Colloidal particles form a face‐centered‐cubic (FCC) lattice on crystallization; however, recent studies have indicated that the cluster with the icosahedral symmetry can be formed under spherical confinement. The icosahedral cluster appears only when the number of constituent particles is small because the FCC lattice is favored thermodynamically for a bulk colloidal crystal. This paper reports the observation of a large icosahedral colloidal cluster comprising ≈4.9 × 105 colloidal particles, which is considerably larger than the previously reported number. This large icosahedral cluster clearly exhibits the triangular reflection pattern that corresponds to one face of the icosahedron. Further, an oblique illumination with sample rotation is found to be an effective method to characterize the icosahedral cluster optically. In this study, larger colloidal particles are used than those in previous studies. It is hypothesized that slow reconfiguration during the evaporation process can result in the formation of a large icosahedral cluster due to the large particle size.

In article number 2100257, Ohnuki and co‐workers investigate the optical properties of massive spherical colloidal clusters with icosahedral structures. The spherical colloidal clusters with the icosahedral structure were discovered several years ago and show a unique triangular reflection pattern, which is expected to be applied to new optical materials such as pigments/dyes and colorimetric sensors.