Browsing by Author "Özçelik, V. O."

Now showing 1 - 19 of 19

Open Access
Atomic structure of the √3 × √3 phase of silicene on Ag (111)
(American Physical Society, 2014-07-28) Cahangirov, S.; Özçelik, V. O.; Xian, L.; Avila, J.; Cho, S.; Asensio, M. C.; Çıracı, Salim; Rubio, A.
The growth of the 3√×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 3√×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 3√×3√ structures from 3×3 structures on Ag substrate. We show that the 3√×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. ©2014 American Physical Society
Open Access
Effects of charging and perpendicular electric field on the properties of silicene and germanene
(Institute of Physics Publishing, 2013) Gürel, H. H.; Özçelik, V. O.; Çıracı, Salim
Using first-principles density functional theory calculations, we showed that electronic and magnetic properties of bare and Ti adatom adsorbed single-layer silicene and germanene, which are charged or subjected to a perpendicular electric field, can be modified to attain new functionalities. In particular, when subjected to a perpendicular electric field, buckled atoms have the symmetry between their planes broken, opening a gap at the Dirac points. The occupation of 3d orbitals of the adsorbed Ti atom changes with charging or applied electric field, inducing significant changes in magnetic moment. We predict neutral silicene uniformly covered by Ti atoms to become a half-metal at a specific value of coverage and hence allow the transport of electrons in one spin direction, but block the opposite direction. These calculated properties, however, exhibit a dependence on the size of the vacuum spacing between periodically repeating silicene and germanene layers, if they are treated using a plane wave basis set within periodic boundary conditions. We clarified the cause of this spurious dependence and show that it can be eliminated by the use of a local orbital basis set. © 2013 IOP Publishing Ltd.
Open Access
Epitaxial growth mechanisms of graphene and effects of substrates
(American Physical Society, 2012) Özçelik, V. O.; Cahangirov, S.; Çıracı, Salim
The growth process of single layer graphene with and without substrate is investigated using ab initio, finite temperature molecular dynamic calculations within density functional theory. An understanding of the epitaxial graphene growth mechanisms in the atomic level is provided by exploring the transient stages which occur at the growing edges of graphene. These stages are formation and collapse of large carbon rings together with the formation and healing of Stone-Wales like pentagon-heptagon defects. The activation barriers for the healing of these growth induced defects on various substrates are calculated using the climbing image nudge elastic band method and compared with that of the Stone-Wales defect. It is found that the healing of pentagon-heptagon defects occurring near the edge in the course of growth is much easier than that of Stone-Wales defect. The role of the substrate in the epitaxial growth and in the healing of defects are also investigated in detail, along with the effects of using carbon dimers as the building blocks of graphene growth. © 2012 American Physical Society.
Open Access
High-performance planar nanoscale dielectric capacitors
(American Physical Society, 2015) Özçelik, V. O.; Çıracı, Salim
We propose a model for planar nanoscale dielectric capacitors consisting of a single layer, insulating hexagonal boron nitride (BN) stripe placed between two metallic graphene stripes, all forming commensurately a single atomic plane. First-principles density functional calculations on these nanoscale capacitors for different levels of charging and different widths of graphene-BN stripes mark high gravimetric capacitance values, which are comparable to those of supercapacitors made from other carbon-based materials. Present nanocapacitor models allow the fabrication of series, parallel, and mixed combinations which offer potential applications in two-dimensional flexible nanoelectronics, energy storage, and heat-pressure sensing systems.
Open Access
Hydrogenated carbon monolayer in biphenylene network offers a potential paradigm for nanoelectronic devices
(American Chemical Society, 2022-09-15) Demirci, S.; Gorkan, T.; Çallıoǧlu, Şafak; Özçelik, V. O.; Barth, J.; Aktürk, E.; Çıracı, Salim
A metallic carbon monolayer in the biphenylene network (specified as C ohs) becomes an insulator upon hydrogenation (specified as CH ohs). Patterned dehydrogenation of this CH ohs can offer a variety of intriguing functionalities. Composite structures constituted by alternating stripes of C and CH ohs with different repeat periodicity and chirality display topological properties and can form heterostructures with a tunable band-lineup or Schottky barrier height. Alternating arrangements of these stripes of finite size enable one to also construct double barrier resonant tunneling structures and 2D, lateral nanocapacitors with high gravimetric capacitance for an efficient energy storage device. By controlled removal of H atom from a specific site or dehydrogenation of an extended zone, one can achieve antidoping or construct 0D quantum structures like antidots, antirings/loops, and supercrystals, the energy level spacing of which can be controlled with their geometry and size for optoelectronic applications. Conversely, all these device functions can be acquired also by controlled hydrogenation of a bare C ohs monolayer. Since all these processes are applied to a monolayer, the commensurability of electronically different materials is assured. These features pertain not only to CH ohs but also to fully hydrogenated Si ohs.
