Masters Degrees (Physics)

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Open Access
Cathodoluminescence degradation and surface characterization of SrGa₂S₄:Ce³⁺ power and thin films
(University of the Free State, 2011-05) Moleme, Pulane Adelaide; Ntwaeaborwa, O. M.; Swart, H. C.
The structure, morphology and luminescent properties of commercial SrGa2S4:Ce3+ phosphor powder and thin films were investigated. The phosphor shows bright blue under ultraviolet (UV) excitation. Measurements were carried out using various characterization techniques such as Xray diffraction (XRD), scanning electron microcopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using Shimadzu Super Scan SSX-550 coupled with EDS. Photoluminescence (PL) data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature, and cathodoluminescence (CL) data were collected with S2000 Ocean Optics Spectrometer. The absorption spectra were recorded using Perkin Elmer Lambda 950 UV-VIS spectrometer. The same characterization tools were used to characterize the thin films. XRD data confirmed the orthorhombic structure of SrGa2S4 that was consistent with the standard JCPDS file no. (77-1189). The SEM images of the SrGa2S4:Ce3+ powder showed particles with irregular shapes and EDS detected presence of the major elements. Both PL and CL showed the broad emission peaks around 444 nm and 485 nm which are due to Ce3+ radiative transitions (5d (T2g) → 4f (2F5/2) and 5d (T2g) → 4f (2F7/2)). Cathodoluminescent ageing characteristics of the SrGa2S4:Ce3+ powder and thin films under prolonged electron beam bombardment were studied and presented. The cathodoluminescent intensity with increasing Coulomb loading was observed to degrade under different primary electron beam voltages for the powder. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes both during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a decrease in S and C Auger peak intensity and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of an SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs). For preparation of films, silicon (Si) (100) substrates were used. A pellet was prepared from the standard SrGa2S4:Ce3+ powder. The Lambda Physik EMG 203 MSC 309 nm XeCl excimer laser was used to grow the films. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 x 10-5 mbar before backfilling to pressures of 1.0 x 10-2 mbar Ar and 1.0 x 10-2 mbar O2, where Ar and O2 were used as cross pulse gases. The films were deposited at different substrate temperatures ranging from 400°C to 600°C with 28 800 and 57 600 pulses respectively. The laser beam was operated at 8 Hz repetitive rate. The substrate temperature, number of pulses and the working pressure are the parameters that were varied during the preparation of the thin films. A highly crystalline SrGa2S4 layer was obtained at the growth temperature of 400°C. XRD patterns also showed that the properties of the films were sensitive to substrate temperature. PL and CL spectra were characterized by a broad band that can be fitted by two Gaussian peaks according to the two Ce3+ radiative transitions. At high substrate temperature a shift to Ce3+ emission in SrS occurred as well as in Ar atmosphere for both UV and high energy electrons excitation. The atomic force microscopy (AFM) images before annealing exhibited smooth surface at low substrate temperature, which became rough at high substrate temperature and after annealing in vacuum at 700°C temperature. Non-uniformity in particles (big and small) of the films and smooth films were observed from the SEM images.
Open Access
Characterization of Y3(Al,Ga)5O12:Ce3+ phosphor thin films prepared by pulsed laser deposition
(University of the Free State, 2013-11) Dlamini, Sipho Thapo Solomon; Swart, H. C.; Ntwaeaborwa, O. M.
The morphological and luminescent properties of Y3(Al,Ga)5O12:Ce3+ powder phosphor were investigated. Scanning Electron Microscopy (SEM) revealed the phosphor’s agglomerated particles with a size ranging from 0.4μm to 1.4μm. The X-ray diffraction (XRD) indicated a cubic polycrystalline phosphor with an average crystal size of 80 nm. Excitation peaks for the powder were obtained at 439, 349, 225 and 189 nm and emission peaks at 512 and 565 nm. Emission wavelength at 512 nm was also used to approximate the Al/Ga ratio within the crystal. Photoluminescence (PL) data also revealed that the addition of the Ga into the YAG:Ce3+ matrix caused a blue-shift in the emission spectra. The UV-VUV excitation and emission spectra of the Y3(Al,Ga)5O12:Ce3+ were also recorded and an energy diagram was constructed from the values. The phosphor powder was used as target material for Pulsed Laser Deposition (PLD). SiO2/Si(100) was used as substrates and thin films were deposited in the presence of different background gases. XRD indicated that better crystallization took place for films deposited in a 20 mTorr O2 atmosphere. Atomic force microscopy (AFM) revealed an RMS value of 0.7 nm, 2.5 nm and 4.8 nm for the films deposited in vacuum, O2 and Ar atmospheres, respectively. The highest PL intensity was observed for films deposited in the O2 atmosphere. The thickness of the films varied from 120 nm to 270 nm with films deposited in vacuum having the thin layer and those in Ar having the thick layer. The stoichiometry of the powder was maintained in the film during the deposition as confirmed by Rutherford backscattering spectroscopy (RBS). Luminescent properties of Y3(Al,Ga)5O12:Ce3+ thin films prepared by PLD at different substrate temperatures in an O2 background atmosphere were also investigated. XRD indicated that the films have the same cubic polycrystalline phase structure as the powder. AFM revealed poorly defined grain growth for films ablated at a substrate temperature of 22°C and 500°C but well defined grain growth was observed for films ablated at a 300°C substrate temperature. Auger electron spectroscopy (AES) depth profile of the film ablated at 500°C indicated that Si has diffused into the thin film. The highest PL intensity was observed for films deposited at the substrate temperature of 300°C. A slight shift in the wavelength of the PL spectra was obtained for the thin films with respect to the powder due to a change in the crystal field. The maximum PL intensity was obtained from the film deposited at the substrate temperature of 300⁰C in an O2 atmosphere. In addition, the films with well-defined grains (rougher surfaces) showed higher PL intensity compared to films with poorly-defined grains (smooth surfaces) as confirmed from AFM data
Open Access
CL degradation of Y2SiO5:Ce thin films coated with SnO2
(University of the Free State, 2006-05) Coetsee, Elizabeth; Swart, H. C.; Terblans, J. J.
