Yasushi Shibuta

Catalyzed growth of carbon nanotube with definable chirality by hybrid molecular dynamics-force biased Monte Carlo simulations

Metal-catalyzed growth mechanisms of carbon nanotubes (CNTs) were studied by hybrid molecular dynamics-Monte Carlo simulations using a recently developed ReaxFF reactive force field. Using this novel approach, including relaxation effects, a CNT with definable chirality is obtained, and a step-by-step atomistic description of the nucleation process is presented. Both root and tip growth mechanisms are observed. The importance of the relaxation of the network is highlighted by the observed healing of defects.

A molecular dynamics study of the phase transition in bcc metal nanoparticles.

The phase transition between liquid and solid phases in body-centered cubic (bcc) metal nanoparticles of iron, chromium, molybdenum, and tungsten with size ranging from 2000 to 31,250 atoms was investigated using a molecular dynamics simulation. The nucleation from an undercooled liquid droplet was observed during cooling in all nanoparticles considered. It was found that a nucleus was generated near one side of the particle and solidification spread toward the other side the during nucleation process. On the other hand, the surface melting and subsequent inward melting of the solid core of the nanoparticles were observed during heating. The depression of the melting point was proportional to the inverse of the particle radius due to the Gibbs-Thomson effect. On the other hand, the depression of the nucleation temperature during cooling was not monotonic with respect to the particle radius since the nucleation from an undercooled liquid depends on the event probability of an embryo or a nucleus.

Molecular dynamics simulation of generation process of SWNTs

The formation mechanism of single-walled carbon nanotubes (SWNTs) was studied with the molecular dynamics simulation. Starting from randomly distributed carbon and Ni atoms, random cage structures of carbon atoms with a few Ni atoms were obtained after 6 ns simulation. In the next process the cell size was artificially shrunk for realization of proceeding collisions of precursor clusters within the computational time limit. A Ni atom on the random cage prohibited the complete closure and anneal of the cage structure. Collisions of such imperfect random-cage clusters lead to an elongated cage structure, which can be regarded as an imperfect SWNT.

Direction Control of Chemical Wave Propagation in Self-Oscillating Gel Array

A chemomechanical actuator utilizing a reaction-diffusion wave across gap junction was constructed toward a novel mircoconveyer by micropatterned self-oscillating gel array. Unidirectional propagation of the chemical wave of the Belousov-Zhabotinsky (BZ) reaction was induced on gel arrays. In the case of using a triangle-shaped gel as an element of the array, the chemical wave propagated from the corner side of the triangle gel to the plane side of the other gel (C-to-P) across the gap junction, whereas it propagated from the plane side to the corner side (P-to-C) in the case of the pentagonal gel array. Numerical analysis based on the Keener-Tyson model was done for understanding the mechanism of unidirectional propagation in triangle and pentagonal gel arrays. The swelling and deswelling changes of the gels followed the unidirectional propagation of the chemical wave.

Homogeneous nucleation and microstructure evolution in million-atom molecular dynamics simulation.

Homogeneous nucleation from an undercooled iron melt is investigated by the statistical sampling of million-atom molecular dynamics (MD) simulations performed on a graphics processing unit (GPU). Fifty independent instances of isothermal MD calculations with one million atoms in a quasi-two-dimensional cell over a nanosecond reveal that the nucleation rate and the incubation time of nucleation as functions of temperature have characteristic shapes with a nose at the critical temperature. This indicates that thermally activated homogeneous nucleation occurs spontaneously in MD simulations without any inducing factor, whereas most previous studies have employed factors such as pressure, surface effect, and continuous cooling to induce nucleation. Moreover, further calculations over ten nanoseconds capture the microstructure evolution on the order of tens of nanometers from the atomistic viewpoint and the grain growth exponent is directly estimated. Our novel approach based on the concept of “melting pots in a supercomputer” is opening a new phase in computational metallurgy with the aid of rapid advances in computational environments.

Global minima of transition metal clusters described by Finnis–Sinclair potentials: A comparison with semi-empirical molecular orbital theory

We present putative global minimum energy structures for nanoscopic transition metal clusters, with sizes ranging from N = 3 to 100 atoms, described by the original embedded atom potential of Finnis and Sinclair (FS), using their parameter sets for molybdenum and iron, and compare selected results with predictions from semi-empirical molecular orbital (SE-MO) theory via further optimization using the AM1* and PM6 Hamiltonians. We find that, for Fe clusters, the global minima found for the FS potential consist mainly of polyicosahedral structures with magic numbers N = 13, 19, 23, 26, 29, 39, 60 and 78, whereas, for Mo clusters with sizes N > 30, they are more likely to be bcc terminated by {110} and {100}-type surface facets. We find that the global minimum energy structures obtained for the FS potential are, in general, very good starting points for further SE-MO optimization, although the relative ordering of the resulting structures by energy compared to those obtained from global minima of other potentials used to model metal clusters does not, in general, agree.

