Hiroyuki Nishikawa

Self-Assembling Process of Colloidal Particles into Two-Dimensional Arrays Induced by Capillary Immersion Force : A Simulation Study With Discrete Element Method

This paper presents a simulation study for self-assembling process of colloidal particles into two-dimensional arrays due to capillary immersion force. Discrete element method is used to simulate the dynamics of colloidal particles trapped in a thin liquid film. The previous model is improved in the following two points: a modification of the screening effect of capillary immersion force and introduction of periodic boundary condition. Snapshots provided by the simulations agree well with experimental images taken by atomic force microscopy. The self-assembling process is quantified with pair correlation function and coordination number. At lower coverage, colloidal particles rapidly form small clusters that consist of several particles in the early stage. Subsequently, chain-like structures with some branches are mainly generated. On the other hand, at higher coverage, large domains of hexagonal close-packed (HCP) structures are gradually generated. The rate of the growth of HCP domains is much slower than that of the generation of the small clusters and the chain-like structures.

Multiscale Simulation of Two-Dimensional Self-Organization of Nanoparticles in Liquid Film

A set of numerical models for two-dimensional self-organization of nanoparticles in a liquid film is developed and numerical simulations are carried out to investigate the relationship between process conditions and structures of self-organized nanoparticles. The two-dimensional Langevin equation is employed to track nanoparticles on a substrate over time. Each nanoparticle is subject to multiscale surface forces such as capillary force, contact force, electrostatic force, van der Waals force and friction force, as well as Brownian force and fluid drag force. The modeling shows that no surface force can be neglected in the self-organization process because the magnitude of the surface forces strongly depends on the interparticle distances and the thickness of the liquid film. Three principles of two-dimensional self-organization are proposed on the basis of the unsteady behavior of the nanoparticles. The isotropic ordering factor and the non-dimensional boundary length are introduced to quantify the structures of self-organized nanoparticles. Every structure of the nanoparticles obtained here can be classified according to sets of these criteria.

Polytypes and crystallinity of ultrathin epitaxial films of layered materials studied with grazing incidence X-ray diffraction

Abstract The structure of ultrathin epitaxial films of layered NbSe 2 and TaSe 2 grown on Se-terminated GaAs(111) substrates was determined by grazing incidence X-ray diffraction. It was found that the crystallographic polytypes of the films were dependent on the growth temperatures. The temperature range for the growth of octahedrally coordinated TaSe 2 was different from that of the bulk. Disappearance of 3R-type portion in NbSe 2 at a high growth temperature will be favorable to fabricate superconducting ultrathin epitaxial films.

Epitaxial growth of TiSe2 thin films on Se‐terminated GaAs(111)B

Epitaxial growth of TiSe2 films on Se‐terminated GaAs(111)B substrates were carried out at different growth temperature by molecular beam epitaxy. Grown films have been investigated by reflection high energy electron diffraction, Auger electron spectroscopy, and measurements of resistivity parallel to the surface. TiSe2 epitaxial films have been obtained at various temperatures, but the optimum growth temperature was determined to be 400 °C. A charge density wave transition has been observed in the resistivity of the film grown at 400 °C, indicating that it has a good quality comparable to a bulk single crystal.

Surface morphology of S or Se terminated GaAs(111)B

The surface roughness and stability of S or Se-terminated GaAs(111)B has been investigated by reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES) and atomic force microscopy (AFM). RHEED streak patterns of S-terminated GaAs(111)B surfaces turned into spot patterns with increasing heating temperature. Se-terminated surfaces, on the other hand, kept the streak pattern until 550°C heating. AES measurements showed that the roughness of the S-terminated surface increased by 400°C heating, while the Se-terminated surface was stable up to 500°C heating. AFM images of both surfaces showed that the roughness of the S-terminated surface was larger than that of the Se-terminated surface. These measurements have revealed the difference in thermal stability between the S- and Se-terminated surfaces.