Density functional theory is used to show that the adhesion between single-walled carbon nanotubes (SWNTs) and the catalyst particles from which they grow needs to be strong to support nanotube growth. It is found that Fe, Co, and Ni, commonly used to catalyze SWNT growth, have larger adhesion strengths to SWNTs than Cu, Pd, and Au and are therefore likely to be more efficient for supporting growth. The calculations also show that to maintain an open end of the SWNT it is necessary that the SWNT adhesion strength to the metal particle is comparable to the cap formation energy of the SWNT end. This implies that the difference between continued and discontinued SWNT growth to a large extent depends on the carbon-metal binding strength, which we demonstrate by molecular dynamics (MD) simulations. The results highlight that first principles computations are vital for the understanding of the binding strength's role in the SWNT growth mechanism and are needed to get accurate force field parameters for MD.

Molecular dynamics simulations based on an empirical potential energy surface were used to study iron catalyzed nucleation and growth of single-walled carbon nanotubes (SWNTs). The simulations show that SWNTs grow from iron-carbide particles at temperatures between 800 and 1400 K, whereas graphene sheets encapsulate the particle at temperatures below 600 K and a three-dimensional soot-like structure is formed above 1600 K. Nucleation of these carbon (C) structures can be divided into three stages: (i) at short times the FeC particle is not saturated in C and all C atoms are dissolved in the particle; (ii) at intermediate times the FeC cluster is highly supersaturated in C and carbon strings, polygons and small graphitic islands nucleate on the cluster surface; (iii) at longer times the FeC cluster is supersaturated in C and, depending on the temperature, the graphene sheet, SWNT, or soot-like structure is grown. At low temperatures the kinetic energy is not sufficient to overcome the attractive forces between the particle and the graphitic islands (that are formed in stage ii) and, because these islands cannot lift off the particle, a complete graphene sheet grows around the cluster. At temperatures above 800 K the kinetic energy is sufficiently high to overcome these attractive forces so that the graphitic island lifts off the particle to form a cap. Between 800 and 1400 K theses caps grow into SWNTs, and at temperatures larger than 1600 K the large number of defects in the growing carbon structure produces a soot-like structure. The calculations also reveal that the growing SWNT maintains an open end on the cluster due to the strong bonding between the open nanotube end atoms and the cluster. The number of defects in the SWNT structure can be reduced by lowering the rate of carbon addition to the FeC cluster.

We present the results of coupled quantum mechanics and molecular mechanics (QM/MM) classical molecular dynamics simulations for HCl sticking to the (0001) basal plane of ice Ih. Interatomic forces and energies of hydrogen chloride and up to 24 water molecules in the top ice bilayer were obtained from semiempirical molecular orbital calculations based on the PM3 method. A few PM3 parameters were adjusted so that structural and energetic properties of small neutral and ionic systems match available ab initio and experimental data. This QM region was coupled to the remainder of the ice surface (the MM region), which was treated using the analytic TIP4P force field. The surface temperature was between 0 and 180 K, and the dynamics was followed for 100 ps. On surface impact, HCl binds to a dangling (free) H 2 O oxygen via a ClH-OH 2 hydrogen bond. If the Cl is solvated by one dangling H 2 O hydrogen, HCl adsorbs molecularly. If two dangling hydrogens are available in a surface hexagon, HCl dissociates to a Cl --H 3 O + contact ion pair. The simulations thus predict a mechanism by which HCl can ionize readily on ice surfaces. This mechanism is consistent with a saturation coverage of 0.33 monolayers for ionized HCl on ice surfaces. As a comparison we have also simulated HCl colliding with a cubic (H 2 O) 8 cluster, in which the whole system was treated by the semiempirical method. Hydrogen chloride adsorbs on the cluster and, depending on the temperature, the (H 2 O) 8 cube may open up, thereby initiating HCl ionization. The results are discussed in relation with stratospheric heterogeneous ozone chemistry and available experimental and theoretical results.

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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, that contain at least 20 iron atoms, nucleate SWNTs and 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.

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.

The thermal behavior of free and alumina-supported iron-carbon nanoparticles is investigated via molecular dynamics simulations, in which the effect of the substrate is treated with a simple Morse potential fitted to ab initio data. We observe that the presence of the substrate raises the melting temperature of medium and large Fe1−xCx nanoparticles (x = 0 − 0.16, N = 80 − 1000, nonmagic numbers) by 40-60 K; it also plays an important role in defining the ground state of smaller Fe nanoparticles (N = 50 − 80). The main focus of our study is the investigation of Fe-C phase diagrams as a function of the nanoparticle size. We find that as the cluster size decreases in the 1.1-1.6-nm-diameter range the eutectic point shifts significantly not only toward lower temperatures, as expected from the Gibbs-Thomson law, but also toward lower concentrations of C. The strong dependence of the maximum C solubility on the Fe-C cluster size may have important implications for the catalytic growth of carbon nanotubes by chemical vapor deposition.

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