The Study of Tunable Local Surface Plasmon Resonances on Au-Ag and Ag-Au Core-Shell Alloy Nanostructure Particles With DDA Method

The local surface plasmon resonances (LSPR) of bimetallic Au-Ag core-shell nanostructure particles are studied using discrete-dipole approximation (DDA) method and plasmon hybridization theory. It is found that LSPR is sensitive to the surrounding medium refractive index, showing a distinct redshift with increasing the surrounding medium refractive index. Au-Ag core-shell nanostructure exhibits a strong coupling between the core and shell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the layer thickness and the composition of core and shell metal. LSPR can be tuned over an extended wavelength range by adjusting the ratio of core to shell. The lower energy mode ω − of Au-Ag core-shell nanoparticle shows a redshift with increasing Ag shell thickness or Au core radius, while the higher energy mode ω + shows the opposite behaviors. In addition, Ag-Au core-shell nanostructure compound particles are also studied with the same method, whose properties are different from that of Au-Ag core-shells. For the sake of clarity, the wavelength shifts of LSPR are plotted as functions of surrounding media refractive index, core radius, shell thickness, and core-shell ratio with figures of merits (FOM). The underlying mechanisms are analyzed with the plasmon hybridization theory and phase retardation effect.