Dissertations / Theses on the topic 'Zinc Oxide Nanocrystal'

Advances in fabrication and analysis tools have allowed the synthesis and manipulation of functional materials with features comparable to fundamental physical length scales. Many interesting properties inherently due to quantum size effects have been observed in nanometre scale structures. It is hoped that these nanoscale structures will play a key role in future materials and devices that exploit their unique properties. Zinc oxide (ZnO) is a wide band-gap transparent and piezoelectric semiconductor material. It also has a large exciton binding energy which allows for stable ultraviolet light emission at room temperature. There are therefore foreseeable applications in optoelectronic devices which include ultraviolet photosensitive devices and light emitting diodes. Nanoscale structures formed from ZnO are interesting as they possess many of the properties inherent form the bulk but are also subject to various quantum size effects that may occur at the nanoscale. To date, the study of ZnO nanostructures is a relatively recent endeavour with the vast majority of reports being made within the last five years. ZnO is unique in that it forms a family of nanoscale structures. These structures include nanoscale wires, rods, hexagons, tetrapods, ribbons, rings, flowers and helixes. This work is focussed on the study of zinc oxide tetrapod crystalline nanoscale structures and their devices. We have synthesised ZnO tetrapods using chemical vapour transport techniques. Photoluminescence characterisation revealed the presence of optically active surface defects that could be quenched with a simple surface treatment. We have also for the first time observed resonant cavity modes in a single ZnO tetrapod nanocrystal. An ultraviolet sensitive Schottky diode was fabricated from a single ZnO tetrapod using focussed ion-beam assisted deposition techniques. The device characteristics observed were modelled and successfully shown to result from an illumination induced reduction in the Schottky barrier height at the metal-semiconductor interface.

As a material whose applications are many and growing, zinc oxide still remains a complex system whose photoluminescent (PL), structural, electrical, and photocatalytic properties have not been fundamentally understood. The luminescent properties of zinc oxide (ZnO) nanocrystals (NCs) are very sensitive to crystal structure, and defect states in zinc oxide, which in turn is very sensitive to preparation methods, post-synthesis workup, and thermal treatments. Understanding and managing this rich defect chemistry is critical to controlling ZnO properties. As the surface-to-volume ratio of ZnO increases as materials enter the quantum regime, the surface defects play a stronger role. The exact role of the defect states and their contribution to the physical and chemical properties of ZnO has been studies in great lengths yet still remains controversial.

The subject of this thesis is microwave assisted rapid growth of ZnO nanoparticles from an aqueous solution using different zinc precursors and heating powers, and characterization of these by scanning electron microscopy, atomic force microscopy and optical microscopy. The goal of the experiment performed was to study the effect of the heating power of the microwave oven as well as that of the zinc precursor used on the morphology and size of the grown particles. ZnO nanoparticles has many interesting possible applications in a wide range of areas, such as LED-technology, medicine, antibacterial applications, solar cells and more. Also, there is still a lot of knowledge missing concerning the growth mechanisms and properties of ZnO on the nano-scale. These two facts give good reasons to continue the research and investigations of nano-ZnO. Being able to use the microwave assisted growth method in large scale is highly interesting as it is relatively cheap, safe and easy compared to other presently used methods, so there are good reasons to learn more about this technique as well. In this project it was found that both the heating power and the zinc precursor used had significant effects on the morphology and size of the grown ZnO nanocrystals, and also that adding a zinc seed layer to the surface of the substrate before growth made a big difference in some cases.

