Physics

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Open Access
The anomalous low state of the X-ray binary system Hercules X-1
(University of the Free State, 2008) Jurua, Edward; Meintjes, P. J.
The lightcurve of Hercules X-1 (Her X-1) shows a peculiar 35-day modulation of the X-ray flux cycling between low and high states. The 35-day modulation is believed to result from the occultation of the neutron star by a warped precessing disc around the central neutron star. Since the discovery of the 35-day cycle of Her X-1, it has entered the anomalous low state a number of times, with the most recent being during the 2003 - 2004 period. Using RXTE ASM observations of Her X-1 after the 2003 - 2004 anomalous low state, it is shown that Her X-1 turned on with a new precession period and main-on flux. It is further shown that there is a positive correlation between the precession period and the main-on flux. Using optical observations of Her X-1 during both the anomalous low state and the normal high state it is shown that the orbital (1.7 day) lightcurve of Her X-1 varies systematically over the 35-day precession cycle. It is also shown that there is insignificant change in the 35-day morphology of the lightcurves between the anomalous low state and normal high state of Her X-1, suggesting a very slight change in the disc warp between the two states. Comparison of optical and X-ray lightcurves suggest that the significant amount of X-ray flux during the anomalous low state originates from the companion star. Analysis of both RXTE PCA and XMM-Newton observations of Her X-1 during the 2003 - 2004 anomalous low state, show that Her X-1 was brighter during this period compared to the normal high state brightness, and that there are two components of X-ray flux during the anomalous low state: reflection component from the companion star and coronal component from the accretion disc corona.
Open Access
AutoCal: A software application for calibrating photometric data
(Academy of Science of South Africa (ASSAf), 2016) Wium, Daniël J.; Van Soelen, Brian
We present a software application for the calibration of stellar magnitudes in the absence of standard stars. It uses an existing algorithm to match stars in the target’s field of view to catalogue entries and computes the average offset between the two sets of magnitudes using a weighted least-squares approach. This offset is used to calibrate the target’s instrumental magnitude. The software application was used to calibrate magnitudes for six Be/X-ray binary sources in the Small Magellanic Cloud and the results were compared with published results for these sources. Where comparisons were possible, our results agreed with those results within the uncertainties specified. Infrared variability was found for all six of the sources tested. The interactive outlier removal that was made possible by our software allowed for smaller uncertainties to be reported for our results.
Open Access
Bepaling van die elastisiteitskonstantes van yster enkel-kristalle
(University of the Free State, 1966-10) Lombaard, J. C.
Abstract not available
Open Access
Bi doped LaOCl and LaOF thin films grown by pulsed laser deposition
(Elsevier, 2024) Jaffar, Babiker M.; Swart, H. C.; Seed Ahmed, H. A. A.; Yousif, A.; Kroon, R. E.
Thin films of Bi³⁺ doped LaOCl and LaOF phosphors prepared via the pulsed laser deposition (PLD) technique in vacuum and different argon (Ar) pressures were compared in order to assess their luminescence properties. All peaks of the X-ray diffraction 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. 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 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 the 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. Therefore phosphors based on LaOCl as the host material were found to be preferable over LaOF under the PLD conditions used in this study.
Open Access
Bias and illumination-dependent room temperature negative differential conductance in Ni-doped ZnO/p-Si Schottky photodiodes for quantum optics applications
(Cell Press, 2023) Ocaya, Richard O.; Orman, Yusuf; Al-Sehemi, Abdullah G.; Dere, Aysegul; Al-Ghamdi, Ahmed A.; Yakuphanoglu, Fahrettin
In this article, evidence for the existence of illumination and bias-dependent negative differential conductance (NDC) in Ni-doped Al/ZnO/p-Si Schottky diodes, and the possible mechanism for its origin, are presented. The atomic percentages of Ni doping were 0%, 3%, 5%, and 10%. NDC is observed between -1.5 V to -0.5 V in reverse bias under illumination, but only at certain doping levels and specific forward bias. Furthermore, the devices show excellent optoelectronic characteristics in the photoconductive and photovoltaic modes, with device open circuit voltages ranging from 0.03 V to 0.6 V under illumination.
Open Access
Cathodoluminescence degradation and surface characterization of SrGa₂S₄:Ce³⁺ power and thin films
(University of the Free State, 2011-05) Moleme, Pulane Adelaide; Ntwaeaborwa, O. M.; Swart, H. C.
The structure, morphology and luminescent properties of commercial SrGa2S4:Ce3+ phosphor powder and thin films were investigated. The phosphor shows bright blue under ultraviolet (UV) excitation. Measurements were carried out using various characterization techniques such as Xray diffraction (XRD), scanning electron microcopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using Shimadzu Super Scan SSX-550 coupled with EDS. Photoluminescence (PL) data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature, and cathodoluminescence (CL) data were collected with S2000 Ocean Optics Spectrometer. The absorption spectra were recorded using Perkin Elmer Lambda 950 UV-VIS spectrometer. The same characterization tools were used to characterize the thin films. XRD data confirmed the orthorhombic structure of SrGa2S4 that was consistent with the standard JCPDS file no. (77-1189). The SEM images of the SrGa2S4:Ce3+ powder showed particles with irregular shapes and EDS detected presence of the major elements. Both PL and CL showed the broad emission peaks around 444 nm and 485 nm which are due to Ce3+ radiative transitions (5d (T2g) → 4f (2F5/2) and 5d (T2g) → 4f (2F7/2)). Cathodoluminescent ageing characteristics of the SrGa2S4:Ce3+ powder and thin films under prolonged electron beam bombardment were studied and presented. The cathodoluminescent intensity with increasing Coulomb loading was observed to degrade under different primary electron beam voltages for the powder. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes both during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a decrease in S and C Auger peak intensity and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of an SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs). For preparation of films, silicon (Si) (100) substrates were used. A pellet was prepared from the standard SrGa2S4:Ce3+ powder. The Lambda Physik EMG 203 MSC 309 nm XeCl excimer laser was used to grow the films. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 x 10-5 mbar before backfilling to pressures of 1.0 x 10-2 mbar Ar and 1.0 x 10-2 mbar O2, where Ar and O2 were used as cross pulse gases. The films were deposited at different substrate temperatures ranging from 400°C to 600°C with 28 800 and 57 600 pulses respectively. The laser beam was operated at 8 Hz repetitive rate. The substrate temperature, number of pulses and the working pressure are the parameters that were varied during the preparation of the thin films. A highly crystalline SrGa2S4 layer was obtained at the growth temperature of 400°C. XRD patterns also showed that the properties of the films were sensitive to substrate temperature. PL and CL spectra were characterized by a broad band that can be fitted by two Gaussian peaks according to the two Ce3+ radiative transitions. At high substrate temperature a shift to Ce3+ emission in SrS occurred as well as in Ar atmosphere for both UV and high energy electrons excitation. The atomic force microscopy (AFM) images before annealing exhibited smooth surface at low substrate temperature, which became rough at high substrate temperature and after annealing in vacuum at 700°C temperature. Non-uniformity in particles (big and small) of the films and smooth films were observed from the SEM images.
