Substrate Induced Symmetry Breaking in Penta-twinned Gold Nanorod Probed by Free Electron Impact

Cathodoluminescence (CL) spectroscopy and imaging in a high resolution scanning electron microscope (SEM) is used to probe and directly map the localized surface plasmon resonance (LSPR) modes of a penta-twinned gold nanorod deposited on a silicon substrate. Finite-difference time-domain simulation of CL enables us to gain insight into the origin of the plasmon modes. Our experimental results and simulations demonstrate that the substrate plays a very crucial role in the observed plasmonic property of gold nanorod. We have shown that, in the visible domain of the spectrum, the plasmon mode gets split into two distinct peaks due to substrate induced hybridization of in-plane and out-of-plane modes. With increasing the refractive index of the substrate, the intensity of these hybridized modes increases. We provide a detailed analysis on the origin and coupling of various plasmon modes mediated by the substrate.