LEE- Nano Bilim ve Nano Mühendislik-Doktora
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ÖgeAdvanced anode active materials for next generation lithium ion batteries(Graduate School, 2025-04-08)The demand for high-performance lithium-ion batteries, characterized by high power, capacity, and rate capability, is continuously increasing. To meet these demands, it is essential to develop new anode materials to replace the currently used graphite anodes, which have limited capacity. The main objective of this work is to find a new way to commercialize Si based anodes to obtain high performance rechargeable lithium-ion batteries. For this purpose, we have designed and optimized the cycling performance of electrodeposited Si, increased the loading of active material of electrodeposited Si with the help of photovoltaic method and proposed a Si/C composite to combine the advantages of carbon (long cycle life) and silicon active (high storage capacity) materials and improve the electrochemical performance of the introduced composite anode materials for high-performance rechargeable lithium-ion batteries. We have further examined the possibility of potential crystallographic orientation of commercial graphite to increase the charging rate capability which has promising application in Si/C composite anodes.
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ÖgeComputational design and analysis of nanostructured materials for neuromorphic engineering(Graduate School, 2024-03-04)We study electronic structure of nanometer-scale materials using electronic structure theories like density functional theory. Nanoscale two-dimensional intelligent materials under strain, and their applications as electronic devices for neuromorphic computing is investigated. Combination of two-dimensional materials with other materials are in the scope of this work. Density functional theory calculations are done using Quantum ESPRESSO simulation suite. Designed and analysed neuromorphic electronic materials are simulated and compared with available technological data.Thesis has significance in terms of the choice of materials; secondly, in terms of the realization of homostructures, and heterostructures, and understanding the mechanical strain effects in these structures; thirdly, in terms of the application area; namely, neuromorphic electronic devices for memories. The overall purpose and the scope of the thesis can be listed chapter-wise. In Chapter 2, "Single atom precise, ultrafast, and universal emulation of biological synapses using atomically thin vertical heterostructures": We realize the current voltage-like characteristics of heterostructures using density functional theory and Boltzmann transport methods, which can reveal the hysteresis characteristics. Moreover, we used time dependent density functional theory to show characteristics of synapses. We show a healthy synapse with N vacancy and dysfunctional synapses with pristine heterostructure. We heal the dysfunctional synapses using N intercalation. Here, a single atom can manipulate the behavior of different synapses. Created heterostructures have different abilities of learning, memory, and so on. One can use these heterostructures to realize certain brain regions on chips because every region of the brain is responsible and superb in one ability mostly. In Chapter 3,"Mimicking bacterial learning and memory in tungsten based two-sided single layers of WSeO, WSeS, WSeSe, and WSeTe": The current literature about neuromorphic materials is based on showing how one can resemble to synapses and neurons of Human using two-dimensional materials. In this chapter, we are showing for the very first time to the best of our knowledge how to use 2D materials to mimic the characteristics of the bacterial learning and memory. We developed methods to show this in 2D materials using a quantum memristor and phase-change like mechanisms. Our modeling is directly comply with the gene regulatory network's governing equation of bacteria, also physical model resemble to the real world bacteria, which is supported by experimental biological letters. In Chapter 4,"Single-electron-precise tailoring of a resistive-switching device with transfer printing": We focus on how to replace the electrochemically active top electrode of a conductive filament forming resistive switching device with an inert electrode. For this, we study a molecular junction to design and model a resistive switching device based on a single-electron box effect, where the molecular junction composed of self-assembled monolayers. Resistive switching is establish through the penetrating Au atoms from the inert top electrode after transfer printing process. Here, Coulomb blockade effect through Au island and tunneling through the self-assembled monolayers are the two effective phenomenon to explain the resistive switching. To conclude, a molecular junction is studied using the methods of density functional theory and environmental effects are modeled using the Quantum ESPRESSO's module environ. In Chapter 5, "Computational Analysis of Device-to-Device Variability in Resistive Switching devices through Single-Layer Hexagonal Boron Nitride and Graphene Vertical Heterostructure Model": We show a simple model for defects and their effects at the interfaces of