Doktora Tezleri

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Biological nano silica reinforced polymeric composites
(01. Izmir Institute of Technology, 2022-12) Ülker, Sevkan; Güden, Mustafa
The present thesis study focused on processing nano-silica powders of varying sizes and crystallinities through heat treatment (900–1200 °C), hydrofluoric acid leaching (1–7 N), and ball milling (1 h, 500 rpm) of natural diatom frustules. As-received frustules was composed of amorphous silica (88%) and quartz. The partially ordered crystalline low-quartz and/or precursor to low-cristobalite started to form at ~900 °C. As the heat treatment temperature increased, the crystallinity of the frustules increased from 9.3% at 25 °C to 46% at 1200 °C. Applying a ball milling reduced the mean particle sizes of the as-received and heat-treated frustules from 15.6–13.7 μm to 7.2-6.7 μm, respectively. Acid leaching of the as-received and heat-treated frustules resulted in a further increase in the crystallinity. Furthermore, ball milling applied after an acid leaching was very effective in reducing the particle size of the as-received and heat-treated frustules. The mean particle size of the acid-leached frustules decreased to 774-547 nm with a crystallinity varying between 12 and 48% after ball milling. A partially dissolved amorphous phase was observed in between crystalline silica grains after acid leaching, which resulted in a rapid fracture/separation of the frustules in ball milling. The prepared nano-silica powders were further used as a filler in an epoxy matrix. The tensile strength, fracture strain, and modulus of epoxy increased with increasing the volume percent of nano-silica up to 2%. The increase in the yield strength and elastic modulus was about 50% and 30% with the addition of 2 vol% frustules, respectively. The rule of mixtures showed a very good agreement with the experimental elastic modulus values and a numerical model of the tensile test in LS-DYNA agreed well with the experimental tensile stress-strain behavior. The microscopic observations showed the presence of nano-silica powder, proving an efficient load transfer from matrix to powders on the fracture surfaces, confirming a strong interface between silica powders and matrix.
Determination of vitamin D by sensor technologies based on molecular imprinted polymers
(Izmir Institute of Technology, 2022-07) Ölçer, Yekta Arya; Demir, Mustafa; Eroğlu, Ahmet Emin; Demir, Mustafa Muammer
Vitamin D is an essential nutrient in the body; it plays important roles in human health. Both its lack and excess can have health risks. As a consequence, there is a great demand for development of simple and precise detection methods for vitamin D derivatives in different samples. Molecular imprinting polymers (MIPs) are artificial receptors that can recognize target molecules in solution. In this study, two different polymerization techniques were used to obtain MIP/NIP sorbents/films for the detection of vitamin D3. Firstly, molecular imprinted solid phase extraction (MISPE) method was proposed prior to HPLC-DAD analysis. Optimized parameters were as follows; sorbent amount of 5.0 mg for 5.0 mL of 1.0 mg/L vitamin D3 in 90:10 (v/v) ratio of H2O:MeOH solution, 5 hours sorption time and MeOH:HOAc ratio of 90:10 (v/v) as desorption solution. The accuracy of the method was verified with spike recovery test for PBS:MeOH in a ratio of 90:10 (v/v) and overall recovery was found as 85.1 (±4.3, n=3). In latter case, a quartz crystal microbalance (QCM) method was proposed for determination of vitamin D3. Electrochemical polymerization of poly(4-vinylpyridine) MIP/NIP films were achieved on gold working electrode by cyclic voltammetry (CV). Mass-transfer ability of the polymer films were analyzed by electrochemical impedance spectroscopy (EIS). The electrochemical QCM (eQCM) was used to develop thin polymer films on quartz crystals and vitamin D3 determination was achieved by QCM. In a preliminary test, as small a concentration as 0.0100 mg/L vitamin D was detected with the QCM method.
