Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Fabrication and Characterization of Graphene/Silicon Based Schottky Photodiode(Izmir Institute of Technology, 2019) Dönmez, Gülçin; Çelebi, Cem; Sarı, EmreThis thesis focused on fabrication and characterization of CVD grown p-type graphene and n-type Si Schottky junction photodiode with rectification behavior. The device operated at wavelength range between 390 and 1100 nm at self-powered mode. The device was encapsulated with Epoxy Resin to prevent graphene from atmospheric adsorbates. The electronic and optoelectronic characterizations of the devices were done before and after coating the devices with ER. By encapsulation stability of the device was enhanced in terms of photoresponsivity. The maximum obtained photoresponsivity value of the bare device was 0.56 A/W. Also, time-resolved photocurrent spectroscopy measurements showed that the devices exhibited enhanced photodetector performance in terms of photo-switching characteristics. Furthermore, electrical characteristics of Gr/n-Si Schottky photodiode under various illumination power densities with 850 nm wavelength were investigated. The short circuit current showed linear response to power density. However, open circuit voltage exhibited two phased slow and fast increment with increased power density. Hall effect measurements were conducted in order to investigate hole carrier concentration and mobility of the graphene on n-Si. With increasing the power density the carrier concentration increased and the mobility decreased. Besides, light induced manipulation of the Schottky barrier height of Gr/n-Si photodiode was studied. Schottky barrier height of the graphene measured by KPFM method as 0.4 eV. With increasing power density we found that Schottky barrier height of the device increased from 0.4 eV to 0.5 eV and showed similar trend with the change in open circuit voltage.Master Thesis Modification of Graphene Surfaces for Detection of Biomicroparticles(Izmir Institute of Technology, 2019) Yeşiltaş, Gözde; Bulmuş, VolgaPathogens present in the food we consume and the water we drink pose a major threat to human health. Another major health concern is the metastasis of cancer in which cancer cells spread to new areas of the body, often by way of the lymph system or bloodstream. To minimize the burden on health and economy, the detection of biomicroparticles such as pathogens or circulating cancer cells in a highly sensitive and practical manner is higly desirable. This thesis aims to develop a method to create graphene-based biosensor substrate for detection of biomicroparticles such as bacteria, viruses or mammalian cells. For this aim, graphene surface was first functionalized using a linker molecule. The effect of solvent type on functionalization was investigated via Raman spectroscopy and X-Ray spectroscopy (XPS). AntiCD2 antibodies (Ab), as a model antibody, were then conjugated to the functionalized graphene via NHS/EDC chemistry. The Ab conjugation was verified by Raman spectroscopy and XPS analyses. Finally, Jurkat cells, as model biomicroparticles, were recognized and captured by Abconjugated graphene surface, as evidenced by optical microscopy. The temperature, medium, and method for interaction of cells with graphene surfaces as well as the specificity of the Ab- functionalized graphene surface were investigated. The results overall showed the specific and efficient recognition of model cell line by Abconjugated graphene surfaces.Master Thesis Rkky Interaction and Its Control in Graphene and Related Materials(Izmir Institute of Technology, 2019) Canbolat, Ahmet Utku; Çakır, ÖzgürGraphene got dramatic attention and lead the two-dimensional material physics after its first successful synthesis in 2004. Its unique electronic properties contain great potential for both scientific and technological applications. RKKY (Ruderman-Kittel-Kasuya Yosida) is an indirect exchange interaction mediated by conduction electrons. In graphene, the interaction strength decay as 1/R³ where R is the distance between the magnetic moments. In the first part of this work, we calculated that applying circular potential on a graphene sheet forms quasi-bound states in the potential region. Via these states, the RKKY interaction is enhanced between magnetic moments on the edge of the potential well. This can be thought of an electronic analog of the Purcell effect. We showed that the interaction strength is even more enhanced if the Fermi level is in resonance with the energies of the quasi-bound states. In the second part, we considered zigzag edged hexagonal nanoflakes. It is known that zigzag edged flakes have zero-energy edge-states. It is also known that the states with closer energies contribute more to RKKY interaction. Thus, we calculated that there is an enhancement between these edge-states. In the third part, we investigated the behavior of RKKY interaction for two dimensional materials with quartic dispersion. An energy dispersion is said to be