageoftheun-dopedZnO(1 x)Sx nanoparticles exhibit flake likestructuresthatchangestonearly spherical particleswithMn-doping.TheXRDofundopedandMndopedZnO(1 x)Sx patternrevealsthe formation ofaproductindexedtothehexagonalwurtzitephaseofZnS.Thenanopowdershavecrys- tallite sizesestimatedfromXRDmeasurementswereintherangeof10–20 nm.Allthesamplesshowed absorption maximumofZnO(1 x)Sx at 271nmandhightransmittanceinUVandvisibleregion,re- spectively.TheundopedZnO(1 x)Sx nanoparticles showstrongroom-temperaturephotoluminescence with fouremissionbandscenteringat338nm,384nm,448nmand705nmthatmayoriginatetothe impurity ofZnO(1 x)Sx, existenceofoxiderelateddefects.Thecalculatedbandgapofthenanocrystalline ZnO(1 x)Sx showedablue-shiftwithrespecttotheMn-doping.ThePLspectraoftheMn-dopedsamples exhibitastrongorangeemissionataround594nmattributedtothe 4T1–6A1 transitionoftheMn2þ ions.
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    Trap characteristics of UV-activated Y3(Al,Ga)5O12:Ce3+phosphorsA
    (Elsevier, 2015-12) Wako, Ali Halake; Dejene, F. B.; Swart, H.C.
    tThis paper reports on the trap characteristics of commercially obtained Y3(Al,Ga)5O12:Ce3+phosphorpowder. The effects of UV irradiation time and different heating rates have been discussed using differentthermoluminescence (TL) techniques. In the TL glow curves of the Y3(Al,Ga)5O12:Ce3+phosphor measuredin the temperature range between room temperature (RT) and 250◦C, two TL peaks were observed, amaximum centered around 74◦C and a shoulder at around 163◦C when excited with UV radiation. Varyingthe UV irradiation (exposure) time or hence absorbed dose and heating rate significantly affected the peakintensity (Im) and peak temperature (Tm) of the glow curve. The UV exposure time was varied from 3 minto 20 min and we noted that the TL intensity increased with UV exposure time in the range 3–15 min. Thismay be explained on the basis of second order recombination kinetics taking place due to a re-trappingprocess of the excitation. The intensity of the two TL peaks moved to the higher temperature side withan increase in the heating rate. The kinetic parameters at various heating rates namely activation energy(E), order of kinetics (b) and frequency factor (s) of the Y3(Al,Ga)5O12:Ce3+sample were determined bythe variable heating rate method, peak shape method and TL Anal Glow Curve Deconvolution (GCD)techniques.