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Item 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 SeedLaOCl: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.Item 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.Item 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.Item Open Access Material Properties of RE- Doped Ln (Ln= Y, La) oxides and oxysulfides phosphors for red-emitting devices(University of the Free State, 2015-11) Ali, Abdub Guyo; Dejene, F. B.; Swart, H. C.Structural and optical properties of Eu3+-doped Ln (Ln=Y, La) oxide and oxysulfide nano crystals synthesized by sol-combustion method were analysed as a function of host to fuel ratio. Structural characterization shows crystallite nanosized particles and the hexagonal phase as the dominant structure. The red emission of Eu3+ doped Y 20 2S, La20 2S and Y 20 3 nanocrystals appearing near 624 nm was assigned to the 5Do-7F2 transition of Eu3+. Due to insufficient quantities of thiourea at the higher Ln/S mole ratio, the bright red emission has been quenched. Fourier-transform infrared spectrometry analysis showed that there was a negligible difference in the absorbed impurities with various molar ratios. The Ln/S concentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs for Ln/S=I to 76 μs for the higher concentrations. Structural and optical properties of La20 2S:Eu3+micro crystals synthesized by sol-combustion method were analyzed as a function of La/S concentration. Structural characterization shows a crystallite size of about 178 nm and the hexagonal phase as the dominant crystalline structure. The red emission of Eu3+ doped La20 2S microcrystals appearing near 624 nm was assigned to the 5D0- 7F2 transition of Eu3+. Due to insufficient quantities of thiourea at the higher La/S mole ratio, the bright red emission has been quenched. Fourier-transform infrared spectrometry analysis showed that there was a negligible difference in the absorbed impurities with various molar ratios. The La/S concentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs for La/S= I to 7 6 μs for the higher concentrations. To investigate the effect of co-doping a series of red-emitting phosphors Y 20 3:Eu3+:Ho3+ were prepared by the solution combustion method. X-ray diffraction (XRD) patterns indicate that the Eu3+ and Ho3+ doping do not show obvious effect on the cubic Y 20 3 crystal. Their crystall ite size estimated by x-ray diffractometry and scanning electron microscopy was about 8 nm. Under UV 325 nm excitation, emission wavelengths at 626 nm was quenched at higher mole percent of Ho3+ and energy was transferred from Eu3 ~ to Ho3+. Y20 3:Eu3+: Ho3+ phosphor shows a red-emitting afterglow phenomenon, and the Eu3 ' ions are the luminescent center during the decay process. The bright red emission near 626 nm has been noticeable due to the 5D0- 7F2 transition of Eu3 ... . The intensity of the luminescence has decreased with an increase of concentration of Ho3+. In sufficient quantities of Eu3+ to Ho3+, the bright red emission near 626 nm has been predominant due to 5D0- 7F2 transition of Eu3+. The decay characteristic of Y 20 3:Eu3 +: Ho3 + phosphor is according with the double exponential equation. The as-prepared powder Y 20 2S:Eu3 + was deposited on Si ( 100) substrates by using a pulsed laser deposition technique. The thin films grown under different oxygen deposition pressure conditions have been characterized using structural and luminescent measurements. The Xray diffraction patterns showed mixed phases of cubic and hexagonal crystal structures. As the oxygen partial pressure increased, the crystallinity of the films improved. Further increase of the 0 2 pressure to 140 mtorr reduced the crystallinity of the fi lm. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that an increase in 0 2 pressure affected the morphology of the films. The average band gap of the films calculated from diffuse reflectance spectra using the Kubeika-Munk function was about 4.75 eV. The photoluminescence measurements indicated red emission of Y20 2S:Eu3 + thin films with the most intense peak appearing at 619 nm, whi ch is assigned to the 5Do-7F2 transition of Eu3 +. This most intense peak was totally quenched at higher 0 2 pressures. X-ray photoelectron (XPS) indicated that Y20 3 thin films are formed on the surfaces of the Y20 2S: Eu3 + thin films during prolonged electron bombardment. The films grown in a lower 0 2 ambient consist of smaller but more densely packet particles relative to the films grown at a higher 0 2 ambient. In order to study the effect of annealing temperature on the films, four samples were annealed at various temperatures while one was kept unannealed. X-ray diffraction measurements show that the un-annealed thin film was amorphous, while those annealed were crystalline. At lower annealing temperature of 600 °c to 700 °c cubic bixbyite Y20 3:Eu3 + was formed . As the annealing temperatures were increased to 800 °c, hexagonal phase emerged. The average crystall ite size of the fi lm was 64 nm. Photoluminescence (PL) measurement indicates intense red emission around 612 nm due to the 5 00~ 7F2 transition. Scanning electron microscopy (SEM) indicated that agglomerates of non-crystalline particles with spherical shapes were present for the un-annealed film s. After annealing at high temperature, finer morphology was revealed. Atomic fo rce microscopy (AFM) further confirmed the formation of new morphology at the higher annealing temperatures. UV-vis measurement indicated a band gap in the range of 4.6 to 4.8 eY. It was concluded that the annealing temperature played an important role in the luminescence intensity and crystallinity of these films. To investigate the effect of different species of gases Y20 2S:Eu3 + thin films have been grown on Si ( I 00) substrates by using a pulsed laser deposition technique. The thin films grown under different species of gases have been characterized using structural and luminescent measurements. The X-ray diffraction patterns showed mixed phases of cubic and hexagonal crystal structures. The crystallinity of the film deposited in vacuum is poor, but improved significantly in argon and oxygen atmosphere. