Self-assembly of segmented anisotropic particles: Tuning compositional anisotropy to form vertical or horizontal arrays

Columnar arrays of anisotropic nano- and microparticles, in which the long axes of the particles are oriented perpendicular to the substrate, are of interest for photovoltaics and other applications. Array assembly typically requires applied electric or magnetic fields and/or controlled drying, which are challenging over large areas. Here, we describe a scalable approach to self-assemble multicomponent nanowires into columnar arrays. Self-assembly of partially etched nanowires (PENs) occurred spontaneously during sedimentation from suspension, without drying or applied fields. PENs, which have segments that are either gold or "empty" (solvent-filled) surrounded by a silica shell, were produced from striped metal nanowires by first coating with silica and then removing sacrificial segments by acid etching. Electrostatic repulsion between the particles was necessary for array assembly; however, details of PEN surface chemistry were relatively unimportant. The aspect ratio and relative center of mass (COM) of the PENs were important for determining whether the PEN long axes were vertically or horizontally aligned with respect to the underlying substrate. Arrays with predominantly vertically aligned particles were achieved for PENs with a large offset in COM relative to the geometric center, while other types of PENs formed horizontal arrays. Assemblies were formed over >10 cm² areas, with over 60% of particles standing. We assessed array uniformity and reproducibility by imaging many positions within each sample and performing multiple assemblies of differently segmented PENs. This work demonstrates the versatility of gravity-driven PEN array assembly and provides a framework for designing other anisotropic particle systems that self-assemble into columnar arrays.