The degradation of the cathodoluminescence (CL) intensity of cerium-doped yttrium silicate (Y 2SiO 5:Ce) phosphor thin films and commercially available Y2SiO5:Ce phosphor powders from Phosphor technology, England, were investigated for possible application in low voltage field emission displays (FEDs). Thin films of Y2SiO5:Ce were pulsed laser ablated on Si (100) substrates by using a XeCl (308 nm) excimer laser, in an oxygen (O) ambient gas pressure of 7.5 x 10-4 Torr, with laser energy of 81.81 mJ, repetition rate of 10 Hz, substrate temperature of 400°C, target to substrate distance of 3.7 cm and by using 6600 pulses. Some of the phosphor thin films were coated with tin oxide (SnO2), with the same deposition parameters as for the Y2SiO5:Ce phosphor layer except for the amount of pulses that was reduced to 1200 pulses. A SnO2 layer was ablated onto some of the thin films in order to investigate the effect of the coated layer on the surface and on the degradatio n of the CL intensity. Rutherford backscattering (RBS) was used to measure the film thicknesses. The results showed a non uniform Y2SiO5:Ce layer covered with a 58 nm thick SnO2 layer. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS) were use to study the surface morphology of the thin films. The results indicated that the Y2SiO5:Ce phosphor was ablated onto the Si (100) substrate surface as micron-sized spherical particles and that the SnO2 layer was ablated as a uniform coated layer covering the surface of the substrate and the randomly distributed spherical Y2SiO5:Ce particles. SEM was also use to study the surface morphology of the Y2SiO 5:Ce phosphor powders and the results showed that the particles were agglomerated. X-ray diffraction (XRD), that was used to measure the crystal planes of both the thin films and the powders, revealed the monoclinic crystal structure of Y2SiO5:Ce. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and CL spectroscopy were used to monitor changes in the surface chemical composition and luminous efficiency of the Y2SiO5:Ce phosphor powders and thin films (coated and uncoated) . AES and CL spectroscopy measurementswere done with 2 keV energy electrons and with beam current densities between 26.3 mA.cm-2 and 52.63 mA.cm-2 , in high vacuum and in oxygen pressures of 1 x 10-8, 1 x 10-7 and 1 x 10-6 Torr. AES indicated adventitious carbon (C) on the surface before CL measurements were made. C was depleted from the surface during electron bombardment. Residual gas mass analysis (RGA) showed that C was removed from the surface as volatile gas species. RGA with the electron beam on resulted in a higher intensity of CO2, CO and H2O gas specie s, compared to when the electron beam was off. This is consistent with the electron stimulated surface chemical reaction (ESSCR) model, whereby the electron beam dissociates the oxygen gas species into reactive atomic species, which then reacts with the carbon on the surface to form the volatile CO2 and CO gas species. Auger peak to peak heights (APPH) for oxygen and silicon on both the uncoated thin film and the powder surface stayed almost constant. The CL intensity (measured at 440 nm) increased within the first 300 C.cm-2, which is the result of the depletion of the carbon from the surface, and then it stayed constant for prolonged electron bombardment. The carbon results in an extra layer on the thin film surface that increases the energy loss of the incoming electrons. This results in the creation of fewer electron – hole pairs for photon emission during radiative recombination. The CL emission spectrum resulted in the characteristic double shoulder peak of Y2SiO5:Ce with the two main peak positions at 440 and 500 nm (blue light) before and after 24 hr s of electron bombardment for the uncoated thin film, coated thin film and for the powders. Light emission in the rare earth, Ce3+, is due to the 5d ? 4f transition due to the splitting effects of the 4f energy level. The 4f energy level splits due to the effect of the crystal field in Y2SiO5 as the host material, into the 2F7/2 and the 2F5/2 energy levels. The broad band emission of Y2SiO5:Ce is the result of the different splitting effects due to the crystal field. The relatively high CL intensity of the thin films is attributed to the spherically shaped phosphor particles grown on the surface of the Si (100) substrate. The SnO2 was also successfully ablated as a coating layer. The SnO 2 coating layer increases the energy loss of the incoming electrons which results in a lower CL intensity. The CL intensity for the uncoated thin film was therefore higher than for the coated thin film. The CL intensity stayed almost constant for the 24 hr s of electron bombardment of both the coated and uncoated thin films. The CL intensity for the phosphor powders, however, behaved differently. The intensity showed an increase after about 300 C.cm-2. The CL emission spectrum showed an increase in a second broad band at a wavelength of 650 nm after 24 hr electron bombardment. It was proved with XPS that this second broad band is due to the formation of a luminescent silicon dioxide (SiO2) layer on the surface of the Y2SiO5:Ce phosphor powders, as a result of the electron surface stimulated reactions (ESSCR). The increase in the CL intensity is thus due to the luminescent SiO2 layer that was formed as a result of electron beam irradiation that causes the Si-O bonds to break and to form intrinsic defects at 1.9 eV (650 nm) and 2.7 eV (459 nm). XPS also indicated that the Ce concentration on the surface layer increased during the degradation process and the formation of CeO2 and CeH3 also resulted from the degradation process. The phosphor powders degraded from a blue light emitting phosphor before electron bombardment to a whit ish light emitting phosphor after 24 hr, as a result of the luminescent SiO2 layer formed during degradation.
Open Access
A comparative study between the simulated and measured cathodoluminescence generated in ZnS phosphor powder
(University of the Free State, 2003-08) Chen, Sheng-Hui; Greeff, A. P.; Swart, H. C.
In the past few decades cathode ray tubes (CRTs) have dominated the display market because of their excellent image quality, ease and economy of manufacture. However their bulky packaging and high power consumption make them unsuitable for portable electronic devices. Field emission displays (FEDs) show the most potential amongst all other types of flat panel displays (FPDs). These FEDs have several advantages over the FPD market, which is currently dominated by active matrix liquid crystal displays (AMLCDs) and plasma displays (PDPs). FEDs generate their own light by a process referred to as cathodoluminescence (CL) in which phosphor powders inside the screen are excited in a similar manner to those used in CRTs. However, in contrast to CRTs, the accelerating voltage of electrons in FEDs is lowered in order to reduce the bulky packaging and the power consumption. Electrons with the reduced accelerating voltage have a shallower penetration depth and therefore the surface condition of the phosphor powder is critical in order to ensure proper functioning of the display. During the prolonged exposure of the phosphors to an electron beam, the phosphor surface is oxidised to form a non-luminescent layer. This electron stimulated oxide formation is due a chemical reaction between the phosphor and the residual gases in the sealed vacuum, e.g. oxygen and water vapour. Since the CL is dependent upon the energy loss of electrons in the phosphors, the CL decreases with the growth of the oxide layer on the phosphor surface. For high acceleration voltages, this oxide layer has little effect on the brightness of the CL, but as the accelerating voltage decreases as for FEDs, the layer has a much more profound effect. The ZnS:Cu,Al,Au (P22G) is a standard green phosphor commonly found in CRTs. In this study the P22G phosphor powder was bombarded by an electron beam in an oxygen ambient, argon ambient and other mixture of gases. These mixtures consisted of varying concentrations of oxygen, carbon monoxide and argon gas. Auger electron spectroscopy (AES) and cathodoluminescence spectroscopy were used to monitor changes in surface composition and luminescent properties of the P22G phosphor during electron bombardment. When the P22G phosphor powder was exposed to an electron beam in water-rich oxygen gas, a chemically-limited ZnO layer was formed on the surface. The CL intensity generated from carbon free P22G phosphor decreased linearly with the thickness of the ZnO layer. The experimentally measured thickness of the ZnO layer agrees very well with the calculated value of the theoretical simulation. The theoretical simulation of electron trajectories into the ZnO/ZnS powders was based on a Monte Carlo simulation and the CL intensity was quantified from the electron energy loss profile generated during the simulation. According to the results of the simulation, the effect of a ZnO layer on the CL is minimised by the use of a high energy electron beam at a low incident angle. The electron exposure of P22G phosphor powder was also performed in dry oxygen gas. A layer of ZnSO4 was formed on the surface after electron exposure. The sulphate formation decayed exponentially with time and it is postulated that this was due to the diffusion of the charge reactants through the sulfate film to reaction interfaces. The P22G phosphor exposed to the electron beam in argon gas and gas mixtures degraded more slowly than in oxygen gas. Argon gas and carbon monoxide gas may suppress the degradation of the P22G phosphor powder.
Open Access
The effect of nitrogen on the cosegregation of molybdenum in a Fe-3.5wt%Mo-N (100) single crystal
(University of the Free State, 2006) Jordaan, Werner Albert; Terblans, J. J.; Swart, H. C.
