Scielo RSS <![CDATA[South African Journal of Science]]> vol. 105 num. 7-8 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Guest editorial</b>: <b>nanoscience and nanotechnology in South Africa</b>]]> <![CDATA[<b>Double-blind reviewing</b>]]> <![CDATA[<b>SA vaccine trials launched, but future funding not secure</b>]]> <![CDATA[<b>Setting priorities for health spending in South Africa</b>]]> <![CDATA[<b>Not real funding?</b>]]> <![CDATA[<b>A sputnik moment?The Natural Sciences and Humanities. An interview with Edward L. Ayers</b>]]> <![CDATA[<b>Animals and people</b>]]> <![CDATA[<b>Turning folklore into an ethnomedicinal catalogue</b>]]> <![CDATA[<b>The influence of cation, anion and water content on the rate of formation and pore size distribution of zeolite ZSM-5</b>]]> Rapid synthesis and high mesoporosity of ZSM-5 is promoted by altering a fixed molar regime with respect to anion and cation type and water content only. This study shows that merely changing the cation and anion types, accompanying the hydroxide and alumina (Al) sources, respectively, has an impact on the characteristics of ZSM-5. Under identical and replicated preparation and testing conditions, the synthesis time, Al incorporation, product yield, acidity, morphology, pore size distribution and catalytic activity were affected. An ion pair combination of Na+/NO3- allowed the formation of ZSM-5 with enhanced mesoporosity within 3 h in a high-water environment, whereas ZSM-5 prepared with the Na+/SO4(2-) combination had enhanced n-hexane cracking activity. Use of a K+/NO3- ion pair combination in a high-water environment resulted in slower time of formation and larger ZSM-5 crystals with extra-framework Al, loss of acidity, lower surface areas and reduced n-hexane cracking activity. The sulphate anion inhibited the incorporation of Al in ZSM-5 products in low-water environments. Low-water levels increased the time of formation of ZSM-5. <![CDATA[<b>N,N'-diisopropylthiourea and N,N'-dicyclohexylthiourea zinc(II) complexes as precursors for the synthesis of ZnS nanoparticles</b>]]> The single X-ray crystal structures of zinc (II) complexes of N,N'-diisopropylthiourea and N,N'-dicyclohexylthiourea were determined. These complexes, similar to other alkylthioureas, were found to be effective as precursors for the preparation of hexadecylamine-capped ZnS nanoparticles. The complexes are air-stable, easy to prepare and inexpensive. They pyrolyse cleanly to give high-quality ZnS nanoparticles, which show quantum confinement effects in their absorption spectra and close to band-edge emission. Their broad diffraction patterns are typical of nanosized particles while their transmission electron microscopy images showed agglomerates of needle-like platelet nanoparticles. <![CDATA[<b>Microstructure cantilever beam for current measurement</b>]]> Most microelectromechanical systems (MEMS) sensors are based on the microcantilever technology, which uses a broad range of design materials and structures. The benefit of MEMS technology is in developing devices with a lower cost, lower power consumption, higher performance and greater integration. A free-end cantilever beam with a magnetic material mass has been designed using MEMS software tools. The magnetic material was used to improve the sensitivity of the cantilever beam to an externally-applied magnetic field. The cantilever was designed to form a capacitance transducer, which consisted of variable capacitance where electrical and mechanical energies were exchanged. The aim of this paper was to analyse the system design of the microcantilever when subjected to a magnetic field produced by a current-carrying conductor. When the signal, a sinusoidal current with a constant frequency, was applied, the cantilever beam exhibited a vibration motion along the vertical axis when placed closer to the line current. This motion created corresponding capacitance changes and generated a voltage output proportional to the capacitive change in the signal-processing circuitry attached to the microcantilever. The equivalent mass-spring system theory was used to describe and analyse the effect of the natural frequency of the system vibrations and motion due to the applied magnetic field, in a single-degree of freedom. The main application of this microcantilever is in current measurements to develop a non-contact current sensor mote. <![CDATA[<b>Carbon-nanostructured materials for energy generation and storage applications</b>]]> We have developed and refined a chemical vapour deposition method to synthesise nanotubes using liquid petroleum gas as the carbon source. The nanotubes were thoroughly characterised by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and thermogravimetric analysis. The protocol to grow nanotubes was then adapted to deposit nanotubes on the surface of different substrates, which were chosen based upon how the substrates could be applied in various hydrogen energy conversion systems. Carbon nanotubes are a nanostructured material with an extremely wide range of applications in various energy applications. The methods outlined demonstrate the complete development of carbon nanotube composite materials with direct applications in hydrogen energy generation, storage and