Le but de cette thèse était d'explorer la chimie de surface des systèmes bimétalliques platine-zinc et leur activité catalytique dans la réaction d'oxydation du CO. La recherche sur ce système bimétallique a été menée sur deux fronts: une étude de surface du système modèle , une couche unique de ZnO discontinue épitaxiée sur du Pt (111), utilisant la microscopie à effet tunnel et le rayonnement synchrotron à proximité de la photoémission par rayons X à pression ambiante, et une étude davantage axée sur la «nanomatériau» du même système bimétallique, en utilisant la chimie complexe de la synthèse colloïdale , microscopie électronique à transmission et à balayage, et enfin XPS de laboratoire.Tout d'abord, une surface modèle constituée d'un film monocouche de ZnO supporté sur du Pt (111) a été fabriquée dans des conditions de vide très poussé. Sa chimie de surface a été explorée par STM puis par rayonnement synchrotron NAP-XPS dans des conditions opératoires. Nous avons pu prouver que ce système était bien un cas typique de catalyse inverse. Les effets synergiques dus à la présence des deux matériaux ont été bien observés, mais uniquement à basse température (jusqu'à 410 K). Au-delà de cette température, les effets de transport de masse empêchent la comparaison de la réactivité des surfaces de ZnO / Pt (111) et de Pt (111). Nous avons montré que des intermédiaires de réaction doivent être formés dans la zone frontière entre le ZnO et le platine, lorsque le film de ZnO est discontinu. Nous avons mis en évidence le rôle clé joué par les hydroxyles présents dans les plaques de ZnO, qui sont dus à la dissociation de H2 ou de H2O de l’atmosphère résiduelle des plaques de platine. En particulier, nous avons détecté par NAP-XPS la présence d'une espèce carboxyle (due à l'association de OH avec CO), qui précède la désorption du CO2. Au-dessus de 410 K, un formiate apparaît et cette dernière espèce est probablement spectatrice du processus d'oxydation du CO. Le transfert des connaissances accumulées dans les précédentes études de la science des surfaces et des catalyseurs modèles au cas plus réaliste des nanocristaux de l’alliage PtZn, tout en aidant à identifier certains phénomènes courants, il montre également ses limites. En fait, les nanocristaux revêtus de leurs ligands oléylamine ont des caractéristiques que les surfaces des modèles UHV ne possèdent pas, en raison du processus de fabrication de la CN lui-même: nous avons trouvé des indices spectroscopiques de la présence d’eau (éventuellement un sous-produit de la réaction, résultant d’une entre la cétone et l'amine); de plus, un recouvrement de la surface du platine par des atomes d'hydrogène est actuellement une explication de nombreux phénomènes observés. Trouver les conditions expérimentales pour produire des nano-alliages bimétalliques à partir de deux précurseurs métal-acac2 était une tâche ardue, bien plus que celle de déposer physiquement un film mince sur un monocristal d’UHV. Nos efforts ont été récompensés car nous avons pu produire des CN en alliage PtZn. C'est l'un des principaux points de la présente étude. La présence de Pt(acac)2 empêche le zinc (dont l'oxydation complète en ZnO, comme c'est le cas lorsque le Zn(acac)2 seul est présent dans l'oléylamine. L'XPS monochromatisé montre que le zinc fabrique un alliage avec le platine, où il reste métallique alors qu’une autre fraction est sous la forme de ZnO, il n’est pas clair si deux canaux de réaction sont en concurrence (alliage PtZn versus oxydation de Zn par l’eau), ou Zn est oxydé par la suite, c’est-à-dire après exposition à l’air. Les CN alliés ont été étudiés en détail par des méthodes avancées de microscopie électronique (y compris dans des conditions opératoires), de diffraction et d’EDS [...]
The purpose of this thesis was to explore the surface chemistry of platinum-zinc bimetallic systems, and their catalytic activity in the oxidation reaction of CO. The research on this bimetallic system was carried out on two fronts: a surface science study of the model system, a discontinuous ZnO single layer epitaxied on Pt(111), using scanning tunneling microscopy and synchrotron radiation near ambien pressure x-ray photoemission, and a more “nanomaterial science” oriented study of the same bi-metallic system, using complex colloidal synthesis chemistry, transmission and scanning electron microscopy, and finally laboratory XPS. First, a model surface consisting of a ZnO monolayer film supported on Pt(111) was fabricated under ultra-high vacuum conditions. Its surface chemistry was explored by STM and then by synchrotron radiation NAP-XPS under operando conditions. We were able to prove that this system was indeed a typical case of inverse catalysis. Synergetic effects due to the presence of both materials were well seen, but only at low temperatures (up to 410 K). Beyond that temperature, mass transport effects prevent the reactivity of the ZnO/Pt(111) and Pt(111) surfaces from being compared. We have shown that reaction intermediates must be formed in the border area between ZnO and platinum, when the ZnO film is discontinuous. We have highlighted the key role played by the hydroxyls present only ion the ZnO patches, which are due to the dissociation of H2 or H2O from the residual atmosphere on the platinum patches. In particular, we have detected by NAP-XPS the presence of a carboxyl species (due to the association of OH with CO), which precedes the desorption of CO2. Above 410 K, a formate appears, and the latter species is likely a spectator in the CO oxidation process. The transfer of the knowledge accumulated in the preceding surface science and model catalysts studies, to the more realistic case of nanocrystals of the PtZn alloy, while it helped identify some common phenomena, it also shows its limitations. In fact the NC coated with their oleylamine ligands have characteristics that UHV model surfaces do not possess, due to the NC fabrication process itself: we have found spectroscopic hints of the presence of water (possibly a byproduct of the reaction, arising from a condensation reaction between the ketone and the amine); in addition, a capping of the platinum surface by H atoms, is, at present, explanatory of many observed phenomena. Finding the experimental conditions to produce bimetallic nano-alloys from two metal-acac2 precursors was a daunting task, much more than that of physically depositing a thin film on a UHV monocrystal. Our efforts were rewarded as we were able to produce PtZn alloy NCs. This one of the main points of the present study. The presence of Pt(acac)2 prevents zinc (whose from being fully oxidized to ZnO, which is the case when Zn(acac)2 alone is present in oleylamine. Monochromatized XPS shows that zinc makes an alloy with platinum, where it remains metallic, while another fraction is under the form of ZnO. It is not completely clear whether two reaction channels are in competion (PtZn alloying versus Zn oxidation by water), or Zn is oxidized afterwards, i.e. after exposure to air. The alloyed NCs have been studied in detail by advanced methods of electron microscopy (including under operando conditions), diffraction and EDS. Unlike the case of the surface model where the STM images were particularly telling, we do not have at this stage of the study an exact model of the interface between the metal alloy and the zinc oxide that surrounds it. On the other hand, we know that the core of the NCs is occupied by the PtZn alloy, and that the outer planes are identical to those of pure platinum. [...]

This thesis is a comprehensive study of the synthesis of nanomaterials. It explores the synthetic methods on the control of the size, shape and phase of semiconductor nanocrystals. A number of important conclusions, including the mechanism behind crystal growth and the structure-relationship, have been drawn through the experimental and theoretical investigation. The synthesized nanocrystals have been tested for applications in gas sensing, photocatalysis and solar cells, which exhibit considerable commercialization potential.

Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices.

Polymeric materials are widely used in diverse applications. However, a major weakness in the majority of the thermoplastic polymers is their lack of ability to resist fire. Most of the chemicals and additives currently used to improve fire retardancy have deleterious effects on the environment. This research focuses on developing an environmentally safe and effective fire-retardant system for high density polyethylene (HDPE), using cellulose nanocrystals (CNCs) and zinc oxide (ZnO). The effect of CNCs coated with nano ZnO has been investigated for improving the fire resistance properties of the HDPE. Improved dispersion of CNCs into HDPE matrix was achieved by employing maleic anhydride as a coupling agent. It was found that addition of CNCs-ZnO can introduce a reasonable level of flame retardancy in HDPE matrix in addition to improving the maximum tensile strength and elongation at break.