In this study the cosegregation of molybdenum and nitrogen to the (100) plane of an iron single crystal was investigated. Ternary systems are considerably more complex than binary systems in that there are seven segregation parameters to determine, as opposed to three. However, a novel approach was undertaken to minimize the amount of variables, by first analysing a similar binary system that was exposed to a nitrogen ambient. Two single crystal were selected for this purpose, i.e. a Fe- 3.5wt%Mo(100) binary system and a Fe-3.5wt%Mo-N ternary system. By exposing the binary crystal to a nitrogen ambient at high temperatures it was observed that molybdenum segregated to the surface. The segregation profiles of the two systems were acquired at constant temperatures from 797 K - 888 K and Auger Electron Spectroscopy was used to monitor the surface concentrations of the relevant species. Since accurate surface temperature measurements are essential to segregation studies, a calibrated infrared thermometer was used. The segregation profiles were generated by measuring time and the Auger signal simultaneously. From the segregation profiles, initial estimates for the diffusion coefficients of Mo were first determined for the binary system by applying Fick’s equation to the segregation profiles. From these values the pre-exponential factor, D0, was determined to be 1.2x10−4±2 m2/s and the activation energy, E, as 258±33 kJ/mol. The diffusion coefficients determined thus, were used as estimates for obtaining the Darken seggregation profiles. In this case the D0 value was found to be 2.4x100±1 m2/s and the E value, 323±16 kJ/mol. The segregation energy, G, of Mo was calculated as -38 kJ/mol. In both cases it was observed that the diffusion coefficient of Mo deviated from the expected value at high temperatures due to the desorption of nitrogen from the surface. Using thermodynamic theory, an expression for the segregation energy of Mo in terms of the nitrogen surface concentration was derived. The Darken fits were repeated and it was found that the high temperature diffusion coefficient values fell on the the Arrhenius linear regression lines. For this special case, the D0 value was calculated as 5.5x101±1 m2/s, the E value as 345±18 kJ/mol. The segregation parameters determined for the binary system were then used as initial values for fitting the experimental data of the ternary system. Using Fick’s equation, the diffusion coefficients of Mo and N in Fe were determined. From the Arrhenius linear regression, the pre-exponential factor for Mo was calculated as 3.6x10−2±1 m2/s and that of N as 4.1x10−1±2 m2/s. The activation energies were 308±20 kJ/mol and 210±40 kJ/mol for Mo and N, respectively. The segregation parameters of the ternary system were then determined via the Darken method. In this case the pre-exponential factors were 1.9x10−4±1 m2/s for Mo and 2.8x100±3 m2/s for N. The activation energies were 271±11 kJ/mol and 323±43 kJ/mol. The segregation energy of Mo was calculated as -32 kJ/mol and for N, -19 kJ/mol. The interaction coefficient between Mo and N was calculated as -19 kJ/mol.
Open Access
Experimental and computational study of S segregation in Fe
(University of the Free State, 2012-06) Barnard, Pieter Egbert; Terblans, J. J.; Swart, H. C.; Hoffman, M. J. H.
A systematic study was conducted to investigate the diffusion and segregation of S in bcc Fe using (i) DFT modelling and (ii) the experimental techniques Auger Electron Spectroscopy (AES) and XRay diffraction (XRD). The aim of this study was to obtain the activation energies for the segregation of sulfur (S) in bcc iron (Fe), both computationally and experimentally in order to explain the diffusion mechanism of S in bcc Fe as well as the influence the surface orientation has on surface segregation. The Quantum ESPRESSO code which performs plane wave pseudopotential Density Functional Theory (DFT) calculations was used to conduct a theoretical study on the segregation of S in bcc Fe. To determine the equilibrium lattice sites of S in bcc Fe, the tetrahedral-interstitial, octahedralinterstitial and substitutional lattice sites were considered. Their respective binding energies were calculated as -1.464 eV, -1.660 eV and -3.605 eV, indicating that the most stable lattice site for S in bcc Fe is the substitutional lattice site. The following mechanisms were considered for the diffusion of S in bcc Fe: tetrahedral-interstitial, octahedral-interstitial, nearest neighbour (nn) substitutional and next nearest neighbour (nnn) substitutional with migration energies, Em, of respectively 4.438 kJ/mol (0.046 eV), 22.48 kJ/mol (0.233 eV), 9.938±6.754 kJ/mol (0.103±0.007 eV) and 96.49±0.579 kJ/mol (1.000±0.006 eV). According to the binding and migration energy calculations, S will diffuse via a substitutional mechanism with a migration energy of 9.938±6.754 kJ/mol (0.103±0.007 eV). The three low-index planes of bcc Fe were investigated to determine the stability, the vacancy formation energy and the activation energy for each surface. Structural relaxation calculations showed that the surfaces in order of decreasing stability are: Fe(110)>Fe(100)>Fe(111) which is in agreement with surface energy calculations obtained from literature. The formation of a vacancy in bcc Fe was modelled as the formation of a Schottky defect in the lattice. Using this mechanism, the vacancy formation energies, Evac, for the Fe(110), Fe(100) and Fe(111) surfaces were respectively calculated as 267.4 kJ/mol (2.772 eV), 256.8 kJ/mol (2.662 eV) and 178.2 kJ/mol (1.847 eV). The activation energy, Q, of S diffusing via the substitutional mechanism for the Fe(100), Fe(110) and Fe(111) surfaces were respectively calculated as 277.4 kJ/mol (2.875 eV), 266.8 kJ/mol (2.765 eV) and 188.1 kJ/mol (1.950 eV). Thus it was found that the vacancy formation energy is dependent on the surface orientation and thus the structural stability of the Fe crystal. Experimental values for the activation energy of S in bcc Fe (232 kJ/mol (2.40 eV) and 205 kJ/mol (2.13 eV)) were obtained from literature confirming the nearest neighbour substitutional diffusion mechanism of S in bcc Fe. No indication is given regarding the orientation of the crystal in which the value of 232 kJ/mol (2.40 eV) was obtained while the value of 205 kJ/mol (2.13 eV) is for a Fe(111) crystal orientation. For the experimental investigation of the Fe/S system polycrystalline bcc Fe samples were studied. These samples were prepared by a new doping method by which elemental S is diffused into Fe. In order to prepare the samples by this method a new system was designed and build. Auger depth profile analysis confirms the successful doping of Fe with S using the newly proposed doping method. It was found that the S concentration was increased by 89.38 % when the doping time was doubled from 25 s to 50 s. An Fe sample doped for 50 s was annealed at 1073 K for 40 days after which the effects induced by S and the annealing of the sample were investigated by Secondary Electron Detector (SED) imaging. Results showed a 36±11 % decrease in the grain sizes of the polycrystalline Fe sample due to the presence of S. It was found that the re-crystallization rate of Fe is increased due to the presence of S. Using XRD, the Fe (100), Fe(211), Fe(110), Fe(310) and Fe(111) orientations were detected for both the un-doped and the annealed S doped Fe samples. The annealed sample showed the following percentage changes in the concentrations of the respective orientations compared to the un-doped sample: -5.180, +2.030, +16.41, +0.400, -13.66. Taking the calculated trend in surface stability for the three low-index orientations of Fe into consideration, it was found that the more stable Fe(110) orientation had increased in concentration during annealing, while the less stable Fe(100) and unstable Fe(111) orientations had decreased in concentration during annealing. AES measurements on the two samples were performed using the linear programmed heating method. The segregation parameters of S for the un-doped Fe sample are: D0=4.90×10-2 m2/s, Q=190.8 kJ/mol (1.978 eV), ΔG=-134 kJ/mol (-1.39 eV) and ΩFe/S=20 kJ/mol (0.21 eV). The segregation parameters of P obtained for the un-doped Fe sample are: D0=0.129 m2/s, Q=226.5 kJ/mol (2.348 eV). For the S doped Fe sample the segregation parameters of S were determined as: D0=1.79×10-2 m2/s and Q=228.7 kJ/mol (2.370 eV), ΔG=-145 kJ/mol (-1.50 eV) and ΩFe/S=8 kJ/mol (0.08 eV). These results showed that for the doped sample, with an increased concentration in the stable Fe(110) and a decreased concentration in the less stable Fe(100) and unstable Fe(111) orientations, a higher activation energy was obtained. Comparing the measured activation energies to the calculated values indicates that the diffusion of S occurs via a vacancy mechanism, where the S atom occupies a substitutional lattice site. Despite the fact that polycrystalline samples were analysed, the activation energies are still in the same order as the calculated activation energies of the single crystals. This confirms the theoretical prediction of a substitutional diffusion mechanism of S in bcc Fe. During this study the diffusion mechanism of S was determined as the substitutional diffusion mechanism whereby a S atom would diffuse from a substitutional lattice site to a nearest neighbour vacancy. The different Fe orientations considered in the calculations can be arranged from highest to lowest activation energy as Fe(110)>Fe(100)>Fe(111). These calculations are in agreement with the AES results which showed an increased activation energy for the doped sample having a higher Fe(110) concentration and lower Fe(111) and Fe(100) concentrations.
Open Access
Luminescent properties of combustion synthesized BaAl2O4:Eu²+ and (Ba1-xSrx)Al2O4:Eu²+ phosphors co-doped with different rare earth ions
(University of the Free State, 2011-11) Annah, Lephoto Mantwa; Ntwaeaborwa, O. M.; Mothudi, B. M.; Swart, H. C.
