Symmetry breaking and morphological instabilities in core-shell metallic nanoparticles

Nanoalloys are bi- or multi-component metallic particles in the size range between 1 and 100 nm. Nanoalloys present a wide variety of structures and properties, which make them suitable for many applications in catalysis, optics, magnetism and biomedicine. This topical review is devoted to the structural properties of nanoalloys of weakly miscible metals, which are expected to present phase-separated arrangements of their components, such as core-shell and Janus arrangements. The focus is on singling out size- and composition-dependent transitions between these arrangements, showing that several transitions can be rationalized by a unifying concept, that is symmetry breaking, caused by the accumulation of strain at the atomic level and its subsequent release. The driving forces that rule the interplay between core-shell and other structures and determine the actual shapes of core and shell, and the placement of the core inside the shell are analyzed. Several systems, such as Ag-Cu, Ag-Co, Ag-Ni, Au-Co, Au-Pt, and Ir-Pt are treated, comparing computational results to experimental observations and simple analytical models. After treating the lowest-energy structures, which are representative of the equilibrium configurations at sufficiently low temperatures, high-temperature and growth kinetics effects are considered.