Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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2016 - 20195

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Access Right: Open AccessAuthor: search.filters.author.Senger, Ramazan Tuğrul

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Now showing 1 - 5 of 5
  • Doctoral Thesis
    Optical and Electronic Properties of Atomically Thin Layered Materials: First Principles Calculations
    (Izmir Institute of Technology, 2019) İyikanat, Fadıl; Senger, Ramazan Tuğrul; Şahin, Hasan
    The extraordinary interest in two-dimensional (2D) materials is increasing day by day. Thanks to advances in the experimental techniques, monolayer form of another material is synthesized every day with features not seen in the bulk form. Ab initio methods provide useful tools for characterizing and functionalizing the various properties of these materials. The results obtained through first principles quantum-mechanical calculations can help to predict and understand the experimental data, such as the position and source of the spectroscopic peaks in the Raman or optical absorption spectra. The aim of this thesis is to predict and functionalize the optical and electronic properties of atomically thin layered materials using density functional theory and approaches beyond. Within the scope of this thesis, possible technological applications of various 2D materials ranging from perovskite crystals to transition metal dichalcogenites are investigated by using several functionalization methods. In order to accurately predict the optical properties of these materials, it is very important to use approaches that take into account the many-body effects. Recent studies have shown that many-body perturbation theory in the form of GW approximation is highly reliable to calculate the quasiparticle properties of materials. By solving the Bethe Salpeter equation on top of GW calculation, the quasiparticle energies and excitonic properties, which have dominant effect in the optical properties of ultra-thin materials are examined in detail.
  • Doctoral Thesis
    Thermoelectric Effect in Layered Nanostructures
    (Izmir Institute of Technology, 2019) Özbal Sargın, Gözde; Senger, Ramazan Tuğrul; Sevinçli, Haldun
    In this thesis, ballistic transport and thermoelectric (TE) properties of semiconducting and dynamically stable two-dimensional materials are investigated by combining first-principles calculations with Landauer formalism. Motivated by finding novel promising TE materials, transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are studied systematically in their 2H- and 1T-phases in Chapter 3. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we analyze the thermoelectric properties of the semiconducting ones. We report for the first time that, 2H-phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T-phases. Next, ballistic transport and thermoelectric (TE) properties of group IIImonochalcogenides (group III-VI) are presented in a wide range temperature from 100 K to 1000 K. This large family composed of 25 compounds which stands out with their unique electronic band structures. In addition to Mexican hat shaped (quartic energy-momentum relation) valence band character, some of the structures exhibit valley degeneracies which can occur either in their conduction and valence bands. Moreover, TE and transport calculations are performed for BO and BS monolayers which consist of lightest species in group III-monochalcogenides. Surprisingly, BO and BS monolayers exhibit high TE efficiency at low temperatures. Low thermal conductance at low temperatures and stepwise electronic transmission at the valence band edge are the physical mechanisms behind achieving large ZT.
  • Doctoral Thesis
    Optical Properties of Ultra-Thin Materials
    (Izmir Institute of Technology, 2017) Bacaksız, Cihan; Senger, Ramazan Tuğrul; Şahin, Hasan
    Many years of research effort, after the synthesis of graphene, have revealed that atomically thin two-dimensional materials have mechanical, electronic, and optical properties which are different from their bulk counterparts. Thus, the interest in twodimensional materials is growing which is also fueled by fast advances in synthesis and measurement techniques. In this regard, the theoretical and computational simulations provide physical insight to the experiments in this new and demanded field; a tool for characterizing these materials; and also a reliable prediction approach to possible stable structures. The density functional theory (DFT) is one of the most powerful and commonly used methods for such theoretical investigations. The DFT-based computational determination of optical properties, as compared to other usual DFT-based calculations, is in its early stage due to high computational resource requirements and lack of established documentation. Therefore, the present thesis aims at giving the methodology and computing the optical properties of ultra-thin materials by using DFT and beyond-DFT approaches. More precisely, the thesis provides an overview of light matter interaction; basics of DFT, GW approximation for many-body effects, Bethe-Salpeter equation for excitonic effects; and several applications of these on atomically-thin systems.
