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A highly durable optical disk has been developed for data archiving. This optical disk uses tellurium as the write layer and carbon as a dielectric and oxidation prevention layer. The sandwich style CTeC film was deposited on polycarbonate and silicon substrates by plasma sputtering. These films were then characterized with SEM, TEM, EELS, ellipsometry, ToF-SIMS, etc, and were tested for writability and longevity. Results show the films were uniform in physical structure, are stable, and able to form permanent pits. Data was written to a disk and successfully read back in a commercial DVD drive.

The high degree of control over the morphology and optoelectronic properties of semiconductor nanowires (NWs) makes them attractive for applications such as thermoelectrics, quantum emitters, and photodetectors. However, NW growth is still not fully understood as many parameters play a role in the determination of NW morphology and crystal structure, which in turn governs resulting optoelectronic properties. We report tellurium-doped GaP NWs with positive tapering and radii measuring as low as 5 nm grown by the self-assisted vapor–liquid–solid mechanism using selective-area molecular beam epitaxy. The occurrence of ultrathin nanoantenna showed a dependence on pattern pitch (separation between NWs) with a predominance at 600 nm pitch, and exhibited radius oscillations that correlate with polytypic zincblende (ZB)/wurtzite (WZ) segments. A growth model explains the positive tapering of the NW leading to an ultrathin tip from the suppression of surface diffusion of Ga adatoms on the NW sidewalls by Te dopant flux. The model also provides a relationship between the radius modulations and the oscillations of the droplet contact angle, predicting the quasi-periodic radius oscillations and corresponding crystal phase transitions. Photoluminescence and cathodoluminescence at 10 K reveal distinct spectra corresponding to either the ZB or WZ phase. Emission above and below ~2.15 eV are associated with ZB and WZ, respectively. The characteristic WZ spectrum arises from a bound exciton and its phonon replicas, consistent with published results. The origin of emission in the ZB regime is less conclusive, but may originate from the splitting of a bound exciton by the field of an axial defect. The results presented in this thesis establish a link between NW growth, morphology, and optoelectronic properties to inform future work involving ultrathin NWs. / Thesis / Master of Applied Science (MASc) / A nanowire (NW) is a tiny rod with a length on the order of one millionth of a meter and diameter on the order of one billionth of a meter. We made gallium phosphide (GaP) NWs by stacking gallium and phosphorus atoms in a column. The NWs were separated by a constant distance. In some cases, we also added beryllium and tellurium atoms to our NWs. The addition of tellurium caused our NWs to grow into extremely sharp points, which we measured with a microscope that uses electrons instead of light. The microscope images also revealed that the arrangement of the atoms in the NW changes along its length. By detecting the light emission from the NWs, it was possible to distinguish between two unique arrangements. Overall, the small dimensions of our GaP NWs make them interesting for applications that require the emission or detection of single particles of light.

With the advancements in the field of wireless sensor networks (WSNs), more and more applications require the sensor nodes to have long lifetime. Energy harvesting sources, e.g. thermoelectric generators (TEGs), can be used to increase the lifetime and capability of the WSNs. Integration of energy harvesters into sensor nodes of WSNs can realize self powered systems, providing the possibility for maintenance free WSNs. TEGs can convert the existing temperature differences into electricity. The efficiency of TEGs is directly related to the dimensionless figure of merit (ZT) of materials, which is given as ZT=σS^2 T/k, where σ is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, T is the temperature and σS^2 is the power factor. Traditional thermoelectric (TE) materials are based on inorganic materials, of which the thermal conductivity is high. Over the past decade, the use of nanostructuring technology, e.g. superlattice, could decrease the thermal conductivity in order to enhance the efficiency of TE materials. However, the high cost and the rigidity of inorganic TE materials are limiting factors. As alternatives, polymer based materials have become the research focus due to their intrinsic low thermal conductivity, high flexibility and high electrical conductivity. Moreover, polymer based materials could be fabricated in solution form, giving the possibility for employing printing techniques hence a decrease in the production cost. Unlike the typical approach, in which secondary dopants are added into PEDOT:PSS solutions to modify the power