303 research outputs found
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
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
Current-voltage (I-V) characteristics of armchair graphene nanoribbons under uniaxial strain
The current-voltage (I-V) characteristics of armchair graphene nanoribbons
under a local uniaxial tension are investigated by using first principles
quantum transport calculations. It is shown that for a given value of
bias-voltage, the resulting current depends strongly on the applied tension.
The observed trends are explained by means of changes in the band gaps of the
nanoribbons due to the applied uniaxial tension. In the course of plastic
deformation, the irreversible structural changes and derivation of carbon
monatomic chains from graphene pieces can be monitored by two-probe transport
measurements.Comment: please see the published version at
http://prb.aps.org/abstract/PRB/v81/i20/e20543
Monolayer honeycomb structures of group IV elements and III-V binary compounds
Using first-principles plane wave calculations, we investigate two
dimensional honeycomb structure of Group IV elements and their binary
compounds, as well as the compounds of Group III-V elements. Based on structure
optimization and phonon mode calculations, we determine that 22 different
honeycomb materials are stable and correspond to local minima on the
Born-Oppenheimer surface. We also find that all the binary compounds containing
one of the first row elements, B, C or N have planar stable structures. On the
other hand, in the honeycomb structures of Si, Ge and other binary compounds
the alternating atoms of hexagons are buckled, since the stability is
maintained by puckering. For those honeycomb materials which were found stable,
we calculated optimized structures, cohesive energies, phonon modes, electronic
band structures, effective cation and anion charges, and some elastic
constants. The band gaps calculated within Density Functional Theory using
Local Density Approximation are corrected by GW0 method. Si and Ge in honeycomb
structure are semimetal and have linear band crossing at the Fermi level which
attributes massless Fermion character to charge carriers as in graphene.
However, all binary compounds are found to be semiconductor with band gaps
depending on the constituent atoms. We present a method to reveal elastic
constants of 2D honeycomb structures from the strain energy and calculate the
Poisson's ratio as well as in-plane stiffness values. Preliminary results show
that the nearly lattice matched heterostructures of ...Comment: 12 Pages, 7 Figures, 1 Table;
http://link.aps.org/doi/10.1103/PhysRevB.80.15545
Spin confinement in the superlattices of graphene ribbons
Cataloged from PDF version of article.Based on first-principles calculations, we showed that repeated heterostructures of zigzag graphene nanoribbons of different widths 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 unique features for spintronic applications. (c) 2008 American Institute of Physics
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