Doctoral Degrees (Physics)

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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
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
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
A combined ab initio and experimental study of lanthanides and/or transition metal doped oxides
(University of the Free State (Qwaqwa Campus), 2017-01) Mulwa, Winfred Mueni; Dejene, B. F.; Ouma, C. N. M.; Onani, Martin
Ab initio modelling techniques have produced a notable contribution in analysing semiconductor metal oxides properties by use of first principles. These techniques have transformed to a high level of accuracy, owing to the development in algorithms and improved computational ability. In the study of structural, electronic and optical properties of metal oxides, ab initio techniques have been used with a lot of success to illustrate these properties. Ab initio studies therefore can complement experimental findings or even provide reliable results on properties which have not yet been experimentally investigated. Properties which can be calculated with the use of density functional theory (DFT) include spectroscopic, energetic, electronic and geometric properties. In this combined experimental and ab initio work on metal oxides doped with transition metals, the used of local density approximation with the Hubbard U correlation to compute the structural, electronic and optical properties of ZnA12O4 and Cu2+:ZnA12O4 was used. The powders of doped and undoped ZnA12O4 were effectively synthesized by use of the sol-gel technique. The X-ray diffraction (XRD) pattern for ZnA12O4 displayed crystalline peaks corresponding to cubic structure and phase dissociation was not observed. It also showed negligible lattice distortion and a slight shift to higher angles with increase of Cu2+ percentage doping. Energy dispersive X-rays spectroscopy (EDS) confirmed pure samples of ZnA12O4 components. Scanning electron microscopy (SEM) micrographs showed a uniform, well distributed and spherical morphology. The high resolution transmission electron microscopy (HRTEM) showed the influence of varying Cu2+ concentration on the particle agglomeration as well as on the crystallite sizes. The average crystallite sizes of ZnA12O4 powders almost remained constant with the increase of Cu2+ doping concentration. The lattice spacing approximated from selected area electron diffraction (SAED) was 0.242 nm corresponding to (311) lattice of ZnA12O4. Setting excitation at 283 nm, the photoluminescence (PL) emission peaks were at 388 nm, 425 nm and 480 nm in undoped ZnA12O4 which was due to oxygen vacancies while the peak at 586 nm was due to Cu2+ ions. Computationally, introduction of Cu2+ ions did not lead to significant lattice distortion and the PL emission peak was at 435 nm with a transition from Cu_3d to Cu_4p. The substitutional energies in Cu2+:ZnA12O4 predicted negative formation energies for oxygen vacancies suggesting that these vacancies are easily formed in ZnA12O4. The two point defects (oxygen vacancy and Cu2+ dopant) existed singly as the binding energies were found to be negative. Both experimental and computational work were carried out on lanthanide-doped metal oxide (ꝩ-A12O3 in this case). The powders of doped and undoped (ꝩ-A12O3 were successfully prepared using the sol-gel technique. The A12O3 as well as Ce3+: A12O3 were modelled where the Kohn- Sham equations were solved by the use of local density approximation with the Hubbard U correction. In ꝩ-A12O3:Ce3+, introduction of the dopant caused lattice strain as well as reduction in band gap. The formation energies in all the charge states were negative, suggesting that the ꝩ - A12O3 lattice could easily accommodate Ce3+. The PL emission peak was reported to be at 502 nm with a transition from O_2p to Ce_4f. The X-ray diffraction (XRD) pattern exhibited crystalline peaks corresponding to cubic structure. Due to difference in ionic radius between A13+ and Ce3+, lattice distortion was realized. As the doping concentration increased, there was a slight shift to lower angles. Only aluminium and oxygen elements were detected in the EDS analysis. SEM analysis revealed agglomeration on doping. From the HRTEM findings, the crystallite size of 16.0 nm was realized. The lattice spacing approximated from SAED was 0.138 nm corresponding to (440) lattice plane of  -A12O3. With excitation at 240 nm, the PL emission peaks at 440 nm and 462 nm were due to oxygen vacancies while the peak at 560 nm was due to Ce3+ doping. This result shows that Ce3+ doping of  -A12O3 improves its luminescence property therefore making it a possible candidate for blue light emitting diodes application. DFT work on both transition metal and lanthanide-doped metal oxides was investigated in undoped TiO2, lanthanides-doped TiO2 as well as transition metal (Cr3+) doped TiO2 by the use of local density approximation with the Hubbard U correlation to compute the substitutional energies, thermodynamic transition levels, optical properties and magnetic properties of Cr3+:TiO2 and lanthanide-doped TiO2. Unlike ZnAl2O4 and  -A12O3, TiO2 was not experimentally synthesized but was modelled theoretically. Lanthanide doping was found to cause red shift of the band gap from the ultraviolet region to the visible region of the optical absorption spectra in TiO2. The value of the computed substitutional energy implied that lanthanide ions are easily incorporated in TiO2 crystal lattice. The most favorable doping percentage was anticipated to be approximately 3%. On doping TiO2 with chromium, a transition was observed from paramagnetism to ferromagnetism at 6% doping. The magnetic moment per chromium atom was 2.59 μB for rutile phase of TiO2 and 2.49 μB for anatase phase. This result makes Cr3+ doped TiO2 a possible candidate for application in memory devices.
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
Effect of broadband excitation ions in the luminescence of Ln.³+ doped SrF₂ nanophosphor for solar cell application
(University of the Free State, 2015-06) Yagoub, Mubarak Yagoub Adam; Coetsee, E.; Swart, H. C.
