Ryuji Miura

Atomic‐scale formation of ultrasmooth surfaces on sapphire substrates for high‐quality thin‐film fabrication

The atomically ultrasmooth surfaces with atomic steps of sapphire substrates were obtained by annealing in air at temperatures between 1000 and 1400 °C. The terrace width and atomic step height of the ultrasmooth surfaces were controlled on an atomic scale by changing the annealing conditions and the crystallographic surface of substrates. The obtained ultrasmooth surface was stable in air. The topmost atomic structure of the terrace was examined quantitatively by atomic force microscopy and ion scattering spectroscopy as well as a theoretical approach using molecular dynamics simulations.

Grand canonical Monte Carlo simulation of the adsorption of CO2 on silicalite and NaZSM-5

The adsorption of carbon dioxide in silicalite and NaZSM-5 zeolite has been studied using new Monte Carlo software. In this program, sodium cations and framework are movable during the simulation. The calculated adsorption isotherms are in good agreement with the experimental results. The energy distribution of carbon dioxide over silicalite and NaZSM-5 shows that the increase of the adsorption energy for NaZSM-5 is mainly due the electric field generated by sodium cations.

A Computational Chemistry Study on Friction of h-MoS2. Part II. Friction Anisotropy

In this work, the friction anisotropy of hexagonal MoS(2) (a well-known lamellar compound) was theoretically investigated. A molecular dynamics method was adopted to study the dynamical friction of two-layered MoS(2) sheets at atomistic level. Rotational disorder was depicted by rotating one layer and was changed from 0° to 60°, in 5° intervals. The superimposed structures with misfit angle of 0° and 60° are commensurate, and others are incommensurate. Friction dynamics was simulated by applying an external pressure and a sliding speed to the model. During friction simulation, the incommensurate structures showed extremely low friction due to cancellation of the atomic force in the sliding direction, leading to smooth motion. On the other hand, in commensurate situations, all the atoms in the sliding part were overcoming the atoms in counterpart at the same time while the atomic forces were acted in the same direction, leading to 100 times larger friction than incommensurate situation. Thus, lubrication by MoS(2) strongly depended on its interlayer contacts in the atomic scale. According to part I of this paper [Onodera, T., et al. J. Phys. Chem. B 2009, 113, 16526-16536], interlayer sliding was source of friction reduction by MoS(2) and was originally derived by its material property (interlayer Coulombic interaction). In addition to this interlayer sliding, the rotational disorder was also important to achieve low friction state.

Molecular dynamics simulation of enhanced oxygen ion diffusion in strained yttria-stabilized zirconia

The application of strain to yttria-stabilized zirconia (YSZ), which can be realized by sandwiching a thin YSZ film epitaxially between layers of a material with larger lattice constants, is proposed as a means to enhance oxygen ion mobility. The possible mechanism of such an enhancement was investigated by molecular dynamics using a CeO2–YSZ superlattice. The calculated diffusion coefficient of oxygen ions in the superlattice is some 1.7 times higher than in YSZ alone due to a decreased activation barrier from the strain of the YSZ structure.

Development of RYUGA for three-dimensional dynamic visualization of molecular dynamics results

Abstract We have developed a new computer graphics (CG) code RYUGA for three-dimensional dynamic visualization of molecular dynamics (MD) results. The applicability of RYUGA for visualizing and analyzing the dynamics of atomic motions in various materials was demonstrated. RYUGA supports various functions, such as solid-modeling CG pictures (called the CPK model), CG animation of the MD results, Miller plane cutting of crystal structures, building graphs, etc., similar to other CG codes for MD simulation. In addition, RYUGA has a number of advantages as follows: (i) a perspective is employed for drawing CG pictures, (ii) three-dimensional trajectories of atoms can be constructed, (iii) an operator can travel inside the materials, (iv) graphic speed and animation speed are enhanced because of the specific algorithms, and (v) it works on any workstations, moreover even personal computers with a UNIX operating system and an X window system are available.

Periodic density functional study on V2O5 bulk and (001) surface

Abstract Density functional calculations on periodic models are performed to investigate the structural and electronic properties of both V2O5 bulk and (001) surface. Full geometry optimizations of both V2O5 bulk and (001) surface are presented. For the bulk, the optimized structure is very close to the experimental one, the calculated band gap and binding energy are in very good agreement with experimental values, from population analysis it is observed that vanadyl oxygens are least ionic (O−0.37), doubly coordinated oxygens are ionic (O−0.56), while triply coordinated oxygens become the most ionic (O−0.68). The structural and electronic properties of the surface are very close to those of the bulk. The interlayer interaction is mainly electrostatic and is found to be 4 kcal/mol. Surface acidic and basic properties are described in terms of projected density of states analysis.

