Browsing by Author "Ataca, C."

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Open Access
Adsorption of carbon adatoms to graphene and its nanoribbons
(AIP Publishing, 2011) Ataca, C.; Aktürk, E.; Şahin, H.; Çıracı, Salim
This paper investigates the adsorption of carbon adatoms on graphene and its nanoribbons using first-principles plane wave calculations within density functional theory. The stability at high carbon adatom coverage, migration, and cluster formation of carbon atoms are analyzed. Carbon adatoms give rise to important changes in electronic and magnetic properties even at low coverage. While bare graphene is nonmagnetic semimetal, it is metallized and acquires magnetic moment upon coverage of carbon adatoms. Calculated magnetic moments vary depending on the coverage of adatoms even for large adatom-adatom distances. Electronic and magnetic properties of hydrogen passivated armchair and zigzag nanoribbons show strong dependence on the adsorption site. We also predict a new type of carbon impurity defect in graphene, which has a small formation energy. Interactions between distant carbon adatoms imply a long ranged interaction.
Open Access
Atomic and electronic structures of doped silicon nanowires: A first-principles study
(2007) Durgun, Engin; Akman, N.; Ataca, C.; Çıracı, Salim
We have investigated the atomic and electronic structures of hydrogen saturated silicon nanowires doped with impurity atoms (such as Al, Ga, C, Si, Ge, N, P, As, Te, Pt) using a first-principles plane wave method. We considered adsorption and substitution of impurity atoms at the surface and also their substitution at the core of the nanowire. In the case of adsorption to the surface, we determined the most energetic adsorption geometry among various possible adsorption sites. All impurities studied lead to nonmagnetic ground state with a significant binding energy. Impurity bands formed at high impurity concentration are metallic for group IIIA and VA elements but are semiconductor and modify the band gap for group IVA and VIA elements. While low substitutional impurity concentration leads to usual n - and p -type behaviors reminiscent of bulk Si, this behavior is absent if the impurity atom is adsorbed on the surface. It is shown that the electronic properties of silicon nanowires can be modified by doping for optoelectronic applications.
Open Access
A comparative study of lattice dynamics of three-and two-dimensional MoS2
(American Chemical Society, 2011) Ataca, C.; Topsakal, M.; Aktürk, E.; Çıracı, Salim
This paper presents a comparative study of the lattice dynamics of three-dimensional layered MoS2 and two-dimensional single layer MoS2 based on the density functional theory. A comprehensive analysis of energetics and optimized structure parameters is performed using different methods. It is found that the van der Waals attraction between layers of three-dimensional (3D) layered MoS2 is weak but is essential to hold the layers together with the equilibrium interlayer spacing. Cohesive energy, phonon dispersion curves, and corresponding density of states and related properties, such as Born-effective charges, dielectric constants, Raman and infrared active modes are calculated for 3D layered as well as 2D single layer MoS2 using their optimized structures. These calculated values are compared with the experimental data to reveal interesting dimensionality effects. The absence of a weak interlayer interaction in 2D single layer MoS2 results in the softening of some of Raman active modes. © 2011 American Chemical Society.
Open Access
Dissociation of H2O at the vacancies of single-layer MoS2
(American Physical Society, 2012) Ataca, C.; Çıracı, Salim
Based on first-principles density functional theory and finite temperature molecular dynamics calculations, we predict that H 2O can be dissociated into its constituents O and H at specific vacancy defects of single-layer MoS 2 honeycomb structure, which subsequently are bound to fourfolded Mo and twofolded S atoms surrounding the vacancy, respectively. This exothermic and spontaneous process occurs, since the electronegativity and ionization energy of Mo are smaller than those of H. Once desorbed from twofolded S atoms, H atoms migrate readily on the MoS 2 surface and eventually form free H 2 molecules to be released from the surface. Present results are critical for acquiring clean and sustainable energy from hydrogen. © 2012 American Physical Society.
Open Access
Effects of silicon and germanium adsorbed on graphene
(A I P Publishing LLC, 2010) Aktürk, E.; Ataca, C.; Çıracı, Salim
Based on the first-principles plane wave calculations, we studied the adsorption of Si and Geon graphene. We found that these atoms are bound to graphene at the bridge site with a significant binding energy, while many other atoms are bound at the hollow site above the center of hexagon. It is remarkable that these adatoms may induce important changes in the electronic structure of graphene even at low coverage. Semimetallic graphene becomes metallized and attains a magnetic moment. The combination of adatom orbitals with those of ππ- and π∗π∗-states of bare graphene is found responsible for these effects.
Open Access
Electronic and magnetic properties of graphane nanoribbons
(American Physical Society, 2010) Şahin, H.; Ataca, C.; Çıracı, Salim
In this study, we investigate the electronic and magnetic properties of graphane nanoribbons. We find that zigzag and armchair graphane nanoribbons with H-passivated edges are nonmagnetic semiconductors. While bare armchair nanoribbons are also nonmagnetic, adjacent dangling bonds of bare zigzag nanoribbons have antiferromagnetic ordering at the same edge. Band gaps of the H-passivated zigzag and armchair nanoribbons exponentially depend on their width. Detailed analysis of adsorption of C, O, Si, Ti, V, Fe, Ge, and Pt atoms on the graphane ribbon surface reveal that functionalization of graphane nanoribbons is possible via these adatoms. It is found that C, O, V, and Pt atoms have tendency to replace H atoms of graphane. We showed that significant spin polarizations in graphane can be achieved through creation of domains of H vacancies and CH divacancies.
