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.

M.Sc. (Chemistry) / In this study the stability as well as the fate and behavior of ZnO engineered nanoparticles (ENPs) in municipal wastewater systems were investigated. The first part of the study examined the influence of pH and ionic strength on the stability of ZnO ENPs in domestic wastewater to elucidate the dynamic changes on the ENPs physicochemical characteristics (e.g. aggregation or dissolution). The ZnO ENPs obtained commercially were characterized using transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), dynamic light scattering (DLS), BET surface area determination, and energy dispersive X-ray spectrometry (EDS). Results derived from inductively coupled plasma optical emission spectrometry (ICP-OES) for zinc analysis indicated a decrease on the released zinc concentration from wastewater as the pH and ionic strength increased. Conversely, an increase on zinc concentration from the sludge was observed. The findings suggest the removal of ZnO ENPs from the influent wastewater as the sludge settled out, and the removal efficiency was directly proportional to ionic strength and pH. In addition, the ZnO ENPs suspension in the wastewater was used to monitor the particle size distribution using the dynamic light scattering analysis (DLS). The formation of agglomerates was observed which the TEM and EDS analysis confirmed to be ZnO aggregates. The distribution of zinc in the sludge was investigated using XRD analysis and the findings indicated partial sedimentation of ZnO ENPs as the sludge settled out. The second part of the study assessed the fate and behavior of ZnO ENPs in wastewater treatment systems. This study was carried out in a simulated activated sludge wastewater treatment plant (AS WWTP), constructed in accordance to the Organization for Economic Co-operation and Development (OECD 303 A) guidelines. Results from the ICP-OES analysis for zinc indicated 50 – 200 μgL-1 and about 3 000 mgkg-1 were released into the effluent and sludge, respectively, after spiking the influent wastewater with 5 mgL-1 ZnO ENPs. Moreover, we noted that increasing the ZnO ENPs concentration up to 20 mgL-1 resulted in a linear increase in the zinc releases into effluent wastewater. However, the increase was insignificant in comparison to the zinc found in the control unit. Therefore, the findings indicated that ZnO ENPs had stronger affinity for the suspended bio-solids during wastewater treatment, and postulated that the ENPs removal from the influent wastewater was due to bio-sorption, and bio-solid settling mechanisms. These were confirmed by results from XRD and diffuse reflectance spectroscopy (DRS) analysis of the sludge as they showed the presence of ZnO in the sludge. The dissolved organic carbon (DOC) and chemical oxygen demand (COD) were concurrently monitored during the ENPs exposure studies to indirectly assess the ZnO ENPs impact to the bacterial degradation of the organic matter. An average of 43 and 91 % for DOC and COD removal efficiencies, respectively, were observed throughout the study. Overall, from results obtained indicated the suitability of the OECD 303 A method to assess the fate and behavior of ZnO ENPs in WWTPs. Secondly, in light of low concentrations of ZnO ENPs found in the treated effluent due to their removal with the waste activated sludge (WAS), suggest low likelihood of ZnO ENPs release and dispersion into the aquatic systems from WWTPs as point sources. Finally, the elevated concentrations of ZnO ENPs in the sludge therefore necessitates additional treatment steps to ensure mitigation of possible dispersion of ENPs from various disposal mechanisms such as landfilling, incineration, and agricultural applications.

