Mode Mixing and Substrate Induced Effect on the Plasmonic Properties of an Isolated Decahedral Gold Nanoparticle

We report new results on the localized surface plasmon (LSP) assisted optical effects of a single noble metal nanoparticle (MNP) in nm level spectral and spatial domain which is related to the phase retardation of electromagnetic signal with larger particle size. Site selective electron beam excitation in a scanning electron microscope (SEM) show multiple resonance peaks in the cathodoluminescence (CL) spectra of an isolated gold decahedron of side edge length 230 nm sitting on a silicon (Si) substrate. Apart from a substrate induced LSP mode in the near-infrared (750 nm) region, finite-difference time-domain (FDTD) numerical analysis also identifies two prominent LSP modes in the visible region. While the shorter wavelength (560 nm) mode has a mixture of in-plane quadrupolar and out-of-plane quadrupolar charge distribution pattern, the longer wavelength (655 nm) mode has the dipolar charge pattern in both the direction. We also analyze numerically to show the critical size of the side edge length of the decahedron particle where mode mixing is initiated

Effect of Intertip Coupling on the Plasmonic Behavior of Individual Multitipped Gold Nanoflower

We report here, the first experimental realization on the selective excitation of two closely lying tips from the same spherical core of a multitipped gold nanoparticle with flower-like morphology. This gives strong multipeaked resonance in the near-infrared region of the far-field emission spectra showing a clear signature of tip to tip coupling. The cathodoluminescence (CL) technique in a scanning electron microscope (SEM) combined with finite-difference time-domain (FDTD) simulation has helped us to identify the coupled plasmon modes to be originated from the interaction between two closely spaced tips with a narrow angular separation. Our analysis further estimates a range of angular separation between the tips that triggers the onset of the intertip coupling

Efficient Excitation of Higher Order Modes in the Plasmonic Response of Individual Concave Gold Nanocubes

Recently, concave nanocube (CNC) shaped metal nanoparticles (MNPs) with high index facets have drawn special attention due to their high chemical activity and large electromagnetic (EM) field enhancements, making them good candidates for multifunctional platforms. However, most of the previously published works focused on the plasmonic properties of silver simple nanocubes of smaller dimension, i.e., within the quasi-static limit, hardly supporting efficient excitation of high-order plasmonic modes. Site-selective electron beam excitation of individual Au CNC particles gives rise to simultaneous excitation of edge and corner localized surface plasmon (LSP) modes. We show that spatial variation of the radiative modes is strongly localized at the corners and extended along the edges of the top surface of the CNCs. Extensive finite-difference time-domain (FDTD) numerical analysis reveals that the substrate-induced plasmon hybridization leads to the activation of corner octupolar and corner quadrupolar LSP modes, in agreement with the cathodoluminescence (CL) measurements. Remarkably, the strength of the hybridization is shown to depend on the CNC size. Furthermore, we show that the edge quadrupolar mode becomes prominent with increasing concaveness, thus opening up a new way of engineering the LSP modes

Probing Localized Surface Plasmons of Trisoctahedral Gold Nanocrystals for Surface Enhanced Raman Scattering

Trisoctahedral (TOH) shaped gold (Au) nanocrystals (NCs) have emerged as a new class of metal nanoparticles (MNPs) due to their superior catalytic and surface enhanced Raman scattering (SERS) activities caused by the presence of high density of atomic steps and dangling bonds on their high-index facets. We examine the radiative localized surface plasmon resonance (LSPR) modes of an isolated single TOH Au NC using cathodoluminescence (CL), with high resolution spatial information on the local density of optical states (LDOS) across the visible spectral range. Further, we show pronounced enhancement in the Raman scattering by performing Raman spectroscopic measurements on Rhodamine 6G (R6G)-covered TOH Au NPs aggregates on a Si substrate. We believe that the hot spots between two adjacent MNP surfaces (“nanogaps”) can be significantly stronger than single particle LSPRs. Such “nanogaps” hot spots may have crucial role on the substantial SERS enhancement observed in this report. Consequently, the present study indicates that MNPs aggregates are highly desirable than individual plasmonic nanoparticles for possible applications in SERS based biosensing