Enhancement of spectral shift of plasmon resonances in bimetallic noble metal nanoparticles in core–shell structure

Core-shell metal nanoparticles (NPs) exhibit unique optical, especially plasmonic, properties in the visible range of electromagnetic spectrum. Here we report the plasmonic properties of bimetallic core-shell spherical NPs consisting of noble metals in pair (such as Au-Ag, Ag-Cu, and Au-Cu, and their interchanging combinations) embedded in silica employing Mie theory. The core radius and shell thickness were considered in the range of 2–20 nm and 1–5 nm, respectively. In the case of varied core radius, shell thickness was kept fixed, and in the case of varied shell thickness, the core radius was kept fixed. The optical extinction spectra exhibited by these NPs have two surface plasmon resonance (SPR) absorption bands in the visible region of the electromagnetic spectrum, one corresponding to the core metal and the other corresponding to shell metal, respectively. The appearance of two absorption peaks is due to the fact that in the core-shell configuration there exists two SPR modes one at the interface between the core and shell, and other at the outer surface of the shell and embedding medium. These two SPR modes interact together and determine the shape of the extinction spectra. The position of the SPR band is found to shift as the radius or the thickness of the shell is increased. The shift of the SPR peak with high range of tunability has been observed in all the cases. Such tunability is beneficial to applications in surface-enhanced Raman scattering, fluorescence, optical sensing, bio-medicine, optoelectronic devices, and others.