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that different species of gases affected the morphology of the fi lms. The average band gap of the films calculated from diffuse reflectance spectra using the Kubeika-Munk function was about 4.69 eV. The photoluminescence measurements indicated red emission of Y20 2S:Eu3+ thin films with the most intense peak appearing at 6 12 nm, which is assigned to the 5D0- 7F2 transition of Eu3+. The intensities of this most intense peak greatly depend on the species of gas with argon having the highest peak. This phosphor has applications in the flat panel displays.Item Open Access Study on luminescence and structural properties of vanadates phosphors(University of the Free State, 2016-01) Foka, Kewele Emily; Dejene, B. F.; Swart, H. C.A self-activated yellow emitting zinc vanadate (Zn2 Y20 1) was synthesized by combustion method. The influence of the processing parameters such as synthesis temperature and dopants concentration on the structure, morphology and luminescence properties was investigated. The X-ray diffraction (XRD) analysis confirmed that the samples have a tetragonal structure and no significant structural change was observed in varying both the synthesis temperature and the dopants concentration. The estimated average grain size was 78 nm for the samples synthesized at different temperatures and 77 nm for the doped samples. Scanning electron microscope (SEM) images show agglomerated hexagonal-like shape particles with straight edges at low temperatures and the shape of the particles changed to cylindrical-like structures at moderate temperatures but were destroyed at higher temperatures. The microstructure retained its original structure when the phosphor was doped with Ba, Ca and Sr. The photoluminescence (PL) of the product exhibited broad emission bands ranging from 400 to 800 nm. The best luminescence intensity was observed for the undoped Zni Y20 1 samples and those synthesized at 600°C. Any further increase in synthesis temperature and concentration of dopants, respectively, led to a decrease in the luminescence intensity. The broad band emission peak of Zn2 Y201 consist of two broad band's corresponding to emission from the Em1 (3T2-1A1) and Em2(3T1 - 1A1) transitions. The Zn2 Y20 1 phosphor was prepared by a sol-gel method. The effect of annealing temperature on the structure and photoluminescence of Zn2 Y201 was investigated. The XRD results showed the single monoclinic phase of Zn2Y201. The crystallinity of the Zn2Y201 phosphor improved while the full width at half maximum of (022) XRD peak was decreased with the increase in annealing temperature. SEM showed that the grains size increased with the increase in annealing temperature, which is due to the improvement in crystallinity of Zn2 Y201. Thermal behaviour of the Zni Y20 1 phosphor was investigated by Thermogravimetric analysis (TOA) and differential scanning calorimetry (DSC), respectively. TOA results showed a total weight loss of 65.3% when temperature was tisen from 35 to 500°C. The photoluminescence emission spectra of annealed Zn2 V201 powders showed a broad band emission from 400 to 800 nm. The PL intensity enhanced as the annealing temperature was increased, resulting to an improvement of the crystallinity. PL emission peaks shift from green emission towards a yellow emission. Dy doped YVQ4:0y3+ phosphors were produced by the combustion method at 600°C. The structure and optical properties of the powders were investigated. The XRD patterns showed the tetragonal phase similar to the standard JCPD file (1 7-0341). SEM shows that the particle sizes were small and agglomerated, and the size increased with the o y3+ dopant concentration and its shape changed to bulk-like particles. In PL, the emission spectra exhibited a weak band at 663 nm for the 4F 912 - 6H1 1/2 transition and a peak at 483 nm (blue) for the4F912 - 6H1 s12 transition and a 574 run (yellow) peak with higher intensity for the 4F912 - 6H 1312 transition. The dependence of the properties of YV04:Dy3+ phosphor upon urea:nitrate concentration was investigated. The samples were synthesized by combustion method. The single tetragonal phase was observed by x-ray diffraction spectra. A highly crystalline YV04:Dy3+ sample was observed when increasing the ratio of the urea to 2. The estimated crystalline size were found to be 20, 39, 33, 30, and 27 nm for the sample prepared with the ratio of 1, 2, 2.5, 3 and 4, respectively. The formation of agglomerated particles was observed by SEM images and it was observed that when increasing the concentration of urea further the flake-like particles formed. The UV diffuse reflectance spectra of YVQ4:Dy3+ with various ratios of urea showed the determined optical band gap ranging from 3.3 to 2.3 eV. Luminescence properties of YV04:0y3+showed that the phosphor emit yellow colour at 573 nm and blue colour at 482 nm corresponding to 4 F912~6Hn12 and 4f912~6H1 s12. respectively. A very week band at 663 nm which correspond to 4 F912~6H1 1/2 transition was also observed. It was found that the PL emission intensity increases with an increase in the ratio of urea and reached maximum at 2 then decreases when increasing the ratio of urea further. YV04:Eu thin films were well deposited by pulse laser deposition at deposition temperature of 200, 300 and 400 °C. The oxygen pressure and deposition time were held constant. The films deposited at higher temperature showed a tetragonal phase. The XRD spectra for the sample deposited at 200 °C showed a very small peak at (200) orientation. Phosphor thin film showed a crystalline structure when the temperature increased. SEM images indicated larger particles at higher temperature. Atomic force microscopy (AFM) results showed the smooth surface with small particles at lower temperature and surface roughness at higher temperature due to the crystallinity. The PL shows the typical emission peaks of Eu in a red region at the 594 and 618 nm attributed to 5Do-7F 1 and 5Do-7F2, transitions. Also the peaks at 652 and 699 nm corresponding to 5Do-7F3 and 5Do-7f4 are observed. The spectra showed an increase in intensity when deposition temperature was increased. YV04:Eu3+ thin films were prepared by pulse laser deposition (PLD). YV04:Eu3+ thin films were deposited at room