Atomistic simulation of the growth of defect-free carbon nanotubes

Atomistic simulation of defect-free single-walled carbon nanotube (SWCNT) growth is essential for the insightful understanding of the SWCNT''s growth mechanism. Despite the extensive effort paid in the past two decades, the goal has not been completely achieved, due to the huge timescale discrepancy between atomistic simulation and the experimental synthesis of SWCNTs, as well as the lack of an accurate classical potential energy surface for large scale simulation. Here, we report atomistic simulations of defect-free SWCNT growth by using a new generation of carbon–metal potential and a hybrid method, in which a basin-hopping strategy is applied to facilitate the defect healing during the simulation. The simulations reveal a narrow diameter distribution and an even chiral angle distribution of the growth of SWCNTs from liquid catalyst, which is in agreement with most known experimental observations.     

Homogeneous nucleation and growth of melt in copper

Molecular dynamics simulations are conducted to investigate homogeneous nucleation and growth of melt in copper described by an embedded-atom method (EAM) potential. The accuracy of this EAM potential for melting is validated by the equilibrium melting point obtained with the solid-liquid coexistence method and the superheating-supercooling hysteresis method. We characterize the atomistic melting process by following the temperature and time evolution of liquid atoms. The nucleation behavior at the extreme superheating is analyzed with the mean-first-passage-time (MFPT) method, which yields the critical size, steady-state nucleation rate, and the Zeldovich factor. The value of the steady-state nucleation rate obtained from the MFPT method is consistent with the result from direct simulations. The size distribution of subcritical nuclei appears to follow a power law similar to three-dimensional percolation. The diffuse solid-liquid interface has a sigmoidal profile with a 10%-90% width of about 12 A near the critical nucleation. The critical size obtained from our simulations is in reasonable agreement with the prediction of classical nucleation theory if the finite interface width is considered. The growth of melt is coupled with nucleation and can be described qualitatively with the Johnson-Meh-Avrami law. System sizes of 10(3)-10(6) atoms are explored, and negligible size dependence is found for bulk properties and for the critical nucleation.     

Understanding the barriers to crystal growth: dynamical simulation of the dissolution and growth of urea from aqueous solution

Both the dissolution and growth of a molecular crystalline material, urea, has been studied using dynamical atomistic simulation. The kinetic steps of dissolution and growth are clearly identified, and the activation energies for each possible step are calculated. Our molecular dynamics simulations indicate that crystal growth on the [001] face is characterized by a nucleation and growth mechanism. Nucleation on the [001] urea crystal face is predicted to occur at a very high rate, followed by rapid propagation of the steps. The rate-limiting step for crystallization is actually found to be the removal of surface defects, rather than the initial formation of the next surface layer. Through kinetic Monte Carlo modeling of the surface growth, it is found that this crystal face evolves via a rough surface topography, rather than a clean layer-by-layer mechanism.     

Assisted desolvation as a key kinetic step for crystal growth

The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.     

Molecular dynamics simulations of amphiphilic graft copolymer molecules at a water/air interface

Fully atomistic molecular dynamics simulations of amphiphilic graft copolymer molecules have been performed at a range of surface concentrations at a water/air interface. These simulations are compared to experimental results from a corresponding system over a similar range of surface concentrations. Neutron reflectivity data calculated from the simulation trajectories agrees well with experimentally acquired profiles. In particular, excellent agreement in neutron reflectivity is found for lower surface concentration simulations. A simulation of a poly(ethylene oxide) (PEO) chain in aqueous solution has also been performed. This simulation allows the conformational behavior of the free PEO chain and those tethered to the interface in the previous simulations to be compared.     

Experimental and computational growth morphology of two polymorphs of a yellow isoxazolone dye

We report on the crystal structures and the experimentally found and the computationally predicted growth morphologies of two polymorphs of a yellow isoxazolone dye. The stable polymorph I has a blocklike habit, and the metastable polymorph II grows as fine needles, nucleating only by heterogeneous or contact nucleation. The habits of both polymorphs depend on the supersaturation during growth. The experimental observations are compared with predictions of the attachment energy model and kinetic Monte Carlo lattice simulations in which the growth is modeled as an "atomistic process", governed by surface energetics. These Monte Carlo simulations correctly predict the shape and the dependence on supersaturation of the crystal morphology for both polymorphs: for polymorph I, a strong dependence on supersaturation is found from the simulations. For polymorph II, the order of morphological importance is reproduced correctly, as well as the needlelike morphology.     

