Tahir Cagin

Energetics, structure, mechanical and vibrational properties of single-walled carbon nanotubes

In this paper, we present extensive molecular mechanics and molecular dynamics studies on the energy, structure, mechanical and vibrational properties of single-wall carbon nanotubes. In our study we employed an accurate interaction potential derived from quantum mechanics. We explored the stability domains of circular and collapsed cross section structures of armchair (n,n), zigzag(n,0) , and chiral (2n,n) isolated single-walled carbon nanotubes (SWNTs) up to a circular cross section radius of 170 A. We have found three different stability regions based on circular cross section radius. Below 10 A radius only the circular cross section tubules are stable. Between 10 and 30 A both circular and collapsed forms are possible, however, the circular cross section SWNTs are energetically favorable. Beyond 30 A (crossover radius) the collapsed form becomes favorable for all three types of SWNTs. We report the behavior of the SWNTs with radii close to the crossover radius ((45, 45), (80, 0), (70, 35)) under uniaxial compressive and tensile loads. Using classical thin-plane approximation and variation of strain energy as a function of curvature, we calculated the bending modulus of the SWNTs. The calculated bending moduli are [kappa][sub](n,n)=963.44 GPa, [kappa][sub](n,0)=911.64 GPa, and [kappa][sub](2n,n)=935.48 GPa. We also calculated the interlayer spacing between the opposite sides of the tubes and found d[sub](n,n)= 3.38 angstroms, d[sub](2n,n)= 3.39 angstroms, and d[sub](n,0)= 3.41 angstroms. Using an enthalpy optimization method, we have determined the crystal structure and Youngs modulus of (10,10) armchair, (17,0) zigzag and (12, 6) chiral forms (which have similar diameter as (10,10)). They all pack in a triangular pattern in two dimensions. Calculated lattice parameters are a[sub](10,10)= 16.78 angstroms, a[sub](17,0)= 16.52 angstroms and a[sub](12,6)= 16.52 angstroms. Using the second derivatives of potential we calculated Youngs modulus along the tube axis and found Y[sub](10,10)=640.30 GPa, Y[sub](17,0)=648.43 GPa, and Y[sub](12,6)=673.94 GPa. Using the optimized structures of (10,10), (12,6) and (17,0), we determined the vibrational modes and frequencies. Here, we report the highest in-plane mode, compression mode, breathing mode, shearing mode and relevant cyclop mode frequencies.

Melting and crystallization in Ni nanoclusters: The mesoscale regime

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms ~5.7 nm! using molecular dynamics with the quantum-Sutten‐Chen many-body force field. We find a transition from cluster or molecular behavior below ;500 atoms to a mesoscale nanocrystal regime ~well-defined bulk and surface properties! above ;750 atoms ~2.7 nm!. We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N5Tm,bulk2aN 21/3 , dropping from Tm,bulk51760 K to Tm,336 5980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N.400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character.

Thermal conductivity of diamond and related materials from molecular dynamics simulations

Based on the Green‐Kubo relation from linear response theory, we calculated the thermal current autocorrelation functions from classical molecular dynamics ~MD! simulations. We examined the role of quantum corrections to the classical thermal conduction and concluded that these effects are small for fairly harmonic systems such as diamond. We then used the classical MD to extract thermal conductivities for bulk crystalline systems. We find that ~at 300 K! 12 C isotopically pure perfect diamond has a thermal conductivity 45% higher than natural ~1.1% 13 C! diamond. This agrees well with experiment, which shows a 40%‐50% increase. We find that vacancies dramatically decrease the thermal conductivity, and that it can be described by a reciprocal relation with a scaling as n va , with a50.6960.11 in agreement with phenomenological theory (a51/2 to 3/4!. Such calculations of thermal conductivity may become important for describing nanoscale devices. As a first step in studying such systems, we examined the mass effects on the thermal conductivity of compound systems, finding that the layered system has a lower conductivity than the uniform system.

Thermal conductivity of BN-C nanostructures

Chemical and structural diversity present in hexagonal boron nitride ((h-BN) and graphene hybrid nanostructures provide new avenues for tuning various properties for their technological applications. In this paper we investigate the variation of thermal conductivity (

Molecular dynamics simulations of thermal resistance at the liquid-solid interface

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Calculation of Mechanical, Thermodynamic and Transport Properties of Metallic Glass Formers

) of hybrid graphene/h-BN nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are investigated using equilibrium molecular dynamics. To simulate these systems, we have parameterized a Tersoff type interaction potential to reproduce the ab initio energetics of the B-C and N-C bonds for studying the various interfaces that emerge in these hybrid nanostructures. We demonstrate that both the details of the interface, including energetic stability and shape, as well as the spacing of the interfaces in the material exert strong control on the thermal conductivity of these systems. For stripe superlattices, we find that zigzag configured interfaces produce a higher

Factors affecting molecular dynamics simulated vitreous silica structures

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MOLECULAR DYNAMICS WITH A VARIABLE NUMBER OF MOLECULES

in the direction parallel to the interface than the armchair configuration, while the perpendicular conductivity is less prone to the details of the interface and is limited by the

A bottom-up route to enhance thermoelectric figures of merit in graphene nanoribbons

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Grand Molecular Dynamics: A Method for Open Systems

of h-BN. Additionally, the embedded dot structures, having mixed zigzag and armchair interfaces, affects the thermal transport properties more strongly than superlattices. Though dot radius appears to have little effect on the magnitude of reduction, we find that dot concentration (50% yielding the greatest reduction) and composition (embedded graphene dots showing larger reduction that h-BN dot) have a significant effect.