arXiv subjects

Salim Ciraci

Publications and source records attributed to Salim Ciraci.

At least 19 recordsLinked to original sources

arXiv2017

Modification of electronic, magnetic structure and topological phase of bismuthene by point defects

This paper reveals how electronic, magnetic structure and topological phase of 2D, single-layer structures of bismuth are modified by point defects. We first showed that free standing, single-layer, hexagonal structure of bismuth, named as h-bismuthene exhibits non-trivial band topology. We then investigated interactions between single foreign adatoms and bismuthene structures, which comprise stability, bonding, electronic and magnetic structures. Localized states in diverse location of the band gap and resonant states in band continua of bismuthene are induced upon the adsorption of different adatoms, which modify electronic and magnetic properties. Specific adatoms result in reconstruction around the adsorption site. Single and divacancies can form readily in bismuthene structures and remain stable at high temperatures. Through rebondings Stone-Whales type defects are constructed by divacancies, which transform into a large hole at high temperature. Like adsorbed adatoms, vacancies induce also localized gap states, which can be eliminated through rebondings in divacancies. We also showed that not only optical and magnetic properties, but also topological features of pristine h-bismuthene can be modified by point defects. Modification of topological features depends on the energies of localized states and also on the strength of coupling between point defects.

cond-mat.mes-hall↗

arXiv2014

Silicite: the layered allotrope of silicon

Based on first-principles calculation we predict two new thermodynamically stable layered-phases of silicon, named as silicites, which exhibit strong directionality in the electronic and structural properties. As compared to silicon crystal, they have wider indirect band gaps but also increased absorption in the visible range making them more interesting for photovoltaic applications. These stable phases consist of intriguing stacking of dumbbell patterned silicene layers having trigonal structure with $\sqrt{3} \times \sqrt{3}$ periodicity of silicene and have cohesive energies smaller but comparable to that of the cubic diamond silicon. Our findings also provide atomic scale mechanisms for the growth of multilayer silicene as well as silicites.

cond-mat.mes-hall↗

arXiv2014

The atomic structure of the $\sqrt{3} \times \sqrt{3}$ phase of silicene on Ag(111)

The growth of the $\sqrt{3} \times \sqrt{3}$ reconstructed silicene on Ag substrate has been frequently observed in experiments while its atomic structure and formation mechanism is poorly understood. Here by first-principles calculations we show that $\sqrt{3} \times \sqrt{3}$ reconstructed silicene is constituted by dumbbell units of Si atoms arranged in a honeycomb pattern. Our model shows excellent agreement with the experimentally reported lattice constant and STM image. We propose a new mechanism for explaining the spontaneous and consequential formation of $\sqrt{3} \times \sqrt{3}$ structures from $3 \times 3$ structures on Ag substrate. We show that the $\sqrt{3} \times \sqrt{3}$ reconstruction is mainly determined by the interaction between Si atoms and have weak influence from Ag substrate. The proposed mechanism opens the path to understanding of multilayer silicon.

cond-mat.mes-hall↗

arXiv2012

Chlorine Adsorption on Graphene: Chlorographene

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 sustain due to strong Cl-Cl interaction resulting in the desorption through the formation of Cl$_2$ 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 $sp^3$-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.

cond-mat.mtrl-sci↗

arXiv2011

Graphene Nanomeshes: Existence of Defect-Induced Dirac Fermions on Graphene Host Matrix

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 state with the bands crossing linearly at the Fermi level. We showed that forming periodically repeating superstructures in graphene matrix can develop a promising technique to engineer nanomaterials with desired electronic and magnetic properties.

cond-mat.mes-hall↗

arXiv2011

Static charging of graphene and graphite slabs

The effect of external static charging of graphene and its flakes are investigated by using first-principles calculations. While the Fermi level of negatively charged graphene rises and then is quickly pinned by the parabolic, nearly free electron like bands, it moves down readily by removal of electrons from graphene. Excess charges accumulate mainly at both surfaces of graphite slab. Even more remarkable is that Coulomb repulsion exfoliates the graphene layers from both surfaces of positively charged graphite slab. The energy level structure, binding energy and and spin-polarization of specific adatoms adsorbed to a graphene flake can be monitored by charging.