A Combustion method was used to prepare all the alkaline earth aluminates (rare-earths doped BaAl2O4, BaSrAl2O4 and BaZnAl2O4) phosphor powders in this study. Measurements of these phosphor powders were carried out using various characterization techniques such as X-ray diffraction (XRD), Scanning Electron Microcopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using JEOL- JSM 7500F Scanning Electron Microscope. The stretching mode frequencies data were collected using Perkin Elmer Spectrum 100 FTIR spectrometer and the elemental composition on the surfaces of the phosphor powders were monitored by the PHI 5400 Versaprobe scanning X-ray photoelectron spectrometer. Photoluminescence (PL) data were collected using 325nm He-Cd laser and decay data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer coupled with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature. The thermoluminescence (TL) data were collected using a Thermoluminescence Reader (Integral-Pc Based) Nucleonix TL 1009I. BaAl2O4:Eu2+ phosphor powders co-doped with different trivalent rare-earth (RE= Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+ and Tb3+) ions were prepared at an initiating temperature of 600oC and annealed at 1000oC for 3 hours. The X-ray diffraction (XRD) data shows hexagonal structure of BaAl2O4 for both as prepared and post annealed samples. All samples exhibited bluish-green emission associated with the 4f65d1→4f7 transitions of Eu2+ at 504 nm. The longest afterglow was observed from the BaAl2O4:Eu2+ co-doped with Nd3+. BaAl2O4:Eu2+, Nd3+, Gd3+ phosphor powders were prepared at different initiating temperatures of 400-1200oC. X-ray diffraction data show the formation of the hexagonal BaAl2O4 structure at the temperatures of 500oC-1200oC. The crystal size calculated from the phosphor powder prepared at 1200oC was found to be 63 nm. Blue-green photoluminescence with persistent/long afterglow, was observed at 502 nm and the highest PL intensity was observed from the sample prepared at 600oC. The phosphorescence decay curves showed that the rate of decay was faster in the case of the sample prepared at 600oC compared to that prepared at 1200oC. The TL glow peaks of the samples prepared at 600oC and 1200oC were both stable at 72oC suggesting that the traps responsible for the long afterglow were not affected by the temperature. Barium-substituted phosphor powders of (Ba1-xSrx)Al2O4:Eu2+;Nd3+ composition were prepared at an initiating temperature of 500oC. The X-ray diffraction with the composition of x = 0 shows the hexagonal phase of BaAl2O4 and the one for x = 1 shows the monoclinic phase of SrAl2O4. The XRD with the composition of x = 0.4, 0.5 and 0.6 shows the admixture of BaAl2O4 and SrAl2O4 structures. SEM investigations showed some changes on the surface morphology for different compositions. Photoluminescence (PL) studies showed the (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 0) and (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 1) with blue-green to bright-green emissions with peaks at 505 nm and 520 nm respectively. The mixed composition with x = 0.4, 0.5 and 0.6 showed two peaks at 447 nm and 517 nm. Phosphorescence showed higher luminescence for (Ba1-xSrx)Al2O4:Eu2+;Nd3+ at (x = 0) compared to other compositions. (Ba1-xZnx)Al2O4:Eu2+;Nd3+ phosphor powders with the compositions x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1 were prepared at an initiating temperature of 500oC. The X-ray diffraction showed the cubic structure for the compositions of x = 0 and x = 1. The SEM images of the phosphor samples showed different kinds of morphologies for the compositions x = 0, 0.5 and 1. The PL emission of the phosphor powder clearly showed a shift from green to blue regions. The highest PL emission and the long afterglow ascribed to trapping and detrapping of charge carriers were observed from (Ba1-xZnx)Al2O4:Eu2+;Nd3+ with x = 0.2.
Open Access
A Monte Carlo program for simulating segregation and diffusion utilizing chemical potential calculations
(University of the Free State, 2004) Joubert, Heinrich Daniel; Terblans, J. J.; Swart, H. C.
Bulk-to-surface segregation plays a major role in the engineering of alloy surfaces. An increase in surface sensitive analysis techniques in recent years have led to big advances in the engineering of surface properties. The focus of this study is the development of a Chemical Potential Monte Carlo (CPMC) model which is based on the modified Darken model. This model is capable of simulating diffusion and segregation in crystals with a uniform concentration as well as crystals consisting of thin layers. The chemical potential equations used for the calculations by the modified Darken model are rewritten to include the segregation energy associated with the surface layer. The change in chemical potential directs atomic motion and simulations involving the change in chemical potential are performed on a 2-dimensional matrix containing two elements: the solute and the solvent elements. A random selection of an atom inside the matrix initiates the model. The change in chemical potential due to an atomic jump of a randomly selected atom to an adjacent layer is calculated. The largest change in chemical potential directs the atomic motion, complying with the conditions associated with the lowering of the Gibbs free energy; the driving force of atomic motion is therefore the lowering of the total crystal energy. Inclusion of the segregation energy (for jumps involving the surface layer) limits the number of atomic jumps from the surface layer to the bulk. Simulated segregation profiles generated by the CPMC model were compared with profiles calculated with both the modified Darken and Fick model. The comparisons show that the CPMC successfully describes both the kinetic and equilibrium conditions associated with surfa ce segregation. A reduction in calculation time was also achieved by implementing the CPMC model in parallel.
Open Access
Narrowband Ultraviolet B emission from gadolinium and praseodymium co-activated calcium phosphate phosphors for phototherapy lamps
(University of the Free State, 2014-01) Mokoena, Puseletso Pricilla; Ntwaeaborwa, O. M.; Swart, H. C.
Different phases of calcium phosphates co-doped with gadolinium and praseodymium were prepared by co-precipitation, urea combustion, citrate-gel combustion and microwave-assisted methods. Ca5(PO4)3OH:Gd3+,Pr3+ phosphors were prepared by the co-precipitation and citrate-gel methods, and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), High resolution transmission electron microscopy (HRTEM), Energy dispersive x-ray spectrometer (EDS) and photoluminescence (PL) spectroscopy. The XRD pattern was consistent with the hexagonal phase of Ca5(PO4)3OH referenced in JCPDS Card Number 73-0293. The XPS data indicated that Ca2+ occupied two different lattice sites referred to as Ca1 and Ca2. The P5+ is surrounded by O2- ions in the tetrahedral arrangements. Each tetrahedron contains oxygen atoms designated as O1, O2, and O3. The particle morphology was analyzed using SEM and HRTEM. SEM shows that the powder was composed of an agglomeration of irregular particles. HRTEM revealed faceted edges forming a hexagonal shape. PL data exhibited a narrowband emission located at 313 nm, which is associated with the 6P7/2→8S7/2 transition of the Gd3+ ion. This emission is classified as ultraviolet B (UVB) and it is suitable for use in phototherapy lamps to treat various skin diseases. The PL intensity of the 313 nm emission was enhanced considerably by Pr3+ co-doping. The crystallographic structure of Ca5(PO4)3OH:Gd3+,Pr3+ and possible mechanism of energy transfer from Pr3+ to Gd3+ are discussed. Ca5(PO4)3OH:Gd3+,Pr3+ phosphor exhibited a single thermoluminescence peak between 339-363 K. The peak shifted towards high temperature with an increase in dose. The shift shows that the trap system is more complicated than a single trap obeying first order kinetics. The calculated activation energy (EA) was found to be 0.91 eV when the using initial rise method. The activation energy values were further calculated using the peak shape method. The calculated activation energies for , and , were 0.75, 1.03, and 0.42 eV respectively. There was a peak shifting to higher temperatures with an increase in heating rate which is attributed to recombination that is slowing down due to electron-phonon interactions. The peak intensity increased with an increase in heating rate from 0.6 to 2.0 °C.s-1 and started to decrease from 3.0 to 5.0 °C.s-1, the decrease maybe due to thermal quenching as the peak shift to higher temperatures. The calculated activation energy by heating rate method was found to be 0.60 eV. This value is comparable to other calculated values of activation energies by various methods mentioned above. Ca3(PO4)2:Gd3+,Pr3+ phosphors with different concentrations of Gd3+ and Pr3+ were successfully prepared by urea combustion process using metal nitrates as precursors and urea as fuel and also by the microwave assisted method. XRD exhibited a rhombohedral phase of Ca3(PO4)2 referenced in JCPDS Card No. 70-2065. The PL excitation spectra of Ca3(PO4)2:Gd3+ and Ca3(PO4)2:Pr3+exhibited peaks at 220-280 nm and 300-490 nm associated with the f-f transitions of Gd3+and Pr3+ respectively. The UVB emission resulting from the 6P7/2→8S7/2 transition of Gd3+ was observed at 313 nm when the Ca3(PO4)2:Gd3+ phosphor was excited at a wavelength of 274 nm using a monochromatized xenon lamp. Upon Pr3+ co-doping, the excitation peaks due to Gd3+ and Pr3+ f-f transitions were suppressed and an intense broad excitation peak ascribed to the 4f4f5d transitions of Pr3+ was observed at 227 nm. The peak intensity of the UVB emission at 313 nm was shown to improve considerably when the Gd3+ and Pr3+ co-doped systems were excited at the wavelength of 227 nm suggesting that the Pr3+ is a good sensitizer of the 313 nm narrow line UVB emission from Gd3+.
