Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Green synthesis of silver nanowires and novel assembly technique for iron oxide nanocubes(01. Izmir Institute of Technology, 2024) Güvenç, Tuğçe Aybüke Arıca; Balcı, Sinan; Adem, UmutGümüş nanoteller, mükemmel elektriksel iletkenlikleri, plazmonik davranışları ve esneklikleriyle bilinir ve bu nedenle çeşitli gelişmiş uygulamalar için oldukça uygundurlar. Bu tez çalışması, gümüş nanotellerin hem çevre dostu sentezini, hem de biyomedikal uygulamalarda gelişmiş manyetik nanoyapılar için bir şablon olarak ikili rolünü araştırmıştır. Çalışmanın ilk bölümünde, geçiş metali tuzlarının gümüş nanotellerin morfolojisini kontrol etmedeki etkisi araştırılmıştır. Çevre dostu indirgeyici madde olarak gliserolün kullanılması, belirli geçiş metali tuzlarının nanotel morfolojisini önemli ölçüde etkileyebileceğini göstermiştir. Deneysel koşulların dikkatli bir şekilde optimize edilmesiyle, gümüş nanoteller yüksek en boy oranlarıyla başarıyla sentezlenmiştir. Bu yeşil sentez yaklaşımı, esnek elektronik, sensörler ve daha fazlası için nanoteller üretmek için sürdürülebilir bir yol sağlar. İkinci bölümde, gümüş nanoteller, demir oksit nanoküplerin manyetik hipertermi performansını artırmak için yüksek yüzey alanlı bir şablon olarak sunulmuştur. Demir oksit nanoküpler, manyetik anizotropilerini artırmak için gümüş nanotellerin yüzeyine dekore edilmiştir. Manyetik karakterizasyon ve özgül emilim oranı analizi, bu yapının uygulanan manyetik alan koşullarına bağlı olarak değişen manyetik alanlar altında ısıtma verimliliğini artırdığını ve tek başına demir oksit nanopartiküllerinden daha üstün performans gösterdiğini ortaya koymaktadır.Doctoral Thesis Optimization of Zinc Oxide Based Metal - Semiconductor Junction Interface Properties and Applications for Optoelectronic Devices(01. Izmir Institute of Technology, 2024) Güzelaydın, Abdurrahman Halis; Tarhan, EnverThis thesis manifests an experimental investigation on the optoelectronic characteristics of wide band gap thin film zinc oxide semiconductor – metal junction and performance enhancement of ultraviolet photo detectors fabricated utilizing this metal-semiconductor interface. Pristine zinc oxide, aluminum doped zinc oxide and amorphous In-Ga-Zn-O thin film samples with thicknesses varying between 50-250 nm were fabricated from 2' ceramic targets via magnetron sputtering method. Surface properties and thus the zinc oxide – metal junction interface was optimized by altering sputtering parameters. Sputtering gas pressure, power and temperature was varied between 1.5 – 5 mTorr, 50 – 120 W and 25 – 500 °C, respectively. To determine the effects of energetic ion bombardment on the films' surface properties, biases ranging from 5 to 15 W were applied to the substrates during depositions. A 5 nm thick silicon dioxide passivation layer was deposited on zinc oxide thin films to suppress persistent photoconductivity effect. Furthermore, a thermal treatment under ultraviolet irradiation and was applied specifically to amorphous In-Ga-Zn-O thin films after device fabrication to improve their ultraviolet sensing capabilities. Optoelectronic spectral responses of devices were assessed experimentally by using transient photocurrent spectroscopy method. An ultraviolet light source with a 275 nm peak wavelength at 500 µW power was used as illumination source. All devices exhibited photoconductor behavior with ohmic metal-semiconductor junctions under 5 V bias. Amorphous In-Ga-Zn-O Sample 10 attained a dark current of 140 nA and reached a photocurrent level of 3.8 µA with a photo-to-dark current ratio of 27, yielding a spectral response of 1830 A/W. The calculated external quantum efficiency for this device was 825000%.Doctoral Thesis Development of Computational Models To Predict the Toxicity of Advanced Materials(01. Izmir Institute of Technology, 2023) Bilgi, Eyüp; Karakuş, Ceyda Öksel; Bedir, ErdalThe aim of this study is to harness computational power to enhance existing knowledge on NM safety and to optimize the use of existing nanotoxicity data. The primary goal is to support the safe(r)-by-design concept, necessitating early integration of safety considerations into NM design through structural manipulation strategies. This thesis focuses on three case studies: zinc oxide, silver, and gold NP, using data manually