SrF2:Pr3+-Yb3+ phosphor powder was previously investigated for down-conversion application in solar cells. The rst surface, structural and optical characterization results indicated that the Pr3+-Yb3+ couple requires a sensitizer for effective enhancement in energy conversion. Broadband excitation ions of Ce3+ and Eu2+, that could be used as sensitizers, were therefore doped and co-doped in the SrF2 crystal. Detailed characterizations and investigations were then done on the surface, structure and optical aspects to see the effect on the energy conversion. Initially, the influence of different synthesis techniques on the surface, structure and concentration quenching of Pr3+ doped SrF2 was studied. The singly doped SrF2:Pr3+ was prepared by the hydrothermal and combustion methods. Scanning electron microscope (SEM) images showed different morphologies which was an indication that the morphology of the SrF2:Pr3+ phosphor strongly depended on the synthesis procedure. Both the SrF2:Pr3+ samples exhibited blue-red emission under a 439 nm excitation wavelength at room temperature. The emission intensity of Pr3+ was also found to be dependent on the synthesis procedure. The dipole-dipole interaction was found to be responsible for the concentration quenching effects at high Pr3+ concentration in both methods. SrF2:Eu nano-phosphors were successfully synthesized by the hydrothermal method. The crystalline size of the phosphors was found to be in the nanometre scale. The photoluminescence and high resolution x-ray photoelectron spectroscopy (XPS) results indicated that the Eu was in both Eu2+ and Eu3+ valance states. The presence of Eu2+ and Eu3+ in the system largely enhanced the response of the Eu3+ under ultraviolet excitation. Time of flight secondary ion mass spectrometry (tof-SIMS) results suggested that the energy transfer between these two ions was likely occurred. The relative photoluminescence intensity of the Eu2+ rapidly decreased with an increasing laser beam irradiating time. This result would make the current Eu2+ doped SrF2 samples unsuitable candidates for several applications, such as white light-emitting diodes and wavelength conversion films for silicon photovoltaic cells. The effect of Ce3+ ions on the SrF2:Eu nano-phosphor was also studied. Ce3+ largely enhanced the Eu3+ emission intensity via energy transfer mechanism. The calculated energy transfer efficiency was relatively effcient at high Eu concentration. The results suggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Eu ions in SrF2 host. Eu2+ co-doped Pr3+, Yb3+ and Pr3+-Yb3+ couple in SrF2 were successfully prepared. XPS confirmed that all Eu contents were in Eu2+ oxidation states. Initially, Eu2+ co-doped SrF2:Pr3+ was studied. From PL and decay curve results, an efficient energy transfer was demonstrated in SrF2:Eu2+, Pr3+ phosphors. The energy transfer process was effective until a concentration quenching between Pr3+ ions occurred. The results proposed that Eu2+ could be a good sensitizer for absorbing the UV photons and hence efficiently enhancing the Pr3+ emission intensity. SrF2:Eu2+ (1.5 mol%) co-doped with Na+ (0.5 mol%) and various concentrations of Yb3+ were also investigated. XRD results showed a mixture of the cubic SrF2 and NaYbF4 phases. The NaYbF4 phase gradually formed with increasing Yb3+ doping concentration. Emission spectra and the fluorescence decay curve measurements were utilized to demonstrate the cooperative energy transfer. Energy transfer occurred subsequently from Eu2+ to Yb3+ followed by intense NIR emission. The energy transfer was completed at high concentrations but the Yb3+ emission intensity was reduced as a result of concentration quenching. In addition, from the photoluminescence data it was evident that Na+ induced significant change to NIR emission. The possibility of using the broadband absorption of Eu2+ to sensitize the Pr3+-Yb3+ down-conversion couple in SrF2 matrix was also investigated. The energy transfer process was demonstrated by the decrease of Eu2+ and Pr3+ related photoluminescence and lifetime with increasing Yb3+ concentration. Upon 325 nm excitation into the 5d levels of Eu2+, the samples yield intense near infrared emission corresponding to Pr3+:4f-4f and Yb3+:4f-4f transition. Yb3+ emission was clearly observed only at high Yb3+ concentrations after the emission intensity of Pr3+ was quenched. The PL lifetime results of Eu2+ confirmed the the second-order cooperative energy transfer also occurred between Eu2+ and Yb3+ ions.
Open Access
Effect of host anion or cation substitution on the luminescence and stability of lanthanum oxide based phosphors doped with bismuth
(University of the Free State, 2015) Jabraldar, Babiker Mohammed Jaffar; Kroon, R. E.; Swart, H. C.; Ahmed, Hassan Abdel Halim Abdallah Seed
LaOCl:Bi3+ phosphor powders were successfully prepared via the solid-state reaction method and compared to LaOBr:Bi3+. Photoluminescence measurements revealed that the phosphors displayed ultraviolet emission around 344 nm for LaOCl:Bi and 358 nm for LaOBr:Bi under excitations of 266 nm and 273 nm, respectively. The optimum Bi doping concentration and annealing temperature were established as 0.7 mol% and 900 °C for both hosts. The scanning electron microscopy data showed that the particles are randomly oriented flat thin plates with diameters close to or larger than 100 nm. The chemical composition of the phosphor was studied by using energy dispersive X-ray spectroscopy, while diffuse reflectance spectroscopy was employed to study the absorption. The ultraviolet emission peaks were assigned to the 3P1 → 1S0 transition of the Bi3+ ions, while additional relatively weak emissions in the visible range were attributed to Bi clustering. LaOCl:Bi was found to undergo a little degradation while exposed to the atmosphere for several months, while LaOBr:Bi was significantly less stable. The LaOBr:Bi phosphor was also found to degrade rapidly under 254 nm ultraviolet excitation, while LaOCl:Bi did not degrade under the same conditions. X-ray photoelectron spectroscopy measurements indicated that similar surface changes occurred for both samples during UV-exposure, suggesting that the cause of degradation is not primarily related to surface changes, but may be due to the greater probability of forming non-radiative defects in the bulk of LaOBr having the smaller bandgap. Luminescence properties of bismuth (Bi) doped lanthanum oxyfluoride (LaOF) were investigated experimentally with samples prepared via the solid-state reaction method using ammonium fluoride, lanthanum oxide and bismuth oxide as precursors. Energy dispersive X-ray spectroscopy data showed that the samples were not completely stoichiometric and that the F/O ratio decreased with increasing synthesis temperature. The X-ray diffraction data confirmed that the phosphor powder crystallized in the tetragonal structure for lower synthesis temperatures up to 1000 C. When prepared at 1100 C, the LaOF took the rhombohedral form which showed almost no luminescence when doped with Bi. Therefore the investigation focussed on the tetragonal phase of LaOF for which scanning electron microscopy data showed that the shape of the particles did not change when it was doped with Bi. A single broad white luminescence emission band centred at 518 nm was observed by exciting the phosphor at 263 nm. The optimum Bi doping concentration was found to be La1-xOF:Bix=0.005. This sample was stable when exposed to the atmosphere for up to six months, although its luminescence degraded slowly under a 254 nm ultraviolet lamp. XPS data confirmed that the annealing at 1100 °C and exposure of the sample to UV radiation result in segregation of the Bi3+ on the surface of the sample. Bi3+ doped LaOCl and LaOF phosphor thin films were successfully prepared via the pulsed laser deposition technique in vacuum