Doctoral Degrees (Physics)
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Browsing Doctoral Degrees (Physics) by Author "Dejene, F. B."
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Item Open Access Characterization of Gd2O2S: Tb³+ phosphor powder and thin films(University of the Free State, 2011-06) Dolo, Jappie Jafta; Swart, H. C.; Dejene, F. B.; Terblans, J. J.Under Ultra violet (UV), cathode-ray and X-ray excitation, terbium activated rare earth oxysulphide (Gd2O2S:Tb3+) phosphors shows bright green luminescence. Due to its superior luminescent performance, Gd2O2S:Tb3+ phosphor is used in the manufacturing of TV screens. The degradation of commercially available Gd2O2S:Tb3+ phosphor powder and pulsed laser deposited (PLD) thin films were studied with Auger Electron Spectroscopy (AES) and Cathodoluminescence (CL). The surface reactions were monitored with AES while the light output was measured with a PC2000-UV spectrometer. The CL of the Gd2O2S:Tb3+ was excited with a 2 keV energy electron beam with a beam current density of 26 mA/cm2. The CL and AES were measured simultaneously while the sample was bombarded with the electrons in an oxygen atmosphere. A comparison between the low energy peaks of the AES spectra before and after degradation showed significant differences in the shape of the peaks. A linear least squares (LLS) method was applied to resolve the peaks. Elemental standards from Goodfellow were used in conjunction with the measured data to subtract the S and Gd peaks. A direct correlation between the surface reactions and the CL output was found for both the thin films and the powder. The adventitious C was removed from the surface as volatile gas species, which is consistent with the electron stimulated surface chemical reactions (ESSCR) model. The CL decreased while the S was removed from the surface during electron bombardment. A new non-luminescent surface layer that formed during electron bombardment was responsible for the degradation in light intensity. X-ray photoelectron (XPS) indicated that Gd2O3 and Gd2S3 thin films are formed on the surfaces of the Gd2O2S:Tb3+ powder and thin films during prolonged electron bombardment. Luminescent Gd2O2S:Tb3+ thin film phosphors were successfully grown by the PLD technique. The effects of oxygen pressure and substrate temperature on the morphology and the PL emission intensity were investigated. The films grown in a higher O2 ambient consist of smaller but more densely packet particles relative to the films grown at a lower O2 ambient. The PL intensity of the films increased relatively with an increase in deposition O2 pressure. The PL of the films grown at a higher substrate temperature was generally also more intense than those grown at a lower substrate temperature. It was clear from the Atomic Force Microscopy (AFM) images that spherical nanoparticles were deposited during the deposition process. X-ray diffraction (XRD) indicated that the broadening of the XRD peaks is reduced with an increase in annealing temperature.Item Open Access Material Properties of RE- Doped Ln (Ln= Y, La) oxides and oxysulfides phosphors for red-emitting devices(University of the Free State, 2015-11) Ali, Abdub Guyo; Dejene, F. B.; Swart, H. C.Structural and optical properties of Eu3+-doped Ln (Ln=Y, La) oxide and oxysulfide nano crystals synthesized by sol-combustion method were analysed as a function of host to fuel ratio. Structural characterization shows crystallite nanosized particles and the hexagonal phase as the dominant structure. The red emission of Eu3+ doped Y 20 2S, La20 2S and Y 20 3 nanocrystals appearing near 624 nm was assigned to the 5Do-7F2 transition of Eu3+. Due to insufficient quantities of thiourea at the higher Ln/S mole ratio, the bright red emission has been quenched. Fourier-transform infrared spectrometry analysis showed that there was a negligible difference in the absorbed impurities with various molar ratios. The Ln/S concentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs for Ln/S=I to 76 μs for the higher concentrations. Structural and optical properties of La20 2S:Eu3+micro crystals synthesized by sol-combustion method were analyzed as a function of La/S concentration. Structural characterization shows a crystallite size of about 178 nm and the hexagonal phase as the dominant crystalline structure. The red emission of Eu3+ doped La20 2S microcrystals appearing near 624 nm was assigned to the 5D0- 7F2 transition of Eu3+. Due to insufficient quantities of thiourea at the higher La/S mole ratio, the bright red emission has been quenched. Fourier-transform infrared spectrometry analysis showed that there was a negligible difference in the absorbed impurities with various molar ratios. The La/S concentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs for La/S= I