Doctoral Degrees (Physics)

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Open Access
'n ISS studie van die (110), (111) en (100)-enkel-kristalvlakke van NiAI
(University of the Free State, 2000-11) Mostert, Jacob Cornelis; Van Wyk, G. N.; Roos, W. D.
English: In this study the [110]-, the [111]- and the [1OO]-single crystal surfaces of Ni Al were investigated. The aim was to firstly harmonize the existing knowledge of these surfaces and secondly to create a clear and coherent overview of their properties. The essence of a problem of this kind is the determination of the structure and composition of the surface of a material. In this study, low energy ion scattering (LEISS) was used as primary investigative technique. Other techniques such as ICISS ("Impact Collision" ISS", Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) were used in conjunction with LEISS. Several intensive investigations of the NiAI(IIO)-surface were launched in recent times. The results of these investigations were largely similar and are widely accepted. In this study the results of several previous investigations are discussed. It is shown that the surface is largely a simple truncation of the bulk in the (110)-direction, but that the surface exhibits a ripple effect due to the particular way in which the Al and Ni atoms in the surface and second layer relaxes. In contradiction with the NiAI(ll O)-surface, there is still a measure of uncertainty regarding the properties of the NiAI(ll I)-surface. As part of this study, the NiAI(111)-surface was investigated using LEISS. The results of this investigation are given and is compared with the results of previous studies of the surface from the literature. It is shown that the surface consists of small Al-areas on top of a Ni-layer. One of the studies showed that the Al-areas could be removed by heating the surface to 1300 K. From other studies however, including this one, it seems that the removal of the Al-atoms from the surface may be inhibited or prevented by the presence of oxygen. As in the case of the NiAI(110)- and NiAI(l1 l j-surfaces, a large number of investigations were done on the NiAI( 1OO)-surface. Contradictory results were however a commonplace occurrence. In this study, the NiAI(lOO)-surface was subjected to a comprehensive LEED, LEISS, ICISS and AES investigation with the aim of clarifying the structure and composition of the surface. An overview of results of previous, as well as this investigation is given. It is shown that the NiAI(100)-surface is very sensitive to changes in temperature and that the surface composition and sometimes also the surface structure undergo several changes during heating. At low temperatures (approximately 500 K), a largely AItermination can be found, but with heating from 500 K to 873 K the surface composition changes to 65 at. % Al and 35 at. % Ni and voids. At this temperature the surface undergoes a reconstruction process so that it exhibits a c (Ji x 3.J2)R45°- LEED pattern. Further heating leads to another change in composition to 75 at. % Al and 25 at. % Ni and voids at 1073 K. At this temperature the surface exhibits a p(1 x 1)-LEED pattern. At temperatures above 1073 K the Al concentration in the surface starts to decline until the Al atoms are complete removed at approximately 1300 K. After this point the surface consists of only Ni atoms.
Open Access
Degradation of ZnS:Cu,Au,Al phosphor powder and thin films under prolonged electron bombardment
(University of the Free State, 2001-10) Hillie, Kenneth Thembela; Swart, H. C.; Berning, G. L. P.
Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in surface composition and luminous efficiency during electron bombardment. ZnS:Cu,Al,Au phosphor powders and thin films were subjected to prolonged electron beam bombardment of varying beam energies and different electron beam current densities in two different (O2 and CO2) vacuum gas ambients. The thin film phosphors were grown on Si (100) substrates by using XeCl (308nm) pulsed laser deposition (PLD) method. X-ray diffraction (XRD) measurements revealed that ZnS (100) films were preferentially grown on a Si (100) substrate. The RBS results show that the growth rate, increased with an increase of the N2 pressure in the deposition chamber during deposition. Degradation on both the powder and the thin film phosphors was manifested by a nonluminescent ZnO layer that formed on the surface of the phosphor according to the electron stimulated surface chemical reactions (ESSCR) mechanism. Lower current densities lead to a higher surface reaction rate, due to a lower local temperature beneath the beam, which resulted into a more severe CL degradation. A lower temperature beneath the electron beam may lead to an increase in the surface reaction rate due to the longer time spent by the adsorbed molecules on the surface, with a direct increase in the ESSCR probability. Low current densities would also lead to surface charging due to a lower electron conductivity of the phosphor resulting in an increase in the CL degradation rate due to band-bending. In the studies conducted between room temperature and 310 oC, an increase in the temperature led to a decrease in the surface reaction rate due to a decrease in the mean surface lifetime of the oxygen molecules on the surface, with a direct decrease in the ESSCR probability. Without the presence of the electron beam no chemical reactions, up to 310 oC, occurred on the surface. Therefore, local heating due to the electron beam irradiation is not responsible for the chemical reactions on the ZnS phosphor surface. At -125 °C the degradation was controlled by the residual small amount of water vapour in the system that is frozen at this low temperature. The thermoluminescence (TL) curves of the phosphor powder before and after degradation showed the influence of the O substitutional atoms that are created during electron bombardment in an O2 ambient. The O substitutional atoms acted as electron traps. On the electron beam bombardment of thin film phosphors, the degradation was more severe under O2 ambient compared to the same partial pressure of CO2 during electron beam bombardment, which is attributed to the free energy of formation of ZnO from ZnS when these respective gases are used. The degradation rate also depended on the energy of the electron beam, decreasing with increasing beam energy. This was interpreted according to the ionisation energy cross-section profile. The CL brightness increased exponentially with the increasing energy beam as more free carriers that will subsequently recombine yielding CL, are excited at higher beam energies. The thin film phosphor was also subjected to the electron beam bombardment after the phosphor film was coated with a CdO film by using a chemical bath deposition (CBD) method. The surface reactions were electron beam stimulated, resulting in the desorption of both Cd and S from the surface which happened as soon as the surface adventitious C was depleted. Sulphur from the ZnS accumulated on the surface but was soon depleted as volatile SOx compounds. The CdO was reduced by an electron beam assisted mechanism in the presence of non-reducible ZnO in the CdO-ZnO system as the Zn from the underlying ZnS layer emerged to the surface. The CL intensity degradation of the coated film showed a dependence on the surface composition. The intensity remained constant until the Cd was reduced on the surface before a slight decrease was observed. The effect of the CdO capping layer on the intensity of the phosphor was evident until the CdO eventually disintegrated.
Open Access
Surface segregation of Sn and Sb in the low index planes of Cu
(University of the Free State, 2005-05) Asante, Joseph Kwaku Ofori; Roos, W. D.; Terblans, J. J.
