Influence of Geometries on the Assembly of Snowman-Shaped Janus Nanoparticles

The self-assembly of micro/nanoparticles into suprastructures is a promising way to develop reconfigurable materials and to gain insights into the fundamental question of how matter organizes itself. The geometry of particles, especially those deviating from perfectly spherical shapes, is of significant importance in colloidal assembly because it influences the particle "recognition", determines the particle packing, and ultimately dictates the formation of assembled suprastructures. In order to organize particles into desired structures, it is of vital importance to understand the relationship between the shape of the colloidal building blocks and the assembled suprastructures. This fundamental issue is an enduring topic in the assembly of molecular surfactants, but it remained elusive in colloidal assembly. To address this issue, we use snowman-shaped Janus nanoparticles (JNPs) as a model to systematically study the effect of colloidal geometries on their assembled suprastructures. Ten types of JNPs with identical chemical compositions but with different geometries were synthesized. Specifically, the synthesized JNPs differ in their lobe size ratios, phase separation degrees, and overall sizes. We show that by altering these parameters, both finite suprastructures, such as capsules with different curvatures, and nonfinite suprastructures, including free-standing single-layered or double-layered JNPs sheets, can be obtained via self-assembly. All these different types of suprastructures are constituted by highly oriented and hexagonally packed JNPs. These findings demonstrate the significance of geometries in colloidal assembly, such that slightly changing the building block geometries could result in a large variety of very different assembled structures, without altering the chemistry of the particles.