to 7 6 μs for the higher concentrations. To investigate the effect of co-doping a series of red-emitting phosphors Y 20 3:Eu3+:Ho3+ were prepared by the solution combustion method. X-ray diffraction (XRD) patterns indicate that the Eu3+ and Ho3+ doping do not show obvious effect on the cubic Y 20 3 crystal. Their crystall ite size estimated by x-ray diffractometry and scanning electron microscopy was about 8 nm. Under UV 325 nm excitation, emission wavelengths at 626 nm was quenched at higher mole percent of Ho3+ and energy was transferred from Eu3 ~ to Ho3+. Y20 3:Eu3+: Ho3+ phosphor shows a red-emitting afterglow phenomenon, and the Eu3 ' ions are the luminescent center during the decay process. The bright red emission near 626 nm has been noticeable due to the 5D0- 7F2 transition of Eu3 ... . The intensity of the luminescence has decreased with an increase of concentration of Ho3+. In sufficient quantities of Eu3+ to Ho3+, the bright red emission near 626 nm has been predominant due to 5D0- 7F2 transition of Eu3+. The decay characteristic of Y 20 3:Eu3 +: Ho3 + phosphor is according with the double exponential equation. The as-prepared powder Y 20 2S:Eu3 + was deposited on Si ( 100) substrates by using a pulsed laser deposition technique. The thin films grown under different oxygen deposition pressure conditions have been characterized using structural and luminescent measurements. The Xray diffraction patterns showed mixed phases of cubic and hexagonal crystal structures. As the oxygen partial pressure increased, the crystallinity of the films improved. Further increase of the 0 2 pressure to 140 mtorr reduced the crystallinity of the fi lm. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that an increase in 0 2 pressure affected the morphology of the films. The average band gap of the films calculated from diffuse reflectance spectra using the Kubeika-Munk function was about 4.75 eV. The photoluminescence measurements indicated red emission of Y20 2S:Eu3 + thin films with the most intense peak appearing at 619 nm, whi ch is assigned to the 5Do-7F2 transition of Eu3 +. This most intense peak was totally quenched at higher 0 2 pressures. X-ray photoelectron (XPS) indicated that Y20 3 thin films are formed on the surfaces of the Y20 2S: Eu3 + thin films during prolonged electron bombardment. The films grown in a lower 0 2 ambient consist of smaller but more densely packet particles relative to the films grown at a higher 0 2 ambient. In order to study the effect of annealing temperature on the films, four samples were annealed at various temperatures while one was kept unannealed. X-ray diffraction measurements show that the un-annealed thin film was amorphous, while those annealed were crystalline. At lower annealing temperature of 600 °c to 700 °c cubic bixbyite Y20 3:Eu3 + was formed . As the annealing temperatures were increased to 800 °c, hexagonal phase emerged. The average crystall ite size of the fi lm was 64 nm. Photoluminescence (PL) measurement indicates intense red emission around 612 nm due to the 5 00~ 7F2 transition. Scanning electron microscopy (SEM) indicated that agglomerates of non-crystalline particles with spherical shapes were present for the un-annealed film s. After annealing at high temperature, finer morphology was revealed. Atomic fo rce microscopy (AFM) further confirmed the formation of new morphology at the higher annealing temperatures. UV-vis measurement indicated a band gap in the range of 4.6 to 4.8 eY. It was concluded that the annealing temperature played an important role in the luminescence intensity and crystallinity of these films. To investigate the effect of different species of gases Y20 2S:Eu3 + thin films have been grown on Si ( I 00) substrates by using a pulsed laser deposition technique. The thin films grown under different species of gases have been characterized using structural and luminescent measurements. The X-ray diffraction patterns showed mixed phases of cubic and hexagonal crystal structures. The crystallinity of the film deposited in vacuum is poor, but improved significantly in argon and oxygen atmosphere. