1,175 research outputs found
Static charging of graphene and graphite slabs
The effect of external static charging of graphene and its flakes are
investigated by using first-principles calculations. While the Fermi level of
negatively charged graphene rises and then is quickly pinned by the parabolic,
nearly free electron like bands, it moves down readily by removal of electrons
from graphene. Excess charges accumulate mainly at both surfaces of graphite
slab. Even more remarkable is that Coulomb repulsion exfoliates the graphene
layers from both surfaces of positively charged graphite slab. The energy level
structure, binding energy and and spin-polarization of specific adatoms
adsorbed to a graphene flake can be monitored by charging.Comment: accepted for publication in APL (after 226 days
Elastic and plastic deformation of graphene, silicene, and boron nitride honeycomb nanoribbons under uniaxial tension: A first-principles density-functional theory study
This study of elastic and plastic deformation of graphene, silicene, and
boron nitride (BN) honeycomb nanoribbons under uniaxial tension determines
their elastic constants and reveals interesting features. In the course of
stretching in the elastic range, the electronic and magnetic properties can be
strongly modified. In particular, it is shown that the band gap of a specific
armchair nanoribbon is closed under strain and highest valance and lowest
conduction bands are linearized. This way, the massless Dirac fermion behavior
can be attained even in a semiconducting nanoribbon. Under plastic deformation,
the honeycomb structure changes irreversibly and offers a number of new
structures and functionalities. Cagelike structures, even suspended atomic
chains can be derived between two honeycomb flakes. Present work elaborates on
the recent experiments [C. Jin, H. Lan, L. Peng, K. Suenaga, and S. Iijima,
Phys. Rev. Lett. 102, 205501 (2009)] deriving carbon chains from graphene.
Furthermore, the similar formations of atomic chains from BN and Si nanoribbons
are predicted.Comment: http://prb.aps.org/abstract/PRB/v81/i2/e02410
Effects of static charging and exfoliation of layered crystals
Using first-principle plane wave method we investigate the effects of static
charging on structural, elastic, electronic and magnetic properties of
suspended, single layer graphene, graphane, fluorographene, BN and MoS2 in
honeycomb structures. The limitations of periodic boundary conditions in the
treatment of charged layers are clarified. Upon positive charging the band gaps
between the conduction and valence bands increase, but the single layer
materials become metallic owing to the Fermi level dipping below the maximum of
valence band. Moreover, their bond lengths increase and their in-plane
stiffness decreases. As a result, phonons are softened and frequencies of Raman
active modes are lowered. High level of charging leads to instability. We
showed that wide band gap BN and MoS2 slabs are metallized as a result of
electron removal and their outermost layers are exfoliated once the charging
exceeds a threshold value.Comment: http://link.aps.org/doi/10.1103/PhysRevB.85.04512
Domain formation on oxidized graphene
Using first-principles calculations within density functional theory we
demonstrate that the adsorption of single oxygen atom results in significant
electron transfer from graphene to oxygen. This strongly disturbs the charge
landscape of the C-C bonds at the proximity. Additional oxygen atoms adsorbing
to graphene prefer always the C-C bonds having highest charge density and
consequently they have tendency to form domain structure. While oxygen
adsorption to one side of graphene ends with significant buckling, the
adsorption to both sides with similar domain pattern is favored. The binding
energy displays an oscillatory variation and the band gap widens with
increasing oxygen coverage. While a single oxygen atom migrates over the C-C
bonds on graphene surface, a repulsive interaction prevents two oxygen adatoms
from forming an oxygen molecule. Our first-principles study together with
finite temperature ab-initio molecular dynamics calculations concludes that
oxygen adatoms on graphene cannot desorb easily without influence of external
agents.Comment: under revie
Structures of Fluorinated Graphenes and Their Signatures
Recent synthesis of fluorinated graphene introduced interesting stable
derivatives of graphene. In particular, fluorographene (CF), namely fully
fluorinated chair conformation, is found to display crucial features, such as
high mechanical strength, charged surfaces, local magnetic moments due to
vacancy defects and a wide band gap rapidly reducing with uniform strain. These
properties, as well as structural parameters and electronic densities of states
are found to scale with fluorine coverage. However, most of the experimental
data reported to date neither for CF, nor for other CnF structures complies
with the results obtained from first-principles calculations. In this study, we
attempt to clarify the sources of disagreements.Comment: Phys. Rev. B 83, 115432 (2011
Armchair nanoribbons of silicon and germanium honeycomb structures
We present a first-principles study of bare and hydrogen passivated armchair
nanoribbons of the puckered single layer honeycomb structures of silicon and
germanium. Our study includes optimization of atomic structure, stability
analysis based on the calculation of phonon dispersions, electronic structure
and the variation of band gap with the width of the ribbon. The band gaps of
silicon and germanium nanoribbons exhibit family behavior similar to those of
graphene nanoribbons. The edges of bare nanoribbons are sharply reconstructed,
which can be eliminated by the hydrogen termination of dangling bonds at the
edges. Periodic modulation of the nanoribbon width results in a superlattice
structure which can act as a multiple quantum wells. Specific electronic states
are confined in these wells. Confinement trends are qualitatively explained by
including the effects of the interface. In order to investigate wide and long
superlattice structures we also performed empirical tight binding calculations
with parameters determined from \textit{ab initio} calculations.Comment: please find the published version in
http://link.aps.org/doi/10.1103/PhysRevB.81.19512
The response of mechanical and electronic properties of graphane to the elastic strain
Based on first-principles calculations, we resent a method to reveal the
elastic properties of recently synthesized monolayer hydrocarbon, graphane. The
in-plane stiffness and Poisson's ratio values are found to be smaller than
those of graphene, and its yielding strain decreases in the presence of various
vacancy defects and also at high ambient temperature. We also found that the
band gap can be strongly modified by applied strain in the elastic range.Comment: accepted version at: http://link.aip.org/link/?APL/96/09191
Graphene coatings: An efficient protection from oxidation
We demonstrate that graphene coating can provide an efficient protection from
oxidation by posing a high energy barrier to the path of oxygen atom, which
could have penetrated from the top of graphene to the reactive surface
underneath. Graphene bilayer, which blocks the diffusion of oxygen with a
relatively higher energy barrier provides even better protection from
oxidation. While an oxygen molecule is weakly bound to bare graphene surface
and hence becomes rather inactive, it can easily dissociates into two oxygen
atoms adsorbed to low coordinated carbon atoms at the edges of a vacancy. For
these oxygen atoms the oxidation barrier is reduced and hence the protection
from oxidation provided by graphene coatings is weakened. Our predictions
obtained from the state of the art first-principles calculations of electronic
structure, phonon density of states and reaction path will unravel how a
graphene can be used as a corrosion resistant coating and guide further studies
aiming at developing more efficient nanocoatings.Comment: under review in PRB;
http://link.aps.org/doi/10.1103/PhysRevB.85.15544
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