In this study, the segregation parameters for Sn and Sb in Cu were determined for the first time using novel experimental procedures. Sn was first evaporated onto the three low index planes of Cu(111), Cu(110) and Cu(100) and subsequently annealed at 920°C for 44 days to form three binary alloys of the same Sn concentration. Experimental quantitative work was done on each of the crystals by monitoring the surface segregation of Sn. Auger electron spectroscopy (AES) was used to monitor the changes in concentration build up on the surface by heating the sample linearly with time (positive linear temperature ramp, PLTR) from 450 to 900 K and immediately cooling it linearly with time (negative linear temperature ramp, NLTR) from 900 to 650 K at constant rates. The usage of NLTR, adopted for the first time in segregation measurements, extended the equilibrium segregation region enabling a unique set of segregation parameters to be obtained. The experimental quantified data points were fitted using the modified Darken model. Two supportive models - the Fick integral and the Bragg- Williams equations - were used to extract the starting segregation parameters for the modified Darken model that describes surface segregation completely. The Fick integral was used to fit part of the kinetic section of the profile, yielding the pre-exponenrial factor and the activation energy. The Bragg- Williams equations were then used to fit the equilibrium profiles yielding the segregation and interaction energies. For the first time, a quantified value for interaction energy between Sn and Cu atoms through segregation measurements was determined (ΩCuSn = 3.8 kJ/mol). The different Sn segregation behaviours in the three Cu orientations were explained by the different vacancy formation energies (that make up the activation energies) for the different orientations. The profile of Sn in Cu(110) lay at lowest temperature which implies that Sn activation energy was lowest in Cu(110). Sb was evaporated onto the binary CuSn alloys and annealed for a further 44 days resulting in Cu(111)SnSb and Cu(100)SnSb ternary alloys. Sn and Sb segregation measurements were done via AES. The modified Darken model was used to simulate Sn and Sb segregation profiles, yielding all the segregation parameters. Guttman equations were also used to simulate the equilibrium segregation region that was extended by the NLTR runs to yield the segregation and interaction energies. These segregation values obtained from the modified Darken model for ternary systems completely characterize the segregation behaviours of Sn and Sb in Cu. For the ternary systems, it was found that Sn was the first to segregate to the surface due to its higher diffusion coefficient, which comes about mainly from a smaller activation energy (ESn(100)= 175 kJ/mol and ESb(100) 186 kJ/mol). A repulsive interaction was found between Sn and Sb (ΩSnSb = - 5.3 kJ/mol) and as a result of the higher segregation energy of Sb, Sn was displaced from the surface by Sb. This sequential segregation was found in Cu(100) (∆GSb(100)= 84 kJ/mol; ∆GSn(100)= 65 kJ/mol) and in Cu(111) (∆GSb(111) = 86 kJ/mol; ∆GSn(l1l) = 68 kJ/mol). It was also found that the profile of Sn in the ternary systems lay at lower temperatures due the higher pre-exponential factor (DoSn(binary) = 9.2 x 10-4 m2/mol and DoSn(ternary) = 3.4 X 10³ m2/mol) if compared to the binary systems. This study successfully and completely describes the segregation behaviour of Sn and Sb in the low index planes of Cu.
Open Access
Surface segregation of Sn and Sb in the low index planes of Cu
(University of the Free State, 2005-05) Asante, Joseph Kwaku Ofori; Roos, W. D.; Terblans, J. J.
In this study, the segregation parameters for Sn and Sb in Cu were determined for the first time using novel experimental procedures. Sn was first evaporated onto the three low index planes of Cu(111), Cu(110) and Cu(100) and subsequently annealed at 920 oC for 44 days to form three binary alloys of the same Sn concentration. Experimental quantitative work was done on each of the crystals by monitoring the surface segregation of Sn. Auger electron spectroscopy (AES) was used to monitor the changes in concentration build up on the surface by heating the sample linearly with time (positive linear temperature ramp, PLTR) from 450 to 900 K and immediately cooling it linearly with time (negative linear temperature ramp, NLTR) from 900 to 650 K at constant rates. The usage of NLTR, adopted for the first time in segregation measurements, extended the equilibrium segregation region enabling a unique set of segregation parameters to be obtained. The experimental quantified data points were fitted using the modified Darken model. Two supportive models – the Fick integral and the Bragg-Williams equations - were used to extract the starting segregation parameters for the modified Darken model that describes surface segregation completely. The Fick integral was used to fit part of the kinetic section of the profile, yielding the pre-exponential factor and the activation energy. The Bragg-Williams equations were then used to fit the equilibrium profiles yielding the segregation and interaction energies. For the first time, a quantified value for interaction energy between Sn and Cu atoms through segregation measurements was determined (Ω.CuSn = 3.8 kJ/mol). The different Sn segregation behaviours in the three Cu orientations were explained by the different vacancy formation energies (that make up the activation energies) for the different orientations. The profile of Sn in Cu(110) lay at lowest temperature which implies that Sn activation energy was lowest in Cu(110). Sb was evaporated onto the binary CuSn alloys and annealed for a further 44 days resulting in Cu(111)SnSb and Cu(100)SnSb ternary alloys. Sn and Sb segregation measurements were done via AES. The modified Darken model was used to simulate Sn and Sb segregation profiles, yielding all the segregation parameters. Guttman equations were also used to simulate the equilibrium segregation region that was extended by the NLTR runs to yield the segregation and interaction energies. These segregation values obtained from the modified Darken model for ternary systems completely characterize the segregation behaviours of Sn and Sb in Cu. For the ternary systems, it was found that Sn was the first to segregate to the surface due to its higher diffusion coefficient, which comes about mainly from a smaller activation energy (ESn(100) = 175 kJ/mol and ESb(100) = 186 kJ/mol). A repulsive interaction was found between Sn and Sb (Ω.SnSb = - 5.3 kJ/mol) and as a result of the higher segregation energy of Sb, Sn was displaced from the surface by Sb. This sequential segregation was found in Cu(100) (ΔGSb(100) = 84 kJ/mol; ΔGSn(100) = 65 kJ/mol) and in Cu(111) (ΔGSb(111) = 86 kJ/mol; ΔGSn(111) = 68 kJ/mol). It was also found that the profile of Sn in the ternary systems lay at lower temperatures due the higher pre-exponential factor (DoSn(binary) = 9.2 × 10-4 m2/mol and DoSn(ternary) = 3.4 × 10-3 m2/mol) if compared to the binary systems. This study successfully and completely describes the segregation behaviour of Sn and Sb in the low index planes of Cu.
Open Access
Kinetic Monte Carlo simulation of the growth of gold nanostructures on a graphite substrate
(University of the Free State, 2006-06) Claassens, Christina Hester; Hoffman, M. J. H.; Terblans, J. J.
