253 research outputs found
Paramagnetic adsorbates on graphene: a charge transfer analysis
We introduce a modified version of the Hirshfeld charge analysis method and
demonstrate its accurateness by calculating the charge transfer between the
paramagnetic molecule NO2 and graphene. The charge transfer between
paramagnetic molecules and a graphene layer as calculated with ab initio
methods can crucially depend on the size of the supercell used in the
calculation. This has important consequences for adsorption studies involving
paramagnetic molecules such as NO2 physisorbed on graphene or on carbon
nanotubes.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter
Graphene: a perfect nanoballoon
We have performed a first-principles density functional theory investigation
of the penetration of helium atoms through a graphene monolayer with defects.
The relaxation of the graphene layer caused by the incoming helium atoms does
not have a strong influence on the height of the energy barriers for
penetration. For defective graphene layers, the penetration barriers decrease
exponentially with the size of the defects but they are still sufficiently high
that very large defects are needed to make the graphene sheet permeable for
small atoms and molecules. This makes graphene a very promising material for
the construction of nanocages and nanomembranes.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter
Stacking Order dependent Electric Field tuning of the Band Gap in Graphene Multilayers
The effect of different stacking order of graphene multilayers on the
electric field induced band gap is investigated. We considered a positively
charged top and a negatively charged back gate in order to independently tune
the band gap and the Fermi energy of three and four layer graphene systems. A
tight-binding approach within a self-consistent Hartree approximation is used
to calculate the induced charges on the different graphene layers. We found
that the gap for trilayer graphene with the ABC stacking is much larger than
the corresponding gap for the ABA trilayer. Also we predict that for four
layers of graphene the energy gap strongly depends on the choice of stacking,
and we found that the gap for the different types of stacking is much larger as
compared to the case of Bernal stacking. Trigonal warping changes the size of
the induced electronic gap by approximately 30% for intermediate and large
values of the induced electron density
Magneto-exciton in planar type II quantum dots
We study an exciton in a type II quantum dot, where the electron is confined
in the dot, but the hole is located in the barrier material. The exciton
properties are studied as a function of a perpendicular magnetic field using a
Hartree-fock mesh calculation. Our model system consists of a planar quantum
disk. Angular momentum (l) transitions are predicted with increasing magnetic
field. We also study the transition from a type I to a type II quantum dot
which is induced by changing the confinement potential of the hole. For
sufficiently large magnetic fields a re-entrant behaviour is found from
to and back to , which results in a transition
from type II to type I.Comment: 6 pages, 12 figure
Induced order and reentrant melting in classical two-dimensional binary clusters
A binary system of classical charged particles interacting through a dipole
repulsive potential and confined in a two-dimensional hardwall trap is studied
by Brownian dynamics simulations. We found that the presence of small particles
\emph{stabilizes} the angular order of the system as a consequence of radial
fluctuations of the small particles. There is an optimum in the increased
rigidity of the cluster as function of the number of small particles. The small
(i.e. defect) particles melt at a lower temperature compared to the big
particles and exhibit a \emph{reentrant} behavior in its radial order that is
induced by the intershell rotation of the big particles.Comment: 7 pages, 3 figure
Electron-electron interactions in bilayer graphene quantum dots
A parabolic quantum dot (QD) as realized by biasing nanostructured gates on
bilayer graphene is investigated in the presence of electron-electron
interaction. The energy spectrum and the phase diagram reveal unexpected
transitions as function of a magnetic field. For example, in contrast to
semiconductor QDs, we find a novel valley transition rather than only the usual
singlet-triplet transition in the ground state of the interacting system. The
origin of these new features can be traced to the valley degree of freedom in
bilayer graphene. These transitions have important consequences for cyclotron
resonance experiments.Comment: 5 pages, 5 figures, to appear in Phys. Rev.
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