and different Argon (Ar) pressures. All peaks of the XRD patterns of the films were consistent with the tetragonal structure of the LaOCl and LaOF, but in the case of LaOF the signal was weaker and not all peaks were present, suggesting some preferred orientation. The elemental composition of the films were studied by using energy dispersive X-ray spectroscopy, while plan-view and cross-sectional scanning electron microscopy was used to study the morphology of the films. Photoluminescence measurements revealed that the films exhibited emission around 344 nm for LaOCl:Bi and 518 nm for LaOF:Bi under excitations of 266 nm and 263 nm, respectively. The emission peaks were attributed to the 3P1 → 1S0 transition of the Bi3+ ions. The luminescence from the LaOF:Bi sample was less intense compared to the LaOCl:Bi sample prepared under the same conditions, which was also the case for powder samples. The amount of ablated material present on the substrate was much less for LaOF:Bi compared to LaOCl:Bi, which is attributed to the greater bandgap and hence weaker absorption of the laser pulses for LaOF:Bi. La2-yYyO3 phosphor powders were prepared via the citric acid sol-gel combustion method and their stability was checked after four months of storage in the atmosphere using X-ray diffraction. The material, like La2O3, was prone to hydroxylation unless a high proportion of Y was added, which also caused changes in the phases present. Based on its stability and single phase (cubic), the composition La0.5Y1.5O3 was selected for further study with Bi doping. Under an excitation of 333 nm the phosphor presented two emission peaks located at 424 and 529 nm. This corresponded to excitation of Bi3+ ions in the non-centrosymmetric C2 symmetry sites. Blue (424 nm) emission could be excited from the same samples when excited at 371 nm from Bi3+ ions on the centrosymmetric S6 sites, which was similar in colour to the emission of Bi-doped La2O3. The optimum doping concentration for La0.5-xY1.5O3:Bi powder was found to be x = 0.005. The luminescence from this material was not as intense as that from Y2-xO3:Bix=0.005, but there was a significant shift in colour as a result of the added La which meant that emission from the Bi3+ ions at the S6 sites of the alloy was close to that of unstable La2O3 on the colour chart. Bismuth (Bi3+) doped lanthanum gallate (LaGaO3) phosphor powder and thin films were successfully synthesised via the citric acid sol-gel combustion synthesis and pulsed laser deposition (PLD), respectively. Firstly the stability of La2-yGayO3 powders under ambient conditions was assessed. The optical properties of La1-xGaO3:Bix phosphor powders were then investigated. X-ray diffraction data confirmed that the structure of LaGaO3 belong to the orthorhombic perovskite structure with Pbnm space group, while diffuse reflectance spectroscopy data showed that the band gap energy of the LaGaO3 host was 4.65 eV. Scanning electron microscopy data showed that the grain size increased with increasing annealing temperature. There was no change in the particle size and morphology when dopant was added to the host. Photoluminescence (PL) and cathodoluminescence (CL) measurements indicated that the phosphor presented efficient ultraviolet emission around 370 nm, which was attributed to transitions between the 3P1 excited state and 1S0 ground state of the Bi3+ ions. The optimum Bi doping concentration and annealing conditions for photoluminescence were found to be for La1-xGaO3:Bix=0.003 and 1200 °C. Thin films of the optimized powder were prepared via PLD. The PL and CL of the LaGaO3:Bi3+ thin films were similar to the powder.
Open Access
Experimental studies and modelling of surface loss during segregation
(University of the Free State, 2015-01) Cronje, Shaun; Roos, W. D.; Kroon, R. E.
Afrikaans: Metallurgiese produkte speel ‘n onontbeerlike rol in die alledaagse lewe. Die soeke na metale met beter eienskappe soos sterkte, slytasie- en korrosieweerstand asook metodes om vervaardigingskostes en – tye te verminder duur steeds voort. Die invloed van onsuiwerhede in metale is van besonderse belang. Tydens die vervaardigingsproses van metallurgiese produkte is dit algemeen om een of ander hittebehandelingsproses te volg. By hoë temperature is die atome van onsuiwerhede, sonder uitsondering, mobiel en kan dus na korrelgrense en ander oppervlakke diffundeer, wat weer ‘n groot invloed op die eienskappe van die materiale het. Hierdie herverdeling van atome tussen die oppervlak en die bulk van die materiaal staan bekend as segregasie. Die segregasie van Sb uit ‘n Cu(100)-kristal, gedoteer met 0.05 at% Sb, is eksperimenteel gemeet. Tydens ‘n liniêre temperatuurveeg, is die oppervlak-konsentrasie van Sb met behulp van Augerelektronspektroskopie gemeet. Die segregasieprofiel is gepas met behulp van die Gemodifiseerde Darkenmodel en die volgende segregasieparameters is onttrek: 𝐷0 = 1.5x10-5 m2.s-1, 𝐸 = 177.0 kJ.mol-1, Δ𝐺 = -89 kJ.mol-1 en Ω = -3 kJ.mol-1. Hierdie data is ‘n waardevolle toevoeging tot vorige gemete segregasieparameters van Sb na Cu-oppervlakke met verskillende oriëntasies. In ‘n aantal binêre-legerings is die oppervlakverdamping tydens segregasie-eksperimente gerapporteer. Die invloed van verdamping tydens segregasiestudies het egter baie min aandag gekry. In besonder het onlangse segregasiemodelle die invloed van segregant-verdamping geïgnoreer. In hierdie studie is ‘n gemodifiseerde weergawe van die Hertz-Knudsenvergelyking gebruik om die Gemodifiseerde Darkenmodel te verbeter. Hierdie rekenaarprogrammetuur stel navorsers in staat om beide die kinetika- en ewewigsegregasie, met inagneming van die oppervlakverdamping van die segregant, te voorspel. Die invloed van die verdampingsparameter wat in die Hertz-Knudsenvergelyking ingevoer is, word bespreek en toon die sensitiwiteit van die segregasieprofiel vir selfs baie klein verdampingstempos. Dit word duidelik getoon dat indien verdamping geïgnoreer word onakkurate segregasieparameters gegenereer kan word. Riglyne word gegee hoe om segregasieparameters, wat met behulp van die Gemodifiseerde Darkenmodel, sonder verdamping onttrek is, te korrigeer. Die interpretasie van die segregasieparameters, in terme van verdamping, word ook bespreek. Veranderinge is aangebring aan ‘n Augerelektronspektroskopie-instrument om verdamping met behulp van ‘n Inficon XTC/3s deponeringsbeheerder te meet. Hierdie veranderinge en voorgestelde eksperimentele prosedures, maak voorsiening vir die verstuiwing en analise van die oppervlak wat vir die verdampingsstudies gebruik word. Deur van hierdie sisteem gebruik te maak is die verdampingstempo van suiwer Sb gemeet. Alhoewel daar kwalitatiewe ooreenkomste rakende die verdampingsgedrag verkry is, is gevind dat die oorspronklike Hertz-Knudsenvergelyking die verdampingstempo ver oorskat. Geen verdamping van Sb kon vanaf die gedoteerde Cu(100)-oppervlak gemeet word nie, aangesien die verdampingsvloed laer as die deteksielimiet van die instrument was. Dit word toegeskryf aan die baie lae verdampingstempo van Sb, soos in hierdie studie vir suiwer Sb waargeneem is, indien vergelyk word met voorspellings deur die Hertz-Knudsenvergelyking. Nogtans toon vergelykings tussen die eksperimentele segregasieprofiel en segregasieprofiele wat met behulp van die opgedateerde rekenaarprogram gegenereer is, dat Sb verdamping wel vanaf die Cu(100)-oppervlak tydens segregasie plaasvind. Hierdie studie toon duidelik hoe belangrik dit is om verdamping tydens segregasiestudies in ag te neem. Die studie lê ‘n belangrike fondament vir toekomstige verdampingstudies vanaf die oppervlakke van metale tydens segregasie, deur die nodige apparatuur in ‘n Augersisteem te monteer, ‘n eksperimentele protokol daar te stel en die opgradering van ‘n rekenaarprogram van die bestaande Gemodifiseerde Darkenmodel om segregasieprofiele te simuleer.