Nanotechnology has, without a doubt, ushered in a new era of technological convergence and holds the promise of making a profound impact on the way research in physics, chemistry, materials science, biotechnology etc. are conducted. The novel properties of materials at the nanoscale (or nanostructures) make them useful in a variety of applications, from catalysis to the medical field and electronics industry. However, to exploit these properties at the nanoscale, precise control over the morphology and size of nanostructures is required. One strategy that may be explored to tailor nanostructure morphology and size is vapour deposition. A lot of further insight can be gained from computer simulations of the processes governing the growth of nanostructures during vapour deposition. A method that shows promise in simulating thin film growth through vapour deposition is kinetic Monte Carlo (KMC). Therefore, in this study, a KMC model was developed to describe growth through vapour deposition. A gold on graphite system was simulated to test the model. In this KMC model, substantial effort was devoted to developing the model in different stages, each stage being more robust than the previous one. The assumptions made at each stage and possible artefacts (unphysical consequences) arising from them are discussed in order to distinguish real physical effects from artificial ones. In the model, data structures, search algorithms and a random number generator were developed and employed in an object-orientated code to simulate the growth. Several simulations were performed at different growth conditions for each of the stages. The results are interpreted based on the kinetic constraints imposed during the growth.
Open Access
Surface analysis of white spot formation on industrial electrogalvanised automotive steel
(University of the Free State, 2006-11-30) Conradie, Rochelle; Swart, H. C.; Terblans, J. J.; Roos, W. D.
MSSA (Mittal Steel South Africa), which produces electrogalvanised steel for the local automobile industry, experiences a problem with white spot formation when their steel is phosphated. The addition of nickel inhibits white spot formation but produces an unacceptable discolouration of the surface layer. Furthermore the locally produced steel exhibits blister formation when heated to 300ºC. The substrates, electrogalvanised coatings, phosphated samples and annealed samples are studied with Glow Discharge Optical Emission Spectroscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, as well as X-ray diffraction. Combining the results allows for an interpretation of the morphology, topography, composition, crystalline structure, quantitative depth profile as well as the spatial distribution of the elements The white spot formation on the electrogalvanised surfaces is closely related to the presence of contaminants on the electrogalvanised surface and at the interface between the substrate and the electrogalvanised coatings. The accelerated phosphate reaction results in complete dissolution of the electrogalvanised surface, thereby exposing the iron substrate. The white spot consists of an anomalous protruding perimeter with elongated crystals that grow towards the centre of the spot, present inside the spot. Partial dissolution is required in order for the phosphate process to occur. Complex phosphates deposit on the surface comprised of various cations, such as zinc, manganese and nickel. The zinc dissolution is the preferred reaction and therefore there is a slight enrichment of nickel in the sublayers of the phosphate. The manganese deposited on the surface must not be confused with the manganese present in the substrate. The addition of other cations to the electrogalvanised layer results in a change in the structure of the phosphated layer. The presence of cobalt and copper in the electrolyte results in an increase in the deposition of manganese phosphates on the surfaces. The manganese phosphates grow upward, away from the surface as opposed to the zinc phosphates that grow along the sample surface. The growth of the zinc phosphates only continues until the surface is covered. The structure of the electrogalvanised deposits changes with changes in the composition of the electrolyte. The morphology changes from a well-defined rigid structure (as for the zinc electrolyte) to a complex structure consisting of both grains and a fine intricate network of small deposits as various cations such as nickel, copper and cobalt are added to the electrolyte. The surface of the steel substrate clearly shows the rolling direction, as well as numerous dislocations. This compromises the epitaxial growth of the electrogalvanised layer. The alloy elements added to the steel are also present on the surface. These react differently compared to the steel and will therefore impact on the nature of the deposition at these sites. The annealing of the electrogalvanised samples causes both structural and compositional changes in the samples. The movement of the zinc and possible dezincification are most likely responsible for the blister formation. This is further affected by the presence of hydrogen in the sample and the subsequent hydrogen blistering. It is of paramount importance for all the surfaces and parameters to be controlled and monitored carefully to ensure the best coating quality. The presence of any contamination on the surfaces or in the solutions will cause adverse reactions and compromise the final product.
Open Access
The role of MHD instabilities in the magnetospheric propeller outflow and emission in the nova-like variable star AE Aquarii
(University of the Free State, 2007-02) Venter, Louis Albert; Meintjes, P. J.
English: AE Aquarii is a close binary consisting of a white dwarf primary star and a K4-5 red dwarf, the secondary. Mass is transferred from the Roche lobe filling secondary to the Roche lobe of the white dwarf. The white dwarf has a spin period Pspin 33 s and a fast corotating magnetosphere. The trajectory of the flow brings it to a closest approach rcl 1010 cm which is outside the corotation radius. Observational studies of the emisssion lines, in conjuction with the observed spin-down of the white dwarf, suggests that the bulk of the mass transfer is propelled from the system. The ejection of the flow is proposed to result from the interaction of the mass flow with the fast rotating magnetosphere. The interaction transfers angular momentum from the magnetosphere to the mass flow. The unique contribution of this study lies therein that this plasma-magnetosphere interaction is modelled as being driven by the Kelvin-Helmholtz (KH) instability, which is assumed to grow at the interface between the mass flow and the magnetosphere. The process can be quantified by evaluating the Poynting flux S, of the magnetospheric field at the radius of closest approach. The energy dissipation rate of the field across the surface of the stream A is PMHD = S x A 1034 erg s−1. Furthermore, if the mass transfer is ejected by the propeller at the escape velocity vesc 1550 km s−1, the energy carried by the outflow is Pout = 12 ˙M v2 esc 5 × 1033 erg s−1, where ˙M is the mass transfer rate. It is therefore plausible that the magnetospheric propeller is responsible for the ejection of the mass transfer. The ultimate energy source for the propeller is the spin of the white dwarf which has been shown to be losing rotational kinetic energy at the rate Pspin 1034 erg s−1. The KH driven magnetospheric propeller also results in the formation of magnetized plasmoids of energized electrons that emit synchrotron emission between infra-red and 1GHz radio frequencies as they are ejected from the system and expand. A large diffuse remnant, which emits in the MHz frequency range, is expected to form as the ejected bubbles coalesce outside the system. Furthermore, the KH instability triggers turbulence in the outflow, which eventually heats the gas and results in optical flares outside the white dwarf ’s Roche lobe. On its trajectory outwards, the outflow disrupts the magnetic field of the secondary and currents are induced that may heat plasma trapped in the field to X-ray emitting temperatures. This Joule heated plasma can account for the observed non-pulsed X-ray emission from AE Aqr.
