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Item Open Access Effects of charging on two-dimensional honeycomb nanostructures(Bilkent University, 2012) Topsakal, MehmetIn this thesis we employ state-of-the-art first-principles calculations based on density functional theory (DFT) to investigate the effects of static charging on two-dimensional (2D) honeycomb nanostructures. Free standing, single-layer graphene and other similar single-layer honeycomb structures such as boron nitride (BN), molybenum disulfide (MoS2), graphane (CH) and fluorographene (CF) have been recently synthesized with their unusual structural, electronic and magnetic properties. Through understanding of the effects of charging on these nanostructures is essential from our points of view in order to better understand their fundamental physics and developing useful applications. We show that the bond lengths and hence 2D lattice constants increase as a result of electron removal from the single layer. Consequently, phonons soften and the frequencies of Raman active modes are lowered. Three-layer, wide band gap BN and MoS2 sheets are metalized while these slabs are wide band semiconductors, and excess positive charge is accumulated mainly at the outermost atomic layers. Excess charges accumulated on the surfaces of slabs induce repulsive force between outermost layers. With increasing positive charging the spacing between these layers increases, which eventually ends up with exfoliation when exceeded the weak van der Waals (vdW) attractions between layers. This result may be exploited to develop a method for intact exfoliation of graphene, BN and MoS2 multilayers. In addition we also show that the binding energy and local magnetic moments of specific adatoms can be tuned by charging. We have addressed the deficiencies that can occur as an artifact of using plane-wave basis sets in periodic boundary conditions and proposed advantages of using atomicorbital based methods to overcome these deficiencies. Using the methods and computation elucidated in this thesis, the effects of charging on periodic as well as finite systems and the related properties can now be treated with reasonable accuracy. The adsorption of oxygen atoms on graphene have been investigated extensively before dealing with the charging of graphene oxide (GOX). The energetics and the patterns of oxygen coverage trends are shown to be directly related with the amount of bond charge at the bridge sites of graphene structure. We finally showed that the diffusion barriers for an oxygen atom to migrate on graphene surface is significantly modified with charging. While the present results comply with the trends observed in the experimental studies under charging, we believe that there are other factors affecting the reversible oxidation-reduction processes.Item Open Access Electro-magnetic properties and phononic energy dissipation in graphene based structures(Bilkent University, 2008) Sevinçli, HaldunWith the synthesis of a single atomic plane of graphite, namely graphene honeycomb structure, active research has been focused on the massless Dirac fermion behavior and related artifacts of the electronic bands crossing the linearly at the Fermi level. This thesis presents a theoretical study on the electronic and magnetic properties of graphene based structures, and phononic energy dissipation. First, functionalization of these structures by 3d-transition metal (TM) atoms is investigated. The binding energies, electronic and magnetic properties have been investigated for the cases where TM-atoms adsorbed to a single side and double sides of graphene. It is found that 3d-TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 to 1.95 eV depending on their species and coverage density. Upon TM-atom adsorption graphene becomes a magnetic metal. TM-atoms can also be adsorbed to graphene nanoribbons with armchair edge shapes (AGNRs). Binding of TM-atoms to the edge hexagons of AGNR yield the minimum energy state for all TM-atom species examined in this work and in all ribbon widths under consideration. Dependingon the ribbon width and adsorbed TM-atom species, AGNR, a non-magnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or anti-ferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNRs half-metallic with a 100% spin polarization at the Fermi level. These results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. Second, repeated heterostructures of zigzag graphene nanoribbons of different widths are shown to form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide novel features for spintronic applications. Third, as a possible device application, a resonant tunnelling double barrier structure formed from a finite segment of armchair graphene nanoribbon with varying widths has been proposed based on first-principles transport calculations. Highest occupied and lowest unoccupied states are confined in the wider region, whereas the narrow regions act as tunnelling barriers. These confined states are identified through the energy level diagram and isosurface charge density plots which give rise to sharp peaks originating from resonant tunnelling effect. Finally, we studied dynamics of dissipation of local vibrations to the surrounding substrate. A model system consisting of an excited nano-particle which is weakly coupled with a substrate is considered. Using three different methods, the dynamics of energy dissipation for different types of coupling between the nano-particle and the substrate is studied, where different types of dimensionality and phonon densities of states were also considered for the substrate. Results of this theoretical analysis are verified by a realistic study. To this end the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated performing first-principles calculations.Item Open Access Electronic structure of graphene nano-ribbons(Bilkent University, 2008) Şen, Hüseyin ŞenerGraphite is a known material to human kind for centuries as the lead of a pencil. Graphene as a two dimensional material, is the single layer of