Browsing by Author "Gürel, H. H."

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Open Access
Effects of charging and electric field on graphene functionalized with titanium
(Institute of Physics Publishing, 2013) Gürel, H. H.; Çıracı, Salim
Titanium atoms are adsorbed to graphene with a significant binding energy and render diverse functionalities to it. Carrying out first-principles calculations, we investigated the effects of charging and static electric field on the physical and chemical properties of graphene covered by Ti adatoms. When uniformly Ti covered graphene is charged positively, its antiferromagnetic ground state changes to ferromagnetic metal and attains a permanent magnetic moment. Static electric field applied perpendicularly causes charge transfer between Ti and graphene, and can induce metal-insulator transition. While each Ti adatom adsorbed to graphene atom can hold four hydrogen molecules with a weak binding, these molecules can be released by charging or applying electric field perpendicularly. Hence, it is demonstrated that charging and applied static electric field induce quasi-continuous and side specific modifications in the charge distribution and potential energy of adatoms absorbed to single-layer nanostructures, resulting in fundamentally crucial effects on their physical and chemical properties. © 2013 IOP Publishing Ltd.
Open Access
Effects of charging and electric field on graphene oxide
(American Chemical Society, 2013) Topsakal, M.; Gürel, H. H.; Çıracı, Salim
We present a first-principles study of various effects of charging and electric field on the oxidation/deoxidation of graphene oxide consisting of only epoxy groups. We first determined the proper basis set, which hinders the spurious spilling of electrons of graphene oxide when negatively charged or exerted by perpendicular electric field, and treated with periodic boundary conditions. We then showed that the electric field perpendicularly applied to graphene surface provide side-specific functionalization. We found that the bonds between oxygen and graphene are weakened under applied electric field. For specific values of excess charge or electric field, an oxygen atom that is normally adsorbed to the bridge site in equilibrium moves to the top site. By directly charging and/or by applying electric field, one can monitor this migration as well as desorption of the oxygen adatom. In spite of the negative formation energy, an energy barrier prevents individually adsorbed oxygen atoms from forming oxygen molecules. This energy barrier is dramatically weakened upon negative charging or exertion of an electric field. Our results explain why the reduction of graphene oxide can be facilitated by these external effects. © 2013 American Chemical 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
Enhanced reduction of graphene oxide by means of charging and electric fields applied to hydroxyl groups
(Institute of Physics Publishing, 2013) Gürel, H. H.; Çıracı, Salim
We present a first-principles study of the effects of charging and perpendicular electric fields on hydroxyl groups, both of which mediate the reduction of graphene oxide through the formation of H2O and H 2O2. Starting with an investigation of the interaction between the hydroxyl groups and graphene, we determine the equilibrium binding geometry, binding energy, and the diffusion path with a minimum energy barrier and show that those equilibrium properties are strongly affected by external agents. While co-adsorbed H and O form bound OH, co-adsorbed H and OH in close proximity form H2O with almost no energy barrier. When negatively charged or subjected to a perpendicular electric field, the energy barrier between two OH co-adsorbed in close proximity is weakened or totally suppressed, forming an oxygen atom strongly bound at the bridge site, together with a water molecule. The water molecule by itself is very weakly bound to graphene and is prone to desorb from the surface, leading to the reduction of graphene oxide. It is therefore demonstrated that the reduction of graphene oxide is promoted to a large extent by negative charging or an applied perpendicular electric field, through the formation of weakly bound water molecules from hydroxyl groups. © 2013 IOP Publishing Ltd.
Open Access
Temperature, strain and charge mediated multiple and dynamical phase changes of selenium and tellurium
(Royal Society of Chemistry, 2020) Demirci, Salih; Gürel, H. H.; Jahangirov, Seymur; Çıracı, Salim
Semiconducting selenium and tellurium in their 3D bulk trigonal structures consist of parallel and weakly interacting helical chains of atoms and display a number of peculiarities. We predict that thermal excitations, 2D compressive strain and excess charge of positive and negative polarity mediate metal–insulator transitions by transforming these semiconductors into different metallic crystal structures. When heated to high temperature, or compressed, or charged positively, they change into a simple cubic structure with metallic bands, which is very rare among elemental crystals. When charged negatively, they transform first into body-centered tetragonal and subsequently into the body-centered orthorhombic structures with increasing negative charging. These two new structures stabilized by excess electrons also have overlapping metallic bands and quasi 2D and 1D substructures of lower dimensionality. Since the external charging of crystals can be achieved through their surfaces, the effects of charging on 2D structures of selenium and tellurium are also investigated. Similar structural transformations have been mediated also in 2D nanosheets and free-standing monolayers of these elements. These phase changes assisted by phonons are dynamical, reversible and tunable; the resulting metal–insulator transitions can occur within very short time intervals and may offer important device applications.
Open Access
Tunable dynamics of a flake on graphene: libration frequency
(American Physical Society, 2017-03) Aktürk, O.; Aktürk, E.; Gürel, H. H.; Çıracı, Salim
In this paper we investigated the interaction between a graphene nanoflake anchored to the 2D graphene monolayer. This interaction is attractive but weak and is capable of setting a well defined registry in equilibrium. Rotational and linear displacements from equilibrium registry generate restoring forces, which can be controlled by external agents. Similar flakes can be self-assembled and can also execute simple harmonic motion as if a physical pendulum. Oscillation of a nanoflake about their equilibrium registries resulting in a characteristic libration frequency is predicted. This frequency depends on the size and geometry of the flake. Moreover, the libration frequency, as well as the electronic and magnetic properties of the flake+monolayer systems, can be tuned by a foreign molecule anchored to the flake, by electric charging and applied parallel and perpendicular electric and magnetic fields. When the sliding of the flake is combined with rotation, the friction force can be reduced dramatically. It is surprising that weak interaction can offer such features at nanoscale, which may offer potential applications. Our predictions are obtained by first-principles calculations based on density functional theory.