Open Access
Novel ZnO nanostructures: synthesis, growth mechanism, and applications
(University of the Free State (Qwaqwa Campus), 2014-12) Molefe, Fokotsa Victor; Koao, L. F.; Dejene, B. F.; Swart, H. C.
The ZnO nanostructures were successfully synthesized by chemical bath deposition method (CBD) to study the influence of parameters such as reaction temperature, time, precursor concentration and the annealing temperature respectively. The main motivation for this thesis is to successfully synthesise novel ZnO nanostructures and understand the growth mechanism. In this work, the thermal, structural, morphology, optical, and luminescence properties of ZnO were investigated in details by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray photoelectron spectroscopy (XPS), ultraviolet visible (UV-vis) spectroscopy and photoluminescence (PL) spectroscopy techniques. From TGA results when increasing both reaction and annealing temperature we observed the increase in thermal stability of ZnO due to the removal of adsorbed species in the material. The melting temperatures (as determined through DSC) decreased due to crystallization of ZnO with the increase in both reaction and annealing temperature. X-ray diffraction (XRD) indicated that all the ZnO nanostructures prepared at 80 ℃ crystallizes in the wurtzite structure with the mean lattice parameters a = b = 3.25 Å and c = 5.18 Å and there is an increment in the particle size resulting into the improvement of crystallinity of the material. In materials prepared at lower reaction temperature, reaction time, and precursor concentration, traces of zinc hydroxide Zn(OH)2 were observed. When Zn(OH)2 decomposes into ZnO, the entire surface morphology through the study of ZnO consisted of agglomerated nanoflakes. The EDS results confirmed the presence of Zinc (Zn) and Oxygen (O) as the major product, and the ratio of Zn to O increased as ZnO becomes more crystalline. The UV-Vis reflectance spectra showed that the absorption band edges shift to the higher wavelength with an increase in reaction time, temperature, molar concentration precursors, and annealing temperature. As a result the band gap energy of ZnO nanostructures determined using Kubelka Munk’s equation was found to decrease due to quantum confinement effects and the increase in particle size. In general, the photoluminescence (PL) analysis showed that ZnO nanoflakes prepared at different parameters have almost the same characteristics. PL measurements revealed broad emission that extends from UV region to the visible region. The luminescence intensity of this emission was quenched when increasing parameters mentioned above, and these quenching is attributed to the decrease in concentration of defect related emissions. It is well known that when using chemical reaction methods such as CBD the emission intensity quenches as Zn(OH)2 dehydrates into ZnO. The slight red-shift in the emission band is also observed which is attributed to band gap narrowing.
Open Access
Oxidation of a segregated MoN layer grown on Fe(100)-3.5wt%Mo-N
(University of the Free State, 2001-06) Conradie, Rochelle; Roos, W. D.; Swart, H. C.
English: The oxidation behaviour of the segregated MoN layer on the Fe(100)-3.5wt% Mo-N substrate was investigated in this study. Previous studies suggested the synergetic segregation of the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. It has also been shown that the segregated Mo and N form a MoN surface compound. As an alloy element in stainless steels, the Mo aids in the inhibition of the oxidation and thus prevents corrosion Auger electron spectroscopy (AES) was used to obtain the experimental results. For this study the oxidation of a Fe(100) specimen and a Fe(100)-3.5wt% Mo-N specimen were investigated to establish a point of reference to describe the oxidation behaviour of the segregated MoN layer. Linear temperature ramping was used to segregate the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. The specimens were exposed to an oxygen environment at various temperatures. The partial pressure of the oxygen was monitored with a mass spectrometer and was kept constant at 2 x 10-10 torr. The Auger peak-to-peak heights for the relevant elements in the specimens were measured as a function of the exposure time. Upon oxidation, the low energy Fe AES peak (47 eV) undergoes shape changes. The iron oxide has a dual peak with 42 eV and 52 eV kinetic energy respectively. The Fe(100) specimen surface reacted rapidly with the oxygen environment at room temperature to form an iron oxide, as depicted by the change in the low energy Fe AES peak. The exposures performed at 100°C and 200°C also resulted in oxide formation although the extent of the oxidation decreased with an increase in the temperature. Above 300°C indication of the Mo and N reacting with the oxygen environment. At 100°C and 200°C less oxide formation was detected and above 300°C there was only oxygen adsorption. The segregated MoN layer had a markedly different response to the oxygen exposure. The oxygen exposure performed at room temperature had a strikingly different course of the 0 Auger peak-to-peak height increase compared to that of the Fe(100) and Fe(100)- 3.5wt% Mo-N specimens exposure at the same temperature. The segregated MoN layer retards the surface reaction. A hypothesis formulated describes the MoN layer as a perforated layer that has some Fe exposed. The oxygen reacts rapidly with the exposed Fe. Longer exposures result in the dissociation of the MoN layer and the desorption of the Mo03 and NxOy compounds from the surface. Once the layer has dissociated completely the Fe will continue to react as for the other specimens. Oxidation occurs up to 300°C and at higher temperatures no oxide formation is detected. The changes in the low energy Fe AES peak are used to calculate the fraction oxide and metal contributing to the peak by using the Linear Least Squares method. The low energy Fe AES peak cannot be used for thickness calculations as it is subject to the backscattering term. The experimental data suggests that the backscattering term is a function of the exposure time. A first approximation is to assume a linear change with time. This approximation was applied successfully to the room temperature oxidation of the segregated MoN layer, but the same function could not be applied to the other two specimens, The thickness of the oxide was calculated using the change in the high energy Fe AES peak intensity. The O2 sticking coefficient for the exposure of the Fe(100) and the exposure of the segregated layer was also calculated and the differences in the values were attributed to the effect of the dissociation of the MoN layer on the adsorption of the O2 on the specimen surface. there was no oxide formation detected and therefore there is only oxygen adsorption at these temperatures. The Fe(100)-3.5wt% Mo-N specimen showed similar oxidation behaviour as was seen for the Fe(100) specimen. At room temperature the surface of the specimen reacted rapidly with the oxygen environment to form an iron oxide. There was no
Open Access
Oxygen-induced segregation during batch annealing of industrial steel coils
(University of the Free State, 2006-05) Wurth, Etienne; Swart, H. C.; Terblans, J. J.
