Yazar "Gokoglu, Gokhan" seçeneğine göre listele
Listeleniyor 1 - 20 / 25
Sayfa Başına Sonuç
Sıralama seçenekleri
Öğe Ab initio electronic structure of NiCoCrGa half-metallic quaternary Heusler compound(Elsevier, 2012) Gokoglu, GokhanIn this study, ab initio calculation results of electronic structure and elastic properties of NiCoCrGa quaternary Heusler compound are presented. Plane wave pseudopotential method is used with spin-polarized Generalized Gradient Approximation (sigma-GGA) scheme of the Density Functional Theory (DFT). Static elastic constants of the cubic system satisfy mechanical stability criteria. The cubic phase of the system remains stable under tetragonal distortion. The spin-polarized electronic band structures and density of electronic states indicate a metallic band structure for majority spins, while minority spin structure has semiconducting character. This situation displays a slightly disturbed half-metallic behavior with high-spin polarization ratio (P = 0.961) at Fermi level E-F. Two electronic bands of minority spins resulting from d-states of cobalt atom cross Fermi level at Gamma-point. This situation gives a finite but very low density of states at E-F. The material can be classified as a new half-metallic ferromagnet for spintronic applications. (C) 2012 Elsevier Masson SAS. All rights reserved.Öğe Ab initio lattice dynamics of Ni2MnX (X = Sn, Sb) magnetic shape memory alloys(Elsevier Science Sa, 2012) Agduk, Savas; Gokoglu, GokhanIn this study, we present the results of first principles calculations of elastic constants and phonon properties of nickel-manganese based magnetic shape memory compounds Ni2MnSn and Ni2MnSb 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 is applied. In investigation of the phonon dispersion spectra, linear response technique of the Density Functional Perturbation Theory is used. Phonon softening is observed in dispersion spectra at the transverse acoustic mode (TA(2)) in [zeta zeta 0] direction as an indication of the structural instability of these systems to shear deformation. The vibrational instability of Ni2MnSb system is larger than that of Ni2MnSn yielding negative phonon frequencies. This vibrational anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. The minority spin Fermi surfaces of both systems exhibit strong nesting features. (C) 2011 Elsevier B. V. All rights reserved.Öğe Above Room Temperature Ferromagnetism in Gd2B2 Monolayer with High Magnetic Anisotropy(Amer Chemical Soc, 2020) Gorkan, Taylan; Vatansever, Erol; Akinci, Umit; Gokoglu, Gokhan; Akturk, Ethem; Ciraci, SalimThe realization of 2D ultrathin crystals with a ferromagnetic ground state that is sustainable at room temperature has been a real challenge now. By combining ab initio density functional theory with Monte Carlo simulations, we predicted a new 2D structure, Gd2B2 monolayer, which maintains its mechanical stability at elevated temperatures. More remarkably, it has a ferromagnetic ground state with high permanent magnetic moment, which persists far above room temperature. It exhibits high magnetocrystalline anisotropy along particular directions. We find also that both its magnetic anisotropy and Curie temperature can largely be altered by applied strain providing an excellent magnetoelastic tunability. This novel 2D crystal with high magnetic moment and Curie temperature combined with high structural and thermal stability can offer critical applications in magnetoelectronics.Öğe Adsorption and Diffusion of Lithium on Monolayer Transition Metal Dichalcogenides (MoS2(1-x)Se2x) Alloys(Amer Chemical Soc, 2015) Ersan, Fatih; Gokoglu, Gokhan; Akturk, EthemOn the basis of first-principles plane-wave calculations, we examine the adsorption and diffusion of lithium on the hexagonal MoS2(1-x)Se2x monolayers with variation of x for 0.00, 0.33, 0.50, 0:66, and 1.00. We find that the lowest energy adsorption positions of Li adatom is at the top site of Mo atom in both MoS2 and MoSe2 monolayers, while Li moves through the Mo-S bond for MoS2(1-x)Se2x. While bare MoS2(1-x)Se2x compounds are nonmagnetic semiconductor and its energy band gap varies with x, they can be metallized by Li adsorption. NEB calculation results show that their energy barriers make them suitable for using in electrode materials. The lithium adsorption energy is sensitive to the external strain, when we elongate the lattice constants, the adsorption energy decreases quickly. We also examine the penetration energy barrier for single lithium atom to pass through the MoS2(1-x)Se2x monolayers; this barrier is, decreasing from similar to 2.5 eV to similar to 1.3 eV with increasing selenium concentration.