Zinc oxide and graphene nanostructures are important technological materials because of their unique properties and potential applications in future generation of electronic and sensing devices. This dissertation investigates a brief account of the strategies to grow zinc oxide nanostructures (thin film and nanowire) and graphene, and their applications as enhanced field effect transistors, chemical sensors and transparent flexible electrodes. Nanostructured zinc oxide (ZnO) and low-gallium doped zinc oxide (GZO) thin films were synthesized by a magnetron sputtering process. Zinc oxide nanowires (ZNWs) were grown by a chemical vapor deposition method. Field effect transistors (FETs) of ZnO and GZO thin films and ZNWs were fabricated by standard photo and electron beam lithography processes. Electrical characteristics of these devices were investigated by nondestructive surface cleaning, ultraviolet irradiation treatment at high temperature and under vacuum. GZO thin film transistors showed a mobility of ~5.7 cm 2/ V•s at low operation voltage of ~0.5 V with a sub threshold swing of ~85 mV/decade. Bottom gated FET fabricated from ZNWs exhibit a very high on-to-off ratio (~10 6) and mobility (∼28 cm 2 /V•s). A bottom gated FET showed large hysteresis of ~5.0 to 8.0 V which was significantly reduced to ~1.0 V by the surface treatment process. The results demonstrate charge transport in ZnO nanostructures strongly depends on its surface environmental conditions and can be explained by formation of depletion layer at the surface by various surface states. A nitric oxide (NO) gas sensor using single ZNW, functionalized with Cr nanoparticles was developed. The sensor exhibited average sensitivity of ~46% and a minimum detection limit of ~1.5 ppm for NO gas. The sensor also is selective towards NO gas as demonstrated by a cross sensitivity test with N2, CO and CO2 gases. Graphene film on copper foil was synthesized by chemical vapor deposition method. A hot press lamination process was developed for transferring graphene film to flexible polymer substrate. The graphene/polymer film exhibited a high quality, flexible transparent conductive structure with unique electrical-mechanical properties; ~88.80 % light transmittance and ~1.1742 kΩ/sq sheet resistance. The application of a graphene/polymer film as a flexible and transparent electrode for field emission displays was demonstrated.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES.

Zinc oxide (ZnO) semiconductors have been utilized by many researchers, due to its unique properties beneficial for functional devices. In particular, gadolinium (Gd)–doped ZnO exhibits high ferromagnetic and electrical properties, which is attributed to defect/impurity bands mediated by Gd dopants. In this dissertation, I study the effects of Gd concentration, oxygen pressure using pulsed laser deposition (PLD), and thermal annealing on the optical and structural properties of undoped and Gd-doped ZnO films and nanostructures. Moreover, as the growth of practical ZnO nanostructures-based devices without catalyst, while presently challenging, is highly important for many applications. Thus, for the first time, a novel method is developed for growing well aligned ZnO nanorods (NRs) by optimizing PLD conditions using Gd-doped ZnO target without any catalyst in a single step. This study shows that, both the lattice orientation of the substrate and the Gd characteristics are significant in enhancing the NR growth. Our findings reveal that precise control of the NR density can be achieved by changing the oxygen partial pressure. Furthermore, due to the Gd incorporation, these NRs possess favorable electrical properties with a significant mobility of 177 cm2 (V.s)-1 compared to that reported in literature. Nonetheless significant challenges need to be overcome to achieve reproducible and stable p-type ZnO for commercial applications. Hence, several attempts based on n-type ZnO grown on foreign p-type substrates were made to achieve high-performance devices and overcome the issues arising when p-type doped ZnO is employed. Moreover, Growth of ZnO nanostructures on a foreign p-type substrates does not require a lattice-matched p-type substrate. Thus, for the first time, PLD conditions are improved to grow high quality ZnO nanotubes (NTs) with high optical, structural and electrical properties on a p-type Si (100) substrate without catalyst for high-performance devices. A fabrication of high performance UV photodetector (PD) based on ZnO NT/p-Si is demonstrated with superior responsivity (up to ~ 101.2 AW-1) compared to that reported in literature. This new and simple method demonstrates that the PLD system has a significant potential for improving the performance of materials used in a wide range of electronic and optoelectronic applications.

This thesis is dealing with nanowires of zinc oxide and of cesium lead bromide. Main goal was a preparation of ZnO nanowires using MBE. This goal was partially achieved and growth of needle-like structures was observed. Another goal was characterization of already prepared ZnO nanowires. We successfully determined polarity of their surfaces, examined the influence of lattice defects and assessed the result of their doping. Side goal of this work was characterization of nanostructures of cesium lead bromide. However, preparation of cesium lead bromide nanowires remains an open issue.