Towards understanding the nucleation mechanism for multi-component systems: an atomistic simulation of the ternary nucleation of water/n-nonane/1-butanol

Inspired by the previous finding of some unusual vapour/liquid nucleation results on the ternary water/n-nonane/1-butanol system, atomistic simulations were carried out for a detailed investigation of this mixture. These simulations reproduced the experimentally-reported non-ideal nucleation behaviour for this system, including both onset activities and the average compositions of the critical nuclei. Close examination of the nucleation free energy data and the structure of the critical nuclei reveals two types of phase separation. One occurs internally inside the cluster via formation of a multi-layered structure. The other takes place externally, leading to the coexistence of multiple nucleation channels, characterized by critical clusters of different compositions. Such mechanistic and structural heterogeneity is the microscopic origin of the complex nucleation behaviour observed for this ternary mixture.     

Design of anti-icing coatings using supercooled droplets as nano-to-microscale probes

A multiscale simulation-based approach is presented for predicting anti-icing properties of nanocomposite coatings. Development of robust anti-icing coatings is a challenging task. An anti-icing coating that can prevent in-flight icing is of particular interest to the aircraft industry. A multiscale simulations based approach is developed to provide insights into the complex effect of coating material and surface topology on the prevention of in-flight icing. Chemical properties of different coatings and kinetics of icing or inhibition of ice nucleation are calculated from nanoscale atomistic simulations. In addition, in-flight icing environments including impingement and rolling of supercooled microdroplet and nucleation of ice under wind shear have been implemented using fluid dynamics methodologies. A model for icing in nano-to-microscale for surfaces with known chemical composition and surface topology is used for developing predictive capabilities regarding anti-icing performance of potential coatings. In this work, fluorinated polyhedral oligomericsilsesquioxanes molecules have been used to increase nanoscale roughness when embedded in a polycarbonate polymeric matrix. The findings suggest that a successful anti-icing coating will require precise control over nanoscale and microscale roughness. The multiscale methodology presented therefore can potentially help in identifying coupled effects of material, surface topology, and icing environment for promising coatings before performing icing tunnel experiments.     

Simulation of plastic deformation in glassy polymers: Atomistic and mesoscale approaches

The mechanism of deformation in glasses is very different from that of crystals, even though their general behavior is very similar. In this study, we investigated the deformation of polycarbonate on the atomistic scale with molecular dynamics and on the continuum scale with a new simulation approach. The results indicated that high atomic/segmental mobility and low local density enabled the formation (nucleation) of highly deformed regions that grew to form plastic defects called plastic shear transformations. A continuum-scale simulation was performed with the concept of plastic shear transformations as the basic region of deformation. The continuum simulations were able to predict the primary and secondary creep behavior. The slope of the secondary creep depended on the interactions between the plastic shear transformations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 994-1004, 2005     

Polymer-clay nanocomposites: a multiscale molecular modeling approach

A hierarchical procedure bridging the gap between atomistic and mesoscopic simulation for polymer-clay nanocomposite (PCN) design is presented. The dissipative particle dynamics (DPD) is adopted as the mesoscopic simulation technique, and the interaction parameters of the mesoscopic model are estimated by mapping the corresponding energy values obtained from atomistic molecular dynamics (MD) simulations. The predicted structure of the nylon 6 PCN system considered is in excellent agreement with previous experimental and atomistic simulation results.     

Multiscale approach to CO2 hydrate formation in aqueous solution: phase field theory and molecular dynamics. Nucleation and growth

A phase field theory with model parameters evaluated from atomistic simulations/experiments is applied to predict the nucleation and growth rates of solid CO(2) hydrate in aqueous solutions under conditions typical to underwater natural gas hydrate reservoirs. It is shown that under practical conditions a homogeneous nucleation of the hydrate phase can be ruled out. The growth rate of CO(2) hydrate dendrites has been determined from phase field simulations as a function of composition while using a physical interface thickness (0.85+/-0.07 nm) evaluated from molecular dynamics simulations. The growth rate extrapolated to realistic supersaturations is about three orders of magnitude larger than the respective experimental observation. A possible origin of the discrepancy is discussed. It is suggested that a kinetic barrier reflecting the difficulties in building the complex crystal structure is the most probable source of the deviations.     

Electrochemical nucleation: Part II. The electrodeposition of silver on vitreous carbon.

A study has been made of the electrochemical nucleation of silver on vitreous carbon from aqueous solutions of silver nitrate.The nucleation is shown to be progressive and mass transferred controlled. The rate of nucleation is somewhat better described bythe atomistic theory than by classical theories. The addition of EDTA reduces the rate of nucleation as the result, it is suggested,of the adsorption of EDTA by the graphite surface. The sensitivity of the rate of the nucleation to the condition of the graphitesurface is also shown by the effects of changing the salvation acidity.     