Öğe Adsorption of alkali and alkaline-earth metal atoms on stanene: A first-principles study(Elsevier Science Sa, 2016) Kadioglu, Yelda; Ersan, Fatih; Gokoglu, Gokhan; Akturk, Olcay Uzengi; Akturk, EthemThis paper presents a study on the adsorption of alkali and alkaline-earth metal atoms on single-layer stanene with different levels of coverage using first-principles plane wave calculations within spin polarized density functional theory. The most favorable adsorption site for alkali atoms (Li, Na, K) were found to be the hollow site similar to other group IV single-layers, but the case of alkaline-earths on stanene is different from silicene and germanene. Whereas Mg and Ca are bound to stanene at hollow site, the bridge site is found to be energetically favorable for Be adatom. All adsorbed atoms are positively charged due to the charge transfer from adatom to stanene single-layer. The semimetallic bare stanene become metallic except for Be adsorption. The Beryllium adsorption give rise to non-magnetic semi-conducting ground state. Our results illustrate that stanene has a reactive and functionalizable surface similar to graphene or silicene. (C) 2016 Elsevier B.V. All rights reserved.Öğe Effect of adatoms and molecules on the physical properties of platinum-doped and -substituted silicene: A first-principles investigation(Elsevier Science Bv, 2016) Ersan, Fatih; Arslanalp, Ozlem; Gokoglu, Gokhan; Akturk, EthemIn this study, we investigate the adsorption properties of Pt-doped and-substituted silicene by means of first-principles computations. The Pt adsorption and substitution on silicene are able to change the electronic structure remarkably. We find that Pt atom prefers to bind silicene on hollow site with high adsorption energy -5.815 eV. For the substitution of single Pt atom in silicene lattice, 4.401 eV energy is required. Both of the systems have good catalytic properties and have a potential for sensor technology. Especially, catalytic oxidation of CO on Pt-embedded silicene is investigated and it is found that Eley-Rideal (ER) mechanism with a two step route is a suitable way to produce CO2 from CO and activated O-2. (C) 2016 Elsevier B.V. All rights reserved.Öğe Effects of gold based dimers on structural and electronic properties of MoS2(Elsevier, 2017) Kadioglu, Yelda; Gokoglu, Gokhan; Akturk, Olcay UzengiIn view of first principles calculations, we investigate the electronic structure redecoration of monolayer MoS2 upon adsorptions of AuAg, AuPt, AuPd, AuCu, and AuAl bimetallic dimers. Geometrical structure, band structures, electronic density of states, charge density differences of dimer adsorbed MoS2 systems are presented and discussed. All the systems studied have non-magnetic ground states. Charge transfers occur from dimer to surface except for AuPt adsorption. Our results indicate that the semiconductor MoS2 maintains its semiconductor character with decreased band gaps upon AuAg, AuCu, and AuAl adsorptions. However, MoS2 shows metallic behaviour by AuPt and AuPd adsorptions, so Pt-d and Pd-d states cross Fermi level yielding metallic character. AuPt adsorbed system has the highest Eads value of 3.15 eV indicating the most stable structure energetically among the dimer adsorbed MoS2 systems considered. (C) 2016 Elsevier B.V. All rights reserved.Öğe Electronic structure and magnetic properties of PbMO3 (M = Fe, Co, Ni) magnetic perovskites: An ab initio study(World Scientific Publ Co Pte Ltd, 2014) Erkisi, Aytac; Yildirim, Erdem Kamil; Gokoglu, GokhanWe present the electronic, magnetic and structural properties of the magnetic transition metal oxides PbMO3 (M= Fe, Co, Ni) in cubic perovskite structure. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and local spin density approximation (LSDA) of the exchange-correlation functional. Onsite Coulomb interaction is also included in calculations (LSDA + U). The systems are considered in ferromagnetic (FM) and G-type antiferromagnetic (G-AFM) order. FM structures are energetically more favored than G-AFM and than non-magnetic states for all the systems studied. The spin-polarized electronic band structures show that all the structures have metallic property in FM order without Hubbard-U interaction (U-eff = 0). However, the inclusion of on-site Coulomb interaction (U-eff = 7 eV) opens a semiconducting gap for majority spin channel of PbFeO3 and of PbNiO3 resulting in a half-metallic character. PbCoO3 system remains as metallic with LSDA + U scheme. Bonding features of all structures are largely determined by the hybridizations between O-p and d-states of transition metal atoms. The partial magnetic moment of Fe atom in PbFeO3 is enhanced by inclusion of Hubbard-U interaction (2.55 mu B double right arrow 3.78 mu B). Total magnetic moments of half-metallic PbFeO3 and of PbNiO3 compounds are very close to integer values.Öğe Electronic structure and surface properties of cubic perovskite oxide BaMnO3(Elsevier, 2011) Gokoglu, Gokhan; Yildirim, HasanWe present the electronic, magnetic, and structural properties of the cubic perovskite oxide BaMnO3 in both bulk and surface geometry. BaMnO3 is reported as keeping the cubic phase even at low temperatures. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and spin-polarized Generalized Gradient Approximation (GGA) of the exchange-correlation functional. The systems studied are treated in ferromagnetic order. The structures of electronic bands and density of states of the systems show half-metallic behavior in both bulk and BaO- and MnO2-terminated (001) surfaces of E2(1) structure. The calculated magnetic moment of bulk structure is 3.00 mu(B), which is largely conserved at surface geometries. Average surface and relaxation energies are also calculated. The rumpling of atoms in relaxed surfaces is determined. It is seen that the relaxation of oxygen relative to metal ion is always in outward direction for both terminations indicating a positive rumpling. (C) 2010 Elsevier B.V. All rights reserved.Öğe Electronic structure of antiferromagnetic PbCrO3 (001) surfaces(Elsevier Science Sa, 2011) Yildirim, Hasan; Agduk, Savas; Gokoglu, GokhanSurfaces of cubic perovksite PbCrO3 in (0 0 1) plane are investigated through density functional theory. The plane wave pseudopotential method is applied with generalized gradient approximation scheme. Hubbard U correction (GGA + U) is included in all calculations in order to simulate on-site Coulomb interactions between Cr-d states. Two types of terminations, namely, PbO-and CrO2-terminations are considered in construction of the surfaces. Surfaces of both terminations show convergence at 9-layer slab geometry. The density of states calculations on the converged slab geometry yield a metallic behavior for both PbO-and CrO2-terminations. Both metal atoms, Pb and Cr, in the uppermost layer of the respective terminations, have inward atomic relaxations much larger in magnitude than the oxygen atoms of the respective layer. However, Cr atoms which are labeled as up and down according to their spin orientation show different relaxations. The interlayer distance between the uppermost layer and the first one next to it decreases in both PbO- and CrO2-terminated surface geometries. The calculations of the relative movement of the oxygen atom with respect to the Pb or Cr atom in each terminations give a positive rumpling in the uppermost layer. (C) 2011 Elsevier B.V. All rights reserved.