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Item Open Access Approximate computation of DFT without performing any multiplications: application to radar signal processing(IEEE, 2014) Arslan, Musa Tunç; Bozkurt, Alican; Sevimli, Rasim Akın; Akbaş, Cem Emre; Çetin, A. EnisShow more In many radar problems it is not necessary to compute the ambiguity function in a perfect manner. In this article a new multiplication free algorithm for approximate computation of the ambiguity function is introduced. All multiplications (a × b) in the ambiguity function are replaced by an operator which computes sign(a × b)(a + b). The new transform is especially useful when the signal processing algorithm requires correlations. Ambiguity function in radar signal processing requires high number of correlations and DFT computations. This new additive operator enables an approximate computation of the ambiguity function without requiring any multiplications. Simulation examples involving passive radars are presented.Show more Item Open Access Characterization of two-dimensional Ga1−xAlxN ordered alloys with varying chemical composition(Elsevier, 2019) Kanlı, Muammer; Önen, Abdullatif; Mogulkoc, A.; Durgun, EnginShow more Similar to bulk semiconductors, alloying suggests a promising strategy to tailor the fundamental properties oftwo-dimensional (2D) systems with constituent composition. In that sense, detailed understanding of atomicstructure and stability analysis are required to predict and design new 2D alloys. In this paper, we analyze thestructural, mechanical, electronic, thermal, and optical properties of monolayer Ga1−xAlxN ordered alloys forvarying concentration by usingab initiomethods. Following the determination of ground state geometries bytaking into account the possibility of segregation, we investigate the stability of the considered structures byphonon spectrum analysis and high temperature molecular dynamics calculations. Our results indicate that theproperties of 2D Ga1−xAlxN can be modified continuously by controlling the Al concentration. Tunability of thedesired properties broadens the possible usage of 2D semiconductors in nanoscale applications.Show more Item Open Access Cubane cluster surface for pyrimidine nucleobases relaxation: DFT approach(Danishgah-i Azad-i Islami, 2020-12-27) Mirzaei, M.; Hadipour, N.; Gülseren, OğuzShow more Density functional theory (DFT) approach was employed to investigate relaxation processes of each of pyrimidine nucleobases (NBs); cytosine (C), thymine (T) and uracil (U), at the Cubane Cluster Surface (CCS). The main idea was about providing a material for recognition of NBs, in which a nanostructure form of cubane (CCS) was first generated by optimization process. In the next step, relaxation processes of each of NBs at the surface were investigated to examine the function of such system for NBs recognition. The results indicated that the electronic based molecular properties could work as proper parameters for recognizing such molecular system, in which energy gap (EG) could be referred for the purpose. Measuring EG could help to recognize the complexes of CCS-C, CCS-T and CCS-U from each other. Strength of such complex formations was investigated using values of binding energy (BE); CCS-U > CCS-C > CCS-T. Total results of EG, BE and additional atomic scale properties indicated that the investigated CCS could work very well to recognize U as the characteristic NB of RNA.Show more Item Open Access Does the donor-acceptor concept work for designing synthetic metals? III. theoretical investigation of copolymers between quinoid acceptors and aromatic donors(2006) Salzner, U.; Karaltı, O.; Durdaǧi, S.Show more Homopolymers of quinoxaline (QX), benzothiadiazole (BT), benzobisthiadiazole (BBT), thienopyrazine (TP), thienothiadiazole (TT), and thienopyrazinothiadiazole (TTP) and copolymers of these acceptors with thiophene (TH) and pyrrole (PY) were investigated with density functional theory. Theoretical band-gap predictions reproduce experimental data well. For all but six copolymers, band-gap reductions with respect to either homopolymer are obtained. Four of the acceptors, BBT, TP, TT, and TTP, give rise to copolymers with band gaps that are smaller than that of polyacetylene. BBT and TTP copolymers with PY in 1:2 stoichiometry are predicted to be synthetic metals. Band-gap reductions result from upshifts of HOMO energies and much smaller upshifts of LUMO values. The smallest band gaps are predicted with TTP, since changes in LUMO energies upon copolymerization are particularly small. The consequence of the small interactions between LUMO levels of donor and acceptor are vanishingly small conduction bandwidths. © Springer-Verlag 2006.Show more Item Open Access Electronic properties of graphene nanoribbons doped with zinc, cadmium, mercury atoms(Elsevier B.V., 2018) Ömeroğlu, O.; Kutlu, E.; Narin, P.; Lisesivdin, S. B.; Özbay, EkmelShow more The effect of substitutional impurities as Zinc (Zn), Cadmium (Cd) and Mercury (Hg) on electronic properties of graphene nanoribbons (GNRs) was investigated by using Density