Atomic-scale 3D insights into heteroepitaxial mechanisms in hcp/fcc, bcc/fcc, and fcc/fcc core-shell nanoparticles.

Heteroepitaxy on nanocrystals of diverse crystal structures-hexagonal close-packed (hcp), body-centered cubic (bcc), and face-centered cubic (fcc)-is a powerful approach for engineering atomic arrangements. However, the mechanisms by which interfacial defects and strain relaxation accommodate structural mismatch remain poorly understood at the three-dimensional (3D) atomic scale. Here, we use atomic-resolution electron tomography to determine the 3D atomic structures and reveal heteroepitaxial mechanisms in Pt-based core-shell nanoparticles with diverse crystal structures including hcp/fcc Ru@Pt, bcc/fcc PdCu@Pt, and fcc/fcc Ru@Pt. We have identified distinct heteroepitaxial modes in these model systems, each mitigating the structural mismatch within a single particle: mixed coherent/semi-coherent growth in hcp/fcc Ru@Pt, coherent growth with strain sharing between core and shell in bcc/fcc PdCu@Pt, and oriented growth combined with twin proliferation in decahedral fcc/fcc Ru@Pt. The various semi-coherent growth modes in the hcp/fcc nanoparticle yield a 3D dislocation network at a scale below 10 nm, interconnected by Lomer-Cottrell, Hirth, and Shockley dislocations. The key heteroepitaxial mechanisms in these examined systems are quantitatively validated by interfacial energies derived from molecular statics simulations. We anticipate these insights will inform the targeted design of interfacial structures and enable precise strain engineering in core-shell nanomaterials.