Öğe Electronic structure, cohesive and magnetic properties of iridium oxide clusters adsorbed on graphene(Elsevier Science Inc, 2020) Aysan, Isil Ilgaz; Gorkan, Taylan; Ozdemir, Ilkay; Kadioglu, Yelda; Gokoglu, Gokhan; Akturk, EthemIn this study, we investigated and revealed the electronic properties, geometric structures and binding behavior of small (IrO)(n) and (IrO2)(n) (n = 1-5) clusters within first principles calculations based on the density functional theory. The electronic and magnetic properties of small nanoclusters displayed significant size dependency due to strong quantum confinement effect. Moreover we considered the binding and structural modification of the clusters on graphene surface as a substrate. The cohesive energy per atom of isolated clusters increased with size of the cluster n. This shows that the increase in coordination number results in a more stable nanocluster with increased number of saturated bonds. Pristine (IrO)(n) and (IrO2)(n) clusters presented different structural motives at equilibrium. The ground states of (IrO) n and (IrO2)(n) clusters considered in this study were all magnetic except for (IrO)(4), (IrO2)(2), and (IrO2)(4). HOMO-LUMO gap E-HLG values displayed large variations due to size of the cluster, hence bond saturation. The structural configurations of free standing nanoclusters are slightly modified, when adsorbed on graphene. The adsorption behavior of a cluster on graphene was improved by an applied electric field yielding larger binding energy and larger charger transfer. We observed that electronic and magnetic ground state of the clusters strongly depend on optimized structural configuration for both bare and adsorbed on graphene monolayer. (C) 2020 Elsevier Inc. All rights reserved.Öğe First principles electronic structure calculations of Co2CrBi Heusler system(Elsevier, 2010) Gokoglu, GokhanFirst principles calculation results of a new full Heusler system Co2CrBi in stoichiometric composition were presented. The calculations are based on the density functional theory (OFT) within the spin-polarized generalized gradient approximation (sigma-GGA) and plane wave pseudopotential method. The system shows nearly half-metallic behavior with very low electronic density of states of minority spins at Fermi level yielding high spin polarization ratio R=0.96. The total magnetic moment of the system was calculated as 5.05 mu(B). which is largely localized at chromium site with mu(Cr) = 3.35 mu(B). The electronic character of the compound is determined by the 3d electronic states of cobalt and chromium atoms. f-Electronic states of bismuth atom were also included in calculations although it has no distinct effect on electronic structure. The system can be labelled as a new half-metallic material for technological applications. (C) 2010 Elsevier B.V. All rights reserved.Öğe First principles investigation of Mg7XH16 (X = Ti, Zn, Pd, and Cd) ternary hydrides for hydrogen storage applications(Iop Publishing Ltd, 2018) Erkisi, Aytac; Gokoglu, GokhanWe present the electronic structure, energetic stability, mechanical, and electronic properties of Mg7XH16(X = Ti, Zn, Pd, Cd) ternary hydride systems using a first principles approach in the framework of density functional theory. The possible usage of these systems in hydrogen storage applications is discussed. The systems under investigation have lower formation enthalpy than conventional MgH2 material indicating lower thermodynamic stability and improved hydrogen releasing capacity. All the systems are mechanically stable and compressible materials with relatively low elastic moduli values. In the electronic aspect, Mg7XH16 (X = Ti, Zn, Cd) systems are metal, but Mg7PdH16 is a zero-gap semiconductor with bands nearly touching at Fermi level (E-F) and zero density of state at E-F. This material can also have possible applications in optoelectronics.Öğe First-principles investigation of LaGaO3 and LaInO3 lanthanum perovskite oxides(Taylor & Francis Ltd, 2016) Erkisi, Aytac; Gokoglu, Gokhan; Surucu, Gokhan; Ellialtioglu, Recai; Yildirim, Erdem KamilAmong the class of ABO(3)-type perovskite oxides, LaMO3 (M=Ga and In) compounds are investigated in cubic (Pm-3m), tetragonal (P4mm), hexagonal (P-3m1), rhombohedral (R-3c) and orthorhombic (Pbnm) phases using generalised gradient approximation (GGA) within the density functional theory. On-site Coulomb interaction is also included in the calculations (GGA+U). After the determination of the stable phase, phase transition pressures have also been calculated. Then, their full structural, mechanical, electronic, optical and vibrational properties have been studied in stable orthorhombic (Pbnm) phase. Both compounds are non-magnetic insulators in their ground states. The energy gaps (E-g) of LaGaO3 and LaInO3 compounds have been found as 3.14 and 2.55eV, respectively. The calculated elastic constants and phonon dispersion curves confirm the stability of orthorhombic phase mechanically and dynamically.