The development of diffusion welds between spirals of steel coils, during batch annealing, is of particular interest because it preve nts the coils from being unwound for further use. The physical metallurgy of iron and steel is exceedingly complicated and many of the complications arise from the behaviour of solutes, which segregate to surfaces and interfaces, which alter the mechanical behaviour. Segregation studies were done by measuring the APPH’s (Auger Peak to Peak Heights) of the segregating species (P, S, C and Ti) against annealing time during the annealing of an ultra low carbon (ULC) Ti stabilized steel between 550 and 800oC. The modified Darken model was used to describe the complex segregation behaviour of the species involved during annealing of the industrial steel. This was done by comparing the initial changes in fractional surface concentration of the segregating species against annealing time to the trends in the surface concentration changes as describe by the Darken model for a ternary alloy. Calculations were done, using Langmuir-McClean equations, to determine the change in effective segregation energy as a function of oxygen surface coverage. Oxidation was allowed after sputtered cleaning and segregation, these oxidation results were compared with each other. No C segregation occurred without oxygen in the system. Oxygen induced-segregation of Ti and C occurred at 700oC and 800oC. Oxidation occurred at 700oC and 800oC. It was found that the adsorption of oxygen on the surface profoundly influence the segregation rate of the species involved. The modified Darken model was successfully used to describe the oxygen induced-segregation process. The induced segregation may act as a possible source of the diffusion welds during batch annealing.
Open Access
Preparation and properties of long afterglow CaAl₂O₄ phosphors activated by rare earth metal ions
(University of the Free State, 2011) Wako, Ali Halake; Dejene, F. B.; Swart, H. C.
This work comprises of several aspects of calcium-aluminate phosphor activated with rare earth metal ions i.e. (CaAl2O4:Eu2+, Nd3+, and Dy3+). In particular the luminescent and structural properties of the long afterglow CaAl2O4:Eu2+,Nd3+,Dy3+ phosphors prepared by urea-nitrate solution-combustion method were investigated. The solution-combustion method is more efficient because phosphors with high efficiency were obtained at low temperature (500 oC) in a very short period of time (5 min). The effects of varying concentration of host matrix composition (Ca:Al), flux i.e. boric acid (H3BO3), activator (Eu2+) and co-activator (Nd3+/Dy3+) mass ratios and urea ((NH2)2CO) on the structural, luminescent, and thermoluminescent(TL) properties of the CaAl2O4:Eu2+, Nd3+, Dy3+ phosphors were studied. It was observed that Ca:Al mass ratios greatly affect the crystalline structure of the material. The results of the X-ray diffraction (XRD) analysis reveal that the formation of several crystalline phases depends on the ratios of the host material. The XRD peaks show the presence of other phases such as Ca3Al2O6 and CaAl4O7 but the predominant phase formed was that of CaAl2O4. However it was found that the crystalline structure is generally not affected by the variation of the co-dopants concentration. Photoluminescence (PL) studies revealed a general rise in intensity with an increase in the mass ratio of Ca:Al. The highest PL intensity was observed with 0.7% Ca. The luminescent intensities vary from each other when co-doped with various proportions of Nd3+ and Dy3+. The addition of H3BO3 favored the formation of pure monoclinic CaAl2O4 phase while the variation of the amount of ((NH2)2CO) showed mixed phases although still predominantly monoclinic. Both boric acid and urea to some extent influence the luminescence intensity of the obtained phosphor but unlike the case of CO(NH2), the emission peak for H3BO3, does not shift evidently because the energy level difference of 4f-5d does not change obviously. The broad blue emissions consisting mainly of symmetrical bands having maxima between 440–445 nm originate from the energy transitions between the ground state (4f7) and the excited state (4f65d1) of Eu2+ ions while the narrow emissions in the red region 600-630 nm arise from the f-f transitions of the remnant unreduced Eu3+ions. High concentrations of H3BO3 generally reduce both intensity and lifetime of the phosphor powders. The optimized content of H3BO3 is 5.8 mol % for the obtained phosphor with excellent properties. XRD analysis of the influence of Eu2+ and Nd3+ doping concentrations on the morphological, structural and PL properties of the CaAl2O4: Eu2+; Nd3+ phosphor, depict a dominant monoclinic phase that indicates no change in the crystalline structure of the phosphor even with high concentration of Eu2+ or Nd3+. The Energy Dispersive x-ray Spectroscopy (EDS) and Fourier Transform Infra-Red Spectroscopy (FTIR) spectra showed the expected chemical components of the phosphor. The excitation spectra show one broadband from 200 nm to 300 nm centered around 240 nm corresponding to the crystal field splitting of the Eu2+ d-orbital. The prepared phosphor compositions exhibit PL emission in the blue region with a maximum around 440 nm. This is a strong indication that there was dominantly one luminescence centre, Eu2+ which represents emission from transitions between 4f7 (8S7/2) ground state and the 4f6-5d1 excited state configuration. Two other, minor peaks, at 580 and 614 nm indicate the presence of remnants of Eu3+ ions as a result of incomplete reduction during sample preparation. High concentrations of Eu2+ and Nd3+ generally reduce both intensity and lifetime of the phosphor powders. The optimized content of Eu2+ is 0.36 mol % and for Nd3+ is 0.09 mol % for the obtained phosphors with good properties. The decay characteristics exhibit a significant rise in initial intensity with increasing Eu2+ doping concentration while the decay time increased with Nd3+ co-doping. Analysis of the TL glow curves is one of the most significant ways to measure the number of traps and also the activation energy of the trap levels in luminescent materials. In the present study TL properties of the CaAl2O4:Eu2+, Nd3+,Dy3+ phosphors were investigated above room temperature by use of Nucleonix 1009I TL reader. The trap depths were estimated with the aid of the peak shape method. The glow curve of CaAl2O4:Eu2+ with a first peak at 50 °C was found to correspond to several traps. The ratio of Nd3+:Dy3+ ions were observed to influence the position, concentration and type of traps formed. The observed afterglow can be ascribed to the generation of suitable traps due to the presence of the Nd3+ trap levels. Trivalent rare earth ions (Nd3+/Dy3+) are thought to play the role of hole traps in calcium aluminate phosphors (CaAl2O4:Eu2+). In these phosphors, Eu2+ ions act as luminescent centre emitting in the blue (λ max = 440 nm) region. Despite a large number of research on the phenomenon the mechanism of the persistent luminescence of CaAl2O4:Eu2+,Nd3+,Dy3+ has not been well presented. A proper understanding of the exact luminescence mechanisms and the identification of trap levels or locations in long phosphorescent materials is required for their use in areas such as detection of radiation, sensors for cracks in buildings, fracture of materials and temperature among others.
Open Access
Red emission of Praseodymium ions (Pr ³⁺)
(University of the Free State, 2011-11) Noto, Luyanda Lunga; Swart, H. C.; Terblans, J. J.
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.
Open Access
The segregation of indium from polycrystalline copper crystals
(University of the Free State, 2012-02) Madito, Moshawe Jack; Terblans, J. J.; Swart, H. C.
