Growth and Characterisation of ZnO Nanostructures: Excitonic Properties and Morphology

The growth mechanism of aligned ZnO nanostructures, grown by the vapour phase transport (VPT) growth method, specifically with nanorod and nanowall morphologies has been studied. The thesis begins with an introductory chapter on ZnO nanostructures and related topics, and the second chapter introduces the various experimental techniques used. The main thesis work involves five distinct studies. Firstly, the conditions to grow nanorod and nanorod/nanowall structures on sapphire using a ZnO/graphite powder mixture as a growth source are studied and the optimum conditions for each morphology identified. Secondly, the effects on ZnO nanostructure growth on sapphire of using activated carbon and carbon black powders, rather than graphite powder are studied. Nanostructures can be grown at significantly lower temperatures with carbon black and activated carbon, though with different morphologies, compared to graphite. Thirdly, low temperature cathodoluminescence spectroscopy measurements of ZnO nanostructures grown on Si substrate are presented. These data show significant inhomogeneity in the spatial distribution of emission throughout the sample for the Al-related donor bound exciton emission at 3.3605 eV and the Al-related emissions are compared to the other spectral features seen for these samples. The possible origin of this inhomogeneity is discussed. Fourthly, the microscopic origin of a unique photoluminescence peak at ~3.367 eV, which is known as surface exciton peak, has been studied in detail and its behaviour is studied after samples have been subjected to various post-growth treatments such as plasma treatment, UV exposure in vacuum and exposure to high voltages. Finally, post-growth passivation of nanostructures has been done using PVP and HF on ZnO nanostructure samples. The effects of these chemicals on the optical emission from these samples are studied and the potential for these to act as effective passivation agents is discussed. The thesis concludes with a summary of the work done, some general conclusions and comments on possible future directions

Influence of ZnO nanowire array morphology on field emission characteristics

In this work the growth and field emission properties of vertically aligned and spatially ordered and unordered ZnO nanowires are studied. Spatially ordered nanowire arrays of controlled array density are synthesised by both chemical bath deposition and vapour phase transport using an inverse nanosphere lithography technique, while spatially unordered arrays are synthesised by vapour phase transport without lithography. The field emission characteristics of arrays with 0.5 µm, 1.0 µm, and 1.5 µm inter-wire distances, as well as unordered arrays, are examined, revealing that with the range of values examined field emission properties are mainly determined by variations in nanowire height, and show no correlation with nanowire array density. Related to this, we find that a significant variation in nanowire height in an array also leads to a reduction in catastrophic damage observed on samples during field emission because arrays with highly uniform heights are found to suffer significant arcing damage. We discuss these results in light of recent computational studies of comparable nanostructure arrays and find strong qualitative agreement between our results and the computational predictions. Hence the results presented in this work should be useful in informing the design of ZnO nanowire arrays in order to optimise their field emission characteristics generally

Growth of spatially ordered ZnO nanowire arrays for field emission applications

In this work the growth of spatially ordered and vertically aligned ZnO nanowires is examined. Nanowire arrays are grown using chemical bath deposition (CBD) and carbothermal reduction vapour phase transport (CTR-VPT) techniques. Nanosphere lithography (NSL) was used to achieve spatial ordering of these arrays; arrays with inter-wire distances of 500 nm, 1.0 μm, and 1.5 μm were grown using both CBD and CTR-VPT techniques. Two distinct implementations of the NSL technique are investigated, one which relied on the deposition of a catalyst material and one which involved the deposition of a secondary mask which prevents ZnO deposition from occurring in undesired areas. The field emission (FE) characteristics of these arrays were examined, revealing a significant dependence of the FE properties on both nanowire morphology and array density. A geometric factor is calculated which is dependent on both nanowire aspect ratio and the density of nanowires in an array and this factor has been found to correlate with other indicators of FE properties. The results presented in this work may be useful in informing the design of ZnO nanowire arrays in order to maximise their FE efficiency and uniformity

Theoretical and experimental studies of ZnO nanowires grown by vapour phase transport

This thesis discusses the growth atmosphere, condensing species and nucleation conditions relevant to vapour phase transport growth of ZnO nanowires. The partial pressure of molecular ZnO in a Zn/O2 mix at normal ZnO growth temperatures is 6 x 10e-7 of the Zn partial pressures. In typical vapour phase transport growth conditions, using carbothermal reduction, the Zn vapour is always undersaturated while the ZnO vapour is always supersaturated. In the case of the ZnO vapour, our analysis suggests that the barrier to nucleation is too large for nucleation of ZnO to take place, which is consistent with experimental evidence that nanostructures will not grow on unseeded areas of substrates. In the presence of suitable accommodation sites, due to ZnO seeds, growth can occur via Zn vapour condensation (followed by oxidation) and via direct condensation of molecular ZnO. The balance between these two condensing species is likely to be a sensitive function of growth parameters. This thesis also examines the relationship between the length and radius of ZnO nanowires grown via VPT and nds that the lengths of the nanowires increase with decreasing radius, supporting the inclusion of a diusion term in a model for the incorporation of molecules into a growing nanowire

