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Now showing items 1 - 16 of 73

  • Advances in phosphors based on organic materials for light emitting devices

    Kashma Sharma   Vijay Kumar   Vinod Kumar   Hendrik C. Swart  

    Abstract A brief overview is presented in the light emitting diodes (LEDs) based on purely organic materials. Organic LEDs are of great interest to the research community because of their outstanding properties and flexibility. Comparison between devices made using different organic materials and their derivatives with respect to synthetic protocols, characterizations, quantum efficiencies, sensitivity, specificity and their applications in various fields have been discussed. This review also discusses the essential requirement and scientific issues that arise in synthesizing cost-effective and environmental friendly organic LEDs diodes based on purely organic materials. This mini review aims to capture and convey some of the key current developments in phosphors formed by purely organic materials and highlights some possible future applications. Hence, this study comes up with a widespread discussion on the various contents in a single platform. Also, it offers avenues for new researchers for futuristic development in the area.
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  • Enhanced Terbium Emission Due to Plasmonic Silver Nanoparticles in Bismuth Silicate

    Abd Ellateef Abbass   Hendrik C. Swart   Elizabeth Coetsee   Robin E. Kroon  

    A simple way to select a suitable host material, when doped with any rare‐earth (RE) ion and incorporated with silver nanoparticles (NPs), to cause overlap between an excitation band of the RE ions and the localized surface plasmon resonance (LSPR) of the metallic NPs to study possible plasmonic enhancement is presented using Mie theory calculations. Unlike in previous studies, plasmonic enhancement was studied in a crystalline host instead of amorphous hosts. Bismuth silicate was synthesized using the sol–gel method and successfully doped with only terbium or silver, or co‐doped with terbium and silver. The formation of silver NPs was investigated using diffuse reflectance spectroscopy, transmission electron microscopy, and Auger electron spectroscopy (AES). Luminescence properties of the terbium‐doped bismuth silicate containing silver NPs were explored in detail and an enhancement of the emission from terbium ions at 545 nm when excited at 485 nm of about 2.5 times is attributed to amplification of the electric field associated with the LSPR of the silver NPs.
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  • Gas bubble formation in the cytoplasm of a fermenting yeast

    Chantel W. Swart   Khumisho Dithebe   Carolina H. Pohl   Hendrik C. Swart   Elizabeth Coetsee   Pieter W.J. van Wyk   Jannie C. Swarts   Elizabeth J. Lodolo and Johan L.F. Kock  

    Current paradigms assume that gas bubbles cannot be formed within yeasts although these workhorses of the baking and brewing industries vigorously produce and release CO2 gas. We show that yeasts produce gas bubbles that fill a significant part of the cell. The missing link between intracellular CO2 production by glycolysis and eventual CO2 release from cells has therefore been resolved. Yeasts may serve as model to study CO2 behavior under pressurized conditions that may impact on fermentation biotechnology.
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  • Glowing synthetic chlorohectorite: The luminescent features of a trioctahedral clay mineral

    Hellen S. Santos   José M. Carvalho   Antti Viinikanoja   Iko Hyppänen   Tero Laihinen   Eric C. Romani   Dunieskys G. Larrudé   Marjukka Tuominen   Pekka Laukkanen   Hendrik C. Swart   Hermi F. Brito   Jorma Hölsä   Mika Lastusaari  

    Abstract Clay minerals are versatile materials with numerous industrial applications. Their unique layered structure can act as a host for many optically active species. The synthesis and luminescence properties of chlorohectorite clays are now reported for the first time. The synthesized clays are composed of quasi-circular nanoparticles with diameters around 20–30 nm showing blue/green emission and persistent luminescence with a duration of ca. 5 s. The luminescence features are associated with titanium impurities in the chlorohectorite structure. It is shown that Ti can act as activator ion even at low concentrations: the XRF and ICP-MS measurements of the chlorohectorite materials indicate a Ti concentration around 60 ppm originating from the precursors. The XPS measurements of the clay materials showed the Ti 2p 3/2 signals at 457.8 and 458.9 eV associated with Ti 3+ and Ti IV , respectively. The excitation spectra confirm Ti 3+ as the luminescent center in the chlorohectorites through the bands at 287 and 370 nm, related to the e − ( O 2 − ( 2 p ) ) → Ti IV charge transfer and the electronic transitions from the t 2g to eg levels of Ti 3+ , respectively. The persistent luminescence properties are due to structural defects common for layered silicates. Graphical abstractfx1
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  • Luminescence investigation of visible light emitting Ho3+ doped tellurite glass

