Unique Reflection Patterns of Spherical Colloidal Clusters with Decahedral Type

Spherical colloidal clusters exhibit structural colors, owing to the interference of light between colloidal particles. They can be easily prepared using water-in-oil-type emulsions; small water droplets disperse colloidal particles, and evaporation of water into the oil phase results in the aggregation of colloidal particles. Some of the prepared clusters exhibit shell-like stacks of hexagonally arranged particle layers beneath the spherical surface. These layers correspond to the (111) planes of the face centered cubic (fcc) lattice, and this type of spherical cluster is called an onion-like structure because the stacked layers are similar to onion peels^[1]. Recent studies have shown that spherical colloidal clusters have different particle arrangements, such as fcc and icosahedral-type structures, and the structural differences can be attributed to the experimental conditions applied during the preparation processes.When a water droplet rapidly shrinks upon heating, clusters with onion-like structures are formed. However, at a low shrinking speed, which can be controlled via presaturation of water-in-oil systems, highly ordered structures, such as single-crystal spherical fcc structures, are formed^[2,3,4,5]. Thus, elucidating the exact particle arrangements and optical properties of each structural type is crucial for the application of spherical colloidal clusters in optical materials and for understanding the formation process.<br/><br/>A decahedral structure is one of the structural type with a 5-fold axis and is commonly found in clusters of nanometer-sized colloidal particles. However, research on decahedral-type spherical clusters is limited. For example, Mbah et al. used confined self-assembly to investigate the interplay of thermodynamics and reaction kinetics in the crystallization pathways of finite clusters in icosahedral and decahedral structures^[6]. Molecular dynamics simulations performed by using hard spheres show that thermodynamics alone is not sufficient to explain the experimental results; instead, the suggested kinetic factors play a crucial role. The optical properties of the decahedral type appear unique because interesting reflection patterns with teardrop and half-moon shapes have been observed using optical microscopy^[7]. However, detailed investigations have not yet been conducted.<br/><br/>In this study, the structural and optical properties of decahedral spherical clusters were investigated. In addition to surface observations via scanning electron microscopy (SEM), the internal particle arrangement was investigated by using sequential cross-sectioning and transmission electron microscopy (TEM). Based on these results, we propose a detailed structural model for decahedral-type spherical colloidal clusters. The model comprises five tetrahedral units that differ slightly from the Mackay structure, which is known as the unit to form the icosahedral structure. Next, the optical properties of the decahedral clusters were carefully examined by using an optical microscope equipped with a microspectrophotometer. We observed various reflection patterns depending on the orientation of the cluster, including previously reported patterns, namely, half-moon and teardrop patterns, and a newly discovered edge-ring pattern. In addition, line-like reflections of different colors were observed. We show that these patterns originate from various combinations of two reflection mechanisms with reflectance peaks at different wavelengths^[8].<br/> <br/>References<br/>[1] N. Vogel <i>et al.</i>, <i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>112</b>, 10845, (2015).<br/>[2] C. Kim <i>et al.</i>, <i>Chem. Mater.</i>, <b>32</b>, 9704, (2020).<br/>[3] R. Ohnuki <i>et al.</i>, <i>ACS Appl. Nano Mater.</i>, <b>6</b>, 13137, (2023).<br/>[4] R. Ohnuki <i>et al.</i>, <i>Part. Part. Syst. Charact.</i>, <b>39</b>, 2100257, (2022).<br/>[5] R. Ohnuki <i>et al.</i>, <i>Engineering Chemistry</i>, <b>1</b>, 39, (2023).<br/>[6] C. F. Mbah <i>et al.</i>, <i>Nat. Commun.</i>, <b>14</b>, 5299, (2023).<br/>[7] J. Wang <i>et al.</i>, <i>Adv. Funct. Mater.</i>, <b>30</b>, 1907730, (2020).<br/>[8] R. Ohnuki <i>et al.</i>, <i>Chem. Mater.</i>, <b>36</b>, 2953, (2024).