Solid lubricant coatings with controlled microstructures are good candidates in providing lubricity in moving mechanical assembly applications, such as orthopedics and bearing steels. Nanocrystalline ZnO coatings with a layered wurtzite crystal structure have the potential to function as a lubricious material by its defective structure which is controlled by sputter deposition. The interrelationships between sputtered ZnO, its nanocrystalline structure and its lubricity will be discussed in this thesis. The nanocrystalline ZnO coatings were deposited on silicon substrates and Ti alloys by RF magnetron sputtering with different substrate adhesion layers, direct current biases, and temperatures. X-ray diffraction identified that the ZnO (0002) preferred orientation was necessary to achieve low sliding friction and wear along with substrate biasing. In addition, other analyses such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were utilized to study the solid lubrication mechanisms responsible for low friction and wear.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
Currently there are many studies involving Nanocrystals (NC) incorporated into different types of matrices, including, matrices with porous surface. However, little information is known about the incorporation of NC in Diatomite matrix, and there is a lack of studies on the use of this material. In this context, zinc oxide (ZnO) semiconductor nanoparticles were prepared using two sol-gel methods, microwave and autoclave, at 100 ºC and 180 ºC, respectively. The nanocrystals of ZnO obtained by microwaves were incorporated into a matrix of diatomite (DE) in natura and modified. The modifiers used were APTES (3- aminopropyltriethoxysilane) and MPTS (3-Mercaptopropyltrimetoxysilane) for the study. The material DE/ ZnO, in which ZnO was synthesized with mercaptoethanol (ZnO: ME), was applied for the degradation of Methylene Blue (AM) dye, while ZnO, synthesized with diethyleneglycol (ZnO: DEG), was used for degradation of Rhodamine 6G dye (R6G) by photocatalysis. The results of UV-vis and FTIR spectra show that synthesis carried out by heating under adsorption method is more efficient for the incorporation of ZnO in Diatomite matrix. The FTIR spectra showed that the use of modifiers had no significant influence on the structure. According to the UV-Vis spectra, the DE / ZnO material was successful for application to AM photocatalysis and follows a pseudo-first order kinetics. The ZnO:DEG material used for degradation of R6G obtained higher efficiency due to the wide absorption in the UV-Vis of the photocatalyst material.
Atualmente existem diversos estudos envolvendo Nanocristais (NC) incorporados em matrizes de diferentes tipos, incluindo matrizes com uma superfície porosa. Entretanto, pouco se conhece sobre a incorporação de NC em matriz de Diatomita, além de ser limitado a presença de estudos sobre aplicação desse material. Neste contexto, foram preparadas nanopartículas semicondutoras de óxido de Zinco (ZnO) utilizando dois métodos sol-gel, por micro-ondas e autoclave, numa temperatura de 100 ºC e 180 ºC, respectivamente. Os nanocristais de ZnO obtidos por micro-ondas foram incorporados em matriz de Diatomita (DE) in natura e modificada. Foram utilizados os modificadores APTES (3-Aminopropiltrietoxissilano) e MPTS (3- Mercaptopropiltrimetoxissilano) para o estudo. O material de DE/ZnO, no qual o ZnO foi sintetizado com mercaptoetanol (ZnO:ME), foi aplicado para a degradação do corante Azul de Metileno (AM), enquanto o ZnO, sintetizado com dietilenoglicol (ZnO:DEG), não incorporado foi utilizado para degradação do corante Rodamina 6G (R6G) por fotocatálise. Os resultados de UV-Vis e FTIR mostram que a síntese realizada pelo método de adsorção sob aquecimento é mais eficiente para a incorporação de ZnO na matriz de Diatomita. Os espectros de FTIR mostraram que a utilização de modificadores não exerceu influência significativa na estrutura da DE. Segundo os espectros de UV-Vis, o material de DE/ZnO foi bem-sucedido para aplicação em fotocatálise de AM e segue uma cinética de pseudo-primeira ordem. O material de ZnO:DEG utilizado para degradação de R6G obteve maior eficiência devido a ampla absorção no UV-Vis do material fotocatalisador.

This thesis presents an investigation into the factors affecting the morphology of hybrid inorganic/organic photoactive layers used in photovoltaic cells. Although optimisation of the organic (polymer) phase has received substantial attention, research into the morphology of the inorganic phase (semiconducting nanocrystals) remains limited. It is believed that there is a strong link between the morphology of the final photoactive film and the quality of the initial nanocrystal dispersion. To this end, two nanocrystal systems were investigated; zinc oxide (ZnO) and lead sulphide (PbS). ZnO nanocrystals were synthesised and found to possess reproducible characteristics. It was determined that colloid stability was initially dependent upon the presence of acetate groups bound to the surface, which in turn required a small quantity of methanol to be present in the organic dispersant. It was also discovered that while methanol evaporated readily from the surface of the nanocrystals, another molecule, 1-propylamine (1-PA), did not. Further investigations showed that while methanol only weakly physisorbed to the surface of ZnO nanocrystals, 1-PA formed strong, dative covalent bonds with Zn2+, preventing evaporation despite a low boiling point. Subsequent investigations into the effects of different ligands upon colloid stability found that amine-based groups typically possessed superior stabilising capabilities compared to alcohol-based analogues. The characteristics of nanocrystal / polymer blends were also investigated. It was determined that the nanocrystal dispersion became unstable at higher concentrations of polymer due to depletion aggregation. Films of nanocrystal / polymer blends were cast from dispersions containing either alcohol or amine-based ligands, and it was observed that dispersions stabilised with 1-PA possessed smooth morphologies on the micrometer scale. Investigations at the nanometer scale, however, revealed aggregates large enough to favour recombination.The latter half of this thesis regards the characterisation of PbS nanocrystals and investigations into triggered aggregation. It was determined that while PbS nanocrystals possessed reproducible characteristics, the stabilising molecule, oleic acid (OA) was insulating. The effects of exchanging the OA groups for a shorter ligand, butylamine (BA) were investigated.Finally, PbS nanocrystals were treated with a bidentate ligand, 1,2-ethanedithiol (EDT) to induce triggered aggregation. It was observed that the system was highly sensitive to the concentration of EDT in dispersion, forming small, relatively dispersed aggregates at low [EDT], and micrometer-sized crystalline structures at high [EDT]. The characterisation and entrapment of these nanocrystal structures within semi-conducting polymer films is also discussed.