temperature by varying the deposition time from 30, 45 to 60 minutes. The XRD analysis confirmed that the samples have a tetragonal phase. The improved on crystallinity of the films was observed when increasing deposition time. The estimated grain particle size increased from 52 to 69 nm as the deposition time increased from 30 to 60 minutes, respectively. SEM images showed that when increasing the deposition time, particles were agglomerated and the formation of homogeneous surface was observed for a film deposited at 45 minutes. The rough surface with larger particles was observed for the sample deposited at 60 minutes. PL emission spectra of YV04:Eu3+ showed the main emission peaks which are due to the Eu3+ transition 5Dj-7fj. The strongest red emission peak at 618 nm is due to transition 5Do-7F2. The increased in deposition time showed the improvement in intensity of the thin films.Item Open Access Theoretical and experimental investigation on surface segregation of Cu-Ni(S) bulk and thin film alloys(University of the Free State, 2017-07) Yan, Xin Liang; Terblans, J. J.; Swart, H. C.In this study, the surface segregation in a Ni-Cu alloywasinvestigated by modeling the segregation process theoreticallyand also measuring it experimentally. This was performedfor abulkcrystaland thin films. In addition to the segregation measurements, the Ni/Cumultilayer thin films werealso used to study the interdiffusion of Cu and Ni with AES depth profiling.TheMRI model was used toquantitatively evaluate the sputtering-induced surface roughness and depth resolution for AES and SIMS depthprofiling. Depth profiles ofthe Ni/Cu polycrystalline multilayer thin films were performedby AES and SIMSin combination with ion sputtering. The measured depth profiles data were quantitative analyzed with the MRI model. The sputtering-induced surface roughness and depth resolutionwere evaluatefor sputtering with (i) a stationary sample, (ii) a rotating sample and (iii)a stationary sample with two ion beams simultaneously. The results showthat the depth resolution is smaller when profiling with dual-ion beam vs. a single-ion beam. It was also found that profiling with a lower ion energy result in a better (smaller) depth resolution. Rotation of the sample during ion sputtering had the better (smaller) depth resolution. Depth profiling with Cs+ ion sputtering had the best depth resolution compared to Xe+ and O2+ ion sputtering. The MRI model was also used for extracting the interdiffusion coefficients for the AES depth profiles of a Ni/Cumultilayer. The interdiffusion parameter for Cu/Ni multilayer thin films was characterized for the first interface Do =6.2×10-13 m2/s and Q =101.4 kJ/mol, and the last Cu/Ni interface Do=6.3×10-14 m2/s and Q =79.0 kJ/mol. It was clearly showing that the depth-dependent interdiffusion coefficients are depth-dependent. The segregation of Cu and S from a ternary Ni-Cu(S) bulk alloy was measured with AES using linear temperature programmed heating and constant temperature heating. The segregation data were fitted with the modified Darken model and the segregation parameters were DoCu in Ni = 8.6×10-14 m2/s, QCu in Ni = 145.2 kJ/mol, DoS in Ni = 9.2×10-2 m2/s, QS in Ni = 224.0 kJ/mol, ΔGCu = -36.0 kJ/mol, ΔGs = -136.0 kJ/mol, ΩCu-Ni = 7.6 kJ/mol, ΩS-Ni = 28.1 kJ/mol and ΩCu-S = -10.3 kJ/mol. The segregationsof Cu from a Ni-Cu thin film alloy were measured using AES with linear temperature programmed heating. The segregation measured profile data were fitted with a modified Fick’s model and the segregation parameters obtained Do= 2.8×10-13m2/s and Q= 135.3kJ/mol for the 26nm thin film and for thicker film (52nm) Do= 2.9×10-13m2/s and Q= 140.5 kJ/mol. The modified Darken model is there adapted to simulate segregation from thin films and also showed that the thickness of the thin film significantly affects the segregation.Item Open Access Origin of pulsar timing noise: possible correlations between pulsar spin-down rates and magnetospheric processes(University of the Free State, 2017-03) Maritz, Jacques; Meintjes, P. J.English: Pulsars are extremely accurate clocks that allow us to explore certain unanswered questions in the elds of exotic compact forms of matter and gravitational wave as- trophysics. Pulsars that form part of a timing array can be used to detect stochastic gravitational wave (GWs) backgrounds produced by merging super-massive black holes by searching for systematic correlated delays in the arrival times of the pulses over decades. However, these GW backgrounds produce a small amplitude variation in the timing residuals of a pulsar over decades. Similar to the residual signature produced by GW, timing noise also exhibits a quasi-periodic timing residual signa- ture due to some unidenti ed variations in the pulsar's spin parameters. This study focused on the analysis of the timing noise phenomena observed in PSR J1326-5859. Several decades of timing and polarization data were analyzed and correlated in an attempt to model the observed timing noise signature. We propose that PSR J1326-5859 is coupled to a fossil disk that torques the star in a quasi stable manner and produces the observed spin-down evolution and polarization state changes.Item Open Access Upconversion of infrared to visible light in rare-earths doped phosphate phosphors for photodynamic therapy application(University of the Free State, 2017) Mokoena, Puseletso Pricilla; Ntwaeaborwa, O. M.; Swart, H. C.