Plasmon Spectroscopy and Imaging of Individual Gold Nanodecahedra: A Combined Optical Microscopy, Cathodoluminescence, and Electron Energy-Loss Spectroscopy Study

Abstract

Imaging localized plasmon modes in noblemetal nanoparticles is of fundamental importance for applications such as ultrasensitive molecular detection. Here, we demonstrate the combined use of optical dark-field microscopy (DFM), cathodoluminescence (CL), and electron energy-loss spectroscopy (EELS) to study localized surface plasmons on individual gold nanodecahedra. By exciting surface plasmons with either external light or an electron beam, we experimentally resolve a prominent dipole-active plasmon band in the far-field radiation acquired via DFM and CL, whereas EELS reveals an additional plasmon mode associated with a weak dipole moment. We present measured spectra and intensity maps of plasmon modes in individual nanodecahedra in excellent agreement with boundary-element method simulations, including the effect of the substrate. A simple tight-binding model is formulated to successfully explain the rich plasmon structure in these particles encompasing bright and dark modes, which we predict to be fully observable in less lossy silver decahedra. Our work provides useful insight into the complex nature of plasmon resonances in nanoparticles with pentagonal symmetry. KEYWORDS: Localized plasmons, gold nanodecahedra, dark-field microscopy and spectroscopy, cathodoluminescence spectral imaging, electron energy-loss spectroscopy, boundary-element method L ight-matter interaction at the nanoscale has attracted much attention during the past decade because of its scientific and practical relevance. In particular, an impressive amount of work on metal nanoparticles has been triggered by their ability to host localized surface plasmons (SPs), which are quantized collective oscillations of conduction electrons mediated by their coulomb interaction. 1 These excitations can be tailored over a broad spectral range by controlling the size, morphology, and composition of the particles. 1−5 In particular, plasmons in noble metal nanocrystals oscillate at frequencies within the visible and near-infrared (vis-NIR), that is, the spectral ranges of interest for optical engineering, spectroscopy, and microscopy. Actually, SPs couple efficiently to external electromagnetic radiation, 6,7 thus offering a convenient way of concentrating and enhancing the electromagnetic field intensity around nanoparticle volumes well below the light diffraction limit. These unique properties of nanoparticle plasmons play a leading role in a wide range of applications such as waveguiding 8−11 and light manipulation at the nanoscale, 12 optical trapping, 13 enhanced fluorescence, 14 single-molecule detection, and surface-enhanced Raman scattering (SERS). 15−18 Experimental access to the electromagnetic field distribution associated with nanoparticle SPs, with a sufficient degree of energy and spatial resolution, is of major importance in the

Photon-induced near-field electron microscopy: Mathematical formulation of the relation between the experimental observables and the optically driven charge density of nanoparticles

“…It was also demonstrated that energy-filtered transmission electron microscopy (EFTEM) can be equivalently employed to obtain the same information [8]. Plasmons in nanoparticles of other geometries were also visualized with SI methods [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. In particular, to investigate SERS activity, Guiton et al employed STEM-EELS to probe the fields of plasmons in silver nanorods and correlated this with far-field scattering spectroscopy [15].…”

Comparison of electron energy-loss and quantitative optical spectroscopy on individual optical gold antennas

“…Later experimental work on higher-order plasmonic resonances of silver nanorods did not show a clear correlation between the resonances of measured EELS and measured optical dark-field scattering spectra [5]. More recent experimental work on gold nanodecahedra found a blue shift of the fundamental EELS resonance compared to the fundamental optical-scattering and cathodoluminescence resonance on the order of 100 meV [6]. They compared their results to a recent paper using a simple harmonicoscillator model [7], referring to earlier numerical calculations [8].…”

“…Within the harmonic-oscillator model [7], near-field probes reveal resonances that are red-shifted with respect to far-field probes. If one thinks of EELS as a near-field probe, experiment [6] and theory [7] actually give shifts of opposite sign. Other theoretical work found no significant shift between EELS and optics [9].…”

Micro-Extinction Spectroscopy (MExS): a versatile optical characterization technique

“…This serves as a map in data post-processing. d Representative point spectra extracted as shown in a for absorbance, reflectance, and darkfield scattering decahedra, bipyramids, and other shapes, and, together with the NP's LSPR energy and stability, are critical design rules for plasmon sensors [37][38][39][40][41]. An even more powerful expression is the sensing figure of merit (FOM), [42] defined as the RIS divided by the LSPR full width at half maximum (FWHM) in eV, i.e.,…”