245 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
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.
Wigner crystallization in transition metal dichalcogenides: A new approach to correlation energy
We introduce a new approach for the correlation energy of one- and two-valley
two-dimensional electron gas (2DEG) systems. Our approach is based on a random
phase approximation at high densities and a classical approach at low
densities, with interpolation between the two limits. This approach gives
excellent agreement with available Quantum Monte Carlo (QMC) calculations. We
employ the two-valley 2DEG model to describe the electron correlations in
monolayer transition metal dichalcogenides (TMDs). The zero-temperature
transition from a Fermi liquid to a quantum Wigner crystal phase in monolayer
TMDs is obtained using density-functional theory within the local-density
approximation. Consistent with QMC, we find that electrons crystallize at
in one-valley 2DEG. For two-valleys, we predict Wigner
crystallization at , indicating that valley degeneracy has little
effect on the critical , in contrast to an earlier claim.Comment: 5 pages, 3 figure
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
Adsorption and absorption of Boron, Nitrogen, Aluminium and Phosphorus on Silicene: stability, electronic and phonon properties
Ab initio calculations within the density-functional theory formalism are
performed to investigate the chemical functionalization of a graphene-like
monolayer of silicon - silicene - with B, N, Al or P atoms. The structural,
electronic, magnetic and vibrational properties are reported. The most
preferable adsorption sites are found to be valley, bridge, valley and hill
site for B, N, Al and P adatoms, respectively. All the relaxed systems with
adsorbed/substituted atoms exhibit metallic behaviour with strongly bonded B,
N, Al, and P atoms accompanied by an appreciable electron transfer from
silicene to the B, N and P adatom/substituent. The Al atoms exhibit opposite
charge transfer, with n-type doping of silicene and weaker bonding. The
adatoms/substituents induce characteristic branches in the phonon spectrum of
silicene, which can be probed by Raman measurements. Using molecular dynamics
we found that the systems under study are stable up to at least T = 500 K. Our
results demonstrate that silicene has a very reactive and functionalizable
surface.Comment: 9 pages, 5 figure
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