𝑬𝒏𝒈𝒍𝒊𝒔𝒉 Phosphate phosphors have emerged as an important family of luminescent material due to their low sintering temperature, broad band gaps, high thermal and chemical stability, and moderate phonon energies. Their structure can provide a wide range of possible cationic substitutions since there are different inequivalent sites of metal ions presenting a large-scale of size and coordination spheres. Rare earth ions doped phosphate compounds as luminescence materials have been widely investigated in different host lattices including phosphates. In this study, the luminescent properties of different phosphate phosphors doped with Er3+, Eu3+, and Yb3+ were investigated. Er3+ and Yb3+ singly doped, and Er3+/Yb3+ co-doped Ba5(PO4)3OH phosphor powders were successfully synthesized by the urea combustion method. The X-ray Diffraction (XRD) patterns exhibited hexagonal structure for Ba5(PO4)3OH referenced in the ICDD (International Center for Diffraction Data) Card Number 00-024-0028. There were no peak shifts nor secondary peaks observed suggesting that pure phases were crystallized. The Scanning Electron Microscope (SEM) image showed that the particles were agglomerated together forming ellipsoidal shapes. The Energy Dispersive x-ray Spectroscopy (EDS) spectra with intense peaks of Ba, P, and O were observed confirming the formation of Ba5(PO4)3OH. The particle size distribution of the Ba5(PO4)3OH powder was estimated from a statistical analysis by measuring approximately 10 particles. The average particles length and width were 867 and 169 nm, respectively. Upon excitation using a 980 nm laser, multiple emission peaks in the green region and red region were observed corresponding to the transition of the Er3+ ion. By further co-doping with Yb3+ the red emission was enhanced due to energy transfer from Yb3+ to Er3+. Ba5(PO4)3OH co-doped with Eu3+ and Yb3+ phosphors were prepared by the urea combustion method. The diffraction peaks of Ba5(PO4)3OH were indexed to the pure hexagonal phase, referenced in ICDD Card Number 00-024-0028. The SEM images showed a change (ranging from rods, spherical, needle-like to non-uniform particles) in surface morphology which was due to annealing and addition of dopants. The size of the particles appeared to be larger/bigger when comparing as-prepared and annealed phosphor powders. This could be due to the annealing-induced expansion. The broad intense excitation peak at 240 nm and other excitation peaks located at ~319, 360, 382, 395 and 465-537 nm were assigned to transitions of Eu3+ ion. The emission peaks were observed at ~589, 614, 651 and 699 nm. Upon co-doping with Yb3+, the strong emission peak was observed at 657 nm assigned to the Eu3+ transitions. This was due to the cooperative energy transfer process. Er3+ and Yb3+ co-doped Ca5(PO4)3OH samples were synthesized by urea the combustion method. The XRD patterns of Ca5(PO4)3OH powders for both as-prepared and those annealed at 800 0C were attributed to the hexagonal phase of Ca5(PO4)3OH referenced in ICDD Card No. 00-073-0293. The SEM micrographs exhibited rod or plate-like morphology forming flowers, plate-like structures and small agglomerated particles on top of the plates. For Er3+ singly doped phosphors emission peaks were observed in the green region ranging from 517 -573 nm and red region in the range of 653- 679 nm. Ca5(PO4)3OH:Er3+ phosphors were prepared using different concentrations of Er3+ ranging from 1-7 mol.%. The photoluminescence intensity increased with increasing concentrations from 1 to 3 mol%, and decreased at high concentrations of 5 and 7 mol.% due to concentration quenching effects. Adding different concentrations (5-15 mol.%) of Yb3+. The emission intensities on both the green and red region increased with increasing concentrations of Yb3+ ions. The enhancement of green emission can be due to increasing of the three-photon energy transfer process probability between the Yb3+ and Er3+ ions. Ca5(PO4)3OH:Eu3+, Yb3+ phosphor powders were synthesized by the combustion method using urea as a fuel. The XRD patterns of Ca5(PO4)3OH powders for both as-prepared and those annealed at 800 0C were assigned to the hexagonal phase of Ca5(PO4)3OH referenced in ICDD Card No. 00-073-0293. The crystal sizes calculated for as-prepared and annealed powders were found to be 27 and 44 nm, respectively. UC emission spectrum of Ca5(PO4)3OH:Eu3+,Yb3+ phosphor powder was observed under 980 nm excitation. Prominent red emission from Eu3+ ion was clearly observed at 613 nm together with minor emission peaks at 547, 591, 654 and 697 nm. The prominent red emission from Eu3+ was due to energy transfer from Yb3+ ion. A cooperative energy transfer from Yb3+ ion pair to a single Eu3+ ion occurred by fast non-radiative relaxation to the metastable 5D0 state, and the red Eu3+ emission was observed. Sr5(PO4)3OH co-doped Er3+/Yb3+ phosphor powders were synthesized by combustion method. The XRD pattern diffraction peaks were consistent with the standard data referenced in ICDD Card No. 00-033-1348. The average crystallite size calculated was 43 ± 2 nm. The SEM micrographs showed that the powder was composed of agglomerated particles with edges forming hexagonal shapes. The agglomeration showed a porous structure resulting from the nature of the combustion reaction associated with the evolution of large volume of gases. Upon 980 nm excitation, Sr5(PO4)3OH:Er3+ exhibited multiple emission bands in the green region and a less intense peak in the red region. The strong red emission peak with two minor splits were observed at 661 nm, and (651 and 679 nm), by co-doping with Yb3+ ion. Sr5(PO4)3OH co-doped Eu3+/Yb3+ phosphor powders were synthesized by the combustion method. All the diffraction patterns matched with the standard data referenced by ICDD Card No. 00-033-1348. The SEM image showed that the powder composed of a network of particles with irregular shapes and small bright particles encrusted on the surface of the bigger particles. The particles containing heavy atoms in backscattered electron detector were stronger than light particles and they appear brighter. UC emission spectrum of Sr5(PO4)3OH:Eu3+,Yb3+ phosphor powder was observed under 980 nm excitation. Prominent red emission from Eu3+ ion was clearly observed at 658 nm due to cooperative energy transfer process. Photodynamic therapy uses special drugs, called photosensitizers, along with light to kill cancer cells. The drugs only works after been activated by certain kinds of light. Most drugs are activated by red light. The enhanced red luminescence from the above mentioned phosphors suitable to activate different photosensitizers for treatment of cancer or photodynamic therapy. Photodynamic therapy activity was performed using red emitting phosphors prepared in this study together with phthalocyanine as a photosensitizer. Phthalocyanine is activated by the wavelength ~670 nm. The activity results are discussed in chapter 10. ___________________________________________________________________Item Open Access Theoretical