arXiv subjects

M. Neek-Amal

Publications and source records attributed to M. Neek-Amal.

At least 37 records · Page 2Linked to original sources

arXiv2014

Electronic structure of graphene hexagonal flake subjected to triaxial stress

The electronic properties of a triaxially strained hexagonal graphene flake with either armchair or zig-zag edges are investigated using molecular dynamics simulations and tight-binding calculations. We found that: i) the pseudo-magnetic field in the strained graphene flakes is not uniform neither in the center nor at the edge of zig-zag terminated flakes, ii) the pseudo-magnetic field is almost zero in the center of armchair terminated flakes but increases dramatically near the edges, iii) the pseudo-magnetic field increases linearly with strain, for strains lower than 15$%$ while growing non-linearly beyond this threshold, iv) the local density of states in the center of the zig-zag hexagon exhibits pseudo-Landau levels with broken sub-lattice symmetry in the zero'th pseudo-Landau level, and in addition there is a shift in the Dirac cone due to strain induced scalar potentials. This study provides a realistic model of the electronic properties of inhomogeneously strained graphene where the relaxation of the atomic positions is correctly included together with strain induced modifications of the hopping terms up to next-nearest neighbors.

cond-mat.mes-hall↗

arXiv2014

Electronic properties of graphene nano-flakes: Energy gap, permanent dipole, termination effect and Raman spectroscopy

The electronic properties of graphene nano-flakes (GNFs) with different edge passivation is investigated by using density functional theory. Passivation with F and H atoms are considered: C$_{N_c}$ X$_{N_x}$ (X=F or H). We studied GNFs with $10<N_c<56$ and limit ourselves to the lowest energy configurations. We found that: i) the energy difference $Δ$ between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with $N_c$, ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination and bilayer on the electronic properties of small GNFs.This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy.

cond-mat.mtrl-sci↗

arXiv2014

Stabilized silicene within bilayer graphene: A proposal based on molecular dynamics and density-functional tight-binding calculations

Free standing silicene is predicted to display comparable electronic properties as graphene. However, the yet synthesized silicene-like structures have been only realized on different substrates which turned out to exhibit versatile crystallographic structures that are very different from the theoretically predicted buckled phase of freestanding silicene. This calls for a different approach where silicene is stabilized using very weakly interacting surfaces. We propose here a novel route by using graphene bilayer as a scaffold. The confinement between the flat graphene layers results in a planar clustering of Si atoms with small buckling, which is energetically unfavorable in vacuum. Buckled hexagonal arrangement of Si atoms similar to free-standing silicene is observed for large clusters, which, in contrast to Si atoms on metallic surfaces, is only very weakly van der Waals coupled to the graphene layers. These clusters are found to be stable well above room temperature. Our findings, which are supported by density functional tight-binding calculations, show that intercalating bilayer graphene with Si is a favorable route to realize silicene.

cond-mat.mtrl-sci↗

arXiv2013

Melting of Partially Fluorinated Graphene: From Detachment of Fluorine Atoms to Large Defects and Random Coils

The melting of fluorographene is very unusual and depends strongly on the degree of fluorination. For temperatures below 1000 K, fully fluorinated graphene (FFG) is thermo-mechanically more stable than graphene but at T$_m\approx$2800 K FFG transits to random coils which is almost twice lower than the melting temperature of graphene, i.e. 5300 K. For fluorinated graphene (PFG) up to 30 % ripples causes detachment of individual F-atoms around 2000 K while for 40-60 % fluorination, large defects are formed beyond 1500 K and beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The results agree with recent experiments on the dependence of the reversibility of the fluorination process on the percentage of fluorination.

cond-mat.mtrl-sci↗

arXiv2013

Boron Nitride Monolayer: A Strain-Tunable Nanosensor

The influence of triaxial in-plane strain on the electronic properties of a hexagonal boron-nitride sheet is investigated using density functional theory. Different from graphene, the triaxial strain localizes the molecular orbitals of the boron-nitride flake in its center depending on the direction of the applied strain. The proposed technique for localizing the molecular orbitals that are close to the Fermi level in the center of boron nitride flakes can be used to actualize engineered nanosensors, for instance, to selectively detect gas molecules. We show that the central part of the strained flake adsorbs polar molecules more strongly as compared with an unstrained sheet.

