  Deciphering the three-dimensional atomic structure of solid-solid interfaces
in core-shell nanomaterials is the key to understand their remarkable
catalytical, optical and electronic properties. Here, we probe the
three-dimensional atomic structures of palladium-platinum core-shell
nanoparticles at the single-atom level using atomic resolution electron
tomography. We successfully quantify the rich structural variety of core-shell
nanoparticles including bond length, coordination number, local bond
orientation order, grain boundary, and five-fold symmetry, all in 3D at atomic
resolution. Instead of forming an atomically-sharp boundary, the core-shell
interface is atomically diffuse with an average thickness of 4.2 A,
irrespective of the particle's size, morphology, or crystallographic texture.
The high concentration of Pd in the interface is highly related to the electric
double layers of the Pd seeds. These results advance our understanding of
core-shell structures at the fundamental level, providing potential strategies
into nanomaterial manipulation and chemical property regulation.
