arXiv subjects

S. Ciraci

Publications and source records attributed to S. Ciraci.

At least 55 records · Page 3Linked to original sources

arXiv2008

Light Transition Metal Monatomic Chains

In this paper we investigated structural, electronic and magnetic properties of 3d (light) transition metal (TM) atomic chains using first-principles pseudopotential plane wave calculations. Periodic linear, dimerized linear and planar zigzag chain structures and 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 geometry. Dimerization in both infinite and finite chains are much stronger than the usual Peierls distortion and appear 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 zigzag structure. An interplay between the magnetic ground state and atomic as well as electronic structure of the chain has been revealed. The end effects influence the geometry, energetics and 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 are found to mimic the bulk behavior pointed out by Friedel. The spin-orbit coupling of finite chains are found to be negligibly small.

cond-mat.mtrl-sci↗

arXiv2007

Hydrogen Absorption Properties of Metal-Ethylene Complexes

Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a single ethylene molecule can form stable complexes with light transition metals (TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and 14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles molecular dynamics (MD) simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to 500 K. The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties.

cond-mat.mtrl-sci↗

arXiv2005

Ab-initio electron transport calculations of carbon based string structures

First-principles calculations show that monatomic strings of carbon have high cohesive energy and axial strength, and exhibit stability even at high temperatures. Due to their flexibility and reactivity, carbon chains are suitable for structural and chemical functionalizations; they form also stable ring, helix, grid and network structures. Analysis of electronic conductance of various infinite, finite and doped string structures reveal fundamental and technologically interesting features. Changes in doping and geometry give rise to dramatic variations in conductance. In even-numbered linear chains strain induces substantial decrease of conductance. The double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties.

cond-mat.mtrl-sci↗

arXiv2005

Chiral single-wall gold nanotubes

Based on first-principles calculations we show that gold atoms can form both free-standing and tip-suspended chiral single-wall nanotubes composed of helical atomic strands. Free-standing, infinite (5,5) tube is found to be energetically the most favorable. While energetically less favorable, the experimentally observed (5,3) tube stretching between two tips corresponds to a local minimum in the string tension. Similarly, the (4,3) tube is predicted as a favorable structure yet to be observed experimentally. Analysis of band structure, charge density, and quantum ballistic conductance suggests that the current on these wires is less chiral than expected, and there is no direct correlation between the numbers of conduction channels and helical strands.

cond-mat.mes-hall↗

arXiv2005

Silicon and III-V compound nanotubes: structural and electronic properties

Unusual physical properties of single-wall carbon nanotubes have started a search for similar tubular structures of other elements. In this paper, we present a theoretical analysis of single-wall nanotubes of silicon and group III-V compounds. Starting from precursor graphene-like structures we investigated the stability, energetics and electronic structure of zigzag and armchair tubes using first-principles pseudopotential plane wave method and finite temperature ab-initio molecular dynamics calculations. We showed that (n,0) zigzag and (n,n) armchair nanotubes of silicon having n > 6 are stable but those with n < 6 can be stabilized by internal or external adsorption of transition metal elements. Some of these tubes have magnetic ground state leading to spintronic properties. We also examined the stability of nanotubes under radial and axial deformation. Owing to the weakness of radial restoring force, stable Si nanotubes are radially soft. Undeformed zigzag nanotubes are found to be metallic for 6 < n < 11 due to curvature effect; but a gap starts to open for n > 12. Furthermore, we identified stable tubular structures formed by stacking of Si polygons. We found AlP, GaAs, and GaN (8,0) single-wall nanotubes stable and semiconducting. Our results are compared with those of single-wall carbon nanotubes.

cond-mat.mtrl-sci↗

arXiv2005

Half-metallic properties of atomic chains of carbon-transition metal compounds

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 and type of transition metal atoms. These chains, which are stable even at high temperature and some of which keep their spin-dependent electronic properties even under moderate axial strain, hold the promise of potential applications in nanospintronics.

cond-mat.mtrl-sci↗

arXiv2005

Molecular and dissociative absorption of multiple hydrogens on transition metal decorated C60

