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

Abstract

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, finitedifference 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 analyse numerically to show the critical size of the side edge length of the decahedron particle where mode mixing is initiated.

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

“…It is established for a single particle that the higher order modes (like quadrupolar) are less affected by the substrate compared to the dipolar modes, which is consistent with the present analyses where the substrate effect is negligible for the lower wavelength LSP mode with respect to the free-standing case. Insignificant substrate effect is also reported even in the case of a mixed mode, which appears at higher energy or lower wavelength region of spectrum. Moreover, the broadening of a LSPR peak is generally regarded as the mixing of different plasmon modes for complex shaped nano-objects studied by CL spectroscopy .…”

“…While the experimentally observed low wavelength LSP mode (548/560 nm) agrees well with that for free-standing modeled TOH Au particle, the observed high wavelength LSP mode (650/670 nm) is blue-shifted substantially with that for free-standing case, which will be discussed later in the context of substrate effect. The nature of the plasmon modes is assigned by observing the standing wave formation through the calculated near electric field intensity maps and the direction of the corresponding electric field vectors. − ,, Electric field converging and diverging points are designated as the negatively and positively charged points, respectively. − ,, Now, for electron beam excitation at point B, the vector plot of Figure a,b clearly depicts that the LSP mode at 550 nm has dipolar nature of oscillation for both in-plane as well as out-of-plane directions. The charges of opposite polarities (±) are observed to be distributed from base point to nearest apex point, e.g., between the points B–A/B–C (in XY plane) or B–H (in XZ plane).…”

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

“…In this context, a focused beam of fast electrons deliverable within a scanning or transmission electron microscope (SEM/TEM) has shown to be an emerging tool for studying plasmonic materials. , Following the bombardment of a MNP by the energetic electron beam within the electron microscope, the LSP modes are excited on the surface of the nanostructure. The LSP excitations can be probed by detecting the energy loss suffered by the inelastically scattered transmitted electrons (electron energy-loss spectroscopy, EELS) − or by detecting the optical radiation that is subsequently emitted by the material (cathodoluminescence, CL). − High spatial nanometer resolution CL of individual nanostructures can be achieved with focused electron beams. The resulting CL images provide direct information on the wavelength-dependent local density of optical states (LDOS) , of the imaged individual plasmonic nanostructures.…”

Structure and Thermodynamics of Metal Clusters on Atomically Smooth Substrates

“…19 Substrates are also known to affect the properties of plasmonic nanoparticles. 20,21 Hence, some theoretical studies of metal clusters have considered substrate models, including magnesia, 22−25 alumina, 13,26 ceria, 27 graphite, 28−30 graphene, 14,15 or more elaborate carbon nanostructures, 31−33 or a laterally averaged half-plane based on the Lennard-Jones potential. 9−12 However, in most of these reports, the supporting surface is treated as a fixed entity, and we are aware of just one previous study 15 where parameters specifying the substrate microstructure are systematically varied.…”

“…Many experiments and applications in nanotechnology require clusters to be supported on a surface, which in turn can influence the cluster morphology − and thermal behavior. − In heterogeneous catalysis, − for example, tuning the cluster–substrate interface has been identified as an important strategy in the design of novel catalysts . Substrates are also known to affect the properties of plasmonic nanoparticles. , Hence, some theoretical studies of metal clusters have considered substrate models, including magnesia, − alumina, , ceria, graphite, − graphene, , or more elaborate carbon nanostructures, − or a laterally averaged half-plane based on the Lennard-Jones potential. − However, in most of these reports, the supporting surface is treated as a fixed entity, and we are aware of just one previous study where parameters specifying the substrate microstructure are systematically varied. Our goal here is to explore the substrate parameter space more extensively, focusing on changes in the equilibrium morphology of selected metal clusters as the character of the supporting surface is adjusted.…”