Open Access
The anomalous low state of the X-ray binary system Hercules X-1
(University of the Free State, 2008) Jurua, Edward; Meintjes, P. J.
The lightcurve of Hercules X-1 (Her X-1) shows a peculiar 35-day modulation of the X-ray flux cycling between low and high states. The 35-day modulation is believed to result from the occultation of the neutron star by a warped precessing disc around the central neutron star. Since the discovery of the 35-day cycle of Her X-1, it has entered the anomalous low state a number of times, with the most recent being during the 2003 - 2004 period. Using RXTE ASM observations of Her X-1 after the 2003 - 2004 anomalous low state, it is shown that Her X-1 turned on with a new precession period and main-on flux. It is further shown that there is a positive correlation between the precession period and the main-on flux. Using optical observations of Her X-1 during both the anomalous low state and the normal high state it is shown that the orbital (1.7 day) lightcurve of Her X-1 varies systematically over the 35-day precession cycle. It is also shown that there is insignificant change in the 35-day morphology of the lightcurves between the anomalous low state and normal high state of Her X-1, suggesting a very slight change in the disc warp between the two states. Comparison of optical and X-ray lightcurves suggest that the significant amount of X-ray flux during the anomalous low state originates from the companion star. Analysis of both RXTE PCA and XMM-Newton observations of Her X-1 during the 2003 - 2004 anomalous low state, show that Her X-1 was brighter during this period compared to the normal high state brightness, and that there are two components of X-ray flux during the anomalous low state: reflection component from the companion star and coronal component from the accretion disc corona.
Open Access
Luminescent properties of synthesized PbS nanoparticle phosphors
(University of the Free State, 2008-08) Dhlamini, Mokhotjwa Simon; Swart, H. C.; Terblans, J. J.
Luminescent lead sulphide (PbS) nanoparticles embedded in an amorphous silica (SiO2) matrix were synthesized at room temperature by a sol-gel process. The prepared nanocomposite materials were crushed into powders and annealed in air at 200oC. The chemical composition of the powders was analyzed with an energy dispersive x-ray spectrometer. Particle sizes, crystalline structure and morphology of the PbS nanoparticles were determined with transmission electron microscopy (TEM) and x-ray diffraction (XRD). The crystal particle sizes estimated from the XRD peaks and the TEM images were in the range of 10 to 50 nm in diameter. The SiO2:PbS powders were then irradiated with 325 nm (He-Cd) and 458 nm (Ar+) lasers for photoluminescence (PL) measurement. PL spectra were obtained for pure SiO2 as well as the encapsulated PbS nanoparticles at room temperature. Two strong broad bands, blue (450 nm) and yellow-orange (560 nm) from bulk SiO2 and PbS nanoparticles, respectively, were observed. The PL data show a blue shift from the normal emission wavelength at 3200 nm in bulk PbS to 560 700 nm in nanoparticulate PbS powders. The blue-shift of the emission wavelengths is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles. Energy transfer from ZnO nanoparticles to PbS nanoparticles was also observed. The possible mechanism for the energy transfer is reported. The powders were also subjected to prolonged 2 keV electron beam irradiation in a vacuum chamber at and different O2 pressures (5 × 10-8 2 × 10-7 Torr O2). The cathodoluminescence (CL) was measured with Ocean Optics S2000 spectrometer, and showed the emission peak to be at a wavelength of 680 nm. Changes in the CL brightness and the corresponding change in the surface chemical composition were investigated with Ocean Optics S2000, Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). The oxygen Auger peak-to-peak height decreased simultaneously with the CL intensity. XPS analysis on the degraded spot showed the development of characteristic SiO, SiOx (0
Open Access
Synthesis and characterization of strontium (Sr), barium (Ba) and calcium (Ca) aluminate phosophors doped with rare earth ions
(University of the Free State, 2009-08) Mothudi, Bakang Moses; Ntwaeaborwa, O. M.; Swart, H. C.
The luminescent and structural properties of the alkaline earth aluminate phosphors prepared by solid state reaction, combustion and sol-gel methods are discussed. The Solid state and Sol-gel methods required much longer time (3-9 hours) for preparation of the phosphors. The annealing processes in both methods are performed at very high temperatures (1000 - 1300 0C). Furthermore in order to reduce Eu3+ to Eu2+ toxic gases such as N2 and H2 were introduced during the annealing process. The combustion method is more efficient because the phosphors of high efficiency were obtained at low temperatures (500 – 600 0C) in a very short period of time (5 min). The Eu2+ was obtained by adding a small amount of urea to the mixture during synthesis. SrAl2O4:Eu2+,Dy3+ phosphors prepared by a solid state reaction method at different annealing temperatures (1000 - 1200 0C), in a reducing atmosphere of N2 and 25%H2, were irradiated with an excitation wavelength of 365 nm. The optimum PL intensity was shown by a sample prepared at 1100 0C. The broad emission spectra symmetric at 497 nm can be attributed to the 4f65d1- 4f7 transition of the Eu2+. Eu2+ and Dy3+ co– doped calcium aluminate, barium aluminate and strontium aluminate phosphors were synthesized at an initiating combustion temperature of 500 0C using urea as an organic fuel. The crystallinity of the phosphors was investigated by using X- ray diffraction (XRD) and the morphology was determined by a scanning electron microscope (SEM). The low temperature monoclinic structure for both CaAl2O4 and SrAl2O4 and the hexagonal structure of BaAl2O4 were observed. Photoluminescent (PL) and phosphorescent properties were investigated by using a 325 nm He-Cd Laser and a Cary Eclipse fluorescence spectrophotometer, respectively. The broad band emission spectra with maxima at 449 nm for CaAl2O4:Eu2+,Dy3+, 450 nm (with a shoulder at 500 nm) for BaAl2O4:Eu2+,Dy3+ and 528nm for SrAl2O4:Eu2+,Dy3+ were observed. Ca0.97Al2O4:Eu2+ 0.01,Dy3+ 0.02 phosphors prepared at different initiating combustion temperatures were characterized by XRD), SEM and PL systems (He-Cd Laser and Cary Eclipse Fluorescence spectrophotometer). The PL emission spectra symmetric at 450 nm, in all the phosphors, confirms that only one emitting center, is present (Eu2+). The optimum PL intensity and Phosphorescence was observed from the sample prepared at an initiating combustion temperature of 600 0C. Ba0.97Al2O4:Eu2+ 0.01,Dy3+ 0.02 powder phosphors were prepared at different initiating temperatures ranging from 500 - 800 0C by the combustion method using urea as a comburent. The most crystalline hexagonal structure of BaAl2O4 was observed from samples prepared at the initiating combustion temperature of 500 - 600 0C. Blue-green persistent/long afterglow emission was observed from all the samples. Red photoluminescence was observed from a nanocrystalline SrAl2O4:Eu3+ powder phosphor prepared by a sol-gel process. The preparation process was carried out using Sr(CH3CO2)2. 2 1 H2O, Al (i-OC3H7)3 and Eu2O3 as starting materials. The white foamy gel of SrAl2O4:Eu3+ was dried at 60 – 200oC and calcined at 1000 0C. Nanocrystals of SrAl2O4 exhibited an agglomeration of nano-rodlike particles with an edge thickness of ~27 nm on average. Based on the X-ray diffraction and photoluminescence data, SrAl2O4 was found to crystallize as metastable monoclinic rather than hexagonal phase. The crystalline structure and photoluminescent properties of SrAl2O4:Eu3+ were reported.
