Browsing by Author "Şahin, H."

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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
Chlorine adsorption on graphene: chlorographene
(American Chemical Society, 2012) Şahin, H.; Çıracı, Salim
We perform first-principles structure optimization, phonon frequency, and finite temperature molecular dynamics calculations based on density functional theory to study the interaction of chlorine atoms with graphene predicting the existence of possible chlorinated graphene derivatives. The bonding of a single chlorine atom is ionic through the transfer of charge from graphene to chlorine adatom and induces negligible local distortion in the underlying planar graphene. Different from hydrogen and fluorine adatoms, the migration of a single chlorine adatom on the surface of perfect graphene takes place almost without barrier. However, the decoration of one surface of graphene with Cl adatoms leading to various conformations cannot be sustained due to strong Cl-Cl interaction resulting in the desorption through the formation of Cl2 molecules. On the contrary, the fully chlorinated graphene, chlorographene CCl, where single chlorine atoms are bonded alternatingly to each carbon atom from different sides of graphene with sp3-type covalent bonds, is buckled. We found that this structure is stable and is a direct band gap semiconductor, whose band gap can be tuned by applied uniform strain. Calculated phonon dispersion relation and four Raman-active modes of chlorographene are discussed. © 2012 American Chemical Society.
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
First-principles calculations of spin-dependent conductance of graphene flakes
(The American Physical Society, 2008) Şahin, H.; Senger, R. T.
Using ab initio density-functional theory and quantum transport calculations based on nonequilibrium Green's function formalism we study structural, electronic, and transport properties of hydrogen-terminated short graphene nanoribbons (graphene flakes) and their functionalization with vanadium atoms. Rectangular graphene flakes are stable, having geometric and electronic structures quite similar to that of extended graphene nanoribbons. We show that a spin-polarized current can be produced by pure hydrogenated rectangular graphene flakes by exploiting the spatially separated edge states of the flake using asymmetric nonmagnetic contacts. Functionalization of the graphene flake with magnetic adatoms such as vanadium also leads to spin-polarized currents even with symmetric contacts. We observe and discuss sharp discontinuities in the transmission spectra which arise from Fano resonances of localized states in the flake.
Open Access
Graphene coatings: an efficient protection from oxidation
(American Physical Society, 2012) Topsakal, M.; Şahin, H.; Çıracı, Salim
We demonstrate that graphene coating can provide efficient protection from oxidation by posing a high-energy barrier to the path of oxygen atom, which could have penetrated from the top of the graphene to the reactive surface underneath. A graphene bilayer, which blocks the diffusion of oxygen with a relatively higher energy barrier, provides even better protection from oxidation. While an oxygen molecule is weakly bound to a bare graphene surface and hence becomes rather inactive, it can easily dissociate into two oxygen atoms adsorbed to low-coordinated carbon atoms at the edges of a vacancy. For these oxygen atoms the oxidation barrier is reduced and hence the protection from oxidation provided by graphene coatings is weakened. Our predictions obtained from the state-of-the-art first-principles calculations of the electronic structure, phonon density of states, and reaction path will unravel how graphene can be used as a corrosion-resistant coating and guide further studies aimed at developing more efficient nanocoatings. © 2012 American Physical Society.
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
Monolayer honeycomb structures of group-IV elements and III-V binary compounds: First-principles calculations
(American Physical Society, 2009) Şahin, H.; Cahangirov, S.; Topsakal, M.; Bekaroglu, E.; Akturk, E.; Senger, R. T.; Çıracı, Salim
Using first-principles plane-wave calculations, we investigate two-dimensional (2D) honeycomb structure of group-IV elements and their binary compounds as well as the compounds of group III-V elements. Based on structure optimization and phonon-mode calculations, we determine that 22 different honeycomb materials are stable and correspond to local minima on the Born-Oppenheimer surface. We also find that all the binary compounds containing one of the first row elements, B, C, or N have planar stable structures. On the other hand, in the honeycomb structures of Si, Ge, and other binary compounds the alternating atoms of hexagons are buckled since the stability is maintained by puckering. For those honeycomb materials which were found stable, we calculated optimized structures, cohesive energies, phonon modes, electronic-band structures, effective cation and anion charges, and some elastic constants. The band gaps calculated within density functional theory using local density approximation are corrected by GW0 method. Si and Ge in honeycomb structure are semimetal and have linear band crossing at the Fermi level which attributes massless Fermion character to charge carriers as in graphene. However, all binary compounds are found to be semiconductor with band gaps depending on the constituent atoms. We present a method to reveal elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson’s ratio as well as in-plane stiffness values. Preliminary results show that the nearly lattice matched heterostructures of these compounds can offer alternatives for nanoscale electronic devices. Similar to those of the three-dimensional group-IV and group III-V compound semiconductors, one deduces interesting correlations among the calculated properties of present honeycomb structures.
Open Access
Spintronic properties of zigzag-edged triangular graphene flakes
(AIP Publishing LLC, 2010) Şahin, H.; Senger, R. T.; Çıracı, Salim
We investigate quantum transport properties of triangular graphene flakes with zigzag edges by using first principles calculations. Triangular graphene flakes have large magnetic moments which vary with the number of hydrogen atoms terminating its edge atoms and scale with its size. Electronic transmission and current-voltage characteristics of these flakes, when contacted with metallic electrodes, reveal spin valve and remarkable rectification features. The transition from ferromagnetic to antiferromagnetic state under bias voltage can, however, terminate the spin polarizing effects for specific flakes. Geometry and size dependent transport properties of graphene flakes may be crucial for spintronic nanodevice applications.
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, mechanical, and electronic properties of defect-patterned graphene nanomeshes from first principles
(American Physical Society, 2011) Şahin, H.; Çıracı, Salim
Motivated by the state of the art method for fabricating high-density periodic nanoscale defects in graphene, the structural, mechanical, and electronic properties of defect-patterned graphene nanomeshes including diverse morphologies of adatoms and holes are investigated by means of first-principles calculations within density functional theory. It is found that various patterns of adatom groups yield metallic or semimetallic, even semiconducting, behavior and specific patterns can be in a magnetic state. Even though the patterns of single adatoms dramatically alter the electronic structure of graphene, adatom groups of specific symmetry can maintain the Dirac fermion behavior. Nanoholes forming nanomesh are also investigated. Depending on the interplay between the repeat periodicity and the geometry of the hole, the nanomesh can be in different states ranging from metallic to semiconducting including semimetallic states with the bands crossing linearly at the Fermi level. We showed that forming periodically repeating superstructures in a graphene matrix can develop a promising technique for engineering nanomaterials with desired electronic and magnetic properties.
Open Access
Structures of fluorinated graphene and their signatures
(American Physical Society, 2011) Şahin, H.; Topsakal, M.; Çıracı, Salim
Recent synthesis of fluorinated graphene introduced interesting stable derivatives of graphene. In particular, fluorographene (CF), namely, fully fluorinated chair conformation, is found to display crucial features, such as high mechanical strength, charged surfaces, local magnetic moments due to vacancy defects, and a wide band gap rapidly reducing with uniform strain. These properties, as well as structural parameters and electronic densities of states, are found to scale with fluorine coverage. However, most of the experimental data reported to date neither for CF nor for other CnF structures complies with the results obtained from first-principles calculations. In this study, we attempt to clarify the sources of disagreements. © 2011 American Physical Society.
Open Access
Two-and one-dimensional honeycomb structures of silicon and germanium
(American Physical Society, 2009) Cahangirov, S.; Topsakal, M.; Aktürk, E.; Şahin, H.; Çıracı, Salim
First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their π and π* bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.