56 research outputs found
A Tight-Binding Grand Canonical Monte Carlo Study of the Catalytic Growth of Carbon Nanotubes
The nucleation of carbon nanotubes on small nickel clusters is studied using
a tight binding model coupled to grand canonical Monte Carlo simulations. This
technique closely follows the conditions of the synthesis of carbon nanotubes
by chemical vapor deposition. The possible formation of a carbon cap on the
catalyst particle is studied as a function of the carbon chemical potential,
for particles of different size, either crystalline or disordered. We show that
these parameters strongly influence the structure of the cap/particle interface
which in turn will have a strong effect on the control of the structure of the
nanotube. In particular, we discuss the presence of carbon on surface or in
subsurface layers
Nickel assisted healing of defective graphene
The healing of graphene grown from a metallic substrate is investigated using
tight-binding Monte Carlo simulations. At temperatures (ranging from 1000 to
2500 K), an isolated graphene sheet can anneal a large number of defects
suggesting that their healing are thermally activated. We show that in presence
of a nickel substrate we obtain a perfect graphene layer. The nickel-carbon
chemical bonds keep breaking and reforming around defected carbon zones,
providing a direct interaction, necessary for the healing. Thus, the action of
Ni atoms is found to play a key role in the reconstruction of the graphene
sheet by annealing defects
Long-range interactions between substitutional nitrogen dopants in graphene: electronic properties calculations
Being a true two-dimensional crystal, graphene has special properties. In
particular, a point-like defect in graphene may have effects in the long range.
This peculiarity questions the validity of using a supercell geometry in an
attempt to explore the properties of an isolated defect. Still, this approach
is often used in ab-initio electronic structure calculations, for instance. How
does this approach converge with the size of the supercell is generally not
tackled for the obvious reason of keeping the computational load to an
affordable level. The present paper addresses the problem of substitutional
nitrogen doping of graphene. DFT calculations have been performed for 9x9 and
10x10 supercells. Although these calculations correspond to N concentrations
that differ by about 10%, the local densities of states on and around the
defects are found to depend significantly on the supercell size. Fitting the
DFT results by a tight-binding Hamiltonian makes it possible to explore the
effects of a random distribution of the substitutional N atoms, in the case of
finite concentrations, and to approach the case of an isolated impurity when
the concentration vanishes. The tight-binding Hamiltonian is used to calculate
the STM image of graphene around an isolated N atom. STM images are also
calculated for graphene doped with 0.5 % concentration of nitrogen. The results
are discussed in the light of recent experimental data and the conclusions of
the calculations are extended to other point defects in graphene
Exciton interference in hexagonal boron nitride
In this letter we report a thorough analysis of the exciton dispersion in
bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation
at finite , and we compare our results with
recent high-accuracy electron energy loss data. Simulations reproduce the
measured dispersion and the variation of the peak intensity. We focus on the
evolution of the intensity, and we demonstrate that the excitonic peak is
formed by the superposition of two groups of transitions that we call and
from the k-points involved in the transitions. These two groups
contribute to the peak intensity with opposite signs, each damping the
contributions of the other. The variations in number and amplitude of these
transitions determine the changes in intensity of the peak. Our results
contribute to the understanding of electronic excitations in this systems along
the direction, which is the relevant direction for spectroscopic
measurements. They also unveil the non-trivial relation between valley physics
and excitonic dispersion in h--BN, opening the possibility to tune excitonic
effects by playing with the interference between transitions. Furthermore, this
study introduces analysis tools and a methodology that are completely general.
They suggest a way to regroup independent-particle transitions which could
permit a deeper understanding of excitonic properties in any system
Size dependent phase diagrams of Nickel-Carbon nanoparticles
The carbon rich phase diagrams of nickel-carbon nanoparticles, relevant to
catalysis and catalytic chemical vapor deposition synthesis of carbon
nanotubes, are calculated for system sizes up to about 3 nanometers (807 Ni
atoms). A tight binding model for interatomic interactions drives the Grand
Canonical Monte Carlo simulations used to locate solid, core/shell and liquid
stability domains, as a function of size, temperature and carbon chemical
potential or concentration. Melting is favored by carbon incorporation from the
nanoparticle surface, resulting in a strong relative lowering of the eutectic
temperature and a phase diagram topology different from the bulk one. This
should be taken into account in our understanding of the nanotube growth
mechanisms
A Tight-Binding Grand Canonical Monte Carlo Study of the Catalytic Growth of Carbon Nanotubes [Working paper]
The nucleation of carbon nanotubes on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. This technique closely follows the conditions of the synthesis of carbon nanotubes by chemical vapor deposition. The possible formation of a carbon cap on the catalyst particle is studied as a function of the carbon chemical potential, for particles of different size, either crystalline or disordered. We show that these parameters strongly influence the structure of the cap/particle interface which in turn will have a strong effect on the control of the structure of the nanotube. In particular, we discuss the presence of carbon on surface or in subsurface layers
Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization
Hexagonal boron nitride (hBN) has been attracting great attention because of
its strong excitonic effects. Taking into account few-layer systems, we
investigate theoretically the effects of the number of layers on quasiparticle
energies, absorption spectra, and excitonic states, placing particular focus on
the Davydov splitting of the lowest bound excitons. We describe how the
inter-layer interaction as well as the variation in electronic screening as a
function of layer number affects the electronic and optical properties.
Using both \textit{ab initio} simulations and a tight-binding model for an
effective Hamiltonian describing the excitons, we characterize in detail the
symmetry of the excitonic wavefunctions and the selection rules for their
coupling to incoming light. We show that for , one can distinguish
between surface excitons that are mostly localized on the outer layers and
inner excitons, leading to an asymmetry in the energy separation between split
excitonic states. In particular, the bound surface excitons lie lower in energy
than their inner counterparts. Additionally, this enables us to show how the
layer thickness affects the shape of the absorption spectrum.Comment: 24 pages, 10 figure
Excitons in boron nitride single layer
Boron nitride single layer belongs to the family of 2D materials whose
optical properties are currently receiving considerable attention. Strong
excitonic effects have already been observed in the bulk and still stronger
effects are predicted for single layers. We present here a detailed study of
these properties by combining \textit{ab initio} calculations and a
tight-binding-Wannier analysis in both real and reciprocal space. Due to the
simplicity of the band structure with single valence () and conduction
() bands the tight-binding analysis becomes quasi quantitative with only
two adjustable parameters and provides tools for a detailed analysis of the
exciton properties. Strong deviations from the usual hydrogenic model are
evidenced. The ground state exciton is not a genuine Frenkel exciton, but a
very localized "tightly-bound" one. The other ones are similar to those found
in transition metal dichalcogenides and, although more localized, can be
described within a Wannier-Mott scheme
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