Developing graphene-organic hybrid electrodes for silicon based Schottky devices
(Izmir Institute of Technology, 2018-07) Aydın, Hasan; Çelebi, Cem; Varlıklı, Canan
This thesis focused on developing graphene-organic hybrid electrodes for silicon based Schottky devices. Two different sets of carboxylic acid based SAMs were used to improve the rectification character of the Schottky junction formed at graphene/Si interface. While the first set of SAMs consists of MePIFA and DPIFA, the second set of SAMs contains TPA and CAR. In addition to this, P3HT, which is known to be an electron donor and absorb light in the visible spectrum, was utilized to form P3HT-graphene bilayer electrode. Current-voltage characteristics of bare and SAMs modified devices showed rectification behavior confirming a Schottky junction formation at the graphene/Si interface. The DPIFA SAMs device exhibited better diode performance compare to MePIFA SAMs due to the absence of methyl group which hinders π-π interaction between SAMs molecule and graphene. Furthermore, the CAR-based device indicates better diode characteristic with respect to the TPA-based device due to smaller energy differences between graphene and CAR. The effect of P3HT-graphene bilayer electrode on the photoresponsivity characteristics of Silicon based Schottky photodetectors have been also investigated. Current-voltage measurements of graphene/Si and P3HT-graphene/Si revealed rectification behavior confirming Schottky junction formation at the graphene/Si interface. Time-resolved photocurrent measurements exhibited excellent durability and fast response speed. Moreover, the maximum photoresponsivity of P3HT-graphene/Si photodetector increased compared to that of bare graphene/Si photodetector. The observed increment in the photoresponsivity of P3HT-graphene/Si devices was attributed to the charge transfer doping from P3HT to graphene within the spectral range between near-ultraviolet and near-infrared. Finally, P3HT-graphene electrode was found to improve the specific detectivity and noise equivalent power of graphene/Si photodetectors.
Development of carbon nanotube embedded polyacrilonitrile/polypyrrole electrospun nanofibrous scaffolds
(Izmir Institute of Technology, 2017-12) İnce Yardımcı, Atike; Yılmaz, Selahattin
In this study, electrospun polyacrilonitrile (PAN)/ polypyrrole (PPy) nanofibers containing different PPy content (10, 25, and 50 wt%) were prepared. Different carbon nanotube (CNT) amounts (1, 2, 3, and 4 wt%) were embedded into PAN/PPy nanofibers to improve their mechanical and electrical properties. CNT functionalization was carried out to solve agglomeration problem and functional CNTs effects on PAN/PPy nanofiber morphology was examined. Alignment of nanofibers was studied to improve mechanical properties of nanofibers. Obtained PAN/PPy and PAN/PPy/CNT nanofibers were utilized as kerotinocytes scaffold. PAN/PPy/CNT and aligned and randomly oriented PAN/PPy nanofibers were examined for bone marrow osteogenic differentiation of mesenchymal stem cells (MSCs). 10 wt% PPy content was optimum in terms of mechanical properties and usage with CNTs. Higher strain was observed for 10 wt% PPy content which was 23.3 %. When as-grown MWCNTs were added into PAN/PPy, disordered nanofibers were formed. To improve interficial properties of these composites, as-grown CNTs were functionalized with H2SO4/HNO3/HCl solution. Upon functionalization, formation of hydroxylic and carboxylic groups were detected on the CNT surfaces. TEM examination of the nanofibers obtained with these CNTs showed decrease in beads formation. The functionalized CNTs were well dispersed within the electrospun nanofibers and aligned along the direction of nanofibers. The electroactivity of the fibers indicated that these nanofibers could be used as electrochemical actuator in acidic solutions. PAN/PPy and PAN/PPy/CNT nanofibers supported the attachment and proliferation of keratinocytes and osteogenic differentiation of MSCs. It was found that these nanofibers could be utilized as scaffolds for both cell types.
