Browsing by Author "Cahangirov, S."
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Item Open Access 2D vibrational properties of epitaxial silicene on Ag(111)(IOP Publishing, 2016-10) Solonenko, D.; Gordan, O. D.; Lay, G. L.; Sahin, H.; Cahangirov, S.; Zahn, D. R. T.; Vogt, P.The two-dimensional silicon allotrope, silicene, could spur the development of new and original concepts in Si-based nanotechnology. Up to now silicene can only be epitaxially synthesized on a supporting substrate such as Ag(111). Even though the structural and electronic properties of these epitaxial silicene layers have been intensively studied, very little is known about its vibrational characteristics. Here, we present a detailed study of epitaxial silicene on Ag(111) using in situ Raman spectroscopy, which is one of the most extensively employed experimental techniques to characterize 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous. The vibrational fingerprint of epitaxial silicene, in contrast to all previous interpretations, is characterized by three distinct phonon modes with A and E symmetries. Both, energies and symmetries of theses modes are confirmed by ab initio theory calculations. The temperature dependent spectral evolution of these modes demonstrates unique thermal properties of epitaxial silicene and a significant electron-phonon coupling. These results unambiguously support the purely two-dimensional character of epitaxial silicene up to about 300°C, whereupon a 2D-to-3D phase transition takes place. The detailed fingerprint of epitaxial silicene will allow us to identify it in different environments or to study its modifications.Item Open Access Anisotropic electronic, mechanical, and optical properties of monolayer WTe2(American Institute of Physics Inc., 2016) Torun, E.; Sahin, H.; Cahangirov, S.; Rubio, A.; Peeters, F. M.Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe2. Atomic structure and ground state properties of monolayer WTe2 (Td phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, WSe2, and MoTe2, which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the Td phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1% strain. By solving the Bethe-Salpeter equation on top of single shot G0W0 calculations, we predict that the absorption spectrum of Td-WTe2 monolayer is strongly direction dependent and tunable by tensile strain.Item Open Access Armchair nanoribbons of silicon and germanium honeycomb structures(American Physical Society, 2010) Cahangirov, S.; Topsakal, M.; Çıracı, SalimWe present a first-principles study of bare and hydrogen passivated armchair nanoribbons of the puckered single layer honeycomb structures of silicon and germanium. Our study includes optimization of atomic structure, stability analysis based on the calculation of phonon dispersions, electronic structure, and the variation in band gap with the width of the ribbon. The band gaps of silicon and germanium nanoribbons exhibit family behavior similar to those of graphene nanoribbons. The edges of bare nanoribbons are sharply reconstructed, which can be eliminated by the hydrogen termination of dangling bonds at the edges. Periodic modulation of the nanoribbon width results in a superlattice structure which can act as a multiple quantum well. Specific electronic states are confined in these wells. Confinement trends are qualitatively explained by including the effects of the interface. In order to investigate wide and long superlattice structures we also performed empirical tight-binding calculations with parameters determined from ab initio calculations.Item Open Access Atomic and electronic structure of silicene on Ag: a theoretical perspective(Springer, 2018) Cahangirov, S.; Rubio, A.The isolation of graphene sheets from its parent crystal graphites has given the kick to experimental research on its prototypical 2D elemental cousin, silicene [1]. Unlike graphene, silicene lacks a layered parent material from which it could be derived by exfoliation. Hence, the efforts of making the silicene dream a reality were focused on epitaxial growth of silicene on substrates. The first synthesis of epitaxial silicene on silver (111) [27, 46] and zirconium diboride templates [16] and next on an iridium (111) surface [31], has boosted research on other elemental group IV graphene-like materials, namely, germanene and stanene [30, 48]. The boom is motivated by several new possibilities envisaged for future electronics, typically because of the anticipated very high mobilities for silicene and germanene [49], as well as potential optical applications [30]. It is also fuelled by their predicted robust 2D topological insulator characters [14, 28] and potential high temperature superconductor character [5, 50]. One of the most promising candidates as a substrate is Ag because from the studies of the reverse system, where Ag atoms were deposited on silicon substrate, it was known that Ag and silicon make sharp interfaces without making silicide compounds [24]. Indeed, studies on synthesiz and characterization of silicene is mainly focused on using Ag(111) as substrates and hence we think it is important to understand this particular system. In this Chapter, we present a theoretical perspective on the studies investigating the atomic and electronic structure of silicene on Ag substrates.Item 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 SocietyItem Open Access Bilayer SnS2: tunable stacking sequence by charging and loading pressure(American Physical Society, 2016-03) Bacaksiz, C.; Cahangirov, S.; Rubio, A.; Senger, R. T.; Peeters, F. M.; Sahin, H.Employing density functional theory-based methods, we investigate monolayer and bilayer structures of hexagonal SnS2, which is a recently synthesized monolayer metal dichalcogenide. Comparison of the 1H and 1T phases of monolayer SnS2 confirms the ground state to be the 1T phase. In its bilayer structure we examine different stacking configurations of the two layers. It is found that the interlayer coupling in bilayer SnS2 is weaker than that of typical transition-metal dichalcogenides so that alternative stacking orders have similar structural