    Anurag Pandey   Hendrik C. Swart  

    Abstract Dual mode (upconversion and downshifting) emissions from the holmium activated TeO 2 –ZnO glass is the focus point of the present study. The glasses were prepared by the well-known melt quenching technique and identified by X-ray diffraction analysis. The thermal stability and glass transition characteristics were detailed on the basis of differential thermal analysis. The luminescence behaviour of the prepared glasses was explained on the basis of optical absorption and emission spectra recorded at different excitations. Intense green and yellowish-green emissions were detected upon excitations with blue and near-infrared wavelength photons, respectively. The upconversion process and colour tuning emission properties were discussed in detail with the help of a power dependence study, energy level diagram and CIE calculations. Highlights • Downshifting and upconversion emission were detected from the Ho 3+ doped TZO glasses. • Power tunable yellowish-green light was obtained upon 453 and 980 nm excitation. • A two photon upconversion process was responsible for the UC emissions.
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  • Microwave assisted synthesis of ZnO nanoparticles for lighting and dye removal application

    Vijay Kumar   M. Gohain   S. Som   Vinod Kumar   B.C.B. Bezuindenhoudt   Hendrik C. Swart  

    Abstract In this study, we report on the synthesis of ZnO nanoparticles (NPs) via the microwave-assisted technique. The as-synthesized ZnO nanoparticles were annealed at 500 °C for three hours. The ZnO NPs were characterized by X-ray diffraction (XRD) and scanning electron microscopic techniques. XRD results confirmed the formation of as-synthesized ZnO powder oriented along the (101) direction. The Kubelka–Munk function has been employed to determine the band gap of the ZnO powder. ZnO powder has been studied by photoluminescence (PL) before and after annealing to identify the emission of defects in the visible range. The intensity of the PL emission has decreased after annealing. The synthesized ZnO samples were also studied for methyl orange dye removal from waste water. It has been found that the as-synthesized ZnO shows better adsorption behaviour as compared to the annealed sample.
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  • Surface Characterization and Photoluminescence Properties of Ce3+,Eu Co-Doped SrF2 Nanophosphor

    Mubarak Y. A. Yagoub   Hendrik C. Swart   Luyanda L. Noto   Peber Bergman   Elizabeth Coetsee  

    SrF2:Eu,Ce3+ nanophosphors were successfully synthesized by the hydrothermal method during down-shifting investigations for solar cell applications. The phosphors were characterized by X-ray diffraction (XRD), scanning Auger nanoprobe, time of flight-secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. XRD showed that the crystallite size calculated with Scherrer’s equation was in the nanometre scale. XPS confirmed the formation of the matrix and the presence of the dopants in the SrF2 host. The PL of the nanophosphor samples were studied using different excitation sources. The phenomenon of energy transfer from Ce3+ to Eu2+ has been demonstrated.
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  • Electrocatalytic biofuel cell based on highly efficient metal-polymer nano-architectured bioelectrodes

    Prashant Mishra   G.B.V.S. Lakshmi   Sachin Mishra   D.K. Avasthi   Hendrik C. Swart   Anthony P.F. Turner   Yogendra K. Mishra   Ashutosh Tiwari  

