Size-Dependent Transition to High-Symmetry Chiral Structures in AgCu, AgCo, AgNi, and AuNi Nanoalloys

A class of nanomaterials possessing the highest degree of chiral symmetry, the chiral icosahedral symmetry, is found by a combination of global optimization searches and first-principle calculations. These nanomaterials are core-shell nanoalloys with a Cu, Ni, or Co core and a chiral Ag or Au shell of monatomic thickness. The chiral shell is obtained by a transformation of an anti-Mackay icosahedral shell by a concerted rotation of triangular atomic islands which breaks all mirror symmetries. This transformation becomes energetically favorable as the cluster size increases. Other chiral nanoalloys, belonging to a different structural family of C(5) group symmetry, are found in the size range between 100 and 200 atoms. High-symmetry chiral nanoalloys associate strong energetic stability with potential for applications in optics, catalysis, and magnetism.