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Electronic properties of two-dimensional carbon systems

Properties of graphene-based nanoelectronic devices are found to be limited by disorder, e.g.,
vacancies, impurities and ripples on the surface. We investigate the speci c e ect of defects
concentration as well as the structural modulation (ripples) on the electronic properties of layers
of graphene-based electronic devices. We show the promise of a possible route for improvement
of the current-voltage characteristics by incorporating nitrogen atoms in the defective graphene
(which has limited device applications). In this work, we develop the tight-binding model of
two-dimensional (2D) carbon and use the recursive Green's function method to study the e ect
of defects concentration as well as periodic structural disorder (ripples) where it has already been
studied using the Dirac Hamiltonian.
The combined e ect of vacancies and ripples on the electronic transport of graphene devices was
studied. The presence of vacancies results in quasi-localized states at the Fermi energy. This
is also found to be common in the presence of ripples, but in that case they are Landau levels
originating from the gauge eld induced by the ripples. In contrast, resonant states emerge when
charged impurities are substituted. The density of these resonances was found to be tunable by
controlling the ratio between the impurity-carbon coupling to impurity ons-site potential as well
as the concentration.
With regard to the mesoscopic phenomena, the system gains zero conductance due to the opening
of an energy gap when the ripples as well as vacancies are present. In particular, the transport
becomes di usive rather than ballistic in the case of ripples, which has already been found
previously within the Dirac Hamiltonian approach. On the other hand, the impurity enhances
the transport properties due to augmentation of the resonant states in the vicinity of the Fermi
level. Moreover, the increase of the sample-lead coupling was found to broaden the levels and
increase the current by over one order of magnitude. The study shows the possibility of tuning the
electronic transport of 2D carbon systems by controlling the structural and topological defects,
which can be extended toward the understanding of experimental observations such as enhanced
transport properties in 2D graphitic carbon lms incorporated with nitrogen.

Identiferoai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/11958
Date18 September 2012
CreatorsMohammed, Faris Siedahmed
Source SetsSouth African National ETD Portal
LanguageEnglish
Detected LanguageEnglish
TypeThesis
Formatapplication/pdf, application/pdf

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