cond-mat.mes-hall↗

arXiv2013

Melting of graphene clusters

Density-functional tight-binding and classical molecular dynamics simulations are used to investigate the structural deformations and melting of planar carbon nano-clusters $C_{N}$ with N=2-55. The minimum energy configurations for different clusters are used as starting configuration for the study of the temperature effects on the bond breaking/rotation in carbon lines (N$<$6), carbon rings (5$<$N$<$19) and graphene nano-flakes. The larger the rings (graphene nano-flake) the higher the transition temperature (melting point) with ring-to-line (perfect-to-defective) transition structures. The melting point was obtained by using the bond energy, the Lindemann criteria, and the specific heat. We found that hydrogen-passivated graphene nano-flakes (C$_{N}$H$_M$) have a larger melting temperature with a much smaller dependence on its size. The edges in the graphene nano-flakes exhibit several different meta-stable configurations (isomers) during heating before melting occurs.

cond-mat.mtrl-sci↗

arXiv2013

Thermomechanical properties of a single hexagonal boron nitride sheet

Using atomistic simulations we investigate the thermodynamical properties of a single atomic layer of hexagonal boron nitride (h-BN). The thermal induced ripples, heat capacity, and thermal lattice expansion of large scale h-BN sheets are determined and compared to those found for graphene (GE) for temperatures up to 1000 K. By analyzing the mean square height fluctuations $< h^2>$ and the height-height correlation function $H(q)$ we found that the h-BN sheet is a less stiff material as compared to graphene. The bending rigidity of h-BN: i) is about 16% smaller than the one of GE at room temperature (300 K), and ii) increases with temperature as in GE. The difference in stiffness between h-BN and GE results in unequal responses to external uniaxial and shear stress and different buckling transitions. In contrast to a GE sheet, the buckling transition of a h-BN sheet depends strongly on the direction of the applied compression. The molar heat capacity, thermal expansion coefficient and the Gruneisen parameter are estimated to be 25.2 J\,mol$^{-1}$\,K$^{-1}$, 7.2$\times10^{-6}$K$^{-1}$ and 0.89, respectively.

cond-mat.mes-hall↗

arXiv2013

Spiral graphone and one sided fluorographene nano-ribbons

The instability of a free-standing one sided hydrogenated/fluorinated graphene nano-ribbon, i.e. graphone/fluorographene, is studied using ab-initio, semiempirical and large scale molecular dynamics simulations. Free standing semi-infinite arm-chair like hydrogenated/fluorinated graphene (AC-GO/AC-GF) and boat like hydrogenated/fluorinated graphene (B-GO/B-GF) (nano-ribbons which are periodic along the zig-zag direction) are unstable and spontaneously transform into spiral structures. We find that rolled, spiral B-GO and B-GF are energetically more favorable than spiral AC-GO and AC-GF which is opposite to the double sided flat hydrogenated/fluorinated graphene, i.e. graphane/fluorographene. We found that the packed, spiral structures exhibit unexpected localized HOMO-LUMO at the edges with increasing energy gap during rolling. These rolled hydrocarbon structures are stable beyond room temperature up to at least $T$=1000\,K.

cond-mat.mes-hall↗

arXiv2013

Thermal properties of fluorinated graphene

Large scale atomistic simulations using the reactive force field approach (ReaxFF) are implemented to investigate the thermomechanical properties of fluorinated graphene (FG). A new set of parameters for the reactive force field potential (ReaxFF) optimized to reproduce key quantum mechanical properties of relevant carbon-fluor cluster systems are presented. Molecular dynamics (MD) simulations are used to investigate the thermal rippling behavior of FG and its mechanical properties and compare them with graphene (GE), graphane (GA) and a sheet of BN. The mean square value of the height fluctuations $< h^2>$ and the height-height correlation function $H(q)$ for different system sizes and temperatures show that FG is an un-rippled system in contrast to the thermal rippling behavior of graphene (GE). The effective Young's modulus of a flake of fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG under uniaxial strain along arm-chair and zig-zag direction, respectively.