Öğe Functionalisation of hexagonal boron phosphide (h-BP) monolayer via atomic adsorption(Taylor & Francis Ltd, 2020) Cakmak, Nergul; Kadioglu, Yelda; Gokoglu, Gokhan; Uzengi Akturk, OlcayIn this study, we investigate the adsorption properties of Fe, Co, Ni, Cu, Zn, In, Tl, Ar atoms on hexagonal boron phosphide monolayer (h-BP) using density functional theory within both GGA and LDA functionals. Bare h-BP is a direct gap semiconductor with planar structure. The adsorption of the atoms on h-BP exhibits a large variety of electronic properties like semiconducting, metallic, and half-metallic states. Fe- and Ni-adsorbed h-BPs show semiconducting character with decreased band gaps. Ni atom is strongly adsorbed on the surface giving largest adsorption energy observed in this work. Fe-adsorbed system is a semiconducting ferromagnet with 1.95 magnetic moment. Co adsorption results in a half-metallic behaviour with 1.00 net magnetic moment and a perfect spin polarisation at Fermi level. Cu, In, and Tl adsorbed h-BP systems show metallic character. The results obtained show that h-BP surface can be functionalised via adsorption of related single atoms and can be suitable for various applications in optoelectronics and spintronics.Öğe Functionalization of monolayer MoS2 with transition metal oxide nanoclusters(Elsevier, 2021) Akpinar, Ese; Kadioglu, Yelda; Ozdemir, Ilkay; Gokoglu, Gokhan; Akturk, EthemThe surface functionalization of 2D materials provides the tunability of electronic structure as well as catalytic activity. In this study, we investigate the adsorption of various transition metal dioxide molecules (MO); i.e. TiO2, VO2, CrO2, MnO2, FeO2, CoO2, NiO2, CuO2, ZnO2, on pristine MoS2 monolayer by using firstprinciples spin-polarized density functional calculations. The binding of H2O molecule on MO+MoS2 system is also considered. We observe that MO molecules are adsorbed on MoS2 surface with a slight reduction of the electronic bandgap of bare MoS2. The interactions between the MoS2 surface and molecules can be strong and the formation of chemisorption bonds is possible with binding energies between approximate to 1.2 and approximate to 2.2 eV. MO-adsorbed MoS2 systems are all magnetic except for TiO2, FeO2, and ZnO2 adsorptions. The nonmagnetic ZnO2+MoS2 system displays a strong interaction yielding the largest charge transfer among the systems considered (2.07 e(-)) and the shortest equilibrium bond length between the metal atom and sulphur. However, CuO2+MoS2 is the most stable system energetically with 2.21 eV binding energy. H2O molecule binds only to MoS2+TiO2 structure with very tiny charge transfer from MoS2+TiO2 to H2O, while other systems result in negative binding energy. The results further reveal that metal oxides can be used to alter the electronic and magnetic nature of surfaces, even though weak van der Waals interactions occur between them.Öğe Half metallicity and pressure-induced electronic structure of monolayer FeX2 (X = S, Se)(Iop Publishing Ltd, 2017) Gokoglu, Gokhan; Akturk, EthemIn this paper, we present the electronic structure and magnetic properties of two members of transition metal dichalcogenides (TMD) family, FeS2 and FeSe2 compounds, in two dimensional monolayer form (2H) using first principles density functional calculations. Iron dichalcogenide compounds are not an intensively studied group of materials within TMDs, although they have an intrinsic magnetic moment and a large variety electronic properties like metallic, half metallic, and semiconducting in equilibrium and under strain. We find that FeS2 system is found to be metallic in equilibrium and turns into a half-metal, with vanishing density of states of minority spins at Fermi