Doctoral Degrees (Physics)
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Browsing Doctoral Degrees (Physics) by Subject "Annealing"
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Item Open Access Growth of antimony on copper : a scanning tunneling microscopy study(University of the Free State, 2012-01) Ndlovu, Gebhu Freedom; Hillie, K. T.; Roos, W. D.English: The thesis deals with adsorption, self–assembly and surface reactions of Sb atoms on solid Cu(111) substrates. It is of genuine interest in materials science and technology to develop strategies and methods for reproducible growth of extended atomic and molecular assemblies with specific and desired chemical, physical and functional properties. When the mechanisms controlling the self-organized phenomena are fully disclosed, the self-organized growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials. The experimental technique used to study ordered phases and phase transitions of Sb on Cu(111) substrates was the Scanning Tunneling Microscopy (STM) – an outstanding method to gain real space information of the atomic scale realm of adsorbates on crystalline surfaces. It is a general trend to conduct studies on well known structures before one begins working on complicated systems. Therefore, in this study, Si(111) Cu(111) and HOPG surfaces were studied in atomic detail to confirm the calibration and the resolution capabilities of the instrument. The acquired data were comparable to the reported theoretical and experimental data in literature. The investigated Cu(111) – Sb system is characterized by a complex interplay between adsorbate interactions and adsorbate substrate interactions which in this study manifests through self–assembly processes. Both low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were utilized to determine the substrate cleanliness prior to the growth of a submonolayer Sb coverage (0.43 ± 0.02 ML Sb as calculated from the acquired STM data). The freely diffusing Sb adatoms on the copper surface were thermally excited from a random distribution of Sb atoms after growth to a finally rearrangement to more energetically stable configuration. The experimental results illustrated the presence of a surface alloy after annealing at ~360°C. The Cu – Cu spacing increased from 0.257 ± 0.01 nm (atomically clean Cu(111)) to 0.587 ± 0.02 nm after annealing at 360°C. At that temperature, the STM images showed the surface protrusions of different sizes and contrast, attributed to Cu and Sb atoms. In addition to the conventional ( 3 × 3)R30°–Sb structural phase acquired at ~400°C, new metastable structural phases: (2 3 × 2 3) R30°–Sb and (2 3 × 3)R30°– Sb were obtained for the first time after annealing at 600°C and 700°C, respectively. STM data after annealing at 600°C and 700°C was best described by a structural model involving an ordered p(2×2) and p(2×1) overlayer structures superimposed onto the ( 3 × 3)R30°–Sb surface, respectively. At elevated temperatures LEED showed ring shaped diffraction patterns composed of twelve equidistant spots which are consistent with the growth of a hexagonal film forming three equivalent rotational domains. All the superstructures were found to favour a structural model based on Sb atoms occupying substitutional rather than overlayer sites within the top Cu(111) layer. Other than the dissolution of Sb onto Cu(111), the study report also on the segregation of Sb on Cu together with STS measurements. The surface chemical reactivity on an atom–by–atom basis of the Cu sample surface was studied by current imaging tunneling spectroscopy (CITS). The local density of states (LDOS) were derived from dI/dV maps at low tunneling voltages by a simultaneous measurement of high resolution topographic micrographs. The use of surface sensitive techniques (LEED, AES, STM, STS) in studying the surface alloy in question has enabled more precise statements to be made about the surface structure of the system at various temperatures. Based on the experimental results, a comprehensive study of the adsorption and segregation behaviour of Sb on Cu(111), including the mechanisms for phase formation at the atomic scale is presented in this study.Item Open Access Luminescence from lanthanide ions and the effect of co-doping in silica and other hosts(University of the Free State, 2012-07) Ahmed, Hassan Abdelhalim Abdallah Seed; Kroon, E. R.; Ntwaeaborwa, O. M.Amorphous