graphite. Many theoretical works have been done about it so far, however, it newer took attention as it takes nowadays. In 2004, Novoselov et al. was able to produce graphene in 2D. Now that, making experiments on graphene is possible scientists have to renew their theoretical knowledge about systems in two dimension because graphene, due to its electronic structure, is able to prove the ideas in quantum relativistic phenomena. Indeed, recent theoretical studies were able to show that, electrons and holes behave as if they are massless fermions moving at a speed about 106m/s (c/300, c being speed of light) due to the linear electronic band dispersion near K points in the brillouin zone which was observed experimentally as well. Having zero band gap, graphene cannot be used directly in applications as a semiconductor. Graphene Nano-Ribbons (GNRs) are finite sized graphenes. They can have band gaps differing from graphene, so they are one of the new candidates for band gap engineering applications such as field effect transistors. This work presents theoretical calculation of the band structures of Graphene NanoRibbons in both one (infinite in one dimension) and zero dimensions (finite in both dimensions) with the help of tight binding method. The calculations were made for Zigzag, Armchair and Chiral Graphene Nano-Ribbons (ZGNR,AGNR,CGNR) in both 1D and 0D. Graphene nano-ribbons with zero band gap (ZGNR and AGNR) are observed in the calculations as well as the ribbons with finite band gaps (AGNR and CGNR) which increase with the decrease in the size of the ribbon making them much more suitable and strong candidate to replace silicon as a semiconductor.Item Open Access Electronic structure of graphene under the influence of external fields(Bilkent University, 2012) İslamoğlu, SelcenIn this thesis, the electronic structure of graphene under the influence of external fields such as strain or magnetic fields is investigated by using tight-binding method. Firstly, we study graphene for a band gap opening due to uniaxial strain. In contrast to the literature, we find that by considering all the bands (both σ and π bands) in graphene and including the second nearest neighbor interactions, there is no systematic band gap opening as a function of applied strain. Our results correct the previous works on the subject. Secondly, we examine the band structure and Hall conductance of graphene under the influence of perpendicular magnetic field. For graphene, we demonstrate the energy spectrum in the presence of magnetic field (Hofstadter Butterfly) where all orbitals are included. We recover both the usual and the anomalous integer quantum Hall effects depending on the proximity of the Dirac points for pure graphene and the usual integer quantum Hall effect for pure square lattice. Then, we explore the evolution of electronic properties when imperfections are introduced systematically to the system. We also demonstrate the results for a square lattice which has a distinct position in cold atom experiments. For the energy spectrum of imperfect graphene and square lattice under magnetic field (Hofstadter Butterflies), we find that impurity atoms with smaller hopping constants result in highly localized states which are decoupled from the rest of the system. The bands associated with these states form close to E = 0 eV line. On the other hand, impurity atoms with higher hopping constants are strongly coupled with the neighboring atoms. These states modify the Hofstadter Butterfly around the minimum and maximum values of the energy and for the case of graphene they form two self similar bands decoupled from the original butterfly. We also show that the bands and gaps due to the impurity states are robust with respect to the second order hopping. For the Hall conductance, in accordance with energy spectra, the localized states associated to the smaller hopping constant impurities or vacancies do not contribute to Hall conduction. However the higher hopping constant impurities are responsible for new extended states which contribute to Hall conduction. Our results for Hall conduction are also robust with respect to the second order interactions.Item Open Access First-principles investigation of functionalization of graphene(Bilkent University, 2013) Korkmaz, YaprakThe graphene sheet is a single-atom thick novel material and attracts great interest due to its unique features. However, it is a metallic material with no bandgap, which makes it difficult to integrate in electronic applications. Adatom adsorption is one of the promising ways to make this structure functional. To this end, electronic and structural properties of graphene have been investigated by using density functional theory formalism in order to understand atomic level interaction between halogen adatoms and graphene layer. The most common adatom, hydrogen, has also been studied. In this study, plane-wave, pseudopotential density functional theory calculations were carried out using generalized gradient approximations for the exchange correlation potential with the Quantum Espresso package. In order to obtain fully relaxed structures, geometry optimization has been performed in all of the calculations. The adatom-graphene system is modelled with a 4 × 4 graphene supercell. Adsorption energies of halogen adatoms and dimers adsorbed on highly symmetric positions on graphene layer are calculated. Different configurations of adatoms have been tested. Specific properties such as band structure and density of states of these system have been investigated. The results show that a fully covered graphene layer is stable and optimized structures exhibit a band gap of a few eV. The most stable structure among halogen adatoms is the fluorine adsorbed on graphene. It has the highest electronegativity, which is the reason for high electron transfer from the graphene layer. This is the reason of the formation of covalent bonds. Furthermore, the most stable configuration is found to be chair configuration with the halogen atoms alternating in both sides of