Ikuya Kinefuchi

Inhomogeneous decomposition of ultrathin oxide films on Si(100): Application of Avrami kinetics to thermal desorption spectra

Thermal decomposition of ultrathin oxide layers on silicon surface was investigated with temperature programed desorption. Oxide layers were formed on Si(100) at 400 degrees C by exposure to O(2) molecular beam. Desorption spectrum for oxygen coverages between 1.7 and 2.6 ML exhibits a single dominant peak with an additional broad peak at lower temperatures. The former peak corresponds to stable binding states of O atoms at dimer bridge sites and dimer backbond sites. The high peak intensity indicates that most O atoms are at stable states. The latter peak corresponds to an unstable binding state, where O atoms are presumably trapped at dangling bonds. The SiO desorption rate from the stable binding states is well described by Avrami kinetics, suggesting that the decomposition process is spatially inhomogeneous with void formation and growth. The rate-determining step is the reaction at void perimeter even if the overlap between voids becomes quite large. The Avrami exponents determined from our experiment indicate that the increase in the initial coverage makes the oxide layer more stable and suppresses the rate of void formation at the potential nucleation sites.

Development of Ultra Small Shock Tube for High Energy Molecular Beam Source

A molecular beam source exploiting a small shock tube is described for potential generation of high energy beam in a range of 1–5 eV without any undesirable impurities. The performance of a non‐diaphragm type shock tube with an inner diameter of 2 mm was evaluated by measuring the acceleration and attenuation process of shock waves. With this shock tube installed in a molecular beam source, we measured the time‐of‐flight distributions of shock‐heated beams, which demonstrated the ability of controlling the beam energy with the initial pressure ratio of the shock tube.

Molecular beam study of the scattering behavior of water molecules from a graphite surface.

Gas flow in nanospaces is greatly affected by the scattering behavior of gas molecules on solid surfaces, resulting in unique mass transport properties. In this paper, the molecular beam scattering experiment of water molecules on a graphite surface was conducted to understand their scattering dynamics in an incident energy range that corresponds to their thermal velocity distribution at room temperature (35-130 meV). Because of the large adsorption energy (∼100 meV), the scattering behavior is quite sensitive to the incident energy even within this narrow energy range. For relatively large incident energies, the direct-inelastic and trapping-desorption channels have comparable contributions to the scattering process on the surface at 300 K. In contrast, when the incident energy decreases well below the adsorption energy on the surface, the trapping-desorption channel becomes dominant, changing the scattering pattern from directional to diffusive scattering. As a result, the tangential momentum accommodation coefficient (TMAC), which significantly impacts the mass transport in nanospaces, largely depends on the incident energy. A decrease in the incident energy from 130 to 35 meV doubles the TMAC (0.42 to 0.86). In addition to the incident energy, the TMAC shows a strong dependence on the surface temperature. With increasing the surface temperature from 300 to 500 K, the scattering becomes more directional because of the increasing contribution of the direct-inelastic channel, which reduces the TMAC for the incident beam energy of 35 meV to 0.48.

Out‐of‐plane Scattering Distribution of Nitrogen Molecular Beam on Graphite (0001) Surface

The interaction of N2 molecules with a graphite (0001) surface was investigated employing the molecular beam experiment. The experimental setup involving a quadrupole mass spectrometer fixed on a rotary platform and a three‐axis substrate manipulator enabled us to measure the out‐of‐plane scattering as well as the in‐plane one. We examined a dependence of the scattering distribution on the translational energy of the incident beam. The obtained scattering distribution showed the lobular pattern with a small diffusive part due to the trapping‐desorption process. The distribution had a spread over the out‐of‐plane direction to the similar extent of a spread over the in‐plane direction. We compared the experimental results with the multistage model, which is based on the analysis of molecular dynamics simulations and suitable for the direct simulation Monte Carlo (DSMC) calculations of rarefied gas flows. The qualitative behavior of the model agreed very well with the experimental results.

Development of High Energy Molecular Beam Source Using Small Shock Tube

High energy molecular beam source for the molecular beam scattering experiment was developed using a diaphragmless shock tube, which works at a high repetition rate, and its performance was evaluated. Helium and nitrogen were used as the driver gas and the sample gas respectively. The diaphragmless small shock tube generated shock waves with Mach numbers of up to 2.8 and the temperature of the gas behind the reflected shock wave reached 1200 K, which was calculated from the shock wave velocity. The high temperature gas was produced at the repetition rate of 5 Hz.