  • Doctoral Thesis
    Electronic, Magnetic, and Mechanical Properties of Novel Two Dimensional Monolayer Materials
    (Izmir Institute of Technology, 2017) Yağmurcukardeş, Mehmet; Senger, Ramazan Tuğrul; Şahin, Hasan
    Layered materials exhibit different properties when they are thinned down to a few monolayers. Following the successful isolation of 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), and in-plane anisotropic monolayers (Redichalcogenides and blackphosphorus). In addition to their electronic, optical, and magnetic properties, mechanical properties of 2D materials are of fundamental importance. Measurements of elastic constants of 2D materials are still challenging. Therefore, theoretical investigation of the mechanical properties is particularly important. Moreover, investigation of Raman spectra of these materials requires a through understanding of their vibrational properties. In these regards, we investigate the electronic, magnetic, and mechanical properties of some novel monolayer 2D materials (such as, auxetic pentagonal monolayers, flexible monolayers of holey graphene crystals, ultra-flexible monolayers of PTMCs, and in-plane anisotropic monolayers of ReS2 and blackphosphorus) by means of first-principles calculations based on density functional theory (DFT). In addition, tuning electronic properties of a van der Waals heterobilayer structure composed of monolayers of Mg(OH)2 and WS2 upon an external out-of-plane electric field is studied. The effect of biaxial strain on the vibrational properties of novel 2D materials is also studied through their off-resonant Raman activities. Our findings will be useful to clarify several issues related to the experiments of novel 2D materials.
  • Doctoral Thesis
    Modelling Electronic and Structural Properties of Graphene and Transition Metal Chacogenide Nanostructures
    (Izmir Institute of Technology, 2016) Özaydın, Hediye Duygu; Senger, Ramazan Tuğrul
    The purpose of this thesis is to investigate the electronic and structural properties of one- and two-dimensional materials such as graphene, graphene-like transition metal chalcogenides by using density functional theory. The single-atom thickness of graphene sheet is a novel material and attracts great interest due to its unique features. In recent years, theoretical and experimental studies on graphene provide quick knowledge and have opened up possibilities for many other two-dimensional new materials. Among these materials, especially transition metal chalcogenides have recently been the focus of studies of condensed matter physics. Unlike many superior properties of graphene, lack of band gap in electronic structure have highlighted the necessity of such transition metal chalcogenides materials for electronic applications. As compared to graphene, transition metal chalcogenides have various physical properties and possess sizable band gaps, for this reason they are promising candidate for many applications. Many experiments have revealed that the surfaces of graphene and graphene-like structures can play an active role as a host surface for clusterization of metal atoms. Motivated by these observations, we investigate characteristic properties of Pt atoms on graphene, MoS2 and TaS2. Similarly, TiSe2 is very recently synthesized two-dimensional transition metal dichalcogenide material and stable in 1T phase. Two-dimensional TiSe2 has a metallic electronic property and widely studied material. We analyze how to change the structural and electronic properties of TiSe2 by functionalization with hydrogen atom. Again to the effects of hydrogenation on two-dimensional TiSe2 monolayer we also study the structural and electronic properties of this material in nanoribbon form. At the same time, PtSe2 which is also very recently synthesized two-dimensional transition metal dichalcogenide and stable in 1T phase like TiSe2, its nanoribbon structural and electronic properties have also been investigated and compared with TiSe2 nanoribbons. Finally, TiS3 which is also transition metal chalcogenide but entirely different crystal structure, is recently widely studied materials. The structural and electronic properties as well as carrier mobility and strain response of TiS3 nanoribbons have been investigated. Besides many comprehensive theoretical studies, a lot of experimental studies are avaibale about the synthesis of these materials. In brief, these materials which tackles a contemporary and rapidly developing field, the nanoribbon form and functionalization of them that hold promise for many other applications.