碩士
國立臺北科技大學
材料及資源工程系所
94
The purpose of this experiment is to synthesize 1-D zinc oxide nanorods using micro-emulsion and calcination. The precursor of nano-sized zinc precipitate was first synthesized using micro-emulsion process. Nano-sized zinc oxide with equi-axial particle form could be synthesized using calcinations, but 1-D zinc oxide was failed to obtain. For producing 1-D zinc oxide nanorods, zinc precipitate mixed with active carbon was calcined, and 1-D zinc oxide was formed at the temperature of above 500℃. The vapor-solid mechanism can be used to interpret the nucleation and crystal growth.

This thesis deals with the research work carried out on the properties and applications such as GaN nanoparticles, Graphene etc. Chapter 1 of the thesis gives introduction to nanomaterials and various aspects of the thesis. Chapter 2 of the thesis describes the synthesis of GaN nanocrystals and their use as white light sources and as room temperature gas sensors. It also discusses negative differential resistance above room temperature exhibited by GaN. Electroluminescence from GaN-polymer heterojunction forms the last section of this chapter. Chapter 3 demonstrates the role of defect concentration on the photodetecting properties of ZnO nanorods with different defects prepared at different temperatures. Chapter 4 presents remarkable infrared and ultraviolet photodetector properties of reduced graphene oxide and graphene nanoribbons. Chapter 5 presents the infrared detecting properties of graphene-like few-layer MoS2. The summary of the thesis is given at the end of the thesis.

A simple bottom-up-based synthetic strategy named a solvothermal technique is introduced as the primary synthetic approach and its crystal growth mechanism is scrutinized. N-type cobalt-doped ZnO-based DMS nanocrystals are employed as a model system, and characterized by a broad spectrum of advanced microscopic and spectroscopic techniques. It is found that the self-orientation growth mechanism, imperfect oriented attachment, is intimately correlated with the high-temperature ferromagnetism via defects. The influence of processing on the magnetic properties, such as compositional variations, reaction conditions, and post-growth treatment, is also studied. In this way, an in-depth understanding of processing-structure-property interrelationships and origins of magnetism in DMS nanocrystals are obtained in light of the theoretical framework of a spin-split impurity band model. In addition, a nanoscale spinodal decomposition phase model is also briefly discussed.
Following the similar synthetic route, copper- and manganese-doped ZnO nanocrystals have been synthesized and characterized. They both show high-temperature ferromagnetism in line with the aforementioned theoretical model(s). Moreover, they display interesting exchange biasing phenomena at low temperatures, revealing the complexity of magnetic phases therein.
Spintronics (spin transport electr onics), in which both spin and charge of carriers are utilized for information processing, is believed to challenge the current microelectronics and to become the next-generation electronics. Nanostructured spintronic materials and their synthetic methodologies are of paramount importance for manufacturing future nanoscale spintronic devices. This thesis aims at studying synthesis, characterization, and magnetism of transition-metal-doped zinc oxide (ZnO) nanocrystals---a diluted magnetic semiconductor (DMS)---for potential applications in future nano-spintronics.
The crystal growth strategy demonstrated in this work not only provides a more convenient approach to directly tailor magnetic properties of advanced multifunctional spintronic materials on a nanometer scale but also contributes to a deeper insight into the microscopic origin of magnetism in wide-band-gap oxide DMSs.
Wang, Xuefeng.
"August 2007."
Adviser: J. B. Xu.
Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1230.
Thesis (Ph.D.)--Chinese University of Hong Kong, 2007.
Includes bibliographical references.
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Abstract in English and Chinese.
School code: 1307.

碩士
國立中央大學
化學工程與材料工程研究所
94
Abstract A highly effective and large scale method was found to transfer ZnO nanocrystals (~5.7nm NCs) prepared in EG to non-polar solvents. This was accomplished by capping the ZnO NCs with Oleic acid (OA) or Linoleic acid (LinOA). FTIR and TGA analysis indicated that OA is bonded to surface of ZnO by bidentate chelating and controlled chelating types could be possible.The amount of capping agent required was ~8 molecule/nm2. The capped NCs formed a highly transparent (T>90%) dispersion in n-hexane even at 16wt%. The dispersed NCs could be further coated with silica or alkyl silane in the non-polar solvent via the nano-emulsion method then transfer back to polar solvent. The so obtained ZnO@SiO2 nanoparticles showed a very high Photoluminescence efficiency in aqueous suspension. The ZnO@SiO2 nanoparticles obtained exhibited a broad green emission band at about 535nm in aqueous suspension and yellow-orange emission band at 590nm as bulk pieces. The further application were disirable for functional nanocomposites and optical material.

