34 research outputs found
On the mechanical properties of novamene: a fully atomistic molecular dynamics and DFT investigation
We have investigated through fully atomistic reactive molecular dynamics and
DFT simulations, the mechanical properties and fracture dynamics of novamene, a
new 3D carbon allotrope structure recently proposed. Our results showed that
novamene is an anisotropic structure with relation to tensile deformation.
Although novamente shares some mechanical features with other carbon
allotropes, it also exhibits distinct ones, such as, extensive structural
reconstructions (self-healing effect). Novamene presents ultimate strength (~
100 GPa) values lower than other carbon allotropes, but it has the highest
ultimate strain along the z-direction (~ 22.5%). Although the Young's modulus
(~ 600 GPa) and ultimate strength values are smaller than for other carbon
allotropes, they still outperform other materials, such as for example silicon,
steel or titanium alloys. With relation to the fracture dynamics, novamene is
again anisotropic with the fracture/crack propagation originating from deformed
heptagons and pentagons for x and y directions and broken sp3 bonds connecting
structural planes. Another interesting feature is the formation of multiple and
long carbon linear chains in the final fracture stages
Mechanical Properties of Protomene: A Molecular Dynamics Investigation
Recently, a new class of carbon allotrope called protomene was proposed. This
new structure is composed of sp2 and sp3 carbon-bonds. Topologically, protomene
can be considered as an sp3 carbon structure (~80% of this bond type) doped by
sp2 carbons. First-principles simulations have shown that protomene presents an
electronic bandgap of ~3.4 eV. However, up to now, its mechanical properties
have not been investigated. In this work, we have investigated protomene
mechanical behavior under tensile strain through fully atomistic reactive
molecular dynamics simulations using the ReaxFF force field, as available in
the LAMMPS code. At room temperature, our results show that the protomene is
very stable and the obtained ultimate strength and ultimate stress indicates an
anisotropic behavior. The highest ultimate strength was obtained for the
x-direction, with a value of ~110 GPa. As for the ultimate strain, the highest
one was for the z-direction (~25% of strain) before protomene mechanical
fracture
Carbon Schwarzites Behavior Under Ballistic Impacts
Schwarzites are 3D crystalline porous materials exhibiting the shape of
Triply Periodic Minimal Surfaces (TPMS). They possess negative Gaussian
curvature, created by the presence of rings with more than six sp2-hybridized
carbon atoms. Recently, new routes to their synthesis have been proposed. Due
to its foam-like structure, schwarzites are interesting for mechanical energy
absorption applications. In this work, we investigate through fully atomistic
reactive molecular dynamics the mechanical response under ballistic impacts of
four structures from primitive (P) and gyroid (G) families (two structures
within each family). The two structures in the same family differ mainly by the
ratio of hexagons to octagons, where this ratio increases the 'flatness' of the
structures. Although the penetration depth values are higher in the 'flatter'
structures (P8bal and G8bal), the absorbed kinetic energy by them is
considerably higher, which yields them a better energy-absorption performance
The Influence of Morphology on the Charge Transport in Two-Phase Disordered Organic Systems
In this work we use a three-dimensional Pauli master equation to investigate
the charge carrier mobility of a two-phase system, which can mimic
donor-acceptor and amorphous- crystalline bulk heterojunctions. Our approach
can be separated into two parts: the morphology generation and the charge
transport modeling in the generated blend. The morphology part is based on a
Monte Carlo simulation of binary mixtures (donor/acceptor). The second part is
carried out by numerically solving the steady-state Pauli master equation. By
taking the energetic disorder of each phase, their energy offset and domain
morphology into consideration, we show that the carrier mobility can have a
significant different behavior when compared to a one-phase system. When the
energy offset is non-zero, we show that the mobility electric field dependence
switches from negative to positive at a threshold field proportional to the
energy offset. Additionally, the influence of morphology, through the domain
size and the interfacial roughness parameters, on the transport was also
investigated.Comment: Submitted to 2014 Fall MRS Symposia Proceeding
Structural Transformations of Carbon and Boron Nitride Nanoscrolls at High Impact Collisions
The behavior of nanostructures under high strain-rate conditions has been
object of theoretical and experimental investigations in recent years. For
instance, it has been shown that carbon and boron nitride nanotubes can be
unzipped into nanoribbons at high velocity impacts. However, the response of
many nanostructures to high strain-rate conditions is still not completely
understood. In this work we have investigated through fully atomistic reactive
(ReaxFF) molecular dynamics (MD) simulations the mechanical behavior of carbon
(CNS) and boron nitride nanoscrolls (BNS) colliding against solid targets at
high velocities,. CNS (BNS) nanoscrolls are graphene (boron nitride) membranes
rolled up into papyrus-like structures. Their open-ended topology leads to
unique properties not found in close-ended analogues, such as nanotubes. Our
results show that the collision products are mainly determined by impact
velocities and by two impact angles, which define the position of the scroll
(i) axis and (ii) open edge relative to the target. Our MD results showed that
for appropriate velocities and orientations large-scale deformations and
nanoscroll fracture can occur. We also observed unscrolling (scrolls going back
to quasi-planar membranes), scroll unzipping into nanoribbons, and significant
reconstruction due to breaking and/or formation of new chemical bonds. For
particular edge orientations and velocities, conversion from open to
close-ended topology is also possible, due to the fusion of nanoscroll walls
On the Structural Stability and Optical Properties of Germanium-based Schwarzites: A Density Functional Theory Investigation
Since graphene was synthesized the interest for building new 2D and 3D
structures based on the carbon allotropes has been growing every day. One of
these 3D structures is know as carbon schwarzites. Schwarzites consist of
carbon nanostructures possessing the shape of Triply-Periodic Minimal Surfaces
(TPMS), which is characterized by a negative Gaussian curvature introduced by
the presence of carbon rings with more than six atoms. Some examples of
schwarzite families include: primitive (P), gyroid (G) and diamond (D).
