Tarek Ragab

Joule heating in single-walled carbon nanotubes

Joule heating in single-walled carbon nanotubes (CNTs) using a quantum mechanical approach is presented in this paper. The modeling is based on the energy transfer between the electrons and both acoustic and optical phonons. In this formulation, only the knowledge of the full energy dispersion relation, phonon dispersion relation, and the electron-phonon coupling potential is required for the calculations. For verification of the proposed model, the current-voltage relation for extremely long nanotubes is calculated and the results are compared with the experimental data. The electric field dependence of the amount of energy generated by Joule heating is plotted. Moreover the effect of the thermal environment on the behavior of Joule heating is studied. The formulation proposed in this paper can also be used for structures other than CNTs. Computations indicate that, contrary to popular opinion, metallic CNT does not follow Joule’s law of P=IV. Joule heating in CNT is significantly less than what is predi...

Mechanical Properties of Hydrogen Edge–Passivated Chiral Graphene Nanoribbons

AbstractUniaxial tension of chiral graphene nanoribbons (GNR) with and without edge hydrogen passivation are simulated using molecular dynamics (MD) simulations to study their mechanical properties. The results demonstrate that hydrogen saturation generally weakens chiral GNRs, although its influence on armchair GNRs is almost negligible. Mechanical properties of GNRs depend on chiral angles. Zigzag GNRs (chiral angle 0°) are always the strongest, whereas armchair GNRs (chiral angle 30°) are weaker. The mechanical properties of other chiral GNRs evolve gradually from these two distinct cases from chiral angles of 30° to 0°, with the smallest value of failure stress and failure strain happening around a chiral angle of 20°. As for the width size effect, wider GNRs always have lower failure strains and failure stress regardless of having edge hydrogen passivation or not.

Hot phonons contribution to Joule heating in single-walled carbon nanotubes

The influence of hot phonons on the electron-phonon scattering rate and Joule heating is studied via an ensemble Monte Carlo (EMC) simulation with a step by step update of the phonon occupation number to account for the generation of hot phonons. The hot phonon contribution to Joule heating appears to be a function of the applied electric force field at room temperature, while it becomes independent of the applied electric force field for higher temperatures. The influence of hot phonons on Joule heating is more pronounced around room temperature and diminishes for higher temperatures. The results of the ensemble Monte Carlo simulation at very high temperatures (around 1800 K and above) suggest that the presence of non-equilibrium phonons may reduce the Joule heating of single-walled carbon nanotubes (SWCNTs).

Phonon dispersion and quantization tuning of strained carbon nanotubes for flexible electronics

Graphene and carbon nanotubes are materials with large potentials for applications in flexible electronics. Such devices require a high level of sustainable strain and an understanding of the materials electrical properties under strain. Using supercell theory in conjunction with a comprehensive molecular mechanics model, the full band phonon dispersion of carbon nanotubes under uniaxial strain is studied. The results suggest an overall phonon softening and open up the possibility of phonon quantization tuning with uniaxial strain. The change in phonon quantization and the resulting increase in electron-phonon and phonon-phonon scattering rates offer further explanation and theoretical basis to the experimental observation of electrical properties degradation for carbon nanotubes under uniaxial strain.

The Unravelling of Open-Ended Single Walled Carbon Nanotubes Using Molecular Dynamics Simulations

The unravelling of (10, 10) and (18, 0) single-walled carbon nanotubes (SWCNTs) is simulated using molecular dynamics simulations at different temperatures. Two different schemes are proposed to simulate the unravelling; completely restraining the last atom in the chain and only restraining it in the axial direction. The forces on the terminal atom in the unravelled chain in the axial and radial directions are reported till the separation of the atomic chain from the carbon nanotube structure. The force-displacement relation for a chain structure at different temperatures is calculated and is compared to the unravelling forces. The axial stresses in the body of the carbon nanotube are calculated and are compared to the failure stresses of that specific nanotube. Results show that the scheme used to unravel the nanotube and the temperature can only effect the duration needed before the separation of some or all of the atomic chain from the nanotube, but does not affect the unravelling forces. The separation of the atomic chain from the nanotube is mainly due to the impulsive excessive stresses in the chain due to the addition of a new atom and rarely due to the steady stresses in the chain. From the simulations, it is clear that the separation of the chain will eventually happen due to the closing structure occurring at the end of the nanotube that would not be possible in multiwalled nanotubes.