This thesis consists of two parts. The first part deals with the visible emission of ZnO Nanocrystals and its possible application in Resonance Energy Transfer (RET) studies. The second part of the thesis is on the magnetic properties of the layered transition metal Thiophosphates MPS3 (M = Mn, Fe), their solid solutions and intercalation compounds. Recent advances in semiconductor nanocrystals or quantum dots (QDs) as inorganic fluorophores have pioneered a new direction in the fluorescent based techniques to investigate fundamental processes in lifesciences. Their broad absorption spectra with narrow, Size-tunable emissions with high quantum e±ciency and stability under relative harsh environments have made inorganic QD's the fluorophores of choice in many applications. Among inorganic fluorophores the II-VI semiconductors based on cadmium chalcogenides are the front-runners. The cytotoxicity associated with these QDs is, however, a major drawback and has lead to the search for new nanocrystalline fluorophores that are non-toxic and possess the same favorable fluorescence properties as the Cd based QDs, viz, tunability and narrow spectral profile. ZnO Nano particles are known to exhibit two emission bands; a narrow emission band in UV around 380 nm (3.25 eV) at a wavelength just below the onset of the band gap excitation in the absorption spectra and a broad emission band in the blue-green part of the visible spectrum, with a maximum between 500 and 550 nm (2.5-2.2 eV). The UV Emission originates from the recombination of bound excitons - excited electrons in the Conduction band with holes in the valence band. The visible emission of ZnO nanocrystals is known to involve deep trap states that lie approximately midway between the Conduction and valence bands and surface defects that exist as shallow traps. In principle, visible-light-emitting ZnO nanocrystals would be ideal candidates as replacement for Cd-based fluorescent labels since they are nontoxic, less expensive, and chemically stable in air. Nanoscale ZnO, however, tends to aggregate and/or undergo Ostwald ripening be-Cause of high surface free energy resulting in an increase in crystallite size and consequent Disappearance of the visible emission. Most attempts to stabilize the ZnO nanocrystals by Capping has usually resulted in the quenching of the visible trap emission. The objective of the present study was to stabilize the visible light emission of ZnO nanocrystals, to Understand the origin and mechanism of the visible emission and to explore the possibility Of using the visible emission of ZnO in RET studies. The stabilization of visible light emission in ZnO nanocrystals was achieved by forming ZnO:MgO core-shell nanocrystals. The nanocrystals were synthesized by a sequential preparative procedure that involved formation of a ZnO core followed by an MgO shell. The Nanocrystals were characterized by using XRD, TEM, optical absorption and photoluminescence spectroscopy. These are described in Chapter 2 of the thesis. The ZnO: MgO Core-shell nanostructures exhibit stable emission in the visible for extended periods. Application of the ZnO: MgO nanocrystals either as fluorescent probes or RET studies require that they be dispersible in both polar and non-polar solvents. This as realized by appropriate choice of the capping agents (Chapter 3). ZnO: MgO nanocrystals with hydrophobic surface were obtained by capping the nanocrystals with oleic acid. The oleate capped ZnO: MgO nanocrystals are soluble in a variety of non-polar organic solvents with no change in their emission properties. Water-soluble ZnO: MgO nanocrystals were obtained by capping the ZnO:MgO nanocrystals with carboxymethyl-β-cyclodextrin (CMCD). The hydroxyl groups located at the rim of the cyclodextrin cavity renders the surface hydrophilic. The integrity of the CMCD molecules are preserved on capping and their by hydrophobic cavities available for host-guest chemistry. The visible emission of The ZnO: MgO nanocrystals are unaltered by the nature of the capping agent. The origin and mechanism of the visible emission from ZnO: MgO nanocrystals has been Investigated using time-resolved emission spectroscopy technique (Chapter 4). The time-evolution of the photoluminescence spectra show that there are, in fact, two features in the visible emission whose relative importance and efficiencies vary with time. These features originate from recombination involving trapped electrons and holes, respectively, And with efficiencies that depend on the occupancy of the trap density of states. The application of the visible emission of ZnO: MgO nanocrystals as resonance energy transfer (RET) donors in water and hydrophobic media are demonstrated. In aqueous media, the carboxymethyl β-cyclodextrin (CMCD) capped ZnO: MgO nanocrystals is able to accommodate the organic dye Nile Red by an inclusion in the anchored hydrophobic cyclodextrin cavity forming a 1:1 complex. Nile Red was chosen as the guest molecule because its absorption has appreciable overlap with ZnO: MgO visible emission, a prerequisite for RET to occur. The resonance energy transfer on the band gap excitation of The ZnO core to included Red molecules in the CMCD-ZnO: MgO-Nile Red supramolecular assembly is demonstrated in aqueous media. A similar RET process is shown to occur in the non-polar media in the oleate capped ZnO: MgO nanocrystals when Nile Red is partitioned from the solvent into hydrophobic anchored oleate chains. The wavelength dependent energy transfer in the system has been studied using time-resolved emission spectroscopic technique. The importance of trap states in giving rise to non-Forster distance dependence for the RET is highlighted. The second part of the thesis deals with magnetism in low dimensional layered transition metal thiophophates, MPS3 (M = Mn, Fe). Low dimensional magnetic systems continue to be a fertile ground for discovering new phenomena and properties. Among two-dimensional magnetic systems the insulating transition metal thiophosphates are one of the few known layered systems, in which both magnetic and crystallographic lattices are two dimensional (2D). In the metal chalcogenophosphates, the magnetic MPX3 layers are separated by a van der Waals gap that effectively rules out interlayer exchange and hence these systems are nearly perfect 2D magnetic systems, with the magnetic ions forming a honeycomb arrangement within the layer. Due to the crystallographic two-dimensional nature these materials may be intercalated by variety of molecules or ions leading to change in magnetic properties. The objective of this thesis work is to try and modify the magnetic properties of the transition metal thiophosphates either by forming solid solutions of the type, M1-xMxPS3, (M, M = Mn, Fe) or by intercalating hydrated metal ions within the layers. The structure, Bonding, reactivity and magnetic properties are briefly introduced in Chapter 7. The Scope and nature of the present work in presented towards the end of the chapter. MnPS3 and FePS3 have identical crystal structures and both order antiferromagnetically at low temperatures, TN. The in-plane magnetic structures of the antiferromagnetically ordered the Neel state in the two compounds are, however, different. In MnPS3 the spins Alternate up-down whereas in FePS3 the spins are arranged as ferromagnetic chains with Alternate chains coupled antiferromagnetically. Since the crystal structures are identical, These two compounds can form solid solutions, Mn1-xFexPS3(0≤x≥1) over the entire concentration range. The magnetic properties of the single crystals of the solid solutions was measured by using a SQUID magnetometer. This system is of interest since the contrasting Neel states of the end-members may give rise to new magnetic phenomena at intermediate composition. It is shown that the magnetic behavior falls into three distinct categories. The Mn-rich compositions behave like a dilute MnPS3 lattice, the Fe-rich compositions behave like dilute FePS3 and in the intermediate compositions a spin-glass like phase appears. The phase boundaries for these regime in Mn1-xFexPS3, 0≤x≥1 is shown to be related to the percolation threshold for a honeycomb lattice. MnPS3 is known to undergo a unusual ion-exchange intercalation reaction. Intercalation occurs by the inclusion of hydrated metal ions in the galleries of MnPS3 with charge neutrality maintained by loss of the Mn2+ ions from the layer (Equation). MnPS3 + 2xG+ (aq) → Mn1-xPS3 [G (H2O) y] 2x + xMn2+ (aq) Where G is a neutral guest species. This chemistry has been exploited to intercalate hydrated Mn2+ ions in the interlamellar space to give Mn1-xPS3[Mn(H2O)6]x. the magnetic properties of this 3D analogue of MnPS3 has been investigated in Chapter 9.