Functional Theory (DFT). A substantial change in the electronic properties of GNR structures was observed while changing the position of dopant atom from the edge to the center of armchair graphene nanoribbons (AGNRs) and zigzag graphene nanoribbons (ZGNRs). The calculations are shown that the electronic band gap of GNRs can be controlled depending on the position of dopant atoms. The calculated electronic band structures for both AGNRs and ZGNRs show spin-dependent metallic or semiconductor behavior according to the position of dopant atoms. From the Density of States (DOS) information, quasi-zero-dimensional (Q0D) and quasi-one-dimensional (Q1D) type behaviors are observed. It is shown that because the doped ZGNRs had the lowest total energies, ZGNRs are energetically more stable than AGNRs.Show more Item Open Access Electronic properties of Li-doped zigzag graphene nanoribbons(Elsevier B.V., 2016) Narin, P.; Kutlu, E.; Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Özbay, EkmelShow more Zigzag graphene nanoribbons (ZGNRs) are known to exhibit metallic behavior. Depending on structural properties such as edge status, doping and width of nanoribbons, the electronic properties of these structures may vary. In this study, changes in electronic properties of crystal by doping Lithium (Li) atom to ZGNR structure are analyzed. In spin polarized calculations are made using Density Functional Theory (DFT) with generalized gradient approximation (GGA) as exchange correlation. As a result of calculations, it has been determined that Li atom affects electronic properties of ZGNR structure significantly. It is observed that ZGNR structure exhibiting metallic behavior in pure state shows half-metal and semiconductor behavior with Li atom.Show more Item Open Access Electronic properties of zigzag ZnO nanoribbons with hydrogen and magnesium passivations(Elsevier, 2018) Abbas, J. M..; Narin, P.; Kutlu, E.; Lisesivdin, S. B.; Özbay, EkmelShow more In this study, the electronic properties of ZnO nanoribbons with zigzag edges (ZZnONr) have been investigated with Density Functional Theory (DFT). After a geometric optimization, the electronic band structures, the density of states (DOS) of ZZnONr passivated with Hydrogen (H) and Magnesium (Mg) atoms were calculated ZZnONr. It is shown that the increasing width of ZZnONrs has led to a decrement in energy band gap of the studied structures. While ZZnONr passivated with Mg for Zn-rich edge have not been shown a spin dependency, the structure passivated with Mg for O-rich edge have exhibited spin-dependent band structure. The energetically most stable structures have been determined as ZZnONr passivated with Mg for Zn-rich edge. ZZnONr passivated with Mg atoms for both edges have a graphene-like band structure especially for 8 and 10 atom width structures and this property of ZZnONrs could be important in terms of the electron transport for ZZnONrs.Show more Item Open Access Electronic structure of β-Si3N4 crystals with substitutional icosagen group impurities(National Institute of Optoelectronics, 2017) Kutlu, E.; Narin, P.; Atmaca, G.; Sarıkavak-Lişesivdin, B.; Lişesivdin, S. B.; Özbay, EkmelShow more The β-Si3N4 crystals are widely used in industrial and electronics areas. Therefore, β-Si3N4 has drawn the attention of researchers for many years. In this study, effects of icosagen group impurity atoms in the IIIA group on the electronic properties of the β-Si3N4 crystal were analyzed by using the density functional theory. As a result of these analyses, it was determined that the electronic properties of the crystal change significantly. Basic electronic characteristics for pure β-Si3N4 crystal and icosagen group impurity β-Si3N4 crystals, such as band structures, densities of states, binding energies, and formation energies were investigated. We identified that the band gap of the β-Si3N4 crystal was affected significantly by the impurity, and this change was varying linearly in line with the formation energy for the impurity cases. As a result of calculations, the Al-impurity was found to be the lowest-energy impurity state.Show more Item Open Access First principles calculations of electronic and optical properties of InSe nanosheets doped with noble metal atoms(Elsevier, 2023-04-05) Narin, P.; All, J. M. Abbas; Kutlu, E. Narin; Lisesivdin, S.B.; Özbay, EkmelShow more Monolayer Indium Selenide (ML-InSe) is studied for 4x4 supercell structure through ab initio calculations. The electronic and optical properties of ML-InSe for both pristine and substitutional doped ML-InSe with Palladium (Pd), Platinum (Pt), Silver (Ag), and Gold (Au) atoms have been calculated. With substitutional doping, ML-InSe has been observed to have a spin-dependent electronic structure. The flat energy bands near the Fermi level are observed in ML-InSe with doping elements placed in In site. The flat bands of d orbitals of some noble metal atoms are formed by the projected density of states (PDOS). The PDOS calculations show that the s-orbital of In and p-orbital of Se form the conduction band edge. The energetically favorable position of