    Abstract Bioenergy based devices are rapidly gaining significant research interest because of growing quest for future alternative energy resources, but most of the existing technologies suffer from poor electron transfer and slow mass transport, which hinder the fabrication of realistic high-power devices. Using a versatile strategy, here we have demonstrated the fabrication of nanoparticle-polymer framework based bioelectrocatalytic interfaces which facilitate a high mass-transport and thus offers the simple construction of advanced enzyme-based biofuel cells. It has been shown that a gold nanoparticle-structured polyaniline network can be effectively used as an electrical cabling interface providing efficient electron transfer for bio- anode and cathode. The resulting bioelectrodes are capable of excellent diffusional mass-transport and thus can easily facilitate the design of new and highly efficient membrane-less advanced bioenergy devices. The biofuel cell delivers a high-power density of about 2.5 times (i.e., 685 µW cm −2 ) and open circuit voltage of 760 mV compared to conventional conducting polymer-based biofuel cells. Graphical abstract The illustration of the biofuel cell set-up using bioanode, GOx/Au@PANI and biocathode, Laccase/Au@PANI nanoarchitectured bioelectrodes. fx1 Highlights • A high mass-transport bioelectrocatalytic interfaces based on a nanoparticle-polymer framework is investigated. • The compatibility of bio- anode and cathode was tested with electrochemical methods. • The bioelectrodes can offer highly efficient membrane-less bioenergy devices. • The biofuel cell delivers a high-power density and open circuit voltage compared to conventional polymer-based biofuel cells. • The nanoparticle-polymer framework could significantly accelerate developments in the field of high-performance bio-energy devices.
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  • Recent advances in rare earth doped alkali-alkaline earth borates for solid state lighting applications

    Shefali Verma   Kartikey Verma   Deepak Kumar   Babulal Chaudhary   Sudipta Som   Vishal Sharma   Vijay Kumar   Hendrik C. Swart  

    Abstract As a novel class of inorganic phosphor, the alkali-alkaline earth borate phosphors have gained huge attention due to their charming applications in solid-state lighting (SSL) and display devices. The current research drive shows that phosphors based on the alkali-alkaline earth borates have transformed the science and technology due to their high transparency over a broad spectral range, their flexibility in structure and durability for mechanical and high-laser applications. Recent advances in various aspects of rare-earth (RE) doped borate based phosphors and their utilizations in SSL and light emitting diodes are summarized in this review article. Moreover, the present status and upcoming scenario of RE-doped borate phosphors were reviewed in general along with the proper credential from the existing literature. It is believed that this review is a sole compilation of crucial information about the RE-doped borate phosphors in a single platform.
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  • XPS investigation of the photon degradation of Znq<sub>2</sub> green organic phosphor

    Mart-Mari Duvenhage   Jacobus J. Terblans   Martin Ntwaeaborwa   Hendrik C. Swart  

    Abstract By substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq 2 ), the device performance of organic light emitting diodes (OLED) can be improved. Znq 2 also has a more closed packed crystal structure that makes it less vulnerable to reactions with atmospheric oxygen and moisture leading to more stable and longer lasting devices. In this work the effect of photon degradation of Znq 2 in air was investigated. Znq 2 powder was synthesized using a co-precipitation method and recrystallized in acetone. The structure of the sample was confirmed to be Znq 2 ·2H 2 O by X-ray diffraction. The photoluminescence (PL) emission data also confirmed that the Znq 2 ·2H 2 O crystal form of Znq 2 was present. To study the photon degradation, the sample was irradiated with a UV lamp for 400 h. The emission data was collected and the change in PL intensity with time was monitored. X-ray photoelectron spectroscopy was performed on the as prepared and photon-degraded samples. The Zn2p and N1s peaks showed no change after degradation. The O1s and C1s peaks confirmed that the phenoxide ring ruptured and that C=O and C–O species had formed.
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  • Pd<sup>2+</sup> doped ZnO nanostructures: Structural, luminescence and gas sensing properties

    Gugu H. Mhlongo   David E. Motaung   Hendrik C. Swart  

    Abstract We report on synthesis of Pd-doping of ZnO-nanostructures and its role on the gas sensing properties of NH 3 and H 2 S. The complementary investigation aimed at understanding, explaining and confirming the role of Pd-doping on the surface morphology, photoluminescence (PL) and gas sensing behaviour exhibited at low gas concentrations were conducted. X-ray diffraction (XRD) analysis revealed a single-phase wurtzite structure of ZnO. X-ray photo-electron spectroscopy (XPS) results showed that partial Pd ions aggregate on the surface of ZnO while the rest are doped into the ZnO lattice. Surface morphology analysis revealed the increase in surface to volume ratio with the incorporation of the Pd-dopant into ZnO. The dominant violet-blue emission observed in the PL spectra of undoped and Pd-doped ZnO indicated the presence of Zni shallow donors resulting to improved sensing response. Highlights • Pd doped ZnO nanostructures prepared using sol–gel method. • Strong violet-blue emission was observed. • Sensing properties of undoped and Pd doped ZnO nanostructures towards NH 3 and H 2 S gases.
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  • Photon downshifting in strong NIR emitting Er3+–Yb3+ embedded tungsten tellurite glass

