Surface Plasmon Resonance in Bimetallic Core–Shell Nanoparticles

Colloidal metal nanoparticles have unique surface plasmon resonance (SPR) properties for applications in optics, medicine, photocatalysis, and photovoltaics. In this work, we use the Mie theory to investigate the SPR properties of bimetallic core–shell nanoparticles having a spherical shape and consisting of Drude metals. We find that there exists a special SPR mode whose energy is concentrated at the interface between the core and the shell, and we call it the extraordinary SPR mode. This mode can interact with the conventional SPR mode whose energy is concentrated at the outer surface of the bimetallic nanoparticle, which is called the ordinary SPR mode. The ordinary and extraordinary SPR modes together determine the line shape of the extinction spectrum as well as the shift of SPR peak (to shorter or longer wavelengths) as a function of various geometric parameters. When extended to practical noble metals such as Au, Ag, Pd, and Pt, we find that the SPR of both Au@Pt and Au@Pd nanoparticles can occur in the visible region with high tunability, which is beneficial to the enhancement of photocatalytic properties of both Pd and Pt. The theoretical studies would open up new avenues for engineering the plasmonic properties of bimetallic nanoparticles to enhance their applications related to fluorescence, Raman spectroscopy, optical sensing, photocatalysis, photovaltaics, and biomedical research.