Morphology, Nanocrystallinity, and Elastic Properties of Single Domain ε‑Co Supracrystals

We report on the elastic modulus of 3D superlattices of single domain ε-Co nanocrystals self-assembled in face-centered cubic structures, so-called supracrystals. The effects of nanocrystal size, probe size, and supracrystal morphology on the elastic properties of supracrystals are investigated by nanoindentation measurements using an atomic force microscope. We show that elastic moduli of supracrystals of single domain ε-Co nanocrystals are invariant on varying the nanocrystal size and supracrystal morphology (films or faceted shape) produced through heterogeneous (solvent evaporation) growth process. A mechanical model is proposed to analyze elastic properties of supracrystals that differ from classical composite materials. These results are explained in terms of nanocrystallinity, that is, the crystalline structure of nanocrystals, which is a key parameter in achieving supracrystals of different nanocrystal sizes with robust elastic properties.