Large-Area Ordered Superlattices from Magnetic Wüstite/Cobalt Ferrite Core/Shell Nanocrystals by Doctor Blade Casting

Although a large diversity of single-component and binary superlattices from colloidal nanocrystals have been demonstrated, applications of such ordered nanocrystal assemblies are still hampered due to a lack of control over the self-assembly processes over large areas. A reel-to-reel compatible large-area coating technique for solutions is given by doctor blade casting, which is applied here to deposit colloidal nanocrystals onto various substrates. The self-assembly process is demonstrated for magnetic nanocrystals, having a high potential for applications in magnetic memory devices. Shape-controlled (spherical and cubic) and monodisperse nanocrystals with a Wustite core and a cobalt ferrite shell are used in particular. Doctor blade casting of these colloidal nanocrystals results in films exhibiting hexagonally closely packed arrangements, which are formed by a top-down growth, as is evidenced by cross sectional transmission electron microscopy. The ordering in the topmost layer extends over large areas, although some defects and irregularities are found. The degree and quality of self-assembly is quantified by analyzing plan view images of the assemblies by means of the decay of their autocorrelation function. This analysis reveals that the degree of ordering obtained by doctor blade casting outperforms those provided by alternative deposition techniques such as inkjet printing or drop casting. The results for the coherent lengths deduced from the autocorrelation analysis are shown to be consistent with those from grazing-incidence small-angle X-ray scattering, giving coherence length on the order of 1000 nm.