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that different species of gases affected the morphology of the fi lms. The average band gap of the films calculated from diffuse reflectance spectra using the Kubeika-Munk function was about 4.69 eV. The photoluminescence measurements indicated red emission of Y20 2S:Eu3+ thin films with the most intense peak appearing at 6 12 nm, which is assigned to the 5D0- 7F2 transition of Eu3+. The intensities of this most intense peak greatly depend on the species of gas with argon having the highest peak. This phosphor has applications in the flat panel displays.Item Open Access Structural and luminescence properties of re doped fluoride and silicate phosphors(University of the Free State (Qwaqwa Campus), 2017-04) Debelo, Nebiyu Gemechu; Dejene, F. B.This work covers several aspects of rare earth activated silicate and fluoride commercial phosphor powders and thin films. All the films were synthesized by pulsed laser deposition technique using Nd- YAG laser and characterized by different techniques with the sole aim of studying their structural and luminescence properties for possible applications in dosimetry and display devices. The Thermoluminescence (TL) properties of Y2SiO5 ∶ Ce3+ phosphor powder and thin films were reported. For the phosphor powder, the TL intensity increases with an increase in UV dose for up to 20 minutes and then decreases. The TL intensity peak shifts slightly to higher temperature region at relatively high heating rates, but with reduced peak intensity. Important TL kinetic parameters, such as the activation energy (E) and the frequency factor (s) were calculated from the glow curves using a variable heating rate (VHR) method and it was found that the glow peaks obey first order kinetics. For the films, broad TL emissions over a wide temperature range with low intensity as compared to that of the powder were observed. The maxima of the TL glow peaks of the films deposited in oxygen ambient and vacuum shift towards higher temperature relative to the TL peak position of the film deposited in an argon environment. Vacuum environment resulted in the formation of a deep trap as compared to oxygen and argon environments. Furthermore, the structure of Y2SiO5 ∶ Ce3+ phosphor powder transformed from x2-monoclinic polycrystalline phase to x1-monoclinic polycrystalline phase at low substrate temperature deposition. TL and photoluminescence (PL) properties of KY3F10: Ho3+ phosphor powder is also reported. The TL measurements were done for different heating rates and for various duration of UV exposure. The TL intensity increases with duration of UV exposure up to 20 minutes and then decreases. Decrease of the glow peak height was observed for the glow curves with increasing heating rate. The area under TL-time plot is calculated for each heating rate at constant UV dose and it is found to be constant and independent of the heating rate. It is therefore, the observed decrement in intensity of each glow curve following increment in heating rate is not attributed to the thermal quenching effect. Important TL kinetic parameters namely, the activation energy (E) and the frequency factor (s) were calculated using variable heating rate (VHR) method. The glow peaks obey first order kinetics. KY3F10: Ho3+ thin films were deposited by a pulsed-laser deposition technique with Nd-YAG laser radiation (λ= 266 nm) on (100) silicon substrate. The influence of background gas pressure, target to substrate distance, and substrate temperature on structural, morphological and luminescence properties of the films have been investigated. For the film grown under different background gas pressure, the XRD and FE-SEM results show improved crystalline structure for the film deposited at a pressure of 1 Torr. The AFM results show that the RMS roughness of the films increases with rise in argon gas pressure. The EDS elemental mapping shows Y-excess for all the films deposited under all pressures and this is attributed to its higher mass and low volatility as compared to K and F. XPS analysis further confirmed Y-excess in the deposited films. XRD analysis of the films deposited under various target to substrate distances in the range of 4-7 cm shows that high crystalline quality film with largest grain size is obtained for target to substrate distance of 4 cm. Decrease in the thickness of the films is observed at larger target to substrate distances. This is attributed to the increased hemispherical expansion of the laser induced plasma plume at larger distances reducing the particle flux of the target species over a substrate area. Moreover, all the films are characterized by low reflectance and high absorption in the visible region. Furthermore, for the films deposited under various substrate temperatures, the crystallinity is improved following increment in deposition temperature and the calculated average crystallite size is in the range of 39-74 nm. For all the KY3F10: Ho3+ commercial phosphor powder and thin films, PL emission spectra were also investigated at four main excitation wavelengths; namely, 362, 416, 454 and 486 nm. Green emission at 540 nm and faint red emission at 750 nm were observed for all the excitations. The green emission at 540 nm is ascribed to the 5F4−5I8 and 5S2−5I8 transitions and the faint red emission at 750 nm is due to the 5F4 −5I7 and 5S2−5I7 transitions. In addition to the sharp green emission at 540 nm, a broad emission centered at 600 nm was observed for excitation wavelength