cond-mat.mes-hall↗

arXiv2011

Structures of Fluorinated Graphenes and Their Signatures

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.

cond-mat.mtrl-sci↗

arXiv2010

Confined states in multiple quantum well structures of Si$_{n}$Ge$_{n}$ nanowire superlattices

Mechanical properties, atomic and energy band structures of bare and hydrogen passivated Si$_{n}$Ge$_{n}$ nanowire superlattices have been investigated by using first-principles pseudopotential plane wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. We found that Si$_{n}$ nanowires are stiffer than Ge$_{n}$ nanowires. Hydrogen passivation makes these nanowires and Si$_{n}$Ge$_{n}$ nanowire superlattice even more stiff. Upon heterostructure formation, superlattice electronic states form subbands in momentum space. Band lineups of Si and Ge zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. The electronic structure of the nanowire superlattice depends on the length and cross section geometry of constituent Si and Ge segments. Since bare Si and Ge nanowires are metallic and the band gaps of hydrogenated ones varies with the diameter, Si$_{n}$Ge$_{n}$ superlattices offer numerous alternatives for multiple quantum well devices with their leads made from the constituent metallic nanowires.

cond-mat.mes-hall↗

arXiv2010

Spintronic Properties of Zigzag-Edged Triangular Graphene Flakes

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.

cond-mat.mtrl-sci↗

arXiv2010

Current-voltage (I-V) characteristics of armchair graphene nanoribbons under uniaxial strain

The current-voltage (I-V) characteristics of armchair graphene nanoribbons under a local uniaxial tension are investigated by using first principles quantum transport calculations. It is shown that for a given value of bias-voltage, the resulting current depends strongly on the applied tension. The observed trends are explained by means of changes in the band gaps of the nanoribbons due to the applied uniaxial tension. In the course of plastic deformation, the irreversible structural changes and derivation of carbon monatomic chains from graphene pieces can be monitored by two-probe transport measurements.

cond-mat.mes-hall↗

arXiv2010

Graphane Nanoribbons: A Theoretical Study

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 ribbons are also nonmagnetic, adjacent dangling bonds of bare zigzag ribbons 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, Pt, Ti, V and Fe atoms on the graphane ribbon surface reveal that functionalization of graphane ribbons 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.

cond-mat.mtrl-sci↗

arXiv2010

Elastic and plastic deformation of graphene, silicene, and boron nitride honeycomb nanoribbons under uniaxial tension: A first-principles density-functional theory study

This study of elastic and plastic deformation of graphene, silicene, and boron nitride (BN) honeycomb nanoribbons under uniaxial tension determines their elastic constants and reveals interesting features. In the course of stretching in the elastic range, the electronic and magnetic properties can be strongly modified. In particular, it is shown that the band gap of a specific armchair nanoribbon is closed under strain and highest valance and lowest conduction bands are linearized. This way, the massless Dirac fermion behavior can be attained even in a semiconducting nanoribbon. Under plastic deformation, the honeycomb structure changes irreversibly and offers a number of new structures and functionalities. Cagelike structures, even suspended atomic chains can be derived between two honeycomb flakes. Present work elaborates on the recent experiments [C. Jin, H. Lan, L. Peng, K. Suenaga, and S. Iijima, Phys. Rev. Lett. 102, 205501 (2009)] deriving carbon chains from graphene. Furthermore, the similar formations of atomic chains from BN and Si nanoribbons are predicted.

cond-mat.mtrl-sci↗

arXiv2009

Magnetization of Graphane by Dehydrogenation

Each single hydrogen vacancy created at the surface of graphane gives rise to 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 pi-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. Our results based on the first-principles calculations suggest that the size and ordering of magnetic moments of hydrogen vacancy domains with thin walls can be used for future data storage and spintronics applications.