Construction of non-Markovian coarse-grained models employing the Mori–Zwanzig formalism and iterative Boltzmann inversion

We propose a new coarse-grained (CG) molecular simulation technique based on the Mori-Zwanzig (MZ) formalism along with the iterative Boltzmann inversion (IBI). Non-Markovian dissipative particle dynamics (NMDPD) taking into account memory effects is derived in a pairwise interaction form from the MZ-guided generalized Langevin equation. It is based on the introduction of auxiliary variables that allow for the replacement of a non-Markovian equation with a Markovian one in a higher dimensional space. We demonstrate that the NMDPD model exploiting MZ-guided memory kernels can successfully reproduce the dynamic properties such as the mean square displacement and velocity autocorrelation function of a Lennard-Jones system, as long as the memory kernels are appropriately evaluated based on the Volterra integral equation using the force-velocity and velocity-velocity correlations. Furthermore, we find that the IBI correction of a pair CG potential significantly improves the representation of static properties characterized by a radial distribution function and pressure, while it has little influence on the dynamic processes. Our findings suggest that combining the advantages of both the MZ formalism and IBI leads to an accurate representation of both the static and dynamic properties of microscopic systems that exhibit non-Markovian behavior.

Incident energy dependence of scattering behavior of water molecules on Si (100) and graphite surfaces

The interaction between water molecules and solid surfaces has a great impact on water vapor flows in nanostructures. We conduct molecular beam scattering experiments covering the incident energy range corresponding to the thermal energy at room temperature to investigate the scattering behavior of water molecules on silicon and graphite surfaces. The incident energy dependence of the scattering distributions exhibits opposite trends on these surfaces. Molecular dynamics simulations reveal that the difference is caused by the inertia effect of the incident molecules and the surface corrugations.

A non-diaphragm type small shock tube for application to a molecular beam source.

A non-diaphragm type small shock tube was developed for application to a molecular beam source, which can generate beams in the energy range from 1 to several electron volts and beams containing dissociated species such as atomic oxygen. Since repetitive high-frequency operation is indispensable for rapid signal acquisition in beam scattering experiments, the dimensions of the shock tube were miniaturized to reduce the evacuation time between shots. The designed shock tube is 2-4 mm in diameter and can operate at 0.5 Hz. Moreover, a high shock Mach number at the tube end is required for high-energy molecular beam generation. To reduce the shock attenuation caused by the wall boundary layer, which becomes significant in small-diameter tubes, we developed a high-speed response valve employing the current-loop mechanism. The response time of this mechanism is about 100 μs, which is shorter than the rupture time of conventional diaphragms. We show that the current-loop valve generates shock waves with shorter formation distances (about 200-300 mm) than those of conventional shock tubes. In addition, the converging geometry efficiently accelerates shock wave in the small-diameter tubes. The optimal geometry of the shock tube yields shock Mach number around 7, which indicates that the translation energy of molecular beams can exceed 1 eV even in the presence of the real gas effect.

Scattering of Monatomic Gas Molecules on Vertically Aligned Single‐Walled Carbon Nanotubes

The scattering process of He molecules on vertically aligned single‐walled carbon nanotubes (VA‐SWNTs) was investigated using the molecular beam technique. The accommodation coefficients for VA‐SWNT films on quartz substrates at room temperature are remarkably high compared to those for bare surfaces, demonstrating the effectiveness of the surface modification technique with VA‐SWNT films for enhancing the energy transfer between gas molecules and surfaces. The enhanced energy transfer suggests that gas molecules can easily penetrate deep into the films because of their high porosity and suffer multiple collisions with bundles of SWNTs. The less effective energy accommodation at elevated temperatures, however, implies that the average number of collisions which gas molecules undergo before leaving the films is not large enough to maintain the perfect accommodation even at high temperature.

Mutual influence of molecular diffusion in gas and surface phases

We develop molecular transport simulation methods that simultaneously deal with gas- and surface-phase diffusions to determine the effect of surface diffusion on the overall diffusion coefficients. The phenomenon of surface diffusion is incorporated into the test particle method and the mean square displacement method, which are typically employed only for gas-phase transport. It is found that for a simple cylindrical pore, the diffusion coefficients in the presence of surface diffusion calculated by these two methods show good agreement. We also confirm that both methods reproduce the analytical solution. Then, the diffusion coefficients for ink-bottle-shaped pores are calculated using the developed method. Our results show that surface diffusion assists molecular transport in the gas phase. Moreover, the surface tortuosity factor, which is known to be uniquely determined by physical structure, is influenced by the presence of gas-phase diffusion. This mutual influence of gas-phase diffusion and surface diffusion indicates that their simultaneous calculation is necessary for an accurate evaluation of the diffusion coefficients.