level, at an in-plane pressure of approximate to 13.7 kbar which corresponds to the slight contraction of the lattice. FeSe2 is an intrinsic half-metal both in equilibrium and under strain in a wide pressure range up to approximate to 56 kbar. Both structures have increased magnetic moments of 2.00 mu B under pressure compared to their equilibrium phases in which FeS2 and FeSe2 have 1.56 and 1.98 mu B magnetic moments, respectively. Half-metallic gaps in minority spin channels are surrounded by localized Fe-d states in all the systems. The bands crossing Fermi level in majority spins are due to d-states of Fe and p-states of S or Se atoms. Spin-orbit effects are not remarkable on the electronic structure of these systems.Öğe Half-metallic magnetism of Co2CrX (X=As, Sb) Heusler compounds: An ab initio study(Elsevier, 2011) Kanbur, Ulvi; Gokoglu, GokhanIn this study, we present the electronic, magnetic, and structural properties of two novel half-metallic full-Heusler compounds, Co2CrAs and Co2CrSb, in cubic L2(1) geometry. The calculations are based on the density functional theory within plane-wave pseudopotential method and spin-polarized generalized gradient approximation of the exchange-correlation functional. The electronic band structures and density of states of the systems indicate half-metallic behavior with vanishing electronic density of states of minority spins at Fermi level, which yields perfect spin polarization. The calculated magnetic moments of both systems in L2(1) structure are 5.00 mu(B), which are largely localized on the chromium site. The energy gaps in minority spin states are restricted by the 3d-states of cobalt atoms on two different sublattices. The formation enthalpies for both structures are negative indicating stability of these systems against decomposition into stable solid compounds. (C) 2010 Elsevier B.V. All rights reserved.Öğe High-pressure elasticity and lattice dynamics of Mg2La from first principles(Elsevier Science Sa, 2012) Agduk, Savas; Gokoglu, GokhanIn this study, we present the results of first principles calculations of elastic constants and structural properties of the Mg2La cubic Laves phase (C15) up to approximate to 150 GPa pressure together with vibrational properties at equilibrium geometry. projector augmented wave (PAW) potentials are used with generalized gradient approximation (GGA) scheme of the density functional theory. The linear response technique of the density functional perturbation theory is applied in investigation of the phonon dispersion spectra. The static equation of states of the system has been studied with Vinet formulation. The values of applied pressure have also been obtained from the Vinet formulation of equation of states. The cubic phase of the system remains stable within the studied pressure range. The elastic anisotropy ratio indicates an electronic topological transition (ETT) around approximate to 100 GPa. This situation is also confirmed by electronic band structures. (C) 2011 Elsevier B. V. All rights reserved.Öğe Molecular adsorption properties of CO and H2O on Au-, Cu-, and AuxCuy-doped MoS2 monolayer(Elsevier, 2017) Kadioglu, Yelda; Gokoglu, Gokhan; Akturk, Olcay UzengiIn this study, we investigate the adsorption properties of Au, Cu, and AuxCuy nanoclusters on MoS2 sheet and the interactions of the adsorbed systems with CO and H2O molecules by using first principles calculations. Results indicate that Au, Cu, or AuxCuy strongly binds to MoS2 monolayer resulting in enhanced chemical activity and sensitivity toward CO and H2O molecules compared to bare MoS2 monolayer. Although both CO and H2O molecules bind weakly to pristine MoS2 monolayer, CO strongly binds to MoS2 sheet in the presence of Au, Cu atoms or AuxCuy clusters. Semiconductor MoS2 monolayer turns into metal upon Au or Cu adsorption. AuxCuy nanocluster adsorption decreases the band gap of MoS2 monolayer acting as a n-type dopant. AuxCuy-doped MoS2 systems have improved adsorption properties for CO and H2O molecules, so the conclusions provided in this study can be useful as a guide for next generation device modeling. (C) 2017 Elsevier B.V. All rights reserved.