Open Access
Characterization of Gd2O2S: Tb³+ phosphor powder and thin films
(University of the Free State, 2011-06) Dolo, Jappie Jafta; Swart, H. C.; Dejene, F. B.; Terblans, J. J.
Under Ultra violet (UV), cathode-ray and X-ray excitation, terbium activated rare earth oxysulphide (Gd2O2S:Tb3+) phosphors shows bright green luminescence. Due to its superior luminescent performance, Gd2O2S:Tb3+ phosphor is used in the manufacturing of TV screens. The degradation of commercially available Gd2O2S:Tb3+ phosphor powder and pulsed laser deposited (PLD) thin films were studied with Auger Electron Spectroscopy (AES) and Cathodoluminescence (CL). The surface reactions were monitored with AES while the light output was measured with a PC2000-UV spectrometer. The CL of the Gd2O2S:Tb3+ was excited with a 2 keV energy electron beam with a beam current density of 26 mA/cm2. The CL and AES were measured simultaneously while the sample was bombarded with the electrons in an oxygen atmosphere. A comparison between the low energy peaks of the AES spectra before and after degradation showed significant differences in the shape of the peaks. A linear least squares (LLS) method was applied to resolve the peaks. Elemental standards from Goodfellow were used in conjunction with the measured data to subtract the S and Gd peaks. A direct correlation between the surface reactions and the CL output was found for both the thin films and the powder. The adventitious C was removed from the surface as volatile gas species, which is consistent with the electron stimulated surface chemical reactions (ESSCR) model. The CL decreased while the S was removed from the surface during electron bombardment. A new non-luminescent surface layer that formed during electron bombardment was responsible for the degradation in light intensity. X-ray photoelectron (XPS) indicated that Gd2O3 and Gd2S3 thin films are formed on the surfaces of the Gd2O2S:Tb3+ powder and thin films during prolonged electron bombardment. Luminescent Gd2O2S:Tb3+ thin film phosphors were successfully grown by the PLD technique. The effects of oxygen pressure and substrate temperature on the morphology and the PL emission intensity were investigated. The films grown in a higher O2 ambient consist of smaller but more densely packet particles relative to the films grown at a lower O2 ambient. The PL intensity of the films increased relatively with an increase in deposition O2 pressure. The PL of the films grown at a higher substrate temperature was generally also more intense than those grown at a lower substrate temperature. It was clear from the Atomic Force Microscopy (AFM) images that spherical nanoparticles were deposited during the deposition process. X-ray diffraction (XRD) indicated that the broadening of the XRD peaks is reduced with an increase in annealing temperature.
Open Access
Characterization of SrAl₂O₄:Eu²⁺,Dy³⁺ nano thin films prepared by pulsed laser deposition
(University of the Free State, 2010-11) Nsimama, Patrick Damson; Swart, H. C.; Ntwaeaborwa, O. M.
English: Thin films of SrAl2O4:Eu2+,Dy3+ phosphor were deposited on silicon (Si (100)) substrates using a 248 nm KrF pulsed laser. Deposition parameters, namely; substrate temperature, pulse repetition rate, number of laser pulses, base pressure and the working atmosphere were varied during the film deposition processes. Atomic force microscopy (AFM), Scanning electron microscopy (SEM), X-ray Diffraction (XRD), energy dispersive x-ray spectroscopy (EDS), and the fluorescence spectrophotometry were used to characterize the thin films. The surface characterization was done by using Auger electron spectroscopy (AES) combined with CL spectroscopy and X-ray photoelectron spectroscopy (XPS). PL data were collected in air at room temperature using a 325 nm He-Cd laser PL system and the UV Xenon lamp Cary Eclipse fluorescence spectrophotometer. The particle morphologies, surface topographies and photoluminescence (PL) properties were varying with the deposition parameters. Rougher film surfaces gave better PL properties. The optimum substrate temperature for SrAl2O4:Eu2+,Dy3+ films with intense PL emission was in the 350-400o C range. SrAl2O4:Eu2+,Dy3+ thin films ablated using a higher number of pulses gave superior PL properties to those deposited at lower number of pulses. As-deposited films prepared in the gas atmospheres gave AFM images with well defined particles and better PL properties than those deposited in vacuum. The average particle sizes for films deposited in gas atmospheres were ranging from 25 nm to 40 nm. The results from XRD and HRTEM showed that the as-deposited SrAl2O4:Eu2+,Dy3+ thin films were amorphous. Upon annealing at 800o in vacuum for 2 hours, the PL of the films deposited in the gas atmospheres decreased. However, the crystallinity and the PL properties of the annealed vacuum deposited thin film improved considerably. The CL spectra gave only green emission peaks ranging from 507 nm to 522 nm. Both the PL and CL emissions were ascribed to the 4f65d1 → 4f7 Eu2+ ion transitions. The AES elemental composition results for the undegraded and electron degraded thin films gave all the main elements in the SrAl2O4:Eu2+,Dy3+ material, i.e. Sr, Al and O. The ratios of Al and Sr APPHs to that of O increased slightly during removal of the C from the surface. The C/O ratio decreased with an increase in electron dose. Results from the RBS showed thin film SrAl2O4:Eu2+,Dy3+ stoichiometric ratios comparable to the commercial powder. The sharp decrease in the C/O APPH ratio was due to removal of C from the surface due to the electron stimulated surface chemical reactions (ESSCRs) which took place during electron bombardment. During the ESSCR process, the electron beam dissociates the O2 and other background species such as H2O to atomic species which subsequently react with C to form volatile compounds (COX, CH4, etc.). The CL intensity degraded during prolonged electron beam irradiation due to the ESSCR process. The CL degradation increased with the increase in the chamber base pressure. The XPS data collected from the degraded films proved that strontium oxide (SrO) and aluminium oxide (Al2O3) were formed on the surface of the films as a result of the ESSCR in line with the increase of Sr/O and Al/O from the AES results.
Open Access
Characterization of Y3(Al,Ga)5O12:Ce3+ phosphor thin films prepared by pulsed laser deposition
(University of the Free State, 2013-11) Dlamini, Sipho Thapo Solomon; Swart, H. C.; Ntwaeaborwa, O. M.
