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Now showing items 1 - 16 of 93

  • Enhanced green luminescence from ZnO nanorods

    Hasabeldaim, Emad H. H.   Ntwaeaborwa, Odireleng M.   Kroon, Robin E.   Coetsee, Elizabeth   Swart, Hendrik C.  

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  • Conducting Polymer Hydrogels and Their Applications

    Sharma, Kashma   Kumar, Vijay   Kaith, B. S.   Kalia, Susheel   Swart, Hendrik C.  

    The industrial demand for novel and smart materials as well as the urge for basic understanding has led to a notable improvement in the area of polymer science. Recently, significant consideration has been given to the modifications of biodegradable hydrogels based on natural polymers with conducting polymers (CPs), because this extends a straightforward process to couple the better features of CPs with the highly cross-linked hydrogels. The final hydrogels have been developed to change mainly electrical and structural properties to a larger degree. Herein, we present a comprehensive survey of the existing and current literature on conducting hydrogels based on natural polymers. This chapter also highlights ample of methods used to synthesize conducting hydrogel, properties, and characterization techniques. Conducting hydrogels have been used in the removal of dyes from wastewater, drug delivery, fuel cells, supercapacitors, dye-sensitized solar cells, rechargeable lithium batteries, etc. All these justify the rising interest in both academia and industrial development. In this analysis, an overview of potential applications of these conducting hydrogels and current challenges in the field are discussed; some new and futuristic advances in this captivating area are also provided.
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  • Calculated Nanocube Vacancy Formation Energy and Cohesion Energy at o K

    van der Walt, Cornelia   Terblans, Jacobus J.   Swart, Hendrik C.  

    Nanoparticles of face-centered cubic Cu are modeled using the Sutton-Chen potential. Shapes ranging from perfect cubes through to octahedrons are modeled and characterized. Bulk properties, surface energies, vacancy formation energy, E-v, and cohesive energies, E-coh, are investigated for particles simulated to up to 5 nm in diameter. Below the subsurface layers, particles larger than 1 nm diameter are compared well to bulk. Of the different shapes, rhombicuboctahedrons are both more stable and have more reactive surfaces. As E-v is dependent on surface orientation, there is a little correlation with size and E-v is mostly dependent on nanoparticle shape. E-coh is not as dependent on surface orientation and shows both size and shape dependency.
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  • Luminescence properties and cathodoluminescence degradation of Bi doped SrO powder

    Abdelrehman, Mogahid H. M.   Kroon, Robin E.   Yousif, Abdelrhman   Seed Ahmed, Hassan A. A.   Swart, Hendrik C.  

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  • The "firing cannons" of Dipodascopsis uninucleata var. uninucleata

    Olivier, Andries P. S.   Swart, Chantel W.   Pohl, Carolina H.   van Wyk, Pieter W. J.   Swart, Hendrik C.   Coetsee, Elizabeth   Schoombie, Schalk W.   Smit, Jon  

    According to literature, the elongated ascospores of Dipodascopsis uninucleata var. uninucleata exhibit smart movement when forcefully ejected from bottle-shaped asci. This type of movement is defined as the unique patterns of non-random movement of ascospores with specialized morphology thereby facilitating release from asci. Smart movement is required to actively release ascospores individually through the narrow ascus neck, without causing an obstruction and blocking ascospore release. However, little is known about the propulsion mechanism of this cannon-type release system. We show that asci of this yeast contain a central channel (barrel) filled with ascospores. These are surrounded by a sheath-like structure that lines the inner surface of the ascus wall. We found that this sheath is responsible for forcing the naked ascospores out of the ascus by exerting turgor pressure from the bottom towards the tip of the ascus. This cannon firing system is in contrast to that found in Dipodascus geniculatus, where no sheaths lining the ascus interior were observed. Instead, sheaths were found enveloping each ascospore.
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  • Low temperature photoluminescence study of Ce3+ and Eu2+ ions doped SrF2 nanocrystals

    Yagoub, Mubarak Y. A.   Swart, Hendrik C.   Kroon, R. E.   Coetsee, Elizabeth  

    The photoluminescence properties of Eu2+ and Ce3+ doped SrF2 have been reported at room temperature and 77 K. The emission spectra of Eu2+ ions in SrF2 consisted of a broad band assigned to the 5d-4f transition of the Eu2+ ion. Ce3+ ions in SrF2 showed a weak emission intensity. The emission intensity of Eu2+ ion was greatly enhanced at 77 K. At room temperature and 77 K, the fluorescence decay of Eu2+ and Ce3+ ions in SrF2 have been measured and the fluorescence decay curve have been reported.
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  • White light emitting LaGdSiO5:Dy3+ nanophosphors for solid state lighting applications

    Ogugua, Simon N.   Swart, Hendrik C.   Ntwaeaborwa, Odireleng M.  

