  Bimetallic nanoparticles (often known as nanoalloys) with core-shell
arrangement are of special interest in several applications, such as in optics,
catalysis, magnetism and biomedicine. Despite wide interest in applications,
the physical factors stabilizing the structures of these nanoparticles are
still unclear to a great extent, especially for what concerns the relationship
between geometric structure and chemical ordering pattern. Here
global-optimization searches are performed in order to single out the most
stable chemical ordering patterns corresponding to the most important geometric
structures, for a series of weakly miscible systems, including AgCu, AgNi, AgCo
and AuCo. The calculations show that (i) the overall geometric structure of the
nanoalloy and the shape and placement of its inner core are strictly
correlated; (ii) centered cores can be obtained in icosahedral nanoparticles
but not in crystalline or decahedral ones, in which asym- metric quasi-Janus
morphologies form; (iii) in icosahedral nanoparticles, when the core exceeds a
critical size, a new type of morphological instability develops, making the
core asymmetric and extending it to- wards the nanoparticle surface; (iv)
multi-center patterns can be obtained in polyicosahedral nanoalloys. Analogies
and differences between the instability of the core in icosahedral nanoalloys
and the Stranski- Krastanov instability occurring in thin-film growth are
discussed. All these issues are crucial for designing strategies to achieve
effective coatings of the cores.
