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Öğe Adsorption of CO and O2 on AumCun clusters: First principles calculations(Elsevier Science Sa, 2015) Kadioglu, Y.; Gokoglu, G.; Akturk, O. UzengiThe adsorption of CO and O-2 molecules on Au-n (n = 2-5), Cu-n(n= 2-5), and AumCun (m= 1, 2, 3; 1 = n = 6) clusters is investigated in the framework of density functional theory. The equilibrium atomic geometry and electronic structure of each cluster are determined. It is found that Au-n (n= 2-5) clusters show strong reactivity toward CO, while they do not bind to molecular oxygen. High adsorption energy values can be obtained for odd number of Cu-n clusters for both CO and O-2 adsorptions. Cu top site is energetically preferred for the adsorption of both CO and O-2 molecules in AumCun clusters. The results of electronic density of states show an sp hybridization between Cu and O atoms in O-2 adsorbed AumCun clusters. This interaction plays a major role in their binding. The highest occupied molecular orbital and the lowest unoccupied molecular orbital energy gap values and charge density differences are presented. There are charge transfers from s and d orbital electrons of Cu to p orbitals of C and of O atoms. It is observed that O-2 adsorption on some clusters can induce half metallicity which is an ideal property for possible applications in spintronics. (C) 2015 Elsevier B.V. All rights reserved.Öğe Bimetallic two-dimensional PtAg coverage on h-BN substrate: First-principles calculations(Elsevier, 2014) Ersan, F.; Gokoglu, G.; Akturk, E.This paper presents a study on the coverage of PtAg layer on h-BN 2D system using plane-wave pseudopotential method within density functional theory. There emerge interesting electronic and magnetic properties by the coverage of PtAg on h-BN. FM (ferromagnetic) and AFM (antiferromagnetic) states are considered for PtAg. As the most stable configuration, Pt atom is bound to the top site of N and Ag is adsorbed to hollow site in the (2 x 2) coverage with a binding energy about -1.013 eV. While bare h-BN is nonmagnetic semiconductor with a band gap of 4.58 eV, the band gap becomes 0.18 eV with an AFM semiconductor ground state upon coverage of PtAg adlayer. The electronic structure calculations reveal that the electronic band gap of the composite system is controlled by d-states of Pt atom. The material can have possible applications in spintronics and in catalysis with decreased and engineered band gap. (C) 2014 Elsevier B. V. All rights reserved.Öğe Effects of silver adatoms on the electronic structure of silicene(Elsevier Science Bv, 2014) Ersan, F.; Arslanalp, O.; Gokoglu, G.; Akturk, E.This paper presents the adsorption of Ag adatoms on silicene surface using first-principles plane wave calculations within density functional theory. It is obtained that silver adatoms form strong bonds with the silicene yielding significant binding energies. The bare silicene, which is a nonmagnetic semimetal, becomes either nonmagnetic metal or semiconductor depending on the number of adsorbed silver atoms. Because of the charge transfer from adatoms to silicene, bonding and antibonding pi bands crossing linearly at the Fermi level shift 0.35 eV below the Fermi level for both single and trimer Ag adsorption. Ag dimer adsorbed silicene becomes a narrow gap semiconductor with E-g = 0.112 eV. (C) 2014 Elsevier B.V. All rights reserved.Öğe Electronic structure of BSb defective monolayers and nanoribbons(Iop Publishing Ltd, 2014) Ersan, F.; Gokoglu, G.; Akturk, E.In this paper, we investigate two- and one-dimensional honeycomb structures of boron antimony (BSb) using a first-principles plane wave method within the density functional theory. BSb with a two-dimensional honeycomb structure is a semiconductor with a 0.336 eV band gap. The vacancy defects, such as B, Sb, B + Sb divacancy, and B + Sb antisite disorder affect the electronic and magnetic properties of the 2D BSb sheet. All the structures with vacancies have nonmagnetic metallic characters, while the system with antisite disorder has a semiconducting band structure. We also examine bare and hydrogen-passivated quasi-one-dimensional armchair BSb nanoribbons. The effects of ribbon width (n) on an armchair BSb nanoribbon and hydrogen passivation on both B and Sb edge atoms are considered. The band gaps of bare and H passivated A-Nr-BSb oscillate with increasing ribbon width; this property is important for quantum dots. For ribbon width n = 12, the bare A-Nr-BSb is a nonmagnetic semiconductor with a 0.280 eV indirect band gap, but it becomes a nonmagnetic metal when B edge atoms are passivated with hydrogen. When Sb atoms are passivated with hydrogen, a ferromagnetic half-metallic ground state is observed with 2.09 mu(B) magnetic moment. When both B and Sb edges are passivated with hydrogen, a direct gap semiconductor is obtained with 0.490 eV band gap with disappearance of the bands of edge atoms.