Yasutaka Yamaguchi

A MOLECULAR DYNAMICS SIMULATION OF THE FULLERENE FORMATION PROCESS

A molecular dynamics simulation starting from 500 isolated carbon atoms resulted in several closed caged structures under suitable temperature control. A caged C 70 cluster that appeared in the simulation was traced back to study the dynamics and the structure of C n precursors: simple chain and ring for n 30. Furthermore, it was found that the final caged structure was obtained when the control temperature was roughly in the range from 2500K to 3000K, and a graphitic flat structure resulted in lower control temperatures.

A molecular dynamics demonstration of annealing to a perfect C60 structure

Abstract The formation process of imperfect fullerenes C60 and C70 was simulated using the molecular dynamics method as described in our previous report. These imperfect fullerenes were kept at 2500 K to evaluate annealing effects. Through successive Stone–Wales transformations, perfect fullerene structures were achieved both for C60 and C70 in about 200 ns. The annealing temperature and time scale were plausible, compared to experimental conditions as seen in an Arrhenius plot. Similar simulations were performed for smaller clusters at each stage of the clustering process. Based on these simulations, a fullerene formation model is proposed.

Analysis on wetting and local dynamic properties of single water droplet on a polarized solid surface: A molecular dynamics study

Molecular dynamics simulations of single water droplets on a solid surface were carried out in order to investigate the effects that the Coulomb interaction between liquid and solid molecules has on wetting behavior by appending vertical electric polarization on a solid surface. The water droplet became more wettable both on upward and downward polarized surfaces, although structures of the adsorption layer appearing near the solid surface were clearly different, and the relation between droplet contact angle and surface polarization was also different for upward and downward polarization directions. The probability density distribution of molecular orientation around the adsorption layer indicated that preferable water molecule orientations varied largely by the surface polarization, and the rotational mobility around the preferable orientations was also affected. The dynamic property due to this rotational mobility was clearly captured by means of distribution of rotational diffusion coefficient, which potentially corresponded to local viscosity distribution.

Molecular dynamics analysis on wetting and interfacial properties of water-alcohol mixture droplets on a solid surface.

Molecular dynamics simulations of single water, water-methanol, or water-IPA (isopropyl-alcohol) mixture droplets on a solid surface were performed with various mixture ratios. An increase in alcohol fraction generally gave an increase in droplet wettability. Both methanol and IPA molecules showed a strong preference to gather at various interfaces, with methanol molecules also showing a tendency to diffuse into the droplet bulk. Specific interfacial tensions were investigated using quasi-one-dimensional simulation systems, and liquid-vapor and solid-liquid interfacial tensions were found to decrease greatly due to the presence of interfacial alcohol, while solid-vapor interfacial tensions were proved to have little influence on wettability. Youngs relation was found to hold quantitatively well for both water-methanol and water-IPA droplets. The validity of using Bakkers equation on solid-liquid interfaces was also investigated, and it was shown that for tightly spaced crystal surfaces, the introduced uncertainly is small.

Molecular dynamics analysis of multiphase interfaces based on in situ extraction of the pressure distribution of a liquid droplet on a solid surface

Molecular dynamics simulations of a nanoscale liquid droplet on a solid surface are carried out in order to examine the pressure tensor field around the multiphase interfaces, and to explore the validity of Youngs equation. By applying the virial theorem to a hemicylindrical droplet consisting of argon molecules on a solid surface, two-dimensional distribution of the pressure tensor is obtained. Tensile principal pressure tangential to the interface is observed around the liquid-vapor transition layer, while both tensile and compressive principal pressure tangential to the interface exists around the solid-liquid transition layer due to the inhomogeneous density distribution. The two features intermix inside the overlap region between the transition layers at the contact line. The contact angle is evaluated by using a contour line of the maximum principal pressure difference. The interfacial tensions are calculated by using Bakkers equation and Young-Laplace equation to the pressure tensor distribution. The relation between measured contact angle and calculated interfacial tensions turns out to be consistent with Youngs equation, which is known as the description of the force balance at the three-phase interface.

Formation Process of Empty and Metal-Containing Fullerenes—Molecular Dynamics and FT-ICR Studies

Abstract The formation mechanism of empty and metal-containing fullerene was studied through MD (molecular dynamics) simulations and FT-ICR (Fourier transform ion cyclotron resonance) mass spectroscopy of laser vaporized carbon cluster. Multi-body classical potential functions for metal-carbon and metal-metal interactions were constructed based on DFT (density functional theory) calculations of various forms of small clusters MCn and Mn (M = La, Sc, Ni). Using the modified Brenner potential for carbon-carbon interaction, the clustering process starting from 500 isolated carbon atoms and 5 metal atoms in gas phase was simulated under the controlled temperature condition at 3000K. The difference of clustering process of La@Cn, Sc@Cn and NiCn were compared with empty fullerene formation simulation. FT-ICR mass spectrometer directly connected to the laser vaporization cluster beam source was implemented in order to experimentally study the clustering process. The increase of cluster nozzle pressure roughly co...

