Phase formation, magnetic and optical properties of epitaxially grown icosahedral Au@Ni nanoparticles with ultrathin shells

The synergistic effect between the metallic elements in the core-shell nanostructures has attracted increasing interest. In the system of Au@Ni nanostructure, it is challenging to epitaxially grow Ni atoms on Au nanocrystals due to the large lattice mismatch. In this paper, Au@Ni core-shell nanostructures have been synthesized by a facile one-pot wet chemical method. The Ni shell is epitaxially grown on the (111) planes of the icosahedral Au cores. The diameter of the icosahedral Au core is about 10-20 nm and the thickness of the Ni shell is of only several nanometers, providing an ideal structure for the study of synergistic effect. The Curie temperature of the Ni shells is estimated to be lower than 400 K by the field-cooling/zero field-cooling M(T) measurements. It is suppressed considerably from that of the bulk phase, mainly attributed to the finite size effect. The optical properties of the Au@Ni core-shell nanostructures are studied by absorption spectroscopy. The spectral blue-shift tendency is consistent with the results described by the plasmon hybridization theory.