Development of novel anticounterfeiting technologies using heavy metal free nanoparticles
(Izmir Institute of Technology, 2021-07) Taşcıoğlu, Didem; Demir, Mustafa; Özçelik, Serdar; Demir, Mustafa Muammer; Izmir Institute of Technology
Counterfeiting, the act of illegally copying a product, document or currency, is a growing problem and causes economic losses. Anticounterfeiting technology uses fluorescent inks that are invisible to the naked eye in daylight, but become visible under UV light. However, these inks have problems such as fading when exposed to sunlight or room light for a long time and disappear completely over time. This is due to the relevant inks are made using organic dyes that fade. The inks used in anticounterfeiting application preventing copying of secure documents such as banknotes, passports and ID cards must be health-friendly and chemically and optically stable for years. All of the existing security materials and equipments for ID cards, driver's licenses, passports, banknotes used in our country are imported. In this study, our aim is to create a new generation of security materials and codes to combat counterfeiters and to verify the generated security codes in a simple, efficient and fast way. In this study, it is aimed to produce nanoparticles, which do not contain heavy metals and show optical stability for a long time, emitting in visible region, on the basis of the security codes created. For this purpose, water and solvent-based nanoparticles synthesized which are non-toxic should have a long-term optical stability. The synthesized nanoparticles act like a pigment in security codes. The photoluminescence (emission color) of the security codes can be adjusted by size and chemical composition of nanoparticles. In this study, colloidally monodispersed and highly photoluminescent InP based nanoparticles were synthesized by the hot-injection approach under an inert atmosphere. In addition, a protective shell (ZnS, ZnSe) coating methods have been applied to provide optical stability to InP nanoparticles. Moreover, carbon-based nanoparticles with high optical stability and being dispersible in water were synthesized using the bottom-up method. Security codes that cannot be detected in daylight have been created on different subtrates (paper, polymer, glass, etc.) by using screen printing and inkjet printing methods, which are well known printing methods using the synthesized nanoparticles. In addition, the authenticity of the security codes was checked using a commercial fiber optic based spectrometer (Ocean Optics spectrometer) and a handy hand-held optical device called the Quantag sensor developed by Quantag Nanotechnologies. Thus, a verification method that can be distinguished by a simple detection device is proposed. The synthesized nanoparticles were furthermore dispersed in a polymer solution to create random droplet and droplet/fiber patterns by electrospinning method. Thus, unique and inimitable security codes detectable under UV light were created which may be used in the fight against counterfeiting. To check the authenticity of the original security codes created; images collected with a simple smartphone microscope and a database was created in which the original patterns were recorded. The originality of the random patterns obtained was checked by comparing it with the patterns recorded in the database. In addition, the spectral information of the particle from the droplet/fiber pattern obtained was determined with a simple hand-held device (Ocean Optics optical spectrometer). Thus, by reading spectral information from the pattern, the spectral signature of the nanoparticles was determined and thus a second-step security was created. In this way, a two-stage anticounterfeiting technology that is impossible to imitate has been developed. As a conclusion, it is believed that the security codes developed in this study will pave the way for the commercialization of quantum labeling technology.
Development of plasmonic nanostructures for photothermal therapy of prostate and breast cancer
(Izmir Institute of Technology, 2019-12) Tomak, Aysel; Bulmuş, Volga; Şahin, Hasan
The aim of this thesis is to synthesize gold nanorods (AuNRs) and lipid-stabilized nanobubbles containing AuNRs and investigate the potential of these plasmonic nanostructures as photothermal therapy agents for breast and prostate cancer through in vitro cell culture experiments. For this aim, firstly, AuNRs were synthesized at varying aspect ratios (ARs) and characterized via several techniques including UV-Vis/NIR spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), inductively coupled plasma-mass spectroscopy (ICP-MS), electrophoretic light scattering (ELS) and X-ray photoelectron spectroscopy (XPS). The surface of AuNRs was modified with a biocompatible polymer, poly(ethylene glycol) (PEG), via ligand exchange method. Cytotoxicity, cell uptake and photothermal effects of AuNRs were investigated via in vitro cell culture experiments using human prostate cancer (DU 145) and epithelial (RWPE-1), breast cancer (MCF7) and epithelial (MCF 10A) cell lines. It was concluded that AuNRs (AR=4.0) were superior than AuNRs (AR=7.0) in terms of cell viability and photothermal effect. Separately, a non-commercial antibody (Ab) targeting a specific sialic acid derivative on the plasma membrane of DU 145 and MCF7 cancer cells was conjugated to AuNRs. Conjugations were characterized with the same techniques and investigated via in vitro cytotoxicity and cell uptake experiments. The Ab-conjugated AuNRs displayed the capability of selective targeting prostate cancer cells. Additionally, lipid-stabilized AuNRs and lipid-stabilized nanobubbles containing AuNRs (AuNBs) were synthesized for the first time and characterized using UV-Vis/NIR spectroscopy, SEM, ICP-MS and ELS techniques. Lipid-stabilized AuNRs were successfully synthesized using varying lipid mixtures instead of cationic, toxic surfactant. Separately, AuNBs were synthesized by combining PEG modified AuNRs with DPPC: DSPE-PEG lipid film under sonication and gas stream. AuNBs showed the same or significantly lower toxicity depending on the cell types and the same photothermal effect with respect to AuNRs (AR=4.0) upon irradiation under laser at 808 nm.
