  Nanomaterials with core-shell architectures are prominent examples of
strain-engineered materials, where material properties can be designed by
fine-tuning the misfit strain at the interface. Here, we elucidate the full 3D
atomic structure of Pd@Pt core-shell nanoparticles at the single-atom level via
atomic electron tomography. Full 3D displacement fields and strain profiles of
core-shell nanoparticles were obtained, which revealed a direct correlation
between the surface and interface strain. It also showed clear Poisson effects
at the scale of the full nanoparticle as well as the local atomic bonds. The
strain distributions show a strong shape-dependent anisotropy, whose nature was
further corroborated by molecular statics simulations. From the observed
surface strains, the surface oxygen reduction reaction activities were
predicted. These findings give a deep understanding of structure-property
relationships in strain-engineerable core-shell systems, which could pave a new
way toward direct control over the resulting catalytic properties.