atomically thin heterostructures by varying the interlayer distance between two atomically thin materials forming the heterostructure. This model will allow us to gain insights into variations in current-voltage characteristics of resistive switching devices compose of metal-insulator-metal vertical structures using the methods of density functional theory. We believe this approach of modeling interfaces with varying distances and showing how it affects the current-voltage characteristics is first time interpreted in this way. Also, the last subsection reveals a very important and easy way to mimic the neurons. In Chapter 6,"Atomistic origins of compound semiconductor synthesis with computational neuromorphic engineering": This article can be considered a follow-up to our previously published article in the Journal of Applied Physics titled 'Mimicking Bacterial Learning and Memory in Tungsten-based Two-sided Single Layers of WSeO, WSeS, WSeSe, and WSeTe.' In this new chapter, we apply a similar methodology to another bacterium. However, the primary objective of our new article extends beyond this. Our aim is to demonstrate the potential for memristivity during compound semiconductor synthesis. It is widely recognized that the fabrication process using chemical vapor deposition based methods lacks a comprehensive understanding in terms of chemical kinetics. We endeavor to elucidate how growth processes exhibit learning behavior and possess memory. This is achieved through the analysis of the smallest potentially meaningful subunit of the growth: the resonant tunneling diode structure. Furthermore, we illustrate how the convergence of various computational methods—such as tight-binding, density functional theory, transfer matrix, and Boltzmann transport theory—can contribute to the design of future multinary memristors. Specifically, $sp^3s^*$ semiempirical tight-binding methods are predicting energy band gaps more accurately than density functional theory at a low computational cost and allow the investigation of compound semiconductors. Our work throughout this thesis is mainly based on density functional theory. Here, we will consider electron density to investigate the electronic structure of the materials. In this way, researchers found functionals, which are connecting the density with the energy. Density functional theory is based on interacting electrons in an external potential. The ground state energy is determined as the unique functional of the electronic density, which means that the ground state electron density is enough to construct the Hamiltonian of the system, and it verify that the construction of many electron wave function is not required to calculate the ground state properties. To conclude, we believe that the computational studies of nanoscale two-dimensional materials using the methods of applied computational materials science can enhance the performance of the neuromorphic devices.
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ÖgeDevelopment of novel aflatoxin B1 biosensors by carbon nanotube integrated microfluidic systems(Graduate School, 2024-05-08)Aflatoxin B1 (AFB1), which contaminates food and feed via molds, is carcinogenic to humans and animals. In addition to adverse effect on human and animal health, it results in product losses due to the difficulties of decontamination. When negative health consequences and product losses are studied economically, it is clear that they result in significant losses and issues for developing countries. To preserve human and animal health, as well as prevent product and economic losses, AFB1 contamination must be regulated at all phases of the food production system, from field to fork. Effective risk assessment and risk management techniques are required to prevent food contamination. Risk assessment requires scientific methodologies. The first stage is to use scientific procedures to detect, measure, or analyze. Traditional AFB1 detection/analysis methods such as microbiological cultivation, chromatographic analysis and ELISA test involve complicated laboratory procedures and difficult, long processes requiring expert personnel. It is expensive because it requires a lot of sample and chemical solvents, and causes environmental pollution. It forces our country to be dependent on foreign sources due to the import of analytical devices. It is conceivable to reduce such long and complex procedures into a chip and realize the lab-on-chip (LOC) concept with the ultimate technology of microfluidic systems. Today, with the advancement of microfluidic technology, it is possible to actualize such long and complex laboratory operations by miniaturizing them to a chip, thereby bringing the notion of laboratory-on-chip (LOC) to reality. In our country, it is vital to profit from the advantages of such advanced technology in items with competitive power in exports, such as olive oil. Where several governmental entities have taken measures toward quality improvement and branding by resolving existing problems, and comprehensive R&D studies must be conducted to address such issues. The submitted thesis describes the development of an integrated microfluidic system that extracts, detects, and quantifies AFB1 in olive oil by combining microfluidic technology and nanotechnology. The