cond-mat.mes-hall↗

arXiv2012

Thermal rippling behavior of graphane

Thermal fluctuations of single layer hydrogenated graphene (graphane) are investigated using large scale atomistic simulations. By analyzing the mean square value of the height fluctuations $ $ and the height-height correlation function $H(q)$ for different system sizes and temperatures we show that hydrogenated graphene is an un-rippled system in contrast to graphene. The height fluctuations are bounded, which is confirmed by a $ H(q) $ tending to a constant in the long wavelength limit instead of showing the characteristic scaling law $ q^{4-η} (η\simeq 0.85)$ predicted by membrane theory. This unexpected behaviour persists up to temperatures of at least 900 K and is a consequence of the fact that in graphane the thermal energy can be accommodated by in-plane bending modes, i.e. modes involving C-C-C bond angles in the buckled carbon layer, instead of leading to significant out-of-plane fluctuations that occur in graphene.

cond-mat.mtrl-sci↗

arXiv2012

Strain engineered graphene using a nanostructured substrate: I Deformations

Using atomistic simulations we investigate the morphological properties of graphene deposited on top of a nanostructured substrate. Sinusoidally corrugated surfaces, steps, elongated trenches, one dimensional and cubic barriers, spherical bubbles, Gaussian bump and Gaussian depression are considered as support structures for graphene. The graphene-substrate interaction is governed by van der Waals forces and the profile of the graphene layer is determined by minimizing the energy using molecular dynamics simulations. Based on the obtained optimum configurations, we found that: (i) for graphene placed over sinusoidally corrugated substrates with corrugation wave lengths longer than 2\,nm, the graphene sheet follows the substrate pattern while for supported graphene it is always suspended across the peaks of the substrate, (ii) the conformation of graphene to the substrate topography is enhanced when increasing the energy parameter in the van der Waals model, (iii) the adhesion of graphene into the trenches depends on the width of the trench and on graphene's orientation, i.e. in contrast to a small width (3 nm) nanoribbon with armchair edges, the one with zig-zag edges follows the substrate profile, (iv) atomic scale graphene follows a Gaussian bump substrate but not the substrate with a Gaussian depression, and (v) the adhesion energy due to van der Waals interaction varies in the range [0.1-0.4] J/m^2.

cond-mat.mes-hall↗

arXiv2012

Strain engineered graphene using a nanostructured substrate: II Pseudo-magnetic fields

The strain induced pseudo-magnetic field in supported graphene deposited on top of a nanostructured substrate is investigated by using atomistic simulations. Step, elongated trench, one dimensional barrier, spherical bubbles, Gaussian bump and Gaussian depression are considered as support structures for graphene. From the obtained optimum configurations we found very strong induced pseudo-magnetic fields which can reach up to $\sim$ 1000\,T due to the strain-induced deformations in the supported graphene. Different magnetic confinements with controllable geometries are found by tuning the pattern of the substrate. The resulting induced magnetic fields for graphene on top of a step, barrier and trench are calculated. In contrast to the step and trench the middle part of graphene on top of a barrier has zero pseudo-magnetic field. This study provides a theoretical background for designing magnetic structures in graphene by nanostructuring substrates. We found that altering the radial symmetry of the deformation, changes the six-fold symmetry of the induced pseudo-magnetic field.

cond-mat.mes-hall↗

arXiv2012

Nano-engineered non-uniform strain in graphene

Recent experiments showed that non-uniform strain can be produced by depositing graphene over pillars. We employed atomistic calculations to study the non-uniform strain and the induced pseudo-magnetic field up to 5000 Tesla in graphene on top of nano-pillars. By decreasing the distance between the nano-pillars a complex distribution for the pseudo-magnetic field can be generated. Furthermore, we performed tight-binding calculations of the local density of states (LDOS) by using the relaxed graphene configuration obtained from the atomistic calculations. We find that the quasiparticle LDOS are strongly modified near the pillars, both at low energies showing sub-lattice polarization, and at high energies showing shifts of the van Hove singularity. Our study shows that changing the specific pattern of the nano-pillars allows us to create a desired shape of the pseudo-magnetic field profile while the LDOS maps provide an input for experimental verifications by scanning tunneling microscopy.

cond-mat.mes-hall↗

arXiv2012

Induced polarization and electronic properties of carbon doped boron-nitride nanoribbons