Down-converting polymer composites and their white light applications
(Izmir Institute of Technology, 2018-12-05) Güner, Tuğrul; Demir, Mustafa; Demir, Mustafa Muammer
Down-converting materials, which are luminescent materials that can emit at lower energies than their excitation energies, have been employed in a broad range of application area including phosphor-converted white LEDs, display technologies, solar cells, etc. In general, they can be obtained in the form powder or in dispersion. Therefore, to use them in an application, various methods such as spin coating can be applied or they can be prepared in the form of ceramic plate or polymer composite. In this thesis, we fabricate polymeric composite of some of these down-converting materials to use them in white light applications. In this sense, we have prepared these down-converting materials together with suitable polymers to form polymer/down-converting material composite in order to obtain free-standing film. Among those, polydimethylsiloxane was used in most of our applications especially the ones that involve inorganic downconverting materials. Second, polystyrene was employed to produce electrospun fibers in the case of when organic down-converting materials were used. After obtaining these composites in the form of free-standing film, we have focused specifically their use as color conversion layers over blue or UV LED chip to produce white light. The last part of the thesis describes our future prospects. We think that halide perovskites, which have high quantum yield, low cost, ease of synthesis, and wavelength tunability, can be promising materials in order to be employed as color conversion layer in white light applications. The details of these material systems, and their recent use in phosphorconverted white LED applications were summarized.
Fabrication of metamaterial filters for terahertz wave applications by e-beam patterning
(Izmir Institute of Technology, 2017-07) Demirhan, Yasemin; Özyüzer, Lütfi; Aral, Gürcan
In the electromagnetic spectrum, terahertz (THz) radiation falls into the region among microwaves and the far-IR. The unique properties of superconducting materials allow them to be utilized in a number of ways for THz device applications. In the first part of this thesis, the spectral performance of THz bandpass filters were produced from titanium, copper and indium tin oxide thin films with a metal-mesh shape on fused silica substrates by uv lithography were investigated. For metamaterial filter fabrication, Bi2Sr2CaCu2O8+x thin films were deposited by DC and RF magnetron sputtering system on saphirre and MgO substrates. After thin film characterizations, it is obtained that the films were not in single phase and could not be used for filter fabrication. A new, unique fourcross shaped metamaterial THz filter was designed and fabricated from both gold thin films and YBa2Cu3O7-d high Tc superconducting thin films. The designed fourcross shaped rectangular filter structure consists of periodic metallic rings where strip lines are located at the sides of the ring. CST Microwave Studio, is used to design and optimize the metamaterial filter structures. To investigate the temperaturedependent resonance behavior and confirm the measurements, simulations are carried out at temperatures above and well below Tc. Fourcross metamaterial filters are fabricated by using e-beam lithography and ion beam etching techniques. The resonance switching of the transmission spectra was investigated by lowering the temperature below the critical transition temperature. This novel fourcross filter holds great potential for active, tunable and lowloss THz devices for imaging, sensing, and detection applications.
The fabrication of plasmonic/photonic nanostructures in polymers: Mechanical sensor applications
(Izmir Institute of Technology, 2019-07) Topçu, Gökhan; Demir, Mustafa; Demir, Mustafa Muammer; Eanes, Mehtap
Functional polymer nanocomposites offer futuristic properties by the association of inorganic additive micro-/nanostructures into the polymers. With the growing knowledge of the physical fundamentals, stimuli-responsive polymeric composites enable detection of chemical, thermal, and mechanical changes by optical sensors and probes. Since the accurate real-time detection of the change in mechanical loading is vital for construction and industrial fields, the use of colorimetric pressure elements in a static body is important for the prediction of catastrophic failures. In this thesis, strain/pressure responsive colorimetric films were produced. A number of polymer nanocomposite-based mechanical sensors are presented. By transferring the optical activity (coherent reflection and plasmonic coupling) of the additives into various polymeric matrices having different mechanical features, the strain and pressure sensors are developed for practical applications. There are two approaches used for the fabrication of polymeric mechanical sensors: i) PDMS/SiO2 composites, ii) PAAm/Au NP composites. The coherent reflectivity of SiO2 colloidal particle arrays was used to develop strain sensors while controllable localized surface plasmon resonance of Au NPs was employed for pressure sensors. These optical systems were separately associated with viscoelastic and elastic polymeric systems, and sensor properties were discussed.