The effects of vacancy defect on the fracture behaviors of zigzag graphene nanoribbons

Zigzag graphene nanoribbons with and without single vacancy defect are strained under uniaxial tension using molecular dynamics simulations. In order to understand the influence of vacancy defect on the damage mechanics, the graphene nanoribbons are categorized into six groups based on their width, ranging from 2.5 nm to 15 nm. In each group, the length of GRNGNR also varied from 2.5 nm to 15 nm. The comparison of the stress–strain relationship and the fracture behavior of pristine and defective graphene nanoribbon demonstrate that single vacancy defect has little influence on the elastic modulus and the ultimate strength of graphene nanoribbons. However, size effect does have an influence on the ultimate failure stress of the graphene nanoribbon.

Unraveling mechanics of armchair and zigzag graphene nanoribbons

The unraveling process of armchair and zigzag graphene nanoribbons (GNRs) was studied with molecular dynamics simulations using the Adaptive Intermolecular Reactive Empirical Bond Order Potential for carbon–carbon bond. Simulations were performed at 300°K, with GNR length and width varying from 2.5 nm to 15 nm in 2.5 nm increments. In these simulations, the unraveling of the GNRs was started from two positions; the corner or the middle of the top side. Force–displacement relationship was analyzed for the terminal atom of the unraveling chains. For armchair GNRs (AGNRs) that were unraveled from the corner, the force required for the onset of the unraveling is in the range of 4.279–5.045 eV/Å, and the observed failure force in the carbon chain is in the range of 5.553–5.963 eV/Å. Unraveling will not happen when AGNRs are unraveled from the middle, and zigzag GNRs (ZGNRs) are unraveled either from corner or middle. For the latter cases, the bond between the terminal atom and GNR sheet breaks under the stretching force, and only one carbon atom can be pulled out from the GNR sheet. The size effect of width and length on the unraveling process was also studied. Simulations show that size has a trivial effect on unraveling. Comparison between unraveling of AGNRs and ZGNRs indicates that AGNRs are perfect structure to produce Monatomic Carbon Chains, while ZGNRs are more stable and are good candidate for graphene nanodevices that are free from unraveling disintegration.

Comparison of fracture behavior of defective armchair and zigzag graphene nanoribbons

Molecular dynamics simulations of armchair graphene nanoribbons and zigzag graphene nanoribbons with different sizes were performed at room temperature. Double vacancy defects were introduced in each graphene nanoribbon at its center or at its edge. The effect of defect on the mechanical behavior was studied by comparing the stress–strain response and the fracture toughness of each pair of pristine and defective graphene nanoribbon. Results show that the effect of vacancies in zigzag graphene nanoribbon is more profound than in armchair graphene nanoribbon. Also, the effect of double vacancy defect on the ultimate failure stress is greater in zigzag graphene nanoribbons than in armchair graphene nanoribbon due to bond orientation with respect to loading direction. Strength reduction can be as high as 17.5% in armchair graphene nanoribbon with no significant difference between single and double vacancies, while for zigzag graphene nanoribbon, the strength reduction is up to 30% for single vacancy and 43% for double vacancy defects. It is observed that for zigzag graphene nanoribbon with double vacancy at the edge, the direction of the failure plane is oriented at ±30° with respect to the loading direction while it is always perpendicular to the direction of loading in armchair graphene nanoribbon. Results have been verified through studying the fracture toughness parameters in each case as well.

Modeling Joule heating in carbon nanotubes with Monte Carlo simulations

The ensemble Monte Carlo simulation is used to calculate the Joule heating per unit length of single-walled carbon nanotubes under an electric field applied through the nanotube axis. The electronic system and the ionic system are decoupled from each other. The rate of energy transferred from the electronic system to the ionic system in the form of the emission or absorption of longitudinal acoustic and longitudinal optical phonons is calculated stochastically to determine the Joule heating. Complete unabridged energy and phonon dispersion relations are included in order to obtain more accurate results. The effect of the temperature and the electric field magnitude on the heat generated is also taken into account. Results are compared with a prediction based on quantum mechanical integral form that calculates the electron occupation probability based on a modified Fermi-Dirac distribution. Results show a quantitative agreement between the two methods, however, the method proposed in here we believe is more accurate, because it does not make simplifications for the electron occupation probability as in the modified Fermi-Dirac distribution.