Master Degree / Yüksek Lisans Tezleri

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Department: search.filters.department.Thesis (Master)--İzmir Institute of Technology, Materials Science and EngineeringSubject: search.filters.subject.Graphene

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Now showing 1 - 4 of 4
  • Master Thesis
    Nanotribological Properties of Graphene Grown on Silicon Carbide Semiconductor
    (Izmir Institute of Technology, 2018) Keskin, Yasemin; Çelebi, Cem; Çelebi, Cem; Ünverdi, Özhan; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    In 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.
  • Master Thesis
    Synthesis and Nitrogen Doping of Graphene by Chemical Vapor Deposition
    (Izmir Institute of Technology, 2017) Yanılmaz, Alper; Çelebi, Cem; Adem, Umut; Çelebi, Cem; Adem, Umut; 03.09. Department of Materials Science and Engineering; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of Technology
    Controllable carrier transport due to charged impurities in the graphene lattice is still lacking. Doping of graphene by foreign atoms leads to modify its band structure and electro chemical properties. Among numerous potential dopants, nitrogen (N2) is considered to be an excellent candidate to form strong valence bonds with carbon atoms, which would provide n or p-doping according to bonding character of charged-impurity atom. Exposure of graphene lattice to nitrogen gas leads to a change in the carrier concentration and opens a bandgap due to symmetry breaking. Furthermore, this seems to be an effective way to customize the properties of graphene and exploit its potential for various applications. This thesis focuses on the growth of graphene by low pressure chemical vapor deposition (LPCVD) and doping it with N2 by using N2 plasma treatment. Here, copper foil was used as the catalytic substrate to grow large area graphene at LPCVD system. The grown graphene was transferred onto SiO2, Au (111) and Sapphire substrates. The effect of different plasma time and power on doping process was investigated while keeping the N2 flow rates constant by using N2 plasma. The nitrogen doped graphene (N-graphene) was characterized via Raman Spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy/spectroscopy (STM/STS), Kelvin probe force microscopy (KPFM). Raman mapping of N-graphene was also conducted to show the homogeneity of N2 incorporation into graphitic lattice. STM results were theoretically modelled by using density functional theory (DFT). Our results provide the opportunity to produce N-graphene with homogenous and effective doping which would be valuable in electronic and optoelectronic applications.
  • Master Thesis
    Self-Organized Network of Silicon Oxide on Epitaxial Graphene
    (Izmir Institute of Technology, 2017) Özkendir, Dilce; Çelebi, Cem; Çelebi, Cem; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    In this thesis, I studied the formation and characterization of self-organized hexagonal-shaped SiO2 wrinkle structures on epitaxial graphene that was grown on SiC substrate. Monolayer graphene was grown by annealing the SiC substrate at high temperatures under ultra-high vacuum conditions. Following the growth process, SiO2 thin film was deposited on epitaxial graphene layer at different deposition temperatures by thermal evaporation method. We found that SiO2 film wrinkles on epitaxial graphene. The origin of the hexagonal shaped wrinkle structures were derived from the thermal expansion coefficient difference between epitaxial graphene and SiO2 thin film. The mesh density of these SiO2 hexagonal wrinkle structures was controlled by changing the cooling rate of the substrate after the thin film deposition. To make a comparison, SiO2 thin film was also deposited on CVD grown graphene and on bare SiC substrate. Unlike on the bare SiC surface, SiO2 thin film on epitaxial graphene exhibited a self-assembled network of hexagonally shaped wrinkles due to thermally induced compressive strain between the two materials. The observed network of wrinkles were found to be comprised of line shaped primary and secondary types of protrusions with distinct topographic characteristics as determined by optical microscopy, Scanning Electron Microscopy and Atomic Force Microscopy measurements. The wrinkle to wrinkle spacing and mesh density of the wrinkle network were modified simply by changing the SiO2 deposition temperature. Our experimental results imply that epitaxial graphene with its high chemical inertness on SiC offers a great potential to be used as a conventional substrate in the realm of thin film metrology.
  • Master Thesis
    Developing Epitaxial Graphene Electrodes for Silicon Carbide Based Optoelectronic Devices
    (Izmir Institute of Technology, 2015) Kuşdemir, Erdi; Çelebi, Cem; Sevinçli, Haldun; Çelebi, Cem; Sevinçli, Haldun; 04.05. Department of Pyhsics; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of Technology
    In this thesis work, I studied the fabrication and characterization of graphene-semiconductor-graphene ultraviolet photodetector based on the rectifying character of Schottky junction at the interface between epitaxial graphene and silicon carbide semiconductor. As-grown single layer epitaxial graphene is interdigitated as transparent conductive electrode to probe photo-generated charge carriers in a semi-insulating 4H-silicon carbide substrate. The fabricated device exhibits the typical current-voltage characteristics of a conventional metal-semiconductor-metal type photodetector with low leakage current. Time-resolved photocurrent measurements suggest an excellent photocurrent reversibility and high response speed of the device. The measurements performed for different illumination wavelengths showed that the sample reveals higher responsivity values when it is exposed to the light with 254 nm wavelength. The obtained results imply that epitaxial graphene can be used readily as transparent conductive electrode for SiC based optoelectronic device applications. Finally, in the last chapter, I discuss how the photoresponsivity of the graphene-semiconductor-graphene photodetector can be enhanced by CdTe/CdS quantum dots. The drop casted CdTe/CdS quantum dots have been shown to increase the photoconductivity of the device. The thickness of the quantum dots is found to effect the enhancement factor of the photoresponsivity of the device.