Open Access
Frictional figures of merit for single layered nanostructures
(American Physical Society, 2012) Cahangirov, S.; Ataca, C.; Topsakal, M.; Sahin, H.; Çıracı, Salim
We determine the frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS 2 and WO 2 moving over each other, by carrying out ab initio calculations of interlayer interaction under constant loading force. Using the Prandtl-Tomlinson model we derive the critical stiffness required to avoid stick-slip behavior. We show that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby the materials avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays a much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers. © 2012 American Physical Society.
Open Access
Functionalization of BN honeycomb structure by adsorption and substitution of foreign atoms
(2010) Ataca, C.; Çıracı, Salim
We carried out first-principles calculations within density-functional theory to investigate the structural, electronic, and magnetic properties of boron-nitride (BN) honeycomb structure functionalized by adatom adsorption, as well as by the substitution of foreign atoms for B and N atoms. For periodic high-density coverage, most of 3d transition metal atoms and some of group 3A, 4A, and 6A elements are adsorbed with significant binding energy and modify the electronic structure of bare BN monolayer. While bare BN monolayer is nonmagnetic, wide band-gap semiconductor, at high coverage of specific adatoms it can achieve magnetic metallic, even half-metallic ground states. At low coverage, the bands associated with adsorbed atoms are flat and the band structure of parent BN is not affected significantly. Therefore, adatoms and substitution of foreign atoms at low coverage are taken to be the representative of impurity atoms yielding localized states in the band gap and resonance states in the band continua. Notably, the substitution of C for B and N yield donorlike and acceptorlike magnetic states in the band gap. Localized impurity states occurring in the gap give rise to interesting properties for electronic and optical application of the single-layer BN honeycomb structure. © 2010 The American Physical Society.
Open Access
Functionalization of single-layer Mos2 honeycomb structures
(American Chemical Society, 2011) Ataca, C.; Çıracı, Salim
Based on first-principles plane-wave calculations, we studied the functionalization of the two-dimensional single-layer MoS2 structure through adatom adsorption and vacancy defect creation. Minimum-energy adsorption sites were determined for 16 different adatoms, each of which gives rise to diverse properties. Bare, single-layer MoS2, which is normally a nonmagnetic, direct-band-gap semiconductor, attains a net magnetic moment upon adsorption of specific transition-metal atoms, as well as silicon and germanium atoms. The localized donor and acceptor states in the band gap expand the utilization of MoS2 in nanoelectronics and spintronics. Specific adatoms, such as C and O, attain significant excess charge upon adsorption onto single-layer MoS2, which might be useful for tribological applications. Each MoS2 triple vacancy created in a single layer of MoS 2 gives rise to a net magnetic moment, whereas other vacancy defects related to Mo and S atoms do not influence the nonmagnetic ground state. The present results are also relevant for the surface of graphitic MoS2. © American Chemical Society.
Open Access
High-capacity hydrogen storage by metallized graphene
(AIP Publishing, 2008) Ataca, C.; Aktürk, E.; Çıracı, Salim; Ustunel H.
First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a uniform and stable coverage on both sides. A significant part of the electronic charge of the Li 2s orbital is donated to graphene and is accommodated by its distorted π* -bands. As a result, semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium with each adsorbed Li absorbing up to four H2 molecules amounting to a gravimetric density of 12.8 wt %.
Open Access
Hydrogen storage of calcium atoms adsorbed on graphene: First-principles plane wave calculations
(American Physical Society, 2009) Ataca, C.; Aktürk, E.; Çıracı, Salim
Based on first-principles plane wave calculations, we showed that Ca adsorbed on graphene can serve as a high-capacity hydrogen storage medium, which can be recycled by operations at room temperature. Ca is chemisorbed by donating part of its 4s charge to the empty π∗ band of graphene. At the end the adsorbed Ca atom becomes positively charged and the semimetallic graphene changes into a metallic state. While each of the adsorbed Ca atoms forming the (4×4) pattern on the graphene can absorb up to five H2 molecules, hydrogen storage capacity can be increased to 8.4 wt % by adsorbing Ca to both sides of graphene and by increasing the coverage to form the (2×2) pattern. Clustering of Ca atoms is hindered by the repulsive Coulomb interaction between charged Ca atoms.
Open Access
Magnetization of graphane by dehydrogenation
(AIP Publishing, 2009) Şahin, H.; Ataca, C.; Çıracı, Salim
Using first principles calculations, we show that each hydrogen vacancy created at graphane surface results in a local unpaired spin. For domains of hydrogen vacancies the situation is, however, complex and depends on the size and geometry of domains, as well as whether the domains are single or double sided. In single-sided domains, hydrogen atoms at the other side are relocated to pair the spins of adjacent carbon atoms by forming ππ-bonds. Owing to the different characters of exchange coupling in different ranges and interplay between unpaired spin and the binding geometry of hydrogen, vacancy domains can attain sizable net magnetic moments.