A thesis submitted to the Faculty of Engineering and the Built Environment in fulfilment of the requirement for the Degree of Masters of Science, University of the Witwatersrand, Johannesburg, 2019
Drilling Mud holds an important role in the drilling process in such a way that it is a determinant key to the success of the operation as well as the money spent throughout the process. Indeed the success and the cost of the operation can be severely impacted by some challenges experienced while drilling such as temperature and pressure conditions which leads to fluid loss, fluid deterioration...As a result there is a need to formulate a fluid with desirable rheological properties to withstand such undesirable parameters. Therefore this work was aimed to improve emulsion drilling fluids (EDFs) based nanoparticles with enhanced properties. Many investigations were performed to find a proper emulsion stability as well as a good drilling fluid performance. The stability of the prepared emulsion drilling fluids was done using surfactant with different concentrations for several days. After several days of preparation, the EDFs containing DTAB as surfactant have showed a better emulsion stabilizer compared to the Triton X-100 ones. In addition an investigation combining both NPs and surfactants confirmed the used of NPs to improve DF and revealed the effective use of ZnO NPs for drilling fluids application and preferentially with DTAB as surfactant. Following that result, the 2nd part of the work was based on the synthesis and characterization of CNCs as NPs to formulate EDF with DTAB as surfactant. The CNCs NPS were successfully obtained via the method of oxidation of microfibrillated cellulose through TEMPO-mediate and after characterization using TEM, spherical NPs with small size varying from 10-50nm were observed. The FANN® Model 35 viscometer served to display the behavior of the shear stress and viscosity of the prepared fluids against variable shear rate at variable NPs and temperature concentration. The rheological and filtration properties were increase with increase in CNCs content from 0.8 to 1.2% of fluid in room temperature and with an increase in temperature.
PH2021

Doping in bulk semiconductors (e.g., n- or p- type doping in silicon) allows for precise control of their properties and forms the basis for the development of electronic and photovoltaic devices. Recently, there have been reports on the successful synthesis of doped semiconductor nanocrystals (or quantum dots) for potential applications in solar cells and spintronics. For example, nanocrystals of ZnSe (with zinc-blende lattice structure) and CdSe and ZnO (with wurtzite lattice structure) have been doped successfully with transition-metal (TM) elements (Mn, Co, or Ni). Despite the recent progress, however, the underlying mechanisms of doping in colloidal nanocrystals are not well understood. This thesis reports a comprehensive theoretical analysis toward a fundamental kinetic and thermodynamic understanding of doping in ZnO, CdSe, and ZnSe quantum dots based on first-principles density-functional theory (DFT) calculations. The theoretical predictions of this thesis are consistent with experimental measurements and provide fundamental interpretations for the experimental observations. The mechanisms of doping of colloidal ZnO nanocrystals with the TM elements Mn, Co, and Ni is investigated. The dopant atoms are found to have high binding energies for adsorption onto the Zn-vacancy site of the (0001) basal surface and the O-vacancy site of the (0001) basal surface of ZnO nanocrystals; therefore, these surface vacancies provide viable sites for substitutional doping, which is consistent with experimental measurements. However, the doping efficiencies are affected by the strong tendencies of the TM dopants to segregate at the nanocrystal surface facets, as indicated by the corresponding computed dopant surface segregation energy profiles. Furthermore, using the Mn doping of CdSe as a case study, the effect of nanocrystal size on doping efficiency is explored. It is shown that Mn adsorption onto small clusters of CdSe is characterized by high binding energies, which, in conjunction with the Mn surface segregation characteristics on CdSe nanocrystals, explains experimental reports of high doping efficiency for small-size CdSe clusters. In addition, this thesis presents a systematic analysis of TM doping in ZnSe nanocrystals. The analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets, as well as dopant-induced nanocrystal morphological transitions, and leads to a fundamental understanding of the underlying mechanisms of dopant incorporation into growing nanocrystals. Both surface kinetics (dopant adsorption onto the nanocrystal surface facets) and thermodynamics (dopant surface segregation) are found to have a significant effect on the doping efficiencies in ZnSe nanocrystals. The analysis also elucidates the important role in determining the doping efficiency of ZnSe nanocrystals played by the chemical potentials of the growth precursor species, which determine the surface structure and morphology of the nanocrystals.

