Self-Assembled Magnetite Mesocrystalline Films: Toward Structural Evolution from 2D to 3D Superlattices

This study describes synthesis and detailed characterization of 2D and 3D mesocrystalline films produced by self-assembly of iron oxide (magnetite) truncated nanocubes. The orientational relations between nanocrystals within the superlattice are examined and atomistic models are introduced. In the 2D case, two distinct superstructures (i.e., translational order) of magnetite nanocubes can be observed with p4mm and c2mm layer symmetries while maintaining the same orientational order (with [100](magnetite) perpendicular to the substrate). The 3D structure can be approximated by a slightly distorted face-centered cubic (fcc) superlattice. The most efficient space filling within the 3D superstructure is achieved by changing the orientational order of the nanoparticles and following the bump-to-hollow packing principle. Namely orientational order is determined by the shape of the nanoparticles with the following orientational relations: [001](SL)||[310](magnetite), [001](SL)||[301](magnetite), [001](SL)||[100](magnetite). Overall the presented data provide a fundamental understanding of a mesocrystal formation mechanism and their structural evolution. Structure, composition, and magnetic properties of the synthesised nanoparticles are also characterized.