Size dependent elastic moduli of CdSe nanocrystal superlattices predicted from atomistic and coarse grained models

Nanocrystal superlattices are materials formed by assembly of monodisperse nanocrystal building blocks that are tunable in composition, size, shape, and surface functionalization. Such materials offer the potential to realize unprecedented combinations of physical properties, but theoretical prediction of such properties, particularly elastic properties, remains a challenge. Here we report the Young's moduli, bulk moduli, and Poisson's ratios of CdSe nanocrystal superlattices computed from fully atomistic molecular dynamics simulations, coarse grained models, and effective medium theory. The atomistic simulations yield Young's moduli in the 4-5 GPa range, in agreement with previously reported results for similar nanocrystal superlattice systems. A clear increase of Young's modulus and bulk modulus with increasing nanocrystal core size is observed, while Poisson's ratio decreases slightly with core size. Effective medium theory overpredicts the moduli, and it is surmised that this arises from its neglect of the atomic-level details of the of the core-ligand interface. The coarse grained calculations, using existing nanocrystal interaction models from the literature, also show similar increases with core size but predict moduli that are two orders of magnitude lower than the present atomistic results and previous literature. It is concluded that coarse grained models, in their current form, are not appropriate for calculating elastic properties of nanocrystal superlattices and that fully atomistic models are better suited for this purpose.