Metal oxide nanocrystals have attracted significant interests due to their unique chemical, physical, and electrical properties which depend on their size and structure. In this study, a continuous flow microreactor system was employed to synthesize metal oxide nanocrystals in aqueous solution. Assembly of nanocrystals is considered one of the most promising approaches to design nano-, microstructures, and complex mesoscopic architectures. A variety of strategies to induce nanocrystal assembly have been reported, including directed assembly methods that apply external forces to fabricate assembled structures. In this study ZnO nanocrystals were synthesized in an aqueous solution using a continuous flow microreactor. The growth mechanism and stability of ZnO nanocrystals were studied by varying the pH and flow conditions of the aqueous solution. It was found that convective fluid flow from Dean vortices in a winding microcapillary tube could be used for the assembly of ZnO nanocrystals. The ZnO nanocrystal assemblies formed three-dimensional mesoporous structures of different shapes including a tactoid, a retangle and a sphere. The assembly results from a competing interaction between electrostatic forces caused by surface charge of nanocrystals and collision of nanocrystals associated with Dean vortices. The as synthesized colloidal ZnO nanocrystals or assembly were directly deposited onto a substrate to fabricate ZnO nanostructured surfaces. The rectangular assembly led to flower-like ZnO nanostructured films, while the spherical assembly resulted in amorphous ZnO thin film and vertical ZnO nanowire (NW) arrays. In contrast to the formation of flower structure or amorphous thin film, only colloidal ZnO nanocrystals were used as the building blocks for forming vertical ZnO NW arrays. This study demonstrates the versatility of the microreactor-assisted nanomaterial synthesis and deposition process for the production of nanostrucuturesres with various morphologies by tuning the physical parameters while using the same chemical precursors for the synthesis. ZnO flower structure was coated on a microwick structure to improve the capillary flow. The coated microwick structure showed an enhanced capillary rise, which was attributed to the hydrophilic property and geometrical modification of ZnO nanostructure. Two-phase boiling heat transfer was performed using ZnO nanostructured surfaces. ZnO nanocoating altered the important characteristics including surface roughness and wettability. Hydrophilic nature of the ZnO nanocoating generally enhanced the boiling heat transfer performance, resulting in higher heat transfer coefficient (HTC), higher critical heat flux (CHF), and lower surface superheat comparing to the bare surface. Octahedral SnO and porous NiO films, fabricated by a continuous flow microreactor system, were suggested as potential boiling surfaces for the high porosity and irregularity of their structures.
Graduation date: 2013

The development of radio frequency integrated circuits (RFICs), especially the dream of integrating analog, digital and radio frequency (RF) components on the same chip that is commonly known as System-on-a-Chip (SoC), is crucial to mobile communications of the future. Such SoC approach offers enhanced performance, greater reliability, and substantially less power consumption of integrated circuits while reducing overall physical size and thus manufacturing cost. However, the progress has been stalled by the lack of miniaturized inductor elements. Rise of unwanted parasitic effects limits down-scaling of the inductor structures and leaves the use of magnetic coating as a viable and attractive option to enhance the inductance and thus inductance density. It is also essential to shift from perm alloy and other amorphous alloys to ferrites and hex ferrites as the core material because of their very high electrical resistivity so as to keep losses in check, a criterion that cannot be compromised on in GHz frequency applications. This is viable, however, only if the integration of the magnetic core (film), particularly a ferrite film, is fully compatible with the CMOS fabrication process. Various approaches have been taken to meet this requirement, including investigations of employing layers of ferrite materials to envelop the inductor loop. However, the deposition of thin films of ferrites, whether by PVD or CVD, usually calls for the deposited ferrite layer to be annealed at an elevated temperature to crystallize the layer so that its magnetic characteristics are appropriate for the optimum performance of the circuit element. Such annealing is incompatible with CMOS process flow required for aggressive device geometries, as the inductor element is added after the active semiconductor circuit is processed, and any exposure of the processed circuit to elevated temperatures risks disturbing precise doping profiles employed and the integrity of the inter-layer dielectrics. What is called for is a low-temperature process for the deposition of a ferrite layer on top of the patterned inductor element – a layer of thickness such that most of the fringe field is encapsulated – while ensuring that the layer comprises crystallites of uniform size that leads to uniform magnetic behaviour. Recognizing the difficulty of meeting the various stringent requirements, it has recently been remarked that such a goal is a formidable challenge. In an attempt to address this challenge, in this work, we have adopted a counter-intuitive approach - the deposition of the desired ferrite composition on a processed die (that contains the inductor structures along with active semiconductor circuits) by immersing it into a chemical (reactant) solution, followed by a brief irradiation of microwave frequency. However, to identify the desired ferrite composition and the appropriate recipe to deposit them, a systematic effort had to be made first, to understand the inter-relationship between synthesis process, structure of resulting material, and its physical and chemical properties. Therefore, at the beginning, a general introduction in which key concepts related to the magnetic-core inductors, the microwave-irradiation-assisted synthesis of nanostructures, the ‗state of the art‘ in the field of integration of appropriate magnetic material to the RFICs, are all outlined. As a proof of concept, microwave-irradiation-assisted solution-based deposition of zinc ferrite thin films on the technologically important Si (100) substrate is demonstrated. The highlight of the process is the use of only non-toxic metal organic precursors and aqua-alcoholic solvents for the synthesis, which is complete in 10 minutes @< 100 °C, without any poisonous by-products. Effects of various process parameters such as solute concentrations, surfactant types, and their concentrations are investigated. A wide range of deposition rates (10 - 2000 nm/min) has been achieved by tweaking the process parameters. The simultaneous formation of zinc ferrite nanocrystallites (ZFNC) along with deposition of thin film is the hallmark of this synthesis technique. Unlike its bulk counterpart, both film and powder are found upon investigation to be rich in magnetic behavior– owing to plausible cationic distribution in the crystal lattice, induced by the inherently quick and far-from-equilibrium nature of the process. The accurate estimation of magnetic characteristics in film is, however, found to be difficult due to the high substrate-to-film mass ratio. The simultaneously prepared ZFNC is examined to arrive at the optimized process recipe that imparts the desired magnetic properties to the zinc ferrite system. The crystallographic cationic distribution in zinc ferrite powder is, however, difficult to study due to the nanoscale dimension of the as prepared material. To enable crystal growth, slow and rapid annealing in air at two different temperatures are employed. The effects of these annealing schemes on various attributes (magnetic properties in particular) are studied. Rapid annealing turns out to be an interesting pathway to promote rapid grain-growth without disturbing the crystallographic site occupancies. The presence of inversion, i.e., the amount of Fe3+ in the ‗A‘-sites in the spinel structure that ideally is zero in normal spinel structure of zinc ferrite, is evident in all annealed ZFNC, as determined by Riveted analysis. Such partially inverted ZFNC exhibits soft magnetic behavior with high saturation magnetization, which can easily be ―tuned‖ by choosing appropriate annealing conditions. However, a few unique strategic modifications to the same microwave-irradiation-assisted solution-based synthesis technique are tried for the formation of nanocrystalline powder with desired sizes and properties without the necessity of anneal. The approach eventually appears to pave a way for the formation of oriented structures of zinc ferrite. The effects of anneal, nevertheless, are studied with the help of neutron powder diffractometry and magnetic measurements. The magnetic ordering at various temperatures is analyzed and connected to the magnetic measurements. The study shows that long-range magnetic ordering, present even at room temperate, originates from the distribution of cations in the partially inverted spinel structures, induced by the rapid and kinetically driven microwave synthesis. Keeping the mild nature (<200 °C) of the processing in mind, a large degree of inversion (~0.5) is a surprise and results in a very high saturation magnetization, as much as 30 emu/g at room temperature (paramagnetic in bulk), in the ZFNC system. Based on the knowledge of process-structure-property interrelationship, a recipe for the deposition of ferrite thin films by the microwave-assisted deposition technique is optimized. Successful deposition of smooth and uniform zinc ferrite thin films on various substrates is, then, demonstrated. The mystery behind the strong adherence of the film to the substrate - an unexpected outcome of a low-temperature process - is probed by XPS and the formation of silicates at the interface is identified as the probable reason. The uniformity and consistency of film composition is also examined in this chapter. Another salient feature of the process is its capability to coat any complex geometry conformally, allowing the possibility of depositing the material in a way to ―wrap around‖ the three-dimensional inductor structures of RF-CMOS. Integration of nanostructure zinc ferrite thin films onto on-chip spiral inductor structures has been demonstrated successfully. The magnetic-core inductors so obtained exhibit the highest inductance density (700 nH/mm2) and the highest Q factor (~20), reported to date, operate at 5 GHz and above, by far the highest reported to date. An increase in inductance density of as much as 20% was achieved with the use of just 1 µm thick film of zinc ferrite covering only the ―top‖ of the spiral structure, i.e., up to 20% of chip real estate can potentially be freed to provide additional functionality. The microwave-assisted solution-based deposition process described in this thesis is meant for ‗post-CMOS‘ processing, wherein the film deposited on some specific electronic components can add desired functionality to or improve the performance of a component (circuit) underneath. However, the effect of such ‗post-CMOS‘ processing on the active MOS devices, interconnects, and even inter-layer-dielectrics fabricated prior to the deposition has to be mild enough to leave the performance of delicate MOS characteristics intact. Such CMOS-compatibility of the present deposition process has been tested with a satisfactorily positive result.