Open Access
Mechanical and electronic properties of MoS2 nanoribbons and their defects
(American Chemical Society, 2011) Ataca, C.; Şahin, H.; Aktürk, E.; Çıracı, Salim
We present our study on atomic, electronic, magnetic, and phonon properties of the one-dimensional honeycomb structure of molybdenum disulfide (MoS 2) using the first-principles plane wave method. Calculated phonon frequencies of bare armchair nanoribbon reveal the fourth acoustic branch and indicate the stability. Force constant and in-plane stiffness calculated in the harmonic elastic deformation range signify that the MoS2 nanoribbons are stiff quasi one-dimensional structures, but not as strong as graphene and BN nanoribbons. Bare MoS2 armchair nanoribbons are nonmagnetic, direct band gap semiconductors. Bare zigzag MoS2 nanoribbons become half-metallic as a result of the (2 × 1) reconstruction of edge atoms and are semiconductor for minority spins, but metallic for the majority spins. Their magnetic moments and spin-polarizations at the Fermi level are reduced as a result of the passivation of edge atoms by hydrogen. The functionalization of MoS2 nanoribbons by adatom adsorption and vacancy defect creation are also studied. The nonmagnetic armchair nanoribbons attain net magnetic moment depending on where the foreign atoms are adsorbed and what kind of vacancy defect is created. The magnetization of zigzag nanoribbons due to the edge states is suppressed in the presence of vacancy defects. © 2011 American Chemical Society.
Open Access
Perpendicular growth of carbon chains on graphene from first-principles
(American Physical Society, 2011) Ataca, C.; Çıracı, Salim
Based on first-principles calculations we predict a peculiar growth process, where carbon adatoms adsorbed to graphene readily diffuse above room temperature and nucleate segments of linear carbon chains attached to graphene. These chains grow longer on graphene through insertion of carbon atoms one at a time from the bottom end and display a self-assembling behavior. Eventually, two allotropes of carbon, namely graphene and cumulene, are combined to exhibit important functionalities. The segments of carbon chains on graphene become chemically active sites to bind foreign atoms or large molecules. When bound to the ends of carbon chains, transition metal atoms, Ti, Co, and Au, attribute a magnetic ground state to graphene sheets and mediate stable contacts with interconnects. We showed that carbon chains can grow also on single-wall carbon nanotubes. © 2011 American Physical Society.
Open Access
Stable, single-layer MX 2 transition-metal oxides and dichalcogenides in a honeycomb-like structure
(American Chemical Society, 2012) Ataca, C.; Şahin, H.; Çıracı, Salim
Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX 2) might offer properties superior to those of graphene. So far, only very few MX 2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX 2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA + U calculations. Our analysis of stability was extended to include in-plane stiffness, as well as ab initio, finite-temperature molecular dynamics calculations. Some of these single-layer structures are direct- or indirect-band-gap semiconductors, only one compound is half-metal, and the rest are either ferromagnetic or nonmagnetic metals. Because of their surface polarity, band gap, high in-plane stiffness, and suitability for functionalization by adatoms or vacancies, these single-layer structures can be utilized in a wide range of technological applications, especially as nanoscale coatings for surfaces contributing crucial functionalities. In particular, the manifold WX 2 heralds exceptional properties promising future nanoscale applications. © 2012 American Chemical Society.
Open Access
Structural, electronic, and magnetic properties of 3d transition metal monatomic chains: First-principles calculations
(American Physical Society, 2008) Ataca, C.; Cahangirov, S.; Durgun, Engin; Jang, Y. -R.; Çıracı, Salim
In this paper we investigated structural, electronic, and magnetic properties of 3d (light) transition metal atomic chains using first-principles pseudopotential plane-wave calculations. Infinite periodic linear, dimerized linear, and planar zigzag chain structures, as well as their short segments consisting of finite number of atoms have been considered. Like Cu, the periodic, linear chains of Mn, Co, and Ni correspond to a local shallow minimum. However, for most of the infinite periodic chains, neither linear nor dimerized linear structures are favored; to lower their energy the chains undergo a structural transformation to form planar zigzag and dimerized zigzag geometries. Dimerization in both infinite and finite chains is much stronger than the usual Peierls distortion and appears to depend on the number of 3d electrons. As a result of dimerization, a significant energy lowering occurs which, in turn, influences the stability and physical properties. Metallic linear chain of vanadium becomes half-metallic upon dimerization. Infinite linear chain of scandium also becomes half-metallic upon transformation to the zigzag structure. An interplay between the magnetic ground state and the atomic as well as the electronic structure of the chain has been revealed. The end effects influence the geometry, the energetics, and the magnetic ground state of the finite chains. Structure optimization performed using noncollinear approximation indicates significant differences from the collinear approximation. Variation of the cohesive energy of infinite- and finite-size chains with respect to the number of 3d electrons is found to mimic the well-known bulk behavior. The spin-orbit coupling of finite chains is found to be negligibly small.