Effects of charging on two-dimensional honeycomb nanostructures
Author(s)
Advisor
Çıracı, SalimDate
2012Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
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.