Recently we have predicted [Phys. Rev. Lett. May 2005(cond-mat/0504694)] that Ti-decorated carbon nanotubes can absorb up to 8-wt% hydrogen at ambient conditions. Here we show that similar phenomena occurs in light transition-metal decorated C60. While Sc and Ti prefers the hexagon (H) sites with a binding energy of 2.1 eV, V and Cr prefers double-bond (D) sites with binding energies of 1.3 and 0.8 eV, respectively. Heavier metals such as Mn, Fe, and Co do not bond on C60. Once the metals are absorbed on C60, each can bind up to four hydrogen molecules with an average binding energy of 0.3-0.5 eV/H2. At high metal-coverage, we show that a C$_{60}$ can accommodate six D-site and eight H-site metals, which can reversible absorb up to 56 H2 molecules, corresponding to 7.5 wt%.

cond-mat.mtrl-sci↗

arXiv2005

Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes

We investigated interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes (SWNT) using first-principles plane wave method. We found that the binding energy of the H$_{2}$ physisorbed on the bare SWNT is very weak, and can be enhanced neither by increasing the curvature of the surface through radial deformation, nor by the coadsorption of Li atom that makes the semiconducting tube metallic. Though the bonding is strengthened upon adsorption directly to Li atom, yet its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of Pt atom. Single H$_{2}$ is chemisorbed to Pt atom on the SWNT either dissociatively or molecularly. If Pt-SWNT bond is weakened either by displacing Pt from bridge site to a specific position or by increasing number of the adsorbed H$_{2}$, the dissociative adsorption of H$_{2}$ is favored. For example, out of two adsorbed H$_{2}$, first one can be adsorbed dissociatively, second one is chemisorbed molecularly. The nature of bonding is weak physisorption for the third adsorbed H$_{2}$. Palladium also promotes the chemisorption of H$_{2}$ with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.

cond-mat.mtrl-sci↗

arXiv2005

Titanium-decorated carbon nanotubes: a potential high-capacity hydrogen storage medium

We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first H$_2$ adsorption is dissociative with no energy barrier while other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8-wt% hydrogen. The system is quite stable and exhibits associative desorption upon heating, a requirement for reversible storage. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.

cond-mat.mtrl-sci↗

arXiv2003

Electronic Structure of Te and As Covered Si(211)

Electronic and atomic structures of the clean, and As and Te covered Si(211) surface are studied using pseudopotential density functional method. The clean surface is found to have (2 X 1) and rebonded (1 X 1) reconstructions as stable surface structures, but no π-bonded chain reconstruction. Binding energies of As and Te adatoms at a number of symmetry sites on the ideal and (2 X 1) reconstructed surfaces have been calculated because of their importance in the epitaxial growth of CdTe and other materials on the Si(211) surface. The special symmetry sites on these surfaces having the highest binding energies for isolated As and Te adatoms are identified. But more significantly, several sites are found to be nearly degenerate in binding energy values. This has important consequences for epitaxial growth processes. Optimal structures calculated for 0.5 ML of As and Te coverage reveal that the As adatoms dimerize on the surface while the Te adatoms do not. However, both As and Te covered surfaces are found to be metallic in nature.

cond-mat.mtrl-sci↗

arXiv2002

Effects of hydrogen adsorption on single wall carbon nanotubes: Metallic hydrogen decoration

We show that the electronic and atomic structure of carbon nanotubes undergo dramatic changes with hydrogen chemisorption from first principle calculations. Upon uniform exohydrogenation at half coverage, the cross sections of zigzag nanotubes become literally square or rectangular, and they are metallic with very high density of states at the Fermi level, while other isomers can be insulating. For both zigzag and armchair nanotubes, hydrogenation of each carbon atom from inside and outside alternatively yield the most stable isomer with a very weak curvature dependence and a large band gap.

cond-mat.mtrl-sci↗

arXiv2002

Pentagonal nanowires: a first-principles study of atomic and electronic structure

We performed an extensive first-principles study of nanowires in various pentagonal structures by using pseudopotential plane wave method within the density functional theory. Our results show that nanowires of different types of elements, such as alkali, simple, transition and noble metals and inert gas atoms, have a stable structure made from staggered pentagons with a linear chain perpendicular to the planes of the pentagons and passing through their centers. This structure exhibits bond angles close to those in the icosahedral structure. However, silicon is found to be energetically more favorable in the eclipsed pentagonal structure. These quasi one dimensional pentagonal nanowires have higher cohesive energies than many other one dimensional structures and hence may be realized experimentally. The effect of magnetic state are examined by spin-polarized calculations. The origin of the stability are discussed by examining optimized structural parameters, charge density and electronic band structure, and by using analysis based on the empirical Lennard-Jones type interaction. Electronic band structure of pentagonal wires of different elements are discussed and their effects on quantum ballistic conductance are mentioned. It is found that the pentagonal wire of silicon exhibits metallic band structure.