silica powders doped with lanthanide ions were synthesised by the sol-gel method and their cathodoluminescence (CL) and photoluminescence (PL) emissions were compared. Interesting differences depending on the type of excitation were observed for Tb and Ce-doped samples. For Tb-doped samples blue 5D3®7FJ emission was measured during CL in samples for which PL results showed this emission to be concentration quenched due to cross-relaxation, while for Ce-doped samples luminescence occurred for CL but not during PL measurements. Unlike the other lanthanides, Tb and Ce ions are sometimes found in the tetravalent rather than the trivalent state, and these differences were attributed to the possibility of electron capture of tetravalent ions possible during CL but not PL. A scheme for the energy levels of divalent and trivalent lanthanide ions relative to the conduction and valence bands in silica was proposed, making use of experimental data and the known relative positions of the energy levels for the lanthanides. Although the location of the divalent europium ion f-level above the valence band can be located by using the charge transfer energy of trivalent europium, this process cannot be generalized to find the location of the trivalent cerium ion f-level above the valence band using the charge transfer energy of tetravalent cerium as has been suggested. Initial investigations of the luminescence properties of Ce doped silica were complicated by overlapping luminescence from oxygen deficiency defects from the host itself and the fact that Ce took the tetravalent state which is nonluminescent for PL measurements. Spectra obtained using a wide variety of excitation methods, including synchrotron radiation, were compared and evaluated in the light of previously published data. Radically improved results were obtained by annealing in a reducing atmosphere instead of air. X-ray photoelectron spectroscopy as well as ultraviolet reflectance spectroscopy provided evidence of the conversion of Ce from the tetravalent to trivalent state and this was accompanied by strong luminescence of these sample during PL measurements. Ce,Tb co-doped silica was used to study the energy transfer from Ce to Tb ions. Initial results were disappointing when measurements showed that adding Ce quenched the Tb emission intensity instead of increasing it. However, after annealing the samples in a reducing atmosphere, a quantum efficiency of 97% for energy transfer from Ce to Tb was achieved. The mechanism for energy transfer was investigated by comparing experimental measurements of the changes in donor (Ce) emission intensity and lifetime as a function of the amount of acceptor (Tb) with numerical simulations of various models. Measurements correlated well to models for dipole-dipole and exchange interactions, but the critical transfer distance obtained was not appropriate for the exchange interaction, hence dipole-dipole interaction was identified as the interaction mechanism. Nanocrystalline LaF3 powders were synthesized by the hydrothermal method and strong luminescence was obtained from samples doped with Ce and Tb. Photoluminescence spectra from co-doped samples revealed that the emission from Ce was quenched and the emission from Tb was enhanced, showing that energy transfer from Ce to Tb occurred. The energy transfer mechanism was investigated in a similar way as for the silica samples, but in this case the experimental results fitted models for the quadrupolequadrupole and exchange interactions. Much higher concentrations of Tb were required to significantly affect the Ce luminescence properties than in the case for silica, and the critical transfer distance obtained was appropriate for the exchange interaction. Either or both of these interaction mechanisms are therefore possible. The results show that the interaction mechanism for energy transfer between lanthanide ions depends not only on the ions themselves, but also on the lattice hosting them.