Open Access
Local reconstructions of silicene induced by adatoms
(American Chemical Society, 2013) Özçelik, V. O.; Çıracı, Salim
The interaction of silicene with Si, C, H, O, and Ti atoms along with H2, H2O, and O2 molecules are investigated and the induced functionalities thereof are analyzed using first principles density functional theory. Si adatom initially adsorbed at the top site of silicene pushes down the Si atom underneath to form a dumbbell like structure with 3 + 1 coordination. This prediction is important for silicene research and reveals new physical phenomena related to the formation of multilayer Si, which is apparently the precursor state for the missing layered structure of silicon. We found that dumbbell structure attributes coverage-dependent electronic and magnetic properties to nonmagnetic bare silicene. Even more interesting is that silicene with dumbbells is energetically more favorable than the pristine silicene: The more dense the dumbbell coverage, the stronger is the cohesion. Incidentally, these structures appear to be intermediate between between silicene and silicon. The carbon adatom, which is initially adsorbed to the bridge position, substitutes one Si atom, if it overcomes a small energy barrier. The oxygen molecule can dissociate on the silicene surface, whereby constituent oxygen atoms oxidize silicene by forming strong bonds. By varying the concentration and decoration of carbon, hydrogen and oxygen atoms, one can tune the band gap of silicene. Through the adsorption of hydrogen or titanium adatom, silicene acquires spin-polarized state. A half-metallic ferromagnetic behavior is attained at specific uniform coverage of Ti adatom, which may function as a spin valve. © 2013 American Chemical Society.
Open Access
Modulation of electronic properties in laterally and commensurately repeating graphene and boron nitride composite nanostructures
(American Chemical Society, 2015) Özçelik, V. O.; Durgun, Engin; Çıracı, Salim
Graphene and hexagonal boron nitride (h-BN) nanoribbons of diverse widths and edge geometries are laterally repeated to form commensurate, single-layer, hybrid honeycomb structures. The resulting composite materials appear as continuous, one atom thick stripes of graphene and BN having the average mechanical properties of constituent structures. However, depending on the widths of constituent stripes they can be metal or semiconductor with band gaps in the energy range of the visible light. These two-dimensional (2D) composite materials allow strong dimensionality in electrical conductivity and undergo transition from 2D to one-dimensional (1D) metal in a 2D medium, resulting in multichannel narrow conductors. As for the composite ribbons, such as one dielectric BN stripe placed between two graphene stripes with bare zigzag edges, charge separation of opposite polarity is possible under applied electric field and they exhibit resonant tunneling effects at nanoscale. Graphene/BN composite materials also form stable single-wall nanotubes with zigzag or armchair geometries.
Open Access
Nanoscale dielectric capacitors composed of graphene and boron nitride layers: a first-principles study of high capacitance at nanoscale
(American Chemical Society, 2013) Özçelik, V. O.; Çıracı, Salim
We investigate a nanoscale dielectric capacitor model consisting of two-dimensional, hexagonal h-BN layers placed between two commensurate and metallic graphene layers using self-consistent field density functional theory. The separation of equal amounts of electric charge of different sign in different graphene layers is achieved by applying an electric field perpendicular to the layers. The stored charge, energy, and the electric potential difference generated between the metallic layers are calculated from the first principles for the relaxed structures. Predicted high-capacitance values exhibit the characteristics of supercapacitors. The capacitive behavior of the present nanoscale model is compared with that of the classical Helmholtz model, which reveals crucial quantum size effects at small separations, which in turn recede as the separation between metallic planes increases. © 2013 American Chemical Society.