quartic if it is of the form E = α(k² - kc² )². Here, α and kc are material dependent constants. There are many materials exhibiting the quartic dispersion such as nitrogene, phosphorene, and arsenene. These materials are also sharing two-dimensional hexagonal lattice structure with graphene. What makes quartic dispersion special is that it has van-Hove singularity in its density of states near the band-edge. RKKY interaction is sensitive to the density of states because it depends on the number of electrons contributing spin exchange. Thus, the larger the number of electrons, the stronger the coupling. In this part, we tuned the Fermi level so that it lies on the DOS singularity and then we calculated the interaction strength as a function of R. We found a slowly decaying RKKY interaction for quartic dispersion. If the energy dispersion is pure quartic (i.e. E = ak4), we found the interaction strength depends on 1/(kf R) instead of 1/R which makes the RKKY interaction long range for arbitrarily small Fermi level.Master Thesis Spin-Spin Interactions of Magnetic Impurities in Graphene Nanoribbons(Izmir Institute of Technology, 2019) Kolay, Anıl; Güçlü, Alev DevrimIn this thesis, we investigate the interaction between two impurity adatoms with high magnetic moment which are located on zigzag graphene nanoribbons that consist of 10516 atoms. The magnetic adatoms communicate with other trough the host electrons such as Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. Firstly, in order to numerically calculate the two impurity Anderson model, we use quantum Monte Carlo technique. When the impurity adatoms are located far from edges, the results we obtained are consistent whit the bulk graphene results in the literature. On the other hand, the specific location and orientation of adatoms on the sublattices, significantly affects the spin-spin correlations of the two impurities. However, we observe that while the adatoms approach to the edges, significant differences occur due to the edge effect of zigzag graphene nanoribbon. As a results of this, we found that the magnetic correlations can be also enhanced if the adatoms belong to the same sublattice as the edga atoms, since the states of the adatoms hybridize with edge states. Moreover, we show that chaning chemical potential can crucially affect the magnitude of the correlations of the adatoms, and may lead to aphase transitions from ferromagnetic to antiferromagnetic or vice versa. Besides, we observe that when the width of the zigzag graphene nanoribbons is decreased, the spin-spin correlations are affected.On the other hand, we also calculated spin-spin correlations using mean-field approximation for themean-field Anderson model. We found that results significantly differ from quantum Monte Carlo results. In addition, when the electron-electron interations of he host atoms are taken into account, crucial differences are obtained at the impurity correlations.Master Thesis Graphene Transfer Approaches With Different Support Materials on the Substrates With Cavities(Izmir Institute of Technology, 2019) Duman, Sinem; Balantekin, Müjdat; Çelebi, CemA micro capacitive sensor characteristically embraces a thin conductive membrane which is freely suspended above an immovable counter electrode in a parallel plate geometry. Such capacitive structures are found in broad range of applications as a transducer like capacitive micro-machined ultrasonic transducer (CMUT), pressure sensor, resonator and biological or chemical material sensing element. The input can be an ultrasound wave, pressure, chemical or biological mass attachment which result in the deflection of the membrane. Emerging nano materials have shown great potential as candidates for generation of nano and micro electromechanical systems (NEMS, MEMS). Among these nano materials, graphene is regarded as a promising material because of its ultra low mass, thickness, high surface to volume ratio, flexibility, and extraordinary electrical and mechanical properties. However, the transfer of graphene on substrates with micro scale cavities is challenging since the fabrication of large area membranes with a smaller air gap often results in membrane tearing or collapse driven by capillary or electrostatic forces. This study presents a research on the fabrication and the characterization of graphene membranes to be used in micro capacitive sensor applications. Substrates which span a large array of circular and hexagonal micro cavities between 2-100 μm in diameter are fabricated. Graphene transfer with different support materials are studied to fabricate graphene micro membranes. Up to 5 μm diameter membranes on 300 nm deep cavities are demonstrated via scanning electron microscope (SEM) and atomic force microscope (AFM) tools.Master Thesis Disorder Induced Electronic and Magnetic Properties of Graphene Quantum Dots(Izmir Institute of Technology, 2019) Kul, Erdoğan Kul; Güçlü, Alev DevrimIn this thesis, we aim to study magnetic properties of hexagonal shaped