factor of polymer films, this thesis is focused on a more efficient method to improve TE properties. This thesis demonstrates for the first time that post treatment of PEDOT:PSS films with the secondary dopant DMSO as the medium results in a much larger power factor than the traditional addition method. The post treatment method also avoids the usually required mixing step involved in the addition method. Different solvents were selected to discuss the impact factors in the modification of the power factor by this post treatment approach. The post treatment of PEDOT:PSS films was then extended to utilize a green solvent EMIMBF_4 (an ionic liquid) as the medium. EMIMBF_4 is found to exchange ions with PEDOT:PSS films. As a result, the EMIM^+ cations remain in the films and reduce the oxidation level of PEDOT chains, which affects the Seebeck coefficient and the electrical conductivity. Furthermore, TE materials based on hybrid composites with polymer as the matrix and Te nanostructures as the nanoinclusions were investigated. This thesis successfully developed a green synthesis method to obtain Te nanostructures, in which a non toxic reductant and a non toxic Te sources were used. Well controlled Te nanostructures including nanorods, nanowires and nanotubes were synthesized by wet chemical and hydrothermal synthesis. Those as synthesized Te nanowires were then integrated into PEDOT:PSS solution for composite films fabrication. A high Seebeck coefficient up to 200 μV/K was observed in the composite film. / Mit den Weiterentwicklungen der Drahtlosen Sensornetzwerke (engl. WSN, wireless sensor networks) stellen immer mehr Anwendungen die Forderung einer langen Lebensdauer der Sensorknoten. Energiegewinnungssysteme (engl. Energy Harvesters) wie z.B. thermoelektrische Generatoren (TEGs) können genutzt werden, um die Lebensdauer und Leistungsfähigkeit der WSN zu steigern. Mit der Integration von Energy Harvesters können WSN ohne äußere Stromversorgung realisiert und somit die Möglichkeit zur Wartungsfreiheit geschaffen werden. TEGs liefern Energie durch die Umwandlung einer Temperaturdifferenz in Elektrizität. Die Effektivität der TEG ist direkt verbunden mit der Material-Kennzahl ZT und ist gegeben durch ZT=σS^2 T/k, wobei σ die elektrische Leitfähigkeit ist, S der Seebeck Koeffizient, k die thermische Leifähigkeit, T die Temperatur und σS^2 der Leistungsfaktor. Herkömmliche thermoelektrische (TE) Materialien basieren auf anorganischen Materialien, von denen die thermische Leitfähigkeit hoch ist. Im Laufe des letzten Jahrzehnts konnte durch den Einsatz der Nanostrukturierung die thermische Leitfähigkeit verringern werden um damit die Effizienz von TE-Materialien zu steigern. Die Steifigkeit dieser Materialien ist ein anderer Aspekt. Als Alternative für anorganische TE Materialien sind Polymer basierte TE Materialien zum Fokus der Forschung geworden aufgrund einer intrinsisch niedrigen thermischen Leitfähigkeit, hohen Flexibilität und hohen elektrischen Leitfähigkeit. Des Weiteren können diese Polymere in gelöster Form verarbeitet werden, was die Möglichkeit für den Einsatz von Drucktechnologien und damit geringeren Produktionskosten gibt. Anders als der herkömmliche Ansatz den Leistungsfaktor der Polymerfilme durch die Ergänzung von sekundären Dotanten in PEDOT:PSS Lösungen zu verändern, wurde in dieser Arbeit eine effizientere Methode zur Verbesserung der TE Eigenschaften gesucht. In dieser Arbeit wird zum ersten Mal gezeigt, dass die Nachbehandlung von PEDOT:PSS Schichten mit sekundären Dotanten Dimethylsulfoxid (DMSO) als Medium der Nachbehandlung zu einem viel höheren Leistungsfaktor führt als bei der Zugabemethode und außerdem die sonst erforderliche Mischprocedur vermeidet. Es wurden verschiedene Lösungsmittel ausgewählt um die Einflussfaktoren bei der Modifikation des Leistungsfaktors durch die Nachbehandlung von Polymerschichten zu diskutieren. Die Nachbehandlung von PEDOT:PSS Schichten wurde nachfolgend erweitert um das umweltfreundliche Lösungsmittel EMIMBF4 (eine ionische Flüssigkeit) als das Medium einzusetzen. EMIMBF4 ist bekannt für den Austausch von Ionen mit PEDOT:PSS Schichten, so dass EMIM Kationen in der Schicht verbleiben, die Oxidationsstufe der PEDOT-Ketten senken und damit den Seebeck-Koeffizient und die elektrische Leitfähigkeit beeinflussen. Des Weiteren konzentriert sich diese Arbeit auf TE Materialien basierend auf Kompositen aus Polymeren mit Nanoeinlagerungen. Erfolgreiche Syntheseansätze wurden für Tellur-Nanostrukturen entwickelt, bei denen keine giftigen Reduktionsmittel und keine giftigen Tellur-Quellen zur Verwendung kamen. Es erfolgte die Erzeugung von kontrollierten Tellur-Nanostrukturen, einschließlich Nanostäben, Nanodrähten und Nanoröhren, mit nass-chemischer und hydrothermaler Synthese. Die so hergestellten Nanodrähte wurden dann in PEDOT:PSS Lösungen integriert für die Herstellung von Komposite-Schichten. Dabei konnte ein hoher Seebeck-Koeffizienten, bis zu 200 μV/K, festgestellt werden.