collected from the literature. Analyses with zinc oxide and silver NP revealed a correlation between their toxicity and both internal (intrinsic properties, size, shape, surface charge) and external (cell and analysis-related properties) factors. For zinc oxide, it was found that coating had significant influence on cell viability, with a critical threshold identified at 20 µg/ml concentration and 10 nm size. Similarly, for silver NPs, concentration, size, and exposure time were significant factors. Coating with organic macromolecules increased cell viability, whereas green-synthesized NPs (using bacteria, plant extracts, algae) decreased it. The gold NP study highlighted that ensemble methods were more effective in elucidating complex relationships, with cellular uptake linked to particle size, zeta potential, concentration, and exposure time. Overall, this thesis contributes to safer-by-design strategies, crucial for developing commercially viable and safe NMs. The findings advocate for a broader toxicity evaluation approach, considering various physicochemical aspects and experimental procedures. The complex interactions observed suggest that advanced algorithms are necessary for accurate modeling, supporting the optimization of experimental parameters in NP engineering for biomedical applications.Doctoral Thesis Synthesis, Properties and Applications of Tungsten Oxide Nanostructures(01. Izmir Institute of Technology, 2023) Kahraman, Zeynep; Balcı, Sinan; Genç, AzizIn this study, tungsten oxide nanostructures, which are n-type semicon ductors with a band gap between 2.6-2.8, have been studied extensively. The hydrothermal method was used as the synthesis technique and the phases and morphologies were optimized in a stable and controllable manner. Firstly, sto ichiometric tungsten oxide nanowires with certain ratios were synthesized, and then cobalt doping was made using this synthesis technique. Subsequently, sub-stoichiometric tungsten oxide nanowires, which have oxygen gaps and can show plasmonic properties due to the increased carrier density, were synthe sized, and tungsten oxides with a flower-like hierarchical structure with oxygen gaps were synthesized and grouped according to possible application areas. Accordingly, how oxygen vacancies and hierarchical structures affect pho tocatalysis applications have been examined and it has been seen that ub stoichiometric tungsten oxide works faster until it reaches a certain saturation than stoichiometric tungsten oxide. According to this study, how the system can be manipulated by adding low pH to the system and hydrogen peroxide as an electron acceptor, respectively. It has been observed that it can be done. Hierarchical tunsten oxide has been found to be an ideal catalyst that can work quickly in photocatalysis studies due to its hierarchical structure, which has oxygen vacancies and can absorb light well. Additionally, tungsten oxide attracts attention as a material used in su percapacitor applications. Supercapacitors are long-lasting and fast-reacting electrochemical devices that can provide high power in energy storage and dis charge processes. The use of tungsten oxide in supercapacitor applications can be summarized as follows: when nanoparticles with large surface area are used as electrode material, they increase the interaction with the electrochemical surface and can increase the energy storage capacity. It shows high electro chemical activity as an electrode material. This feature contributes to the high performance of the supercapacitor. Tungsten oxide has a structure suitable for electron and ion conduction. This allows the supercapacitor to have fast charge/discharge capabilities and low internal resistance. Tungsten oxide can show stable performance during electrochemical cycles. This feature ensures the long life of the supercapacitor. In supercapacitor applications, in addition to these features, the electrical conductivity of the material can be increased by increasing the number of electrons carried in the material due to its oxygen gap. Accordingly, we investigated the comparative electrochemical properties and cycling stability of stoichiometric and sub-stoichiometric nanowires. Thanks to its electrochromic properties, the latest application has observed electrochromic changes of oxygen vacancies and cobalt doping