In this study the surface segregation of In from a dilute Cu(In) alloy to the surface was investigated. The Cu(In) alloy was prepared by diffusion doping. A thin layer of In was deposited onto the backside of a polycrystalline Cu crystal. The In layer was covered with a thin Cu layer to prevent the In from melting and evaporating from the Cu crystal. The Cu crystal was annealed in an Ar gas atmosphere. During annealing the temperature was increased stepwise from 150 to 900 to prevent the In layer from melting. Finally the Cu crystal was annealed at 900 for 456 hours. As a result, the polycrystalline Cu crystal was successfully doped with an In concentration of 0.059 at%. Using Auger electron spectroscopy (AES) coupled with an Ar+ ion gun (for sur- face sputter cleaning) and a programmed crystal heater inside the ultra high vacuum (UHV) chamber, the segregation profiles where recorded by monitoring the surface condition of In and S on the Cu crystal during linear heating and constant temperatures heating. The constant temperature segregation measurements were performed in the tem- perature range 460 to 580 . The segregation profiles showed that both In and S segregates from the Cu crystal. In segregates to the surface at a much higher rate than S. In reached a maximum surface coverage of 25 % for the temperatures 460 ,490 and 520 and 16 % for temperatures 550 and 580. Both the In and S segregation profiles were fitted with semi-infinite solution of Fick’s equation and the segregation parameters (Q and D0) were obtained for In as Q = 191.9 kJ.mol−1, D0 = 1.1 × 10−5 m2.s−1 and for S as Q = 201.1 kJ.mol−1, D0 = 4.4 × 10−2 m2.s−1. The linear temperature measurements were carried out from the temperature range 100 to 800 at heating rates of 0.025; 0.05; 0.1 and 0.2.s−1. All seg- regation profiles showed In segregation to the surface that reached a surface maxi- mum coverage of 23−25 %. The segregation of In was accompanied by a slow seg- regating S that replaced the In completely from the surface. The surface concen- tration of In reached a maximum coverage of 29−32 %. The replacement of In by S is due to a large difference between the segregation energies of In and S with S hav- ing the more negative segregation energy. The segregation profiles were fitted with the modified semi-infinity model of Fick to obtain the segregation parameters (pre- exponential factor (D0) and activation energy (Q)). The segregation profiles were also fitted with the Guttmann model to obtain the segregation energies (G) and interaction energies ( ) for In and S segregation in Cu. Finally, using segregation parameters obtained from these fits (Fick and Guttmann) the segregation profiles were fitted with modified Darken model to yield the segregation parameters for In and S segregation in Cu crystal. The segregation parameters for In that were ob- tained were Q = 184.1 kJ.mol−1, D0 = 1.6 × 10−5 m2.s−1,G = −60.4 kJ.mol−1 and In-Cu = 3.0 kJ.mol−1. The segregation parameters for S that were obtained were Q = 212.4 kJ.mol−1, D0 = 9.1 × 10−3 m2.s−1,G = −120.0 kJ.mol−1, S-Cu = 23.0 kJ.mol−1. The interaction energy for In and S was determined as In-S = −4.0 kJ.mol−1. The segregation parameters (Q and D0) obtained in this study for In segregation from a Cu crystal compare well with those reported in literature for In tracer diffusion in a Cu(In) bulk system. The segregation parameters (Q and D0) ob- tained in this study for S compare well with those reported in literature for both S segregation from a Cu crystal and S tracer diffusion in a Cu(S) bulk system.
Open Access
Simulating the formation of Pt nanostructures utilizing molecular dynamic calculations
(University of the Free State, 2014) Wessels, Leon Adolf Leopold; Terblans, J. J.; Swart, H. C.
Platinum (Pt) is an important catalyst for applications such as catalytic converters. In this thesis the formation of platinum nanoparticles was investigated by means of simulations. For the first part of the thesis a molecular dynamics simulation using the Sutton-Chen potential was implemented. This program was used for the simulations. Low energy structures were found. It was found that the number of nearest neighbours are maximised in the low energy structures. The energy barriers that have to be overcome as atoms move around the structures were also calculated. A model is proposed for the prediction of energy barriers. The model is useful for understanding the factors that influence the energy barriers and thus the mobility of atoms. The model will also be useful for Monte Carlo simulations. Simulations were done modelling physical vapour deposition onto the Pt(111) surface and a graphite surface represented by the Steele potential. It was found that higher temperatures and lower evaporation rates lead to lower energy structures. The smaller interaction between the graphite surface and the Pt leads to structures that have more layers. The parameters of the Steele potential that determine nearest neighbour distance and interaction strength between Pt and the substrate were adjusted to simulate other materials. It was found that a mismatch between the nearest neighbour distance of the substrate and Pt causes an increase in the mobility of the Pt atoms on the surface. The results of the simulations will enable the choice of suitable substrate and experimental parameters for the growth of Pt nanoparticles of desired shapes.
Open Access
Sol-gel synthesis of and luminescent properties of Pr³⁺ in different host matrices
(University of the Free State, 2009-11) Mbule, Pontsho Sylvia; Ntwaneaborwa, O. M.; Swart, H. C.
Luminescent ZrO2:Pr3+ , SiO2:Pr3+, ZnO:Pr3+ and ZnS:Pr3+ nanophosphors were synthesized by a sol-gel method, dried, ground and annealed in air at 600oC (SiO2:Pr3+, ZrO2:Pr3+, ZnO:Pr3+ and ZnS:Pr3+) or 280oC (ZrO2:Pr3+). The chemical composition of the powder phosphors was analyzed by energy dispersive x-ray spectrometer (EDS). The structure and particle sizes were determined with x-ray diffraction (XRD) and particle morphology was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SiO2:Pr3+ was amorphous even after annealing at 600oC. ZrO2:Pr3+ annealed at 280oC showed an amorphous structure but the material crystallized when the annealing temperature was increased to 600oC. The particle sizes estimated from the XRD peaks were ∼2±0.2 nm (dried ZnS and ZnO) and ∼8±0.1 nm (ZrO2:Pr3+ annealed at 600oC). Particle sizes increased to ∼17-20±0.2 nm in diameter for annealed ZnS:Pr3+ and ZnO:Pr3+. The UV-Vis spectrophotometer was used to determine the absorption properties of the nanophosphors and their band absorption showed a blue shift compared to their bulk counterparts. Powder phosphors were also irradiated with 325 nm (He-Cd) laser to study photoluminescence (PL) properties. PL spectra were obtained for both undoped and Pr3+ -doped nanophosphors. A broad emission band was observed at 498 nm with a shoulder at 416 nm from SiO2:Pr3+ annealed at 600oC. ZrO2:Pr3+ annealed at 280oC showed two emission bands in the visible range at 459 nm and 554 nm. A broad green emission band at 567 nm and a shoulder at 607 nm were observed for dried ZnO:Pr3+ nanophosphor and the shoulder at 607 nm was enhanced significantly when the Pr3+ concentration was increased. Annealed ZnO:Pr3+ (280oC) nanophosphor showed a green emission band centered around 533 nm and a shoulder at 624 nm. Dried ZnS:Pr3+ nanophosphor showed a blue emission centered at 445 nm and the PL intensity increased with an increase of Pr3+ ions concentration. All these emissions were coming from the host matrices and not from the Pr3+ ion when the powders were excited by 325 nm (3 eV) photons. SiO2 and SiO2:Pr3+ powder phosphors were subjected to prolonged 2 keV electron beam irradiation in an ultra high vacuum (UHV) chamber at a base pressure of 1x10-9 torr. The surface reactions and degradation of cathodoluminescence intensity were monitored using Auger electron spectroscopy (AES) and cathodoluminescence (CL) spectroscopy respectively. CL emission of SiO2 showed a maximum emission peak at 451 nm and a shoulder at 478 nm and SiO2:Pr3+ showed a multiple peak emissions located at 510 nm, 614 nm, 730 nm, 780 nm and 970 nm which are attributed to the transitions in the Pr3+ ions. The SiO2:Pr3+ CL intensity decreased with time as a result of continuous exposure to 2 keV electrons. The Auger peak-to-peak height as a function of energy spectrum showed that there were changes on the surface chemistry of the powders as a result of prolonged irradiation by 2 keV electrons. It is most likely that non-luminescent layers were formed on the surface and they contributed to the CL intensity degradation. A high concentration of volatile gas species, which might have contributed to the CL degradation, was detected with a residual gas analyzer (RGA). Cathodoluminescence was not measured for ZnO:Pr3+,ZnS:Pr3+ and ZrO2:Pr3+ due to charging of the powder phosphors and ZrO2:Pr3+ did not emit light under high energy electron exposure (2 keV).
Open Access
Synthesis and characterization of Ce³⁺ doped silica (SiO₂) ) nanophosphors co-doped with Al³⁺ or Mg²⁺ ions
(University of the Free State (Qwaqwa Campus), 2009-11) Koao, Lehlohonolo Fortune; Dejene, B. F.; Swart, H. C.
