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Identification of Single-Layer Crystalline Structures Through Their Electronic and Optical Properties
(01. Izmir Institute of Technology, 2021) Sözen, Yiğit; Şahin, Hasan; Balcı, Sinan
A large number of two-dimensional (2D) van der Waals type materials have become a focus of interest in many scientific fields, ever since the thinnest carbon compound, graphene, takes to the stage with its exceptional electronic properties. The outstanding electronic behavior resulting from quantum size effects requires an investigation of the electronic and optical features of materials at the atomic scale. The understanding of such properties of matter within the framework of the theoretical approaches is the first step to shed light on the discovery of electronic and optoelectronic devices including brand new features. This thesis discusses the identification of electronic and optical properties of several types of atomically thin crystals, consisting of 2D and lead-free perovskite structures, by means of density functional theory (DFT). In the first study, primarily, the strong interaction mechanism between Ge atom and single-layer GaAs was studied starting from single atom adsorption to detached germanene layer formation. Following that dynamically stable metallic structures of Janus and alloy type GaGeAs crystals are discovered by performing one-sided and alternated decoration of GaAs single-layer with Ge atoms, respectively. %The Raman spectroscopy is found to be applicable for phase detection as the theoretically calculated Raman spectra of each polytype exhibit distinctive signals. In the second study, bulk and dynamically stable ultra-thin structures of lead-free CsMnCl$_3$ are discussed. According to total energy and electronic band structure calculations, bulk, bilayer, and single-layer structures of CsMnCl$_3$ are robust antiferromagnetic insulators. In third and fourth chapters are devoted to the identification of different stacking types of GaP/GaSe heterobilayers, and two different hexagonal phases of single-layer Germanium Oxide by means of electronic and optical characterization tools, respectively. In these studies, for the purpose of providing an accurate solution for the prediction of absorption, reflectance, and transmission spectra of materials, excitonic effects are considered by employing Bethe-Salpeter formalism following the $G_0W_0$ approximation. Wide range of atomically thin crystal structures studied within the framework of this thesis are verified to be promising candidates for the development of future nano-sized electronic and optoelectronic device applications thanks to their attractive electronic and optical properties arising from strong quantum confinement effects.
Graphene-Like Materials for Electronic Applications
(01. Izmir Institute of Technology, 2020) Başkurt, Mehmet; Şahin, Hasan; Balcı, Sinan
Two-dimensional (2D) materials have gained vast interest in nanotechnology since these materials exhibit extraordinary properties due to electron confinement. Starting with graphene, many other 2D materials with characteristics of metals, semiconductors, insulators, and their magnetic analogues have been studied over the years. Insulators show importance as dielectric layers. Low dimensional metallic materials are used in electrical conduction. Ultra-thin semiconductors have variety of potential applications due to their characteristic band gap. Magnetic analogues of low dimensional materials are used in spintronics, offering high frequency, controllable switching. In addition, defects in these materials alter their physical properties and the concept can be adopted in order to use in different practices. Therefore it is important to study array of such materials and consider the alteration in their lattice theoretically and experimentally. In this thesis, first-principles calculations are used to predict insulating calcium halide single-layers are predicted, determine the effects of strain and V dopant in recently synthesized magnetic semiconducting VI3 single-layers, propose synthesis of magnetic, semiconducting manganese fluorides from manganese dichalcogenides, investigate the affects of defects and simulate scanning tunneling microscopy images in order to compare with experimental results, and finally to determine rather the detection of volatile organic compounds (VOC) such as methanol and ethanol by graphene-based sensors is feasible or not. Experiments are carried out to construct and further investigate the mechanism of VOC detection and working, highly sensitive alcohol sensors.

Master Degree / Yüksek Lisans Tezleri

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