Nanoparticle-based superchain networks formed by the side-by-side self-assembly of soft magnetite nanorods

Abstract

Mesophases are structures of higher order formed by anisotropic building blocks, the most prominent case being thermotropic molecular liquid crystals. The self-assembly of anisotropic inorganic nanoparticles is attracting significant interest to design novel and complex functional materials, obtaining superstructures resembling these mesophases. A key question is whether, and how, magnetic interactions between magnetic nanorods affect the self-assembly process compared to their non-magnetic counterparts. Existing studies often rely on polycrystalline magnetic nanorods or silica shell coatings, thus reducing the magnetic interactions and shape anisotropy and introducing additional electrostatic repulsion. Here, we present the synthesis of single-crystalline magnetite nanorods stabilized only by organic ligands, allowing for unhindered magnetic interactions, showing that the aspect ratio can be precisely adjusted by the ligand choice. Assembling these nanorods revealed the formation of unusual superchain structures in ferrite nanorods, distinct from the classical nematic or cholesteric phases. In the assembled samples, the superchain formation results in an antiparallel dipole–dipole alignment of neighboring nanorods attaching side-by-side to reduce the magnetostatic energy through flux closure. At a sufficient length of the superchains, percolation takes place, forming a network enclosing cavities containing nanorods in a disordered state. Longer nanorods exhibit a higher tendency for superchain formation, leading to a denser network with a higher fraction of nanorods included into the superchains. The resulting assembly preserves the ferrimagnetic-superparamagnetic behaviour of the building blocks while exhibiting direction-dependent magnetic properties. Without the need of a silica shell to obtain this alignment, the close interparticle contact offers exciting perspectives for magnetoelectrical applications.