Heterogeneity in homogeneous nucleation from billion-atom molecular dynamics simulation of solidification of pure metal

Can completely homogeneous nucleation occur? Large scale molecular dynamics simulations performed on a graphics-processing-unit rich supercomputer can shed light on this long-standing issue. Here, a billion-atom molecular dynamics simulation of homogeneous nucleation from an undercooled iron melt reveals that some satellite-like small grains surrounding previously formed large grains exist in the middle of the nucleation process, which are not distributed uniformly. At the same time, grains with a twin boundary are formed by heterogeneous nucleation from the surface of the previously formed grains. The local heterogeneity in the distribution of grains is caused by the local accumulation of the icosahedral structure in the undercooled melt near the previously formed grains. This insight is mainly attributable to the multi-graphics processing unit parallel computation combined with the rapid progress in high-performance computational environments.Nucleation is a fundamental physical process, however it is a long-standing issue whether completely homogeneous nucleation can occur. Here the authors reveal, via a billion-atom molecular dynamics simulation, that local heterogeneity exists during homogeneous nucleation in an undercooled iron melt.

Phase-field modeling for electrodeposition process

Abstract A novel phase-field model for electrochemical processes, in which cations were driven by an electrostatic potential coupled with a thermodynamic potential, was formulated from a variation of the Ginzburg–Landau free-energy functional. Using this model, an electrodeposition process of copper deposits from copper-sulfate solution was studied using a phase-field simulation. The dependence of the growth velocity of the electrode on the applied voltage was examined in a one-dimensional system. Then, the morphological transition of the electrodeposits as functions of the applied voltage and the composition ratio of copper ion in electrolyte was examined using a two-dimensional system. Thin and dense branches were observed at a low applied voltage. The shape of the branches became more complicated as the composition ratio was lowered.

A multiscale approach for modeling the early stage growth of single and multiwall carbon nanotubes produced by a metal-catalyzed synthesis process

A parametrized mesoscale model for the early stage growth of isolated single or multiwall carbon nanotubes (CNTs) has been developed in order to investigate the effects of metal catalyst particle size and composition on CNT growth mechanism during synthesis via a substrate-supported, catalytic chemical vapor deposition process. The model is based on a coarse-grained graphene sheet, represented by a two-dimensional simply connected triangular mesh, with parameters for the surface curvature, bond stretching, carbon-carbon interaction, and carbon-catalyst interaction determined by classical molecular dynamics simulations using a bond-order potential derived from ab initio calculations. The mesoscale simulations show that the initial type of CNT growth is strongly influenced by the surface interaction energy between the graphene sheet and metal catalyst particle, rate of carbon deposition, and particle size. As expected, single wall tubes are produced from small catalyst particles at low deposition rates, but increasing the strength of carbon-catalyst interaction energy or carbon deposition rate results in double or even multiwall CNT structures, formed by folding or involution of the graphene sheet. For the range of model parameters investigated, all single wall CNTs with a diameter greater than 6.6 nm exhibited a kink-collapse transition once a certain critical tube length was reached.

Wafer-scale fabrication and growth dynamics of suspended graphene nanoribbon arrays

Adding a mechanical degree of freedom to the electrical and optical properties of atomically thin materials can provide an excellent platform to investigate various optoelectrical physics and devices with mechanical motion interaction. The large scale fabrication of such atomically thin materials with suspended structures remains a challenge. Here we demonstrate the wafer-scale bottom–up synthesis of suspended graphene nanoribbon arrays (over 1,000,000 graphene nanoribbons in 2 × 2 cm2 substrate) with a very high yield (over 98%). Polarized Raman measurements reveal graphene nanoribbons in the array can have relatively uniform-edge structures with near zigzag orientation dominant. A promising growth model of suspended graphene nanoribbons is also established through a comprehensive study that combined experiments, molecular dynamics simulations and theoretical calculations with a phase-diagram analysis. We believe that our results can contribute to pushing the study of graphene nanoribbons into a new stage related to the optoelectrical physics and industrial applications.