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 Synthesis and characterization of down−conversion nanophosphors(University of the Free State, 2014-10) Tshabalala, Kamohelo George; Ntwaeaborwa, O. M.; Swart, H. C.Luminescent zinc aluminate (ZnAl2O4) nanoparticles, crystalline–low quartz and amorphous silica powders were incorporated with Ce3+ and Tb3+ ions. These powders were successfully synthesized by the solution combustion and sol-gel routes. Phase analysis, particle sizes and morphology of the ZnAl2O4 nanoparticles were determined with X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Similarly, both low-quartz and amorphous phases of silica were determined the same way. The photoluminescence (PL) data were collected at room temperature using a variable UV Xenon lamp mounted into the F7000 Fluorescence and Cary Eclipse fluorescence spectrophotometers. The cathodoluminescence (CL) data were collected at room temperature using Ocean Optics CL spectrometer attached to the vaccum chamber of the Physical Electronics PHI 549 Auger electron spectrometer. The surface characterization was carried out using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The average crystallite sizes for zinc aluminate powder phosphors reduced in the hydrogen atmosphere were ranging from 20 nm to 50 nm. The results from XRD and HRTEM showed that ZnAl2O4:Ce3+, Tb3+ powder phosphors were crystalline and the lattice spacing estimated form SAED was 0.24 nm, corresponding to the (311) lattice of ZnAl2O4. The PL intensity of the green line emission from Tb3+ at 544 nm (5D4→7F5 transition) increased as a result of Ce3+ co-doping. The fact that the increase was simultaneous with the decrease in blue emission from Ce3+ (5d → 4f transition) suggests that excitation energy was transferred from Ce3+ to Tb3+. The AES and CL data were collected simultaneously when the powders were irradiated with a beam of electrons ( for 10 hours) in a vacuum chamber maintained at 1 × 10−7 TorrO2 atmosphere. The AES elemental composition data for the degraded powder phosphors gave all the main elements in the ZnAl2O4:Ce3+, Tb3+, namely; Zn, Al, O and adventitious C. The ratio of Zn APPH to that of oxygen was almost stable during the electron beam irradiation. The Al/O ratio increased from 0 – 300 C.cm−2 and then stabilized while the adventitious C peak decreased drastically from 0 – 600 C.cm−2 before stabilizing. The simultaneous increase of the CL intensity with the removal of C between 0 – 600 C.cm−2 suggests that the presence of C on the surface inhibited light emission from the surface. The decrease in the C/O APPH ratio was due to removal of C from the surface due to the presence of Al2O3 investigated using electron stimulated surface chemical reactions (ESSCRs) model. The CL intensity then decreased slightly after 600 C.cm−2 electron dose and then remained stable. According to ESSCR model, electron beam irradiation may dissociate the O-O (from O2 introduced in the vacuum chamber) and Zn-Al-O bonds resulting in highly reactive O2−, Zn2+, and Al3+. The XPS data collected from the sample of ZnAl2O4:Ce3+, Tb3+ proved that there was structural readjustment from inversion to normal spinel as a result of annealing in reduced H2 atmosphere. In a low quartz and amorphous silica samples, efficient energy transfer from Ce3+ to Tb3+ ions was observed when the powder phosphors were excited at the wavelength of 322 nm. The transfer rate was shown to be more efficient for samples reduced in a mixture of N2 and H2 compared to those annealed in air. Thus, the maximum energy transfer was observed from the sample co-doped with SiO2: 2 mol%Ce3+, 4 mol%Tb3+. The excitation of 322 nm is ascribed to direct excitation of Ce3+ ions from 4f → 5d transition of Ce. The improved down-converted emission indicates that our materials can be used as wavelength shifting layer in Si photovoltaic cells to improve their power conversion efficiency.Item Open Access Synthesis and characterization of MgAl₂O₄ and (MgxZn₁-x)Al₂O₄) mixed spinel phospors(University of the Free State, 2014-01) Tabaza, Wael Abed Ibrahim; Kroon, R. E.; Swart, H. C.Magnesium aluminate (MgAl2O4) has received special attention as a technologically important material because of its attractive properties, such as mechanical strength, chemical inertness, a wideband gap, relatively low density, high melting point, high thermal shock resistance, low thermal expansion coefficient, resistance to neutron irradiation and low dielectric loss. It has also been used as a phosphor host activated by a variety of transition metal and lanthanide ions. Doped and undoped MgAl2O4 nanocrystalline powders were successfully prepared by a simple combustion method. The structure of the powders was analyzed with x-ray diffraction (XRD). The XRD data showed that