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
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
An image guided adaptive radiotherapy strategy for cervix cancer treatment based on equivalent uniform dose
(University of the Free State, 2021-11) O’Reilly, Frederika Hendrika Jacoba; Shaw, W.; Du Plessis, F. C. P.
Aim: This study aims to develop an image-guided adaptive strategy to compensate for temporal effects of tumour shrinkage and organ motion in cervix cancer treatment based on the equivalent uniform dose (EUD). The strategy should be flexible enough to retain a minimum workload system. Materials and Methods: Patients receiving radical radiotherapy treatment for cervix cancer with daily pre-treatment CBCT imaging were included in this retrospective study. A thorough investigation of bladder volume variation and its influence on primary and nodal tumor motion using both empty and full bladder protocols during cervix cancer RT was performed. 6 treatment strategies; fixed margins (7, 10, 15, and 20 mm), offline and online adaptive ART strategies, an ITV approach, variable margin (VBM) and coverage probability (CoVP) strategy were simulated for both groups. Planning strategies were scored based on the equivalent uniform and dose volume histogram parameters. Results and Discussion: Although empty bladder volumes were more reproducible no significant difference in movement between EB and FB patients was observed. Our study confirmed dosimetric advantages in bladder and small bowel sparing when a full bladder protocol is used. Occupancy probability maps were used to quantify primary and nodal movement. The best target coverage and OAR sparing were seen when online and offline adaptive strategies were used. Fixed margins <10 mm caused underdosages in most patients. Although adequate target coverage was noted for the ITV and VBM strategy, OAR sparing was inferior. CovP planning resulted in dosimetric outcomes similar and even better than the online strategy. Conclusion: Considering bladder volume reproducibility and protocol executability, an empty bladder protocol appears simpler in our department. However, variations in target shape and position cannot be attributed to bladder filling alone. The EUD can be used as a quick and reliable way of scoring treatment plans. Based on EUD metrics this study demonstrated that a personalized on-line adaptive strategy is most effective to account for target motion and deformation, yielding sufficient target coverage with adequate OAR sparing.
Open Access
Investigation of the luminescent properties of metal quinolates (Mqx) for use in OLED devices
(University of the Free State, 2014-01) Duvenhage, Mart-Mari; Swart, H. C.; Ntwaeaborwa, O. M.
Since Tang and VanSlyke developed the first organic light emitting diode (OLED) in the late 80’s using tris-(8-hydroxyquinoline) aluminium (Alq3) as both the emissive and electron transporting layer, a lot of research has been done on Alq3 and other metal quinolates (Mqx). The optical, morphological and electrical properties of these Mqx have been studied extensively. Alq3 has, however, a disadvantage as it tends to degrade when stored under atmospheric conditions. These degraded products are non-luminescent and lead to poor device performance. A good understanding of what happens during the degradation process and ways of eliminating this process are needed. In this study different Mqx compounds were synthesized and their degradation behavior was studied to see what effect it has on their luminescent properties. One way to tune the emissive colour of Alq3 is to introduce electron-withdrawing or electron-donating groups (EWG and EDG) onto the hydroxyquinoline ligands. These groups have an effect on the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. In this study Alq3 powders were synthesized with an EDG (-CH3) substituted at position 5 and 7 ((5,7-dimethyl-8-hydroxyquinoline) aluminium) (5,7Me-Alq3) and EWG (-Cl) at position 5 ((5-chloro-8-hydroxyquinoline) aluminium) (5Cl-Alq3). A broad absorption band at ~ 380 nm was observed for un-substituted Alq3. The bands of the substituted samples were red shifted. The un-substituted Alq3 showed a high intensity emission peak at 500 nm. The 5Cl-Alq3 and 5,7Me-Alq3 samples showed a red shift of 33 and 56 nm respectively. Optical absorption and cyclic voltammetry measurements were done on the samples. The optical band gap was determined from these measurements. The band gap did not vary with more than 0.2 eV from the theoretical value of Alq3. The photon degradation of the samples was also investigated and the 5,7Me-Alq3 sample showed the least degradation to the UV irradiation over the 24 h of continuous irradiation. By encapsulating the Alq3 molecule with glass (SiO2) or a polymer-like polymethyl methacrylate (PMMA), the oxygen and moisture responsible for degradation have a lesser effect on the degradation of the Alq3 molecule. The as prepared SiO2-Alq3 sample’s emission was blue shifted by 10 nm from that of Alq3. The sample was subjected to UV irradiation and after 24 hours, no luminescence intensity was detected. According to literature the SiO2 will decompose into Si and O species under UV irradiation. These O species have reacted with the Alq3 to form non-luminescent products. The Alq3:PMMA samples showed a maximum emission at 515 nm. There was a decrease in luminescence intensity when the sample was irradiated with UV photons. This was due to the decomposition of PMMA into elemental species and the O again reacted with the Alq3 molecule to form non-luminescent products. However, the intensity stabilized after 100 h of irradiation. X-ray photoelectron spectroscopy (XPS) and infra red (IR) measurements were done on the as-prepared and degraded Alq3 samples. It revealed that the Al-O and Al-N bonds stayed intact, but C-O and C=O bonds formed during degradation, indicating that the phenoxide ring ruptures during degradation. It is known that the luminescent centre of the molecule is located on the quinoline rings and the rupturing of one of these rings will destroy this centre, leading to a decrease in luminescence intensity. When the Al3+ ion was replaced with a Zn2+ ion to form Znq2, it showed higher emission intensity and, compared to Alq3, did not degrade as fast. This might be due to the fact that Znq2 only has two quinoline rings. The effect of solvent molecules, in the solid state crystal lattice, on the photoluminescence properties of synthesized mer-[In(qn)3].H2O. 0.5 CH3OH was studied. Single crystals were obtained through a recrystallization process and single crystal x-ray diffraction (XRD) was performed to obtain the unit cell structure. The main absorption peaks were assigned to ligand centered electronic transitions, while the solid state photoluminescence excitation peak at 440 nm was assigned to the 0-0 vibronic state of In(qn)3. Broad emission at 510 nm was observed and was ascribed to the relaxation of an excited electron from the S1-S0 level. A powder sample was annealed at 130 °C for two hours. A decrease in intensity was observed and could possibly be assigned to a loss of solvent species. To study the photon degradation, the sample was irradiated with an UV lamp for ~ 15 hours. The emission data was collected and the change in photoluminescence intensity with time was monitored. High resolution XPS scans of the O-1s peak revealed that after annealing, the binding energy shifted to lower energies indicating a possible loss of the H2O and CH3OH present in the crystal. The O-1s peak of the degraded sample indicated the possible formation of C=O (~ 532.5 eV), C-O-H and O=C-O-H (~ 530.5 eV) on the phenoxide ring. Commercial Alq3 is normally used in the fabrication of OLEDs. In this study Alq3 was synthesized using a co-precipitation method and it was purified using temperature gradient sublimation. The Alq3 was then used to fabricate a simple two layer OLED with a device structure: ITO/NPB/Alq3/Cs2CO3:Al. The electroluminescence (EL) spectrum of the device consisted of a broad band with a maximum at ~520 nm and was similar to the photoluminescence (PL) spectrum observed from the synthesized Alq3 powder. The luminance (L)–current density (J)–voltage (V) characteristics of the device showed a turn on voltage of ~ 2 V, which was lower than the current density of the device fabricated using the commercial Alq3. The external quantum efficiency (ηEQE) and the power conversion efficiency (ηP) of the device were 1% and 2 lm/W, respectively.