Functionalized CVD grown graphene for gas sensing applications
(Izmir Institute of Technology, 2017-07) Yağmurcukardeş, Nesli; Çelebi, Cem; Ünverdi, Özhan
Graphene is a two dimensional one-atom thick sheet of sp2 bonded carbon atoms arranged in a honeycomb lattice structure. It has high electron mobility and it is the material with the lowest resistivity at room temperature. By changing the edge properties with chemical modification, few-layer graphene may gain new magnetic properties. Besides having unusual electronic properties, single-layer graphene has important gas sensing capability. With the adsorption of the gas molecules, the local carrier concentration of graphene is modified and its resistance is altered. The high mobility, large area ohmic contact and metallic conductivity of graphene help to reduce the background noise and thus make it highly sensitive device even small molecular changes at atomic ranges. In this dissertation, Chemical Vapor Deposition (CVD) grown graphene layers were functionalized by self-assembled monolayers (SAMs) and etched anisotropically by H2 for the first time to improve sensor characteristics for toxic gas sensing. CO, CO2, NH3 gases were used as target molecules. Characterization techniques such as Optical Microscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Kelvin Probe Force Microscopy (KPFM), Raman Spectroscopy, Quartz Crystal Microbalance (QCM) and amperometric measurements were used for the investigation of the metal thin film, graphene layers and gas adsorbed film structures. Results indicate that the SAM modification enhanced CO and NH3 absorbing capability of graphene films and also improved their periodic reversible response characteristics. The resistivity results are consistent with frequency change results. Humidity sensitivity of sensors are also decreased significantly due to the applied etching process.
Lead-free ferroelectric ceramics for energy storage and electrocaloric cooling applications
(01. Izmir Institute of Technology, 2022-12) Karakaya, Merve; Adem, Umut
This thesis study consists of five main chapters that include an introduction about the principles and applications of ferroelectrics and four chapters on different lead-free ferroelectric ceramic systems developed for the energy storage and electrocaloric cooling applications. In the first chapter, the principles of ferroelectricity and dielectric, piezoelectric and pyroelectric properties of ferroelectrics are introduced followed by the applications considered in this thesis: Electrocaloric cooling and capacitive energy storage. In the second chapter, the effects of bismuth lithium titanate incorporation into sodium bismuth titanate-barium titanate systems on the energy storage properties have been discussed. In addition, it has been calculated that these compositions have improved energy storage properties close to the high values obtained in the literature. In the third chapter, the effect of two different manganese precursors on the probability of defect dipole formation, ferroelectric aging and resulting manganese valences of ceramics were investigated by manganese doping on barium strontium titanate base ceramics. In addition, their electrocaloric properties were investigated by indirect method. In the fourth chapter, electrocaloric properties of barium titanate - sodium bismuth titanate systems has been investigated. The increase in tetragonality by sodium bismuth titanate incorporation has been verified by Rietveld refinement and those compositions were shown to be suitable for electrocaloric applications. In the fifth chapter, a sodium bismuth titanate - potasium bismuth titanate composition which is in morphotropic phase boundary, was synthesized by templated grain growth method, and the effect of orientation on the electrocaloric effect was investigated by direct and indirect measurements.
Magnetic effect in the biological functioning of hemoglobin: DFT+QMC approach within an effective multi-orbital Anderson impurity model
(Izmir Institute of Technology, 2019-07) Mayda Bacaksız, Selma; Demir, Mustafa; Bulut, Nejat; Demir, Mustafa Muammer
Hemoglobin corresponds to O2 transportation from lungs to the tissues and exhibits high-spin to low-spin transition by binding of O2 to Fe. In this thesis, we study the electronic and magnetic properties of the deoxy and the oxy forms of the human adult hemoglobin (HbA) to investigate the mechanism of the spin transition. We use an effective multi-orbital Anderson model and the parameters of this model are determined by the density functional theory (DFT) calculations. Then, this model is solved by using a quantum Monte Carlo (QMC) algorithm. The DFT+QMC results show that new electronic states named as the impurity bound states (IBS) exist in both deoxy-HbA and oxy-HbA.We also observe that as the temperature decreases, a magnetic gap is opened at the Fermi level for oxy-HbA. This gap arises from the Fe-O2 charge transfer. We find that both the IBS and the opening of the magnetic gap are responsible for the spin transition in hemoglobin. In addition, the DFT+QMC calculations show that antiferromagnetic (AF) correlations between the Fe(3d) and the surrounding orbitals exist in both deoxy-HbA and oxy-HbA. For deoxy-HbA, the anomalous magnetic circular dichrosim signal in the UV region is experimental evidence for these AF correlations. In the light of these magnetic measurements, we propose some explanations for the Bohr effect and the cooperativity which are the fundemental functional properties of the hemoglobin. The results presented in this thesis show that the magnetic effects play a crucial role in the funtioning of the hemoglobin.