doping atoms is found to be the PtIn substitution atom according to formation energy calculations. For each studied structure, the bond length of the first neighbor of doping atoms in doped ML-InSe, static dielectric constant (ε0), refractive index, and energy band gap have been calculated. In the structure ML-InSe with AuSe, ε0 reaches ∼ 8.15. Another important result is that substitutional doping induces some peaks in the lower energy region of the imaginary part of the dielectric function. These peaks mainly refer to the absorption in related regions and may be important for the optoelectronic properties of ML-InSe.Show more Item Open Access A first principles investigation of the effect of aluminum, gallium and indium impurities on optical properties of β-Si3N4 structure(Elsevier GmbH, 2017) Narin, P.; Kutlu, E.; Atmaca, G.; Lişesivdin, S. B.; Özbay, EkmelShow more In this study, effects of some impurity atoms included in IIIA group such as Al, Ga, and In on the optical properties of the β-Si3N4 structure have been discussed. The calculations were made using Density Functional Theory (DFT) in 0–15 eV range and local density approximation (LDA) as the exchange-correlation. Using the real and the imaginary parts of the complex dielectric function, the basic optical properties of β-Si3N4 such as dielectric coefficient, refractive index, absorption, reflection coefficients have been investigated. As a result of the calculations, it is determined that optical properties of structure have been significantly changed with doping.Show more Item Open Access First-principles investigation of functionalization of graphene(Bilkent University, 2013) Korkmaz, YaprakShow more The graphene sheet is a single-atom thick novel material and attracts great interest due to its unique features. However, it is a metallic material with no bandgap, which makes it difficult to integrate in electronic applications. Adatom adsorption is one of the promising ways to make this structure functional. To this end, electronic and structural properties of graphene have been investigated by using density functional theory formalism in order to understand atomic level interaction between halogen adatoms and graphene layer. The most common adatom, hydrogen, has also been studied. In this study, plane-wave, pseudopotential density functional theory calculations were carried out using generalized gradient approximations for the exchange correlation potential with the Quantum Espresso package. In order to obtain fully relaxed structures, geometry optimization has been performed in all of the calculations. The adatom-graphene system is modelled with a 4 × 4 graphene supercell. Adsorption energies of halogen adatoms and dimers adsorbed on highly symmetric positions on graphene layer are calculated. Different configurations of adatoms have been tested. Specific properties such as band structure and density of states of these system have been investigated. The results show that a fully covered graphene layer is stable and optimized structures exhibit a band gap of a few eV. The most stable structure among halogen adatoms is the fluorine adsorbed on graphene. It has the highest electronegativity, which is the reason for high electron transfer from the graphene layer. This is the reason of the formation of covalent bonds. Furthermore, the most stable configuration is found to be chair configuration with the halogen atoms alternating in both sides of the layer.Show more Item Open Access Investigation of new polymorphs of borophene and their functionalization(Bilkent University, 2017-12) Khanifaev, JamoliddinShow more The realization of buckled monolayer sheets of boron (i.e., borophene) and its other polymorphs has attracted signi cant interest in the eld of two-dimensional systems. Motivated by their chameleonic behavior we analyzed di erent polymorphs of borophene and discovered two new phases with unprecedented crystal structures namely symmetric washboard and asymmetric washboard using ab initio methods based on density functional theory. While symmetric washboard borophene is a metal with high electronic density of states in the vicinity of Fermi level asymmetric washboard borophene is a narrow band gap semiconductor. Asymmetric washboard structure is actually a 2 1 reconstructed form of symmetric structure with in plane and out of plane Peierl's distortion along the chains of boron atoms which is the key reason for the contrasting electronic behavior of these phases. Phonons dispersion calculations based on density functional perturbation theory reveal that both structures are stable at 0 K however ab ini- tio molecular dynamics simulations showed that symmetric washboard structure is stable only at temperatures close to absolute zero and at nite temperatures this structure gets deformed transforming into asymmetric washboard structure. Moreover we discovered that asymmetric washboard structure has a positive Poissons's ratio however symmetric one has a negative Poisson's ratio. In the next work, motivated by buckled borophene's tendency to donate electrons, we analyzed the interaction of