Open Access
A theoretical and experimental investigation on the effect that slow heating and cooling has on the inter-diffusion parameters of Cu/Ni thin films
(University of the Free State, 2010) Joubert, Heinrich Daniel; Terblans, J. J.; Swart, H. C.
Thin film diffusion studies often involve a surface sensitive analysis technique combined with ion erosion to produce a depth profile of a sample. Such studies compare the depth profile of a reference sample to the depth profiles of samples that were annealed at different temperatures and times. The extent to which atoms of one layer diffuse into an adjacent layer, for a particular temperature and time, yields information on the diffusion process involved and allows quantification of the diffusion coefficient. The drawback to using an erosion type system is the effect of the incident ions on the surface being probed. The Mixing-Roughness-Information model attempts to compensate for this effect and is often employed as a means of quantification of measured depth profiles by means of profile reconstruction. Used in conjunction with Auger electron spectroscopy, the Mixing-Roughness-Information (MRI) model is a useful tool to reconstruct the ion erosion depth profiles as well as extracting inter-diffusion parameters from these depth profiles. The first part of the study focuses on the extraction of the diffusion coefficient of classically annealed samples of Ni in Cu from Ni/Cu depth profiles obtained from ion erosion Auger electron spectroscopy. The resultant depth profiles were reconstructed with the MRI model. The diffusion coefficient for Ni diffusing in Cu was obtained from the MRI fit and it compared well to values available in literature. From an Arrhenius graph a value of 9 2 -1 D0 6.49 10-9 m2 .s-1 for the pre-exponential factor and Q =130.5 kJ.mol-1 for the activation energy was calculated. The second part of the study involves linear ramping as an annealing technique. In previous studies, linear temperature ramping was used to determine diffusion coefficients from bulk-to-surface segregation experiments of a low concentration solute. Thin film diffusion studies usually employ a classical heating regime, where a sample’s annealing time is taken as the time between insertion and removal from a furnace. The aforementioned study type assumes that the time it takes to heat a sample after insertion is instantaneous, while the sample cools down instantaneously after removal from the furnace. This assumption is incorrect, as it does not compensate for the various mechanisms that govern heat transfer. In order to eliminate the uncertainty, a linear ramping regime is used and samples were annealed inside an UHV environment with a programmed linear heating scheme. After each anneal, a depth profile was obtained by simultaneously bombarding the sample with Ar+ ions and monitoring the exposed surface with an electron beam which excites Auger electrons, among others. The depth profiles were normalised and the time scale converted to depth. In order to compare the diffusion profiles obtained from classical annealing studies to the linearly ramped studies, the diffusion coefficient obtained for a classical study of Ni diffusing in Cu was compared to the diffusion coefficient obtained from a MRI linear ramp analysis of the ramped samples. The linear ramp analysis yielded a pre-exponential factor of 13 2 -1 D0 2.29 10-13 m2 .s-1 and activation energy of Q= 82.5 kJ.mol-1. Comparison of the diffusion profiles calculated with the diffusion coefficients obtained from classical heating and linear heating showed a large discrepancy between the calculated diffusion profiles. Analysis of the calculated profiles showed that classical diffusion studies overestimate the rate of diffusion if compared to the diffusion profile calculated with diffusion parameters obtained from linear ramping experiments. The linear ramping MRI technique was extended even further by changing the heating and cooling rate, thereby decreasing the effective annealing time. Diffusion profiles obtained from the extended linear heating MRI method refined the diffusion parameters for linear ramping even further.
Open Access
Luminescent properties of Y2SiO5:Ce thin films
(University of the Free State, 2010-05) Coetsee, Elizabeth; Swart, H. C.; Terblans, J. J.
