.

Colloidal photonic crystals, which are the ordered assemblies of monodisperse particles, can reflect specific light wavelengths according to Bragg’s law. Three-dimensionally ordered colloidal crystals can be fabricated conveniently via bottom-up methods such as self-assembly. However, in general, dynamically changing the arrangement of the composed particles in self-assembled colloidal crystals is barely possible because they form a close-packed structure. Owing to that, the scattered wavelengths and intensities cannot be tuned easily once the particles are assembled. Colloidal crystals of which composed particles can be reversibly rearranged are expected to have tunable scattering properties applicable to functional optical devices such as displays or e-ink.

In this study, yolk-shell particles (or rattle-type particles) were employed to realize the tunable coherent scattering of colloidal crystals. Yolk-shell particles consist of a hollow particle (shell) enclosing a mobile core particle. By applying an external AC electric field, the core motion changes from Brownian motion to a specific motion with reference to the field direction [1, 2]. Therefore, the core particles are expected to possess tunable mobility even if their shells form close-packed structures like often is the case when colloidal crystals are dried. We synthesized submicron-sized yolk-shell particles of the which cores are composed of silica and titanium oxide, and the shells are composed of silica. Colloidal crystals of yolk-shell particles exhibited switchable scattering intensities (i.e., Bragg reflection intensities) by applying an external AC electric field. This was due to the change in the core arrangement. Via confocal microscopy, it was observed that the core motion changed from Brownian motion to coherently moving parallel to the electric field within the shells, leading to a reduction of the displacement disorder of the cores. The coherence in the Bragg reflection could be further controlled by adjusting the electrolyte concentration in the surrounding medium [3] and/or core-to-shell size ratio, demonstrating the wide tunability in scattering intensities of yolk-shell colloidal crystals.

References:

[1] K. Watanabe et al., Langmuir, 33, 296–302 (2017).

[2] T. A. J. Welling et al., J. Colloid Interface Sci., 627, 761–773 (2022).

[3] T. A. J. Welling et al., ACS Nano, 15, 11137–11149 (2021).