In recent studies, amorphous silica (SiO2) has been used as a host matrix for rare-earth ions to prepare luminescent materials that can be used in various light emitting devices. Sol-gel glasses have the potential to hold up to ≥10% dopants without losing their amorphous structure. However, before rare earth (RE) - doped sol-gel glasses can be used as luminescent material, several fluorescence quenching mechanisms must be overcome. There are several quenching mechanisms which are present in all materials that are more serious in sol-gel glasses. The first is cross relaxation which involves energy transfer between RE elements; the others are energy transfer through lattice vibrations and to hydroxyl (OH) groups which are present due to the use of water as the solvent during the preparation process. A few studies have demonstrated that the luminescence intensity of rare-earth doped silica can be improved through incorporation of co-dopants such as Al, TiO2, B and by annealing at high temperatures (e.g. > 500ºC). Following their footsteps and in order to make comparisons, we used aluminum as the codopant in some samples to investigate the effects on luminescence yield for various RE concentrations. We also investigated the effects of magnesium co-dopant and high temperature annealing on the luminescence intensity of rare-earth doped silica. In this work, the highest emission intensity was observed for the sample with a composition of 0.5 mol% Ce3+. Cerium doped silica glasses had broad blue emission corresponding to the D3/2- FJ transition at 445 nm but exhibited apparent concentration quenching after higher concentrations of 0.5 mol% Ce3+. Silica containing Mg2+ or Al3+ ions displayed an increase in luminescence intensity as the Mg2+ or Al3+ to Ce3+ ratio increases for the range investigated but significant luminescence enhancement was observed for Mg2+:Ce ratio greater than 20, while that of Al3+ co-doping had the highest luminescent intensity when the ratio of Al:Ce is 10:1. This enhanced photoluminescence was assigned to an energy transfer from the Mg nanoparticles, to result in enhanced emission from Ce3+. The Al3+ or Mg2+ ions disperses the Ce3+ clusters, enhancing 2F5/2 and 2F7/2 emissions due to increased ion-ion distances and decreased cross-relation.
Open Access
Synthesis and characterization of long afterglow phosphors (Sr Al2iO4:Ce3+ SrAl2iO4:Tb3+CaAlxOy:Tb3+, Y3Al5O12:Eu3+) using solution combustion method
(University of the Free State, 2011-11) Foka, Kewele Emily; Dejene, B. F.; Swart, H. C.
This work consists of several aspects of phosphor materials. Strontium, calcium and yttrium aluminate doped with rare earth (Ce, Tb and Eu) have been synthesized by solution combustion method using urea as a fuel for investigations of the luminescent, structure and morphological properties. The phosphors were characterized by several techniques such as Xray diffraction (XRD), energy dispersive electroscopy (EDS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Photoluminescence (PL), PL data were collected using a Cary Eclipse Photoluminescence Spectrophotometer equipped with a 150 W xenon lamp. Cerium doped strontium aluminum oxide (SrAhO4:Ce3+) were synthesizes. The effects of different concentration of cerium were investigated. X-ray diffraction results confirmed the formation of the SrAhO4 monoclinic phase (Powder Diffraction Standards (JCPDS) file N0 34-0379). The particle sizes of different peaks were estimated and the average particle size was 47 nm. SEM results showed agglomerated as well as small elongated-egg-like shape on particles when taken to higher magnification. The PL spectra show a broad emission consisting of two bands peaking at 374 and 384 nm, corresponding to the transitions from the lowest 5d excited state to the 2F512 and 2F712 states. The excitation and emission peak position shifted with varying the cerium concentration. This maybe due to uncontrollable electrospinning conditions like air and wetness, which influence the cristal field that surround Ce3+.SrAhO4:Tb3+ XRD peaks confirmed the formation of the SrAhO4 monoclinic phase and some impurities were also observed. The photoluminescence characteristics show the emission peaks at 415, 436 and 459 nm which correspond to the 5D3 to 7F1 (1=5 , 4, and 3) level and 489, 543, 585, and 622 nm corresponding to 5D4 to 7F 1 (J= 6, 5, 4, 3) under excitation at 229 nm and the terbium concentration was varied. The elements of the phosphor SrAhO4:Tb3+ were shown by energy dispersive spectroscopy. The decay curves were also observed and the decay constants show a higher value at a concentration of 0.25 mol% and lower value at a concentration of 2 mol¾. CaAlxOy:Tb3+ green phosphors were obtained at low temperature (500 °C) by a solutioncombustion method. The structural analysis revealed the presence of both monoclinic CaA4O1 and CaAhO4. The main parent structure of CaAhO4 monoclinic was revealed when varying the concentration of terbium. The characteristic luminescence properties were investigated using emission spectra. The emission peaks are from transition of the 5D4 state to the 7F1 (J = 6, 5, 4, 3) state. The optimal intensity was obtained when the concentration of Tb3+ was increased to 2 mol¾. FTIR was used to identify all the chemical bands. Absorption bands of the condensed matter AlO4 located in the range of700 cm·1-900 cm·1 and condensed matter AlO6 at 500 cm·1-680 cm·1 are attributed to AlO4 liberation at 600 cm·1-900 cm·1 The decay curves of the phosphor were investigated and showed a higher intensity and longer afterglow time at higher concentration of terbium 2 mol%.Y3Al5O12 known as Yttrium aluminum garnet (Y AG) phosphor doped with different concentration of Eu was synthesized by the solution combustion method. The crystalline structure, morphology and luminescent properties of the phosphors were studied. The SEM revealed the agglomerated morphology containing small spherical particles around the pores. FTIR spectra reveal all bonds that exist in the phosphor. The emission spectra revealed three major emission peaks at 592, 615, and 628 nm, corresponding to the 5D0-7F1 (592 nm), 5D0-7F2 (615 nm) and 5D0-7F3 (628 nm) transitions respectively. The luminescence intensity increased with an increase in Eu concentration at 0. 7 mol¾ and then decreases with an increasing of concentration further.
Open Access
The synthesis and characterization of the ZnO nanoparticles
(University of the Free State (Qwaqwa Campus), 2011-11) Tshabalala, Modiehi Amelia; Dejene, B. F.; Swart, H. C.
ZnO has been by far the most interesting semiconductor because of its properties. The ZnO nanostructures were synthesized by a sol-gel method and the samples were annealed in air at various temperatures capped with polymers PVP (Polyvinyl Pyrrolidone) and PEG (Polyethylene glycol). Again the ZnO was synthesized using different solvents; ethanol, methanol or water at various temperatures. Characterizations of the powders were carried out using different techniques. The structure and the particle size of the samples were obtained using the XRD (x-ray diffraction). The morphology was determined by the SEM (scanning electron microscopy) and the chemical composition was analyzed using the EDS (energy x-ray dispersed spectroscopy). The PL (photoluminescence) data were collected using the He- Cd (Helium-Cadmium) laser and also using the Cary Eclipse fluorescence spectroscopy at room temperature. The absorption spectra were analyzed using the UV-Vis spectroscopy. The PL spectra for the ZnO nanostructures capped and prepared using polymers showed broad emissions in the visible range. The broad emission in the visible range with maximum intensity peaks at 449 nm and at 530 nm for the PVP capped ZnO nanoparticles were observed annealed at 150°C. This was influenced by the addition of various molar masses of PVP on the Zn(Ac)2. The green emission band at 560 nm and a blue emission at 450 nm were obtained for the PEG encapsulated ZnO nanostructure. The PL of the ZnO nanoparticles prepared using various solvent was shown, the different shifts from the emission peaks were observed and the fluctuation of the intensity which was attributed to an increase and a decrease on the annealing temperatures. The effect of pH values on the ZnO prepared using different solvents. The PL on these samples exhibited a strong broad blue emission, for all the ZnO prepared using ethanol, methanol or water as solvents. The intensities differed with the amount of NaOH which was added onto the Zn(Ac)2 solution. The XRD pattern for all the prepared ZnO nanostructures exhibited the peaks corresponding to that of various planes of ZnO wurtzite structure with the JCPDS (Joint Committee on Powder Diffraction Standards) file no. (13-1451). The absorption spectra of the PVP capped ZnO nanostructures did not show any shifts while the absorption spectra for the PEG encapsulated ZnO nanostructures showed a shifts with an addition of the molar masses of the PEG. The UV-Vis spectroscopy for the ZnO prepared with ethanol, methanol or water as solvents at various temperatures gave the absorption edges and also the blue shifts that occurred with and increase on the annealing temperatures 300, 400, 500 and 600°C. It was observed from the UV absorption of the ZnO using different solvents with various pH values that the band gaps for all the samples were determined to be larger than that of ZnO bulk. The NaOH solution which was slowly added on the Zn(Ac)2 solution took control over the surface of the ZnO surfaces.