    Anurag Pandey   R.E. Kroon   Vinod Kumar   Hendrik C. Swart  

    Abstract Downshifting/quantum cutting behaviour of the Er 3+ –Yb 3+ doped tungsten tellurite glasses, synthesized by a conventional melt-quenching route is discussed. The non-crystalline nature and thermal stability of the prepared glasses were confirmed by X-ray diffraction, scanning electron microscopy and thermo-gravimetric analysis/differential thermal analysis whereas the optical spectroscopy was obtained via optical absorption and photoluminescence excitation/emission spectra. The results displayed a markable reduction in visible emissions observed through the 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , 4 F 9/2 → 4 I 15/2 and 4 I 9/2 → 4 I 15/2 transitions of the Er 3+ ion and a huge enhancement of about 14.2 times in the near-infrared (∼977 nm upon 490 nm and 325 nm excitations) emission range on incorporation of the Yb 3+ ions. A cross relaxation from 4 F 7/2 → 4 I 11/2 (Er 3+ ) to 2 F 7/2 → 2 F 5/2 (Yb 3+ ) and energy transfer from 4 I 11/2 (Er 3+ ) to 2 F 5/2 (Yb 3+ ) states were found to be responsible to cut the blue photon (∼490 nm) into two near infrared (∼977 nm) photons, which put forward the convenience of the present glass as a possible luminescent layer to enhance the efficiency of silicon solar cells. Highlights • Er 3+ /Yb 3+ embedded TeO 2 –WO 3 glasses were prepared successfully. • Strong green and NIR emissions were detected upon excitation with blue photons. • Huge enhancement in NIR emission was found upon codoping of Yb 3+ ions. • Blue to NIR quantum cutting suitable for solar cell applications was observed.
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  • Recent advances in enhanced luminescence upconversion of lanthanide-doped NaYF<sub>4</sub> phosphors

    Deepak Kumar   Kartikey Verma   Shefali Verma   Babulal Chaudhary   Sudipta Som   Vishal Sharma   Vijay Kumar   Hendrik C. Swart  

    Abstract NaYF 4 is regarded as the best upconversion (UC) matrix owing to its low phonon energy, more chemical stability, and a superior refractive index. This review reports on the various synthesis techniques of lanthanide-doped NaYF 4 phosphors for UC application. The UC intensity depends on different properties of the matrix and those are discussed in detail. Plasmon-enhanced luminescence UC of the lanthanide-doped NaYF 4 core-shells structure is discussed based on a literature survey. The present review provides the information about how the UC intensity can be enhanced. The idea about the UC is then deliberately used for versatile applications such as luminescent materials, display devices, biomedical imaging and different security appliances. In addition, the present review demonstrates the recent trends of NaYF 4 UC materials in solar cell devices. The role of NaYF 4 phosphor to eradicate the spectral variance among the incident solar spectrum, semiconductor as well as the sub-band gap nature of the semiconductor materials is also discussed in detail. Considering the fact that the research status on NaYF 4 phosphor for photovoltaic application is now growing, the present review is therefore very important to the researchers. More importantly, this may promote more interesting research platforms to investigate the realistic use of UC nanophosphors as spectral converters for solar cells.
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  • Annealed Ce<sup>3+</sup>-doped ZnO flower-like morphology synthesized by chemical bath deposition method

