We report on a hierarchical mechanical behavior of 500 nm-thick Co nanocrystal 3D superlattices (supracrystals) induced by the crystalline structure (nanocrystallinity) or by the length of the coating agent of Co nanocrystals. Increasing the nanocrystal shape anisotropy of Co nanocrystals through the control of their nanocrystallinity 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 superlattice within supracrystal, is correlated with marked changes in the Young’s modulus of supracrystals: from 0.7 ± 0.4 GPa, to 1.7 ± 0.5 GPa and to 6.6 ± 1.5 GPa as the crystalline structure of Co nanocrystals changes from amorphous-Co, to -Co and to hcp-Co, respectively. Moreover, for supracrystals of 7 nm amorphous Co nanocrystals, the Young’s modulus decreases by one order of magnitude from 0.7 ± 0.4 GPa to 0.08 ± 0.03 GPa, by reducing the alkyl chain length of the ligands coated on Co nanocrystals from C18 (oleic acid) to C12 (lauric acid). The hierarchical mechanical behavior is rationalized using a dimensional model of stress-strain relationship in supracrystals.
Source : Hierarchical mechanical behavior of cobalt supracrystals related to nanocrystallinity.
M.Gauvin, N. Yang, Z. Yang, I.Arfaoui and M.P.Pileni Nanoresearch, 2015, 8, 3480-3487.