Boron nitride and graphene 2D nanostructures from first-principles
Author
Ovalı, Rasim Volga
Advisor
Gülseren, Oğuz
Date
2010Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
In this thesis, the structures as well as mechanical and electronic properties
of various boron nitride (BN) and graphene based two dimensional (2D)
nano-structures are investigated in detail from rst-principle calculations using
planewave pseudopotential method based on density functional theory.
At the beginning of the thesis, essentials of the density functional theory
(DFT) and a guidance for performing ab-initio calculations in the framework
of DFT is presented. In addition, fundamentals about the exchange-correlation
potential as well as approaches approximating it like local density approximation
(LDA) and generalized gradient approximation (GGA) are discussed.
Along with this thesis, rst of all, in order to understand the relation between
the hexagonal boron nitride (h-BN) and cubic boron nitride (c-BN) and
the growth of three dimensional (3D) BN structures, various defect structures
introduce to BN monolayer, including point defects and especially highly curved
defects such as n-fold rings, are investigated in detail. The calculated formation
energies and structural analysis showed that 4-fold BN rings are the transient
phase between h-BN to c-BN during c-BN nucleation. The charge density plots
and density of states analysis further provide information about the electronic
structure of these defect formations.
Second of all, we have studied the formation of boron-nitride-carbon (BNC)
ternary thin lms, so we observed the carbon nucleation in BN monolayer. These
DFT based calculations show that carbon prefers the nitrogen site at rst step
and the calculated defect energy indicates that carbon atoms tends to aggregate
in BN hexagonal network, and hence increases the number of C-C bonds. BNC
structures have magnetization of =1.0 B for odd number of carbon adsorption.
Further substitution of carbon atoms into BN layer showed that carbon atomsform hexagonal rings instead of armchair or zigzag formations. Moreover, we calculated
the vibrational modes of BN monolayer and BNC structures, and phonon
density of states graphs are presented. The phonon frequencies intrinsic to C-C
bond oscillations are observed, which is in good agreement with the experiment.
Finally, point defects and ring formations on graphene are investigated in order
to understand the Y-junction and kink formation in carbon nanotubes (CNTs).
Pentagonal rings are the good candidates to initiate such 2D networks in CNTs.
The curvature increases with increasing number of pentagonal rings. Moreover,
interaction of sulphur atoms with graphene defects is studied. Final geometries
and binding energies suggest that sulphur prefers to adsorb on defected regions,
but it is not responsible for the formation of these structures or defects.
Keywords
Boron nitridedensity functional theory
ab-initio,
Y-junction
n-fold rings
highly curved defects
vacancies
point defects
sulphur
carbon nanotubes
graphene