Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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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.Master Thesis Multiple exciton generation in graphene nanostructures(Izmir Institute of Technology, 2014) Yıldırım, Jülide; Çakır, ÖzgürThis thesis comprises a theoretical study on the role of the inverse-Auger process in graphene nanostructures. Inverse-Auger effect (IAE) is the formation of a multitude of low energy excitons from a single exciton of higher energy. Its mechanism is the conversion of the kinetic energy of the high energy carriers to new excitons via Coulomb interaction. Bulk graphene has zero band gap energy and has two Dirac points which is linearly dependent crystal momentum. Due to quantum confinement, graphene nanoribbons and graphene flakes or the structures having periodically holes develop a band gap. The emergence of a band gap makes these structures eligible for solar cell applications. In bulk structures, due to translational symmetry momentum is conserved which leads to a decreased IAE. However, in nanostructures, in addition to the relaxation of momentum conservation condition, the Coulomb interaction between the carriers increases which leads to an enhanced IAE. In this thesis, a theoretical analysis of inverse-Auger effect is carried out for graphene and armchair graphene nanoribbons. Tight binding method is employed to obtain the electronic structure and to calculate the Coulomb matrix elements for the inverse-Auger effect in this structures. According to our calculations, inverse- Auger effect in the bulk graphene provides the formation of new excitons at a rate which is approximately linearly proportional to the energy of an electron at the conduction band.