The morphological and luminescent properties of Y3(Al,Ga)5O12:Ce3+ powder phosphor were investigated. Scanning Electron Microscopy (SEM) revealed the phosphor’s agglomerated particles with a size ranging from 0.4μm to 1.4μm. The X-ray diffraction (XRD) indicated a cubic polycrystalline phosphor with an average crystal size of 80 nm. Excitation peaks for the powder were obtained at 439, 349, 225 and 189 nm and emission peaks at 512 and 565 nm. Emission wavelength at 512 nm was also used to approximate the Al/Ga ratio within the crystal. Photoluminescence (PL) data also revealed that the addition of the Ga into the YAG:Ce3+ matrix caused a blue-shift in the emission spectra. The UV-VUV excitation and emission spectra of the Y3(Al,Ga)5O12:Ce3+ were also recorded and an energy diagram was constructed from the values. The phosphor powder was used as target material for Pulsed Laser Deposition (PLD). SiO2/Si(100) was used as substrates and thin films were deposited in the presence of different background gases. XRD indicated that better crystallization took place for films deposited in a 20 mTorr O2 atmosphere. Atomic force microscopy (AFM) revealed an RMS value of 0.7 nm, 2.5 nm and 4.8 nm for the films deposited in vacuum, O2 and Ar atmospheres, respectively. The highest PL intensity was observed for films deposited in the O2 atmosphere. The thickness of the films varied from 120 nm to 270 nm with films deposited in vacuum having the thin layer and those in Ar having the thick layer. The stoichiometry of the powder was maintained in the film during the deposition as confirmed by Rutherford backscattering spectroscopy (RBS). Luminescent properties of Y3(Al,Ga)5O12:Ce3+ thin films prepared by PLD at different substrate temperatures in an O2 background atmosphere were also investigated. XRD indicated that the films have the same cubic polycrystalline phase structure as the powder. AFM revealed poorly defined grain growth for films ablated at a substrate temperature of 22°C and 500°C but well defined grain growth was observed for films ablated at a 300°C substrate temperature. Auger electron spectroscopy (AES) depth profile of the film ablated at 500°C indicated that Si has diffused into the thin film. The highest PL intensity was observed for films deposited at the substrate temperature of 300°C. A slight shift in the wavelength of the PL spectra was obtained for the thin films with respect to the powder due to a change in the crystal field. The maximum PL intensity was obtained from the film deposited at the substrate temperature of 300⁰C in an O2 atmosphere. In addition, the films with well-defined grains (rougher surfaces) showed higher PL intensity compared to films with poorly-defined grains (smooth surfaces) as confirmed from AFM data
Open Access
CL degradation of Y2SiO5:Ce thin films coated with SnO2
(University of the Free State, 2006-05) Coetsee, Elizabeth; Swart, H. C.; Terblans, J. J.
The degradation of the cathodoluminescence (CL) intensity of cerium-doped yttrium silicate (Y 2SiO 5:Ce) phosphor thin films and commercially available Y2SiO5:Ce phosphor powders from Phosphor technology, England, were investigated for possible application in low voltage field emission displays (FEDs). Thin films of Y2SiO5:Ce were pulsed laser ablated on Si (100) substrates by using a XeCl (308 nm) excimer laser, in an oxygen (O) ambient gas pressure of 7.5 x 10-4 Torr, with laser energy of 81.81 mJ, repetition rate of 10 Hz, substrate temperature of 400°C, target to substrate distance of 3.7 cm and by using 6600 pulses. Some of the phosphor thin films were coated with tin oxide (SnO2), with the same deposition parameters as for the Y2SiO5:Ce phosphor layer except for the amount of pulses that was reduced to 1200 pulses. A SnO2 layer was ablated onto some of the thin films in order to investigate the effect of the coated layer on the surface and on the degradatio n of the CL intensity. Rutherford backscattering (RBS) was used to measure the film thicknesses. The results showed a non uniform Y2SiO5:Ce layer covered with a 58 nm thick SnO2 layer. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS) were use to study the surface morphology of the thin films. The results indicated that the Y2SiO5:Ce phosphor was ablated onto the Si (100) substrate surface as micron-sized spherical particles and that the SnO2 layer was ablated as a uniform coated layer covering the surface of the substrate and the randomly distributed spherical Y2SiO5:Ce particles. SEM was also use to study the surface morphology of the Y2SiO 5:Ce phosphor powders and the results showed that the particles were agglomerated. X-ray diffraction (XRD), that was used to measure the crystal planes of both the thin films and the powders, revealed the monoclinic crystal structure of Y2SiO5:Ce. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and CL spectroscopy were used to monitor changes in the surface chemical composition and luminous efficiency of the Y2SiO5:Ce phosphor powders and thin films (coated and uncoated) . AES and CL spectroscopy measurementswere done with 2 keV energy electrons and with beam current densities between 26.3 mA.cm-2 and 52.63 mA.cm-2 , in high vacuum and in oxygen pressures of 1 x 10-8, 1 x 10-7 and 1 x 10-6 Torr. AES indicated adventitious carbon (C) on the surface before CL measurements were made. C was depleted from the surface during electron bombardment. Residual gas mass analysis (RGA) showed that C was removed from the surface as volatile gas species. RGA with the electron beam on resulted in a higher intensity of CO2, CO and H2O gas specie s, compared to when the electron beam was off. This is consistent with the electron stimulated surface chemical reaction (ESSCR) model, whereby the electron beam dissociates the oxygen gas species into reactive atomic species, which then reacts with the carbon on the surface to form the volatile CO2 and CO gas species. Auger peak to peak heights (APPH) for oxygen and silicon on both the uncoated thin film and the powder surface stayed almost constant. The CL intensity (measured at 440 nm) increased within the first 300 C.cm-2, which is the result of the depletion of the carbon from the surface, and then it stayed constant for prolonged electron bombardment. The carbon results in an extra layer on the thin film surface that increases the energy loss of the incoming electrons. This results in the creation of fewer electron – hole pairs for photon emission during radiative recombination. The CL emission spectrum resulted in the characteristic double shoulder peak of Y2SiO5:Ce with the two main peak positions at 440 and 500 nm (blue light) before and after 24 hr s of electron bombardment for the uncoated thin film, coated thin film and for the powders. Light emission in the rare earth, Ce3+, is due to the 5d ? 4f transition due to the splitting effects of the 4f energy level. The 4f energy level splits due to the effect of the crystal field in Y2SiO5 as the host material, into the 2F7/2 and the 2F5/2 energy levels. The broad band emission of Y2SiO5:Ce is the result of the different splitting effects due to the crystal field. The relatively high CL intensity of the thin films is attributed to the spherically shaped phosphor particles grown on the surface of the Si (100) substrate. The SnO2 was also successfully ablated as a coating layer. The SnO 2 coating layer increases the energy loss of the incoming electrons which results in a lower CL intensity. The CL intensity for the uncoated thin film was therefore higher than for the coated thin film. The CL intensity stayed almost constant for the 24 hr s of electron bombardment of both the coated and uncoated thin films. The CL intensity for the phosphor powders, however, behaved differently. The intensity showed an increase after about 300 C.cm-2. The CL emission spectrum showed an increase in a second broad band at a wavelength of 650 nm after 24 hr electron bombardment. It was proved with XPS that this second broad band is due to the formation of a luminescent silicon dioxide (SiO2) layer on the surface of the Y2SiO5:Ce phosphor powders, as a result of the electron surface stimulated reactions (ESSCR). The increase in the CL intensity is thus due to the luminescent SiO2 layer that was formed as a result of electron beam irradiation that causes the Si-O bonds to break and to form intrinsic defects at 1.9 eV (650 nm) and 2.7 eV (459 nm). XPS also indicated that the Ce concentration on the surface layer increased during the degradation process and the formation of CeO2 and CeH3 also resulted from the degradation process. The phosphor powders degraded from a blue light emitting phosphor before electron bombardment to a whit ish light emitting phosphor after 24 hr, as a result of the luminescent SiO2 layer formed during degradation.