graphene quantum dots with armchair edge in the case of atomic collapse by modelling two vacancies on it. The measured relativistic electron transport property of the graphene allows us to observe the phenomenon called "atomic collapse" in a small energy scale which existence is proven theoretically before for atoms whose atomic number is higher than 170. First we modelled a Coulomb potential at the center of a hexagonal shaped and armchair edged GQD and examined by using tight-binding method. We obtain similar results with previous works. After that, we started to study magnetic properties of the dot by meanfield Hubbard method which includes spins into calculation. We modelled a vacancy close to the center of the dot and examined electronic and magnetic properties by MFH metod. Also we modelled two vacancies on the dot that we changed the distance between them and the direction respectively. Also by applying Coulomb potential at the center of the vacancies we examined magnetic behaviour at the atomic collapse regime. Also, we compared our results with the works obtained by using RKKY (Ruderman-Kittel- Kasuya-Yosida) interaction method which considers the indirect interactions of magnetic impurities that uses electrons of metallic substrates. We found that increasing Coulomb potential and increasing distance between the vacancies, reduces correlations of electrons around the vacancies. The ground state energy difference between ferromagnetic and antiferromagnetic systems, that proportional to interaction strength, shows similar behaviour that has been observed by using RKKY method. Also if we take out two atoms from the same sublattice and with the same spin property, changing Coulomb potential leads to ferromagnetic-anti-ferromagnetic phase transition, independent from the atomic collapse behaviour. Also we observed that there is no direct link between the magnetic transition and the energy difference of the vacancy states.Master Thesis Thermal Performance of Graphene Coating on Copper(Izmir Institute of Technology, 2019) Ersavaş, Gizem; Toprak, Kasım; Çelebi, Cem; Toprak, Kasım; Çelebi, CemOver heat is always a problem for electronic devices because the locally generated heat cannot be transferred appropriately to the corresponding heat sink fast enough. This situation leads to affect materials’ structures, mechanical properties and conductivities badly. In order to avoid this problem, high thermal conductivity materials are used to dissipate the heat quickly. Thanks to the development of technology, the size of the electronic devices is reduced day by day. This also shrinks the size of the interconnect components. So this situation leads to researchers to investigate nano-sized interconnect components and copper, which is a widely used material, is one of them. Copper is one of the preferred metals for electronic devices because of high thermal conductivity, easy processability, and high use in daily life and industry. For example, copper components, which is used in electronic, are getting so thin and must carry so much current. And that causes to increase friction. Thus heat is occurred. Consequently, cooling problems have arisen. And if the material’s cooling problem won’t be solved then the material can be damaged. It is thought that to overcome this problem, coating with a high thermal conductivity material such as graphene, the thermal conductivity can be improved. In this study, thermal performance of graphene-coated copper were investigated numerically and experimentally. This study consist of two main sections. The first part, MD simulation code was created using C++ programming language to investigate thermal conductivity of copper, different number of graphene layers and these graphene layers were coated on copper in different length, width, height and temperature. In the second part, the thermal performance of pure copper, annealed copper, a layer of graphene-coated copper, and multilayer graphene-coated copper was studied by the experimental setup at three different temperatures and volume flow rates.Master Thesis Monitoring the Diffusion and Degradation Characteristics of Crystals Via Raman Spectroscopy(Izmir Institute of Technology, 2018) Akbalı, Barış; Şahin, HasanLamellar structures, having strong in-plane and weak (van der Waals) out-ofplane bonding, exhibit extraordinary properties when thinned down to their monolayer limit. Following the isolation of single layer graphene in 2004, there has been a rapid increase in the number of studies focusing on other novel two dimensional (2D) materials such as hexagonal Boron Nitride (BN), transition metal dichalcogenides (TMDs), post transition metal chalcogenides (PTMCs), silicene and black-phosphorus. Doping of 2D and bulk crystals is a well-known strategy that may lead to novel functionalities and significantly alters materials’ electronic, optical, and magnetic properties. In this regard, understanding of diffusion characteristic of dopant in a crystal via computational simulation