Browsing by Author "Dag, S."

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Open Access
Ab-initio electron transport calculations of carbon based string structures
(American Physical Society, 2004) Tongay, S.; Senger, R. T.; Dag, S.; Çıracı, Salim
The new stable structures of carbon-based strings and their unusual electronic transport properties were discussed. Total energy and electronic structure calculations using first principles pseudopotential plane wave method within density functional theory (DFT) and supercell geometries were also carried out. It was found that carbon chains were suitable for structural and chemical functionalizations because of their flexibility. These carbon chains also form stable ring, helix, grid and network structures. The results show that the double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties and carbon chains can form stable string structures with impressive physical properties.
Open Access
Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes
(American Physical Society, 2005) Dag, S.; Ozturk, Y.; Çıracı, Salim; Yildirim, T.
Interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes (SWNT) is investigated using first-principles plane wave method. It is found that the binding energy of the H2 physisorbed on the outer surface of the bare SWNT is very weak, and cannot be enhanced significantly either by increasing the curvature of the surface through radial deformation, or by the coadsorption of a Li atom that makes the semiconducting tube metallic. Although the bonding is strengthened upon adsorption directly to the Li atom, its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of a Pt atom. A single H2 is chemisorbed to the Pt atom on the SWNT either dissociatively or molecularly. The dissociative adsorption is favorable energetically and is followed by the weakening of the Pt-SWNT bond. Out of two adsorbed H2, the first one can be adsorbed dissociatively and the second one is chemisorbed molecularly. The nature of bonding is a very weak physisorption for the third adsorbed H2. Palladium also promotes the chemisorption of H2 with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and these results advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.
Open Access
Atomic and electronic structure of carbon strings
(IOP Publishing Ltd., 2005) Tongay, S.; Dag, S.; Durgun, Engin; Senger, R. T.; Çıracı, Salim
This paper presents an extensive study of various string and tubular structures formed by carbon atomic chains. Our study is based on first-principles pseudopotential plane wave and finite-temperature ab initio molecular dynamics calculations. Infinite- and finite-length carbon chains exhibit unusual mechanical and electronic properties such as large cohesive energy, axial strength, high conductance, and overall structural stability even at high temperatures. They are suitable for structural and chemical functionalizations. Owing to their flexibility and reactivity they can form linear chain, ring, helix, two-dimensional rectangular and honeycomb grids, three-dimensional cubic networks, and tubular structures. Metal-semiconductor heterostructures and various quantum structures, such as multiple quantum wells and double-barrier resonant tunnelling structures, can be formed from the junctions of metallic carbon and semiconducting BN linear chains. Analysis of atomic and electronic structures of these periodic, finite, and doped structures reveals fundamentally and technologically interesting features, such as structural instabilities and chiral currents. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies the chemical, mechanical, and electronic properties.
Open Access
Atomic scale study of superlow friction between hydrogenated diamond surfaces
(American Physical Society, 2004) Dag, S.; Çıracı, Salim
Strong attractive interaction between two clean diamond (001) slabs turns repulsive upon the hydrogenation of surfaces. This repulsive interaction serves as if a boundary lubricant and prevents the sliding surfaces from being closer to each other even under high normal forces. As a result, calculated lateral force variation generated during sliding has small magnitude under high constant loading forces. Superlow friction observed earlier between diamondlike carbon-coated surfaces can be understood by the steady repulsive interaction between sliding surfaces, as well as strong and stiff carbon-carbon and carbon-hydrogen bonds which do not favor energy dissipation. In ambient conditions, the steady repulsive interaction is, however, destroyed by oxygen atoms which chemically modify those stiff surface bonds.
Open Access
Carbon string structures: First-principles calculations of quantum conductance
(American Physical Society, 2005) Senger, R. T.; Tongay, S.; Dag, S.; Durgun, Engin; Çıracı, Salim
Carbon forms various nanostructures based on the monatomic chains or strings which show transport properties of fundamental and technological interest. We have carried out first-principles quantum conductance calculations using optimized structures within density functional theory. We treated finite segments of carbon monatomic chain, metal-semiconductor heterostructure, and resonant tunneling double barrier formed of C-BN chains, as well as symmetric and antisymmetric loop devices between two electrodes. We examined the effects of electrode, contact geometry, size of the device, strain, and foreign atoms adsorbed on the chain. Calculated quantum ballistic conductance of carbon chains showing even-odd disparity depending on the number of atoms and strain are of particular interest. Notably, chains consisting of an even number of carbon atoms contacted to metal electrodes display a resonant tunneling-like behavior under axial strain. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies unusual transport properties.