Unknown
A combined ab initio and experimental study of lanthanides and/or transition metal doped oxides
(University of the Free State (Qwaqwa Campus), 2017-01) Mulwa, Winfred Mueni; Dejene, B. F.; Ouma, C. N. M.; Onani, Martin
Ab initio modelling techniques have produced a notable contribution in analysing semiconductor metal oxides properties by use of first principles. These techniques have transformed to a high level of accuracy, owing to the development in algorithms and improved computational ability. In the study of structural, electronic and optical properties of metal oxides, ab initio techniques have been used with a lot of success to illustrate these properties. Ab initio studies therefore can complement experimental findings or even provide reliable results on properties which have not yet been experimentally investigated. Properties which can be calculated with the use of density functional theory (DFT) include spectroscopic, energetic, electronic and geometric properties. In this combined experimental and ab initio work on metal oxides doped with transition metals, the used of local density approximation with the Hubbard U correlation to compute the structural, electronic and optical properties of ZnA12O4 and Cu2+:ZnA12O4 was used. The powders of doped and undoped ZnA12O4 were effectively synthesized by use of the sol-gel technique. The X-ray diffraction (XRD) pattern for ZnA12O4 displayed crystalline peaks corresponding to cubic structure and phase dissociation was not observed. It also showed negligible lattice distortion and a slight shift to higher angles with increase of Cu2+ percentage doping. Energy dispersive X-rays spectroscopy (EDS) confirmed pure samples of ZnA12O4 components. Scanning electron microscopy (SEM) micrographs showed a uniform, well distributed and spherical morphology. The high resolution transmission electron microscopy (HRTEM) showed the influence of varying Cu2+ concentration on the particle agglomeration as well as on the crystallite sizes. The average crystallite sizes of ZnA12O4 powders almost remained constant with the increase of Cu2+ doping concentration. The lattice spacing approximated from selected area electron diffraction (SAED) was 0.242 nm corresponding to (311) lattice of ZnA12O4. Setting excitation at 283 nm, the photoluminescence (PL) emission peaks were at 388 nm, 425 nm and 480 nm in undoped ZnA12O4 which was due to oxygen vacancies while the peak at 586 nm was due to Cu2+ ions. Computationally, introduction of Cu2+ ions did not lead to significant lattice distortion and the PL emission peak was at 435 nm with a transition from Cu_3d to Cu_4p. The substitutional energies in Cu2+:ZnA12O4 predicted negative formation energies for oxygen vacancies suggesting that these vacancies are easily formed in ZnA12O4. The two point defects (oxygen vacancy and Cu2+ dopant) existed singly as the binding energies were found to be negative. Both experimental and computational work were carried out on lanthanide-doped metal oxide (ꝩ-A12O3 in this case). The powders of doped and undoped (ꝩ-A12O3 were successfully prepared using the sol-gel technique. The A12O3 as well as Ce3+: A12O3 were modelled where the Kohn- Sham equations were solved by the use of local density approximation with the Hubbard U correction. In ꝩ-A12O3:Ce3+, introduction of the dopant caused lattice strain as well as reduction in band gap. The formation energies in all the charge states were negative, suggesting that the ꝩ - A12O3 lattice could easily accommodate Ce3+. The PL emission peak was reported to be at 502 nm with a transition from O_2p to Ce_4f. The X-ray diffraction (XRD) pattern exhibited crystalline peaks corresponding to cubic structure. Due to difference in ionic radius between A13+ and Ce3+, lattice distortion was realized. As the doping concentration increased, there was a slight shift to lower angles. Only aluminium and oxygen elements were detected in the EDS analysis. SEM analysis revealed agglomeration on doping. From the HRTEM findings, the crystallite size of 16.0 nm was realized. The lattice spacing approximated from SAED was 0.138 nm corresponding to (440) lattice plane of  -A12O3. With excitation at 240 nm, the PL emission peaks at 440 nm and 462 nm were due to oxygen vacancies while the peak at 560 nm was due to Ce3+ doping. This result shows that Ce3+ doping of  -A12O3 improves its luminescence property therefore making it a possible candidate for blue light emitting diodes application. DFT work on both transition metal and lanthanide-doped metal oxides was investigated in undoped TiO2, lanthanides-doped TiO2 as well as transition metal (Cr3+) doped TiO2 by the use of local density approximation with the Hubbard U correlation to compute the substitutional energies, thermodynamic transition levels, optical properties and magnetic properties of Cr3+:TiO2 and lanthanide-doped TiO2. Unlike ZnAl2O4 and  -A12O3, TiO2 was not experimentally synthesized but was modelled theoretically. Lanthanide doping was found to cause red shift of the band gap from the ultraviolet region to the visible region of the optical absorption spectra in TiO2. The value of the computed substitutional energy implied that lanthanide ions are easily incorporated in TiO2 crystal lattice. The most favorable doping percentage was anticipated to be approximately 3%. On doping TiO2 with chromium, a transition was observed from paramagnetism to ferromagnetism at 6% doping. The magnetic moment per chromium atom was 2.59 μB for rutile phase of TiO2 and 2.49 μB for anatase phase. This result makes Cr3+ doped TiO2 a possible candidate for application in memory devices.
Unknown
A comparative study between the simulated and measured cathodoluminescence generated in ZnS phosphor powder
(University of the Free State, 2003-08) Chen, Sheng-Hui; Greeff, A. P.; Swart, H. C.
