Spectroscopy and Imaging of Plasmonic Modes Over a Single Decahedron Gold Nanoparticle: A Combined Experimental and Numerical Study

Employing cathodoluminescence (CL) spectroscopy and imaging in a field emission gun (FEG) scanning electron microscope (SEM), we study localized surface plasmon (LSP) modes on individual tilted gold nanodecahedron sitting on a silicon substrate. We experimentally resolve three distinct LSP modes in the far-field radiation acquired via CL. The experimental spectra and intensity maps of plasmon modes are in excellent agreement with the spectra and 2D-CL image obtained from finite difference time domain (FDTD) simulations. Detail analysis reveals the signature of a quadrupolar surface plasmon mode in addition to the two dipolar modes along azimuthal and polar direction of the decahedron. The experimental method and the theoretical formalism presented here provide useful insight into the plasmonic behavior of complex shaped metal nanoparticle supported by a high index substrate

Probing Higher Order Surface Plasmon Modes on Individual Truncated Tetrahedral Gold Nanoparticle Using Cathodoluminescence Imaging and Spectroscopy Combined with FDTD Simulations

We report the spatial maps of the localized surface plasmon resonances associated photon emission in a truncated tetrahedral gold nanoparticle on a silicon substrate. Site-specific cathodoluminescence spectroscopy and imaging in a scanning electron microscope shows stronger photon emission in the visible range near the tips of the particle in contact with the substrate compared to the edges of the particle. Strong local field variations on a length scale as short as 19 nm are resolved. We also perform FDTD simulations of both the spectra and, for the first time, the full cathodoluminescence images. Excellent agreement is obtained with the experimental results, and the detailed information available from the simulated results makes it possible to identify the signature of out-of-plane higher order modes in the truncated tetrahedral gold particle

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

Annealing Induced Morphology of Silver Nanoparticles on Pyramidal Silicon Surface and Their Application to Surface-Enhanced Raman Scattering

This paper reports on a simple and cost-effective process of developing a stable surface-enhanced Raman scattering (SERS) substrate based on silver (Ag) nanoparticles deposited on silicon (Si) surface. Durability is an important issue for preparing SERS active substrate as silver nanostructures are prone to rapid surface oxidation when exposed to ambient conditions, which may result in the loss of the enhancement capabilities in a short period of time. Here, we employ the galvanic displacement method to produce Ag nanoparticles on Si(100) substrate prepatterned with arrays of micropyramids by chemical etching, and subsequently, separate pieces of such substrates were annealed in oxygen and nitrogen environments at 550 °C. Interestingly, while nitrogen-annealed Si substrates were featured by spherical-shaped Ag particles, the oxygen annealed Si substrates were dominated by the formation of triangular shape particles attached with the spherical one. Remarkably, the oxygen-annealed substrate thus produced shows very high SERS enhancement compared to the either unannealed or nitrogen annealed substrate. The hitherto unobserved coexistence of triangular morphology with the spherical one and the gap between the two (source of efficient hot-spots) are the origin of enhanced SERS activity for the oxygen-annealed Ag particle-covered Si substrate as probed by the combined finite-difference time domain (FDTD) simulation and cathodoluminesensce (CL) experiment. As the substrate has already been annealed in an oxygen environment, further probability of oxidation is reduced in the present synthesis protocol that paves the way for making a novel long-lived thermally stable SERS substrate

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

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

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