parameters and they are separated with low energy barriers. A possible signature of the stacking order in the SnS2 bilayer has been sought in the calculated absorbance and reflectivity spectra. We also study the effects of the external electric field, charging, and loading pressure on the characteristic properties of bilayer SnS2. It is found that (i) the electric field increases the coupling between the layers at its preferred stacking order, so the barrier height increases, (ii) the bang gap value can be tuned by the external E field and under sufficient E field, the bilayer SnS2 can become a semimetal, (iii) the most favorable stacking order can be switched by charging, and (iv) a loading pressure exceeding 3 GPa changes the stacking order. The E-field tunable band gap and easily tunable stacking sequence of SnS2 layers make this 2D crystal structure a good candidate for field effect transistor and nanoscale lubricant applications.Item Open Access Confined linear carbon chains as a route to bulk carbyne(Nature Publishing Group, 2016-04) Shi, L.; Rohringer, P.; Suenaga, K.; Niimi, Y.; Kotakoski, J.; Meyer, J. C.; Peterlik, H.; Wanko, M.; Cahangirov, S.; Rubio, A.; Lapin, Z. J.; Novotny, L.; Ayala, P.; Pichler, T.Strong chemical activity and extreme instability in ambient conditions characterize carbyne, an infinite sp1 hybridized carbon chain. As a result, much less has been explored about carbyne as compared to other carbon allotropes such as fullerenes, nanotubes and graphene. Although end-capping groups can be used to stabilize carbon chains, length limitations are still a barrier for production, and even more so for application. We report a method for the bulk production of long acetylenic linear carbon chains protected by thin double-walled carbon nanotubes. The synthesis of very long arrangements is confirmed by a combination of transmission electron microscopy, X-ray diffraction and (near-field) resonance Raman spectroscopy. Our results establish a route for the bulk production of exceptionally long and stable chains composed of more than 6,000 carbon atoms, representing an elegant forerunner towards the final goal of carbyne's bulk production.Item Open Access Confined states in multiple quantum well structures of Sin Gen nanowire superlattices(American Physical Society, 2007) Akman, N.; Durgun, Engin; Cahangirov, S.; Çıracı, SalimMechanical properties, atomic and energy band structures of bare and hydrogen-passivated Sin Gen 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 Sin nanowires are stiffer than Gen nanowires. Hydrogen passivation makes these nanowires and Sin Gen 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 vary with the diameter, Sin Gen superlattices offer numerous alternatives for multiple quantum well devices with their leads made from the constituent metallic nanowires.Item Open Access Confinement of electrons in size-modulated silicon nanowires(American Physical Society, 2009) Cahangirov, S.; Çıracı, SalimBased 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.Item Open Access Epitaxial growth mechanisms of graphene and effects of substrates(American Physical Society, 2012) Özçelik, V. O.; Cahangirov, S.; Çıracı, SalimThe 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.Item Open Access First-principles study of defects and adatoms in silicon carbide honeycomb structures(The American Physical Society, 2010) Bekaroglu, E.; Topsakal, M.; Cahangirov, S.; Çıracı, SalimWe present a study of mechanical, electronic and magnetic properties of two-dimensional (2D), monolayer of silicon carbide (SiC) in honeycomb structure and its quasi-one-dimensional (quasi-1D) armchair nanoribbons using first-principles plane-wave method. In order to reveal dimensionality effects, a brief study of three-dimensional (3D) bulk and 1D atomic chain of SiC are also included. Calculated bond-lengths, cohesive energies, charge transfers and band gaps display a clear dimensionality effect. The stability analysis based on the calculation of phonon frequencies indicates that 2D SiC monolayer is stable in planar geometry. We found that 2D SiC monolayer in honeycomb structure and its bare and hydrogen passivated nanoribbons are ionic, nonmagnetic, wide band gap semiconductors. The band gap is further increased upon self-energy corrections. The mechanical properties are investigated using the strain energy calculations. The effect of various vacancy defects, adatoms, and substitutional impurities on electronic and magnetic properties in 2D SiC monolayer and in its armchair nanoribbons is also investigated. Some of these vacancy defects and impurities, which are found to influence physical properties and attain magnetic moments, can be used to functionalize SiC honeycomb structures. © 2010 The American Physical Society.Item Open Access First-principles study of GaAs nanowires(American Physical Society, 2009) Cahangirov, S.; Çıracı, SalimIn this paper we present a detailed analysis of the atomic and electronic structures of GaAs nanowires using first-principles pseudopotential calculations. We consider six different types of nanowires with different diameters all grown along [111] direction, and we reveal interesting trends between cohesive energy and nanowire type with varying diameters. Generally, the average cohesive energy of nanowires with wurtzite stacking is higher than those with zinc-blende stacking for small diameters. We found that most of the bare nanowires considered here are semiconducting and continue to be semiconducting upon the passivation of surface dangling bonds with hydrogen atoms. However, the surface states associated with the surface atoms having two dangling bonds in zinc-blende stacking occur in the band gap and can decrease the band gap to change the nanowire from semiconducting to metallic state. These nanowires become semiconducting upon hydrogen passivation. Even if the band gap of some nanowires decreases with increasing diameter and hence reveals the quantum confinement effect, generally the band-gap variation is rather complex, and depends on the type and geometry, diameter, type of relaxation, and also whether the dangling bonds of surface atoms are saturated with hydrogen.Item Open Access First-principles study of zinc oxide honeycomb structures(American Physical Society, 2009) Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Çıracı, SalimWe present a first-principles study of the atomic, electronic, and magnetic properties of two-dimensional 2D, single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. In order to reveal the dimensionality effects, our study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2D ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temperature ab initio molecular-dynamics calculations. 