Magnetic Assembly of Nanocubes for Orientation-Dependent Photonic Responses

Close packing is the most favorable manner in colloidal self-assembly, producing superstructures with a limited variety of spatial configurations. This challenge can be overcome by incorporating anisotropic interactions into the assembly process. Using magnetite nanocubes as the building blocks, we show that they can be magnetically assembled into one-dimensional nanochains in an edge-to-edge rather than close-packed face-to-face manner. The cubic shape of the building blocks plays a key role: it decouples the easy magnetization from any of the three geometric axes favoring close packing. Therefore, under magnetic fields, the induced competition between the long-range Zeeman coupling and the short-range dipole-dipole coupling determines the assembly of nanocubes along the [110] directions. The photonic properties of the edge-to-edge configuration are dependent on both chain orientation and viewing angle. Unlike nanosphere assemblies where the strongest diffraction occurs parallel to the chain (or field) direction, the nanocubes allow one to define their long-range periodicity in the plane of a thin film while diffracting light out of the plane, making it particularly useful for applications that desire the achievement of structural colors of sufficient intensity in a film with a minimum thickness. For example, their unique photonic property can be taken advantage of to design "magic" patterns whose rotation is perceived to be opposite to the actual rotational direction of the film. It is difficult to reproduce such unusual optical effects by other means; thereby, many new ways for designing novel security devices are provided. This work reveals the enormous potential of the magnetic assembly strategy, when combined with the use of well-defined nonspherical building blocks, for controlling the spatial configurations of colloidal assemblies and exploiting their novel physical properties for intriguing applications.