Browsing by Subject "Ab initio calculations"

Now showing 1 - 13 of 13

Open Access
Band Structure and Optical Properties of Kesterite Type Compounds: First principle calculations
(Institute of Physics Publishing, 2017) Palaz S.; Unver H.; Ugur G.; Mamedov, Amirullah; Özbay, Ekmel
In present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated. © Published under licence by IOP Publishing Ltd.
Open Access
First principles prediction of the elastic, electronic, and optical properties of Sb 2S 3 and Sb 2Se 3 compounds
(2012) Koc H.; Mamedov, A.M.; Deligoz, E.; Ozisik H.
We have performed a first principles study of structural, mechanical, electronic, and optical properties of orthorhombic Sb 2S 3 and Sb 2Se 3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, Debye temperature, and hardness have also been estimated in the present work. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valence electrons and the effective optical dielectric constant are calculated. Our structural estimation and some other results are in agreement with the available experimental and theoretical data. © 2012 Elsevier Masson SAS. All rights reserved.
Open Access
First Principles Study of Barium Chalcogenides
(2008) Gökoǧlu, G.
In this study, ab initio calculation results of the vibrational properties and elastic parameters as well as characteristic Debye temperature and Poisson's ratios of two barium chalcogenides, BaSe and BaS, which crystallize in NaCl-type structure, were presented. Calculations were based on plane wave basis sets together with ultrasoft pseudopotentials in the framework of density functional theory (DFT) with generalized gradient approximation. Phonon dispersion spectra were obtained using the first principles linear response approach of the density functional perturbation theory (DFPT). The detailed total energy calculations were performed in order to obtain elastic constants using distortions on cubic phase. The calculated structural, elastic, and thermal parameters of BaSe and BaS systems agree well with the available experimental data and theoretical calculations.
Open Access
First-principles calculations of Pd-terminated symmetrical armchair graphene nanoribbons
(Elsevier, 2013) Kuloglu, A. F.; Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Özbay, Ekmel
The effects of Palladium (Pd) termination on the electronic properties of armchair graphene nanoribbons (AGNRs) were calculated by using ab initio calculations. After a geometric optimization process, the electronic band structures, density of states, and binding energies of AGNRs with Na = 5-15 were calculated. Pd-termination was found to significantly influence the electronic properties of AGNRs. In DOS, many Q0D and Q1D type states were observed. Binding energy (BE) for single-side or both-side Pd-terminated structures represents characteristic drops with the increasing GNR width. With the increasing GNR width, the BEs of these structures become similar to hydrogenated structures. Because of the GNR width, dependent BE also gave information on the possible stiffness information, in which all of this information can be used in studies where controlled binding to graphene is required.
Open Access
Investigation of anisotropic mechanical, electronic, and charge carrier transport properties of germanium-pnictogen monolayers
(Institute of Physics Publishing Ltd., 2022-02-04) Abboud, Mohammad; Özbey, Doğukan Hazar; Kılıç, M. E.; Durgun, Engin
Recently, novel two-dimensional (2D) GeP and GeAs systems have been fabricated by mechanical exfoliation and utilized in various applications. These developments have brought the 2D germanium-pnictogens, C2/m-GeX (X = N, P, As, Sb, and Bi) structures into the limelight. In this study, we systematically investigate the structural, mechanical, electronic, and charge carrier transport properties of GeX monolayers by using first-principles methods. Our results show that the considered systems are dynamically stable and possess anisotropic physical properties. Examined structures are found to be flexible, and their mechanical strength and stiffness decrease down the group-V, in line with the trends of the bond strength, cohesive energy, charge transfer, and electron localization function. Additionally, the zigzag in-plane direction is mechanically superior to the armchair direction. The electronic band structure calculations based on HSE06 hybrid functional with the inclusion of spin–orbit coupling indicate that GeX monolayers are either direct or quasi-direct semiconductors with band gaps lying within the infrared and visible spectrum. The estimated charge carrier mobilities are highly anisotropic and also differ significantly depending on the structure and carrier type. These unique properties render GeX monolayers as suitable 2D materials for flexible nanoelectronic applications.
Open Access
Janus two-dimensional transition metal dichalcogenide oxides: first-principles investigation of WXO monolayers with X=S, Se, and Te
(American Physical Society, 2021-05-26) Varjovi Jahangirzadeh, Mirali; Yagmurcukardes, M.; Peeters, F. M. F. M.; Durgun, Engin
Structural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases (1H,1T, and 1T′) of Janus WXO (X=S, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles mods. Phonon spectra analysis reveals that while the 1H phase is dynamically stable, the 1T phase exhibits imaginary frequencies and transforms to the distorted 1T′ phase. Ab initio molecular dynamics simulations confirm that 1H- and 1T′−WXO monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of 1H−WXO are found to be isotropic, they are orientation dependent for 1T′−WXO. It is also shown that 1H−WXO monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except 1T′-WSO, 1T′−WXO monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus WXO monolayers together with their large piezoelectric response imply that these systems are interesting for several anoelectronic applications.
Open Access
Koopmans' springs to life
(American Institute of Physics, 2009) Salzner, U.; Baer, R.
