LSPR Imaging of Silver Triangular Nanoprisms: Correlating Scattering with Structure Using Electrodynamics for Plasmon Lifetime Analysis

Abstract

Here, an investigation into the optical response of truncated and nontruncated Ag triangular nanoprisms is presented. Single particle darkfield microspectroscopy and transmission electron microscopy (TEM) are performed to obtain an overview of the relationship between plasmonic properties and nanoparticle structure. A general framework is built to describe the optical response (resonant wavelength λmax and full width at half maximum (fwhm)) of triangle nanoprisms in a size regime (60–140 nm edge length) that has not been investigated previously at this level of detail. The discrete dipole approximation is used to infer the thickness of individual nanoprisms from λmax measurements, thereby determining three-dimensional structures derived from two-dimensional TEM images. This additional structural information allowed the various contributions to the fwhm to be deconvoluted and analyzed. It is shown that electron-interface scattering, radiative damping, and bulk damping each make similar contributions to the plasmon width of the triangular prisms for the sizes considered in this work. Surface scattering is found to be seriously overestimated if it is assumed that electron collisions with all prism surfaces lead to dephasing. Reasonable comparisons between theory and experiment are found if it is assumed that only electron collisions involving motions parallel to the plane of the prism lead to dephasing. Substrate effects make a comparatively small contribution to the fwhm. Additionally, it is shown that the optical constants of Johnson and Christy [Phys. Rev. B 1972, 6, 4370] provide the best match to the experimental data. We conclude by presenting the design rules for producing Ag triangular nanoparticles with narrow plasmon resonances.

Optical dielectric function of silver

“…However, SPR lifetime can be influenced by radiative damping and surface scattering, and in most nano-particles studies, the SPR resonances spectrally overlap with interband transitions. [21,22,25,74] It is worth noting, however, that our measured τ = 17 ± 3 fs is consistent with the longest value of τ = 13 fs based on plasmonic resonance linewidth of a silver nano-particle at room temperature. [21,22,25] Interband effects…”

“…In addition, discrepancies between theoretical and experimental values of different optical and plasmonic properties of silver have raised concerns over the accuracy of some of the most widely used measurements of the dielectric function of silver. [18][19][20][21][22][23][24][25] Accurate values for the dielectric function of silver are needed in the visible and infrared (IR) spectral ranges, because many important parameters, such as surface plasmon propagation length, plasmon lifetime, non-radiative loss, and even the Casimir force, are sensitively linked to small variations of the dielectric function. [16] In this work, we provide a comprehensive measurement of the optical dielectric function (ω) of evaporated and template-stripped polycrystalline silver using spectroscopic ellipsometry, covering a broad spectral range throughout the mid-IR to Vis/UV of two orders of magnitude, from 0.05 eV to 4.14 eV (25 µm to 300 nm).…”

“…While there is a good agreement of ω p and ∞ with many past measurements, our value of τ = 17 ± 3 fs is significantly shorter than the commonly used literature value from Johnson and Christy of 31 ± 12 fs [26] and the value derived from DC conductivity of ∼ 40 fs [27,28], yet consistent with most optical and plasmonic experiments, such as typical surface plasmon propagation length and particle plasmon resonance lifetimes. [21,23,25,29,30] The difference in τ between the DC and the optical frequency measurements is due to the frequency dependence of the scattering rate 1/τ (ω). [31,32] An analysis with extended Drude model extracts this frequency dependence, which is found to be consistent with Fermi liquid theory beyond 0.1 eV.…”

Landau damping of surface plasmons in metal nanostructures

“…Surface-assisted plasmon decay (Landau damping) has been extensively studied experimentally [33][34][35][36][37][38][39][40][41][42][43] and theoretically [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] since the pioneering paper by Kawabata and Kubo [44], who have shown that, for a spherical particle of radius a, the surface scattering rate is γ sp = 3v F /4a, where v F is the electron Fermi velocity. In subsequent quantum-mechanical studies carried within random phase approximation (RPA) [45][46][47][48][49][50][51] and timedependent local density approximation (TDLDA) [52][53][54][55][56][57][58][59] approaches, a more complicated picture has emerged involving the role of confining potential and nonlocal effects.…”

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].…”