Hierarchical mechanical behavior of cobalt supracrystals related to nanocrystallinity

Here, we report on hierarchical mechanical behavior of 500-nm-thick Co-nanocrystal 3D superlattices (supracrystals) induced by either the crystalline structure (nanocrystallinity) or the length of the coating agent of Co nanocrystals. Increasing the nanocrystal shape anisotropy of Co nanocrystals through the control of their nanocrystallinities induces a higher level of ordering with both translational and orientational alignment of nanocrystals within the supracrystals. The hierarchy in ordering at various scales, i.e., from the atomic lattice within the nanocrystals to the nanocrystal superlattices within supracrystals, is correlated with marked changes in the Young’s modulus of supracrystals: From 0.7 ± 0.4 to 1.7 ± 0.5 and to 6.6 ± 1.5 GPa as the crystalline structure of Co nanoparticles changes from amorphous-Co to e-Co and to hexagonal compact packing (hcp)-Co, respectively. Moreover, for supracrystals of 7 nm amorphous Co nanoparticles, the Young’s modulus decreases by one order of magnitude from 0.7 ± 0.4 to 0.08 ± 0.03 GPa upon reducing the alkyl chain length of the ligands coating the Co nanoparticles from C18 (oleic acid) to C12 (lauric acid). The hierarchical mechanical behavior is rationalized using a dimensional model of the stress-strain relationship in supracrystals.