In the past few decades cathode ray tubes (CRTs) have dominated the display market because of their excellent image quality, ease and economy of manufacture. However their bulky packaging and high power consumption make them unsuitable for portable electronic devices. Field emission displays (FEDs) show the most potential amongst all other types of flat panel displays (FPDs). These FEDs have several advantages over the FPD market, which is currently dominated by active matrix liquid crystal displays (AMLCDs) and plasma displays (PDPs). FEDs generate their own light by a process referred to as cathodoluminescence (CL) in which phosphor powders inside the screen are excited in a similar manner to those used in CRTs. However, in contrast to CRTs, the accelerating voltage of electrons in FEDs is lowered in order to reduce the bulky packaging and the power consumption. Electrons with the reduced accelerating voltage have a shallower penetration depth and therefore the surface condition of the phosphor powder is critical in order to ensure proper functioning of the display. During the prolonged exposure of the phosphors to an electron beam, the phosphor surface is oxidised to form a non-luminescent layer. This electron stimulated oxide formation is due a chemical reaction between the phosphor and the residual gases in the sealed vacuum, e.g. oxygen and water vapour. Since the CL is dependent upon the energy loss of electrons in the phosphors, the CL decreases with the growth of the oxide layer on the phosphor surface. For high acceleration voltages, this oxide layer has little effect on the brightness of the CL, but as the accelerating voltage decreases as for FEDs, the layer has a much more profound effect. The ZnS:Cu,Al,Au (P22G) is a standard green phosphor commonly found in CRTs. In this study the P22G phosphor powder was bombarded by an electron beam in an oxygen ambient, argon ambient and other mixture of gases. These mixtures consisted of varying concentrations of oxygen, carbon monoxide and argon gas. Auger electron spectroscopy (AES) and cathodoluminescence spectroscopy were used to monitor changes in surface composition and luminescent properties of the P22G phosphor during electron bombardment. When the P22G phosphor powder was exposed to an electron beam in water-rich oxygen gas, a chemically-limited ZnO layer was formed on the surface. The CL intensity generated from carbon free P22G phosphor decreased linearly with the thickness of the ZnO layer. The experimentally measured thickness of the ZnO layer agrees very well with the calculated value of the theoretical simulation. The theoretical simulation of electron trajectories into the ZnO/ZnS powders was based on a Monte Carlo simulation and the CL intensity was quantified from the electron energy loss profile generated during the simulation. According to the results of the simulation, the effect of a ZnO layer on the CL is minimised by the use of a high energy electron beam at a low incident angle. The electron exposure of P22G phosphor powder was also performed in dry oxygen gas. A layer of ZnSO4 was formed on the surface after electron exposure. The sulphate formation decayed exponentially with time and it is postulated that this was due to the diffusion of the charge reactants through the sulfate film to reaction interfaces. The P22G phosphor exposed to the electron beam in argon gas and gas mixtures degraded more slowly than in oxygen gas. Argon gas and carbon monoxide gas may suppress the degradation of the P22G phosphor powder.
Unknown
Data of electronic, reactivity, optoelectronic, linear and non-linear optical parameters of doping graphene oxide nanosheet with aluminum atom
(Elsevier, 2022) Foadin, Crevain Souop Tala; Nya, Fridolin Tchangnwa; Malloum, Alhadji; Conradie, Jeanet
We have established a design to increase the absorption capacity, optoelectronic, linear and nonlinear optical prop- erties of the graphene oxide nanosheet (GON) based on the coronene molecule [C 24 H 12 ] with the help of doping, using the aluminum atom. The attachment of functional groups to the coronene surface was defined according to the Lerf-Klinowski model, based on experimental predic- tions [1] . Two GON structures (GON1 and GON2 with for- mula (C 24 H 11 )(O)(OH)COOH)) have been proposed for this purpose, and it should be noted that each of them is dis- tinguished by a different distribution of functional groups within their honeycomb lattice. A series of substitutions of the carbon atoms of the two isomers considered GON1 and GON2 were performed with the aluminum atom, resulting in the abbreviated derivative systems GON1-Alx and GON2- Alx ( x = 1–6), respectively to each of the GON1 and GON2 units. In this work, we provide data carried out in the gas phase, from density functional theory (DFT) methods that al- lowed us to understand the effects of aluminum atom dop- ing on the circular graphene oxide nanosheets. First, we report the wavenumber data related to the IR spectrum peak characteristics computed at the B3LYP, B3LYP-D3 and ωB97XD/6–31 + G(d,p) levels of theory, that allowed us to val- idate the designs of both proposed graphene oxide models. Then, we provide electronic, reactivity, optoelectronic, linear and nonlinear optical data parameters of both graphene ox- ide nanosheets and their aluminum-doped derivatives com- puted at the B3LYP, B3LYP-D3 and /6-31 + G(d,p) levels of the- ory. Finally the UV-vis spectra of the investigated compounds evaluated from time-dependent (TD) B3LYP and B3LYP-D3/6- 31 + G(d,p) levels of theory and the HOMO & LUMO orbitals of the derivatives of graphene oxide isomers computed at the B3LYP/6-31 + G(d,p) level of theory are provided. In addi- tion, the raw data of UV-vis spectra, optoelectronic parame- ters, Cartesian coordinates of all studied compounds and also those of IR spectra of both studied graphene oxide models are provided as supplementary file. The data reported in this work are useful to expose some specific positions of alu- minum within circular model of graphene oxide nanosheet that improve its electronic, reactive, optoelectronic, linear and nonlinear optical characteristics. All the formulas and de- tails of calculation performed to obtain the data reported in this work are provided in our previous work (Foadin et al., 2020) and summarized in the experimental section of this paper. To learn more about the ideal doping positions of the aluminum atom within both proposed graphene oxide de- signs that increase their electronic, reactivity, optoelectronic, linear optical and nonlinear optical properties, respectively, please see the corresponding main research paper (Foadin et al., 2022).
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Degradation of ZnS:Cu,Au,Al phosphor powder and thin films under prolonged electron bombardment
(University of the Free State, 2001-10) Hillie, Kenneth Thembela; Swart, H. C.; Berning, G. L. P.
Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in surface composition and luminous efficiency during electron bombardment. ZnS:Cu,Al,Au phosphor powders and thin films were subjected to prolonged electron beam bombardment of varying beam energies and different electron beam current densities in two different (O2 and CO2) vacuum gas ambients. The thin film phosphors were grown on Si (100) substrates by using XeCl (308nm) pulsed laser deposition (PLD) method. X-ray diffraction (XRD) measurements revealed that ZnS (100) films were preferentially grown on a Si (100) substrate. The RBS results show that the growth rate, increased with an increase of the N2 pressure in the deposition chamber during deposition. Degradation on both the powder and the thin film phosphors was manifested by a nonluminescent ZnO layer that formed on the surface of the phosphor according to the electron stimulated surface chemical reactions (ESSCR) mechanism. Lower current densities lead to a higher surface reaction rate, due to a lower local temperature beneath the beam, which resulted into a more severe CL degradation. A lower temperature beneath the electron beam may lead to an increase in the surface reaction rate due to the longer time spent by the adsorbed molecules on the surface, with a direct increase in the ESSCR probability. Low current densities would also lead to surface charging due to a lower electron conductivity of the phosphor resulting in an increase in the CL degradation rate due to band-bending. In the studies conducted between room temperature and 310 oC, an increase in the temperature led to a decrease in the surface reaction rate due to a decrease in the mean surface lifetime of the oxygen molecules on the surface, with a direct decrease in the ESSCR probability. Without the presence of the electron beam no chemical reactions, up to 310 oC, occurred on the surface. Therefore, local heating due to the electron beam irradiation is not responsible for the chemical reactions on the ZnS phosphor surface. At -125 °C the degradation was controlled by the residual small amount of water vapour in the system that is frozen at this low temperature. The thermoluminescence (TL) curves of the phosphor powder before and after degradation showed the influence of the O substitutional atoms that are created during electron bombardment in an O2 ambient. The O substitutional atoms acted as electron traps. On the electron beam bombardment of thin film phosphors, the degradation was more severe under O2 ambient compared to the same partial pressure of CO2 during electron beam bombardment, which is attributed to the free energy of formation of ZnO from ZnS when these respective gases are used. The degradation rate also depended on the energy of the electron beam, decreasing with increasing beam energy. This was interpreted according to the ionisation energy cross-section profile. The CL brightness increased exponentially with the increasing energy beam as more free carriers that will subsequently recombine yielding CL, are excited at higher beam energies. The thin film phosphor was also subjected to the electron beam bombardment after the phosphor film was coated with a CdO film by using a chemical bath deposition (CBD) method. The surface reactions were electron beam stimulated, resulting in the desorption of both Cd and S from the surface which happened as soon as the surface adventitious C was depleted. Sulphur from the ZnS accumulated on the surface but was soon depleted as volatile SOx compounds. The CdO was reduced by an electron beam assisted mechanism in the presence of non-reducible ZnO in the CdO-ZnO system as the Zn from the underlying ZnS layer emerged to the surface. The CL intensity degradation of the coated film showed a dependence on the surface composition. The intensity remained constant until the Cd was reduced on the surface before a slight decrease was observed. The effect of the CdO capping layer on the intensity of the phosphor was evident until the CdO eventually disintegrated.