and experimental study of core-shell structured ZnO/ZnS and growth mechanism of un-doped and doped ZnO nanomaterials(University of the Free State (Qwaqwa Campus), 2017-12) Jule, Leta T.; Dejene, F. B.; Roro, K. T.There is currently widespread interest among researchers in ZnO-ZnS coreshell nanorods as electrodes in prototype solar cells. ZnS has been proposed as a suit- able inorganic sensitizer to ZnO because ZnO and ZnS when in intimate contact, form a type-II (staggered) heterojunction with 1:00 eV valence band o -set. Type II core shell nanorods should therefore act to separate electrons and holes radi- ally. This has been con rmed by density functional theory (DFT) calculations, which revealed an active separation of electron hole pairs after photo-excitation. Therefore these structures are similar to coaxial cables, because they allow the movement of the electrons through the core (i.e. ZnO) in one direction and the holes through the outer shell (i.e. ZnS) in the opposite direction. In this thesis, rapid synthesis of ZnO and controllable growth of ZnO/ZnS core-shell structures has been realized. Moreover, the e ect of dopants on the structural, optical, and its magnetic properties are investigated in detail. The nal product was analyzed using such techniques as scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy(steady and temperature dependent), Ultra-violet visible (UV-Vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), Electron paramagnetic resonance (EPR) and X-ray di raction (XRD). ZnO nanorod arrays were grown by a two-step chemical bath deposition pro- cess on (100) silicon substrates. ZnS coated ZnO nanorods were prepared by a simple, cost e ective, two-step chemical synthesis process. This method provides a continuous, uniform ZnS coating on ZnO nanorods at relatively low temperature. The optical properties of the core-shell(ZnO/ZnS) are explored including the case when the absorption of propagating wave by dissipative component is completely compensated by ampli cation in active (lasing) medium. Rapid synthesis of blue emitting ZnO nanoparticles for uorescent applications has been developed. In this method ZnO nanoparticles (NPs), with size 16 - 20 nm were produced using simple, cost e ective and rapid synthesis method. In this method zinc salt (typically zinc acetate dehydrate) is directly annealed in air at a temperature from 200 - 500 0C for 2 h to form ZnO (NPs). This synthesis method would be ideal for blue light emitting applications as it is catalyst free growth and only requires zinc precursor to produce NPs that can emit visible emission by scalable temperature. Cd doped ZnO nanopowder has been synthesized by facile sol-gel method. The modulation in optical band gap of the samples decreases from 3:15 eV to 2:76 eV are observed and it is believed to be responsible for the red shift in Ultra-violet visible (UV-Vis) spectroscopy with increase in Cd content. This is explained in terms of possibility of engineering band gap and in uencing physical, chemical, and electronic properties which provides a strong impetus to study nanocrystals and other nanodimensional materials. The method employed would be ideal to synthesize materials for devices operating in the visible region as well as for de- veloping heterojunction (Cd:ZnO) structures. Defect-induced room temperature ferromagnetic properties of the Al-doped ZnO (AZO) and undoped ZnO nanostructure synthesized by sol-gel method has been investigated. Electron Paramagnetic Resonance (EPR) spectroscopy which is an e ective tool to investigate the origin and nature of un-paired electrons in an atom shows the electron spin trapped in defected areas become randomly orientated at higher atomic percentages of Al. Based on PL and EPR analysis it was demonstrated that singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the undoped ZnO where as in Al doped ZnO it might be due to Al clustering forming Al-Al short range orders.Item Open Access 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, MartinAb 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.Item Open Access Structural and luminescence properties of re doped fluoride and silicate phosphors(University of the Free State (Qwaqwa Campus), 2017-04) Debelo, Nebiyu Gemechu; Dejene, F. B.This work covers several aspects of rare earth activated silicate and fluoride commercial phosphor powders and thin films. All the films were synthesized by pulsed laser deposition technique using Nd- YAG laser and characterized by different techniques with the sole aim of studying their structural and luminescence properties for possible applications in dosimetry and display devices. The Thermoluminescence (TL) properties of Y2SiO5 ∶ Ce3+ phosphor powder and thin films were reported. For the phosphor powder, the TL intensity increases with an increase in UV dose for up to 20 minutes and then decreases. The TL intensity peak shifts slightly to higher temperature region at relatively high heating rates, but with reduced peak intensity. Important TL kinetic parameters, such as the activation energy (E) and the frequency factor (s) were calculated from the glow curves using a variable heating rate (VHR) method and it was found that the glow peaks obey first order kinetics. For the films, broad TL emissions over a wide temperature range with low intensity as compared to that of the powder were observed. The maxima of the TL glow peaks of the films deposited in oxygen ambient and vacuum shift towards higher temperature relative to the TL peak position of the film deposited in an argon environment. Vacuum environment resulted in the formation of a deep trap as compared to oxygen and argon environments. Furthermore, the structure of Y2SiO5 ∶ Ce3+ phosphor powder transformed from x2-monoclinic polycrystalline phase to x1-monoclinic polycrystalline phase at low substrate temperature deposition. TL and photoluminescence (PL) properties of KY3F10: Ho3+ phosphor powder is also reported. The TL measurements were done for different heating rates and for various duration of UV exposure. The TL intensity increases with duration of UV exposure up to 20 minutes and then decreases. Decrease of the glow peak height was observed for the glow curves with increasing heating