The electronic properties of boron-nitride nanoribbons (BNNRs) doped with a line of carbon atoms are investigated by using density functional calculations. Three different configurations are possible: the carbon atoms may replace a line of boron or nitrogen atoms or a line of alternating B and N atoms which results in very different electronic properties. We found that: i) the NCB arrangement is strongly polarized with a large dipole moment having an unexpected direction, ii) the BCB and NCN arrangement are non-polar with zero dipole moment, iii) the doping by a carbon line reduces the band gap independent of the local arrangement of boron and nitrogen around the carbon line, iv) an electric field parallel to the carbon line polarizes the BN sheet and is found to be sensitive to the presence of carbon dopants, and v) the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital decreases linearly with increasing applied electric field directed parallel to the carbon line. We show that the polarization and energy gap of carbon doped BNNRs can be tuned by an electric field applied parallel along the carbon line.

cond-mat.mes-hall↗

arXiv2012

Effect of grain boundary on the buckling of graphene nanoribbons

The buckling of graphene nano-ribbons containing a grain boundary is studied using atomistic simulations where free and supported boundary conditions are invoked. We found that when graphene contains a small angle grain boundary the buckling strains are larger when the ribbons with free (supported) boundary condition are subjected to compressive tension parallel (perpendicular) to the grain boundary. The shape of the deformations of the buckled graphene nanoribbons depends on the boundary conditions and the presence of the grain boundary and the direction of applied in-plane compressive tension. Large angle grain boundary results in smaller buckling strains as compared to perfect graphene or to a small angle grain boundary.

cond-mat.mtrl-sci↗

arXiv2012

Thermomechanical properties of graphene: valence force field model approach

Using the valence force field model of Perebeinos and Tersoff [Phys. Rev. B {\bf79}, 241409(R) (2009)], different energy modes of suspended graphene subjected to tensile or compressive strain are studied. By carrying out Monte Carlo simulations it is found that: i) only for small strains ($|\varepsilon| \lessapprox 0.02$) the total energy is symmetrical in the strain, while it behaves completely different beyond this threshold; ii) the important energy contributions in stretching experiments are stretching, angle bending, out-of-plane term and a term that provides repulsion against $π-π$ misalignment; iii) in compressing experiments the two latter terms increase rapidly and beyond the buckling transition stretching and bending energies are found to be constant; iv) from stretching-compressing simulations we calculated the Young modulus at room temperature 350$\pm3.15$\,N/m, which is in good agreement with experimental results (340$\pm50$\,N/m) and with ab-initio results [322-353]\,N/m; v) molar heat capacity is estimated to be 24.64\,J/mol$^{-1}$K$^{-1}$ which is comparable with the Dulong-Petit value, i.e. 24.94\,J/mol$^{-1}$K$^{-1}$ and is almost independent of the strain; vi) non-linear scaling properties are obtained from height-height correlations at finite temperature; vii) the used valence force field model results in a temperature independent bending modulus for graphene, and viii) the Gruneisen parameter is estimated to be 0.64.

cond-mat.mtrl-sci↗

arXiv2011

Van der Waals energy surface of carbon nanotubes sheet

The van der Walls interaction between a carbon nanotubes sheet (CNTS) and a rare gas atom, is studied using both atomistic and continuum approaches. We present analytical expressions for the van der Waals energy of continuous nanotubes interacting with a rare gas atom. It is found that the continuum approach dose not properly treat the effect of atomistic configurations on the energy surfaces. The energy barriers are small as compared to the thermal energy, which implies the free motion above the CNTS in heights about one nanometer. In contrast to the energy surface of a graphene sheet, the honeycomb lattice structure in the energy surface of a CNTS is imperceivable. Defects alter the energy surface which therefore influence the gas absorbtion mechanism. (Corresponding author:neekamal@srttu.edu)

cond-mat.mes-hall↗

arXiv2011

Nanoindentation of a circular sheet of bilayer graphene

Nanoindentation of bilayer graphene is studied using molecular dynamics simulations. We compared our simulation results with those from elasticity theory as based on the nonlinear Föppl-Hencky equations with rigid boundary condition. The force deflection values of bilayer graphene are compered to those of monolayer graphene. Young's modulus of bilayer graphene is estimated to be 0.8 TPa which is close to the value for graphite. Moreover, an almost flat bilayer membrane at low temperature under central load has a 14$%$ smaller Young's modulus as compared to the one at room temperature.

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