all the samples had the spinel structure and the average particle size of the as-prepared samples was about 25 nm. The morphology of the samples was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) was used to obtain more information on the conversion of Ce ions from the non-luminescent Ce4+ ion to the luminescent Ce3+ oxidation state. The optical properties of the material were measured using photoluminescence (PL) spectroscopy and ultraviolet-visible (UV-Vis) spectroscopy at room temperature. At a 350 nm excitation wavelength the PL spectra of Ce3+ doped MgAl2O4 showed a broad green emission band centred at 490 nm. The maximum green emission was obtained for the sample doped with 0.75 mol% Ce. However the Ce3+ doped MgAl2O4 requires reducing at a high temperature (1400 °C) to convert the non-luminescent Ce4+ ions to the luminescent Ce3+ ions. For this reason Bi3+ doped MgAl2O4 was also investigated. For an excitation wavelength of 335 nm the Bi3+ doped MgAl2O4 produced a broad blue emission band centred at around 400 nm. The maximum blue emission was obtained for the sample doped with 0.5 mol% Bi. The results indicate that doping MgAl2O4 with Bi ions may be more attractive than doping with Ce ions. The blue emission from Bi doped MgAl2O4 was found to occur in the wavelength region corresponding to the well known host independent weak f-f excitation peaks of Tb. Therefore the possibility of using Bi3+ ions as a sensitizer for Tb3+ ions emitting green light was investigated. Although Tb can generally be excited efficiently using its strong f-d transitions, the possibility of exciting it through energy transfer from Bi to the normally weak f-f transitions is important, since the excitation wavelength of Bi (335 nm) is longer and more accessible than the f-d transition wavelength of Tb in MgAl2O4 (227 nm). The results show that Bi is a good sensitizer for Tb in the MgAl2O4 host and leads to significant enhancement of Tb emissions from the 5D4 level for excitation wavelengths between 300 nm and 380 nm (with a maximum enhancement of almost 100 times near 328 nm), thereby extending the useable excitation band of Tb to much longer wavelengths compared to Tb single doped samples. Zinc aluminate (ZnAl2O4) also has the spinel structure and has been used as the phosphor host. Therefore it is interesting to consider the mixed spinel magnesium zinc aluminate (MgxZn1-xAl2O4) as a host for Tb. The maximum PL intensity of the green emission at 544 nm occurred for Mg0.75Zn0.25Al2O4 and the optimum Tb concentration was found to be 0.5 mol%. The blue emission at 416 nm was almost absent for ZnAl2O4, but increased with the Mg content and was a maximum for MgAl2O4. The absence of blue emission peaks is usually attributed to the concentration quenching, but since the same Tb concentration was used for the MgAl2O4:Tb where blue emissions did occur, it is rather suggested that because of the smaller bandgap of ZnAl2O4, the 5D3 level lies close to or inside the conduction band and this prevents transitions from this level. An important application for phosphors is to make thin films for devices such as plasma displays and light emitting diodes. Tb doped Mg0.75Zn0.25Al2O4 thin films were fabricated on Si (100) substrates by two different techniques, namely sol-gel spin-coating and pulsed laser deposition (PLD) methods. The atomic force microscopy (AFM) and SEM data showed that the film deposited using a spin-coating method was more uniform and smooth compared to the thin film prepared by PLD. The thin film prepared by PLD has particles on the surface while the spincoated film was uniform. For the spin-coated samples annealing was necessary, but the morphology of these films was changed after annealing at 1200 °C and the surface formed ridges. The depth profile results obtained by Auger electron spectroscopy (AES) show that there was already some diffusion of the Si substrate into the material of the thin film for the samples annealed at 1000 °C. The emission spectra were similar for both kind of thin films, but there was a variation in the intensity of luminescence of the thin films. The spin coating process is the more simple method for growing the thin films, while PLD is a good technique for synthesizing of the thin films at lower substrate temperatures which might be important for the fabrication of the devices using glass with a melting point around 700 °C.