Magnetron sputter grown metal doped vanadium oxide thin films for terahertz bolometers
(Izmir Institute of Technology, 2018-01) Alaboz, Hakan; Aygün Özyüzer, Gülnur
Terahertz (THz) studies and hence technological improvements have increased and that caused expansion of application of THz waves. Applications of THz region have been expanded in many areas such as security, medical imaging, detection of explosives, nondestructive tests and wireless communication recently. THz radiation passes through many plastic materials, clothing but it reflects from metals and it is used in the detection of a lot of well known explosive materials. In spite of mentioned advantages and a wide range of application area, constructing a detector which is low cost, compact and uncooled is difficult and this causes the industry to improve slowly. Now, detectors which are widely used in THz region are pyroelectric, Schottky barrier diodes, field effect transistors and they have disadvantages such as low sensitivity, hard to construct an array and low speed. Instead of these detectors, superconducting bolometers are presented but they require liquid helium cooling. It is thought that VOx will be a premium technology for THz region due to its success in the infrared region. In this thesis, VOx:Au thin films were produced by DC magnetron sputtering and properties of these films were optimized for uncooled bolometer that operates at THz region. Polycrystalline VOx:Au thin films which have -1.7 % K-1 temperature coefficient resistance (TCR) and 0.07 Ωcm resistivity values were obtained. Increasing TCR values mostly depend on sputtering parameters such as gas and Au dope rates. These parameters were changed until the best TCR value was achieved and with Au doping, high resistance values of the films decreased to acceptable levels hence it decreased Johnson noise of the bolometer. Originally, these thin films which were sputtered on high resistivity silicon wafers and doped with Au, provided the bolometer to operate in THz region efficiently. The antenna design on the device was made by CST Microwave Studio, the antenna resonance was arranged to 0.6 THz which is related to our THz source operating frequency.
Magnetron sputtering growth of AZO/ZnO/Zn(O,S) multilayers for Cu2ZnSnS4 thin film solar cells: Material and device characterization
(Izmir Institute of Technology, 2017-06) Köseoğlu, Fulya; Özyüzer, Lütfi; Aygün Özyüzer, Gülnur
Cu2ZnSnS4 (CZTS) absorber layer attracts so much attention in photovoltaic industry since it contains earth abundant, low cost and non-toxic elements contrary to other chalcogenide based solar cells such as CuInGa(S,Se)2 (CIGS) and CdTe. Although, CZTS studies have been newly started, recently 9.4 % efficiency has been achieved. In the present thesis, all layers used in the CZTS device structure were investigated using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrophotometry and Raman spectroscopy. For CZTS absorber layer, CZTS films based on a stacked precursor (Cu/Sn/Zn/Cu) were prepared. The effect of sulfurization time and the thickness of top and bottom Cu layer in precursors on the properties of CZTS thin films were investigated. We addressed the importance of Cu layer thickness adjacent to Sn to avoid developing detrimental phases and to get complete formation of kesterite CZTS absorber layer. We also addressed the importance of sulfurization time to restrict the Sn and Zn losses, formation of oxides such as SnO2 and ZnO, formation of MoS2 and voids between Mo/CZTS interface. Effect of the sulfur concentration on the properties of Zn(O,S) thin films were investigated. We showed that key parameters such as energy gap and crystal structure of the Zn(O,S) thin films can be tuned by changing the sulfur concentrations of the films. We succeed substitute conventionally used CdS buffer layer with environmentally friendly alternative Zn(O,S) buffer layer in CZTS solar cells. Effect of substrate position and rotation speed during the deposition of AZO thin films were investigated. We addressed that stress on the films can be significantly reduced by off-center deposition and rotating the sample holder during the deposition. In this way, high transmission in the visible range and metal like resistivity were achieved simultaneously at room temperature. We observed strong dependence of device performances on both sulfurization time and the thickness of Cu layer adjacent to Sn in CZTS absorber. The best device was based on CZTS films sulfurized for 30 minutes and having thicker Cu layer adjacent to Sn layer in precursors.