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The determination of ternary segregation parameters using a linear heating method
(University of the Free State, 2000-11) Asante, Joseph Kwaku Ofori; Roos, W. D.; Du Plessis, J.
In this study the segregation behaviour of the ternary system Cu(lll ), Sb, Sn is investigated experimentally, as well as with the modified Darken segregation model. The model, which describes the kinetics as well as the equilibrium of segregation, had been used successfully in various studies of binary systems. A computer program based on this model was developed for ternary systems. A Cu(lll) single crystal was doped with low concentrations of 0,180 at% Sb and 0.133 at% Sn using evaporation and diffusion process.' The experimental results were gathered with the Auger electron spectroscopy technique. This technique was combined with a linear temperature ramp that makes it possible to obtain the segregation parameters in a single run. The traditional method requires various runs at different temperatures. The overlapping of Sb and Sn Auger peaks in the energy regions of interest necessitated the development of a method to successfully extract the true contributions of the elements from the measured spectra. It is clearly shown that the combination of Auger peaks is not linear and that the true contributions of Sb and Sn can be calculated if the peaks overlap in two energy regions and the standard spectra are available. The segregation profiles resulted from the Auger data show clearly the sequential segregation of the two elements (Sn and Sb). From the equilibrium conditions, it is also concluded that an interaction energy between Sb and Sn is present. By simulating the experimental results, using the theoretical Darken model, values for the segregation parameters can be obtained. The initial values for the fits are found mathematically (highenergy regions) and manually (low energy regions). The calculated profiles fit the experimental results very well. The present study confirms that Sn segregate first to the surface with Do = 1.58x10-5 m²s-¹ and E = 170 kJ/mol. Sb with a lower dimsion coefficient (Do = 1.93x10-8 m²s-¹ and E = 150 kJ/mol) segregates at higher temperatures. A further increase in temperature results in the stronger segregate Sb, (with a higher segregation energy ∆G = -74.6 kJ/mol) to displace the Sn (∆G = -59.0 kJ/mol) from the surface. From the simulations, it is clear that the maximum surface coverage for Sn is determined mainly by the attractive interaction (ΩSnCu = -8.25 kJ/mol) between Sn and Cu. The desegregation rate of Sn in this system is determined by the segregation rate of Sb. The segregation profile of Sb is similar to that in a binary system (Cu,Sb) with the desegregation rate of Sb much slower than the segregation rate. The study also shows definite attractive interaction between Sb and Cu (ΩSbCu= -17.05 kJ/mol) This trend was not observed in the studies of binary systems. There is, however, repulsive interaction between the segregates (ΩSnSb = 3.62 kJ/mol). The repeatability of the segregation parameters at different heating rates shows that this experimental method can be used successfully.
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The development of a UFS-Boyden Photometric pipeline to facilitate the observational study of accretion driven systems
(University of the Free State, 2005-05-30) Calitz, Johannes Jacobus; Meintjes, P. J.
After the retirement of professor A. Jarrett in 1986, the 1.5-m telescope at Boyden Observatory stood idle for a decade. With the appointment of Dr P. Meintjes, steps were taken to refurbish the telescope with an updated drive control and camera system, which would eventually enable the telescope to be operated as an astrophysical research instrument. After funding became available, upgrading of the drive mechanisms were undertaken by DFM during August and September 2001 and the new SpectraVision 1k 1k CCD camera, that was on loan from Lawrence Livermore National Laboratory (LLNL), was installed during February 2002. After 16 years, the telescope was ready to be used for gathering data for research projects. The camera was installed with only demonstration software. Software was needed to control the camera and also for data reduction and a photometry pipeline. During this project, the problems encountered with the baes, electronics and collimation in the telescope were analized and xed where needed and possible. Manuals were written for the general use of the telescope, as well as the reduction and photometry pipeline. Extinction coecients for Boyden Observatory were determined. Software were developed to control the PixelVision CCD camera. A CCD reduction routine that is easy and automatic as far as possible was written and implemented. A photometry pipeline that can be used with vast amounts of data, while producing a high level of accuracy were developed. The research elds that are making use of the software include gravitational microlens observations, accreting compact objects and Gamma Ray Burst afterglows. A brief overview of these elds are given.
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The development of an IRAF-based scientific photometric package for the UFS-Boyden 1.5-m telescope
(University of the Free State, 2009-03-10) Van Heerden, Hendrik Jacobus; Meintjes, P. J.
English: In this dissertation there will be looked at the development of an (Image Reduction and Analysis Facility) IRAF-based scientific photometric package for the UFS / Boyden 1.5-m telescope. The dissertation consist of a discussion on the history of Boyden Observatory and its instruments, with specific emphasis on the Rockefeller 1.5-m telescope. The discussion will include information on the upgrades and improvements the telescope underwent to compete and do research on an international level. In the proceeding chapters charged coupled devices (CCDs) will be discussed, as well as how to characterize CCD photometric observation systems, like the 1.5-m telescope. The chapters will include experimental procedures and results obtained during characterization experiments. Chapters on photometry techniques will follow thereafter as well as the development of the Boyden-IRAF photometric data-analysis system. It will include an overview of IRAF, as well as a more in depth discussion of the Boyden-IRAF package. The discussion will specify as to why and how it was developed and how it works. A final chapter will be presented on the testing of the Boyden-IRAF package through the determining of the Boyden atmospheric extinction coefficients using the newly developed package. With this project, i.e. the development of an IRAF-based photometric program, an attempt is made to fill a void that exists related to the in-house photometric capabilities. A reliable and user-friendly photometric program will definitely also result in Boyden Observatory playing an important role in student training and research programs. Finally a conclusion will be drawn as to the success of the new developments, the IRAF-based photometric package, and what this means for the development of Boyden Observatory and the UFS Astrophysics group i.t.o. research and development.