conversion. <![CDATA[<b>Flat-topped emission centred at 1 250 nm from quantum dot superluminescent diodes</b>]]> We present a method for tailoring a broadband and flat-topped emission spectrum in quantum dot superluminescent diodes based upon modification of the dots-in-compositionally-modulated-well (DCMWELL) technique. We demonstrate flat-topped emission with 95 nm full width at half maximum (FWHM), centred at 1 250 nm, and with output power in excess of 8 mW. <![CDATA[<b>Continuous production of carbon nanotubes and diamond films by swirled floating catalyst chemical vapour deposition method</b>]]> Various techniques for the synthesis of carbon nanotubes (CNTs) are being developed to meet an increasing demand as a result of their versatile applications. Swirled floating catalyst chemical vapour deposition (SFCCVD) is one of these techniques. This method was used to synthesise CNTs on a continuous basis using acetylene gas as a carbon source, ferrocene dissolved in xylene as a catalyst precursor, and both hydrogen and argon as carrier gases. Transmission electron microscopy analyses revealed that a mixture of single and multi-wall carbon nanotubes and other carbon nanomaterials were produced within the pyrolytic temperature range of 900-1 100°C and acetylene flow rate range of 118-370 ml min-1. Image comparison of raw and purified products showed that low contents of iron particles and amorphous carbon were contained in the synthesised carbon nanotubes. Diamond films were produced at high ferrocene concentration, hydrogen flow rate and pyrolysis temperatures, while carbon nanoballs were formed and attached to the surface of the CNTs at low ferrocene content and low pyrolysis temperature. <![CDATA[<b>Second harmonic generation as a technique to probe buried interfaces</b>]]> Since the advances of femtosecond laser technology during the last decade, optical second harmonic generation (SHG) has proven itself a powerful tool to investigate the electronic and structural properties of semiconductor materials. Its advantage lies in the fact that it is a contact-less, non-intrusive method that can be used in situ. It is sensitive to systems with broken symmetry, in particular interfaces and surfaces. The Si/SiO2 system is technologically important since it forms a component of most modern electronic equipment. Furthermore, it has been shown that it is possible to induce an electric field across this interface by means of laser irradiation as a result of defect formation and defect population. This electric field can be measured since it determines the SHG signal. The anisotropy of the SHG signal from the Si/SiO2 interface was measured and showed four-fold symmetry, illustrating that the SHG technique was able to characterise the electrical properties of the interface below the 5 nm thick oxide layer. <![CDATA[<b>Applicability of analytical protocols for the characterisation of carbon-supported platinum group metal fuel cell electrocatalysts</b>]]> The nanoparticulate size of fuel cell electrocatalysts raises significant challenges in the analytical techniques used in their structural and electrochemical characterisation. For this reason, the applicability of analytical protocols in the qualitative and quantitative characterisation of nanophase fuel cell electrocatalysts was investigated. A set of structural and chemical properties influencing the performance of the electrocatalysts was identified. A large range of analytical tools was employed in characterising the electrocatalysts of interest. High accuracy and precision in the quantitative and qualitative structural and electrochemical characterisation of Pt/C and Pt-Ru/C nanophase electrocatalysts was demonstrated. Certain techniques were deemed to be highly applicable in discriminating between high- and low-performance electrocatalysts based on their structural and electrochemical properties. The goal of this effort is to contribute to the development of South Africa's capabilities in the emerging hydrogen economy. <![CDATA[<b>Synthesis of nanocrystalline silicon thin films using the increase of the deposition pressure in the hot-wire chemical vapour deposition technique</b>]]> Nanostructured thin silicon-based films have been deposited using the hot-wire chemical vapour deposition (HWCVD) technique at the University of the Western Cape. A variety of techniques including optical and infrared spectroscopy, Raman scattering spectroscopy, X-rays diffraction (XRD) and transmission electron microscopy (TEM) have been used for characterisation of the films. The electrical measurements show that the films have good values of photoresponse, and the photocurrent remains stable after several hours of light soaking. This contribution will discuss the characteristics of the hydrogenated nanocrystalline silicon thin films deposited using increased process chamber pressure at a fixed hydrogen dilution ratio in monosilane gas. <![CDATA[<b>Numerical solution of multiband k·p model for tunnelling in type-II heterostructures</b>]]> A new and very general method was developed for calculating the charge and spin-resolved electron tunnelling in type-II heterojunctions. Starting from a multiband k·p description of the bulk energy-band structure, a multiband k·p Riccati