rate. The area under TL-time plot is calculated for each heating rate at constant UV dose and it is found to be constant and independent of the heating rate. It is therefore, the observed decrement in intensity of each glow curve following increment in heating rate is not attributed to the thermal quenching effect. Important TL kinetic parameters namely, the activation energy (E) and the frequency factor (s) were calculated using variable heating rate (VHR) method. The glow peaks obey first order kinetics. KY3F10: Ho3+ thin films were deposited by a pulsed-laser deposition technique with Nd-YAG laser radiation (λ= 266 nm) on (100) silicon substrate. The influence of background gas pressure, target to substrate distance, and substrate temperature on structural, morphological and luminescence properties of the films have been investigated. For the film grown under different background gas pressure, the XRD and FE-SEM results show improved crystalline structure for the film deposited at a pressure of 1 Torr. The AFM results show that the RMS roughness of the films increases with rise in argon gas pressure. The EDS elemental mapping shows Y-excess for all the films deposited under all pressures and this is attributed to its higher mass and low volatility as compared to K and F. XPS analysis further confirmed Y-excess in the deposited films. XRD analysis of the films deposited under various target to substrate distances in the range of 4-7 cm shows that high crystalline quality film with largest grain size is obtained for target to substrate distance of 4 cm. Decrease in the thickness of the films is observed at larger target to substrate distances. This is attributed to the increased hemispherical expansion of the laser induced plasma plume at larger distances reducing the particle flux of the target species over a substrate area. Moreover, all the films are characterized by low reflectance and high absorption in the visible region. Furthermore, for the films deposited under various substrate temperatures, the crystallinity is improved following increment in deposition temperature and the calculated average crystallite size is in the range of 39-74 nm. For all the KY3F10: Ho3+ commercial phosphor powder and thin films, PL emission spectra were also investigated at four main excitation wavelengths; namely, 362, 416, 454 and 486 nm. Green emission at 540 nm and faint red emission at 750 nm were observed for all the excitations. The green emission at 540 nm is ascribed to the 5F4−5I8 and 5S2−5I8 transitions and the faint red emission at 750 nm is due to the 5F4 −5I7 and 5S2−5I7 transitions. In addition to the sharp green emission at 540 nm, a broad emission centered at 600 nm was observed for excitation wavelength of 362 nm for the powder. The highest PL intensity occurs at excitation of 454 nm for all samples of this material. The Cathodoluminescence (CL) images of the films deposited under various background gas pressures show non uniform distribution of luminescent centers in the deposited films. Moreover, the CL emission spectra are similar to those of the PL with the main peak at 540 nm, suggesting that the electron beam did not change the electron energy level configuration or transitions of the activator ion in the film.Item Open Access A molecular dynamics study of segregation and diffusion in FCC nanocrystals using the sutton-chen potential(University of the Free State, 2017-02) Van der Walt, Cornelia; Terblans, J. J.; Swart, H. C.English: English: Nanotechnology research has expanded notably, with a wide range of applications from catalysis in fuels, to optics. A key factor in manufacturing these particles is understanding diffusion and segregation of dopants and impurities in the nanocrystals, as segregation of these impurities influences which atom is exposed to the surface of the nano-particle, and able to react. Understanding these processes in terms of the shape and size of the particle, as well as the effects of temperature, are all important factors for nano-material manufacture. Molecular Dynamics software is uniquely able to study the dynamics inside particles of up to several thousand atoms. The Sutton-Chen potential, in particular, is able to simulate the reactions of face-centred cubic (FCC) metals and model bulk modulus, elastic constants, lattice parameters, surface energies, phonon dispersion, cohesion energy and vacancy formation energy. It is ideally suited for studying the diffusion and segregation dynamics of the large clusters of atoms that make up nanocrystals. In this study, a Molecular Dynamics model using the Sutton-Chen potential was built. This model implements the Verlet Velocity scheme to simulate the kinetics of the atoms, and uses the Berendsen thermostat to keep the system at a constant temperature. The model was tested on six FCC metals, namely Al, Ni, Cu, Pd, Ag and Pt, and, making use of periodic boundaries in order to simulate bulk crystals, calculated the cohesion energy to confirm the effectiveness of the model. The model further confirmed surface orientation dependence for low index surfaces. The relationship for vacancy formation energy Ev(111) > Ev(100) > Ev(110) of applied to all the FCC metals studied. The effects of temperature on other diffusion-related energies in the crystals were also studied. It was further found that the diffusion activation energy of FCC metals has the same relationship of Q(110) < Q(100) < Q(111) Equipped with this information, the model was used for in-depth analysis of Cu, and later Ag, nano-cubes, -rhombicuboctahedrons and -octahedrons. A thorough analysis of the surface orientation dependence, size dependence, shape dependence and temperature dependence of key energies involved in diffusion, created a complete picture of nanoparticle stability and surface reactivity. It was found that larger particles are more stable, and that surface reactivity indicates that nano-rhombicuboctahedrons are more reactive than perfect cubes, and that octahedrons are the least surface-reactive. The final part of this study calculated the segregation energy in Ag/Cu systems to confirm the ability of the mixed Sutton-Chen potential to simulate segregation in alloys. In the Ag/Cu system, Ag is known to segregate