This thesis deals with the research work carried out for the development of novel applications by integrating biomolecules with various nanostructures. The thesis is organized as follows: Chapter 1 reviews the properties of nanomaterials which are important to consider while developing them for various biological and other applications. It discusses the factors which affect the cytotoxicity of nanocrystals towards living cells, photocatalytic mechanisms of nanocrystals that work behind the inactivation of bacterial cells and gas sensing properties of nanocrystals. It also mentions about the integration of biomolecules with nanomaterials which is useful for the development of biosensors, materials that are presently used for fabricating biosensors and the challenges associated with designing successful biosensors. Chapter 2 presents the antibacterial and anticancer properties of ZnO/Ag nanohybids. In this study a simple route to synthesize ZnO/Ag nanohybrids by microwave synthesis has been established where ZnO/Ag nanohybrids have shown synergistic cytotoxicity towards mammalian cells. The observed synergism in the cytotoxicity of ZnO/Ag nanohybrids could lead to the development of low dose therapeutics for cancer treatment. Chapter 3 presents photocatalytic inactivation of bacterial cells by pentavalent bismuthates class of materials. AgBiO3 which was obtained from KBiO3 by ion-exchange method was investigated for its photocatalytic inactivation properties towards E.coli and S.aureus cells under dark and UV illumination conditions. Chapter 4 presents the integration of DNA molecules with ZnO nanorods for the observation of Mott-Gurney characteristics. In this study, ZnO nanorods were synthesized hydrothermally and were characterized by TEM and XRD analysis. DNA molecules were immobilized over ZnO nanorods which were confirmed by UV-Vis spectroscopy and confocal florescence microscopy. Solution processed devices were fabricated by using these DNA immobilized nanostructures and I-V characteristics of these devices were taken in dark and under illumination conditions at different wavelengths of light at fixed intensity. Interestingly, Mott-Gurney law was observed in the I-V characteristics of the devices fabricated using DNA immobilized ZnO nanorods. Chapter 5 presents the chemical synthesis of molecular scale ultrathin Au nanowires. These nanostructures were then used for fabricating electronic biosensors. In this study, the devices were fabricated over Au nanowires by e-beam lithography and a methodology to functionalize Au nanowires and then characterize them by florescence microscopy as well as AFM has been established. The fabricated biosensors were employed for the label free, electrical detection of DNA hybridization process. Chapter 6 presents a simple, cost effective and solution processed route to fabricate devices using ultrathin Au nanowires. The devices were then used for sensing ethanol, H2 and NH3. An important property of these devices is that they can sense these gases at room temperature which reduce their operation cost and makes them desirable to use under explosive conditions.