The luminescent properties of yttrium silicate doped with cerium (Y2SiO5:Ce) phosphor thin films were investigated. A detailed investigation (cathodoluminescence (CL), photoluminescence (PL) and Gaussian peak fits) was first done on the luminescent mechanism of Y2SiO5:Ce phosphor powders in order to understand and find a plausible mechanism that could assist in future research to be done. Luminescence in Y2SiO5:Ce occurs due to characteristic transitions in the Ce3+ ion itself. Splitting of the 4f energy level into the 2F5/2 and 2F7/2 energy levels is due to the 4f1 electron in Ce3+ having the ability to exhibit a +1/2 and -1/2 spin. This creates the expectation of a luminescent spectrum with two main peaks in the blue region (between 400 and 500 nm). Y2SiO5:Ce has two different monoclinic crystal structures. A low temperature (synthesized at temperatures less than 1190 °C) X1 - phase (much weaker luminescent intensity, with space group P21/c) and a high temperature (synthesized at temperatures above 1190 °C with a melting temperature at 1980 °C) X2 - phase (space group B2/c). In each of these two phases there are two possible Y3+ sites in the Y2SiO5 matrix. The most plausible explanation for the broad band luminescent spectra obtained from excitation and emission results in this research study is that the two different sites of the Ce3+ ion (Ce can substitute Y) (A1 and A2) in the host matrix are responsible for two sets of visible peaks. The difference in orientation of the neighbour ions in the crystal structure will be responsible for the broadening of the band emission. Three sets of Y2SiO5:Ce thin films were grown with pulsed laser deposition (PLD) by using a 248 nm KrF and a XeCl (λ = 308 nm) excimer laser. The thin films were grown on Si (100) substrates with different process parameters in order to investigate the surface morphology and luminescent properties. Process parameters that were changed during the growth process using a KrF laser were the O2 ambient pressure (vacuum, 10 mTorr and 1 Torr), the fluence (3 ± 0.3 and 1.6 ± 0.1 J.cm-2), the substrate temperature (400 and 600 ºC) and the gas species (N2, O2 and Ar at 455 mTorr). The laser pulse frequency and the amount of pulses were kept constant at 8 Hz at 4000 pulses. The increase in the pressure to 1 Torr O2 shows a definite increase in particle size and roughness. The increased fluence led to bigger particle and grain sizes. The surface structure of the thin film ablated at 400 ºC substrate temperature is less compact (lesser agglomeration of particles than the 600 °C). The increase in substrate temperature definitely resulted in a rougher surface layer. Ablation done in N2 gas resulted in small particles of mostly 20 nm in diameter. Ablation in O2 gas produced bigger particles of 20, 30 and 40 nm as well as an agglomeration of these particles into bigger size clusters of about 80 to a 100 nm. Ablation in Ar gas showed particle sizes of mostly 30 nm. The particles are more spherically defined and evenly distributed on the surface in comparison with the agglomerated particles grown in O2 gas. Thin film morphology and other characteristic properties strongly depend on the gas pressure during PLD. An increase to 1 Torr O2 gas thus resulted in bigger particle sizes and the higher fluence also led to bigger particles with a decrease in particle density. The higher substrate temperature resulted in a rougher surface layer and ablation in Ar gas at 455 mTorr compared to N2 and O2 gas resulted in bigger and less agglomerated particles being formed. CL scanning images were obtained to investigate the effect of a tin oxide (SnO2) coated layer on the light output. The CL scan results of the uncoated and tin oxide coated thin films showed a definite increase in luminescent intensity with the uncoated thin film which indicates the photon absorption effect of the extra tin oxide coated layer. The tin oxide acts as a coated layer to prevent electron stimulated reactions with the phosphor surface and thus inhibits degradation. CL measurements that were done showed that the increased O2 ambient (1 Torr) resulted in a higher CL intensity compared to the thin films ablated in vacuum. This is in agreement with the PL results where the nano – particles’ shape ensure better light output due to fewer photons being totally reflected internally. The ablation in high fluence also showed a higher CL and PL intensity with the vacuum and 1 Torr thin films compared to the low fluence. The higher substrate temperature (600 ºC) results in better intensities due to the rougher surface formed. The thin film ablated in 455 mTorr Ar gas showed a higher CL intensity than the other two thin films. This is due to the spherically shaped and the less agglomerated particles on the surface of the substrate. A 144 hr CL degradation study was done on the thin film ablated in Ar gas (coulomb dose of 1.4 x 104 C.cm-2) at an O2 pressure of 1 x 10-6 Torr, 2 keV electron energy and 10 μA electron beam current. There was a definite decrease in the CL intensity measured at 440 nm while a second broad band peak emerged at 650 nm, which increased with an increase in the degradation time; leading to a broad spectrum ranging from 400 to 850 nm. The blue colour again changed (the same as with the powders) to a whitish colour. The degradation results were again ascribed to the formation of SiO2 with a defect level at 1.9 eV (650 nm). The XPS analysis showed that a SiO2 layer formed on the surface under electron bombardment. The thin films are therefore also degrading but are more chemically stable than the phosphor powders. The light output intensity however; is lower.
Open Access
Monte Carlo simulation and characterisation of phase formation in Pt-based alloy thin films
(University of the Free State, 2010-06) Harris, Richard Anthony; Terblans, J. J.; Swart, H. C.
English: From Icarus’ mythical flight to escape Crete to manned space flight to the moon, mankind’s dream to fly has impacted this world immensely. Technological advancements made in metallurgy and alloy development has played a huge role in realizing this dream. Developing materials and superalloys with higher melting temperatures and greater strength has allowed for the design of the modern turbine jet engines. Economical and (today more than ever) environmental concerns continue to provide ample motivation for operating the engines at ever increasing temperatures, thereby improving the thermodynamic efficiency and reducing pollutant emissions. One of the most aggressive man made environments is that of the high pressure turbine section of a modern gas turbine engine. During operation, after combustion, highly oxidizing gas enters the turbine. This happens at temperatures exceeding 200 °C above the melting point of the superalloy turbine blade. Newer generations of civil aircraft will have turbine entry temperatures (TET) that will exceed 1800 K at take-off. Increased power and improved fuel consumption remains a continuing demand in modern aero-gas turbine engines as this result in an increase in TET. One strategy to achieve this goal is by coating the turbine blades with a thin film composed of alloy material. These films can be engineered to have specific heatresistant, oxidation-resistant properties. Two coating techniques that show promise in achieving these goals are pulsed laser ablation (PLD) and electron beam physical vapour deposition (EB-PVD). These techniques are investigated in this study in particular of platinum-aluminium alloys. The appearances of droplets on the thin film surface that arise due to the pulsed laser ablation technique itself are investigated. A suitable technique to minimize the appearance of these droplets by using ambient gas and ambient gas pressure is discussed. The stoichiometric transfer of material from the target to a substrate was also investigated. A lot of insight into engineering these types of coatings can be gained from computer simulations of the processes governing the diffusion of the individual elements making up the superalloy. Therefore, in this study, a chemical potential Monte Carlo (CPMC) model was developed to simulate diffusion of platinum-aluminium binary alloys. The change in microstructure during diffusion as the pure elements diffuse into each other to form an alloy with a specific composition is investigated. In the model, data structures, search algorithms and a random number generator were developed and employed in an object-orientated code 6 to simulate the diffusion of binary metals during annealing. Several simulations were performed at different compositions. The results are compared to experimentallymeasured elemental maps of EB-PVD prepared thin film samples.
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.
Open Access
Characterization of SrAl₂O₄:Eu²⁺,Dy³⁺ nano thin films prepared by pulsed laser deposition
(University of the Free State, 2010-11) Nsimama, Patrick Damson; Swart, H. C.; Ntwaeaborwa, O. M.