Open Access
An investigation on surface segregation of S in Fe and a Fe-Cr alloy using computational models and experimental methods
(University of the Free State, 2014-11) Barnard, Pieter Egbert; Terblans, J. J.; Swart, H. C.
English: A systematic investigation is conducted to determine the influence of the microscopic effects of the bcc Fe lattice on the segregation parameters, Q, D0, ΔG and Ω. These microscopic effects include the dependence of the surface orientation on the activation energy of diffusion, Q, and the layer dependence of the segregation parameters in the surface (atomic layer 1) and near surface atomic layers (atomic layers 2-4). The formation of vacancies in the low-index orientations of bcc Fe namely: Fe(100), Fe(110) and Fe(111) were considered to form via the Schottky defect mechanism. This mechanism resulted in an orientation dependence of the vacancy formation energy and also the activation energy of diffusion. Bulk activation energies for the segregation of Sulphur (S), as calculated by Density Functional Theory (DFT), for the Fe(110), Fe(100) and Fe(111) orientations are 2.86 eV (276 kJ/mol), 2.75 eV (265 kJ/mol) and 1.94 eV (187 kJ/mol) respectively. Experimental data obtained by Auger Electron Spectroscopy (AES) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) confirmed the orientation dependence of the activation energy of diffusion. Furthermore, AES results revealed the orientation dependence of the pre-exponential factor (D0), the segregation energy (ΔG) and interaction parameter (Ω). DFT calculations are performed to investigate the layer dependence of the segregation parameters in the first 4 atomic layers of Fe(100), a phenomenon termed the “surface effect”. Results indicate that all the segregation parameters depend on the atomic layer in which either the S or Chrome (Cr) impurities reside. Both S and Cr have very small activation energies of respectively 1.39 eV (134 kJ/mol) and 1.62 eV (156 kJ/mol) for segregation from atomic layer 2 to 1. These low activation energies are responsible for the surface “dumping effect”, whereby S and Cr were “dumped” into the surface layer. S segregated from atomic layer 3 to 2 with an activation energy of 2.97 eV (287 kJ/mol), the highest activation energy value for the crystal and the rate limiting factor for S segregation in Fe(100). Cr had the highest activation energy for segregation from atomic layer 4 to 3 with a value of 4.16 eV (401 kJ/mol) forming the rate limiting step for Cr segregation in Fe(100). Segregation energies of S are observed to increase from a 0.00 value in atomic layer 5 to a positive value of 0.07 eV (6.51 kJ/mol) in atomic layer 3 and a value of 0.21 eV (20.7kJ/mol) in atomic layer 2. Atomic layer 1, the surface layer, has a negative segregation energy of -1.93 eV (-186 kJ/mol) indicating the favourable segregation of S to the Fe(100) surface. Cr segregation energies increase monotonically from the bulk up to atomic layer 2, with a value of 0.47 eV (45.3 kJ/mol), and then decrease to a value of 0.18 eV (17.6 kJ/mol) in the surface layer. Thus, segregation of Cr in Fe is observed to be unfavourable due to the positive segregation energies. The interaction energies obtained for S and Cr confirms the behaviour predicted by the segregation energies, with S being a strong segregant and Cr segregation being unfavourable. Simulations incorporating the segregation parameters, calculated by DFT, in combination with the Modified Darken Model (MDM) reveals the macroscopic segregation of S in Fe(100) and the desegregation of Cr in Fe(100). Segregation experiments performed by AES on the Fe(100) and Fe(111) single crystals confirms the layer dependence of the segregation parameters. Fitting of the MDM to the segregation data of S in Fe(100) and Fe(111) shows that the conventional MDM fails to provide a truly accurate description of the segregation profile. Incorporation of the layer dependence, the “surface effect”, of the segregation parameters provides an accurate description of the observed segregation data. Segregation of S and Cr is studied in the ternary Fe-Cr-S alloy by TOF-SIMS measurements. Results reveal the segregation of Cr as a result of Cr and S co-segregating towards the surface. At high temperatures (> 900 K) S desegregates into the bulk lattice while the concentration of Cr in the surface layer is observe to increase. This observed cosegregation of Cr and S in Fe is explained by the interaction parameters between Cr and S as calculated by DFT. In the bulk lattice Cr and S experience a strong positive interaction resulting in S “drawing” Cr from the bulk towards the surface. In the surface layer these two species however experience a strong negative interaction resulting in the desegregation of S. These results provide a possible explanation of the observed discrepancies that exist in literature concerning the desegregation of Cr in Fe. Furthermore it provides evidence for the presence of the “surface effect” responsible for the layer dependency of the segregation parameters.
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
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
Luminescence enhancement of phosphors by doping with silver
(University of the Free State, 2015-10) Abbass, Abd Ellateef; Kroon, R. E.; Swart, H. C.
