Kim Bolton

Molecular dynamics study of the catalyst particle size dependence on carbon nanotube growth

The molecular dynamics method, based on an empirical potential energy surface, was used to study the effect of catalyst particle size on the growth mechanism and structure of single-walled carbon nanotubes (SWNTs). The temperature for nanotube nucleation (800-1100 K), which occurs on the surface of the cluster, is similar to that used in catalyst chemical vapor deposition experiments, and the growth mechanism, which is described within the vapor-liquid-solid model, is the same for all cluster sizes studied here (iron clusters containing between 10 and 200 atoms were simulated). Large catalyst particles, which contain at least 20 iron atoms, nucleate SWNTs that have a far better tubular structure than SWNTs nucleated from smaller clusters. In addition, the SWNTs that grow from the larger clusters have diameters that are similar to the cluster diameter, whereas the smaller clusters, which have diameters less than 0.5 nm, nucleate nanotubes that are approximately 0.6-0.7 nm in diameter. This is in agreement with the experimental observations that SWNT diameters are similar to the catalyst particle diameter, and that the narrowest free-standing SWNT is 0.6-0.7 nm.

Iron-carbide cluster thermal dynamics for catalyzed carbon nanotube growth

Molecular dynamics simulations have been used to study the thermal behavior of FeN−mCm clusters where N, the total number of atoms, extends up to 2400. Comparison of the computed results with experimental data shows that the simulations yield the correct trends for the liquid–solid region of the iron-carbide phase diagram as well as the correct dependence of cluster melting point as a function of cluster size. The calculation indicates that, when carbon nanotubes (CNTs) are grown on large (>3–4 nm) catalyst particles at low temperatures (<1200 K), the catalyst particles are not completely molten. It is argued that the mechanism of CNT growth under these conditions may be governed by the surface melting of the cluster.

Modeling the melting of supported clusters

Molecular dynamics simulations have been used to study the structural and dynamic changes during melting of free and supported iron clusters ranging from 150 to 10000atoms. The results reveal a method for determining effective diameters of supported metal clusters, so that the melting point dependence on cluster size can be predicted in a physically meaningful way by the same analytic model used for free clusters.

Reduced Carbon Solubility in Fe Nanoclusters and Implications for the Growth of Single-Walled Carbon Nanotubes

Fe nanoclusters are becoming the standard catalysts for growing single-walled carbon nanotubes via chemical vapor decomposition. Contrary to the Gibbs-Thompson model, we find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first-principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function of particle size, there are three scenarios compatible with steady state growth, limited growth, and no growth of single-walled carbon nanotubes, corresponding to unaffected, reduced, and no solubility of C in the particles.

Hidden features of the catalyst nanoparticles favorable for single-walled carbon nanotube growth

Combining in situ studies of the catalyst activity during single-walled carbon nanotube (SWCNT) growth by mass spectrometry with differential scanning calorimetry and Raman spectroscopy results, the authors expose the favorable features of small catalyst for SWCNT growth and their relationship with synthesis parameters. The sequential introduction of C12 and C13 labeled hydrocarbon reveals the influence of catalyst composition on its lifetime and the growth termination path. Ab initio and molecular dynamics simulations corroborate “V”-shape liquidus line of metal-carbon nanoparticle binary phase diagram, which explains observed carbon-induced solid-liquid-solid phase transitions during nanotube growth.

Classical trajectory study of argon–ice collision dynamics

Classical trajectory simulations have been used to study Ar–ice Ih collisional energy transfer, trapping coefficients and scattering distributions for initial Ar kinetic energies between 0.1 and 2.0 eV, incident angles between 0 and 70° and surface temperatures between 0 and 300 K. Collisional energy transfer is extremely efficient due to substantial transfer of energy from the Ar atom to the ice surface over typically 2–4 gas-surface encounters, and the rapid dissipation of this energy away from the collision center, preventing energy transfer back to the Ar atom. This leads to large trapping coefficients over this range of Ar collision energies, incident angles and surface temperatures. Scattered gas atoms lose most of their initial kinetic energy and have broad angular distributions. The large trapping coefficients obtained for the Ar–ice collisions are expected to be found for similar reactions under stratospheric conditions (e.g., HCl–ice, HOCl–ice and ClONO2–ice).