The Science behind amorphous Chalcogenide materials opened up new technologies in the arena of Phase Change Memories. The Ovonic universal phase change memory is called universal because it can replace flash memory, DRAM and SRAM. These are not only basic computer memory devices but also are becoming the driving force for the ongoing revolutionary growth of cell phones and other mobile devices, which are in desperate need of memory providing higher density, faster speed and lower power consumption. In this thesis, compositional dependence of various properties of different chalcogenide glasses are investigated, to explore the possibility of their application in Phase Change Memories. Efforts are also made to understand the effect of rigidity and extended rigidity transition on the composition dependence of properties investigated. This thesis comprises of 9 chapters; a brief summary is given below. Chapter 1 deals with fundamental aspects of amorphous semiconductors with a particular reference to chalcogenide glasses. The advantages and applications of chalcogenide glasses are also described. Chapter 2 outlines preparation and characterization of the glasses investigated. The sample preparation and various experimental setup used in the present thesis work like Raman Scattering, Nanoindentation, Alternating Differential Scanning Calorimetry (ADSC), Photo-thermal Deflection Spectroscopy (PDS), Electrical Switching are summarized here. Chapter 3 deals with Micro-Raman studies in Ge15Te85-x Inx Glasses. Micro-Raman studies reveal that as-quenched Ge15Te85-xInx samples exhibit two prominent peaks, at 123 and 155 cm-1. In thermally annealed samples, the peaks at 120 cm-1 and 140 cm-1, which are due to crystalline Te, emerge as the strongest peaks. The Raman spectra of polished samples are similar to those of annealed samples, with strong peaks at 123 cm-1 and 141 cm-1. The spectra of lightly polished samples outside the thermally reversing window resemble those of thermally annealed samples; however, the spectra of glasses with compositions in the thermally reversing window resemble those of as-quenched samples. This observation confirms the earlier idea that compositions in the thermally reversing window are non-ageing and are more stable. Chapter 4 explains nanoindentation studies undertaken on Ge15Te85-xInx glasse (1 ≤ x ≤ 11). Nanoindentation studies on Ge15Te85-xInx glasses indicate that the hardness and elastic modulus of these glasses increase with indium concentration. While a pronounced plateau is seen in the elastic modulus in the composition range 3 ≤ x ≤ 7, the hardness exhibits a change in slope at compositions x = 3 and x = 7. Also, the density exhibits a broad maximum in this composition range. The observed changes in the mechanical properties and density are clearly associated with the thermally reversing window in Ge15Te85-xInx glasses in the composition range 3 ≤ x ≤ 7. In addition, a local minimum is seen in density and hardness around x = 9, the chemical threshold of the system. Chapter 5 deals with crystallization kinetics of Ge15Te85-xInx glasses. The crystallization kinetics of Ge15Te85Inx glasses have been studied by non-isothermal method. The composition dependence of Tg and Tc at different heating rates, is investigated. The activation energy of crystallization is calculated using the Kissinger’s plot. It is found that the composition dependence of the glass transition temperature, Tg and the crystallization temperature, Tc, the activation energy of crystallization, Ec, and the stability factor, (ΔT= Tc-Tg) exhibit specific signatures of intermediate phase in the composition rang 3 ≤ x ≤ 7 and Chemical Threshold at x = 9. Chapter 6 explains Alternating Differential Scanning Calorimetric and XRD studies on silver doped Ge15Te80In5 glasses. X-ray diffraction studies on quaternary Ge15Te80-xIn5Agx glasses (2 ≤ x ≤ 24) reveal the presence of Te, GeTe, Ag8GeTe6, AgTe, In2Te3 and In4Te3. Thermal studies on quaternary Ge15Te80-xIn5Agx glasses exhibit signatures of Intermediate Phase (IP) in the variation of Tg, ∆HNR and ∆Cp with Ag addition in the composition range 8 ≤ x ≤ 16. The composition x = 16 has been identified to be the Chemical Threshold (CT) based on the saturation of flexible Ag-Te bonds. Micro-Raman, molar volume, thermal diffusivity studies on Ge15Te80-xIn5Agx glasses reveal a clear evidence of intermediate phase in the composition range 8 ≤ x ≤ 16 as depicted in the ADSC studies. Chapter 7 deals with Micro-Raman studies on as-quenched Ge15Te80-xIn5Agx glasses