Open Access
New phases of germanene
(American Chemical Society, 2014) Özçelik, V. O.; Durgun, Engin; Çıracı, Salim
Germanene, a graphene-like single-layer structure of Ge, has been shown to be stable and recently grown on Pt and Au substrates. We show that a Ge adatom adsorbed on germanene pushes down the host Ge atom underneath and forms a dumbbell structure. This exothermic process occurs spontaneously. The attractive dumbbell-dumbbell interaction favors high coverage of dumbbells. This Letter heralds stable new phases of germanene, which are constructed from periodically repeating coverage of dumbbell structures and display diversity of electronic and magnetic properties.
Open Access
Prediction of a two-dimensional crystalline structure of nitrogen atoms
(American Physical Society, 2015) Özçelik, V. O.; Aktürk, O. U.; Durgun, Engin; Çıracı, Salim
Based on first-principles density functional calculations, we predict that nitrogen atoms can form a single-layer, buckled honeycomb structure called nitrogene, which is rigid and stable even above room temperature. This 2D crystalline phase of nitrogen, which corresponds to a local minimum in the Born-Oppenheimer surface, is a nonmagnetic insulator with saturated π bonds. When grown on a substrate like Al(111) surface and graphene, nitrogene binds weakly to substrates and hence preserves its free-standing properties, but it can easily be pealed off. Zigzag and armchair nanoribbons of nitrogene have fundamental band gaps derived from reconstructed edge states. These band gaps are tunable with size and suitable for the emerging field of 2D electronics. Nitrogene forms not only bilayer, but also 3D graphitic multilayer structures. Single-layer nitrogene can nucleate and grow on the armchair edges of hexagonal boron nitride.
Open Access
Self-assembly mechanisms of short atomic chains on single-layer graphene and boron nitride
(American Physical Society, 2012) Özçelik, V. O.; Çıracı, Salim
Nucleation and growth mechanisms of short chains of carbon atoms on single-layer, hexagonal boron nitride (h-BN) and short BN chains on graphene are investigated using first-principles plane-wave calculations. Our analysis starts with the adsorption of a single carbon adatom and examines its migrations. Once a C-2 nucleates on h-BN, the insertion of each additional carbon at its close proximity causes a short segment of carbon atomic chain to grow by one atom at at a time in a quaint way: The existing chain leaves its initial position and subsequently is attached from its bottom end to the top of the carbon adatom. The electronic, magnetic, and structural properties of these chains vertically adsorbed to h-BN depend on the number of carbon atoms in the chain, such that they exhibit an even-odd disparity. An individual carbon chain can also modify the electronic structure with localized states in the wide band gap of h-BN. As a reverse situation, we examined the growth of short BN atomic chains on graphene, which attribute diverse properties depending on whether B or N is the atom bound to the substrate. These results together with ab initio molecular dynamics simulations of the growth process reveal the interesting self-assembly behavior of the grown chains. Furthermore, we find that these atomic chains enhance the chemical activity of h-BN and graphene sheets by creating active sites for the bonding of various adatoms and can act as pillars between two and multiple sheets of these honeycomb structures, leaving wider spacing between them to achieve high-capacity storage of specific molecules.
Open Access
Self-healing of vacancy defects in single-layer graphene and silicene
(American Physical Society, 2013) Özçelik, V. O.; Gurel, H. H.; Çıracı, Salim
Self-healing mechanisms of vacancy defects in graphene and silicene are studied using first-principles calculations. We investigated host adatom adsorption, diffusion, vacancy formation, and revealed atomistic mechanisms in the healing of single, double, and triple vacancies of single-layer graphene and silicene. Silicon adatom, which is adsorbed to silicene at the top site forms a dumbbell-like structure by pushing one Si atom underneath. The asymmetric reconstruction of the single vacancy in graphene is induced by the magnetization through the rebonding of two dangling bonds and acquiring a significant magnetic moment through the remaining unsaturated dangling bond. In silicene, three twofold coordinated atoms surrounding the single vacancy become fourfold coordinated and nonmagnetic through rebonding. The energy gained through new bond formation becomes the driving force for the reconstruction. Under the external supply of host atoms, while the vacancy defects of graphene heal perfectly, the Stone-Wales defect can form in the course of healing of silicene vacancy. The electronic and magnetic properties of suspended, single-layer graphene and silicene are modified by reconstructed vacancy defects. © 2013 American Physical Society.