    Powdered dysprosium (Dy3+) doped Lanthanum gadolinium oxyorthosilicate (LaGdSiO5) mixed phosphors were synthesized using urea-assisted solution combustion method. The X-ray diffractometer analysis showed that the samples crystalized in the pure monoclinic mixed phase of LaGdSiO5. The crystallite size and the lattice strain calculated from the X-ray diffraction peaks using Williamson-Hall equation varied from 12 nm to 16 nm and 1.6x10(-2) to 2.43x10(-2) respectively. The photoluminescence (PL) emission spectra recorded using 425, 454 and 475 nm excitation wavelengths exhibit characteristic similar to the YAG:Ce phosphor pumped InGaN LED system, by absorbing portion of the excitation energy and re-emitting it. The emission spectra were characterized by radiative recombination at 425, 454, 475,485 and 575 nm depending on the excitation wavelength. These emission line are ascribed to the f -> f transitions of Dy3+ The peak intensity and hence the color of the emitted visible light were dependent on the concentration of Dy3+ The International Commission on Illumination (CIE) color coordinates of (0.336, 0.313) and (0.359, 0.361) were obtained for Dy3+ molar concentration of 0.05 and 3.0 mol% when the emission was monitored using 454 nm and 475 nm respectively. The band gap measured from the reflectance curve using Tauc plot initially decreased with increasing Dy3+ concentration, but at higher concentration, it started to increase. These materials were evaluated for solid state lighting application. (C) 2015 Elsevier B.V. All rights reserved.
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  • Surface analysis and cathodoluminescence degradation of undoped ZnO and ZnO:Zn phosphors

    Hasabeldaim, Emad   Ntwaeaborwa, Odireleng M.   Kroon, Robin E.   Swart, Hendrik C.  

    Commercial ZnO: Zn and undoped ZnO with a hexagonal wurtzite structure (JCPDS 80-0075) were studied. The direct optical bandgaps were 3.25 and 3.28 eV for the undoped and ZnO: Zn, respectively. Undoped ZnO exhibited strong ultraviolet band to band emission as well as weak deep level defect emission around the green, orange, and red regions. ZnO: Zn emits only green light centered at 514 nm which was attributed to oxygen vacancies. Electron irradiated degradation of the ZnO: Zn sample showed a rapid initial decrease in cathodoluminescence intensity and then a subsequent recovery and increase to up to double the initial value during prolonged electron irradiation. Deconvoluted peaks of the O1s x-ray photoelectron peaks confirmed the presence of oxygen-related defects in both samples. The ZnO: Zn sample was found to be very stable under electron bombardment, which makes it suitable for the use in field emission displays. (C) 2016 American Vacuum Society.
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  • Electron Stimulated Surface Chemical Reaction Mechanism for Phosphor Degradation

    Swart, Hendrik C.   Terblans, Jacobus J.   Nagpure, I.M.   Pitale, S.S.   Ntwaeaborwa, O.M.  

    Borates, phosphates, aluminates, silicates or sulphides are usually used as host lattices for display phosphors. These phosphors reacted similarly under electron bombardment. Nano and micron phosphors normally lose brightness upon bombardment with electron, ion or photon beams. A combination of techniques such as XPS (X-ray photoelectron spectroscopy), AES (Auger electron spectroscopy) and CL (cathodoluminescence) spectroscopy were used to show that the main reason for the degradation in luminescent intensity, of the different phosphors, under electron bombardment is the formation of a non-luminescent layer on the surface due to an electron stimulated surface chemical reaction (ESSCR). The decrease in luminance was found to be the result of the growing of a “dead layer” on the surface. In some cases, however, a thermodynamically stable layer formed on the surface as a result of the electron stimulated surface chemical reactions lead to CL stability of the phosphor. The formation of an altered layer (oxide layer) on the surface of the different phosphors leads to a decrease in the luminescent intensity at that specific wavelength and in some cases to an increase of the intensity at another wavelength.
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  • Synthesis and studies of carbazole-based donor polymer for organic solar cell applications

    Sathiyan, Govindasamy   Siva, Govindasamy   Sivakumar, E. K. T.   Prakash, Jai   Swart, Hendrik C.   Sakthivel, Pachagounder  