integrated system will include three modules: a paper-based microfluidic AFB1 biosensor (µPAD), a PDMS-based microfluidic mixer connected to µPAD, and a syringe pump system for introducing AFB1-treated olive oil and extraction solvents at specific times and volumes. The aim of the thesis is defined as; i. Creating AFB1 detection areas using different techniques with the help of hydrophobic materials on cellulose-based filter paper, ii. Functionalization of MWCNTs by immobilizing antibodies that will specifically identify the target AFB1, iii. Integration of detection areas of functionalized MWCNTs, iv. Production and optimization of PDMS-based microfluidic mixers, v. Integration of microfluidic biosensor and mixer with syringe pumps, vi: Extraction of AFB1 from olive oil with microfluidic mixers, vii. Validation of the microfluidic biosensor and quantitative measurement of extracted AFB1 samples. The first stage of the thesis is to create detection areas and channels on filter papers with the help of various hydrophobic materials. For this purpose, different fabrication techniques were applied for each hydrophobic material. When variables such as ease of manufacture, cost, analysis in which the biosensor will be utilized, and required chemical solvent resistances were considered, it was decided to continue experimental research with the wax dipping approach. The second stage is to transform the detection areas created on paper into biosensor surfaces where we can perform AFB1 detection and quantitative analysis. At this stage, nanotechnology methods were utilized. Multi-walled carbon nanotubes (MWCNT) are used in biosensors because they improve detection sensitivity by increasing surface area and provide an ideal surface for the immobilization of biological components such as antibodies. For biological detection, MWCNT surfaces must be biologically functionalized and distributed uniformly in a dispersant to ensure homogenous integration in detecting areas. Chemical alterations must be made to the surface structures of MWCNTs to achieve both goals. Due to strong van der Waals interactions, MWCNTs disperse as bundles in water, which limits their biosensor applications. MWCNTs are distributed homogeneously in water due to the carboxyl (-COOH) and hydroxyl (-OH) groups formed as a result of the treatment of MWCNTs with chemicals such as acids or peroxides. Additionally, these groups enable antibodies to bind to the MWCNT surface. With this aim, MWCNT solutions were prepared with HNO3, H2SO4 + HNO3, H2O2, H2O2 + H2SO4 to observe the formation of -COOH and -OH groups, and the FTIR method was used to determine if the appropriate functional groups developed on MWCNTs as a result of certain time and temperature applications. According to FTIR measurement results, acid oxidations were continued with HNO3, which provides the formation of -COOH groups. XRD measurements were performed for structural analysis. In the XRD spectrum of MWCNT functionalized with HNO3, the MWCNT crystal plane with added carboxyl groups was seen at 260 and the MWCNT characteristic crystal plane was seen at 430. The presence of the -COOH group was examined by XPS measurements. According to the results obtained from the elemental compositions, the Oxygen (O) ratio increased as a result of the treatment of MWCNTs with acids. Oxygen originates from -COOH groups formed as a result of reactions with acids on the surface. Nitrogen (N) was seen as a result of antibody immobilization, indicating that protein was added to the system. The increase in Oxygen (O) ratio along with Nitrogen (N) shows that protein structures are immobilized on the surface. In other words, the antibody to which AFB1 will specifically bind is immobilized on the MWCNT surface. As understood from the morphological characterizations, there was no significant change occurred in MWCNT morphologies after HNO3 treatment, and their structures were not damaged. With antibody immobilization, a beaded appearance like a cover was formed on the MWCNT surface. This shows that the protein layer bound to MWCNTs was formed and the immobilization was successful. A paper-based biosensor (µPAD) was successfully created by dispersing antibody-functionalized MWCNTs in water and integrating them onto the paper detection areas obtained in the first stage via the drop casting technique. In order to obtain sensitive results in food contamination analyses, the contaminant must be extracted from the food matrix. The other part of the thesis work is AFB1 extraction from olive oil. PDMS-based microfluidic mixers were fabricated for pre-teratments. Separation and mixing abilities were observed with various designs such as straight, sunflower, 1800 and 2400 degree bend angles, 200, 500, and 1000 µm channel widths. After COMSOL simulations and experimental optimizations with different microchannel designs, it was decided to fabricate microfluidic mixers with sunflower geometry. PDMS based mixers were created via UV litography and soft litography techniques. PDMS was preferred as the second surface instead of glass due to its flexibility and ease of micromixing. The last phase of the thesis consists of detection and quantitative analysis studies with µPADs. In AFB1 detection studies, it was observed that AFB1 could be detected at a concentration