Ordered silicon nanowire bundle networks from aqueous dispersions

Many applications of nanowires require their processing in large volumes and assembly as ordered arrays with controlled density. Starting from aqueous dispersions of silicon nanowires stabilized through a cationic surfactant, we carry out vacuum filtration to form macroscopic paper-like networks. We controllably induce the assembly of SiNWs as ordered domains within the network, arising from a 2D nematic phase formed due to the relatively high SiNW concentration and long filtration time comparable to their rotational diffusion. The domains consist of bundles of around 15 nanowires, highly aligned parallel to each other (nematic order parameterS2D=0.71) and extending over several micrometers as micro-fibrils through nanowires linking adjacent domains. In the bundles, van der Waals forces hold the SiNWs together within less than 0.4 nm, which translates into a significant cohesive energy density above 2 J g^-1. Large-scale formation of bundles is visually evident through electron microscopy and detected by x-ray scattering. We measure changes in the structure factor of the SiNW network upon bundling, and find a new scattering peak (q= 16 nm^-1) arising from the inter-wire separation. By packing nanowires in domains with inter-wire separation at van der Waals interaction distance, while extending across lengths well beyond an individual nanowire, we provide a method to bridge scales and build macroscopic networks of ordered nanowires of different chemistries.