Half-raspberry-like bimetallic nanoassembly: Interstitial dependent correlated surface plasmon resonances and surface-enhanced Raman spectroscopy.

Correlated localized surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS) and localized electromagnetic (EM) field distributions of pure and modified gold (Au) nanoassemblies have been demonstrated. The Au nanoassemblies were decorated as half-raspberry-like nanostructures by silver (Ag) mists, and the characteristics of their SPR and SERS were observed at the same spatial position with and without decoration. The decoration of Au nanoassemblies was analyzed in-depth and confirmed by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). Multifunctional and lab-built microscopy was used to capture correlated SPR and SERS imaging and spectral measurements. Without decoration, strong SPR peaks and enhanced SERS signals were observed, whereas intense plasmon excitation deteriorated with a broadening and diminishing peak and the SERS enhancement dropped at least by 10 fold upon the modification. Preferential enhancement near the edge was observed in the correlated SPR and SERS measurements. The variations in localized SPR, subsequent SERS enhancement, and preferential confinement were speculated concerning localized EM near-field deformation. A typical tetramer with five interstitials was modeled and simulated by finite difference time domain (FDTD) analysis at different incident polarizations. The EM near-field distributions were extracted with and without decoration of constituent interstitials by Ag mists. Without the modification of participating interstitials, the EM near-field distributions were found confined, whereas additional EM near-field confinements were observed in the presence of Ag mists. Such EM near-field deformations due to the modification of constituent interstitials were supposed to broaden and deteriorate SPR characteristics of Au nanoassemblies as observed under this investigation.