Magnetic Assembly of Nonuniform Nanocrystal Clusters into Responsive Photonic Crystals

Colloidal assembly of photonic crystals typically relies on uniform building blocks to achieve the required periodicity. However, the difficulties in large-scale production of uniform particles limit the practical use of colloidal photonic crystals in many exciting applications. Here, we show that this uniformity requirement can be relaxed by increasing the interparticle spacing during colloidal magnetic assembly, enabling the formation of responsive photonic crystals from particles with a broader size range. Specifically, we demonstrate that magnetite nanocrystal clusters with a wide size range of 40–60 nm, formed by breaking their large aggregates via ultrasonication in the presence of polymeric ligands, can be magnetically assembled in solution into one-dimensional photonic crystals with tunable optical diffraction across the entire visible spectrum. The strong electrostatic repulsion imparted by the polymeric ligands results in large interparticle separation, reducing the negative impact of low particle uniformity and enabling the required periodicity for notable diffraction and broad spectral tunability.