to the surface, while Cu desegregates, and the model was able to demonstrate this. As this model can successfully reproduce that segregation, it can become a powerful tool for the study of diffusion dynamics in FCC alloy nano-materials.Item 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.Item 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.Item Open Access Sol-gel synthesis and characterization of MAl2O4 (M = Zn or Mg) spinels doped, co-doped and triply doped nano-phosphors(University of the Free State (Qwaqwa Campus), 2014-12) Motloung, Setumo Victor; Dejene, B. F.; Swart, H. C.; Ntwaeaborwa, O. M.The luminescent and structural properties of the MAl2O4 (M = Zn, Mg) (which are thereafter referred as hosts) phosphors prepared by sol-gel methods at a relatively low temperature (~80 °C) are discussed. Zinc, magnesium and aluminium nitrates and citric acid were used as the starting materials for the hosts preparations. The prepared gels were dried in an oven and subsequently annealed in air either at 800 0C for an hour. In order to study the effects of the different dopants into the hosts matrix, the dopants concentration were varied. The host material was either singly doped or co-doped or triply doped. Furthermore, in order to study the effects of the catalyst content on the prepared powders, the optimum concentrations for the singly doped phosphors were prepared and the catalyst content was varied during synthesis. Generally, the surface morphologies, surface topographies, crystal structure, photoluminescence (PL), Ultraviolet-visible (UV-Vis) properties were influenced by the dopant concentration and catalyst content. The incorporation of the foreign atoms seems to populate the hosts with more defects. For the ZnAl2O4: x% Pb2+ samples, the Thermo gravimetric analysis (TGA) showed that the minimum annealing temperature required to obtain single phase ZnAl2O4 is above 400 °C. Undoped and Pb2+-doped ZnAl2O4 nanoparticles exhibit the violet emission at slightly different positions, which suggests the possibilities that the luminescence centre can either be due to the defects level in the host or Pb2+ ions. The emission peaks at 390 and 399 nm are ascribed to the typical ultra-violet (UV) transitions 3P0,1 → 1S0 in Pb2+ ions. On the study of ZnAl2O4: x% Cr3+ (0 ≤ x ≤0.3), Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) analysis confirmed the presence of all expected ions in the powder material. The results showed that Cr3+ can occupy multiple-sites in the host matrix. It was interesting to see, once again, that the PL results showed that the host and the Cr3+-doped exhibit violet emission slightly at different peak positions, which suggests that the luminescence can originate from the host or Cr3+ ion. Emission from the host is attributed to the band-gap defects in the host material, while the emission from the Cr3+ is attributed to the 4T1 → 4A2 transition. At the higher mol% there is an emission at 692 nm, which is attributed to the 2E → 4A2 transition in Cr3+. On the case-study of the co-doped ZnAl2O4: 0.1% Ce3+, x% Eu3+ (0 ≤ x ≤ 2mol%), the results showed that the nanopowders microstructure consists of non-uniform sizes and the loss in lattice fringes as the Eu3+ mol% increase suggest the increase in strain or disorder. The incorporation of the co-activator (Eu3+) at the higher mol% resulted in the radiative energy transfer from Ce3+→ Eu3+. The International Commission on Illumination (CIE) color coordinates show the shift from the blue to orange visible region as the Eu3+ concentration is increased. From the triply doped MgAl2O4: 0.1% Ce3+, 0.1% Eu2+, x% Tb3+ (0 ≤ x ≤ 2%) study, the PL results revealed the existence of the energy transfer from Eu2+ → Tb3+ → Ce3+. CIE colour chromaticity showed that the colour can be tuned from bluish → greenish by changing the Tb3+ mol% and the excitation wavelength. In both studies of the effects of the catalyst content in ZnAl2O4:1.5% Pb2+ and ZnAl2O4:0.01% Cr3+, the results showed that the increase in the catalyst content lead to the morphological evolution and transformation from small particles to rods-like-needles. In addition, at the higher catalyst content, the extra peak associated with the ZnO impurities are observed. The emission intensity was influenced by the catalyst content. The catalysts content does not affect the emission colour in the case of ZnAl2O4:1.25% Pb2+. However, in the case of ZnAl2O4:0.01% Cr3+, the results revealed the possibilities of tuning the emission colour by varying the catalyst content.Item 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.Item Open Access Luminescence properties of Y2O3:Bi3+ as powder and thin film phosphor for solar cell application(University of the Free State, 2015-07) Rasha, Mohmmed Jafer; Coetsee, E.; Swart, H. C.The luminescent properties of the bismuth doped yttrium oxide (Y2-xO3:Bix) phosphor material was investigated as a powder and as thin films for possible application as a down-conversion material for solar cells. The goal of this investigation is to improve the energy conversion efficiency of photovoltaics (PV) by using the solar spectral conversion principle. A downconversion (DC) material converts a high-energy ultraviolet photon to two less energetic redemitting photons to improve the spectral response of Si solar cells. The luminescent properties of Y2-xO3:Bix=0.2% phosphor powder were investigated and the fluorescence spectra show that the luminescence was stimulated by the emission from two types of centers. These two types of centers were associated with the substitution of the Y3+ ion with the Bi3+ ion in two different sites in the crystal lattice of Y2O3 (with point symmetries C2 and S6). The emission of Bi3+ in the S6 site caused blue luminescence with maxima at 360 nm and 407 nm, and in the C2 site it gave green luminescence with the maxima at 495 nm. Both these emissions are related to the 3P1→1S0 transition in Bi3+. The diffuse reflectance was measured for Y2O3 and Y2-xO3:Bix=0.2%. No change in the band gap, when 0.2 mol% of Bi was doped in the Y2O3 host, was observed. X-ray photoelectron spectroscopy (XPS) results provided proof for the blue and green emission of Bi3+ in the Y2O3:Bi3+ phosphor powder. The Y2O3:Bi3+ phosphor