Chemically synthesized PbS, CdSe, and CoPt(3) nanocrystals (NCs) were self-assembled into highly periodic supercrystals. Using the combination of small-angle X-ray scattering, X-ray photoelectron spectroscopy, infrared spectroscopy, thermogravimetric analysis, and nanoindentation, we correlated the mechanical properties of the supercrystals with the NC size, capping ligands, and degree of ordering. We found that such structures have elastic moduli and hardnesses in the range of approximately 0.2-6 GPa and 10-450 MPa, respectively, which are analogous to strong polymers. The high degree of ordering characteristic to supercrystals was found to lead to more than 2-fold increase in hardnesses and elastic moduli due to tighter packing of the NCs, and smaller interparticle distance. The nature of surface ligands also significantly affects the mechanical properties of NCs solids. The experiments with series of 4.7, 7.1, and 13 nm PbS NCs revealed a direct relationship between the core size and hardness/modulus, analogous to the nanoparticle-filled polymer composites. This observation suggests that the matrices of organic ligands have properties similar to polymers. The effective moduli of the ligand matrices were calculated to be in the range of approximately 0.1-0.7 GPa.

To be able to control the functions of engineered multicomponent nanomaterials, a detailed understanding of heterogeneous nucleation at the nanoscale is essential. Here, by using in situ synchrotron X-ray scattering, we show that in the heterogeneous nucleation and growth of Au on Pt or Pt-alloy seeds the heteroepitaxial growth of the Au shell exerts high stress (∼2 GPa) on the seed by forming a core/shell structure in the early stage of the reaction. The development of lattice strain and subsequent strain relaxation, which we show using atomic-resolution transmission electron microscopy to occur through the slip of {111} layers, induces morphological changes from a core/shell to a dumbbell structure, and governs the nucleation and growth kinetics. We also propose a thermodynamic model for the nucleation and growth of dumbbell metallic heteronanostructures.