Processing foam-like porous glass structure using a combined process of glass powder expansion in aqueous environment and sintering process
(Izmir Institute of Technology, 2019-07) Zeren, Doğuş; Güden, Mustafa; Akdoğan, Yaşar
Soda-lime glass foams were formed by the controlled pore structure of inorganic particle-liquid suspensions at room temperature and then sintered at elevated temperatures between 650oC-800 oC. The slurries were prepared using the glass particles below 38 µm (fine), between 38 and 45 µm (medium) and between 45 and 56 µm (coarse) and with 50, 55, and 60 wt% solid content and 2, 3, and 4 wt% carboxymethyl cellulose (CMC) binder addition. The slurries were foamed using an Al-based foaming agent and a calcium hydroxide alkali activator with an amount of 1 wt%. An increase in CMC content and a decrease in particle size shifted the slurries from a Newtonian to a non-Newtonian behavior and slurry stabilization with the CMC addition. The extensively increased initial bubble pressure in high viscosity slurries resulted in higher linear expansion rate initially followed by a bursting of gas bubbles. The maximum foam linear expansion of the slurries increased with CMC addition until about ~5 Pa s and the expansions stayed almost constant over 400% expansion, while the slurries with the viscosity above 50 Pa s could not be foamed. The most effective factor on the maximum expansion was found the solid content followed by CMC content and the least effective factor was determined the particle size. Partial bonding of glass particles and excessive shrinkage of glass particles due to the melting of foam green bodies were seen at 650 and 800 oC sintering temperatures. Prepared foam glasses showed lower compressive strengths and thermal conductivities than the glass foams reported in the literature. Finally, foaming at room temperature with this technique was found to be more advantageous than conventional glass foam production techniques due to ease of pore formation controlling at room temperature.
Production and characterization of porous ceramics for high temperature applications
(Izmir Institute of Technology, 2022-07) Semerci, Tuğçe; Ahmetoğlu, Çekdar Vakıf; Akdoğan, Yaşar
This thesis focuses on the production and characterization of different porous polymer derived ceramic (PDC) components (foams, additively manufactured (AM) honeycombs, and aerogels) and demonstrates their potential for high temperature applications, including gas permeability (up to ~700 o C), molten metal filtration, and heat exchanger. The foams were produced via the replica technique and different pore sizes, ranging from 300 μm to 2 mm, silicon oxycarbide (SiOC) ceramic foams were able to be formed. The average total porosity of the foams was 96 vol% with a specific surface area (SSA) of ~80 m2 /g. AM-made honeycomb-like cellular structures with different cell sizes (578 μm, 1040 μm) were obtained via fused filament fabrication. Finally, SiOC aerogels were synthesized using siloxane resin, then dried at ambient pressure and room temperature. The produced SiOC aerogels showed a total porosity of around 80 vol% and an SSA reaching 250 m2 /g. Regarding the high temperature applications of porous PDC components, initially, the gas permeability of SiOC foams was tested, and the results showed stability up to 700 °C in the air without any loss of functionality, offering reusability even in aggressive environments. In the subsequent studies, filtration of molten aluminum alloy was tested using various porous components. PDC foams demonstrated better performance in comparison to the AM-made cellular structures and commercial SiC foams. Finally, heat exchange analysis was performed to evaluate the heat transfer of SiOC foams, and an increase in pressure drop was found to be directly proportional to the rate of increase in air velocity.
Separation of macromolecules from aqueous systems using electrospun fibers
(Izmir Institute of Technology, 2018-11) Işık, Tuğba; Demir, Mustafa; Demir, Mustafa Muammer
Electrospinning has been recognised as a versatile method for the fabrication of continuous polymeric fibers with various type of morphology. Since it allows changing the fiber diameter, surface morphology and porosity by adjusting the solution and instrumental parameters, electrospun fibers present a wide range of properties that cannot be found in bulk materials. Through this thesis, removal of several types of pollutants from the aqueous systems was studied by using the electrospun fibers fabricated from both virgin and waste polymers. The first part of the dissertation deals with the removal of macromolecular pollutants from aqueous systems by using waste-based electrospun fibers. Electrospun fibers fabricated from CD cover and expanded polystyrene wastes were utilized for the protein-rich medical waste treatment by using Bovine Serum Albumin, Myoglobin and Trypsin as protein models. Electrospun fibers from expanded polystyrene wastes were utilized for the remediation of oily wastewaters. The second part of the dissertation deals with the polyatomic nuclear waste removal using uranyl ions as analyte and amidoxime functionalized PIM-1 electrospun fibers. The last part of this dissertation describes an approach for the fabrication of fluorine-free hydrophobic surfaces by electrospraying of methacrylate based linear and hyperbranched copolymers.