Öğe Electronic structure of half-metallic ferromagnet Co2MnSi at high-pressure(Springer, 2010) Gokoglu, G.; Gulseren, O.In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co2MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (sigma-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L2(1) structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around similar to 70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at I-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.Öğe First-principles study of elastic and vibrational properties of Ni2MnIn magnetic shape memory alloys(Springer, 2011) Agduk, S.; Gokoglu, G.We present the results of ab initio calculations of lattice dynamics and the second order elastic stiffness constants of nickel-based magnetic shape memory alloy Ni2MnIn in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (sigma-GGA) scheme of the density functional theory (DFT) is applied. Elastic constants are calculated by tetragonal and monoclinic isochoric strains on cubic L2(1) structure. The calculated elastic constants agree very well with the recent ultrasonic experimental data. Phonon dispersion spectra are investigated within linear response technique of the density functional perturbation theory (DFPT). A vibrational anomaly is observed in phonon spectra at the transverse acoustic mode (TA(2)) in [zeta zeta 0] direction at wavevector zeta = 0.3 as an indication of the structural instability of the system to shear deformation. This anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. Phonon dispersion curves are in excellent agreement with the results of recent neutron diffraction experiments.Öğe Free-standing and supported phosphorene nanoflakes: Shape- and size-dependent properties(Elsevier, 2020) Bakir, M. Y.; Ozaydin, H. D.; Gorkan, T.; Akturk, O. Uzengi; Gokoglu, G.; Akturk, E.; Ciraci, S.The ultra-small sized nanomaterials are important for basic functional components of future nanoelectronics, spintronics and sensor devices. In this study, based on first-principles density functional theory, the free-standing and supported nanoflakes of bare and hydrogen saturated black and blue phosphorene of diverse size and shape have been investigated. Cohesion, formation energy, thermal stability and electronic structure of these nanoflakes have been revealed. For nanoflakes supported by specific substrates, such as phosphorene, graphene and Mos(2) monolayer, the equilibrium configuration and the binding energy of the flakes, as well as the effects of substrate on the electronic structure have been investigated. While the cohesive and formation energies and HOMO-LUMO gaps of nanoflakes with their edges passivated by hydrogen display clear size, shape and edge geometry dependencies, they are rather dispersed in bare nanoflakes. The binding of phosphorene nanoflakes to two-dimensional (2D) phosphorene, graphene and MoS2 monolayers is generally weak and originate from van der Waals interaction. Accordingly, when supported by these monolayers, the electronic structure of free-standing nanoflakes can be preserved for critical applications.Öğe Interactions of h-AlN monolayer with platinum, oxygen, and their clusters(Elsevier, 2015) Ersan, F.; Akcay, A.; Gokoglu, G.; Akturk, E.In this paper, we investigate the adsorption properties of single Pt and O atoms, and PtO, Pt2O, and PtO2 clusters on hexagonal AlN monolayer as well as several substitutions in AlN structure. We employ density functional theory to study electronic structure and charge transfers by considering nonmagnetic and ferromagnetic states. PtO and Pt2O adsorbed AlN system has ferromagnetic ground state with 2.00 mu(B) magnetic moment, while PtO2, Pt, and O adsorption lead to nonmagnetic structures. Pt adsorbed AlN system has the lowest adsorption energy with -3.175 eV indicating the most stable structure energetically. Oxygen atom largely disrupts the AlN layer due to strong N-O repulsion caused by high electronegativities of N and O atoms. The substitution of AlN monolayer with Pt and O atoms also presents interesting features. The various substitutions are able to yield ferromagnetic structures with semiconducting (AlO), metallic (N-Pt), or half-metallic (Al-Pt) ground states. These properties can lead to possible applications in spintronics and nanoelectronics. (C) 2015 Elsevier B.V. All rights reserved.Öğe Mechanical and Electrical Monitoring in the Dynamics of Twisted Phosphorene Nanoflakes on 2D Monolayers(Amer Chemical Soc, 2019) Gorkan, T.; Kadioglu, Y.; Akturk, O. Uzengi; Gokoglu, G.; Akturk, E.; Ciraci, S.We investigated the rotational and translational dynamics of hydrogen-passivated, black phosphorene and blue phosphorene nanoflakes of diverse size and geometry anchored to graphene, black phosphorene, blue phosphorene, and MoS2 monolayer substrates. The optimized attractive interaction energy between each nanoflake and monolayer substrates are harmonic for small angular displacements, leading to libration frequencies. We showed that the relevant dynamical parameters and resulting libration frequencies, which vary with the size/geometry of nanoflakes, as well as with the type of substrate, can be monitored by charging, external