Phosphor materials doped with noble metals have attracted considerable attention for the past fifty years due to their possible applications in lighting and solar cells with improved efficiency, biology, lasers and novel display technologies. Active research has recently been focused on the interaction between noble metal nanoparticles and rare-earth ions in different phosphor hosts, with the aim of luminescence enhancement. Much attention has been paid to silver nanoparticles due to their strong absorption of electromagnetic radiation, resulting from localized surface plasmon resonance which can enhance the incident electric field by about two orders of magnitude. Although some reports have been published in regard to phosphors doped with silver, there is still room to better understand the interaction between silver and phosphors and to boost the efficiency of such phosphors. In this work, three different types of materials, namely amorphous silica, bismuth silicate and zinc oxide were used as hosts for silver and terbium. These hosts were selected due to their good physical properties and particularly because they have the appropriate refractive index, which is one of the main parameters required to control the plasmon absorption band for plasmonic enhancement. Doped and undoped amorphous silica and bismuth silicate were successfully prepared by the sol-gel method, while zinc oxide was prepared by the combustion method. The sources of the dopants used in this work were silver nitrate and terbium nitrate. The prepared phosphor powders were investigated by many techniques in order to apply appropriate conditions for phosphor enhancement. The structure, morphology and particle size were investigated by X-ray diffraction and transmission electron microscopy. Reflectance and absorption band of localized surface plasmon resonance were measured using a ultraviolet-visible spectrophotometer. X-ray photoelectron spectroscopy was used to investigate the composition of the phosphors, while optical properties were investigated using a fluorescence spectrophotometer having a xenon lamp or by exciting samples with a helium-cadmium laser. Firstly, doped and undoped amorphous silica was synthesized by the sol-gel method. The photoluminescence properties of amorphous silica doped only with silver as a function of annealing temperature were investigated in detail. The obtained results showed that the addition of silver after annealing at low temperature (500 °C) enhanced the luminescence associated with oxygen deficiency centres of the amorphous silica host, which is attributed to the formation of silver oxide. Increasing the annealing temperature to 1000 °C introduced new optically active centres in the amorphous silica. These new emission bands were related to excess oxygen due to decomposition of the silver oxide at high temperature. The additional luminescence band changed the blue emission from pure amorphous silica to near white light from the silver doped material suggesting that the silver doped silica system may be suitable for solid state lighting applications. The stability of this phosphor under ultraviolet irradiation was also investigated. To study the effect of addition of silver on the terbium luminescence, both terbium (5 mol%) and different silver concentrations were incorporated into amorphous silica using the sol-gel method. The obtained results showed significant enhancement of the terbium emission when 1 mol% silver was added after annealing at 500 °C. In previous works, the enhancement of rare-earth ion emission in the presence of silver was assigned to two possibilities, namely plasmonic enhancement associated with silver nanoparticles or energy transfer associated with silver ions. This work shows a third possibility, namely that enhancement of the rare-earth (e.g. terbium) emission is due to energy transfer from defects of the host material to the terbium ions, where the addition of silver influences the silica host defects. Secondly, powder samples of doped and undoped zinc oxide were successfully prepared by the combustion method. The photoluminescence properties of zinc oxide doped only with silver were studied in detail. More than a two fold increase in the intensity of near band edge emission of undoped zinc oxide was observed in the presence of silver nanoparticles. A new mechanism due to interaction between silver nanoparticles and zinc oxide has been proposed as being responsible for the enhancement of near band edge emission which is different from previous reports. In other samples, zinc oxide was doped with both terbium and silver. The addition of 1 mol% silver to the 5 mol% terbium doped zinc oxide system caused significant quenching on the terbium emission intensity instead of enhancement. The quenching effect is attributed to radiative energy transfer from terbium ions to silver nanoparticles (re-absorption) and was studied by means of spectral overlap and lifetime measurements. In the previous reports, researchers focused only on enhancement as a beneficial effect and considered quenching as a deleterious effect. In this work, the obtained results showed that the absorption of energy by silver nanoparticles (acting as energy acceptors) can also be beneficial in biological and polymer applications where local heating is desired i.e. photothermal applications. Another novelty of this work is that one can use the down-converting phosphor properties (containing, for example, rare-earth ions) as effective method to indirectly couple a laser to the plasmon resonance wavelength of metal nanoparticles without the need to change the particle size or shape of the nanoparticles, which requires special synthesis methods. Thirdly, bismuth silicate was synthesized using the sol-gel method and successfully doped with only terbium or silver, or co-doped with both. A simple way to select a suitable host material, when doped with any rare-earth ion and incorporated with silver nanoparticles, to cause overlap between an excitation band of the rare-earth ions and the localized surface plasmon resonance of the metallic nanoparticles in order to study possible plasmonic enhancement is presented using Mie theory calculations. Luminescence properties of the terbium doped bismuth silicate containing silver nanoparticles were explored in detail and an enhancement of the emission from the terbium ions at 545 nm when excited at 485 nm of about two and a half times is attributed to amplification of the electric field associated with the localized surface plasmon resonance of the silver nanoparticles. A particular novelty of the present work is the use of a crystalline host instead of an amorphous host to study plasmonic enhancement as in previous studies.
Open Access
Luminescence from lanthanide ions and the effect of co-doping in silica and other hosts
(University of the Free State, 2012-07) Ahmed, Hassan Abdelhalim Abdallah Seed; Kroon, E. R.; Ntwaeaborwa, O. M.