single halogen atoms (F, Cl, Br, I) with borophene. The possible adsorption sites are tested and the top of the boron atom is found as the ground state con guration. The nature of bonding and strong chemical interaction is revealed by using projected density of states and charge di erence analysis. The migration of single halogen atoms on the surface of borophene is analyzed and high di usion barriers that decrease with atomic size are obtained. The metallicity of borophene is preserved upon adsorption but anisotropy in electrical conductivity is altered. The variation of adsorption and formation energy, interatomic distance, charge transfer, di usion barriers, and bonding character with the type of halogen atom are explored and trends are revealed. Lastly, the adsorption of halogen molecules (F2, Cl2, Br2, I2), including the possibility of dissociation, is studied. The obtained results are substantial for fundamental understanding and possible device implementations of borophenes and their halogenated derivatives.Show more Item Open Access Investigation of new two-dimensional materials derived from stanene(Elsevier, 2017-09) Fadaie, M.; Shahtahmassebi, N.; Roknabad, M. R.; Gulseren, O.Show more In this study, we have explored new structures which are derived from stanene. In these new proposed structures, half of the Sn atoms, every other Sn atom in two-dimensional (2D) buckled hexagonal stanene structure, are replaced with a group- IV atom, namely C, Si or Ge. So, we investigate the structural, electronic and optical properties of SnC, SnGe and SnSi by means of density functional theory based first-principles calculations. Based on our structure optimization calculations, we conclude that while SnC assumes almost flat structure, the other ones have buckled geometry like stanene. In terms of the cohesive energy, SnC is the most stable structure among them. The electronic properties of these structures strongly depend on the substituted atom. We found that SnC is a large indirect band gap semiconductor, but SnSi and SnGe are direct band gap ones. Optical properties are investigated for two different polarization of light. In all structures considered in this study, the optical properties are anisotropic with respect to the polarization of light. While optical properties exhibit features at low energies for parallel polarization, there is sort of broad band at higher energies after 5 eV for perpendicular polarization of the light. This anisotropy is due to the 2D nature of the structures. © 2017 Elsevier B.V.Show more Item Open Access Nanocarbon-assisted biosensor for diagnosis of exhaled biomarkers of lung cancer: DFT approach(Sami Publishing Company, 2021-03) Mirzaei, M.; Gülseren, Oğuz; Rafienia, M.; Zare, A.Show more Density functional theory (DFT) calculations were performed to investigate a nanocarbon-assisted biosensor for diagnosis of exhaled biomarkers of lung cancer. To this aim, an oxidized model of C20 fullerene (OC) was chosen as the surface for adsorbing each of five remarkable volatile organic compounds (VOC) biomarkers including hydrogen cyanide, methanol, methyl cyanide, isoprene, and 1-propanol designated by B1-B5. Geometries of the models were first optimized to achieve the minimum energy structures to be involved in further optimization of B@OC bi-molecular complexes. The relaxation of B counterparts at the surface of OC provided insightful information for capability of the investigated system for possible diagnosis of such biomarkers. In this case, B1 was placed at the highest rank of adsorption to make the strongest B1@OC complex among others whereas the weakest complex was seen for B4@OC complex. The achievement was very much important for differential detection of each of VOC biomarkers by the investigated OC nanocarbon. Moreover, the recorded infrared spectra indicated that the complexes could be very well recognized in complex forms and also among other complexes. As a final remark, such proposed nanocarbon-assisted biosensor could work for diagnosis of remarkable VOC biomarkers of lung cancer.Show more Item Open Access Novel honeycomb nanostructures for energy storage and nanoscale device design(Bilkent University, 2015-06) Özçelik, Veli OngunShow more This thesis presents a variety of new two dimensional honeycomb-like structures and heterostructures; the main objective being to determine their fundamental electronic, magnetic, mechanical and optical properties for new device and material design. Utilization of existing two dimensional materials for nanoscale device design, understanding the fundamental properties of their composite structures, explaining the existing data on known two dimensional materials and using computational simulations to discover new materials are the main concerns of this thesis. We begin by assessing the validity of density functional theory on monolayer composites of graphene and boron nitride. We show that it is possible to grow vertical graphene / boron nitride heterostructures on top of each other and reveal the growth mechanisms at the atomistic level. We then utilize this vertical heterostructure