The meaning of orbital energies (OOEs) in Kohn-Sham (KS) density functional theory (DFT) is subject to a longstanding controversy. In local, semilocal, and hybrid density functionals (DFs) a Koopmans' approach, where OOEs approximate negative ionization potentials (IPs), is unreliable. We discuss a methodology based on the Baer-Neuhauser-Livshits range-separated hybrid DFs for which Koopmans' approach "springs to life." The OOEs are remarkably close to the negative IPs with typical deviances of +/- 0.3 eV down to IPs of 30 eV, as demonstrated on several molecules. An essential component is the ab initio motivated range-parameter tuning procedure, forcing the highest OOE to be exactly equal to the negative first IP. We develop a theory for the curvature of the energy as a function of fractional occupation numbers to explain some of the results.
Open Access
On the structural and electronic properties of Ir-silicide nanowires on Si(001) surface
(American Institute of Physics Inc., 2016) Fatima; Oguz I. C.; Çakır, D.; Hossain, S.; Mohottige, R.; Gulseren, O.; Oncel, N.
Iridium (Ir) modified Silicon (Si) (001) surface is studied with Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Density Functional Theory (DFT). A model for Ir-silicide nanowires based on STM images and ab-initio calculations is proposed. According to our model, the Ir adatom is on the top of the substrate dimer row and directly binds to the dimer atoms. I-V curves measured at 77 K shows that the nanowires are metallic. DFT calculations confirm strong metallic nature of the nanowires.
Open Access
Optical properties and electronic band structure of topological insulators (on A5 2B6 3 compound based)
(Taylor & Francis, 2013-09-20) Koc, H.; Mamedov, A. M.; Özbay, Ekmel
We have performed a first principles study of structural, electronic, and optical properties of rhombohedral Sb2Te3 and Bi 2Te3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valance electrons and the effective optical dielectric constant are calculated and presented in the study.
Open Access
Optical properties and electronic band structure of topological insulators on A2 5B36 compound based
(IEEE, 2012) Koc H.; Mamedov, Amirullah M.; Özbay, Ekmel
We have performed a first principles study of structural, electronic, and optical properties of rhombohedral Sb 2Te 3 and Bi 2Te 3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valance electrons and the effective optical dielectric constant are calculated and presented in the study © 2012 IEEE.
Open Access
Simultaneous measurement of multiple independent atomic-scale interactions using scanning probe microscopy: data interpretation and the effect of cross-talk
(American Chemical Society, 2015) Baykara, M. Z.; Todorović, M.; Mönig, H.; Schwendemann, T. C.; Rodrigo, L.; Altman, E. I.; Pérez, R.; Schwarz, U. D.
In high-resolution scanning probe microscopy, it is becoming increasingly common to simultaneously record multiple channels representing different tip-sample interactions to collect complementary information about the sample surface. A popular choice involves simultaneous scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) measurements, which are thought to reflect the chemical and electronic properties of the sample surface. With surface-oxidized Cu(100) as an example, we investigate whether atomic-scale information on chemical interactions can be reliably extracted from frequency shift maps obtained while using the tunneling current as the feedback parameter. Ab initio calculations of interaction forces between specific tip apexes and the surface are utilized to compare experiments with theoretical expectations. The examination reveals that constant-current operation may induce a noticeable influence of topography-feedback-induced cross-talk on the frequency shift data, resulting in misleading interpretations of local chemical interactions on the surface. Consequently, the need to apply methods such as 3D-AFM is emphasized when accurate conclusions about both the local charge density near the Fermi level, as provided by the STM channel, and the site-specific strength of tip-sample interactions (NC-AFM channel) are desired. We conclude by generalizing to the case where multiple atomic-scale interactions are being probed while only one of them is kept constant.
Open Access
Structural, elastic, and electronic properties of topological insulators: Sb2Te3 and Bi2Te3
(IEEE, 2013) Koc H.; Mamedov, Amirullah M.; Özbay, Ekmel
We have performed a first principles study of structural, elastic, and electronic properties of rhombohedral Sb2Te3 and Bi 2Te3 compounds using the density functional theory within the local density approximation. The lattice parameters of considered compounds have been calculated. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, and Debye temperature have also been estimated in the present work. The calculated electronic band structure shows that Sb2Te3 and Bi2Te 3 compounds have a direct forbidden band gap. Our structural estimation and some other results are in agreement with the available experimental and theoretical data. © 2013 IEEE.
Open Access
Topological Insulators: Electronic Band Structure and Spectroscopy
(Institute of Physics Publishing, 2017) Palaz S.; Koc, H.; Mamedov, Aamirullah M.; Özbay, Ekmel
In this study, we present the results of our ab initio calculation of the elastic constants, density of states, charge density, and Born effective charge tensors for ferroelectric (rhombohedral) and paraelectric phases (cubic) of the narrow band ferroelectrics (GeTe, SnTe) pseudopotentials. The related quantities such as bulk modulus and shear modulus using obtained elastic constants have also been estimated in the present work. The total and partial densities of states corresponding to the band structure of Sn(Ge)Te(S,Se) were calculated. We also calculated the Born effective charge tensor of an atom (for instance, Ge, Sn, Te, etc.), which is defined as the induced polarization of the solid along the main direction by a unit displacement in the perpendicular direction of the sublattice of an atom at the vanishing electric field. © Published under licence by IOP Publishing Ltd.