reveal the presence of tetrahedral structural units. Further, the Raman peak positions are found to shift with silver addition. In addition, specific signatures of the Intermediate Phase (IP) are observed in the composition dependence of Raman frequencies and corresponding intensities of different modes in the composition range, 8 ≤ x ≤ 16. In thermally annealed samples, the observed Raman peaks can be attributed to crystalline tellurium and silver lattice vibrational modes; significant increase in intensity is observed at 93 and 141cm-1 with silver addition in annealed samples, suggesting an increase in silver lattice vibrational modes. Also, the compositional dependence of density, molar volume and thermal diffusivity confirms the presence of the intermediate phase. Chapter 8 contains the current-voltage characteristics and electrical switching behavior of Ge15Te80-xIn5Agx glasses. The glasses are found to exhibit memory type switching for 3mA current in the voltage range 70 -120 V, for a sample thickness 0.3 mm. But when the current is lowered to 1mA the samples exhibit threshold switching. The compositional studies indicate the presence of an intermediate phase in the composition range 8 ≤ x ≤ 16. SET-RESET studies have been carried out using a triangular pulse of 6 mA amplitude for SET and 21 mA amplitude for RESET for a sample thickness 0.3 mm. Raman studies on SET and RESET indicates SET state resemble annealed samples and RESET state resemble as-quenched samples. It is interesting to note that the samples in the intermediate phase, especially compositions at x =10, 12, 14 withstand more set-reset cycles. This indicates compositions in the intermediate phase are suitable for PCM devices. Chapter 9 summarizes the significant results obtained and explains the scope of this thesis.

Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: The copper sulphate leach solutions produced during the final pressure leach stages in base metal refinery processes contain low concentrations of other precious metals (OPMs, namely Rh, Ru and Ir ) and impurities in addition to the base metals (BMs) of interest. Se and Te impurities, in particular, must be removed from the leach solution before it is fed to copper electrowinning because these species have adverse effects on electrowinning efficiency. Currently, these elements are being precipitated from the leach solution with sulphurous acid. Se precipitation is satisfactory but Te removal still proves challenging. Previous studies have shown that tellurium can either be precipitated as cuprous telluride from copper sulphate solutions by reduction with sulphurous acid alone, or by the addition of SO2 as a precipitating agent and metallic copper as an additional precipitating agent. The objective of this study was to evaluate the effects of different process variables on Te and Se recovery in order to propose operating conditions at which increased tellurium precipitation can be achieved with minimal co-precipitation of base metals of interest (notably Cu and Ni). This would also aid in the development of a better understanding of tellurium and selenium precipitation mechanisms in CuSO4-H2SO4 medium. / AFRIKKANSE OPSOMMING: Die kopersulfaat logingsoplossing wat gedurende die finale druklogingstadia in basis metaal raffinaderye produseer word bevat, behalwe vir die basis metale van belang, ook lae konsentrasies ander edelmetale (AEM, naamlik Rh, Ru, en Ir) sowel as onsuiwerhede. Se en Te onsuiwerhede, in die besonder, moet vanuit die logingsoplossing verwyder word voordat die oplossing na die koper elektrowinning gevoer word omdat hierdie spesies negatiewe effekte op die elektrowinning effektiwiteit het. Hierdie elemente word tans met swaweligsuur vanuit die logingsoplossing gepresipiteer. Se presipitasie is voldoende, maar die Te verwydering bly steeds problematies. Vorige studies het getoon dat tellurium as kuprotelluried vanuit kopersulfaat oplossings presipiteer kan word deur middel van reduksie met swaweligsuur alleen, of met die byvoeging van SO2 as presipiteermiddel en metallieke koper as addisionele presipiteermiddel. Die doelwit van hierdie studie was om die effekte van verskillende prosesveranderlikes op Te en Se presipitasie te ondersoek ten einde bedryfstoestande voor te stel wat verbeterde tellurium presipitasie toelaat met minimale kopresipitasie van basis metale van belang (hoofsaaklik Cu en Ni). Dit sal ook bydra tot die ontwikkeling van ʼn beter begrip van die tellurium en selenium presipitasie meganisme in ʼn CuSO4-H2SO4 medium.

