Electro-magnetic properties and phononic energy dissipation in graphene based structures
Author
Sevinçli, Haldun
Advisor
Çıracı, Salim
Date
2008Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
With 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.