Mildred Airo

Structural modification and band-gap crossover in indium selenide nanosheets

Herein, we report on the synthesis of InSe nanosheets. We further report on the interdependency of concentration and time on the evolution of the nanosheets as well as the role of the capping agents on the crystal phase and morphology of the resultant particles. Our results show that hexagonal-like InSe nanosheets were synthesized at different optimum times depending on the amount of indium precursor. Regardless of the amount of indium precursor used, the product remained unchanged, i.e. InSe. This was attributed to the nature of the oleylamine capping agent. Oleylamine is known to be a reducing agent and in this case resulted in the reduction of In3+ to In2− and elemental selenium to Se2−, thus always resulting in the formation of rhombohedral InSe nanosheets. A non-reducing 1-dodecanthiol co-surfactant was thus used. Dodecanthiol was thought to stabilize the In3+ ions by coordinating and forming a complex, thus preserving the 3+ oxidation state of indium, resulting in the formation of the final product of In2Se3 nanocrystals. The morphology of In2Se3 changed depending on the amount of oleylamine used. The optical properties of the InSe were further evidence that nanosheets had been synthesized due to the band gap crossover.

Improved efficiency of organic solar cells using Au NPs incorporated into PEDOT:PSS buffer layer

The University of theWitwatersrand, Material Physics Research Institute, School of Physics & Chemistry; and MMU facilities at Wits, NRF and Material Energy Research Group (MERG).

Colloidal synthesis of pure CuInTe2 crystallites based on the HSAB theory

The colloidal method has extensively been used to synthesize ternary and quaternary copper sulfides and selenides. The formation pathway and the crystallization mechanism of these nanostructures have also been investigated. Although tellurides form part of the chalcogenides, little has been reported on them particularly the crystallization mechanism of these nanostructures. Herein, we report on the colloidal synthesis of CuInTe2. Typically reaction temperatures play a vital role in the formation of colloidal nanostructures. At temperatures below 250 °C, no formation of CuInTe2 was seen. At 250 °C formation of CuInTe2 could be observed with formation of binary impurities. A change in the sequence in which precursors were added at 250 °C yielded pure CuInTe2. Therefore starting with InCl3 and elemental Te dissolved in OLA and TOP, respectively, then adding CuCl dissolved in OLA yielded a pure CuInTe2 phase with agglomerated cubic structures. The pure CuInTe2 crystallites had an optical band gap of 1.22 eV in comparison to 0.93 eV of the impure CuInTe2 phase.

Structural and spectroscopic analysis of ex-situ annealed RF sputtered aluminium doped zinc oxide thin films

Aluminium doped zinc oxide thin films are prepared by Radio Frequency magnetron sputtering in pure argon atmosphere at 100 W. The structural results reveal good film adhesion on a silicon substrate (001). The thin films were then subjected to heat treatment in a furnace under ambient air. The structural, morphological, and optical properties of the thin films as a function of deposition time and annealing temperatures have been investigated using Grazing incidence X-Ray Diffraction (GIXRD), Atomic Force Microscopy, and Scanning Electronic Microscopy. The photoluminescence properties of the annealed films showed significant changes in the optical properties attributed to mid gap defects. Annealing increases the crystallite size and the roughness of the film. The crystallinity of the films also improved as evident from the Raman and XRD studies.

Effect of thermal treatment on ZnO:Tb3+ nano-crystalline thin films and application for spectral conversion in inverted organic solar cells

Down conversion has been applied to minimize thermalization losses in photovoltaic devices. In this study, terbium-doped ZnO (ZnO:Tb3+) thin films were deposited on ITO-coated glass, quartz and silicon substrates using the RF magnetron sputtering technique fitted with a high-purity (99.99%) Tb3+-doped ZnO target (97% ZnO, 3% Tb) for use in organic solar cells as a bi-functional layer. A systematic study of the film crystallization dynamics was carried out through elevated temperature annealing in Ar ambient. The films were characterized using grazing incidence (XRD), Rutherford backscattering spectrometry (RBS), atomic force microscopy, and UV-visible transmittance and photoluminescence measurements at an excitation wavelength of 244 nm. The tunability of size and bandgap of ZnO:Tb3+ nanocrystals with annealing exhibited quantum confinement effects, which enabled the control of emission characteristics in ZnO:Tb3+. Energy transfer of ZnO → Tb3+ (5D3–7F5) was also observed from the photoluminescence (PL) spectra. At an inter-band resonance excitation of around 300–400 nm, a typical emission band from Tb3+ was obtained. The ZnO:Tb3+ materials grown on ITO-coated glass were then used as bi-functional layers in an organic solar cell based on P3HT:PCBM blend, serving as active layers in an inverted device structure. Energy transfer through down conversion between ZnO and Tb3+ led to enhanced absorption in P3HT:PCBM in the 300–400 nm range and subsequently augmented Jsc of a Tb3+-based device by 17%.

Synthesis and characterization of Cu3N nanoparticles using pyrrole-2-carbaldpropyliminato Cu(II) complex and Cu(NO3)2 as single-source precursors: the search for an ideal precursor

Herein, we report the synthesis and characterization of Cu3N nanocrystals using two single-source precursors, bis(pyrrole-2-carbalpropyliminato) Cu(II) (PPC) and Cu(NO3)2·3H2O. The optical and structural properties were investigated and the suitability of the two precursors was studied in terms of producing good quality Cu3N nanocrystals without the detection of Cu or oxidation to CuO. Both precursors resulted in crystalline Cu3N with an anti-ReO3 cubic structure with no presence of copper impurities, however, peaks due to excess capping agent were detected by XRD and confirmed with XPS. The PPC complex resulted in spherical nanocrystals whilst the copper nitrate resulted in nanocubes. The band gaps were in the visible region with the copper nitrate nanocrystals slightly red shifted from the PPC derived particles due to larger crystal sizes. The emission spectra were blue-shifted from the absorption band edges hence indicating an up-conversion process.