In 2018 alone, the global energy demand grew by 2.3% and will rise by 1.3% each year to 2040 [1] making it the fastest growth rate in the last decade predominantly driven by a robust global economy and increased heating and cooling needs. This tremendous need resulted in using fossil fuels to meet nearly 70% of the growth, but also promoted solar and wind generation to have a double-digit growth pace, with solar alone increasing by 31%. Terrestrial solar energy at AM1.5 is generally given at 1kW/m2, which is a vast free source of energy that can be harvested to meet the global demand for electricity [2]. A major obstacle for large scale solar power production is obscuration of sunlight on solar collectors caused by dust deposition or ‘soiling’ as the power plants are located in semi-arid or desert regions. Soiling could result in energy-yield output losses of about a third of the total power output of the installation [3] as water is a scarce commodity, resulting in lesser cleaning cycles of the solar collectors. Electrodynamic Screen (EDS) films can restore the efficiency of solar power installations without the use of water, manual labor or robots and can be retrofitted onto the sun facing surfaces of solar collectors, including concentrating solar power mirrors (CSP) and photovoltaic (PV) modules. Applying a low frequency pulsed voltage to the electrodes which form the central unit of the EDS films charges the dust on the collector surface and ejects it using a travelling wave. The electrodes are of paramount importance to the EDS film as they are the primary working unit of the device. Copper is the current choice of electrode material but it is susceptible to electromigration and has serious environmental disadvantages with respect to corrosion and instability. Copper electrodes also do not meet the transmission efficiency (TE) requirements as the opaque electrodes absorb and obstruct the incident sunlight through shadowing, hurting the output efficiency of the photovoltaic modules. Hence for the EDS film to be a strong candidate as a cleaning technology, it must have environmentally durable electrodes that meet the TE needs. For this purpose (1) I have developed a screen printable ink with zinc oxide (ZnO) and silver nanowire (AgNW) that is transparent in nature which satisfies the TE requirements of the EDS film which were previously unmet; this ink referred to as AgNW_ZnO Hybrid Ink throughout this document is my original contribution for the optimization of the optical performance of the EDS film technology. (2) I have established the environmental durability, stability and functionality of the electrodes of the EDS film through standardized, vigorous testing which were not performed before. To do so, I have followed the testing standards specified under the Accelerated Weathering (QUV) ASTM G154 tests which are used to validate the environmental viability of materials (3) My study and findings on the top surface component of the EDS film proposes measures of action that will enhance the removal of dust and suggests alternative, more cost efficient replacements for the ultrathin glass layer which serves as the current design’s top layer (4) My experiments and results on the charging mechanism/behavior of dust particles upon EDS film activation contribute to optimizing the design parameters used for both the EDS film and the power supply unit used to power it. These contributions allow for increasing the output power restoration of PV modules and specular reflection restoration of CSP mirrors that have the EDS film on their optical surfaces.

ZnO and ZnO-TiO2 ceramics have intriguing electronic and mechanical properties and find applications in many fields. Many of these properties and applications rely on the understanding of defects and defect processes in these oxides as these defects control the electronic, catalytic and mechanical behaviors. The goal of this dissertation is to systematically study the defects and defects behaviors in Wurtzite ZnO and Ilmenite ZnTiO3 by using first principles calculations and classical simulations employing empirical potentials. Firstly, the behavior of intrinsic and extrinsic point defects in ZnO and ZnTiO3 ceramics were investigated. Secondly, the effect of different surface absorbents and surface defects on the workfunction of ZnO were studied using DFT calculations. The results show that increasing the surface coverage of hydrocarbons decreased the workfunction. Lastly, the stacking fault behaviors on ilmenite ZnTiO3 were investigated by calculating the Generalized Stacking Fault (GSF) energies using density functional theory based first principles calculations and classical calculations employing effective partial charge inter-atomic potentials. The gamma-surfaces of two low energy surfaces, (110) and (104), of ZnTiO3 were fully mapped and, together with other analysis such as ideal shear stress calculations.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第18382号 / エネ博第294号 / 新制||エネ||61(附属図書館) / 31240 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 佐川 尚, 教授 八尾 健, 教授 萩原 理加 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

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