09-08-2012, 04:11 PM
Ab nitio calculation of nanomaterial electronic and transport property
[attachment=30824]
INTRODUCTION
First let me start my deepest thanks and glory to my almighty GOD and his Mother St. Mary for all my
existence.
I would like to express my gratitude to my instructor Prof. Javed Mazhar for his dedicated concern and
invaluable ideas and lectures he shared me. I thank him for his continuous assessment and monitoring
whether we are performing our duties or not. I believe that even his screaming on us if we are failed to
perform our duties shows his concern on our learning outcomes.
I would also like to acknowledge my classmates Mesfin Haile; Fasil Amdeslasie;Fetene Fufa; Asegidew
Ergetie and Fistum for their assistance in my work place.
My heart- felt word of thanks goes to director general of ENAO and coworkers for their material and
moral support .
Finally my thanks go to all members of materials science staffs and all materials science senior post
graduate students for countless support they provide me.
Back ground
There was an earlier saying that history cannot be changed. But the technology especially
nanotechnology makes the revolutions in the history. There is no present and future, the development
was such like a dream come true when one gets up from the sleep. Forthcoming developments in
nanotechnology through which the impossible can be made possible are nanomaterials with novel
optical, electrical, and magnetic properties .
The electronic transport properties of monolayer graphene with extreme physical deformation are
Ab nitio calculation of nanomaterial electronic and transport property
studied using ab initio calculations. The aim is to explore the influence of physical bending on transport
properties and identify the most important geometrical parameter. The transmission spectra are
relatively insensitive to the geometrical parameters in low-energy regions even in the extreme case of
uniaxial bending. The results suggest that graphene, with its superb electromechanical robustness, could
serve as a viable nanoscale material platform in a wide spectrum of applications such as photovoltaics,
flexible electronics, and 3D carbon chips.
Carbon nanomaterials reveal a rich polymorphism of various allotropes exhibiting each possible
dimensionality: fullerene molecule (0D), nanotubes and graphene ribbons (1D), graphite platelets (2D),
and nanodiamond (3D) are selected examples. Because of this extraordinary versatility of
nanomaterials exhibiting different physical and chemical properties, carbon nanostructures are playing
an important role in nanoscience and nanotechnology.
The fundamental building block in these carbon nanostructures (except for sp3 diamond) relies on the
theoretical concept of the graphene sheet. Indeed, graphene is the name given to a perfect infinite single
layer of sp2 - bonded carbon atoms densely packed into a benzene-ring structure. This ideal 2D solid
has thus been widely used to describe properties of many carbon-based materials, in- cluding graphite
(where a large number of graphene sheets are stacked), nanotubes (where graphene sheets are rolled up
into nanometer-sized cylinders), large fullerenes (where graphene sheets contain at least 12 pentagons
displaying a spherical shape), and ribbons (where graphene is cut into strips). Planar graphene itself was
presumed not to exist in the free state, being unstable with respect to the formation of curved structures,
such as soot, fullerenes, and nanotubes.
However, a couple of years ago, researchers went on to actually prepare graphene by mechanical
exfoliation (repeated peeling or micro mechanical cleavage) of bulk graphite (highly oriented pyrolytic
graphite: HOPG) or by epitaxial growth through thermal decomposition of SiC [3]. Such a discovery of
a simple method to transfer a single atomic layer of carbon from the c-face of graphite to a substrate
suitable for the measurement of its electrical properties has led to a renewed interest in what was
considered to be a prototypical, yet theoretical, two dimensional system. Graphene displays, indeed,
unusual electronic properties arising from the confinement of electrons in two dimensions and peculiar
geometrical symmetries. Indeed, old theoretical studies of graphene reveal that the specific linear
electronic band dispersion near the Brillouin zone corners (Dirac point) gives rise to electrons and holes
that propagate as if they were massless . Fermions, with a velocity on the order of one hundredth of the
velocity of light. Charge excitations close to the Fermi level can thus be formally described as massless
relativistic particles obeying a Dirac equation, whereas a new degree of freedom reflecting inherent
symmetries (sublattice degeneracy) appears in the electronic states: the pseudo spin.
Ab nitio calculation of nanomaterial electronic and transport property
Because of the resulting pseudospin symmetry, electronic states turn out to be particularly insensitive
to external sources of elastic disorder (topological and electrostatic defects) and, as a result, charge
mobilities in graphene layers as large as 105 cm2 V−1 s−1 have been reported close to the Dirac
point .
Electronic properties of graphene
Graphene is the ideal bidimensional (2D) allotropic form of carbon where the atoms are periodically
arranged in an infinite hexagonal network (Fig. 1a). Such an atomic struc- ture is characterized by two
types of bonds and exhibits the so-called planar sp2 hybridization. Indeed, among the four valence
orbitals of the carbon atom (the 2s, 2px , 2py and 2pz orbitals, where z is perpendicular to the sheet),
the (s, px , py ) orbitals combine to form the in- plane σ (bonding or occupied) and σ ∗ (anti-bonding or
unoccupied) orbitals. Such orbitals are even with respect to the planar symmetry. The σ bonds are
strong covalent bonds responsible for most of the binding energy and for the elastic properties of the
graphene sheet (Fig. 1a). The remaining p z orbital, pointing out of the graphene sheet (Fig. 1a), is odd
with respect to the planar symmetry and cannot couple with the σ states. The lateral interaction
with neighboring pz orbitals (labeled the ppπ interaction) creates the delocalized π (bonding) and π ∗
(anti-bonding) . In addition, in suspended graphene, the minimum conductivity at the Dirac point
approaches a universal (geometry independent) value of 4e2 /πh at low temperature.