of 362 nm for the powder. The highest PL intensity occurs at excitation of 454 nm for all samples of this material. The Cathodoluminescence (CL) images of the films deposited under various background gas pressures show non uniform distribution of luminescent centers in the deposited films. Moreover, the CL emission spectra are similar to those of the PL with the main peak at 540 nm, suggesting that the electron beam did not change the electron energy level configuration or transitions of the activator ion in the film.Item Open Access Theoretical and experimental study of core-shell structured ZnO/ZnS and growth mechanism of un-doped and doped ZnO nanomaterials(University of the Free State (Qwaqwa Campus), 2017-12) Jule, Leta T.; Dejene, F. B.; Roro, K. T.There is currently widespread interest among researchers in ZnO-ZnS coreshell nanorods as electrodes in prototype solar cells. ZnS has been proposed as a suit- able inorganic sensitizer to ZnO because ZnO and ZnS when in intimate contact, form a type-II (staggered) heterojunction with 1:00 eV valence band o -set. Type II core shell nanorods should therefore act to separate electrons and holes radi- ally. This has been con rmed by density functional theory (DFT) calculations, which revealed an active separation of electron hole pairs after photo-excitation. Therefore these structures are similar to coaxial cables, because they allow the movement of the electrons through the core (i.e. ZnO) in one direction and the holes through the outer shell (i.e. ZnS) in the opposite direction. In this thesis, rapid synthesis of ZnO and controllable growth of ZnO/ZnS core-shell structures has been realized. Moreover, the e ect of dopants on the structural, optical, and its magnetic properties are investigated in detail. The nal product was analyzed using such techniques as scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy(steady and temperature dependent), Ultra-violet visible (UV-Vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), Electron paramagnetic resonance (EPR) and X-ray di raction (XRD). ZnO nanorod arrays were grown by a two-step chemical bath deposition pro- cess on (100) silicon substrates. ZnS coated ZnO nanorods were prepared by a simple, cost e ective, two-step chemical synthesis process. This method provides a continuous, uniform ZnS coating on ZnO nanorods at relatively low temperature. The optical properties of the core-shell(ZnO/ZnS) are explored including the case when the absorption of propagating wave by dissipative component is completely compensated by ampli cation in active (lasing) medium. Rapid synthesis of blue emitting ZnO nanoparticles for uorescent applications has been developed. In this method ZnO nanoparticles (NPs), with size 16 - 20 nm were produced using simple, cost e ective and rapid synthesis method. In this method zinc salt (typically zinc acetate dehydrate) is directly annealed in air at a temperature from 200 - 500 0C for 2 h to form ZnO (NPs). This synthesis method would be ideal for blue light emitting applications as it is catalyst free growth and only requires zinc precursor to produce NPs that can emit visible emission by scalable temperature. Cd doped ZnO nanopowder has been synthesized by facile sol-gel method. The modulation in optical band gap of the samples decreases from 3:15 eV to 2:76 eV are observed and it is believed to be responsible for the red shift in Ultra-violet visible (UV-Vis) spectroscopy with increase in Cd content. This is explained in terms of possibility of engineering band gap and in uencing physical, chemical, and electronic properties which provides a strong impetus to study nanocrystals and other nanodimensional materials. The method employed would be ideal to synthesize materials for devices operating in the visible region as well as for de- veloping heterojunction (Cd:ZnO) structures. Defect-induced room temperature ferromagnetic properties of the Al-doped ZnO (AZO) and undoped ZnO nanostructure synthesized by sol-gel method has been investigated. Electron Paramagnetic Resonance (EPR) spectroscopy which is an e ective tool to investigate the origin and nature of un-paired electrons in an atom shows the electron spin trapped in defected areas become randomly orientated at higher atomic percentages of Al. Based on PL and EPR analysis it was demonstrated that singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the undoped ZnO where as in Al doped ZnO it might be due to Al clustering forming Al-Al short range orders.Item Open Access Thermal, structural and luminescent properties of long after-glow MAlxOy:Eu²+,Dy³+ (M: Sr, Ba) phosphors(University of the Free State, 2010-11) Bem, Barasa Daniel; Dejene, F. B.; Luyt, A. S.; Swart, H. C.The optimization of properties for new and potentially useful materials becomes a continuous and