Abstract. As the sizes of electronic and mechanical devices are decreased to the micron and nanometre level, it becomes particularly important to predict the thermal transport properties of the components. Using molecular level theories, such predictions are particularly important for modelling nano-electronic devices where scaling laws may change substantially but it is most difficult to accurately measure the properties. Hence, using the empirical bond order dependent force field, we have studied here the thermal conductivity of nanotubes' dependence on structure, defects and vacancies. The anisotropic character of the thermal conductivity of the graphite crystal is naturally reflected in the carbon nanotubes. We found that the carbon nanotubes have very high thermal conductivity comparable to diamond crystal and in-plane graphite sheet. In addition, nanotube bundles show very similar properties as graphite crystal in which dramatic difference in thermal conductivities along different crystal axis.

The structure and dynamics of poly(amido amide) (PAMAM) dendrimers have been of great interest both scientifically and industrially, but such important features as the distributions of atoms, channels, and strain inside these molecules remain unresolved. This paper reports results from systematic investigations of the atomistic structure of ethylenediamine (EDA) cored PAMAM dendrimer up through the 11th generation (294 852 atoms), at which point the strain energy has risen to a point that limits uniform growth of additional layers. Here we report, as a function of generation, structural properties such as radius of gyration, shape tensor, asphericity, fractal dimension, monomer density distribution, solvent accessible surface area, molecular volume, and end group distribution functions, all evaluated from extensive molecular dynamics (MD) at 300 K. We find that the radius of gyration scales as R g ∼ N 1/3 over the entire range of generations, suggesting rather uniform space filling for all generations. Contrary to common expectation, we find that the outer subgenerations penetrate substantially into the interior of the dendrimer, even for G11. Consequently, the terminal amine groups are distributed throughout the interior, not just on the periphery of the dendrimer. However for G6 through G11 there is a large region of uniform density, supporting the uniform scattering model often used in interpreting the SANS (small-angle neutron scattering) and SAXS (small-angle X-ray scattering) data, which lead to sizes in excellent agreement with the calculations. The calculated single particle form factor approaches that of a sphere as the generation number increases. For the larger generations, we found that the use of continuous configuration biased Monte Carlo (CCBB MC) was essential to construct initial configurations that lead to lower final strain energies.

We report various structural and conformational properties of generations 4, 5, and 6 PAMAM (polyamidoamine) dendrimer [EDA (ethylenediamine) core)] at various protonation levels through extensive molecular dynamics (MD) simulations in explicit solvent. The presence of solvent leads to swelling of the dendrimer (by 33% for G5 compared to the case of no solvent). We find that decreasing the solution from high pH (∼10, no protonation) to neutral (∼7, only primary amines protonated) to low pH (∼4, tertiary amines also protonated) changes the radius of gyration of G5 from 21 to 22 to 25 Å, respectively. We also report such other structural quantities as radial density, distribution of terminal groups, solvent accessible surface area and volume, shape, and structure factors (to compare with SAXS and SANS experiments) at various pH conditions. We find significant back-folding of the outer subgenerations in the interior of the molecules at all levels of pH, contrary to original expectations and some SANS experiments but in agreement with other SANS experiments. We find significant water penetration inside the dendrimer, with ∼3 water/tertiary amine for high pH and ∼6 water/tertiary amine for low pH (all for G5). This indicates that the interior of the dendrimer is quite open with internal cavities available for accommodating guest molecules, suggesting using PAMAM dendrimer for guest−host applications. This estimate of internal waters suggests that sufficient water is available to facilitate metal ion binding.

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms ͑5.7 nm͒ using molecular dynamics with the quantum-Sutten-Chen many-body force field. We find a transition from cluster or molecular behavior below ϳ500 atoms to a mesoscale nanocrystal regime ͑well-defined bulk and surface properties͒ above ϳ750 atoms ͑2.7 nm͒. We find that the mesoscale nanocrystals melt via surface processes, leading to T m,N ϭT m,bulk Ϫ␣N Ϫ1/3 , dropping from T m,bulk ϭ1760 K to T m,336 ϭ980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For NϾ400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character.

A major goal of metal–organic framework (MOF) research is the expansion of pore size and volume. Although many approaches have been attempted to increase the pore size of MOF materials, it is still a challenge to construct MOFs with precisely customized pore apertures for specific applications. Herein, we present a new method, namely linker labilization, to increase the MOF porosity and pore size, giving rise to hierarchical-pore architectures. Microporous MOFs with robust metal nodes and pro-labile linkers were initially synthesized. The mesopores were subsequently created as crystal defects through the splitting of a pro-labile-linker and the removal of the linker fragments by acid treatment. We demonstrate that linker labilization method can create controllable hierarchical porous structures in stable MOFs, which facilitates the diffusion and adsorption process of guest molecules to improve the performances of MOFs in adsorption and catalysis.