Atomistic mechanisms of phase separation and formation of solid solutions: model studies of NaCl, NaCl-NaF, and Na(Cl1-xBrx) crystallization from the melt

The crystallization of sodium chloride from its melt and mixtures with other sodium halides is investigated by means of transition path sampling molecular dynamics simulations. From this we explore the nucleation mechanisms of both the solidification and the melting process at the atomistic level of detail. By incorporation of impurities the nucleation picture of the eutectic mixtures changes considerably. Doping the NaCl crystal with fluoride ions, we observed the substitutional defects to act as favored nucleation centers for the melting transition. This phenomenon plays a critical role during the solidification process of NaCl-NaF melts of low NaF concentration and is demonstrated to account for the segregation of fluoride ions. While NaCl-NaF corresponds to a eutectic system, we also investigated NaCl-NaBr mixtures. The bromide ions were observed to behave very similarly to chloride ions. As a consequence, no phase separation occurs and Na(Cl1-xBrx) solid solutions are formed. At the example of these two prototypes we demonstrate the study of the atomistic mechanisms related to phase separation processes and solid solution formation during the nucleation and growth of crystals from multinary melts.     

From simple surface models to lipid membranes: Universal aspects of the hydration interaction from solvent-explicit simulations

A review of atomistic simulation approaches including explicit water for the study of hydration forces between polar surfaces is presented. In particular, we discuss different methods for keeping the chemical potential of water constant and compare advantages and limitations of each method. It turns out that modifications of hydration forces due to surface softness can be accounted for by a convolution over the surface shape profile. Universal aspects of the hydration interaction observed in simulations of different surface chemistries are highlighted, while special attention is given to hydration forces between self-assembled phospholipid membranes.     

Mechanisms and nucleation characteristics of the pressure-induced B1-B2 transition in potassium halides: a question of ion hardness and softness

The transformation of potassium bromide from the B1 to the high-pressure B2 structure type is investigated by means of molecular dynamics simulations and compared to previous studies of KF and KCl. The underlying simulation scheme is based on the transition path sampling approach, which allows an unbiased investigation of the phase transition and offers a unique perspective for studying the involved mechanisms at the atomistic level of detail. Our analysis reveals identical mechanisms for the overall transition in KF, KCl, and KBr, but rather dissimilar characteristics of the nucleation and growth of phases. The transformation of KCl may be initiated by both K+ and Cl- ion displacement, exhibiting no preference for either species. However, for KF and KBr, we identified a clear favoring of column-wise F- and K+ displacement, respectively. Such tendencies have important implications on the morphogenesis of the phase nuclei and account for the observation of short-ranged coexisting nucleation centers, resulting in the formation of nanosized twin domains separated by mirror planes on completion of the transition. On the basis of a systematic study of potassium halides, we present a conclusive explanation for the observed nucleation characteristics, which is expected to be of general relevance to pressure-induced phase transitions in ionic compounds.     

Kinetic Monte Carlo study of nucleation processes on patterned surfaces

The properties of template-directed nucleation are studied in the transition region where full nucleation control is lost and additional nucleation beyond the prepatterned structure is observed. To get deeper insight into the microscopic mechanisms, Monte Carlo simulations were performed. In this context, the previously used continuous algorithm [F. Kalischewski, J. Zhu, and A. Heuer, Phys. Rev. B 77, 155401, (2008)] was replaced by a discrete one to reduce simulation time and to allow more detailed calculations. The applied method is based on the assumption that the molecules on the surface occupy the sites of a simple fcc lattice. It is shown that a careful mapping of the continuous Monte Carlo technique onto the discrete algorithm leads to a good reproduction of the former results by means of the latter method. Furthermore, the new method facilitates the calculation of the spatial distribution of nuclei on the surface. This provides a detailed comparison with experimental data.     

New insights into zeolite formation from molecular modeling

We review recent molecular modeling efforts to shed light on the mechanisms of zeolite formation. We focus on studies that model the early stages of silica polymerization and zeolite nucleation. Electronic structure calculations, classical molecular dynamics, atomistic Monte Carlo simulations and Monte Carlo simulations of lattice models have been used to probe the formation of zeolites and mesoporous materials. Results from these modeling studies predict that in early stages of formation, the silicate material is amorphous. Cluster growth is predicted to occur primarily through Ostwald ripening, and by aggregation of small fragments.     

Nonclassical nucleation kinetics in the crystallization of a supercooled melt

A molecular dynamics simulation of homogenous nucleation of a crystal in supercooled aluminum melt is performed. Nucleation rates at a temperature of 900 K in the range of pressures from 12 to 15 GPa are calculated. Analysis of the mean first-passage times of crystalline cluster sizes is performed. A stepwise dependence of the mean first-passage time on crystal nucleus size is observed, in contrast to the sigmoidal dependence that follows from classical nucleation theory. Based on the data from atomistic simulations, it is established that the form of the free energy barrier during nucleation differs significantly from the one assumed in classical nucleation theory for a spherically symmetric nucleus. It is supposed that the observed differences are apparently due to the complex structure of the crystalline nucleus.