Surface composition and ordering of binary nanoparticle mixtures in spherical confinement

Abstract

We use coarse-grained Langevin dynamics simulations in shrinking spherical confinement to probe the fabrication of spherical “supraballs” via the emulsion assembly of binary nanoparticle mixtures. We examine a binary mixture of silica and synthetic melanin nanoparticles and discuss the structure and composition of the resulting supraballs particularly in the context of optical nanomaterials applications. Our results demonstrate how particle chemistry, particle size, particle mixture composition, assembly timescale, and supraball size cooperate/compete to control the spatial distribution of particles on the surface and within the supraball. We find strong enrichment of melanin particles at the supraball surface, with the degree of enrichment decreasing with increasing melanin/silica size ratio. We observe appreciable crystalline ordering only in systems where the particles are of similar size, and we note that particle size dispersity, finite assembly timescale, and curvature of the supraball surface all serve to suppress particle ordering. We also report routes toward some interesting hierarchical structures such as core–shell supraballs. These findings provide design rules for the development of optical materials for structural color applications, and they also increase our fundamental understanding of nanoparticle organization near curved surfaces and may find relevance for processes such as spray drying and porous materials fabrication.