arXiv subjects

M. Neek-Amal

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

46 records · Page 3Linked to original sources

arXiv2011

Lattice thermal properties of Graphane: thermal contraction, roughness and heat capacity

Using atomistic simulations we determine the roughness and the thermal properties of a suspended graphane sheet. As compared to graphene we found that hydrogenated graphene has: 1) a larger thermal contraction, 2) the roughness exponent at room temperature is smaller, i.e. $\simeq$ 1.0 versus $\simeq$ 1.2 for graphene, 3) the wave lengths of the induced ripples in graphane cover a wide range corresponding to length scales in the range (30-125)\,Åat room temperature, and 4) the heat capacity of graphane is estimated to be 29.32$\pm$0.23\,J/molK which is 14.8% larger than the one for graphene, i.e. 24.98$\pm$0.14\,J/molK. Above 1500\,K we found that graphane buckles when its edges are supported in the $x-y$ plane.

cond-mat.mes-hall↗

arXiv2011

Directed motion of C60 on a graphene sheet subjected to a temperature gradient

Nonequilibrium molecular dynamics simulations is used to study the motion of a C60 molecule on a graphene sheet subjected to a temperature gradient. The C60 molecule is actuated and moves along the system while it just randomly dances along the perpendicular direction. Increasing the temperature gradient increases the directed velocity of C60. It is found that the free energy decreases as the C60 molecule moves toward the cold end. The driving mechanism based on the temperature gradient suggests the construction of nanoscale graphene-based motors.

cond-mat.mes-hall↗

arXiv2011

Graphene nano ribbons subjected to axial stress

Atomistic simulations are used to study the bending of rectangular graphene nano ribbons subjected to axial stress both for free boundary and supported boundary conditions. The shape of the deformations of the buckled graphene nano ribbons, for small values of the stress, are sine waves where the number of nodal lines depend on the longitudinal size of the system and the applied boundary condition. The buckling strain for the supported boundary condition is found to be independent of the longitudinal size and estimated to be 0.86$%$. From a calculation of the free energy at finite temperature we find that the equilibrium projected two-dimensional area of the graphene nano ribbon is less than the area of a flat sheet. At the optimum length the boundary strain for the supported boundary condition is 0.48$%$.

cond-mat.mes-hall↗

arXiv2010

Buckled circular monolayer graphene: a graphene nano-bowl

We investigate the stability of circular monolayer graphene subjected to a radial load using non-equilibrium molecular dynamics simulations. When monolayer graphene is radially stressed, after some small circular strain ($\sim 0.4%$) it buckles and bends into a new bowl like shape. Young's modulus is calculated from the linear relation between stress and strain before the buckling threshold, which is in agreement with experimental results. The prediction of elasticity theory for the buckling threshold of a radially stressed plate is presented and its results are compared to the one of our atomistic simulation. Jarzynski equality is used to estimate the difference between the free energy of the non-compressed states and the buckled states. From a calculation of the free energy we obtain the optimum radius for which the system feels the minimum boundary stress.

cond-mat.mes-hall↗

arXiv2009

Diffusive motion of C60 on a graphene sheet

The motion of a C60 molecule over a graphene sheet at finite temperature is investigated both theoretically and computationally. We show that a graphene sheet generates a van der Waals laterally periodic potential, which directly influences the motion of external objects in its proximity. The translational motion of a C60 molecule near a graphene sheet is found to be diffusive in the lateral directions. While, in the perpendicular direction, the motion may be described as diffusion in an effective harmonic potential which is determined from the distribution function of the position of the C60 molecule. We also examine the rotational diffusion of C60 and show that its motion over the graphene sheet is not a rolling motion.

cond-mat.mes-hall↗

arXiv2009

Formation of atomic nanoclusters on graphene sheets

The formation of atomic nanoclusters on suspended graphene sheets have been investigated by employing a Molecular dynamics simulation at finite temperature. Our systematic study is based on temperature dependent Molecular dynamics simulations of some transition and alkali atoms on suspended graphene sheets. We find that the transition atoms aggregate and make various size nanoclusters distributed randomly on graphene surface. We also report that most alkali atoms make one atomic layer on graphene sheets. Interestingly, the potassium atoms almost deposit regularly on the surface at low temperature. We expect from this behavior that the electrical conductivity of a suspended graphene doped by potassium atoms would be much higher than the case doped by the other atoms at low temperature.

cond-mat.mtrl-sci↗

arXiv2008

Ground state study of simple atoms within a nano-scale box

Ground state energies for confined hydrogen (H) and helium (He) atoms, inside a penetrable/impenetrable compartment have been calculated using Diffusion Monte Carlo (DMC) method. Specifically, we have investigated spherical and ellipsoidal encompassing compartments of a few nanometer size. The potential is held fixed at a constant value on the surface of the compartment and beyond. The dependence of ground state energy on the geometrical characteristics of the compartment as well as the potential value on its surface has been thoroughly explored. In addition, we have investigated the cases where the nucleus location is off the geometrical centre of the compartment.

cond-mat.other↗

arXiv2007

Roughness of undoped graphene and its short-range induced gauge field

We present both numerical and analytical study of graphene roughness with a crystal structure including $500 \times 500$ atoms. The roughness can effectively result in a random gauge field and has important consequences for its electronic structure. Our results show that its height fluctuations in small scales have scaling behavior with a temperature dependent roughness exponent in the interval of $ 0.6 < χ< 0.7 $. The correlation function of height fluctuations depends upon temperature with characteristic length scale of $ \approx 90 Å$ (at room temperature). We show that the correlation function of the induced gauge field has a short-range nature with correlation length of about $\simeq 2-3 Å$. We also treat the problem analytically by using the Martin-Siggia-Rose method. The renormalization group flows did not yield any delocalized-localized transition arising from the graphene roughness. Our results are in good agreement with recent experimental observations.

cond-mat.mtrl-sci↗

arXiv2007

Ground state properties of a confined simple atom by C$_{60}$ fullerene

We numerically study the ground state properties of endohedrally confined hydrogen (H) or helium (He) atom by a molecule of C$_{60}$. Our study is based on Diffusion Monte Carlo method. We calculate the effects of centered and small off-centered H- or He-atom on the ground state properties of the systems and describe the variation of ground state energies due to the C$_{60}$ parameters and the confined atomic nuclei positions. Finally, we calculate the electron distributions in $x-z$ plane in a wide range of C$_{60}$ parameters.

cond-mat.other↗

arXiv2006

Monte Carlo simulation of size-effects on thermal conductivity in a 2-dimensional Ising system

A model based on microcanonical Monte Carlo method is used to study the application of the temperature gradient along a two-dimensional (2D) Ising system. We estimate the system size effects on thermal conductivity, $K$, for a nano-scale Ising layer with variable size. It is shown that $K$ scales with size as $ K=cL^α$ where $α$ varies with temperature. Both the Metropolis and Cruetz algorithms have been used to establish the temperature gradient. Further results show that the average demon energy in the presence of an external magnetic field is zero for low temperatures.

cond-mat.stat-mech↗