is vital to enlighten physical insights of the experiment. In addition, investigation of degradation mechanisms of crystals at atomic-level is also still open question. In this sense, the density functional theory (DFT) is one of the most powerful and commonly used methods for such theoretical investigations. Moreover, measuring vibrational spectra of a material via Raman spectroscopy is powerful method to understand atomic vibrations that give information about physical properties of a material. In this regards, we investigate diffusion characteristics and degradation mechanism of several crystal (such as, perovskites and MoS2) by means of first-principles calculations based on density functional theory (DFT). In addition, Raman measurements are also carried out to investigate vibrational properties of the crystals. It is shown that few-layer MoS2 can be used for selective nitrogenation of graphene. In addition, red shift in photoluminescence peak of water interacted CsPbBr3 nanowires arise from detachment of surface ligand from surface of nanowire by presense of water molecules. Lastly, time-dependent photoluminescence measurement of Mn-doped CsPbCl3 shows that change in emission color under UV illumination is due to segregation of Mn atoms towards crystal surface. This thesis provides some important results for deeper understanding of degradation and diffusion mechanisms of dopants in 2D materials and perovskites.Master Thesis Effects of Random Atomic Disorder on Electronic and Magnetic Properties of Graphene Nanoribbons(Izmir Institute of Technology, 2018) Çakmak, Korhan Ertan; Güçlü, Alev DevrimIn this thesis, We investigate the effects of randomly distributed atomic defects on the magnetic and electronic properties of graphene nanoribbons with zigzag edges using an extended mean-field Hubbard model. We show that electron-electron interaction effects not only make defect states robust as compared with the tight-binding results,but also make edge states fragile even at low defect concentration for clean edge sites. For a balanced defect distribution among the sublattices of the honeycomb lattice in the bulk region of the ribbon, the ground state antiferromagnetism of the edge states remains unaffected. By analyzing the excitation spectrum, we show that while the antiferromagnetic ground state is susceptible to single spin flip excitations from edge states to magnetic defect states at low defect concentrations, it’s overall stability is enhanced with respect to the ferromagnetic phase. Then, we investigated Anderson localization induced metal to insulator transition by a localization length in nanometer scale up to 5% vacancy concentration by using time dependent results. We found that, Anderson localization is stronger at the vicinity of Fermi level energy states since those states are becoming full of impurity states and edge states, mixed.Master Thesis Nanotribological Properties of Graphene Grown on Silicon Carbide Semiconductor(Izmir Institute of Technology, 2018) Keskin, Yasemin; Çelebi, Cem; Ünverdi, ÖzhanIn this thesis, nanotribological properties of single and multilayer graphene grown on two sides of the Silicon Carbide (SiC) semiconductors were investigated. For this purpose, epitaxial growth technique was used to obtain single-layer graphene on both C-face and Si-face. This thesis consists of two purposes: One of them is to investigate the nanotribological properties of the single and multilayer graphene grown on C-face of SiC and the other one is to compare nanotribological properties of the single layer graphene on two sides of SiC. Graphene, a two-dimensional semi-metal material, was grown epitaxially on the SiC surface under ultra-high vacuum conditions. In epitaxial method, direct current heating is applied to the SiC substrate to vaporize Si atoms from the surface. As the Si atoms evaporate, the remaining C atoms form a graphene layers on top. When single layer graphene is formed on the Si-face, multilayer graphene is formed on the C-face at the same parameters. For this reason, two different samples of graphene were needed in order to compare the tribological properties of the single layer graphene grown on both Si-face and C-face for the secondary objective. A capping method was used to control the rate of Si atoms evaporating from the SiC surface. By this way, single layer graphene on the C-face was obtained too. Number of layers were determined by Raman Spectroscopy. Nanotribological characterizations were done with Atomic Force Microscopy. The experimental results showed that single layer graphene on the Si-face has higher friction coefficient compared to single layer graphene on the C-face. It has been found that the single layer graphene (0.02) formed on the C-surface has a lower coefficient of friction than the multilayer graphene (0.82). It is expected that with the support of the theoretical studies on this results will increase the interest in this study by means of these results are new and original for the literature.