was successfully prepared by the combustion process during the investigation of DC materials for Si solar cell application. The X-ray diffraction (XRD) patterns indicated that a single phase cubic crystal structure with the Ia3 space group was formed. XPS showed that the Bi3+ ion replaces the Y3+ ion in two different coordination sites in the Y2O3 crystal structure. The O 1s peak shows 5 peaks, two which correlate with the O2- ion in Y2O3 in the two different sites, two which correlate with O2- in Bi2O3 in the two different sites and the remaining peak relates to hydroxide. The Y 3d spectrum shows two peaks for the Y3+ ion in the Y2O3 structure in two different sites and the Bi 4f spectrum shows the Bi3+ ion in the two different sites in Bi2O3. The photoluminescence (PL) results showed three broad emission bands in the blue and green regions under ultraviolet excitation, which were also present for panchromatic cathodoluminescence (CL) results. These three peaks have maxima at ~ 365, 412 and 490 nm. The PL emission ~ 407 nm (blue emission) showed two excitation bands centered at ~ 338 and 370 nm while the PL emission at ~ 495.0 nm (green emission) showed a broad excitation band from ~ 310 to 365 nm. The panchromatic CL images were obtained for selected wavelengths at (415 ± 10.5) nm (for blue emission) and (530.0 ± 12.5) nm (for green emission). These luminescence results correlate with the XPS results that show that there are two different Bi3+ sites in the host lattice. The effect of different annealing temperatures on the PL properties of Y2-xO3:Bix phosphor powders were then investigated. Y2-xO3:Bix was synthesized by the combustion method with varying the Bi3+ dopant concentrations (x = 0.08, 0.1, 0.2, 0.3 and 0.5 mol%). The minimum PL emission intensity was observed for the high dopant concentration of 0.5 mol% and can be ascribed to concentration quenching. The effect of different annealing temperatures (800, 1000, 1200, 1400 and 1600 °C) were investigated for this sample in order to increase the emission intensity. Results showed that the emission intensity did increase with an increase in the annealing temperature up to 1400 °C. The increased intensities were attributed to two factors. The first one is the improvement of the Y2O3 crystal structure and second one is the segregation of Bi3+ ions from the bulk to populate the particles’ surfaces. The intensity increase up to 1200 °C is due to the segregation of Bi3+ ions from the bulk to populate the particles’ surfaces as a result of the increased temperature. Temperatures higher than 1200 °C resulted in a Bi3+ deficiency from the sample’s surface and therefore leading to a decrease in the dopant concentration. The decrease in the dopant concentration is creating the second factor, which is the further increase in intensity to 1400 °C due to a lower dopant concentration (then the effect of concentration quenching is lower). A further increase in the annealing temperature up to 1600 °C resulted in a decrease in the intensity because the majority of the Bi3+ ions evaporated from the sample’s surface as volatile species. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and XPS confirmed the segregation of Bi3+ ions to the particles surface with an increase in annealing temperature. These results concluded that the luminescence properties of Y2-xO3:Bix can be affected by different annealing temperatures and different dopant concentrations, Y2O3:Bi3+ phosphor thin films were prepared by PLD in the presence of oxygen (O2) gas. The microstructures and PL of these films were found to be highly dependent on the substrate temperature. XRD analysis showed that the Y2O3:Bi3+ films transformed from amorphous to cubic and monoclinic phases when the substrate temperature was increased up to 600 °C. At the higher substrate temperature of 600 °C the cubic phase became dominant. The crystallinity of the thin films therefore increased with increasing substrate temperatures. Surface morphology results obtained by scanning electron microscope (SEM) and atomic force microscopy (AFM) showed a decrease in the surface roughness. The increase in the PL intensities was attributed to the increase in the crystallinity and to the decrease in the surface roughness. The thin films prepared at substrate temperatures of 450 °C and 600 °C showed a shift in the main peak position to shorter wavelengths of 460 and 480 nm respectively, if compared to the main PL peak position of the powder at 495 nm. The shift was attributed to the change in the Bi3+ ions’ environment in the monoclinic and cubic phases. The reactive radio-frequency (RF) magnetron sputtering and spin coating fabrication techniques were also used to fabricate Y2-xO3:Bix=0.5% phosphor thin films. The two techniques were analyzed and compared as part of investigations being done on the application of DC materials for a Si solar cell. The morphology, structural and optical properties of these thin films are comparatively investigated. The XRD results of the thin films fabricated by both techniques showed cubic structures with different space groups. The optical properties showed different results because the Bi3+ ion is very sensitive towards it’s environment. The luminescence results for the thin film fabricated by the spin coating technique is very similar to the luminescence observed in the powder form. It showed three obvious emission bands in the blue and green regions centered at about 360, 420 and 495 nm. These emissions were related to the 3P1→1S0 transition of the Bi3+ ion situated in the two different sites of Y2O3 matrix with I a- 3(206) space group. Whereas the thin film fabricated by the RF magnetron technique shows a broad single emission band in the blue region centered at about 416 nm. This was assigned to the 3P1→1S0 transition of the Bi3+ ion situated in one of the Y2O3 matrix’s sites with a Fm-3 (225) space group. The spin coating fabrication technique is suggested to be the best technique to fabricate the Y2O3:Bi3+ phosphor thin films.Item 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.Item 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.Item 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.
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