English: Thin films of SrAl2O4:Eu2+,Dy3+ phosphor were deposited on silicon (Si (100)) substrates using a 248 nm KrF pulsed laser. Deposition parameters, namely; substrate temperature, pulse repetition rate, number of laser pulses, base pressure and the working atmosphere were varied during the film deposition processes. Atomic force microscopy (AFM), Scanning electron microscopy (SEM), X-ray Diffraction (XRD), energy dispersive x-ray spectroscopy (EDS), and the fluorescence spectrophotometry were used to characterize the thin films. The surface characterization was done by using Auger electron spectroscopy (AES) combined with CL spectroscopy and X-ray photoelectron spectroscopy (XPS). PL data were collected in air at room temperature using a 325 nm He-Cd laser PL system and the UV Xenon lamp Cary Eclipse fluorescence spectrophotometer. The particle morphologies, surface topographies and photoluminescence (PL) properties were varying with the deposition parameters. Rougher film surfaces gave better PL properties. The optimum substrate temperature for SrAl2O4:Eu2+,Dy3+ films with intense PL emission was in the 350-400o C range. SrAl2O4:Eu2+,Dy3+ thin films ablated using a higher number of pulses gave superior PL properties to those deposited at lower number of pulses. As-deposited films prepared in the gas atmospheres gave AFM images with well defined particles and better PL properties than those deposited in vacuum. The average particle sizes for films deposited in gas atmospheres were ranging from 25 nm to 40 nm. The results from XRD and HRTEM showed that the as-deposited SrAl2O4:Eu2+,Dy3+ thin films were amorphous. Upon annealing at 800o in vacuum for 2 hours, the PL of the films deposited in the gas atmospheres decreased. However, the crystallinity and the PL properties of the annealed vacuum deposited thin film improved considerably. The CL spectra gave only green emission peaks ranging from 507 nm to 522 nm. Both the PL and CL emissions were ascribed to the 4f65d1 → 4f7 Eu2+ ion transitions. The AES elemental composition results for the undegraded and electron degraded thin films gave all the main elements in the SrAl2O4:Eu2+,Dy3+ material, i.e. Sr, Al and O. The ratios of Al and Sr APPHs to that of O increased slightly during removal of the C from the surface. The C/O ratio decreased with an increase in electron dose. Results from the RBS showed thin film SrAl2O4:Eu2+,Dy3+ stoichiometric ratios comparable to the commercial powder. The sharp decrease in the C/O APPH ratio was due to removal of C from the surface due to the electron stimulated surface chemical reactions (ESSCRs) which took place during electron bombardment. During the ESSCR process, the electron beam dissociates the O2 and other background species such as H2O to atomic species which subsequently react with C to form volatile compounds (COX, CH4, etc.). The CL intensity degraded during prolonged electron beam irradiation due to the ESSCR process. The CL degradation increased with the increase in the chamber base pressure. The XPS data collected from the degraded films proved that strontium oxide (SrO) and aluminium oxide (Al2O3) were formed on the surface of the films as a result of the ESSCR in line with the increase of Sr/O and Al/O from the AES results.
Open Access
Luminescence investigation of trivalent rare earth ions in sol-gel derived SiO₂ and ZnO co-doped SiO₂:Pr³⁺
(University of the Free State, 2011-05) Mhlongo, Gugu Hlengiwe; Hillie, K. T.; Ntwaeaborwa, O. M.
Tb3+-Pr3+ , Ce3+-Pr3+ , and Eu3+-Pr3+ ion pairs co-doped in SiO2 were successfully synthesized using a sol gel method to produce rare earth activated oxide nanophosphors. Green emitting ZnO nanoparticles were also successfully embedded into single doped Pr3+ in SiO2 matrix resulting in a red emitting ZnO.SiO2:Pr3+ nanocomposite. The phosphor powders were produced by drying the precursor gels at room temperature followed by annealing at 600 oC in ambient air. Based on the X-ray diffraction results, it was found that the SiO2 was amorphous regardless of the incorporation of Pr3+, Ce3+, Tb3+, Eu3+ ions and nanocrystalline ZnO or annealing at 600 oC. The particle morphology of powder phosphors was observed from field emission scanning electron microscopy and high resolution transmission electron microscope images. The field emission scanning electron microscopy revealed that the particles of the synthesized phosphors were mostly spherical and agglomerated. In addition, the morphology and distribution of SiO2 nanoparticles were not influenced by the presence of different rare affected by the presence of rare-earth ions in the matrix. The high resolution transmission electron microscope on the other hand confirmed the homogenous dispersion of the rare-earth ions incorporated in the amorphous SiO2 matrix. The presence of these ions in SiO2 host was confirmed by the energy dispersive X-ray spectroscopy. The energy transfer from ZnO to Pr3+ which was evidenced by quenching of green emission from ZnO resulting in an enhanced red emission from Pr3+ under both low electron beam and vacuum ultra violet excitation was demonstrated. For Pr3+-Ce3+ ion pair, the red emission form Pr3+ was slowly quenched while that from Ce3+ was slightly enhanced with increasing Ce3+ concentration. Such results indicate the energy transfer from Pr3+ to Ce3+. In the case of SiO2:Tb3+/Eu3+ co-doped with Pr3+, the cathodoluminescence and photoluminescence intensities of Pr3+-Tb3+ and Pr3+-Tb3+ were strongly quenched with Pr3+ co-doping. We also investigated the effect of beam voltage and current on the cathodoluminescence intensity from the powder phosphors as well as their cathodoluminescence intensity degradation under prolonged electron bombardment in the cathodoluminescence spectroscopy.
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.
Open Access
Growth of antimony on copper : a scanning tunneling microscopy study
(University of the Free State, 2012-01) Ndlovu, Gebhu Freedom; Hillie, K. T.; Roos, W. D.