Single-photon generation from defects and manipulation with nanostructures
(Izmir Institute of Technology, 2019-12) Özçeri İyikanat, Elif; Tarhan, Enver; Aygün Özyüzer, Gülnur
Single-photon sources are essential components for several applications in the field of quantum information technologies, such as quantum cryptology and quantum computation. To this aim, efficient generation and detection of single-photons are the crucial to be achieved. Among single-photon sources that are extensively studied in the literature, defect centers in solid are very promising due to their room temperature operation and their stability. The aim of this thesis is to generate single photons at room temperature and control their optical properties by nanostructures. Single-photon emission from TMDCs originates from localized weakly bound excitons at cryogenic temperatures due to their small exciton binding energies. However, room temperature SP emission from WS2 can be obtained by creatingWO3 defects. In our study, room temperature emission from defects in WO3 was investigated. Density functional theory calculations showed that the source of the emission can be oxygen defects. Additionally, the emission was brightened by plasmonic gold nanoparticles. Furthermore, defects in two-dimensional (2D) hexagonal boron nitride (hBN) is offered as an efficient room temperature SPS. HBN is a wide bandgap 2D material, in which defect centers create discrete energy level to generate single photons. In our study, reversible single-photon emission control from defects in hBN was demonstrated by Förster-like resonance energy transfer between the single-photon emitter and a graphene layer. To this aim an ionic liquid based device structure was used.
Structural and electronic properties of organic layers on AU(111)
(Izmir Institute of Technology, 2018-06) Kap, Özlem; Varlıklı, Canan; Çelebi, Cem
Self-assembled monolayers (SAMs) have attracted attention due to their chemical and structural properties providing numerous new applications such as molecular electronics and electrochemistry. SAMs were optimized by experimental techniques including Scanning Tunneling Microscope (STM), Scanning Tunnelling Spectroscopy (STS), X-Ray Photoelectron Spectroscopy (XPS), Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS), Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS). The first part of this dissertation deals with the dynamic behaviour of decanethiol SAMs. The dynamic behaviour alkanethiols SAM is unique for its configuration giving indirect evidence for the structural ordering within the formation. Structural stability of decanethiol (decanesulfonates) SAMs were investigated by space- and time- resolved STM. Decanesulfonate phase shows less dynamic behaviour and is structurally more stable compared to the decanethiol phases. The second part of this dissertation describes the binding properties of alkyne molecules adsorbed on gold. Alkyne oxidation occurs at ambient conditions but it is found that unlike thiols, ordered alkyne SAM structure has still chemical bonding between carbon and gold. Alkyne SAMs are good candidates for the ambient molecular electronics application. The last part of this dissertation presents the study of a monolayer of a Ru(II) complex which is prepared on gold substrate. With the help of STM and XPS methods, the ordered structure and binding properties of the CS28 molecules were characterized by providing a deeper insight into the carboxyl and sulfur groups binding affinity to gold substrate. CV and EIS methods were used to compare the adsorption properties and charge transfer process with the bare gold substrate and SAMs.
Structural engineering of halide perovskites and their association with organics for optoelectronic applications
(Izmir Institute of Technology, 2022-06) Yüce Çakır, Hürriyet; Demir, Mustafa; Demir, Mustafa Muammer; Yüksel Aldoğan, Kıvılcım
Halide perovskites show great potential for next-generation optoelectronic applications due to their unique photophysical properties with low production costs. However, their stability issues still prevent their commercialization on a large scale. The main objective of this dissertation is to understand the additive engineering strategy to improve the quality of halide perovskite films and nanocrystals for solar cells and lightemitting diodes. There are two sections to this dissertation: The first section focuses on halide perovskite films and solar cells while the second one focuses on halide perovskite nanocrystals and white light-emitting diodes. In the first section of this dissertation, in Chapter 2-3, the improvement of Sn-Pb and methylammonium-free Pb-based halide perovskite films by additives are investigated. The suppression of defects via additives is demonstrated through structural, elemental, and optical analyses. The improved performance of perovskite solar cells by decreasing defects is also shown. In Chapter 4-5, the change in stability and optical properties of the halide perovskite nanocrystals by means of additive engineering and their applications in white light-emitting diode are studied. The results in this dissertation represent a new approach to improving the structural and photophysical properties of halide perovskites and introduce a new perspective of additive engineering method in the field of halide perovskite-based optoelectronic applications.

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