equation was derived. The reflection and transmission coefficients were obtained for each channel by integrating the Riccati equation over the entire heterostructure. Numerical instability was reduced through this method, in which the multichannel log-derivative of the envelope function matrix, rather than the envelope function itself, was propagated. As an example, a six-band k·p Hamiltonian was used to calculate the current-voltage characteristics of a 10-nm wide InAs/GaSb/InAs single quantum well device which exhibited negative differential resistance at room temperature. The calculated current as a function of applied (bias) voltage was found to be in semi-quantitative agreement with the experiment, a result which indicated that inelastic transport mechanisms do not contribute significantly to the valley currents measured in this particular device. <![CDATA[<b>Photocatalytic degradation of methyl red dye</b>]]> Silica nanoparticles (SiO2 NPs) are active in the photocatalytic degradation of methyl red dye (MR). SiO2 NPs and SiO2 NPs that have been doped with either silver (Ag NPs) and/or gold nanoparticles (Au NPs) were prepared. The particle size and morphology of the catalysts were assessed by transmission electron microscopy (TEM) imaging. The rate of photocatalytic degradation of MR was found to increase from SiO2 NPs, SiO2 NPs coated with both Au NPs and Ag NPs, SiO2 NPs coated with Ag NPs, SiO2 NPs coated with Au NPs, Ag+-doped SiO2 N Ps to Au3+-doped SiO2 NPs. <![CDATA[<b>Synthesis and study of carbon microspheres for use as catalyst support for cobalt</b>]]> The production of pure carbon spheres was achieved in the absence of a catalyst through the direct pyrolysis of two hydrocarbon sources, acetylene and ethylene. Systematic studies using acetylene as the feedstock indicated that the size distribution of the resulting carbon microspheres can be controlled by pyrolysis temperature, time and feedstock flow rate. The resulting spheres were fully characterised by transmission electron microscopy (TEM) and thermogravimetric analysis. The TEM examination showed that these spheres have a ball-like and chain-like morphology, and the balls have smooth surfaces with a variation in diameter size and distribution determined by the reaction conditions. Carbon microsphere-supported cobalt catalysts were synthesised and have shown good activity in the ethylene hydrogenation reaction. <![CDATA[<b>Circular dichroism as a means to follow DNA gymnastics</b>: <b>on the shoulders of giants</b>]]> This is the first report of DNA stem-loops self-assembled by 'foot-loop' interactions into either two-dimensional strings or three-dimensional spirals, distinguished by circular dichroism spectroscopy. All subunits are linked by cooperative Watson-Crick hydrogen bonds. <![CDATA[<b>An assessment of the likely impact of strain-related phenotypic plasticity on hominin fossil species identification</b>]]> It has been proposed that strain-related phenotypic plasticity may be a major confounding factor in attributing hominin fossils to species. The study reported here tested this hypothesis with craniometric data from the great apes and Colobus guereza. We divided the measurements into three groups: measurements of features subject to high masticatory strain, measurements of features subject to low-to-moderate masticatory strain, and measurements of features that do not remodel and therefore are not prone to strain-related phenotypic plasticity. Next, we used the coefficient of variation and ANOVA to investigate whether masticatory strain is a cause of variability. These analyses partially supported the hypothesis. The predicted differences between the high-strain measurements and the other measurements were found in the majority of the species. However, the coefficient of variation values for the low-to-moderate strain and non-phenotypically plastic measurements were indistinguishable. Thereafter, we used discriminant function analysis to compare the ability of the three groups of measurements to assign specimens to species. This analysis did not support the hypothesis. The high-strain measurements were less effective than the other measurements, but the low-to-moderate strain measurements were more effective than the non-phenotypically plastic measurements. In addition, better discrimination was achieved when all the measurements were employed than when just the non-phenotypically plastic measurements were utilised. We conclude from this that strain-related phenotypic plasticity is unlikely to impede hominin alpha taxonomic research. <![CDATA[<b>Biological survey of the Prince Edward Islands, December 2008</b>]]> A biological survey of the Prince Edward Islands took place in December 2008. The survey repeated an earlier survey of the populations of surface-nesting seabirds on both islands and of fur seals (Arctocephalus spp.) and alien plants on Prince Edward Island in December 2001. Observations on burrowing seabirds, macro-invertebrates and plant communities on Prince Edward Island and an oceanographic survey of surrounding waters were also included. The survey confirmed many of the observations made on the earlier survey and permitted an assessment of trends in the abundance and distribution of biota since 2001.