Effects of charging on two-dimensional honeycomb nanostructures





Çıracı, Salim





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Bilkent University






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In 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.


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