Previous studies considering different element species of schwarzites have
investigated the mechanical, electrical and thermal properties. In this work,
we investigated the stability of germanium (Ge) schwarzites using density
functional theory with GGA exchange-correlation functional. We chose one
structure of each family (P8bal), (G688) and (D688). It was observed that
regions usually flat in carbon schwarzites acquires buckled configurations as
previously observed on silicene and germanene monolayers. The investigated
structures presented a semiconducting bandgap ranging from to eV.
We also performed calculations of optical properties within the linear regime,
where it was shown that Ge schwarzites structures absorb light from infrared to
ultra-violet frequencies. Therefore, our results open new perspectives of
materials that can be used in optelectronics devices application.Comment: 23 pages, 6 figure
A DFT Investigation of the Electronic and Optical Properties of Pentadiamond
Recently, a new carbon 3D carbon allotrope named pentadiamond was proposed.
Pentadiamond is composed of carbon atoms in mixed sp and sp-like
hybridization. In this work, we have carried out a detailed investigation of
the electronic and optical properties of pentadiamond structure using
first-principles (DFT) methods. Our results show that pentadiamond has an
indirect bandgap semiconductor of eV with GGA-PBE and eV with
HSE06. Its static dielectric constant is and the static refractive index
is . Pentadiamond presents low reflectivity, almost 40, for
all-optical spectrum, making it a good structure to be used as a UV collector.
Also, pentadiamond exhibits optical activity in the UV range where other carbon
allotropes, such as diamond and 8-tetra(2,2) tubulane show no activity
Thiophene-Tetrathia-Annulene monolayer (TTA-2D): A new 2D semiconductor material with indirect bandgap
We propose a new 2D semiconductor material (TTA-2D) based on the molecular
structure of Thiophene-Tetrathia-Annulene (TTA). The TTA-2D structural,
electronic, and optical properties were investigated using \textit{ab initio}
methods. Our results show that TTA-2D is a small indirect bandgap semiconductor
( eV). A semiconductor-metal transition can be induced by applying a
uniaxial strain. Our results also show that TTA-2D is thermally stable up to
K. TTA-2D absorbs in a large spectral range, from infrared to
ultraviolet regions. Values of refractive index and reflectivity show that
TTA-2D reflects only of the incident light in the visible region. These
results suggest that TTA-2D is a promising material for solar cell
applications.Comment: 28 pages, 9 figure
Insights on the mechanism of water-alcohol separation in multilayer graphene oxide membranes: entropic versus enthalpic factors
Experimental evidences have shown that graphene oxide (GO) can be impermeable
to liquids, vapors and gases, while it allows a fast permeation of water
molecules. The understanding of filtration mechanisms came mostly from studies
dedicated to water desalination, while very few works have been dedicated to
distilling alcohols. In this work, we have investigated the molecular level
mechanism underlying the alcohol/water separation inside GO membranes. A series
of molecular dynamics and Grand-Canonical Monte Carlo simulations were carried
out to probe the ethanol/water and methanol/water separation through GO
membranes composed of multiple layered graphene-based sheets with different
interlayer distance values and number of oxygen-containing functional groups.
Our results show that the size exclusion and membrane affinities are not
sufficient to explain the selectivity. Besides that, the favorable water
molecular arrangement inside GO 2D-channels forming a robust H-bond network and
the fast water diffusion are crucial for an effective separation mechanism. In
other words, the separation phenomenon is not only governed by affinities with
the membrane (enthalpic mechanisms) but mainly by the geometry and size factors
(entropic mechanisms). We verified that the 2D geometry channel with optimal
interlayer distance are key factors for designing more efficient alcohol-water
separation membranes. Our findings are consistent with the available
experimental data and contribute to clarify important aspects of the separation
behavior of confined alcohol/water in GO membranes
Mechanical Properties of Diamond Schwarzites: From Molecular Dynamics Simulations to 3D Printing
Schwarzites are porous crystalline structures with Gaussian negative
curvature. In this work, we investigated the mechanical behavior and energy
absorption properties of two carbon-based diamond schwarzites (D688 and D8bal).
We carried out fully atomistic molecular dynamics (MD) simulations. The
optimized MD atomic models were used to generate macro-scale models for
3D-printing (PolyLactic Acid (PLA) polymer filaments) through Fused Deposition
Modelling (FDM). Mechanical properties under uniaxial compression were
investigated for both the atomic models and the 3D-printed ones. Mechanical
testings were performed on the 3D-printed schwarzites where the deformation
mechanisms were found to be similar to those observed in MD simulations. These
results are suggestive of a scale-independent mechanical behavior that is
dominated by structural topology. The structures exhibit high specific energy
absorption and crush force efficiency ~0.8, which suggest that the 3D-printed
diamond schwarzites are good candidates as energy-absorbing materials