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Effect of broadband excitation ions in the luminescence of Ln.³+ doped SrF₂ nanophosphor for solar cell application
(University of the Free State, 2015-06) Yagoub, Mubarak Yagoub Adam; Coetsee, E.; Swart, H. C.
SrF2:Pr3+-Yb3+ phosphor powder was previously investigated for down-conversion application in solar cells. The rst surface, structural and optical characterization results indicated that the Pr3+-Yb3+ couple requires a sensitizer for effective enhancement in energy conversion. Broadband excitation ions of Ce3+ and Eu2+, that could be used as sensitizers, were therefore doped and co-doped in the SrF2 crystal. Detailed characterizations and investigations were then done on the surface, structure and optical aspects to see the effect on the energy conversion. Initially, the influence of different synthesis techniques on the surface, structure and concentration quenching of Pr3+ doped SrF2 was studied. The singly doped SrF2:Pr3+ was prepared by the hydrothermal and combustion methods. Scanning electron microscope (SEM) images showed different morphologies which was an indication that the morphology of the SrF2:Pr3+ phosphor strongly depended on the synthesis procedure. Both the SrF2:Pr3+ samples exhibited blue-red emission under a 439 nm excitation wavelength at room temperature. The emission intensity of Pr3+ was also found to be dependent on the synthesis procedure. The dipole-dipole interaction was found to be responsible for the concentration quenching effects at high Pr3+ concentration in both methods. SrF2:Eu nano-phosphors were successfully synthesized by the hydrothermal method. The crystalline size of the phosphors was found to be in the nanometre scale. The photoluminescence and high resolution x-ray photoelectron spectroscopy (XPS) results indicated that the Eu was in both Eu2+ and Eu3+ valance states. The presence of Eu2+ and Eu3+ in the system largely enhanced the response of the Eu3+ under ultraviolet excitation. Time of flight secondary ion mass spectrometry (tof-SIMS) results suggested that the energy transfer between these two ions was likely occurred. The relative photoluminescence intensity of the Eu2+ rapidly decreased with an increasing laser beam irradiating time. This result would make the current Eu2+ doped SrF2 samples unsuitable candidates for several applications, such as white light-emitting diodes and wavelength conversion films for silicon photovoltaic cells. The effect of Ce3+ ions on the SrF2:Eu nano-phosphor was also studied. Ce3+ largely enhanced the Eu3+ emission intensity via energy transfer mechanism. The calculated energy transfer efficiency was relatively effcient at high Eu concentration. The results suggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Eu ions in SrF2 host. Eu2+ co-doped Pr3+, Yb3+ and Pr3+-Yb3+ couple in SrF2 were successfully prepared. XPS confirmed that all Eu contents were in Eu2+ oxidation states. Initially, Eu2+ co-doped SrF2:Pr3+ was studied. From PL and decay curve results, an efficient energy transfer was demonstrated in SrF2:Eu2+, Pr3+ phosphors. The energy transfer process was effective until a concentration quenching between Pr3+ ions occurred. The results proposed that Eu2+ could be a good sensitizer for absorbing the UV photons and hence efficiently enhancing the Pr3+ emission intensity. SrF2:Eu2+ (1.5 mol%) co-doped with Na+ (0.5 mol%) and various concentrations of Yb3+ were also investigated. XRD results showed a mixture of the cubic SrF2 and NaYbF4 phases. The NaYbF4 phase gradually formed with increasing Yb3+ doping concentration. Emission spectra and the fluorescence decay curve measurements were utilized to demonstrate the cooperative energy transfer. Energy transfer occurred subsequently from Eu2+ to Yb3+ followed by intense NIR emission. The energy transfer was completed at high concentrations but the Yb3+ emission intensity was reduced as a result of concentration quenching. In addition, from the photoluminescence data it was evident that Na+ induced significant change to NIR emission. The possibility of using the broadband absorption of Eu2+ to sensitize the Pr3+-Yb3+ down-conversion couple in SrF2 matrix was also investigated. The energy transfer process was demonstrated by the decrease of Eu2+ and Pr3+ related photoluminescence and lifetime with increasing Yb3+ concentration. Upon 325 nm excitation into the 5d levels of Eu2+, the samples yield intense near infrared emission corresponding to Pr3+:4f-4f and Yb3+:4f-4f transition. Yb3+ emission was clearly observed only at high Yb3+ concentrations after the emission intensity of Pr3+ was quenched. The PL lifetime results of Eu2+ confirmed the the second-order cooperative energy transfer also occurred between Eu2+ and Yb3+ ions.
Open Access
Effect of host anion or cation substitution on the luminescence and stability of lanthanum oxide based phosphors doped with bismuth
(University of the Free State, 2015) Jabraldar, Babiker Mohammed Jaffar; Kroon, R. E.; Swart, H. C.; Ahmed, Hassan Abdel Halim Abdallah Seed
LaOCl:Bi3+ phosphor powders were successfully prepared via the solid-state reaction method and compared to LaOBr:Bi3+. Photoluminescence measurements revealed that the phosphors displayed ultraviolet emission around 344 nm for LaOCl:Bi and 358 nm for LaOBr:Bi under excitations of 266 nm and 273 nm, respectively. The optimum Bi doping concentration and annealing temperature were established as 0.7 mol% and 900 °C for both hosts. The scanning electron microscopy data showed that the particles are randomly oriented flat thin plates with diameters close to or larger than 100 nm. The chemical composition of the phosphor was studied by using energy dispersive X-ray spectroscopy, while diffuse reflectance spectroscopy was employed to study the absorption. The ultraviolet emission peaks were assigned to the 3P1 → 1S0 transition of the Bi3+ ions, while additional relatively weak emissions in the visible range were attributed to Bi clustering. LaOCl:Bi was found to undergo a little degradation while exposed to the atmosphere for several months, while LaOBr:Bi was significantly less stable. The LaOBr:Bi phosphor was also found to degrade rapidly under 254 nm ultraviolet excitation, while LaOCl:Bi did not degrade under the same conditions. X-ray photoelectron spectroscopy measurements indicated that similar surface changes occurred for both samples during UV-exposure, suggesting that the cause of degradation is not primarily related to surface changes, but may be due to the greater probability of forming non-radiative defects in the bulk of LaOBr having the smaller bandgap. Luminescence properties of bismuth (Bi) doped lanthanum oxyfluoride (LaOF) were investigated experimentally with samples prepared via the solid-state reaction method using ammonium fluoride, lanthanum oxide and bismuth oxide as precursors. Energy dispersive X-ray spectroscopy data showed that the samples were not completely stoichiometric and that the F/O ratio decreased with increasing synthesis temperature. The X-ray diffraction data confirmed that the phosphor powder crystallized in the tetragonal structure for lower synthesis temperatures up to 1000 C. When prepared at 1100 C, the LaOF took the rhombohedral form which showed almost no luminescence when doped with Bi. Therefore the investigation focussed on the tetragonal phase of LaOF for which scanning electron microscopy data showed that the shape of the particles did not change when it was doped with Bi. A single broad white luminescence