Open Access
Silicite: the layered allotrope of silicon
(American Physical Society, 2014) Cahangirov, S.; Özçelik, V. O.; Rubio, A.; Çıracı, Salim
Based on first-principles calculations, 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 root 3x root 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.
Open Access
Single-layer crystalline phases of antimony: Antimonenes
(American Physical Society, 2015) Aktürk, O. Ü.; Özçelik, V. O.; Çıracı, Salim
The pseudolayered character of 3D bulk crystals of antimony has led us to predict its 2D single-layer crystalline phase named antimonene in a buckled honeycomb structure like silicene. Sb atoms also form an asymmetric washboard structure like black phospherene. Based on an extensive analysis comprising ab initio phonon and finite-temperature molecular dynamics calculations, we show that these two single-layer phases are robust and can remain stable at high temperatures. They are nonmagnetic semiconductors with band gaps ranging from 0.3 eV to 1.5 eV, and are suitable for 2D electronic applications. The washboard antimonene displays strongly directional mechanical properties, which may give rise to a strong influence of strain on the electronic properties. Single-layer antimonene phases form bilayer and trilayer structures with wide interlayer spacings. In multilayers, this spacing is reduced and eventually the structure changes to 3D pseudolayered bulk crystals. The zigzag and armchair nanoribbons of the antimonene phases have fundamental band gaps derived from reconstructed edge states and display a diversity of magnetic and electronic properties depending on their width and edge geometry. Their band gaps are tunable with the widths of the nanoribbons. When grown on substrates, such as germanene or Ge(111), the buckled antimonene attains a significant influence of substrates.
Open Access
Size dependence in the stabilities and electronic properties of α-graphyne and its boron nitride analogue
(American Chemical Society, 2013) Özçelik, V. O.; Çıracı, Salim
We predict the stabilities of α-graphynes and their boron nitride analogues (α-BNyne), which are considered as competitors of graphene and two-dimensional hexagonal BN. On the basis of the first-principles plane wave method, we investigated the stability and structural transformations of these materials at different sizes using phonon dispersion calculations and ab initio finite temperature, molecular dynamics simulations. Depending on the number of additional atoms in the edges between the corner atoms of the hexagons, n, both α-graphyne(n) and α-BNyne(n) are stable for even n but unstable for odd n. α-Graphyne(3) undergoes a structural transformation, where the symmetry of hexagons is broken. We present the structure-optimized cohesive energies and electronic, magnetic, and mechanical properties of stable structures. Our calculations reveal the existence of Dirac cones in the electronic structures of α-graphynes of all sizes, where the Fermi velocities decrease with increasing n. The electronic and magnetic properties of these structures are modified by hydrogenation. A single hydrogen vacancy renders a magnetic moment of one Bohr magneton. We finally present the properties of the bilayer α-graphyne and α-BNyne structures. We expect that these layered materials can function as frameworks in various chemical and electronic applications. © 2013 American Chemical Society.
Open Access
Stable single-layer honeycomblike structure of silica
(American Physical Society, 2014) Özçelik, V. O.; Cahangirov, S.; Çıracı, Salim
Silica or SiO2, the main constituent of Earth's rocks has several 3D complex crystalline and amorphous phases, but it does not have a graphitelike layered structure in 3D. Our theoretical analysis and numerical calculations from the first principles predict a single-layer honeycomblike allotrope, hα silica, which can be viewed to be derived from the oxidation of silicene and it has intriguing atomic structure with reentrant bond angles in hexagons. It is a wide band gap semiconductor, which attains remarkable electromechanical properties showing geometrical changes under an external electric field. In particular, it is an auxetic metamaterial with a negative Poisson's ratio and has a high piezoelectric coefficient. While it can form stable bilayer and multilayer structures, its nanoribbons can show metallic or semiconducting behavior depending on their chirality. Coverage of dangling Si orbitals by foreign adatoms can attribute new functionalities to hα silica. In particular, Si2O5, where Si atoms are saturated by oxygen atoms from top and bottom sides alternatingly can undergo a structural transformation to make silicatene, another stable, single layer structure of silica. © 2014 American Physical Society.