cond-mat.mtrl-sci↗

arXiv2009

Confinement of electrons in size modulated silicon nanowires

Based on first-principles calculations we showed that superlattices of periodically repeated junctions of hydrogen saturated silicon nanowire segments having different lengths and diameters form multiple quantum well structures. The band gap of the superlattice is modulated in real space as its diameter does and results in a band gap in momentum space which is different from constituent nanowires. Specific electronic states can be confined in either narrow or wide regions of superlattice. The type of the band lineup and hence the offsets of valence and conduction bands depend on the orientation of the superlattice as well as on the diameters of the constituent segments. Effects of the SiH vacancy and substitutional impurities on the electronic and magnetic properties have been investigated by carrying out spin-polarized calculations. Substitutional impurities with localized states near band edges can make modulation doping possible. Stability of the superlattice structure was examined by ab initio molecular dynamics calculations at high temperatures.

cond-mat.mes-hall↗

arXiv2009

Two and one-dimensional honeycomb structures of silicon and germanium

Based on first-principles calculations of structure optimization, phonon modes and finite temperature molecular dynamics, we predict that silicon and germanium have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, they are ambipolar and their charge carriers can behave like a massless Dirac fermions due to their pi- and pi*-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.

cond-mat.mtrl-sci↗

arXiv2007

Half-metallic silicon nanowires

From first-principles calculations, we predict that transition metal (TM) atom doped silicon nanowires have a half-metallic ground state. They are insulators for one spin-direction, but show metallic properties for the opposite spin direction. At high coverage of TM atoms, ferromagnetic silicon nanowires become metallic for both spin-directions with high magnetic moment and may have also significant spin-polarization at the Fermi level. The spin-dependent electronic properties can be engineered by changing the type of dopant TM atoms, as well as the diameter of the nanowire. Present results are not only of scientific interest, but can also initiate new research on spintronic applications of silicon nanowires.

cond-mat.mtrl-sci↗

arXiv2007

Dynamics of Phononic Dissipation at the Atomic Scale: Dependence on Internal Degrees of Freedom

Dynamics of dissipation of a local phonon distribution to the substrate is a key issue in friction between sliding surfaces as well as in boundary lubrication. We consider a model system consisting of an excited nano-particle which is weakly coupled with a substrate. Using three different methods we solve the dynamics of energy dissipation for different types of coupling between the nano-particle and the substrate, where different types of dimensionality and phonon densities of states were also considered for the substrate. In this paper, we present our analysis of transient properties of energy dissipation via phonon discharge in the microscopic level towards the substrate. Our theoretical analysis can be extended to treat realistic lubricant molecules or asperities, and also substrates with more complex densities of states. We found that the decay rate of the nano-particle phonons increases as the square of the interaction constant in the harmonic approximation.

cond-mat.other↗

arXiv2006

Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes

We present a systematic study of the electronic and magnetic properties of transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs). We considered the adsorption on the external and internal wall of SWNT and examined the effect of the TM coverage and geometry on the binding energy and the spin polarization at the Fermi level. All those adsorbed chains studied have ferromagnetic ground state, but only their specific types and geometries demonstrated high spin polarization near the Fermi level. Their magnetic moment and binding energy in the ground state display interesting variation with the number of $d-$electrons of the TM atom. We also show that specific chains of transition metal atoms adsorbed on a SWNT can lead to semiconducting properties for the minority spin-bands, but semimetallic for the majority spin-bands. Spin-polarization is maintained even when the underlying SWNT is subjected to high radial strain. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with non-magnetic metal electrodes, these magnetic needles or nanomagnets can perform as spin-dependent resonant tunnelling devices. The electronic and magnetic properties of these nanomagnets can be engineered depending on the type and decoration of adsorbed TM atom as well as the size and symmetry of the tube. Our study is performed by using first-principles pseudopotential plane wave method within spin-polarized Density Functional Method.

cond-mat.other↗