the layer.Item Open Access First-principles investigation of graphitic nanostructures(Bilkent University, 2013) Şen, Hüseyin ŞenerIn this thesis, first-principles investigations of several graphene related nanosystems based on density functional theory are presented. First, the electronic structure of several graphene nano-ribbons both in 1D and 0D (up to systems with more than 1000 atoms) including all types (armchair, zigzag and chiral) are discussed using tight binding calculations. We observed that the band gap of the ribbons depend both on the length of the ribbon and the angle of chirality. Second, the effect of phosphorus and sulfur during the growth of carbon nanotubes is investigated from ab-initio density functional theory based calculations. To this end, we present the binding chemistry of phosphorus and sulfur atoms on graphene with and without vacancies and kink like defect structures. Consequently, the difference between the bindings of these two atoms is discussed in order to understand the reason behind their effects on the growth mechanism. The details of the phosphorus or sulfur binding are important in order to understand the occurrence of Y-junctions and kinks in carbon nanotubes as well. Third, we focus on the interaction of bilayer graphite and multi-walled carbon nanotubes with the Li atom since these materials are prime candidates for the electrodes for battery applications. The need for rechargeable batteries with high capacity increased enormously by the invention of electronic devices like cell phones or MP3 players. Hence, there is a huge effort to develop and improve Li-ion batteries. Therefore, we have investigated interaction of Li with graphene and Li intercalation to bilayer graphene and multi-walled carbon nanotubes from planewave pseudo potential calculations. Finally, super-periodic graphitic structures observed through scanning tunnelling microscope are described and investigated from density functional calculations. The difference between the observed and actual periodicity and the occurrence of the so-called Moire patterns are explained in terms of geometrical calculations and the charge density of these systems.Item Open Access Functionalization of graphene and stoichiometric graphene derivatives(Bilkent University, 2011) Şahin, HasanRe ent developments in experimental te hniques have made the design and produ tion of materials at nanos ale possible. In parti ular, graphene has been the fo us of resear h in diverse elds owing to high mobility arrier transport and other ex eptional properties. Over the past four years experimental studies have demonstrated that hemi al onversion of graphene to its stoi hiometri derivatives is possible by hydrogenation, uorination and hlorination. The aim of this thesis is to predi t stable stoi hiometri graphene derivatives and explore their me hani al, ele troni and magneti properties. Moreover, the fun tionalization of graphene and its derivatives are a hieved, whereby their physi al properties are modi ed to derive novel materials. Our predi tions revealing stable 2D single layer onformers, whi h an be used as novel nano oeting materials, are obtained from state-of-the art rst-prin iples Density Fun tional al ulations of total energy, phonons, transition state analysis and ab-initio mole ular dynami s. An extensive theoreti al study on the stability of hydrogenated graphene (CnH), fully hydrogenated graphane i.e graphane (CH), and their quasi onedimensional nanoribbons is performed. The formation of meshes of dehydrogenated domains on graphane resulted in geometry spe i magneti stru tures showing interesting magneti intera tions. Creation of H and CH va an ies, as well as adsorption of transition metal atoms give rise to signi ant spinpolarization in graphane nanoribbons. It is shown that as a result of one-sided or two-sided uorination of graphene one an obtain nanostru tures with diverse ele troni and magneti properties. Fully uorinated graphene or uorographene CF is a stable, sti and non-magneti semi ondu tor. Additionally, this onformer of bu kled graphene is fun tionalized by alkali, non-metal, metalloid and transition metal atoms, and ea h group leads to diverse adsorption properties. Adsorption of hlorine to graphene is dramati ally di erent from those of hydrogen and uorine. While the binding energy of hlorine is signi ant, its migration on the surfa e of perfe t graphene takes pla e almost without barrier. This is ru ial for energy harvesting on graphene surfa e. Energy optimization and phonon al ulations indi ate that the hair on guration of fully hlorinated graphene ( hlorographene) is energeti ally most favorable and stable. It is a nonmagneti semi ondu tor with 1.2 eV dire t band gap, whi h an be tuned by applied uniform strain. Graphene by itself an be fun tionalized by reating meshes of va an ies or adatoms onserving spe i symmetries. Under these ir umstan es linearly rossing bands and hen e the massless Dira Fermion behavior an be maintained. Finally, it is demonstrated that multilayer, even single layer graphene onstitute an ex ellent nanos ale oating, whi h an prevent a rea tive metal surfa e from oxidation without hanging the size and other physi al properties. Graphene an sti k to at metal surfa es and hinders free oxygen atom and mole ule from penetrating to the metal surfa e. Single layer uorographene an be used also for the same purposes. Design of novel nanomaterials, in parti ular biologi al mole ules and omplexes using rst-prin iples methods derived from quantum theory indi ates a new dire tion in theory, whi h promises a produ tive hybridization with experimental studies.Item Open Access Graphene based high frequency electronics(Bilkent University, 2010) Pinçe, ErçağRecent advances in chemical vapor deposition of graphene on large area substrates stimulate a significant research effort in order to search for new applications of graphene in the field of unusual electronics such as macroelectronics. The primary aim of this work is to use single layer of graphene for applications of high