Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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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 dopingDoctoral Thesis Strong and Weak Light-Matter Interactions in Plasmonic and Optical Cavities(01. Izmir Institute of Technology, 2023) Polat, Nahit; Balcı, SinanThis thesis investigates the interaction of light and matter through both experimental and theoretical approaches, focusing on two key topics: Strong and Weak coupling. We also examine the field of strong light-matter interactions under two headings: plasmonic and optical cavities. In the section on plasmonic cavities, we focus on the strong coupling between surface plasmon polaritons and excitons with different optical properties and localized surface plasmon resonance. We also discuss the strong coupling between localized surface plasmon resonance and excitons in plasmonic nanoparticles with different optical properties due to their different shapes. This thesis focuses on the theoretical and optical characterization of the aforementioned topics. Therefore, details on the manufacturing processes are not given. Instead, it presents a comparative analysis of optical measurement results and related simulation data, to which I have contributed more. In addition, we demonstrate for the first time hyperspectral imaging of exciton polaritons at optical microcavities. Two metal thin films acting as reflectors and a polymer matrix containing a collection of quantum emitters form the hybrid system for polariton imaging. We show a strong exciton-photon interaction between photons trapped in the microcavity and Frenkel excitons of dye molecules placed inside the optical microcavity. We find that exciton polaritons in the optical microcavity can be imaged and spatially mapped using hyperspectral imaging in the visible region. In the field of weak light-matter interactions, the focus of the thesis is on the Fano resonance, which is a unique case in the weak coupling regime. The degree of coupling between interacting systems plays a critical role in determining the extent to which constructive and destructive interference phenomena occur in the system. An important example of destructive interference is the Fano resonance, which is caused by interference between two optical paths that are characterized by different states: one is narrow and discrete, and the other is broad and continuum. This is the first study to theoretically investigate the destructive interference between subwavelength excitonic nanoparticles placed on the surface of a prism for Fano resonance observations and the evanescent wave scattering on the prism surface. In an attempt to understand the intricate dynamics between the evanescent wave and the Fano resonance, we study the effect of changing the angle of incidence of the incident plane wave on the properties of the Fano resonance. Furthermore, we also investigate how the size of the excitonic nanoparticles and the absorption linewidth influence the properties of the Fano resonance.Doctoral Thesis Fabrication and Characterization of Soi Based Photodetectors With Graphene Electrode(01. Izmir Institute of Technology, 2023) Yanılmaz, Alper; Çelebi, Cem; Balcı, SinanThis thesis presents the pioneering methods for the design, fabrication process, and performance evaluation of graphene (G) and n-type silicon (n-Si) based self-powered one dimensional (1D) and two dimensional (2D) photodetector arrays (PDAs) on a silicon on insulator (SOI) substrate. In the device structure, monolayer G is utilized as hole collecting transparent conductive electrode (TCE) and n-Si is used as light absorbing material, respectively. After analyzing the photo-response characteristics of single pixel G/n-Si diode on SOI, we fabricated G/n-Si based Schottky barrier 1D PDAs with common G electrode, separate G electrode and 2D PDA with individual G electrodes on linearly arrayed n-Si channels, respectively. Each G/n-Si diodes exhibited a clear rectifying Schottky character with low dark current and diode parameters were analyzed using the current-voltage measurement. Besides, all diodes demonstrated a clear photovoltaic activity under the light illumination and maximum responsivity at 660 nm peak wavelength. Each diode in PDA revealed similar device performances under self-powered mode in terms of an Ilight/Idark ratio up to 104, a responsivity of ~0.1 A/W and a response speed of ~1.3 μs at 660 nm wavelength. The optical crosstalk was extremely low between neighboring diodes and also it could be greatly minimized when G is used as separated electrode on arrayed Si up to ~0.10% (-60 dB) per array. Time dependent photocurrent spectroscopy measurements revealed an excellent photocurrent reversibility of both device types. In the diode structure, the homogeneity of the graphene film transferred on n-Si were examined by Raman mapping and correlated with the sensitivity of diode to incoming light. This thesis paves the way for the new generation of optoelectronic devices with various potential by integrating G and SOI technology to PDA devices with ease of fabrication.