Open Access
Chiral single-wall gold nanotubes
(American Physical Society, 2004) Senger, R. T.; Dag, S.; Çıracı, Salim
The formation of freestanding and tip-suspended chiral-wall (n,m) nanotubes, which were composed of helical atomic strands, from gold atoms was investigated using first-principles calculations, where (n,m) notation defines the structure of the tube. The tubes with 3≤n≤5 were found to be stable and exhibited electronic and transport properties investigated. The (5,3) gold tube was energetically the most favourable. It was observed from the quantum ballistic conductance, band structure and charge density analysis that the current on these wires was less chiral, and no direct correlation between the numbers of conduction channels and helical strands was found.
Open Access
A comparative study of O2 adsorbed carbon nanotubes
(2003) Dag, S.; Gülseren, O.; Çıracı, Salim
First-principles, density functional calculations show that O2 adsorbed single-wall carbon nanotubes (SWNT) show dramatic differences depending on the type of the tube. Upon O2 physisorption, the zig-zag SWNT remains semiconducting, while the metallicity of the armchair is lifted for the spin-down bands. The spin-up bands continue to cross at the Fermi level, and make the system metallic only for one type of spin. The singlet bound state of O2 occurs at the bridge site of the (6,6) SWNT at small distance from the surface of the tube. However, for the hollow site, the molecule dissociates when it comes close to the surface. © 2003 Elsevier B.V. All rights reserved.
Open Access
Coverage and strain dependent magnetization of titanium-coated carbon nanotubes
(American Physical Society, 2005) Dag, S.; Çıracı, Salim
First-principles, spin-relaxed pseudopotential plane wave calculations show that Ti atoms can form a continuous coating of carbon nanotubes at different amounts of coverage. Fully relaxed geometry has a complex but regular atomic structure. The semiconducting tube becomes ferromagnetic metal with high quantum conductance. However, 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, the electronic structure and the spin-polarization near the Fermi level can also be modified by radial strain.
Open Access
Electronic and magnetic properties of zinc blende half-metal superlattices
(A I P Publishing LLC, 2004) Fong, C. Y.; Qian, M. C.; Pask, J. E.; Yang, L. H.; Dag, S.
Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0mB and 14.0mB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.
Open Access
Electronic structure of the contact between carbon nanotube and metal electrodes
(American Institute of Physics, 2003) Dag, S.; Gülseren, O.; Çıracı, Salim; Yildirim, T.
Our first-principles study of the contact between a semiconducting single-walled carbon nanotube ~s-SWNT! and metal electrodes shows that the electronic structure and potential depend strongly on the type of metal. The s-SWNT is weakly side-bonded to the gold surface with minute charge rearrangement and remains semiconducting. A finite potential barrier forms at the contact region. In contrast, the molybdenum surface forms strong bonds, resulting in significant charge transfer and metallicity at the contact. The radial deformation of the tube lowers the potential barrier at the contact and increases the state density at the Fermi level.
Open Access
Energetics and Electronic Structures of Individual Atoms Adsorbed on Carbon Nanotubes
(American Chemical Society, 2004) Durgun, Engin; Dag, S.; Çıracı, Salim; Gülseren, O.
The adsorption of individual atoms on the semiconducting and metallic single-walled carbon nanotubes (SWNT) has been investigated by using the first principles pseudopotential plane wave method within density functional theory. The stable adsorption geometries and binding energies have been determined for a large number of foreign atoms ranging from alkali and simple metals to the transition metals and group IV elements. We have found that the character of the bonding and associated physical properties strongly depends on the type of adsorbed atoms, in particular, on their valence electron structure. Our results indicate that the properties of SWNTs can be modified by the adsorbed foreign atoms. Although the atoms of good conducting metals, such as Zn, Cu, Ag, and Au, form very weak bonds, transition-metal atoms such as Ti, Sc, Nb, and Ta and group IV elements C and Si are adsorbed with a relatively high binding energy. Owing to the curvature effect, these binding energies are larger than the binding energies of the same atoms on the graphite surface. We have showed that the adatom carbon can form strong and directional bonds between two SWNTs. These connects can be used to produce nanotube networks or grids. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as ID nanomagnets or nanoconductors, conducting connects, and so forth.
