Control over structures and optical properties of colloidal clusters created from emulsion templates

Abstract

Colloidal assembly within emulsion droplets presents an instrumental platform for investigating the optimal organization of particles under spherical constraints. Furthermore, it offers a practical methodology for fabricating photonic micro-particles. Employing these emulsion droplets as a microenvironment, we can manipulate and study the particle dynamics, spatial configurations, and potential photonic applications, advancing both fundamental research and applied sciences in the sphere of colloidal assembly. We develop into the production and optical characterization of supraballs, polycrystals, single crystals, and icosahedral structures. This paper presents a thorough exploration of controlling the structures and optical properties of spherical colloidal clusters. The exploration begins with the novel use of eumelanin nanoparticles as a color saturation enhancer in colloidal crystals. Produced through the oxidative polymerization of precursors in basic solutions, these eumelanin nanoparticles significantly suppress incoherent scattering, thereby enhancing color chroma. By incorporating the nanoparticles into aqueous suspensions with polystyrene particles, we maintain high crystallinity in the resulting colloidal crystals. This presents eumelanin as a valuable tool for color saturation enhancement, expanding the possibilities of vibrant and bright structural colors. Remarkably, the study evolves further as we introduce a droplet-to-droplet osmotic extraction technique to regulate crystalline structures within the colloidal clusters. This technique, which involves the mixture of particle-laden and salt-dissolved droplets, enables us to control the enrichment rate, subsequently influencing the formation of crystalline structures. Our findings demonstrate that varying salt concentrations can yield either single-crystalline or polycrystalline structures within a concise consolidation period. This unprecedented control over the crystalline structures in colloidal clusters marks a significant milestone in the field. By employing the extraction technique, we develop deeper into the self-organization of colloids into icosahedral clusters, an energy-minimizing approach under spherical confinement. We reveal that the sphericity of the outermost layer, created through late-stage particle rearrangement, plays a critical role in determining surface morphology. Our findings also highlight the fact that this layer greatly influences the surface arrangements in icosahedral clusters, irrespective of the particle numbers. This discovery offers novel insights into the formation and structural diversity of colloidal clusters. Taken together, our work represents a major advance in the understanding and control of the structural and optical properties of spherical colloidal clusters, contributing to the broader fields of colloidal and photonic sciences. We have not only pioneered the production of colloidal clusters with enhanced color saturation but also facilitated the exclusive creation of single-crystalline, polycrystalline, and icosahedral structures. The findings presented in this study not only expand our knowledge of colloidal systems but also pave the way for extensive research and practical utilization, deepening our comprehension of their captivating optical characteristics.