    We have developed a carbazole-based electron donor material, containing extended thiophene pi-conjugated through a cyanovinylene spacer in order to enhance the effective intramolecular charge transfer for bulk heterojunction polymer solar cell (BHJ-PSC) applications. The polymer was synthesized via FeCl3 oxidative polymerization method, namely poly(2E,2'E)-3,3'-(9-hexyl-9H-carbazole-3,6-diyl)bis(2-(5-methylthiophen-2yl)acrylonitrile (CN-PICTAN). The CN-PICTAN formation was confirmed by Fourier transformed infrared, nuclear magnetic resonance spectroscopy, and gel permeation chromatography techniques. The CN-PICTAN showed a broad absorption and emission range with optical band gap (E (g) (opt)) of 2.1 eV. The CN-PICTAN exhibited a high thermal stability with 5% weight loss at 356 A degrees C and deep-lying highest occupied molecular orbital level of - 5.23 eV. The BHJ-PSC device fabricated with CN-PICTAN:PC61BM (1:1.5) showed a power conversion efficiency of 1.23%, which was enhanced to 1.73% after the device was annealed at 100 A degrees C.
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  • Luminescence and energy transfer of color-tunable Lu2MgAl4SiO12:Eu2+,Ce3+,Mn2+ phosphors

    Ming, Zhiqiang   Zhao, Jing   Swart, Hendrik C.   Xia, Zhiguo  

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  • Effect of Mn and Cr doping on the up-conversion luminescence from NaYE4:Yb3+,Er3+

    Tuomisto, Minnea   Palo, Emilia   Laihinen, Tero   Hyppanen, Iko   Lastusaari, Mika   Swart, Hendrik C.   Holsa, Jorma  

    The up-conversion luminescence where the absorption of two or more low-energy photons results in emission of a higher-energy photon has been highly effective in certain applications such as bioanalytical assays. Currently one of the most efficient up-converting materials is the NaYF4:Yb3+,Er3+. However, some applications (e.g. solar cells) still require improvement of the up-conversion efficiency in order to have actual practical use. Therefore, it is important to enhance the performance of the materials. In this work, the effect of Mn and Cr doping on the up-conversion luminescence of NaYF4:Yb3+,Er3+ was studied. The materials were prepared using the co-precipitation method and the ratios of the doping ions were optimized for best up-conversion intensity. The as-prepared materials showed very similar sizes and morphologies. Cr co-doping was observed to increase the up-conversion luminescence whereas the addition of Mn resulted in quenching. Up-conversion luminescence was also measured with different excitation power densities and the results suggested that Cr co-doping enables the up-conversion in lower power densities than without Cr co-doping. Furthermore, the reasons for the changes in up-conversion luminescence intensity were discussed. (C) 2016 Elsevier B.V. All rights reserved.
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  • LaBO3 (B= Fe, Co) nanofibers and their structural, luminescence and gas sensing characteristics

    Shingange, Katekani   Swart, Hendrik C.   Mhlongo, Gugu H.  

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  • A review on recent progress of p-type nickel oxide based gas sensors: Future perspectives

    Mokoena, Teboho P.   Swart, Hendrik C.   Motaung, David E.  

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  • Cathodoluminescence properties of monoclinic phased reddish-orange emitting BaY2(MoO4)4:Eu3+ phosphor

    Krishnan, Rajagopalan   Swart, Hendrik C.  

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  • Mechanoluminescence induced by impulsive excitation in CdS and CdSe:Mn,Cu,Ag,Au doped phosphors

    Kaur, Jagjeet   Dubey, Neha   Tiwari, Ratnesh   Dubey, Vikas   Pai, Asha   Dhoble, S. J.   Swart, Hendrik C.  

    In this paper the phenomenon of mechanoluminescence (ML) in CdSe:Mn, CdS:Au, CdS:Mn, CdS:Ag and CdS:Cu (cubic) phosphors is described. When the ML was excited impulsively by the impact of a load on the phosphors the ML intensity increased with time, attained a maximum value and then it decreased. In the ML intensity versus time curve, the peak increased and shifted towards shorter time values with increasing impact velocities and the rising portion of the curve followed an exponential increase and the decaying portion followed an exponential decrease with time. The total ML initially increased with the impact velocity of the load and then it attained a saturation value for higher values of the impact velocities. The total ML intensity increased linearly with the amount (mass) of the phosphors for the higher impact velocities. The ML intensity corresponding to the peak of the ML intensity versus time curve increased linearly with the impact velocities. The time was found to be linearly related to 1000/impact velocity. The ML intensity attained an optimum value for the activator concentration of 0.4% in CdSe:Mn, CdS:Au, CdS:Mn, CdS:Ag and CdS:Cu phosphors. CdSe:Mn phosphors were found to be the most intense ML and the CdS:Cu phosphors exhibited a comparatively weak ML. The ML induced by impulsive excitation in the CdS and CdSe doped phosphors plays a significance role in the understanding of the biological sensors and display device applications.
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