USE OF HIGH SURFACE NANOFIBROUS MATERIALS IN MEDICINE

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Cavity effects in polygonal resonators

In der vorliegenden Arbeit werden ZnO-Mikronadeln bezüglich ihrer Anwendbarkeit als Mikroresonatoren untersucht. Dabei stehen Kavitätsmoden im Fokus der Untersuchungen, die sich nur senkrecht zur Nadelachse ausbreiten, sprich innerhalb der hexagonalen Nadelquerschnittsfläche. Folglich wird der Einfluss der Gestalt der Querschnittsfläche auf Resonatoreigenschaften wie Propagation, Form, Direktionalität und Qualität der Kavitätsmoden sowohl theoretisch simuliert als auch experimentell nachgewiesen. Die dabei beobachteten hohen Qualitätsfaktoren von Flüstergalerie-Moden ermöglichen es darüberhinaus, Wechselwirkungseffekte zwischen Kavität und Mode zu beobachten. Der erste Teil der Arbeit beschäftigt sich mit der regulären, polygonalen Resonatorform und deren Einfluss auf die Dimensionalität von Kavitätsmoden sowie deren mögliche Wechselwirkung mit dem elektronischen System des Resonators. Beispielhaft wird ein hexagonaler Resonator zur Veranschaulichung gewählt, wie er durch ZnO-Mikronadeln gegeben ist, undmittels Finite-Difference-Time-Domain (FDTD)-Simulationen sowie winkelaufgelöster Photolumineszenz (PL)-Spektroskopie untersucht. Die aufgenommenen PL-Spektren können unter Zuhilfenahme photonischer Dispersionskurven von ein- und zwei-dimensionalen Kavitätsmoden reproduziert werden. Basierend auf diesen Ergebnissen wird der Einfluß der Resonatorecken auf die Lichtauskopplung diskutiert und mittels winkelaufgelöster, anregungsabhängiger und temperaturabhängier PL-Spektroskopie nachgewiesen. Desweiteren wird auf die Wechselwirkung zwischen dem Resonator und den Kavitätsmoden eingegangen, imSpeziellen auf die starke Kopplung zwischen Flüstergalerie-Moden und freien Exzitonen imResonatormaterial. Bereits erschienende Publikationen zu diesemThema werden präsentiert und kritisch hinterfragt. Dabei wird ein Leitfaden aufgestellt, der eine Evaluierung möglicher Polaritonen-Phänomene ermöglicht. Um Wechselwirkungen dieser Art auch in den hier untersuchtenMikronadeln nachzuweisen, werden Hochanregungs-PL-Messungen durchgeführt. Dabei werden Messungen in der Mitte der Nadel sowie in der Nähe ihrer Ecken getätigt, um spezielle Polaritonen-Propagationseffekte beobachten zu können. Im zweiten Teil der Arbeit wird der Einfluß von irregulären und inhomogenen Resonatorformen auf die Bildung von Flüstergalerie-Moden diskutiert. Dafür werden elongierte Teile der Nadeln, die durch laterale Auswüchse entstehen, winkelaufgelöst bezüglich einer gerichteten Auskopplung von Kavitätsmoden vermessen und verzerrte Mikronadeln, wie sie beim Biegen entstehen, bezüglich der entstehenden Deformationseffekte und deren Einfluss auf die Kavitätsmoden mittels hochaufgelöster Mikro-PL untersucht. Die experimentellen Ergebnisse zu irregulären Resonatoren können durch FDTD-Simulationen bestätigt werden. Desweiteren wurden Mikronadel- und Nanonadel-Quantengraben-Heterostrukturen hergestellt und deren Lumineszenzeigenschaften diskutiert. Dabei wird speziell auf die Homogenität der Quantengrabenemission eingegangen und Strategien zur Realisierung einer starken Kopplung zwischen Flüstergalerie-Moden und Quantengraben-Exzitonen aufgestellt. Diese Strategien werden experimentell umgesetzt und deren Ergebnisse anhand von Kathodolumineszenzmessungen vorgestellt

