Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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2011 - 20195

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Author: search.filters.author.Senger, Ramazan Tuğrul

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Now showing 1 - 5 of 5
  • Master Thesis
    Electronic, Vibtational and Transport Properties of Quasi-One Dimensional Transition Metal Dichalcogenide Structures
    (Izmir Institute of Technology, 2019) Ünsal, Elif; Sevinçli, Haldun; Senger, Ramazan Tuğrul
    Thermoelectric materials have attracted great attention due to their ability to convert heat to electrical energy. As the application area of nanoscience expands, nanostructuring becomes a promising approach for enhancing thermoelectric properties. In this thesis, thermoelectric enhancement of the T-phase HfSe2 structures is studied via nanostructuring. Density functional theory (DFT) based electronic and vibrational spectra of two-dimensional (2D) and quasi-one dimensional T-phase HfSe2 structures are investigated and their ballistic thermoelectric transport properties are examined within the Landauer formalism. For the first time, it was reported that the nanoribbons of the Tphase HfSe2 are dynamically stable and semiconducting materials. They have promising thermoelectric properties. We reported the enhancement of the p-type ZT parameter of T-phase HfSe2 at both low and high temperatures. Moreover, the width dependency of the thermoelectric properties of the nanoribbons are studied.
  • Master Thesis
    Fröhlich Polaron Calculations in Non-Integer Dimensional Space as a Model of Confinement
    (Izmir Institute of Technology, 2013) Özbal, Gözde; Senger, Ramazan Tuğrul
    Polaron is a quasiparticle describing an electron in interaction with phonons of a medium. A microscopic description of large polaron is given by the Fr¨ohlich Hamiltonian which does not admit exact solutions. For calculating the ground state energy and effective mass of polaron there are several approximation methods, some of which are valid only for large or small values of the electron-phonon coupling constant. In lowdimensional systems, where the polaron is confined by an external potential such as in the form of slab or wire geometries, the polaronic energy and effective mass are known to get enhanced. In this thesis we present an approach towards quantifying the degree of confinement on a large polaron provided by a parabolic potential. On that purpose, first, variation of polaronic ground state energy as a function of the parameters of the confinement potential for both slab- and wire-like geometries and using a methodology valid for all values of electron-phonon coupling constant is calculated. Then, applying a noninteger- dimensional-space algebra the polaron problem has been solved in an isotropic D-dimensional space using the same approch (D varies continuously from 3 to 2 for slab, and from 3 to 1 for wire geometry.) Finally, by matching the polaron ground state energy values obtained from the two calculations in large electron-phonon coupling constant limit, we identify the effective dimensionality D, of the polaron for a given set of confinement and material parameters.
  • Master Thesis
    Calculations of Electric and Magnetic Properties of Triangular Graphene Fragments Using Density Functional Theory: Effects of Edge Functionalization and Electric Field
    (Izmir Institute of Technology, 2013) İyikanat, Fadıl; Senger, Ramazan Tuğrul
    The triangular graphene flakes (N-TGFs) we consider have equilateral triangular shapes with zigzag edges, where N denotes the number of edge hexagonal cells in one side of the triangle. Termination of these N-TGF structures with several elements (of the first two rows of the periodic table) and application of electric field to these flakes alter their electronic and magnetic properties. In accordance with previous studies [1, 2], it is found that bare flakes have large spin magnetic moment values of 4(N − 1) μB, whereas they reduce to (N − 1) μB for full saturation of edges with Hydrogen, Lithium, Beryllium or Flour atoms. Moreover we have studied possible termination of TGF with other elements like Boron, Carbon and Nitrogen. Hydrogen and Flour atoms prefer to bind at the top of an edge Carbon atom. Unlike Hydrogen and Flour, the other atoms prefer to bind at the bridge sites. Recent studies [3, 4] have shown that themagneticmoments of triangular graphene flakes can be controlled by applied electric field. We show that the value of total spin polarization of triangular graphene flakes can be changed by tuning an applied in-plane external field. We demonstrate that, in these flakes total spin polarization can be reduced stepwise with the applied field. The electric field control of ferromagnetism in TGFs promises a new route for spintronic applications.
  • Master Thesis
    Electronic, Spintronic and Transport Properties of Carbon Based Nanowires
    (Izmir Institute of Technology, 2011) Arı, Ozan; Senger, Ramazan Tuğrul
    In this thesis, properties of carbon based nanowires are studied by ab-initio calculations. The aim is to gain a thorough understanding of the electronic, spintronic, transport properties in nanowires and how they are affected by different geometric formations, defects and adatom adsorptions. To this end the non-equilibrium Green's function formalism with first principles pseudopotential density functional theory calculations have been used to describe spin-polarized systems. Firstly, different geometric formations of Cobalt-Benzene nanowires are investigated. Systems with ferromagnetic ordering are calculated as half-metallic while systems with antiferromagnetic ordering behave as metallic. Also the results of the spin polarized current calculations indicate that one of the spin components of current is dominant for the antiferromagnetic systems while both spin components of current are dominant in different bias windows of a specific total applied bias. As second case, alkali atom termination of the zigzag graphene nanoribbons (ZGNR) are studied. In particular, using sodium atoms for the saturation of ZGNR edges at half the concentration of edge-carbon atoms make it a one dimensional, perfect semimetal, where the valance and conduction bands meet at only a single, Dirac-like point. Unlike pristine graphene, the Dirac-"cones" of Na-ZGNR is not symmetric with respect to wave vector, but rather it is tilted. Finally, adsorption up to the graphenic sheets with periodic 5-8 defects is studied. Especially, electronic structure of the V adsorption into 5-8 defects induced graphenic sheets are calculated as half-metallic while formation of linear bands crossing at the Fermi level which form a tilted Dirac cone.
  • Master Thesis
    Excitonic Luminescence in Spherical Core-Multi Quantum Dot Structures
    (Izmir Institute of Technology, 2011) Bacaksız, Cihan; Senger, Ramazan Tuğrul
    In this thesis, we studied excitonic light emission from core-multishell semiconductor hetero-nanocrystals having a spherically symmetric shape as a function of core size and shell thicknesses. Different combinations of semiconductors were considered in the core/shell/shell structure. The multilayer quantum dot structures that we considered were in A/B/A/B form, where A and B denote II-VI binary semiconductor crystals. We used method of finite differences to solve the Schrödinger equation of the confined exciton problem numerically. Several approximations including the effective mass approximation for the electron and hole masses, and treatment of Coulomb interaction as a perturbation, were employed. Normalized oscillator strengths for optical transitions and coexistence of multiple transitions from a single structure at several frequencies have been investigated. In this study, three different pairs of semiconductors are considered. One of them is (CdSe) ZnS) CdSe) ZnSe), other is (CdSe) CdS) CdSe) CdSe) and third one is (CdTe) ZnS) CdTe) ZnSe). According to our calculation, (CdSe)ZnS)CdSe)ZnSe) structure, when the core radius is 1.51 nm with some different shells combinations, has multi-color emissions from higher frequency to lower frequency in the visible spectrum. (CdSe) CdS) CdSe) CdSe) structure has multi-color emissions when the core radii are 2.02 nm and 2.52 nm yet this structure is the most inefficient structures which we analyse Moreover, multi-color emissions are obtained from the structure (CdTe) ZnS) CdTe) ZnS) when the core radii are 1.29 nm, 1.93 nm, 2.58 nm and 3.23 nm. Also, this structure has nearly white light emission for 3.23 nm core radius with 3 ML ZnS shell and 1 ML CdTe shell, and with 4 ML ZnS shell and 1 ML CdTe shell.