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No description available.

The research on phosphorus-chalcogen compounds enjoys a long tradition in the field of inorganic chemistry, which has led to applications such as strike-anywhere matches, precursors for metal chalcogenide thin films and versatile reagents in organic synthesis. Whereas a wide range of phosphorus-sulfur and -selenium systems is known, the literature lacks information about compounds incorporating phosphorus-tellurium bonds. This thesis describes fundamental studies that develop the basic understanding of the synthesis of phosphorus-tellurium systems and the structural characteristics of these species. The focus will be on cyclic structural motifs as these offer novel bonding modes and often an interesting reactivity. In addition, the novel compounds are compared with the properties of the sulfur and selenium analogues. Three different approaches were developed to stabilise and study compounds incorporating phosphorus-tellurium bonds: a) Stabilisation of binary organophosphorus-tellurium heterocycles by bulky substituents, b) the utilisation of P₂N₂ rings based on the dianions [{EP(NtBu)(μ-NtBu)}₂]²⁻ (E = S, Se, Te) and c) the peri-substitution of phosphorus and tellurium atoms on an acenaphthene backbone. The use of sterically demanding substituents led to the isolation of the first series of structurally characterised organophosphorus(III)-tellurium heterocycles of the type (RP)[sub]nTe[sub]m including three- to six-membered ring systems. The mild oxidation of [{EP(NtBu)(μ-NtBu)}₂]²⁻ (E = S, Se, Te) with iodine yielded macrocyclic (S, Se) or oligomeric systems (Te). Furthermore, a collection of novel P₂N₂-supported phosphorus-chalcogen heterocycles incorporating main group elements was synthesised employing [{EP(NtBu)(μ-NtBu)}₂]²⁻ (E = S, Se, Te) in metathetical reactions with main group element halides. Extension of this approach to transition metal halides generated some unusual metallocycles, as well as macrocycles and ladders incorporating coinage metals. The first peri-substituted phosphorus-tellurium species were studied regarding their interatomic and intermolecular forces. Systems of the general formula RTe–Acenap–P(iPr)₂ were shown to exhibit extensive through-space spin-spin coupling. In addition, the influence of oxidation and complexation on these interactions was investigated and the formation of peri-substituted phosphorus-tellurium cations exhibiting P–Te bonds was observed.

This thesis is concerned with the synthesis and evaluation of various tellurium-based chain-breaking antioxidants. The purpose is to find novel regenerable compounds with improved radical-trapping capacity. In the first part of this work, we explore the possibilities to incorporate tellurium into tocopherols and aromatic amines. Overall, tocopherols carrying alkyltelluro groups are better radical-trapping agents than the corresponding sulfur- and selenium analogues. Among them, 7-octyltelluro δ-tocopherol showed a ca. 17-fold higher reactivity than recorded for α-tocopherol and much better regenerability. Even longer inhibition times were recorded for the corresponding bis(tocopheryl) tellurides. In the aromatic amine series, diphenyl amines carrying alkyltelluro groups were shown to function as efficient radical-quenchers capable of inhibiting peroxidation for 460 min in the presence of N-acetylcysteine. Thiol-consumption experiments suggested that the long inhibition times are due to efficient quenching of in-situ formed alkoxyl radicals in a solvent cage. In the second part of the thesis, we study how the antioxidant properties are affected by variations in the electron density at tellurium and the number of alkyltelluro substituents in the molecule. Evaluation of a series of aryltelluro phenols carrying electron donating and electron withdrawing groups in the para-position of the aryl moiety suggested that a high electron density at the heteroatom prolonged the inhibition time. Among alkyltelluro phenols, alkyltelluro resorcinols and bis(alkyltelluro) phenols, phenols carrying alkyltelluro groups in both ortho positions were superior when it comes to radical-trapping activity and regenerability.