Browsing by Subject "Ab initio"

Now showing 1 - 7 of 7

Open Access
Ab initio study of electronic properties of armchair graphene nanoribbons passivated with heavy metal elements
(Elsevier, 2019) Narin, P.; Abbas, J. M.; Atmaca, G.; Kutlu, E.; Lisesivdin, S. B.; Özbay, Ekmel
In this study, electronic properties of graphene nanoribbons with armchair edges (AGNRs) have been investigated with Density Functional Theory (DFT). Effects of heavy metal (HM) elements, including Zinc (Zn), Cadmium (Cd) and Mercury (Hg) atoms on electronic behavior of AGNRs have been calculated by passivating for both one and two edges of AGNRs in detail. To explain the electronic behavior of investigated AGNRs, the electronic band structure, the density of states (DOS), total energy have been calculated. Energetically favorable structures have been determined using calculated binding energy values. The obtained bandgap values of investigated structures changes between 0.30 and 0.64 eV. Increasing atomic number of passivation atoms have led to an increment in the bandgap of AGNRs.
Open Access
Bias in bonding behavior among boron, carbon, and nitrogen atoms in ion implanted a-BN, a-BC, and diamond like carbon films
(2011) Genisel, M. F.; Uddin, M. N.; Say, Z.; Kulakci, M.; Turan, R.; Gulseren, O.; Bengu, E.
In this study, we implanted Nþ and Nþ 2 ions into sputter deposited amorphous boron carbide (a-BC) and diamond like carbon (DLC) thin films in an effort to understand the chemical bonding involved and investigate possible phase separation routes in boron carbon nitride (BCN) films. In addition, we investigated the effect of implanted Cþ ions in sputter deposited amorphous boron nitride (a-BN) films. Implanted ion energies for all ion species were set at 40 KeV. Implanted films were then analyzed using x-ray photoelectron spectroscopy (XPS). The changes in the chemical composition and bonding chemistry due to ion-implantation were examined at different depths of the films using sequential ion-beam etching and high resolution XPS analysis cycles. A comparative analysis has been made with the results from sputter deposited BCN films suggesting that implanted nitrogen and carbon atoms behaved very similar to nitrogen and carbon atoms in sputter deposited BCN films. We found that implanted nitrogen atoms would prefer bonding to carbon atoms in the films only if there is no boron atom in the vicinity or after all available boron atoms have been saturated with nitrogen. Implanted carbon atoms also preferred to either bond with available boron atoms or, more likely bonded with other implanted carbon atoms. These results were also supported by ab-initio density functional theory calculations which indicated that carbon-carbon bonds were energetically preferable to carbon-boron and carbon-nitrogen bonds.
Open Access
Characterization of two-dimensional Ga1−xAlxN ordered alloys with varying chemical composition
(Elsevier, 2019) Kanlı, Muammer; Önen, Abdullatif; Mogulkoc, A.; Durgun, Engin
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.
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, Ekmel
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.
Open Access
High conducting nanowires obtained from uniform titanium covered carbon nanotubes
(2005) Daǧ, Sefa; Durgun, Engin; Çıracı, Salim
We have shown that Ti atoms can form continuous coating of carbon nanotubes at different amount of coverage. The circular cross section changes to a square-like form, and the semiconducting tube becomes ferromagnetic metal with high quantum ballistic conductance. Metallicity is induced not only by the metal-metal coupling, but also by the band gap closing of SWNT at the corners of the square. The magnetic properties of Ti coated tubes depend strongly on the geometry, amount of Ti coverage and also on the elastic deformation of the tube. While the magnetic moment can be pronounced significantly by the positive axial strain, it can decrease dramatically upon the adsorption of additional Ti atoms to those already covering the nanotube. Besides, electronic structure and spin-polarization near the Fermi level can also be modified by radial strain. Our results have been obtained by the first-principles, spin-relaxed pseudopotential plane wave calculations within the density functional theory. © TÜBİTAK.
Open Access
Optical and magnetic properties of some XMnSb and Co2YZ compounds: ab initio calculations
(Wiley-VCH Verlag, 2017) Palaz S.; Koc, H.; Ozisik, H.; Deligoz, E.; Mamedov, A. M.; Özbay, Ekmel
In present work, our research is mainly focused on the electronic structures, optical, and magnetic properties of XMnSb (X = Ni, Cu, Pd), Co2YZ (Y = Ti; Z=Si, Ge, Sn), and Co2YZ (Y =Mn; Z=Al, Ga, Si) Heusler compounds by using ab initio calculations within the generalized gradient approximation. The calculations are performed by using the Vienna ab initio simulation package based on the density functional theory. The band structure of these Heusler alloys for majority spin and minority spin were calculated and 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. We also find that these Heusler compounds have the indirect band gaps in the minority spin channel. 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 XMnSb and Co2YZ compounds were also calculated. In addition, we also show the variations of the total magnetic moment per f.u. and minority spin gap width of these compounds with optimized lattice constants: minority spin gap width decreases with increasing the lattice constants. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Open Access
Strain-induced structural phase transition in GeN monolayer
(Elsevier BV, 2021-11-30) Abboud, Mohammad; Özbey, D.H.; Durgun, Engin
The recent synthesis of SiP, SiAs, GeP, and GeAs monolayers has brought two-dimensional (2D) group IV–V systems into the limelight. To date, all the fabricated structures of this class belong to the C2/m space group which has a low structural symmetry, while the class could exist in more symmetric phases (i.e., P3m1 and P6m2). The realization of more symmetric phases can enhance the intrinsic properties of these materials and increase their potential field of usage. In this study, the possibility of a structural phase transition in GeN monolayer by application of mechanical strain is investigated. Based on ab initio simulations, we first confirm the stability of the GeN monolayer in all phases, then demonstrate how a large enough compressive strain (~%12) can transform C2/m into P3m1 phase. The results are interpreted by analyzing the geometry, bond order, electron localization functions, and net atomic charges of the structures. Upon transition into the P3m1 phase, tensile strength and in-plane stiffness double, while the compressive strength quadruples. On the other hand, the effect of the phase transition on the electronic properties is not substantial and similar band structure profiles with narrowed band gap are obtained. Our study provides insight on how to experimentally achieve the P3m1 phase of the GeN monolayer, which is in principle applicable to other group IV–V monolayers under suitable conditions involving the optimization of pressure, temperature, and impurity concentration. These unique features of the GeN monolayer render them ideal candidates for a variety of high technological nanoscale applications.