    Lehlohonolo F. Koao   Francis B. Dejene   Moges Tsega   Hendrik C. Swart  

    Abstract We have successfully synthesized ZnO: xmol% Ce 3+ (0≤ x ≤10 mol%) doped nanopowders via the chemical bath deposition method (CBD) technique at low temperature (80 °C) and annealed in air at 700 °C. The X-ray diffraction patterns showed that all the undoped and Ce-doped ZnO nanopowders have a hexagonal wurtzite polycrystalline structure with an average crystallite size of about 46 nm. Weak diffraction peaks related mainly to cerium oxide were also detected at higher concentrations of Ce 3+ ( x =5–10 mol%). The scanning electron microscopy study revealed that the nanopowder samples were assembled in flower-shaped undoped ZnO and pyramid-shaped Ce 3+ -doped ZnO nanostructures. The UV–vis spectra showed that the absorption edges shifted slightly to the longer wavelengths with the increase in the Ce 3+ ions concentration. Moreover, the photoluminescence (PL) results showed a relative weak visible emission for the Ce 3+ -doped ZnO nanoparticles compared to the undoped ZnO. The effects of Ce 3+ -doping on the structure and PL of ZnO nanopowders are discussed in detail.
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  • Effects of thermal treatment and depth profiling analysis of solution processed bulk-heterojunction organic photovoltaic cells

    Pontsho S. Mbule   Hendrik C. Swart   Odireleng M. Ntwaeaborwa  

    Graphical abstract Highlights • Highlights the advantages of using ZnO nanoparticles as electron transport layer. • Demonstrates the effects of heat treatment on the power conversion efficiency of solar cells. • Highlights the formation oxide layers at the interface and diffusion of atomic species from the surface to the bulk. Abstract We report the use of solution processed zinc oxide (ZnO) nanoparticles as a buffer layer inserted between the top metal electrode and the photo-active layer in bulk-heterojunction (BHJ) organic solar cell (OSC) devices. The photovoltaic properties were compared for devices annealed before (Device A) or after (Device B) the deposition of the Al top electrode. The post-annealing treatment was shown to improve the power conversion efficiency up to 2.93% and the fill factor (FF) up to 63% under AM1.5 (100 mW/cm 2 ) illumination. We performed the depth profile/interface analysis and elemental mapping using the time-of-flight secondary ion mass spectrometry (TOF-SIMS). Signals arising from 27 Al, 16 O, 12 C, 32 S, 64 Zn, 28 Si, 120 Sn and 115 In give an indication of successive deposition of Al, ZnO, P3HT:PCBM and PEDOT:PSS layers on ITO coated glass substrates. Furthermore, we discuss the surface imaging and visualize the chemical information on the surface of the devices.
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  • The dynamics of the photoluminescence of Pr<sup>3+</sup> in mixed lanthanum yttrium oxyorthosilicate hosts

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

    Graphical abstract Highlights • Successful preparation of mixed rare-earths oxyorthosilicates. • Effects of crystal field on Pr 3+ . • Non-radiative induced variation of branching ratio of the 3 P 0 and 1 D 2 transitions. Abstract Praseodymium (Pr 3+ ) doped mixed lanthanum yttrium oxyorthosilicate (LaYSiO 5 ) powder phosphors were synthesized using urea-assisted combustion method. The molar ratio of La:Y were varied in the following manner: La 2-x Y x SiO 5 ( x = 0, 0.5, 1, 1.5, 2), were x = 0 is pure La 2 SiO 5 , x = 2 is pure Y 2 SiO 5 and x = 0.5, 1, and 1.5 are the admixtures of the two compounds. The X-ray diffractometer results showed that the La 2 SiO 5 and Y 2 SiO 5 crystalized in their pure monoclinic phases, while their admixtures are both present in the same phase. The Burstein–Moss (BM) shift was used to explain the increase observed in the band gap after doping. The influence of the host crystal field on the branching ratios of the photoluminescence emission intensities of the 3 P 0 and 1 D 2 energy levels of Pr 3+ were studied. The electronic transition from the 3 P 0 transition dominated the emission spectra when x = 0, while the 1 D 2 electronic transition dominated when x = 2. The variation in the branching ratios of the 3 P 0 and 1 D 2 emission with the change in the molar ratio of La:Y could be due to 3 P 0 → 1 D 2 non-radiative transitions, which increased with the crystal field of the host as the value of x increased (i.e., as the molar ratio of Y increases). Furthermore, it was shown that the 3 P 0 emission lines emerged from Pr 3+ ions occupying La1 and Y1 sites of La 2 SiO 5 and Y 2 SiO 5 respectively, while the 1 D 2 emission lines emerged from Pr 3+ ions occupying La2 and Y2 sites. The decay curve showed three lifetime components from both 3 P 0 and 1 D 2 emission lines, with the 1 D 2 lines having higher lifetimes in all cases.
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