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Item Open Access The role of thickness on the structural and luminescence properties of Y₂O₃:Ho³⁺, Yb³⁺ upconversion films(Nature Research, 2024) Makumbane, Vhahangwele; Kroon, Robin E.; Yagoub, Mubarak Y. A.; Erasmus, Lucas J. B.; Coetsee, E.; Swart, Hendrik C.The structural, surface, and upconversion (UC) luminescence properties of Y₂O₃:Ho³⁺,Yb³⁺ films grown by pulsed laser deposition, for different numbers of laser pulses, were studied. The crystallinity, surface, and UC luminescence properties of the thin films were found to be highly dependent on the number of laser pulses. The X-ray powder diffraction analysis revealed that Y₂O₃:Ho³⁺,Yb³⁺ films were formed in a cubic structure phase with an Ia ̅3 space group. The thicknesses of the films were estimated by using cross-sectional scanning electron microscopy, depth profiles using X-ray photoelectron spectroscopy (XPS), and the Swanepoel method. The high-resolution XPS was used to determine the chemical composition and oxidation states of the prepared films. The UC emissions were observed at 538, 550, 666, and 756 nm, assigned to the ⁵F₄ → ⁵I₈, ⁵S₂ → ⁵I₈, ⁵F₅ → ⁵I₈, and ⁵S₂ → ⁵I₇ transitions of the Ho³⁺ ions. The power dependence measurements confirmed the involvement of a two-photon process in the UC process. The color purity estimated from the Commission International de I’Eclairage coordinates confirmed strong green UC emission. The results suggested that the Y₂O₃:Ho³⁺,Yb³⁺ UC transparent films are good candidates for various applications, including solar cell applications.Item 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.Item Open Access Experimental and theoretically calculated structural data of different iron(II)-terpyridine complexes – validation of theoretical method(Elsevier, 2024) Athanasopoulos, Evangelia; Conradie, Marrigje Marianne; Conradie, JeanetExperimental structural data for bis(terpyridine)iron(II) and a series of related iron(II) complexes, featuring either substituted terpyridine or tris-azinyl analogues of terpyridine, are presented and analyzed in terms of the Mean Absolute Deviation (MAD) from the average experimental data for each specific complex. The experimental structural data are then juxtaposed with density functional theory (DFT) calculated data obtained using various combinations of DFT functionals and basis sets, with and without the inclusion of Grimme D3 empirical dispersion correction. These diverse computational approaches yield optimized geometries that are subsequently compared against the available experimental structural data to assess their accuracy. The aim is to identify a reliable DFT method for determining the geometries of bis(terpyridine)iron(II) and its related complexes.Item Open Access Web-based application software for Judd-Ofelt analysis of Eu³⁺ ion luminescence(Elsevier, 2024) Hasabeldaim, E.H.H.; Swart, H.C.; Kroon, R.E.This article reports on the development of a new Web-based application software for calculating Judd-Ofelt intensity parameters and derived quantities from the emission spectra of Eu³⁺ doped compounds. The application was entirely developed in JavaScript, and it is compatible with all major browsers. The web application can be accessed via the following link https://sciapps.sci-sim.com/judd_ofelt_analysis_Eu.html. Test samples of ZnO doped with different amounts of Eu³⁺ were synthesized using the combustion method, and their radiative properties and the chemical environment in the Eu surroundings were thoroughly investigated by Judd-Ofelt intensity and the derived parameters using the online application. In contrast to the tediousness and time-consuming process of these types of calculations, by using this online software, all intensity parameters and the derived quantities can be obtained within a short time.Item 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, FahrettinIn 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.Item Open Access The vanishing of the primary emission region in PKS 1510–089(American Astronomical Society, 2023) Aharonian, F.; Ait Benkhali, F.; Aschersleben, J.; Ashkar, H.; Backes, M.; Barbosa Martins, V.; Barnard, J.; Batzofin, R.; Becherini, Y.; Berge, D.; Bernlohr, K.; Bi, B.; de Bony de Lavergne, M.; Bottcher, M.; Boisson, C.; Bolmont, J.; Borowska, J.; Bouyahiaoui, M; Bradascio, F.; Breuhaus, M.; Brose, R.; Brown, A. M.; Brun, F.; Bruno, B.; Bulik, T.; Burger-Scheidlin, C.; Caroff, S.; Casanova, S.; Cecil, R.; Celic, J.; Cerruti, M.; Chand, T.; Chandra, S.; Chen, A.; Chibueze, J.; Chibueze, O.; Cotter, G.; Damascene Mbarubucyeye, J.; Davids, I. D.; Djannati-Atai, A.; Dmytriiev, A.; Doroshenko, V.; Egberts, K.; Einecke, S.; Ernenwein, J.-P.; Fegan, S.; Fontaine, G.; Fussling, M.; Funk, S.; Gabici, S.; Ghafourizadeh, S.; Giavitto, G.; Glawion, D.; Glicenstein, J. F.; Goswami, P.; Grolleron, G.; Haerer, L.; Hofmann, W.; Holch, T. L.; Holler, M.; Horns, D.; Jamrozy, M.; Jankowsky, F.; Joshi, V.; Jung-Richardt, I.; Kasai, E.; Katarzynski, K.; Khatoon, R.; Khelifi, B.; Kluzniak, W.; Komin, N.; Kosack, K.; Kostunin, D.; Lang, R. G.; Le Stum, S.; Leitl, F.; Lemiere, A.; Lenain, J.-P.; Leuschner, F.; Luashvili, A.; Mackey, J.; Marandon, V.; Marchegiani, P.; Marti-Devesa, G.; Marx, R.; Mehta, A.; Meyer, M.; Mitchell, A.; Moderski, R.; Mohrmann, L.; Montanari, A.; Moulin, E.; de Naurois, M.; Niemiec, J.; Priyana Noel, A.; O'Brien, P.; Ohm, S.; Olivera-Nieto, L.; de Ona Wilhelmi, E.; Ostrowski, M.; Panny, S.; Panter, M.; Peron, G.; Prokhorov, D. A.; Ren, H.; Rieger, F.; Rowell, G.; Rudak, B.; Rueda Ricarte, H.; Ruiz-Velasco, E.; Sahakian, V.; Salzmann, H.; Sanchez, D. A.; Santangelo, A.; Sasaki, M.; Schussler, F.; Schutte, H. M.; Schwanke, U.; Shapopi, J. N. S.; Sol, H.; Specovius, A.; Spencer, S.; Stawarz, K.; Steenkamp, R.; Steinmassl, S.; Steppa, C.; Sushch, I.; Suzuki, H.; Takahashi, T.; Tanaka, T.; Terrier, R.; Tsuji, N.; van Eldik, C.; van Soelen, B.; Vecchi, M.; Veh, J.; Vink, J.; Wach, T.; Wagner, S. J.; Wierzcholska, A.; Zacharias, M.; Zargaryan, D.; Zdziarski, A. A.; Zech, A.; Zouari, S.; Zywucka, N.; Buckley, D. A. H.; Cooper, J.; Groenewald, D.; H.E.S.S., CollaborationIn 2021 July, PKS 1510−089 exhibited a significant flux drop in the high-energy γ-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy γ-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy γ-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy γ-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.Item Open Access 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, JeanetWe 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).Item Open Access AutoCal: A software application for calibrating photometric data(Academy of Science of South Africa (ASSAf), 2016) Wium, Daniël J.; Van Soelen, BrianWe 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.