The importance of supersaturated carbon concentration and its distribution in catalytic particles for single-walled carbon nanotube nucleation

The carbon concentration in catalytic metal particles during the initial stages of single-walled carbon nanotube (SWNT) nucleation is studied by thermodynamics modelling and molecular dynamics simulation. Highly supersaturated carbon concentrations are necessary for nucleating the initial small carbon structures (e.g., islands) on the particle surface. A lower, but still supersaturated, concentration is required to enlarge the carbon island, with the required carbon concentration decreasing with increasing island size. This decrease in carbon concentration explains the widely observed phenomenon that only one carbon nanotube is nucleated per catalyst particle in catalytic chemical vapour deposition experiments. It is also shown that an inhomogeneous distribution of carbon in the catalyst particle can lead to the formation of several SWNTs from each particle, as formed in arc discharge and laser ablation experiments.

A classical molecular dynamics study of the intramolecular energy transfer of model trans-stilbene

Abstract An approximate anharmonic potential energy surface for the S1 state of trans-stilbene has been constructed. This model surface, which is based on the potential of toluene and ethene, was optimised to the available trans-stilbene S1 state experimental frequencies and the trans → cis isomerisation barrier height. The surface was employed in classical trajectory studies where the main goal was to investigate the internal energy transfer of the isolated trans-stilbene molecule. Although the applicability of classical dynamics for large molecules at low vibrational energies is uncertain, we find that the beating patterns that have been observed for the isolated photoexcited molecule are qualitatively reproduced by the classical trajectories. The simulated quasiperiodic beating patterns persist at elevated molecular thermal energies where zero point energy effects are approximately taken into account. The simulation data indicate the presence of a bottleneck to intramolecular energy redistribution that is also robust with respect to increased molecular thermal energies. Similarly, the RRKM rate coefficient that is based on our surface overestimates the isomerisation rates that are observed experimentally. The experimental data that have been obtained for the isolated molecule and for the molecule under low pressure thermal conditions are discussed in the light of these results.

Monte Carlo simulations of equilibrium solubilities and structure of water in n-alkanes and polyethylene

Gibbs ensemble Monte Carlo methods based on a force field that combines the simple point charge [Berendsen et al., in Intermolecular Forces, edited by Pullman (Reidel, Dordrecht, 1981), p. 331] and transferable potentials for phase equilibria [Martin and Siepmann, J. Phys. Chem. B 102, 2659 (1998)] models were used to study the equilibrium properties of binary systems consisting of water and n-alkanes with chain lengths from hexane to hexadecane. In addition, systems where extended linear alkane chains (up to 300 carbon units long) were used to represent amorphous polyethylene were simulated in the presence of water using a connectivity altering osmotic Gibbs ensemble. In these simulations the equilibrium between a liquid water phase and a polymer phase into which water was inserted was studied. The predicted solubilities, which were determined between 350 and 550 K, are in good agreement with experiment, where experimental results are available, and the density of water molecules in the hydrocarbons is approximately 63% as high as in saturated water vapor under the same conditions. At the lower temperatures most of the water exists as monomers; increasing the temperature leads to an increase in the density of water in the alkane phase and hence in the fraction of molecules that participate in clusters. Dimers are the most prevalent clusters in all hydrocarbons and at all temperatures studied, and the fraction of clusters of given size decrease with increasing cluster size. A large fraction of trimers, tetramers, and pentamers, which are the cluster sizes for which topologies have been studied, are cyclic at low temperatures, but at higher temperatures linear structures predominate. The same properties are observed for pure water vapor clusters in equilibrium with the liquid phase, showing that the cluster topologies are not significantly affected by the surrounding hydrocarbon.

Simulations of vapor water clusters at vapor–liquid equilibrium

The Gibbs-ensemble Monte Carlo methods based on the extended single point charge [H. J. C. Berendsen, J. R. Grigera, and T. P. Straatsma, J. Phys. Chem. 91, 6269 (1987)] potential-energy surface have been used to study the clustering of vapor phase water under vapor-liquid equilibrium conditions between 300 and 600 K. It is seen that the number of clusters, as well as the cluster size, increase with temperature. This is primarily due to the increase in vapor density that accompanies the temperature increase at equilibrium. In addition, due to entropic effects, the percentage of clusters that have linear (or open) topologies increases with temperature and dominates over the minimum-energy cyclic topologies at the temperatures studied here. These results are insensitive to the number of molecules used in the simulations and the criterion used to define a water cluster.