Jun Nozawa

In an experiment, two types of hexagonal structures, the primitive translation vector of which was parallel to that of substrate or rotated by 30°, were generated during colloidal heteroepitaxy. When colloidal particles smaller than the epitaxial particles were included as additives, the formation ratio of the two hexagonal structures changed depending on the size of the additive particles. To examine the dependence of the formation ratio of the two hexagonal structures on the size of the small colloidal particles, Brownian dynamics simulations were performed. Under a high particle density and strong interaction conditions, the hexagonal structure with a lower formation ratio without additives became the major product when additives much smaller than the epitaxial particles were included. Conversely, the hexagonal structure with a higher formation ratio without additives became difficult to form in the presence of additive particles. The effect of additive particles on the formation rates of hexagonal structures depended on whether the additive particles could intrude into the space between the substrate and epitaxial layer. Both hexagonal structures became more difficult to form as the additive particle size increased. At a specific ratio of the additive particle size to the substrate particle size, the formation ratio of the structure present in a lower content without additive increased because of the generation of a precursor structure.

Brownian dynamics simulations were conducted to investigate the formation of a Kagome lattice in the first epitaxial layer during colloidal heteroepitaxy. When the epitaxial particles were smaller than the substrate particles and the interactions were predominantly governed by the depletion force, a kagome lattice structure was successfully formed in the first epitaxial layer when the epitaxial particles were a specific size. The formation process of the Kagome lattice in the first layer was analyzed in detail. Initially, the epitaxial particles adhered to positions where they interacted with three substrate particles. When the density of epitaxial particles on the substrate increased by sedimentation, the epitaxial particles changed locations to attach to the sites where they interacted with one or two substrate particles, and a hexagonal structure formed. Subsequently, particles interacting with only one substrate particle detached from the substrate, resulting in the formation of a kagome lattice.

Epitaxial growth is one of the most important techniques for the control of crystal growth, especially for growing thin-film semiconductor crystals. Similarly, colloidal epitaxy, a template-assisted self-assembly method, is a powerful technique for controlling the structure of colloidal crystals. In this study, heteroepitaxial growth, which differs from homoepitaxial growth of conventional colloidal epitaxy, using foreign colloidal crystals as a substrate, was used to grow single-component colloidal crystal films. The Frank-van der Merwe (FM), Stranski-Krastanov (SK), and Volmer-Weber (VW) modes were observed, and the mode varied with the lattice-misfit ratio and interparticle interactions between the substrate and epitaxial phase. The transition of the growth mode (from SK to VW) and the coexistence of different growth modes (FM and VW) were observed, and their processes were revealed by in situ observation. Colloidal heteroepitaxy was confirmed to be useful for controlling structure, which will enable exploration of novel colloidal self-assembly structures.

Because ice surfaces catalyse various key chemical reactions impacting nature and human life, the structure and dynamics of interfacial layers between water vapor and ice have been extensively debated with attention to the quasi-liquid layer. Other interfaces between liquid water and ice remain relatively underexplored, despite their importance and abundance on the Earth and icy extraterrestrial bodies. By in situ optical microscopy, we found that a high-density liquid layer, distinguishable from bulk water, formed at the interface between water and high-pressure ice III or VI when they were grown or melted in a sapphire anvil cell. The liquid layer showed a bicontinuous pattern, indicating that immiscible waters with distinct structures were separated on the interfaces in a similar manner to liquid–liquid phase separation through spinodal decomposition. Our observations not only provide a novel opportunity to explore ice surfaces, but also give insight into the two kinds of structured water.