Group III-nitrides: synthesis and sensor applications

Submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the Faculty of science Department of Chemistry University of the Witwatersrand. November 2016.An overview of the evolution of synthesis and applications of indium nitride and gallium nitride in modern science and technology is provided. The working principles and parameters of chemical vapour deposition (CVD) synthesis technique are explored in this study. In this study indium oxide, indium phosphate, indium nitride and gallium nitride materials are prepared by CVD. The versatility of CVD on the fabrication of one-dimensional (1D) structures is portrayed. Both change in dimensionality and change in size are achieved by a CVD technique. 1D indium oxide (In2O3) nanowires, nanonails and nanotrees are synthesised from vapour deposition of three-dimensional In2O3 microparticles. While 1D structures of the novel indium phosphate known as triindium bisphosphate In3(PO4)2 were obtained from reactions of In2O3 with ammonium phosphate. The effect of temperature, activated carbon and the type of indium precursor on dimensionality of the synthesized materials is studied. The inter-dependency between temperature and precursors is observed. The presence of activated carbon at high temperatures encouraged growth of secondary structures via production of excess indium droplets that act as catalysts. The combination of activated carbon and high temperature was found responsible for the novel necklace, nanonail, nanotree and nanocomb structures of In2O3. Indium nitride (InN) has for the first time been made by a combined thermal/UV photoassisted process. In2O3 was reacted with ammonia using two different procedures in which either the ammonia was photolysed or both In2O3 and ammonia were photolysed. A wide range of InN structures were made that was determined by the reaction conditions (time, temperature). Thus, the reaction of In2O3 with photolysed NH3 gave InN rod like structures that were made of cones (6 h/ 750 oC) or discs (6 h/ 800 oC) and that contained some In2O3 residue. Photolysis of In2O3 and NH3 by contrast gave InN nanobelts, InN tubes and pure InN tubes filled with In metal (> 60 %). The transformation of the 3D In2O3 particles to the tubular 1D InN was monitored as a function of time (1-6 h) and temperature (700-800 oC); the product formed was very sensitive to temperature. The band gap of the InN tubes was found to be 2.19 eV and of the In filled InN tubes to be 1.89 eV. Gallium nitride (GaN) and indium gallium nitride (InGaN) nanostructures were synthesized from thermal ammonification of gallium oxide (Ga2O3) as well ammonification of a mixture of In2O3 and Ga2O3 respectively. The effect of temperature on preparation of high purity GaN was studied. The GaN materials synthesized at 800 °C showed a mixture of the gallium oxide and the gallium nitride phases from the XRD analysis. However at temperatures ≥ 900 °C high quality GaN nanorods were obtained. The band-to-band ultraviolet optical emission value of 3.21 eV was observed from the GaN nanorods. However, the preparation of InGaN was complicated by the thermally stable In2O3. At lower temperatures inhomogeneous materials consisting of GaN nanorods and In2O3 were obtained. While at high temperatures (≥ 1050 °C) InGaN was obtained. However because indium has a high vapour pressure and a low melting point only a minute amount of it was incorporated in the crystal lattice. Hexagonally shaped nanoplates of In0.01Ga0.99N were successfully obtained. A shift in optical emission to longer wavelengths was observed for the InGaN alloy. A blue optical emission with the energy value of 2.86 eV was observed for the InGaN nanoplates. The two n-type group III-nitrides (InN, GaN) prepared in this study were used for the detection of CO, NH3, CH4 and NO2 gases in the temperature range between 250 and 350 °C. The InN sensor and GaN sensor responses were compared to the response of the wellestablished n-type SnO2 sensor under the same conditions. All the three sensors responded to all the four gases. However, InN and GaN were much more selective in comparison to SnO2. InN sensitivity to CO at 250 °C surpassed its sensitivity to any other gas at the studied temperature range. Its response towards CO at 250 °C was about five times more than that of SnO2 towards CO at the same temperature. While, GaN was the best CH4 sensor at 300 °C in comparison to InN and SnO2 sensors at all temperatures. Meanwhile SnO2 responded remarkably to both NH3 and CO across the studied temperature range with its performance improving with increasing temperature. The ability for InN to respond to both NH3 and NO2 at 250 °C opens up the possibility for an application of InN as an ammonia sensor in diesel engines. InN and SnO2 sensors were found susceptible to humidity interference in a real environmental situation. On the contrary, GaN sensor presented itself as an ideal candidate for indoor and outdoor environments as well as in bio-sensors because it showed robustness and inertness towards humidity. InN and GaN by showing activity at high temperatures only, presented themselves as good candidates for in-situ high temperate gas sensing applications. Response and recovery times for all sensors showed improvement with increasing temperature.MT201