Amorphous silica powders doped with lanthanide ions were synthesised by the sol-gel method and their cathodoluminescence (CL) and photoluminescence (PL) emissions were compared. Interesting differences depending on the type of excitation were observed for Tb and Ce-doped samples. For Tb-doped samples blue 5D3®7FJ emission was measured during CL in samples for which PL results showed this emission to be concentration quenched due to cross-relaxation, while for Ce-doped samples luminescence occurred for CL but not during PL measurements. Unlike the other lanthanides, Tb and Ce ions are sometimes found in the tetravalent rather than the trivalent state, and these differences were attributed to the possibility of electron capture of tetravalent ions possible during CL but not PL. A scheme for the energy levels of divalent and trivalent lanthanide ions relative to the conduction and valence bands in silica was proposed, making use of experimental data and the known relative positions of the energy levels for the lanthanides. Although the location of the divalent europium ion f-level above the valence band can be located by using the charge transfer energy of trivalent europium, this process cannot be generalized to find the location of the trivalent cerium ion f-level above the valence band using the charge transfer energy of tetravalent cerium as has been suggested. Initial investigations of the luminescence properties of Ce doped silica were complicated by overlapping luminescence from oxygen deficiency defects from the host itself and the fact that Ce took the tetravalent state which is nonluminescent for PL measurements. Spectra obtained using a wide variety of excitation methods, including synchrotron radiation, were compared and evaluated in the light of previously published data. Radically improved results were obtained by annealing in a reducing atmosphere instead of air. X-ray photoelectron spectroscopy as well as ultraviolet reflectance spectroscopy provided evidence of the conversion of Ce from the tetravalent to trivalent state and this was accompanied by strong luminescence of these sample during PL measurements. Ce,Tb co-doped silica was used to study the energy transfer from Ce to Tb ions. Initial results were disappointing when measurements showed that adding Ce quenched the Tb emission intensity instead of increasing it. However, after annealing the samples in a reducing atmosphere, a quantum efficiency of 97% for energy transfer from Ce to Tb was achieved. The mechanism for energy transfer was investigated by comparing experimental measurements of the changes in donor (Ce) emission intensity and lifetime as a function of the amount of acceptor (Tb) with numerical simulations of various models. Measurements correlated well to models for dipole-dipole and exchange interactions, but the critical transfer distance obtained was not appropriate for the exchange interaction, hence dipole-dipole interaction was identified as the interaction mechanism. Nanocrystalline LaF3 powders were synthesized by the hydrothermal method and strong luminescence was obtained from samples doped with Ce and Tb. Photoluminescence spectra from co-doped samples revealed that the emission from Ce was quenched and the emission from Tb was enhanced, showing that energy transfer from Ce to Tb occurred. The energy transfer mechanism was investigated in a similar way as for the silica samples, but in this case the experimental results fitted models for the quadrupolequadrupole and exchange interactions. Much higher concentrations of Tb were required to significantly affect the Ce luminescence properties than in the case for silica, and the critical transfer distance obtained was appropriate for the exchange interaction. Either or both of these interaction mechanisms are therefore possible. The results show that the interaction mechanism for energy transfer between lanthanide ions depends not only on the ions themselves, but also on the lattice hosting them.
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
Luminescence investigations of CaS:Eu2+ powder and pulsed laser deposited thin films for application in light emitting diodes
(University of the Free State, 2015-06) Nyenge, Raphael Lavu; Ntwaeaborwa, O. M.; Swart, H. C.
The main objective of this thesis was to investigate the luminescent properties of commercial CaS:Eu2+ powder and pulsed laser deposited thin films for application in light emitting diodes. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and photoluminescence (PL) spectroscopy data suggest that the CaS:Eu2+ phosphors contain secondary phases that were possibly formed during the preparation or due to unintended contamination. An intense red PL broad band with a maximum at 650 nm was observed when the powder was excited at 484 nm using a monochromatized xenon lamp. When the powder was excited using a 325 nm He-Cd laser an additional PL emission peak was observed at 384 nm. The origin of this emission is discussed. Auger electron spectroscopy and Cathodoluminescence (CL) spectroscopy were used to monitor the changes in the surface chemical composition and CL intensity when the phosphor was irradiated with a 2 keV electron beam in vacuum. Possible mechanism for the degradation of CL intensity is presented. Thermal quenching in CaS:Eu2+ occurred at a relatively low temperature of 304 K. The kinetic parameters, namely activation energy and order of kinetics of γ-irradiated CaS:Eu2+ were determined using initial rise and peak shape methods, respectively. An Edinburgh Instruments FS920 fluorescence spectrometer equipped with a Xe lamp as the excitation source was used to collect emission and excitation spectra at low temperature. The samples were exposed to γ-radiation ranging from 10 to 50 Gy for thermoluminescence studies, from a 60Co source. The thermoluminescence data were obtained using a Harshaw thermoluminescence Reader (Harshaw 3500 TLD Reader). The possible mechanism leading to the decay of luminescence is explored. Pulsed laser deposited thin films of CaS:Eu2+ phosphor were grown on Si (100) or Si (111) substrates using the Q-switched Nd: YAG laser. For the purpose of this work, the deposition parameters which were varied during the film deposition are: laser wavelength, working atmosphere, number of laser pulses, deposition pressure, and substrate temperature. The film thickness, crystalline structure, surface morphology, and the photoluminescent properties of the thin films were found to be a function of the laser wavelength. The results from XRD showed that the as-deposited CaS:Eu2+ thin films were amorphous, except for the (200) diffraction peak observed from the films deposited at the wavelengths of 266 and 355 nm. The Rutherford backscattering (RBS) results indicate that film thickness depends on the laser wavelength used during deposition. Atomic force microscopy and scanning electron microscopy results show that the roughness of the samples is determined by the laser wavelength. The interaction of laser with matter is discussed, and the best wavelength for ablating this material is proposed. With RBS, it was possible to look at the variation of composition with depth as well as to determine the thickness of the thin films. Compositional analysis carried out using the energy dispersive X-ray spectroscopy showed that the films contained oxygen as an impurity. The films prepared in an oxygen atmosphere were amorphous while those prepared in a vacuum and argon atmosphere showed a degree of crystallinity. The roughness of the films has a strong influence on the PL intensity. The PL intensity was better for films in the argon atmosphere; showing bigger surface structures with respect to the other films. The emission detected at around 650 nm for all the films was attributed to 6 1 7 4 f 5d 4 f transitions of the Eu2+ ion. An emission at around 618 nm was observed, and was attributed to 2 7 0 5 D  F transitions in Eu3+, suggesting that Eu2+ was unintentionally oxidized to Eu3+. Results from time-of-flight secondary ion mass spectroscopy study show that all the films contain oxygen although the film prepared in oxygen contain more oxygen. The PL intensity of the CaS:Eu2+ films was found to depend on the pulse rate, with PL intensity increasing as the number of pulses is increased. XRD studies showed that there was an improvement in crystallinity of CaS:Eu2+ thin films upon post-deposition annealing, and subsequently an improvement on the PL intensity . PL intensity also improved significantly at a substrate temperature of 650oC. The best PL intensity as a function of deposition pressure was obtained at an argon pressure of 80 mTorr.
Open Access
Luminescence properties of Gd2O3:Bi3+ co-doped Ln3+ as powder and thin films phosphor for solar cell application
(University of the Free State, 2021-11) Abdelrehman, Mogahid Hassan Mohammed; Swart, H. C.; Kroon, R. E.