Open Access
Half-metallic properties of atomic chains of carbon-transition-metal compounds
(American Physical Society, 2005) Dag, S.; Tongay, S.; Yildirim, T.; Durgun, Engin; Senger, R. T.; Fong, C. Y.; Çıracı, Salim
We found that magnetic ground state of one-dimensional atomic chains of carbon-transition-metal compounds exhibit half-metallic properties. They are semiconductors for one spin direction, but show metallic properties for the opposite direction. The spins are fully polarized at the Fermi level and net magnetic moment per unit cell is an integer multiple of Bohr magneton. The spin-dependent electronic structure can be engineered by changing the number of carbon atoms and type of transition metal atoms. These chains, which are stable even at high temperatures and some of which keep their spin-dependent electronic properties even under moderate axial strain, hold the promise of potential applications in nanospintronics.
Open Access
Initial stages of Pt growth on Ge (001) studied by scanning tunneling microscopy and density functional theory
(American Physical Society, 2004) Gurlu, O.; Zandvliet, H. J. W.; Poelsema, B.; Dag, S.; Çıracı, Salim
We have studied the initial stages of submonolayer Pt growth on the Ge(001). We have observed several stable and meta-stable adsorption configurations of Pt atoms at various temperatures. Calculations indicate relatively high binding energies of Pt atoms onto the Ge lattice, at different adsorption sites. Our results show that through-the-substrate bonding (concerted bonding) of two Pt atoms is more favored on Ge(001) surface then a direct Pt-Pt bond. Both our experiments and calculations indicate the breaking of Ge-Ge bonds on the surface in the vicinity of Pt adsorbates. We have also observed the spontaneous generation of 2 + 1 dimer vacancy defects at room temperature that cause the ejection of Ge atoms onto the surface. Finally we have studied the diffusion of Pt atoms into the bulk as a result of annealing and found out that they get trapped at subsurface sites.
Open Access
Nanospintronic properties of carbon-cobalt atomic chains
(EDP Sciences, 2006) Durgun, Engin; Senger, R. T.; Mehrez, H.; Dag, S.; Çıracı, Salim
Periodic atom chains of carbon-cobalt compounds, (CnCo) ∞, comprise both conducting and insulating electronic properties simultaneously depending on the spin type of electrons, and hence are half-metals. Their band gap and the net magnetic moment oscillate with the number of carbon atoms in a unit cell. Finite segments of these chains also show interesting magnetic and transport properties. When connected to appropriate metallic electrodes the antiferromagnetic CoCnCo segments behave like molecular spin-valves, which can be conveniently manipulated. © EDP Sciences.
Open Access
Oxygenation of carbon nanotubes: Atomic structure, energetics, and electronic structure
(American Physical Society, 2003) Dag, S.; Gülseren, O.; Yildirim, T.; Çıracı, Salim
This paper presents an extensive and systematic analysis of the oxygenation of semiconducting and metallic single-wall carbon nanotubes by using the first principles pseudopotential plane wave method. Our study involves the physisorption of oxygen molecules, chemisorption of oxygen atoms and formation of an oxide, and deals with the equilibrium binding geometry and corresponding electronic energy structure. The binding energies of an oxygen molecule physisorbed at different sites are determined by calculating short and long range interactions. The triplet state of the physisorbed oxygen molecule is energetically favorable, whereas the nonmagnetic (spin paired) state yields a relatively stronger binding energy. An oxygen atom is adsorbed on top of the carbon-carbon bond. The zigzag bonds of the nanotubes are weakened and eventually are replaced by a carbon-oxygen-carbon bridge bond. Chemisorption of atomic oxygen and physisorption of an oxygen molecule modify the electronic energy structure of the bare tube in different ways. For a different coverage and pattern, self-consistent field electronic energy structure calculations using the optimized physisorption geometry corresponding to the triplet ground state result in a small energy gap between unoccupied oxygen levels and the top of the valence band of the semiconducting carbon nanotube. These results invalidate the hole doping of the semiconducting carbon nanotube upon the physisorption of oxygen.
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Six low-strain zinc-blende half metals: An ab initio investigation
(2003) Pask J.E.; Yang L.H.; Fong, C.Y.; Pickett W.E.; Dag, S.