English: The thesis deals with adsorption, self–assembly and surface reactions of Sb atoms on solid Cu(111) substrates. It is of genuine interest in materials science and technology to develop strategies and methods for reproducible growth of extended atomic and molecular assemblies with specific and desired chemical, physical and functional properties. When the mechanisms controlling the self-organized phenomena are fully disclosed, the self-organized growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials. The experimental technique used to study ordered phases and phase transitions of Sb on Cu(111) substrates was the Scanning Tunneling Microscopy (STM) – an outstanding method to gain real space information of the atomic scale realm of adsorbates on crystalline surfaces. It is a general trend to conduct studies on well known structures before one begins working on complicated systems. Therefore, in this study, Si(111) Cu(111) and HOPG surfaces were studied in atomic detail to confirm the calibration and the resolution capabilities of the instrument. The acquired data were comparable to the reported theoretical and experimental data in literature. The investigated Cu(111) – Sb system is characterized by a complex interplay between adsorbate interactions and adsorbate substrate interactions which in this study manifests through self–assembly processes. Both low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were utilized to determine the substrate cleanliness prior to the growth of a submonolayer Sb coverage (0.43 ± 0.02 ML Sb as calculated from the acquired STM data). The freely diffusing Sb adatoms on the copper surface were thermally excited from a random distribution of Sb atoms after growth to a finally rearrangement to more energetically stable configuration. The experimental results illustrated the presence of a surface alloy after annealing at ~360°C. The Cu – Cu spacing increased from 0.257 ± 0.01 nm (atomically clean Cu(111)) to 0.587 ± 0.02 nm after annealing at 360°C. At that temperature, the STM images showed the surface protrusions of different sizes and contrast, attributed to Cu and Sb atoms. In addition to the conventional ( 3 × 3)R30°–Sb structural phase acquired at ~400°C, new metastable structural phases: (2 3 × 2 3) R30°–Sb and (2 3 × 3)R30°– Sb were obtained for the first time after annealing at 600°C and 700°C, respectively. STM data after annealing at 600°C and 700°C was best described by a structural model involving an ordered p(2×2) and p(2×1) overlayer structures superimposed onto the ( 3 × 3)R30°–Sb surface, respectively. At elevated temperatures LEED showed ring shaped diffraction patterns composed of twelve equidistant spots which are consistent with the growth of a hexagonal film forming three equivalent rotational domains. All the superstructures were found to favour a structural model based on Sb atoms occupying substitutional rather than overlayer sites within the top Cu(111) layer. Other than the dissolution of Sb onto Cu(111), the study report also on the segregation of Sb on Cu together with STS measurements. The surface chemical reactivity on an atom–by–atom basis of the Cu sample surface was studied by current imaging tunneling spectroscopy (CITS). The local density of states (LDOS) were derived from dI/dV maps at low tunneling voltages by a simultaneous measurement of high resolution topographic micrographs. The use of surface sensitive techniques (LEED, AES, STM, STS) in studying the surface alloy in question has enabled more precise statements to be made about the surface structure of the system at various temperatures. Based on the experimental results, a comprehensive study of the adsorption and segregation behaviour of Sb on Cu(111), including the mechanisms for phase formation at the atomic scale is presented in this study.
Open Access
Gamma-ray production in the Be-type star-pulsar binary system PSR B1259-63
(University of the Free State, 2012-01-30) Van Soelen, Brian; Meintjes, P. J.
English: The high-mass binary system PSR B1259-63/LS 2883 is one of only six known gamma-ray binaries, and the only one where the compact object is known from the direct observation of a pulsed radio signal to be a 48 ms pulsar. During it’s eccentric 3.4 year orbit, the pulsar moves through the circumstellar disc of the optical companion, approximately twenty days before and after periastron. This results in conditions for complicated interactions between the material in the disc, the fast rotating pulsar, the pulsar wind, and the radiation field from the star and disc. The system has been the object of multi-wavelength campaigns with telescopes such as the VLT, H.E.S.S. and Fermi. The interaction between the stellar and pulsar wind results in the formation of a radiating pulsar wind nebula within the binary system, which has been detected from radio to TeV gamma-ray energies. The spectral energy distribution is dominated by the emission at gamma-ray energies, classifying this system as a gammaray binary. The interaction between the stars is greater near periastron where the pulsar passes closest to the optical companion. Approximately twenty days from periastron the pulsar passes through or behind the Be star’s circumstellar disc, obscuring the pulsed radio signal. During this period there is a corresponding increase in the unpulsed emission from the system. The TeV gamma-rays are believed to be produced by electrons in the pulsar wind which cool via the inverse Compton up-scattering of stellar photons from the optical companion. The circumstellar disc associated with the Be star produces an infrared flux below ! 1015 Hz, which is greater than that expected from the blackbody distribution associated with star, providing additional target photons which could increase the inverse Compton scattering rate. The scattering of infrared photons can occur in the Thomson limit with its significantly larger cross-section and should produce GeV energy gamma-rays in the energy range observed by the Fermi telescope. A curve of growth method is presented to model the infrared free-free and free-bound emission from the circumstellar disc, taking into account the changing viewing angle as observed from the pulsar. The curve of growth model is fitted to archive near-infrared and optical data and mid-infrared data obtained with the Very Large Telescope during January 2011. The effect of this infrared excess on the inverse Compton scattering rate is considered for an isotropic and anisotropic photon distribution, considering pre– and post–shock electron distributions. The anisotropic modelling considers the effects of the changing size and orientation of the circumstellar disc relative to the pulsar, as well as the change in the inverse Compton scattering angle during the orbit. The inverse Compton scattering rate for three disc orientations is modelled over a period of approximately 160 days around periastron, including the disc crossing epochs before and after periastron. The maximum disc contribution is found to occur close to periastron and not near the disc–crossing where the low infrared flux from the disc, at a radius of ! 45 stellar radii, has a less significant effect. It is found that the inclusion of the infrared flux from the circumstellar disc can increase the GeV flux from the system by a factor ! 2 near periastron, for favourable disc orientations. The predicted increase is, however, less than was detected with Fermi during the 2011 periastron passage. The observations showed a flare which cannot be explained by this, or any current model.
Open Access
The propeller driven pulsar-like spin-down and non-thermal emission in the nova-like variable star AE Aquarii
(University of the Free State, 2012-04) Oruru, Bosco; Meintjes, P. J.
English: The nova-like variable AE Aquarii consists of a fast rotating magnetized white dwarf, orbiting a late-type main sequence companion star. It is the most enigmatic among cataclysmic variables, and perhaps the best laboratory to study the physics of accretion and related phenomena, due to its multi-wavelength nature and unique flaring activity. The system is in an accretor-propeller state and most of its properties are associated with the propeller process. Using data observed contemporaneously with Chandra and Swift, the UV and X-ray properties of AE Aquarii have been studied. It is shown that the X-ray emission below 10 keV is predominantly soft and characterized by flares and emission lines. The spectra can be reproduced by multicomponent thermal emission models, and the time-averaged X-ray luminosity is determined to be LX 1031 erg s−1. The thermal soft X-ray emission (below 10 keV) is modelled in terms of plasma heating at the magnetospheric radius, where accretion flow from the secondary star interacts with the magnetosphere of the white dwarf. Both UV and X-ray emission are pulsed at the spin period of the white dwarf. The recently detected hard X-ray emission in AE Aquarii (above 10 keV), with a luminosity of LX,hard 6 5 × 1030 erg s−1, shows a non-thermal nature, possible synchrotron emission of high energy electrons in the white dwarf magnetosphere. It is proposed that these electrons are accelerated by large field aligned potentials of V > 1012 V, a process common in the magnetospheres of fast rotating neutron stars, or pulsars. This places AE Aquarii in a unique category with respect to most members of cataclysmic variables. The ratio of the observed hard X-ray luminosity to the spin-down luminosity of the white dwarf in AE Aquarii lies in the range 0.01-0.1 %, which is the same as observed from young rotation-powered neutron stars in the 2-10 KeV range. In this regard, a pulsar-like model is appropriate to explain the origin of the observed non-thermal hard X-ray emission in AE Aquarii.