emission band centred at 518 nm was observed by exciting the phosphor at 263 nm. The optimum Bi doping concentration was found to be La1-xOF:Bix=0.005. This sample was stable when exposed to the atmosphere for up to six months, although its luminescence degraded slowly under a 254 nm ultraviolet lamp. XPS data confirmed that the annealing at 1100 °C and exposure of the sample to UV radiation result in segregation of the Bi3+ on the surface of the sample. Bi3+ doped LaOCl and LaOF phosphor thin films were successfully prepared via the pulsed laser deposition technique in vacuum and different Argon (Ar) pressures. All peaks of the XRD patterns of the films were consistent with the tetragonal structure of the LaOCl and LaOF, but in the case of LaOF the signal was weaker and not all peaks were present, suggesting some preferred orientation. The elemental composition of the films were studied by using energy dispersive X-ray spectroscopy, while plan-view and cross-sectional scanning electron microscopy was used to study the morphology of the films. Photoluminescence measurements revealed that the films exhibited emission around 344 nm for LaOCl:Bi and 518 nm for LaOF:Bi under excitations of 266 nm and 263 nm, respectively. The emission peaks were attributed to the 3P1 → 1S0 transition of the Bi3+ ions. The luminescence from the LaOF:Bi sample was less intense compared to the LaOCl:Bi sample prepared under the same conditions, which was also the case for powder samples. The amount of ablated material present on the substrate was much less for LaOF:Bi compared to LaOCl:Bi, which is attributed to the greater bandgap and hence weaker absorption of the laser pulses for LaOF:Bi. La2-yYyO3 phosphor powders were prepared via the citric acid sol-gel combustion method and their stability was checked after four months of storage in the atmosphere using X-ray diffraction. The material, like La2O3, was prone to hydroxylation unless a high proportion of Y was added, which also caused changes in the phases present. Based on its stability and single phase (cubic), the composition La0.5Y1.5O3 was selected for further study with Bi doping. Under an excitation of 333 nm the phosphor presented two emission peaks located at 424 and 529 nm. This corresponded to excitation of Bi3+ ions in the non-centrosymmetric C2 symmetry sites. Blue (424 nm) emission could be excited from the same samples when excited at 371 nm from Bi3+ ions on the centrosymmetric S6 sites, which was similar in colour to the emission of Bi-doped La2O3. The optimum doping concentration for La0.5-xY1.5O3:Bi powder was found to be x = 0.005. The luminescence from this material was not as intense as that from Y2-xO3:Bix=0.005, but there was a significant shift in colour as a result of the added La which meant that emission from the Bi3+ ions at the S6 sites of the alloy was close to that of unstable La2O3 on the colour chart. Bismuth (Bi3+) doped lanthanum gallate (LaGaO3) phosphor powder and thin films were successfully synthesised via the citric acid sol-gel combustion synthesis and pulsed laser deposition (PLD), respectively. Firstly the stability of La2-yGayO3 powders under ambient conditions was assessed. The optical properties of La1-xGaO3:Bix phosphor powders were then investigated. X-ray diffraction data confirmed that the structure of LaGaO3 belong to the orthorhombic perovskite structure with Pbnm space group, while diffuse reflectance spectroscopy data showed that the band gap energy of the LaGaO3 host was 4.65 eV. Scanning electron microscopy data showed that the grain size increased with increasing annealing temperature. There was no change in the particle size and morphology when dopant was added to the host. Photoluminescence (PL) and cathodoluminescence (CL) measurements indicated that the phosphor presented efficient ultraviolet emission around 370 nm, which was attributed to transitions between the 3P1 excited state and 1S0 ground state of the Bi3+ ions. The optimum Bi doping concentration and annealing conditions for photoluminescence were found to be for La1-xGaO3:Bix=0.003 and 1200 °C. Thin films of the optimized powder were prepared via PLD. The PL and CL of the LaGaO3:Bi3+ thin films were similar to the powder.
Open Access
The effect of nitrogen on the cosegregation of molybdenum in a Fe-3.5wt%Mo-N (100) single crystal
(University of the Free State, 2006) Jordaan, Werner Albert; Terblans, J. J.; Swart, H. C.
In this study the cosegregation of molybdenum and nitrogen to the (100) plane of an iron single crystal was investigated. Ternary systems are considerably more complex than binary systems in that there are seven segregation parameters to determine, as opposed to three. However, a novel approach was undertaken to minimize the amount of variables, by first analysing a similar binary system that was exposed to a nitrogen ambient. Two single crystal were selected for this purpose, i.e. a Fe- 3.5wt%Mo(100) binary system and a Fe-3.5wt%Mo-N ternary system. By exposing the binary crystal to a nitrogen ambient at high temperatures it was observed that molybdenum segregated to the surface. The segregation profiles of the two systems were acquired at constant temperatures from 797 K - 888 K and Auger Electron Spectroscopy was used to monitor the surface concentrations of the relevant species. Since accurate surface temperature measurements are essential to segregation studies, a calibrated infrared thermometer was used. The segregation profiles were generated by measuring time and the Auger signal simultaneously. From the segregation profiles, initial estimates for the diffusion coefficients of Mo were first determined for the binary system by applying Fick’s equation to the segregation profiles. From these values the pre-exponential factor, D0, was determined to be 1.2x10−4±2 m2/s and the activation energy, E, as 258±33 kJ/mol. The diffusion coefficients determined thus, were used as estimates for obtaining the Darken seggregation profiles. In this case the D0 value was found to be 2.4x100±1 m2/s and the E value, 323±16 kJ/mol. The segregation energy, G, of Mo was calculated as -38 kJ/mol. In both cases it was observed that the diffusion coefficient of Mo deviated from the expected value at high temperatures due to the desorption of nitrogen from the surface. Using thermodynamic theory, an expression for the segregation energy of Mo in terms of the nitrogen surface concentration was derived. The Darken fits were repeated and it was found that the high temperature diffusion coefficient values fell on the the Arrhenius linear regression lines. For this special case, the D0 value was calculated as 5.5x101±1 m2/s, the E value as 345±18 kJ/mol. The segregation parameters determined for the binary system were then used as initial values for fitting the experimental data of the ternary system. Using Fick’s equation, the diffusion coefficients of Mo and N in Fe were determined. From the Arrhenius linear regression, the pre-exponential factor for Mo was calculated as 3.6x10−2±1 m2/s and that of N as 4.1x10−1±2 m2/s. The activation energies were 308±20 kJ/mol and 210±40 kJ/mol for Mo and N, respectively. The segregation parameters of the ternary system were then determined via the Darken method. In this case the pre-exponential factors were 1.9x10−4±1 m2/s for Mo and 2.8x100±3 m2/s for N. The activation energies were 271±11 kJ/mol and 323±43 kJ/mol. The segregation energy of Mo was calculated as -32 kJ/mol and for N, -19 kJ/mol. The interaction coefficient between Mo and N was calculated as -19 kJ/mol.