of 1 ng/mL under the FL microscope, thanks to the FL dye-labeled secondary antibody. Quantitative studies performed with a FL microscope were carried out with AFB1 standard solutions. Standard solutions prepared in the concentration range of 0.01-100 ng/mL were dropped onto the µPAD surface and examined under a microscope. Because clear results could not be obtained from MWCNT alone, silver nanoparticles (AgNP) were introduced to the system during the functionalization stage. Thanks to AgNPs, quantitative measurements were made by image analysis and a calibration graph was created. The lowest detectable concentration was determined as 0.1 ng/mL. Resistance measurements (four point probe) were made on MWCNT surfaces with the same standard AFB1 solutions and a calibration graph was created. The lowest detectable concentration was determined as 0.01 ng/mL. More precise measurements were made with resistance measurements. Both types of studies have shown that µPADs have two different linear working ranges: 0.1-1 ng/mL and 1-10 ng/mL. Microfluidic mixers integrated with syringe pumps were used for pre-teratment studies. After AFB1 spiked olive oil samples passed through the microfluidic mixer, quantitative resistance measurements were conducted. The biosensor recovery rates ranged from 91 to 97%. FL spectroscopic measurements in AFB1 standard solutions served to validate the results. Linear operating range and recovery rates are likewise within acceptable levels, according with the literature. The integrated system has high sensitivity AFB1 detection capacity. It has the potential to be used in every field where AFB1 detection is legally required. When the lowest quantities found are studied, it is clear that detection may be accomplished with a sensitivity below the legal limits. It will be cheaper compared to other technologies due to the materials used, it will reduce the economic burden required by the analysis since it is sufficient to use less chemicals and samples, and it will provide an environmentally friendly technology due to less waste chemicals. AFB1 can be used not only in the laboratory environment but also without the need for expert personnel because it is portable and simple to use. It is a modular analysis system that will enable on-site detection. In addition, it is a domestic product and domestic patent technology that will reduce our country's import dependence on analytical devices and technologies. Due to the combination of microfluidic technology and nanotechnology, the system has many differences and advantages compared to academic studies, patents and commercial products that serve similar purposes. It has the potential to turn into a domestic and high value-added product that has the power to create its own market in the national and international arena if it is patented and commercialized with its originality.
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ÖgeMachine learning analysis on nanomaterials literature data and knowledge exploration(Graduate School, 2024-06-04)In this thesis study, regression- and classification-based machine learning analyzes were used to understand process-property and structure-property relationships of thin film materials. As an aspect of materials paradigm, two distinct efforts were comprehensively performed on poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films, and aluminum doped zinc oxide (AZO) thin films. The extended automated machine learning workflows were used to find proper regression algorithm, and hyperparameters, that can demonstrate high prediction capability by coefficient of determination, 𝑅2. These models can be used as thin film design tools for screening promising design areas before real laboratory experiments to be performed.These models can improve the efficiency of laboratory studies, in terms of both reducing time and financials. In this way, the formed models has been named for two most critical challenges in scientific development. The study is also reducing the reproducibility issue for both film structures.In this endeavor, a tree-based method and a hierarchical agglomerative clustering algorithm were both applied on the dataset compiled from published literature. According to the performance of nanomaterials, the clustering method generated performance classes (for example, field effect charge carrier mobility for transistors, electrical resistivity, energy bang gap characteristics etc.). After creating performance classes, tree-based algorithms looked for the most important process variables, conditions, or structural elements that could improve the performance of nanomaterials. The relationship between the inputs to the fabrication process, structural characteristics, and material properties can be defined in quantitative manner by the relevance each other. Sol-gel deposited aluminum doped zinc oxide films have been shown to be a promising and cost-effective transparent conductive oxide for achieving high performance in solar cells, transistors, and diodes due to their wide energy band gap and low electrical resistivity. The reasons for variations in energy band gap and electrical resistivity of aluminum doped zinc oxide films have been discussed as doping concentration, crystalline size crystallinity, film thickness, and lattice constants.