Open Access
Structure dependent optoelectronic properties of monolayer antimonene, bismuthene and their binary compound
(Royal Society of Chemistry, 2019) Kecik, Deniz; Özçelik, V. O.; Durgun, Engin; Çıracı, Salim
Two-dimensional (2D) antimonene, bismuthene, and their binary compound 2D BiSb possess high spin–orbit coupling (SOC) and potential topological insulator properties upon engineering their structural and chemical properties. Based on many-body first-principles calculations, we show that these materials can exhibit isotropic or anisotropic optoelectronic properties depending on their geometry, i.e. buckled (hb) or asymmetrical washboard (aw) phases. SOC significantly alters their optoelectronic properties, which is predominantly evident in 2D bismuthene. hb-antimonene absorbs light in the visible and partially in the ultraviolet regimes, while the absorption band edge for aw-antimonene, hb- and aw-bismuthene is set at the infrared region, absorption being spread as a broadband optical response through the spectral range. An exciton binding with 0.18 eV energy is detected for hb-bismuthene. Due to their broadband optical response, antimonene, bismuthene, and their binary compound offer possibilities towards applications as 2D materials in solar cells, light-emitting devices, photodetectors and light modulation.
Open Access
Superlubricity through graphene multilayers between Ni (111) surfaces
(American Physical Society, 2013) Cahangirov, S.; Çıracı, Salim; Özçelik, V. O.
A single graphene layer placed between two parallel Ni(111) surfaces screens the strong attractive force and results in a significant reduction of adhesion and sliding friction. When two graphene layers are inserted, each graphene is attached to one of the metal surfaces with a significant binding and reduces the adhesion further. In the sliding motion of these surfaces the transition from stick-slip to continuous sliding is attained, whereby nonequilibrium phonon generation through sudden processes is suppressed. The adhesion and corrugation strength continues to decrease upon insertion of the third graphene layer and eventually saturates at a constant value with increasing number of graphene layers. In the absence of Ni surfaces, the corrugation strength of multilayered graphene is relatively higher and practically independent of the number of layers. Present first-principles calculations reveal the superlubricant feature of graphene layers placed between pseudomorphic Ni(111) surfaces, which is achieved through the coupling of Ni-3d and graphene-π orbitals. The effect of graphene layers inserted between a pair of parallel Cu(111) and Al(111) surfaces is also discussed. The treatment of sliding friction under the constant loading force, by taking into account the deformations corresponding to any relative positions of sliding slabs, is the unique feature of our study. © 2013 American Physical Society.
Open Access
Two-dimensional pnictogens: a review of recent progresses and future research directions
(American Institute of Physics, 2019) Ersan, Fatih; Keçik, Deniz; Özçelik, V. O.; Kadıoğlu, Yelda; Üzengi-Aktürk, O.; Durgun, Engin; Aktürk, Ethem; Çıracı, Salim
Soon after the synthesis of two-dimensional (2D) ultrathin black phosphorus and fabrication of field effect transistors thereof, theoretical studies have predicted that other group-VA elements (or pnictogens), N, As, Sb, and Bi can also form stable, single-layer (SL) structures. These were nitrogene in a buckled honeycomb structure, arsenene, antimonene, and bismuthene in a buckled honeycomb, as well as washboard and square-octagon structures with unusual mechanical, electronic, and optical properties. Subsequently, theoretical studies are followed by experimental efforts that aim at synthesizing these novel 2D materials. Currently, research on 2D pnictogens has been a rapidly growing field revealing exciting properties, which offers diverse applications in flexible electronics, spintronics, thermoelectrics, and sensors. This review presents an evaluation of the previous experimental and theoretical studies until 2019, in order to provide input for further research attempts in this field. To this end, we first reviewed 2D, SL structures of group-VA elements predicted by theoretical studies with an emphasis placed on their dynamical and thermal stabilities, which are crucial for their use in a device. The mechanical, electronic, magnetic, and optical properties of the stable structures and their nanoribbons are analyzed by examining the effect of external factors, such as strain, electric field, and substrates. The effect of vacancy defects and functionalization by chemical doping through adatom adsorption on the fundamental properties of pnictogens has been a critical subject. Interlayer interactions in bilayer and multilayer structures, their stability, and tuning their physical properties by vertical stacking geometries are also discussed. Finally, our review is concluded by highlighting new research directions and future perspectives on the challenges in this emerging field.