frequency electronics. This thesis consists of both theoretical and experimental studies of graphene transistors for the use of radio frequency electronics. We have grown graphene layer using chemical vapor deposition technique on large area copper substrates. The grown graphene layers are then transferred onto dielectric substrates for the fabrication of graphene transistors. The theoretical part of the thesis is focused on the understanding the performance limits of the graphene transistor for high frequency operation. We investigate the intrinsic high frequency performance of graphene field effect transistors using a self consistent transport model. The self-consistent transport model is based on a nonuniversal diffusive transport that is governed by the charged impurity scattering. The output and transfer characteristics of graphene field effect transistors are characterized as a function of impurity concentration and dielectric constant of the gate insulator. These experimental and theoretical studies shape the basis of our research on the graphene based radio frequency electronics.Item Open Access High performance floating gate memories using graphene as charge storage medium and atomic layer deposited high-k dielectric layers as tunnel barrier(Bilkent University, 2013) Kocaay, DenizWith the ongoing development in portable electronic devices, low power consumption, improved data retention rate and higher operation speed are the merits demanded by modern non-volatile memory technology. Flash memory devices with discrete charge-trapping media are regarded as an alternative solution to conventional floating gate technology. Flash memories utilizing Sinitride as charge storage media dominate due to enhanced endurance, better scaling capability and simple fabrication. The use of high-k dielectrics as tunnel layer and control layer is also crucial in charge-trap flash memory devices since they allow further scaling and enhanced charge injection without data retention degradation. Atomic layer deposition (ALD) is a powerful technique for the growth of pinhole-free high-k dielectrics with precisely controlled thickness and high conformality. The application of graphene as charge trapping medium in flash memory devices is promising to obtain improved charge storage capability with miniaturization. Graphene acts as an effective charge storage medium due to high density of states in deep energy levels. In this thesis, we fabricate graphene flash memory devices with ALD-grown HfO2/AlN as tunnel layer and Al2O3 as control layer. Graphene oxide nanosheets are derived from the acid exfoliation of natural graphite by Hummers Method. The graphene layer is obtained by spin-coating of water soluble graphene oxide suspension followed by a thermal annealing process. Memory performance including hysteresis window, data retention rate and program transient characteristics for both electron and hole storage mechanisms are determined by performing high frequency capacitance-voltage measurements. For comparing the memory effect of graphene on device performance, we also fabricate and characterize identical flash capacitors with Si-rich SiN layer as charge storage medium and HfO2 as tunnel oxide layer. The Si-nitride films are deposited with high SiH4/NH3 gas flow ratio by plasma-enhanced chemical vapor deposition system. Graphene flash memory devices exhibit superior memory performance. Compared with Si-nitride based cells, hysteresis window, retention performance and programming speed are both significantly enhanced with the use of graphene. For electron storage, graphene flash memory provides a saturated flat band shift of 1.2 V at a write-pulse duration of 100 ns with a voltage bias of 5 V. The high density of states and high work function of graphene improve the memory performance, leading to increased charge storage capability, enhanced retention rate and faster programming operation at low voltages. The use of graphene as charge storage medium and ALD-grown high-k dielectrics as tunnel and control layers improves the existing flash technology and satisfies the requirements including scalability, at least 10-year retention, low voltage operation, faster write performance and CMOS-compatible fabrication.Item Open Access Size modulation and defects in graphene based ribbons : magnetism and charge confinement(Bilkent University, 2008) Topsakal, MehmetIn this thesis, we investigated the effects of vacancy and heterojunction formation on electronic and magnetic properties of graphene nanoribbons (GNRs) by using first principles pseudopotential plane wave method within Density Functional Theory. Graphene based materials are expected to be very important in future technology. Through understanding of all the factors which influence their physical properties is essential. We have shown that electronic and magnetic properties of graphene nanoribbons can be affected by defect-induced itinerant states. The band gaps of armchair nanoribbons can be modified by hydrogen saturated holes. Defects due to periodically repeating vacancies or divacancies induce metallization, as well as magnetization in non-magnetic semiconducting nanoribbons due to the spin-polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to edges. We also predicted that periodically repeated junctions of graphene ribbons of different widths form multiple quantum well structures having confined states. These quantum structures are unique, since both constituents of heterostructures are of the same material. The width as well as the band gap, for specific superlattices are modulated in direct space. Orientation of constituent nanoribbons, their widths, lengths and the symmetry of the junction are some of the crucial structural parameters to engineer electronic properties of these systems. Our further studies on nanoribbons and nanoribbon superlattices showed the strong dependence of band gaps and magnetic moments on applied uniaxial stress. This thesis presents an extensive study of size modulation and defect formation on graphene nanoribbons.