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ÖgeMixed-anion photochromic yttrium oxyhydride and gadolinium oxyhydride: Research and applications(Graduate School, 2021-02-04)are-earth metal oxides and hydrides are the focal point in literature, promising various high demand solutions in industry and everyday life. Recently, a multi-functional, multi-anion material class called "rare-earth metal oxyhydrides" were shown to be synthesized as a thin film under ambient temperatures with photochromic properties. Oxyhydrides present high level of flexibility for material development through various combination possibilities having multi-anions. The newly realized possibility of ambient temperature thin film synthesis possibility of rare-earth metal oxyhydrides attracted high attention from literature but, there is little to no data is available in conventional databases for rare-earth oxyhydrides. Combination of the oxyhydrides being an under-developed class of materials and the promise of presenting important solutions for a high demanding era we live in, oxyhydrides is an excellent topic to research. In 2011, the synthesis of yttrium hydride thin film in room temperature with wide-spectrum transparency using only one step deposition process was shown. Originally, this research started to search for an alternative material for solar cells but, evolved into another dimension when the material was realized to be a highly responsive photochromic material that can modulate in wide spectrum. The material attracted high attention from literature and initially named as "oxygen containing yttrium hydride". However, it was later found through synchrotron measurements that this material is belong to the emerging class of materials; rare-earth metal oxyhydrides. When exposed to air, yttrium dihydride (YH2) and gadolinium dihydride (GdH2) films turn into insulating and transparent yttrium oxyhydride (YHO) and gadolinium oxyhydride (GdHO), respectively. Oxidation in air, hence bandgap, can be controlled by deposition parameters. MHO (M; rare-earth metal) photodarkens when illuminated with light of adequate energy and intensity, recovers (bleaching) when stored in dark. Photochromic rare-earth metal oxyhydride knowledge and know-how was established around synthesis method, band-gap engineering, optical properties, anion sites in the lattice etc. but the photochromic mechanism and environmental effects either yet to be understood or never even investigated before the present thesis work has been started. Also, yttrium was the only rare-earth element that was shown to have photochromic properties and investigated. Especially the knowledge gap over the interaction of photochromic oxyhydrides with the environment, prevented the realization into a product that sought heavily in industry. Therefore, in the present PhD thesis the interactions of the photochromic rare-earth metal oxyhydride thin films with the environment were first and foremost investigated. This endeavor resulted with solutions that enabled product development. Furthermore, contribution to the knowledge of photochromic rare-earth metal oxyhydrides by developing and studying at least one more rare-earth element next to the yttrium was also targeted. The studies showed that photochromic rare-earth metal oxyhydride thin films interacts with environment heavily. Additionally, we have published one article in Physical Review Materials that tries to shine a light to the understanding of the rare-earth metal oxyhydride photochromic mechanism, related to the environmental interaction: "Light-induced breathing in photochromic yttrium oxyhydrides". The studies showed that during the photodarkening/bleaching cycle of yttrium oxyhydride, material releases/intakes oxygen following lattice contraction/expansion, respectively. We coined the term, breathing, after the accordion-like structural process of yttrium oxyhydride based on oxidation. The article was selected as an editor selection and featured in Physics magazine. Based on these studies, stable IGUs which has been long sought since 2016 was able to be manufactured. Another contribution for the explanation of the photochromic mechanism of yttrium oxyhydride thin films published in Physica Status Solidi Rapid Research Letters with the title of "Temperature-dependent photochromic performance of yttrium oxyhydride thin films". In this article, we have presented the photochromic kinetics of yttrium oxyhydride thin films studied between 5-250K and presented a new approach which would enable new questions. The second part of the PhD thesis plan was contributing to the knowledge of photochromic rare-earth metal oxyhydrides by at least one another rare-earth element. Gadolinium was selected as a member of the rare-earth elements for study for having similar chemical properties and widely accepted by the nuclear industry for large neutron capture diameter. In the span of a year, the production know-how and knowledge related to gadolinium oxyhydride thin films were elevated also. One article was published in the journal Molecules that shows the post-deposition oxidation is related with the preferential lattice orientation which controlled by the deposition parameters: "Preferential Orientation of Photochromic Gadolinium Oxyhydride Films". Additionally, another article based on the environmental interaction of gadolinium oxyhydride thin films will be submitted in an international journal in 2020. Environmental effect on rare-earth metal oxyhydrides was investigated further by systematic study of yttrium oxyhydride thin films under atmospheres with varying relative humidity levels. Correlation between the relative humidity levels and photochromic kinetics was observed and microstructure formation that causes the delamination was shown. One article based on the results is under progress and planned to be submitted in 2021. In the last part, applications developed during this thesis based on yttrium and gadolinium oxyhydride thin films were presented. First, photochromic kinetics of stable IGUs based on yttrium oxyhydride were presented. However, the properties photochromic rare-earth metal oxyhydrides present is much wider than only window applications as a result of their multi-anion nature. Lastly, photocatalytic properties of photochromic gadolinium oxyhydrides were also shown.