The main aim of this project is to study the luminescence properties of Gd2O3:Bi3+ co-doped Ln3+(Yb3+ and Er3+) phosphor powder and thin films to improve the energy conversion efficiency of photovoltaics (PV) by using the solar spectral conversion principle for use in solar cell applications. The luminescence properties of Gd2O3:Bi3+ single doped was investigated. The effect of different annealing temperatures and different concentrations of Bi3+ on the luminescence properties were investigated. The combustion method was used to synthesize all the powders samples. The optimum Bi3+ doping concentration was found to be x = 0.003 and the optimum annealing temperature was found to be 1000 °C (2 h). An increase in the average crystallite size with increasing annealing temperature and a decrease with an increase in Bi3+ doping concentration were obtained. The morphological studies showed small particles (less than 100 nm) that started to agglomerate to form bigger particles with sizes up to close to 400 nm during thermal treatment at higher temperature. Diffuse reflectance measurements of the pure host material showed absorption bands at 250, 275 and 315 nm that were attributed to the 4f-4f transitions of the Gd3+ ions. The bandgap was found to be influenced by the annealing temperature, which was determined to be 5.09 eV for the as prepared Gd2O3 host and then increased with increasing temperature. For all the doped samples the strong band at about 227 nm was observed corresponding to inter-band transitions of the Gd2O3 matrix in addition to three bands located around 260, 335, and 375 nm corresponding to the excitation transitions of Bi3+ ions into the different sites (C2 and S6). The luminescent properties of Gd2-xO3:Bix phosphor powder were investigated and the fluorescence spectra show that the luminescence was stimulated by the emission from two types of centers which exhibited efficient blue-green emission bands. The excitation and emissions that correspond to the 1S0→3P1 transitions in the Bi3+ ion depend on the presence of Bi3+ in the two different sites (S6 and C2) of Gd2O3. Significant changes in the thermoluminescence (TL) intensity for the different annealing temperatures and different dopant concentration in the Gd2O3 were observed. The TL glow curves of the UV-irradiated samples showed a low temperature peak at about 364 K and a high temperature peak at 443 K for all the samples. Auger electron spectroscopy (AES) was employed to analyse the surface chemical composition of the powder at a vacuum base pressure of 1.3 × 10−8 Torr and after back-filling with O2 to a pressure of 1.0 × 10−7 Torr. Simultaneous monitoring of the cathodoluminescence (CL) and AES peak-to-peak heights (APPHs) during prolonged electron bombardment in vacuum and O2 over time for 40 h was done. The CL emission of Gd2O3:Bi powder was found to be stable under electron irradiation. Thin films were successfully prepared using the pulsed laser deposition technique. Gd2-xO3:Bix=0.003 phosphor that was optimized for blue luminescence was deposited on Si (100) substrates in vacuum and an oxygen atmosphere at different substrate temperatures. The background atmosphere and substrate temperature were found to significantly affect the microstructure and luminescence of the thin films. The thicknesses for the thin films were relatively constant around 100 nm. The CL emission intensity degradation in a vacuum and an oxygen atmosphere was checked synchronously with the APPHs using the same electron beam for both measurements. The effect of the electron bombardment on the surface state of the samples was studied by using AES and X-ray photoelectron spectroscopy (XPS). All major elements (gadolinium and oxygen) were located, with additional carbon and chlorine that were removed during the early stages of electron bombardment. The CL intensity of the thin films with a blue-green emission was stable under electron bombardment. The Gd2O3 materials based on the Bi3+,Yb3+ phosphor powders were investigated for possible improvement of the photovoltaic conversion efficiency via spectral modification, utilizing the down-conversion (DC) process. The optical bandgap of Gd2O3 increased with additional doping. The DC emission was obtained successfully from Yb3+ co-doped Gd2-xO3:Bix=0.003 samples. The visible emission has blue and green bands related to the Bi3+ in the S6 and C2 symmetry of Gd2O3. The near-infrared (NIR) of DC emissions were centered at 976 nm and less intense peaks centered around 950 nm, 1025 nm and 1065 nm caused by the crystal field Stark splitting of the 2F5/2 and 2F7/2 energy levels corresponding to the Yb3+: 2F5/2 →2F7/2 transitions after absorption of a single UV photon. It was observed that the visible Bi3+ emission gradually decreased with the addition of Yb3+ ion concentration while the NIR emission increased due to energy transfer from Bi3+ to Yb3+ ions. There was a strong increase in DC emissions with doping by adding of Bi3+ ions compared with the samples doped by Yb3+ ions. Er3+ co-doped Gd2-xO3:Bix=0.003 was investigated for up-conversion (UC) processes for the possible use in applications for c-Si SCs. The 980 nm infrared excitation was successfully converted into visible (green-orange-red) emissions and was confirmed for Er3+ doped Gd2O3 samples with and without the presence of Bi3+. There was an increase in the UC emissions with the addition of Bi3+ compared to the sample doped with single Er3+ ions due to the enhancement in Er3+ emission. The UC emissions were observed at 520, 537, 560, 670 and 870 nm and were assigned to the 2H11/2, 4S3/2, 4F9/2 and 4I9/2→ 4I15/2 transitions of the Er3+ ion. The results showed that the presence of Bi3+ ions improved the UC emission of the Er3+ ions. The effect of different concentrations of Yb3+ on the luminescence of Gd2O3:Bi3+, Er3+ phosphor powder was investigated successfully. The structure and morphology of the surfaces revealed that the phosphors were affected by an increasing concentration of the Yb3+ ions. There was a decrease in the crystallite size with an increase in the Yb3+ doping concentration. The successful UC emission spectra of Gd1.977-xO3:Bi0.003, Er0.02, Ybx (x = 0.0, 0.01, 0.03, 0.06, 0.09 and 0.12) were studied under 980 nm infrared excitation. The UC visible emission spectra consisted of a series of green, red and NIR emission bands at 520, 537, 550, 560 and 670 nm and are ascribed to the 4F5/2, 4F7/2, 2H11/2, 4S3/2 and 4F9/2→ 4I15/2 transitions of the Er3+ ions, respectively. The intensity of the UC emission of the green, red and NIR emission bands of the Gd2O3:Er3+ was dependent on the introduction of the Bi3+ and Yb3+ into the matrix. Co-doping by Bi3+ ions strongly enhancing the UC green emission, while co-doping with Yb3+ ions enhanced the UC red emission intensity. The intensity for both the visible emission spectra of Er3+ monitored by using a 379 nm excitation wavelength and CL at 5 keV exhibited a reduction in the intensity with an increase in the Yb3+ concentration due to the energy transfers from Er3+ to Yb3+ by cross-relaxation in contrast to the UC emission results. Under the excitation at 375 nm the emissions were centered at 418 nm corresponding to the 3P1 → 1S0 transition of the Bi3+ ions and in addition the peaks mentioned earlier corresponding to the transitions of the Er3+ ions. Our results indicated that the obtained phosphors might be possibly used for applications in displays, lighting and as a luminescence layer used to modify the solar spectrum with the aim of improving the efficiency of solar cells.