Ejection of clusters from solid surface by impact of size-selected cluster ion

Ejection of clusters from a solid surface by impact of size-selected clusters was investigated by mass spectroscopy and molecular dynamics (MD) simulation. It was found experimentally that carbon cluster anions, (m = 1–12), are ejected from a graphite surface by impact of (N = 1–25) at collision energies (E col) of 0.2–14.0 keV per CO2 molecule. A yield, η, of the carbon-atom ejection in the form of C m − was measured to increase with the 4th power of E col as well as the 3.6th power of N, that is, η∼N 3.6 , and levels off as E col increases further. The size distribution of the ejected remains unchanged with E col, while the average size, ⟨m⟩, of increases in proportion to N 0.17. On the other hand, MD simulation showed that C m are produced by recombination of carbon atoms evaporated from a hot surface of a cylindrical crater-shaped defect temporarily formed on the graphite surface by the cluster impact. The E col and N dependences of η and ⟨m⟩ were explained by a thermal desorption model parameterized using the results of the MD simulation. It was concluded that the higher efficiency of the cluster ejection by the larger cluster impact is attained in terms of efficient energy localization in a larger surface area and in a shallower region due to the instantaneous energy deposition by the cluster impact and the fragile nature of graphene sheets.

Molecular dynamics analysis of the friction between a water-methanol liquid mixture and a non-polar solid crystal surface

We performed molecular dynamics analysis of the momentum transfer at the solid-liquid interface for a water-methanol liquid mixture between parallel non-polar solid walls in order to understand the strong decrease of the friction coefficient (FC) induced by the methanol adsorption at the surface observed in our previous work [S. Nakaoka et al., Phys. Rev. E 92, 022402 (2015)]. In particular, we extracted the individual contributions of water and methanol molecules to the total FC and found that the molecular FC for methanol was larger than that for water. We further showed that the reduction of the total solid-liquid FC upon the increase of the methanol molar fraction in the first adsorption layer occurred as a result of a decrease in the molecular number density as well as a decrease in the molecular FCs of both molecules. Analysis of the molecular orientation revealed that the decrease of the molecular FC of methanol resulted from changes of the contact feature onto the solid surface. Specifically, methanol molecules near the solid surface had their C-O bond parallel to the surface with both CH3 and O sites contacting the solid at low methanol molar fraction, while they had their C-O bond outward from the surface with only the CH3 site contacting the solid at higher methanol molar fraction. The mechanisms discussed in this work could be used to search for alternative water additives to further reduce the solid-liquid friction.

Numerical analysis of nanograin collision by classical molecular dynamics

We have carried out atomistic simulations of grain-grain collisions for spherical grains of 1.4 and 4 nm radii, with relative velocities of 3.6–6.1 km/s and a number of impact parameters. Since the initial grains are crystallites without any pre-existing defects, grain shattering due to nucleation of cracks was not observed in our simulations. We find grain fusion in some events, but generally melting occurs, leading to nucleation, growth and linkage of voids in the melt, which then leads to production of small clusters. The size distribution does not obey a simple power law and can be considered as having four different regimes, where each regime can be fitted as a power law.

金属内包フラーレン生成の分子動力学シミュレーション ( マイクロスケールの流体・熱流動現象)

The growth process of metal-containing fullerene was studied using the molecular dynamics method. As for metalcontaining fullerene, only the metal atoms such as La, Y or Sc are experimentally assigned to be contained inside the carbon cage. In this paper, the difference of growth process and structures of metal-attached carbon clusters were examined using lanthanum and nickel as the additional metal atoms to clarify the effect of the metal atoms. In order to model the potential function between carbon and metal atoms appropriate for the classical MD simulation, the binding energy and charge state of various forms of small clusters MCn (M: La,Ni; n =1-3) were calculated with the density functional theory. A multi-body potential function was constructed as a function of coordinate number of a scandium atom. Here, the Coulomb term and the Morse-type term were separately described considering the large charge transfer from a lanthanum atom to carbon atoms. By using the potential function, the clustering process starting from 500 isolated carbon atoms and 5 metal atoms in a 342 Å cubic cell was simulated under the controlled temperature condition at Tc = 3000 K, and the growth history of metal-attached clusters was studied in detail. Figure A-1 shows the growth process of La attached clusters obtained in the simulation. The fan-type structure and mono-cyclic or bi-cyclic rings were reproduced around the metal atom for LaCn (n<20). An open cap structure surrounding the La atom was formed for LaC n (20<n<50) due to the strong Coulomb interaction, and that resulted in the lanthanum-containing caged cluster. When Ni atoms were applied as shown in Figure A-2, the structures of small clusters up to NiC n (n<20) were similar to LaCn (n<20), while tangled-poly-cyclic structures and random cage were dominant for NiC n (20<n<50). The nickel atom attached on one face of the caged structure, and could not stay stably inside the carbon cage structure. time (ps) lu te r size