sometimes a lifelong process if future applications are anticipated. Research on luminescent materials is a good example of this statement and rare earth-doped alkaline earth aluminates is at the epicentre of this focus due to the anticipated superior qualities, vis a vis those of classical sulphide phosphorescent materials. The focus in these developments has been to produce a phosphor with high emission intensity, high colour purity, longer afterglow and that is safe and chemically stable. To address some of the issues in these efforts, this study had three major aims: (1) The investigation of the surface morphology, crystallinity, particle size, luminescence, and thermal properties of commercial phosphors by various techniques. (2) The preparation and characterization of two types of phosphors by standard techniques. (3) The preparation and characterization of phosphor/polymer composites. The first commercial phosphor was a green-emitting aluminate phosphor. The properties of this phosphor as well as those of LDPE/phosphor and PMMA/phosphor composites were studied. Polymer/phosphor composite samples were prepared with phosphor concentrations ranging from 1 to 5 volume% and subsequently hot-melt-pressed. Sharp and broad XRD peaks were observed for the LDPE and PMMA composites respectively, reflecting configuration characteristics similar to those of the respective pure polymers. TEM micrographs show a transition from nanosized particles to cluster formation with increase in phosphor concentration. PL was observed in the composites of both polymers for phosphor volume concentrations above 1.0% for PMMA and above 0.5% for LDPE. For each of these samples, a broad PL peak at about 505 nm wavelength was observed after excitation at 350 nm with a xenon lamp. For the LDPE composites, the DSC results show that the presence of the phosphor in the polymer had no major influence on either the melting temperature or enthalpy values of the polymer. LDPE/phosphor composite samples, based on blue-emitting commercial aluminate phosphor, were similarly prepared and characterized for structural, luminescent and thermal properties. XRD analysis revealed the presence of the Sr4Al14O25 phase in the composites. PL spectra have two sets of peaks, major broad bands peaking at about 486 nm and minor ones between 412 nm and 418 nm, attributed to the 4f–5d transition of Eu2+. DSC and TGA results show that the introduction of the phosphor in LDPE matrix caused a slight reduction in the crystallinity of LDPE but a strong increase in the stability of the composites. SrAlxOy:Eu2+,Dy3+ phosphor was synthesized by a combustion method and characterized for luminescent and thermal properties. Phosphor nanocrystallites with high brightness were obtained. The average crystallite sizes, calculated from the Scherrer equation ranged between 34 and 43 nm. Emissions arising from transitions between the 5d and 4f orbital gaps of Eu2+ are manifested in the broad-band excitation and emission spectra with major peaks at 360 and 515 nm, respectively. The decay curves and half-life times show a clear trend in the influence of the phosphor in the improvement of the initial brightness and the afterglow times, which are ascribed to the presence of shallow and deep traps. Thermal results indicate that the phosphor nanoparticles acted as nucleating agents and improved the overall crystallinity in the LDPE/ SrAlxOy:Eu2+,Dy3+ phosphor system. The temperature-dependence of the structural and luminescent properties of sol-gel derived SrAlxOy:Eu2+,Dy3+ phosphor was investigated. Calculations based on XRD results, by means of the Scherrer equation showed the average crystallite sizes increasing from about 42 to 47 nm. Reflections corresponding to both SrAl2O4 and Sr2Al3O6 phases were observed at the various annealing temperatures but with a diminishing contribution from the Sr2Al3O6 phase. PL characterization also shows temperature-dependence through variation of both the peak position and intensity, which indicate emission at low and high annealing temperatures originating from Eu2+ and Eu3+ ions respectively. BaAlxOy:Eu2+,Dy3+ was the second phosphor synthesized by a combustion method. PL results indicate that the LDPE/BaAlxOy:Eu2+,Dy3+ interface, which is considered to have an influence on the composite behaviour, did not significantly change the spectral positions of the phosphor materials, whose major emission peaks occurred at about 505 nm. The improved afterglow results for the composites are probably due to morphological changes due to the increased surface area and defects. Thermal results indicate that the BaAlxOy:Eu2+,Dy3+ particles acted as nucleating centres and enhanced the overall crystallinity in the LDPE nanocomposite while preventing lamellar growth, hence reducing the crystallite sizes in LDPE.