[attachment=30824]
INTRODUCTION
First let me start my deepest thanks and glory to my almighty GOD and his Mother St. Mary for all my
existence.
I would like to express my gratitude to my instructor Prof. Javed Mazhar for his dedicated concern and
invaluable ideas and lectures he shared me. I thank him for his continuous assessment and monitoring
whether we are performing our duties or not. I believe that even his screaming on us if we are failed to
perform our duties shows his concern on our learning outcomes.
I would also like to acknowledge my classmates Mesfin Haile; Fasil Amdeslasie;Fetene Fufa; Asegidew
Ergetie and Fistum for their assistance in my work place.
My heart- felt word of thanks goes to director general of ENAO and coworkers for their material and
moral support .
Finally my thanks go to all members of materials science staffs and all materials science senior post
graduate students for countless support they provide me.
Back ground
There was an earlier saying that history cannot be changed. But the technology especially
nanotechnology makes the revolutions in the history. There is no present and future, the development
was such like a dream come true when one gets up from the sleep. Forthcoming developments in
nanotechnology through which the impossible can be made possible are nanomaterials with novel
optical, electrical, and magnetic properties .
The electronic transport properties of monolayer graphene with extreme physical deformation are
Ab nitio calculation of nanomaterial electronic and transport property
studied using ab initio calculations. The aim is to explore the influence of physical bending on transport
properties and identify the most important geometrical parameter. The transmission spectra are
relatively insensitive to the geometrical parameters in low-energy regions even in the extreme case of
uniaxial bending. The results suggest that graphene, with its superb electromechanical robustness, could
serve as a viable nanoscale material platform in a wide spectrum of applications such as photovoltaics,
flexible electronics, and 3D carbon chips.
Carbon nanomaterials reveal a rich polymorphism of various allotropes exhibiting each possible
dimensionality: fullerene molecule (0D), nanotubes and graphene ribbons (1D), graphite platelets (2D),
and nanodiamond (3D) are selected examples. Because of this extraordinary versatility of
nanomaterials exhibiting different physical and chemical properties, carbon nanostructures are playing
an important role in nanoscience and nanotechnology.
The fundamental building block in these carbon nanostructures (except for sp3 diamond) relies on the
theoretical concept of the graphene sheet. Indeed, graphene is the name given to a perfect infinite single
layer of sp2 - bonded carbon atoms densely packed into a benzene-ring structure. This ideal 2D solid
has thus been widely used to describe properties of many carbon-based materials, in- cluding graphite
(where a large number of graphene sheets are stacked), nanotubes (where graphene sheets are rolled up
into nanometer-sized cylinders), large fullerenes (where graphene sheets contain at least 12 pentagons
displaying a spherical shape), and ribbons (where graphene is cut into strips). Planar graphene itself was
presumed not to exist in the free state, being unstable with respect to the formation of curved structures,
such as soot, fullerenes, and nanotubes.
However, a couple of years ago, researchers went on to actually prepare graphene by mechanical
exfoliation (repeated peeling or micro mechanical cleavage) of bulk graphite (highly oriented pyrolytic
graphite: HOPG) or by epitaxial growth through thermal decomposition of SiC [3]. Such a discovery of
a simple method to transfer a single atomic layer of carbon from the c-face of graphite to a substrate
suitable for the measurement of its electrical properties has led to a renewed interest in what was
considered to be a prototypical, yet theoretical, two dimensional system. Graphene displays, indeed,
unusual electronic properties arising from the confinement of electrons in two dimensions and peculiar
geometrical symmetries. Indeed, old theoretical studies of graphene reveal that the specific linear
electronic band dispersion near the Brillouin zone corners (Dirac point) gives rise to electrons and holes
that propagate as if they were massless . Fermions, with a velocity on the order of one hundredth of the
velocity of light. Charge excitations close to the Fermi level can thus be formally described as massless
relativistic particles obeying a Dirac equation, whereas a new degree of freedom reflecting inherent
symmetries (sublattice degeneracy) appears in the electronic states: the pseudo spin.
Ab nitio calculation of nanomaterial electronic and transport property
Because of the resulting pseudospin symmetry, electronic states turn out to be particularly insensitive
to external sources of elastic disorder (topological and electrostatic defects) and, as a result, charge
mobilities in graphene layers as large as 105 cm2 V−1 s−1 have been reported close to the Dirac
point .
Electronic properties of graphene
Graphene is the ideal bidimensional (2D) allotropic form of carbon where the atoms are periodically
arranged in an infinite hexagonal network (Fig. 1a). Such an atomic struc- ture is characterized by two
types of bonds and exhibits the so-called planar sp2 hybridization. Indeed, among the four valence
orbitals of the carbon atom (the 2s, 2px , 2py and 2pz orbitals, where z is perpendicular to the sheet),
the (s, px , py ) orbitals combine to form the in- plane σ (bonding or occupied) and σ ∗ (anti-bonding or
unoccupied) orbitals. Such orbitals are even with respect to the planar symmetry. The σ bonds are
strong covalent bonds responsible for most of the binding energy and for the elastic properties of the
graphene sheet (Fig. 1a). The remaining p z orbital, pointing out of the graphene sheet (Fig. 1a), is odd
with respect to the planar symmetry and cannot couple with the σ states. The lateral interaction
with neighboring pz orbitals (labeled the ppπ interaction) creates the delocalized π (bonding) and π ∗
(anti-bonding) . In addition, in suspended graphene, the minimum conductivity at the Dirac point
approaches a universal (geometry independent) value of 4e2 /πh at low temperature.