for a nanoscale capacitor design by applying an external electric eld. We test and show how rst principles methods can be used to investigate the properties of materials under electric eld. After explaining the reliable methods, capacitance values are calculated for the model for various thicknesses, which show quantum mechanical size e ects at small separations that recede as the separations get larger; as the later is con rmed by experimental observations. The next part of the thesis, investigates the electronic properties of lateral graphene / boron nitride heterostructures, and show how these composites act di erently depending on the concentrations of graphene and boron nitride in the composite system. Namely, di erent behaviors of alloys, -doping and line compounds are revealed. Following this, these lateral heterostructures are utilized as nanoscale planar capacitors for atomically thin circuitry. As a nal remark on carbon and boron nitride nanocomposites, the next chapter of this thesis describes the growth mechanisms of one dimensional carbon/ boron nitride short atomic chains and show their stabilities at elevated temperatures. The electronic and magnetic properties of these chains exhibit even/odd disparity depending on the number of atoms in the chain. These chains also construct another two dimensional allotrope of graphene, namely graphyne, when connected to each other on the same plane. The properties of graphyne and its boron nitride analogue described in the following chapter introduces a new monolayer allotrope of carbon and boron nitride. The following chapter turns to silicon and germanium analogue of graphene, silicene and germanene. Dumbbell type reconstructions of silicene and germanene are introduced, which lead to layered silicene and germanene. Dumbbell units introduced here form the fundamental building blocks of experimentally observed layered silicene and germanene. The last chapter of the thesis looks at new material design and prediction studies based on computational simulations. Oxygenated silicene leads to a new monolayer piezoelectric material called silicatene. Finally, the monolayer structures of Group V elements nitrogen and antimony are also shown to be stable by phonon calculations and high temperature molecular dynamics simulations.Show more Item Open Access Stability of two dimensional (2D) structures based on GaAs(Bilkent University, 2015-08) Erol, MustafaShow more Graphene is a two dimensional material isolated for the first time in 2004. After this, two dimensional materials has become an appealing research area for the scientists because of their exotic properties. In search for two dimensional materials, both experimental and theoretical investigations have been carried out. First-principles approaches have been used to predict silicene and germane theoretically. A technologically important semiconductor material is GaAs, however there is no report of two dimensional materials which is based on GaAs. We attempted to find a new stable 2D structure which is formed from either Ga and As atoms based on GaAs or its functionalized form with O atoms. In search for such a system, we performed density functional theory based calculations by using a plane-wave pseudopotential method. We used local density approximation for the exchange correlation potential. First, we performed geometrical optimization calculation in order to identify possible stable structures. We obtained electron band diagrams and phonon dispersion relations to check electronic properties and stability of these materials. We found three structures which are based on GaAs (100), (110) and (110) surfaces. We found that these two dimensional materials are geometrically stable but after performing phonon calculations we observe that there are some negative energy modes. In addition we identified one system which is based on Ga, As, and O atoms. Even though this structure is stable after geometry optimization, it has negative phonon modes in its phonon band diagrams.Show more Item Open Access Thickness dependence of solar cell efficiency in transition metal dichalcogenides MX2 (M: Mo, W; X: S, Se, Te)(Elsevier, 2020) Özdemir, Burak; Barone, V.Show more Bulk transition metal dichalcogenides are indirect gap semiconductors with optical gaps in the range of 0.7–1.6 eV, which makes them suitable for solar cell applications. In this work, we study the electronic structure, optical properties, and the thickness dependence of the solar cell efficiencies of MX2 (M: Mo, W; X: S, Se, Te) with density functional theory and GW + BSE. Through this analysis, we find a change in solar cell efficiency trends at slab thicknesses of 3 μm. For thin films solar cells (thicknesses smaller than 3 μm), the tellurides present the highest efficiencies (about 20% for a 100 nm thick slab). In contrast, for thicknesses greater than 3 μm, our results indicate that a maximum solar cell efficiency can be achieved in WS2. For instance, a 100 μm slab of WS2 presents a solar cell efficiency of 36.3%, making this material a promising candidate for solar cell applications.Show more