Mechanism of the formation of ultrafine gold particles on MgO(100) as investigated by molecular dynamics and computer graphics

The applicability of a new molecular-dynamics (MD) code developed by us and the computer graphics technique to investigating the mechanism of the formation of ultrafine Au particles on the MgO(100) plane was demonstrated. MD calculations were performed to understand the effect of temperature of the MgO substrate on the mechanism. Lower temperatures led to more Au atoms being fixed on the MgO(100) plane. The effect of defects, such as point defects and steps, in the MgO(100) plane was also investigated. Au clusters were formed and fixed just over the defect sites at low temperature, namely 300 K, in agreement with the experimental results. This behavior was in marked contrast to that at high temperature, namely 1000 K. In the latter case, there is no single favorable location of Au clusters and the Au clusters were considerably mobile on the surface, similar to the behavior on a smooth MgO(100) plane. Hence the location of Au clusters was not affected by the presence of defects at 1000 K. Furthermore, an Au atom trapped in the defects is shown to play the role of a nucleation center in the formation processes of Au clusters on the MgO(100) plane at low temperatures, such as 300 K. These results suggested that a low temperature of the MgO substrate and the presence of defects on MgO(100) are required for the formation of atomically controlled ultrafine Au particles on the MgO(100) plane.

Development of a quantum chemical molecular dynamics tribochemical simulator and its application to tribochemical reaction dynamics of lubricant additives

Tribology at the atomistic and molecular levels has been theoretically studied by a classical molecular dynamics (MD) method. However, this method inherently cannot simulate the tribochemical reaction dynamics because it does not consider the electrons in nature. Although the first-principles based MD method has recently been used for understanding the chemical reaction dynamics of several molecules in the tribology field, the method cannot simulate the tribochemical reaction dynamics of a large complex system including solid surfaces and interfaces due to its huge computation costs. On the other hand, we have developed a quantum chemical MD tribochemical simulator on the basis of a hybrid tight-binding quantum chemical/classical MD method. In the simulator, the central part of the chemical reaction dynamics is calculated by the tight-binding quantum chemical MD method, and the remaining part is calculated by the classical MD method. Therefore, the developed tribochemical simulator realizes the study on tribochemical reaction dynamics of a large complex system, which cannot be treated by using the conventional classical MD or the first-principles MD methods. In this paper, we review our developed quantum chemical MD tribochemical simulator and its application to the tribochemical reaction dynamics of a few lubricant additives.

Layer-by-layer homoepitaxial growth process of MgO(001) as investigated by molecular dynamics, density functional theory, and computer graphics

We applied molecular dynamics, density functional theory, and computer graphics techniques to the investigation of the homoepitaxial growth process of the MgO(001) surface. MgO molecules are deposited over the MgO(001) plane one by one at regular time intervals with definite velocities. Any deposited MgO molecule migrated on the surface, and later a two-dimensional and epitaxial growth of MgO thin layer was observed at 300 K which is in agreement with the experimental result. However, some defects were constructed in the grown film at low temperature of 300 K, which is in remarkable contrast to that at 1000 K. In the latter case, a single flat and smooth MgO layer without defects was formed, which also agreed with the experimental result. Self-diffusion coefficients and activation energy for the surface diffusion of the deposited MgO molecule on the MgO(001) plane were discussed to clarify the temperature-dependency of the epitaxial growth process.

Molecular dynamics simulation of traction fluid molecules under EHL condition

Abstract A new molecular dynamics (MD) code was developed to simulate the dynamic behaviour of traction fluid molecules kept between 2 solid surfaces under the shear condition. In this methodology, one of the surfaces is slid with a constant velocity in a given direction, while a constant pressure is applied on the 2 solid surfaces vertically in order to provide a normal load. We have applied this new MD code and computer graphics technique to the investigation of the dynamic behaviour of benzene and cyclohexane molecules kept between 2 Fe(001) planes at 400 K. Significant differences were observed between the dynamic behaviour of benzene and cyclohexane molecules. The 2nd cyclohexane layer on the sliding surface moved almost together with the 1 st cyclohexane layer, whereas the 2nd benzene layer moved slower than the 1st benzene layer. These differences can be explained by the interlocking of cyclohexane molecules due to their chair conformation as opposed to the plane-shape of the benzene molecules. Moreover, the traction coefficient of benzene and cyclohexane molecules were estimated; they qualitatively agreed with the experimental results.