A class of spintronic materials, the zinc-blende (ZB) half metals, has recently been synthesized in thin-film form. We apply all-electron and pseudopotential ab initio methods to investigate the electronic and structural properties of ZB Mn and Cr pnictides and carbides, and find six compounds to be half metallic at or near their respective equilibrium lattice constants, making them excellent candidates for growth at low strain. Based on these findings, we further propose substrates on which the growth may be accomplished with minimum strain. Our findings are supported by the recent successful synthesis of ZB CrAs on GaAs and ZB CrSb on GaSb, where our predicted equilibrium lattice constants are within 0.5% of the lattice constants of the substrates on which the growth was accomplished. We confirm previous theoretical results for ZB MnAs, but find ZB MnSb to be half metallic at its equilibrium lattice constant, whereas previous work has found it to be only nearly so. We report here two low-strain half metallic ZB compounds, CrP and MnC, and suggest appropriate substrates for each. Unlike the other five compounds, we predict ZB MnC to become/remain half metallic with compression rather than expansion, and to exhibit metallicity in the minority-rather than majority-spin channel. These fundamentally different properties of MnC can be connected to substantially greater p-d hybridization and d-d overlap, and correspondingly larger bonding-antibonding splitting and smaller exchange splitting. We examine the relative stability of each of the six ZB compounds against NiAs and MnP structures, and find stabilities for the compounds not yet grown comparable to those already grown.
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Spin-polarized ballistic transport in a thin superlattice of zinc blende half-metallic compounds
(The American Physical Society, 2005) Qian, M. C.; Fong, C. Y.; Pickett, W. E.; Pask, J. E.; Yang, L. H.; Dag, S.
We examine theoretically ballistic conduction in thin layers of zinc blende half metals, considering as an example a superlattice consisting of monolayers of GaAs and MnAs, a bilayer of CrAs, and a bilayer of GaAs. By artificially separating bilayers, we show that surface states thwart half metallicity. However, capping the metal-As bilayers restores half metallicity, and ballistic conduction of electrons within ∼0.3 eV of the Fermi level will give nearly 100% spin-polarized transmission in the direction of the superlattice. Recent developments suggest atomic layer epitaxy can be used to produce such thin layers for spintronic applications.
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Systematic study of adsorption of single atoms on a carbon nanotube
(American Physical Society, 2003) Durgun, Engin; Dag, S.; Bagci, V. M. K.; Gülseren, O.; Yildirim, T.; Çıracı, Salim
We studied the adsorption of single atoms on a semiconducting and metallic single-wall carbon nanotube from first principles for a large number of foreign atoms. The stable adsorption sites, binding energy, and the resulting electronic properties are analyzed. The character of the bonding and associated physical properties exhibit dramatic variations depending on the type of the adsorbed atom. While the atoms of good conducting metals, such as Cu and Au, form very weak bonding, atoms such as Ti, Sc, Nb, and Ta are adsorbed with relatively high binding energy. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects, etc.
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Theoretical study of crossed and parallel carbon nanotube junctions and three-dimensional grid structures
(American Physical Society, 2004) Dag, S.; Senger, R. T.; Çıracı, Salim
This work presents a first-principles study of parallel and crossed junctions of single-wall carbon nanotubes (SWNT). The crossed junctions are modeled by two-dimensional grids of zigzag SWNTs. The atomic and electronic structure, stability, and energetics of the junctions are studied for different magnitudes of contact forces pressing the tubes towards each other and hence inducing radial deformations. Under relatively weak contact forces the tubes are linked with intertube bonds which allow a significant conductance through the junction. These interlinking bonds survive even after the contact forces are released and whole structure is fully relaxed. Upon increasing contact force and radial deformation the tube surfaces are flattened but the interlinking bonds are broken to lead to a relatively wider intertube spacing. The intertube conductance through such a junction diminish because of finite potential barrier intervening between the tubes. The linkage of crossing tubes to form stable junctions is enhanced by a vacancy created at the contact. The three-dimensional grid structure formed by SWNTs is also investigated as a possible framework in device integration.
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Theoretical study of Ga-based nanowires and the interaction of Ga with single-wall carbon nanotubes
(American Physical Society, 2004) Durgun, Engin; Dag, S.; Çıracı, Salim
Gallium displays physical properties which can make it a potential element to produce metallic nanowires and high-conducting interconnects in nanoelectronics. Using first-principles pseudopotential plane method we showed that Ga can form stable metallic linear and zigzag monatomic chain structures. The interaction between individual Ga atom and single-wall carbon nanotube (SWNT) leads to a chemisorption bond involving charge transfer. Doping of SWNT with Ga atom gives rise to donor states. Owing to a significant interaction between individual Ga atom and SWNT, continuous Ga coverage of the tube can be achieved. Ga nanowires produced by the coating of carbon nanotube templates are found to be stable and high conducting.