cond-mat.mtrl-sci↗

arXiv2002

Metal nanoring and tube formation on carbon nanotubes

The structural and electronic properties of aluminum covered single wall carbon nanotubes (SWNT) are studied from first-principles for a large number of coverage. Aluminum-aluminum interaction that is stronger than aluminum-tube interaction, prevents uniform metal coverage, and hence gives rise to the clustering. However, a stable aluminum ring and aluminum nanotube with well defined patterns can also form around the semiconducting SWNT and lead to metallization. The persistent current in the Al nanoring is discussed to show that a high magnetic field can be induced at the center of SWNT.

cond-mat.mtrl-sci↗

arXiv2002

A systematic ab-initio study of curvature effects in carbon nanotubes

We investigate curvature effects on geometric parameters, energetics and electronic structure of zigzag nanotubes with fully optimized geometries from first-principle calculations. The calculated curvature energies, which are inversely proportional to the square of radius, are in good agreement with the classical elasticity theory. The variation of the band gap with radius is found to differ from simple rules based on the zone folded graphene bands. Large discrepancies between tight binding and first principles calculations of the band gap values of small nanotubes are discussed in detail.

cond-mat.mtrl-sci↗

arXiv2002

Reversible Band Gap Engineering in Carbon Nanotubes by Radial Deformation

We present a systematic analysis of the effect of radial deformation on the atomic and electronic structure of zigzag and armchair single wall carbon nanotubes using the first principle plane wave method. The nanotubes were deformed by applying a radial strain, which distorts the circular cross section to an elliptical one. The atomic structure of the nanotubes under this strain are fully optimized, and the electronic structure is calculated self-consistently to determine the response of individual bands to the radial deformation. The band gap of the insulating tube is closed and eventually an insulator-metal transition sets in by the radial strain which is in the elastic range. Using this property a multiple quantum well structure with tunable and reversible electronic structure is formed on an individual nanotube and its band-lineup is determined from first-principles. The elastic energy due to the radial deformation and elastic constants are calculated and compared with classical theories.

cond-mat.mtrl-sci↗

arXiv2001

Tunable adsorption on carbon nanotubes

We investigated the adsorption of a single atom, hydrogen and aluminum, on single wall carbon nanotubes from first-principles. The adsorption is exothermic, and the associated binding energy varies inversely as the radius of the zigzag tube. We found that the adsorption of a single atom and related properties can be modified continuously and reversibly by the external radial deformation. The binding energy on the high curvature site of the deformed tube increases with increasing radial deformation. The effects of curvature and radial deformation depend on the chirality of the tube.

cond-mat.mtrl-sci↗

arXiv2001

Structure of aluminum atomic chains

First-principles density functional calculations reveal that aluminum can form planar chains in zigzag and ladder structures. The most stable one has equilateral triangular geometry with four nearest neighbors; the other stable zigzag structure has wide bond angle and allows for two nearest neighbors. An intermediary structure has the ladder geometry and is formed by two strands. All these planar geometries are, however, more favored energetically than the linear chain. We found that by going from bulk to a chain the character of bonding changes and acquires directionality. The conductance of zigzag and linear chains is 4e^2/h under ideal ballistic conditions.

cond-mat.mtrl-sci↗

arXiv2001

Te covered Si(001): a variable surface reconstruction

At a given temperature, clean and adatom covered silicon surfaces usually exhibit well-defined reconstruction patterns. Our finite temperature ab-initio molecular dynamics calculations show that the tellurium covered Si(001) surface is an exception. Soft longitudinal modes of surface phonons due to the strongly anharmonic potential of the bridged tellurium atoms prevent the reconstruction structure from attaining any permanent, two dimensional periodic geometry. This explains why experiments attempting to find a definite model for the reconstruction have reached conflicting conclusions.

cond-mat.mtrl-sci↗