Enhanced refractive index sensitivity and SERS performances of individual body-Ag-nanoshell-encapsulated Au nanorods

Abstract

Individual Au nanorods have been widely investigated in refractive index sensing and surface enhanced Roman spectroscopy (SERS) due to their excitation of localized surface plasmon resonances (LSPRs). It is crucial to unveil the effect of Ag nanoshell encapsulating only the body part of individual Au nanorods to improve their SERS and plasmonic sensing applications, which were investigated numerically by finite element method (FEM) in this paper. We have calculated the extinction spectra of individual body-Ag-nanoshell-encapsulated Au nanorods and optimized their refractive index sensitivity factor S, figure of merit FOM, and enhanced electric fields $\left| E \right|$ by controlling Au nanorod aspect ratio AR and Ag nanoshell thickness $t$. An effective aspect ratio ARʹ combining both AR and $t$ is revealed to control their S, FOM and $\left| E \right|$. The optimized FOM and ARʹ for their plasmon sensitivity applications are predicted, both of which are shown to become larger than that of naked Au nanorod. The optimized $\left| E \right|$ under three normally adopted excitation laser in experiments are also predicted, which are all ~ 17 times that of naked Au nanorods. This work is helpful to choose the appropriate size of partially encapsulated Au nanorods by Ag to facilitate designing of their LSPR based plasmon sensing and SERS nanodevices from visible to infrared.