electric field, pressure, and also by a molecule anchored to the flake. The optimized energy profiles and energy barriers thereof have been calculated in translational and in large angle rotational dynamics. Owing to the weak interaction between the flakes and monolayers the energy barriers are particularly small for incommensurate systems and can renders nearly frictionless rotation and translation, which is crucial for nanoscale mechanics. Even if small for particular combined nanoflake + monolayer heterostructures, the energy band gaps exhibit variations with angular and linear displacements of nanoflakes. However, these band gaps undergo considerable reduction under pressure. With tunable dynamics, electronic structure, and low friction coefficients, individual or periodically repeating nanoflakes on a monolayer substrate constitute critical composite structures offering the design of novel detectors, nanomechanical, electromechanical, and electronic devices.Öğe Silicon-based counterpart of alpha-graphyne(Elsevier Science Bv, 2014) Akturk, E.; Gokoglu, G.We present the first principles density functional calculations of electronic structure and energetics of silicon-based counterpart of alpha-graphyne, labeled as alpha-silicyne. Both LDA and GGA functionals are applied for exchange-correlation potentials. We show that graphyne-like silicon in 2D buckled structure (equilibrium buckling delta z congruent to 0.73 and Delta z congruent to 1.45 A) has approximate to 2.33 eV and approximate to 1.96 eV lower energies than planar geometry for GGA and LDA functionals, respectively. The single and triple bond lengths of silicon are consistent with previously reported values. As a different case from graphyne, which is semimetallic, the electronic band structures of buckled a-silicyne do not show Dirac fermion indicating a metallic nature. The metallic character of the system is largely determined by p-electronic states of the triple bonded silicon atoms. (C) 2014 Elsevier B.V. All rights reserved.Öğe Theoretical investigation of lithium adsorption, diffusion and coverage on MX2 (M = Mo, W; X = O, S, Se, Te) monolayers(Elsevier, 2017) Ersan, F.; Ozaydin, H. D.; Gokoglu, G.; Akturk, E.It is important to improve the high-efficient anode materials for Li batteries, which require the large capacity, high stability and mobility. In this work, we present the adsorption and diffusion properties of lithium atom on MX2 (M = Mo, W; X = O, S, Se, Te) transition metal dichalcogenide structures using first principles calculations within density functional theory. All the MX2 systems considered are semiconductor in bare state with band gaps between 0.93 eV (MoO2) and 1.79 eV (WS2). They turn into metal upon single Li adsorption. Li atom is adsorbed on MoO2 and WO2 rather stronger than other systems. The energy barrier for diffusion of single Li on MX2 varies between 0.15 eV and 0.28 eV which are lower or comparable to that of graphene or silicene. Two Li atoms are preferably adsorbed on MX2 monolayer symmetrically at opposite sides with high adsorption energy. The increasing number of Li atoms does not remarkably affect the adsorption energy per Li atom. This can be attributed to that Li atoms do not accumulate on certain regions of the surface. The systems under investigation provide insights into exploring electronic properties which are rather adequate for possible applications in Li-ion batteries. (C) 2017 Elsevier B.V. All rights reserved.Öğe Two dimensional ruthenium carbide: structural and electronic features(Royal Soc Chemistry, 2020) Gorkan, T.; Demirci, S.; Jahangirov, S.; Gokoglu, G.; Akturk, E.The design and realization of novel 2D materials and their functionalities have been a focus of research inspired by the successful synthesis of graphene and many other 2D materials. In this study, in view of first principles calculations, we predict a novel 2D material ruthenium carbide (RuC) in graphene-like honeycomb hexagonal lattice with planar geometry. Phonon dispersion spectra display a dynamically stable structure. Comprehensive molecular dynamics calculations confirm the stability of the structure up to high temperatures as approximate to 1000 K. The system is a narrow gap semiconductor with a band gap of 53 meV (345 meV) due to GGA-PBE (HSE) calculations. Band gap exhibits significant changes by applied strain. Elastic and optical properties of the system are examined in monolayer form. RuC/RuC bilayer, RuC/graphene and RuC/h-BN heterostructures are also investigated. By calculating the phonon dispersion it is verified that RuC bilayer is the most stable in AA type-stacking configuration where Ru and C atoms of both layers have identical lateral coordinates. The effects of atomic substitutions on electronic band structures, acting as p-type and n-type doping, are revealed. A novel 3D RuCLi structure is also predicted to be stable and the isolation of its monolayer forms are discussed. Ruthenium carbide, as a 2D material which is dynamically and thermally stable, holds promise for applications in nanoelectronics.