2D ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net magnetic moment upon the creation of zinc-vacancy defect. Zigzag ZnO nanoribbons are ferromagnetic metals with spins localized at the oxygen atoms at the edges and have high spin polarization at the Fermi level. However, they change to nonmagnetic metal upon termination of their edges with hydrogen atoms. From the phonon calculations, the fourth acoustical mode specified as twisting mode is also revealed for armchair nanoribbon. Under tensile stress the nanoribbons are deformed elastically maintaining honeycomblike structure but yield at high strains. Beyond yielding point honeycomblike structure undergo a structural change and deform plastically by forming large polygons. The variation in the electronic and magnetic properties of these nanoribbons have been examined under strain. It appears that plastically deformed nanoribbons may offer a new class of materials with diverse properties.Item Open Access Frictional figures of merit for single layered nanostructures(American Physical Society, 2012) Cahangirov, S.; Ataca, C.; Topsakal, M.; Sahin, H.; Çıracı, SalimWe determine the frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS 2 and WO 2 moving over each other, by carrying out ab initio calculations of interlayer interaction under constant loading force. Using the Prandtl-Tomlinson model we derive the critical stiffness required to avoid stick-slip behavior. We show that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby the materials avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays a much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers. © 2012 American Physical Society.Item Open Access Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, SalimPotential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).Item Open Access Hydrogen-induced sp2-sp3 rehybridization in epitaxial silicene(American Physical Society, 2017) Solonenko, D.; Dzhagan, V.; Cahangirov, S.; Bacaksiz, C.; Sahin, H.; Zahn, D. R. T.; Vogt, P.We report on the hydrogenation of (3×3)/(4×4) silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of (3×3)/(4×4) silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed sp2-sp3 to a dominating sp3 state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries. © 2017 American Physical Society.Item Open Access Long-range interactions in carbon atomic chains(American Physical Society, 2010) Cahangirov, S.; Topsakal, M.; Çıracı, SalimBased on first-principles calculations we revealed fundamental properties of infinite and finite-size monatomic chains of carbon atoms in equilibrium and under an applied strain. Distributions of bond lengths and magnetic moments at atomic sites exhibit interesting even-odd disparity depending on the number of carbon atoms in the chain and on the type of saturation of carbon atoms at both ends. It was shown that, the bands of carbon atomic chains behave as a one-dimensional free-electron system. A local perturbation created by a small displacement of the single carbon atom at the center of a long chain induces oscillations of atomic forces and charge density, which are carried to long distances over the chain. These long-ranged oscillations are identified as Friedel oscillations showing 1/r decay rate in one-dimensional systems.Item 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, SalimUsing 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.Item Open Access Polyyne electronic and vibrational properties under environmental interactions(American Physical Society, 2016) Wanko, M.; Cahangirov, S.; Shi, L.; Rohringer, P.; Lapin, Z. J.; Novotny, L.; Ayala, P.; Pichler, T.; Rubio, A.Recently the novel system of linear carbon chains inside double-walled carbon nanotubes has extended the length of sp1 hybridized carbon chains from 44 to thousands of atoms [Shi, Nat. Mater. 15, 634 (2016)1476-112210.1038/nmat4617]. The optoelectronic properties of these ultralong chains are poorly described by current theoretical models, which are based on short chain experimental data and assume a constant environment. As such, a physical understanding of the system in terms of charge transfer and van der Waals interactions is widely missing. We provide a reference for the intrinsic Raman frequency of polyynes in vacuo and explicitly describe the interactions between polyynes and carbon nanotubes. We find that van der Waals interactions strongly shift this frequency, which has been neither expected nor described for other intramolecular C-C stretching vibrations. As a consequence of charge transfer from the tube to the chain, the Raman response of long chains is qualitatively different from the known phonon dispersion of polymers close to the Γ point. Based on these findings we show how to correctly interpret the Raman data, considering the nanotube's properties. This is essential for its use as an analytical tool to optimize the growth process for future applications.Item Open Access The response of mechanical and electronic properties of graphane to the elastic strain(AIP Publishing LLC, 2010) Topsakal, M.; Cahangirov, S.; Çıracı, SalimBased on first-principles calculations, we resent a method to reveal the elastic properties of recently synthesized monolayer hydrocarbon, graphane. The in-plane stiffness and Poisson’s ratio values are found to be smaller than those of graphene, and its yielding strain decreases in the presence of various vacancy defects and also at high ambient temperature. We also found that the band gap can be strongly modified by applied strain in the elastic range.