Multipolar Surface Magnetoplasmon Resonances in Triangular Silver

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Multipolar Surface Magnetoplasmon Resonances in Triangular Silver Nanoprisms Studied by MCD Spectroscopy Hiroshi Yao, and Taisuke Shiratsu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11216 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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The Journal of Physical Chemistry

Multipolar Surface Magnetoplasmon Resonances in Triangular Silver Nanoprisms studied by MCD Spectroscopy Hiroshi Yao* and Taisuke Shiratsu

Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan * To whom correspondence should be addressed.

ABSTRACT

Magneto-optical responses of triangular silver nanoprisms are demonstrated for the first time with magnetic circular dichroism (MCD) spectroscopy. In Ag nanoprisms with the mean aspect ratio ranging from 6.3 to 8.8, optical absorption exhibits a strong main peak at 650–800 nm that is ascribed to the in-plane dipolar localized surface plasmon resonance (LSPR), along with other two peaks at 450–500 nm and ~330 nm, which can be assigned to the in-plane and out-of-plane quadrupolar LSPR, respectively. A derivative-like MCD signal is observed at the energy of in-plane dipolar LSPR, but the position crossing zero is not in correspondence with the absorption maximum. In contrast, interestingly, the MCD response for in-plane or out-of-plane quadrupolar LSPR shows a simple (single) peak that corresponds to the position of its absorption maximum, suggesting that MCD spectroscopy is very effective for easy identification (or rapid determination) whether the LSPR modes are dipolar or quadrupolar. The agreement between the energies of quadrupolar LSPR maxima of absorption and MCD spectra can be explained within the context of a model on the Lorentz force imparted on the collective electrons that circularly move in the nanoprism under an applied magnetic field. We believe the present results will bring a wide range of impact to the field of active plasmonics and magnetoplasmonics. 1 ACS Paragon Plus Environment


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• INTRODUCTION Synthesis of metal (particularly gold or silver) nanostructures has been an active research area because of the importance of these nanomaterials to future catalysis, electronics, photonics, biological imaging and sensing.1–5 The spectroscopic behaviors of such metal nanostructures result from the interaction of their free conduction electrons with the incident light, giving an excitation known as a localized surface plasmon resonance (LSPR) that has a fundamental role in tailoring the optical responses as a function of geometrical parameters such as shape and size.1,6 Interestingly, the shape or dimensional parameter influences the ways of plasmonic excitations (that is, the number and energy position of the LSPR maxima),6,7 so there has been considerable interest in the development of synthetic methods for preparing anisotropic nanostructures including nanorods, nanocubes and nanoprisms.8–11 In particular, triangular nanoprisms of silver favor a high-tunability of LSPR energy, and are known to give a maximum electromagnetic-field enhancement.12 Indeed, Ag nanoprisms mostly exhibit multiple absorption bands associated with the multipolar LSPR, and the observation of dipole and higher multipole (typically quadrupole) resonances at a separated wavelength is typical:12–14 The most intense LSPR mode corresponds to the in-plane dipole resonance, and the other bands, which are generally assigned to quadrupolar resonances, exhibit much weaker intensity due to the split in the electronic cloud and appear at higher energy compared to the dipolar resonance.14 The appropriate identification of multipolar LSPR of Ag nanoprisms still then remains somewhat uncertain because of the low intensity and partial superimposition of the corresponding bands.14 Meanwhile, an external magnetic field can modulate the optical properties of metal nanostructures if they exhibit a magneto-optical (MO) behavior. Despite the fact that MO activity of non-magnetic noble metals is rather low, it has been shown that gold or silver nanoparticles (nanospheres) have an appreciable MO activity in the dipolar LSPR region, which makes recent

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plasmonics research more attractive both from application and fundamental points of view.15–19 In such plasmonic metal nanospheres, a splitting of dipolar LSPR under an external magnetic field occurs upon left- or right-circularly polarized light excitation,20,21 so the magnetic circular dichroism (MCD) is detectable, giving a derivative-like MCD response. For example, the MCD responses of gold nanospheres for the LSPR transition display a significant asymmetric feature.17,18 The phenomenon has been characterized by a full dielectric function of gold, because the plasmon resonance falls in a spectral region of the onset of interband transitions and thus the effect of conduction electrons cannot be extracted from the responses, making the MCD spectral shapes distorted and asymmetric.20 In contrast, silver is particularly suitable for investigating the MO activity in the LSPR regions since its interband transitions are essentially separated from LSPR (typically the imaginary part of the dielectric function exists only below ~325 nm) and thus the effect of purely free or conduction electrons would be examined.19,22 The derivative-like MCD response can be explained by a substantial increase of the Lorentz force induced by the collective clockwise or counterclockwise circular movement of the conduction electrons,19–21 yielding two modes of surface magnetoplasmon that are split in energy by the cyclotron (angular) frequency ωc. In the present study, to understand the MO activity of anisotropic noble metal nanostructures exhibiting multipolar plasmonic resonances, an MCD spectroscopic study is carried out for some triangular Ag nanoprisms having different aspect ratios. As noted above, silver has excellent plasmonic properties of all chemically stable metals, and a full spectral range of plasmonic resonances can be attained with anisotropic silver nanomaterials such as nanoprisms by tuning the dimension (for example, edge-length, thickness and degree of snipping) of the prisms.22–25 A distinct difference is found in the magnetoplasmonic responses between the dipolar and multipolar (typically quadrupolar) LSPR in the Ag nanoprisms. Nanoprisms exhibit a large derivative-like MCD signal in the main dipolar LSPR region, while the quadrupolar LSPR (irrespective whether it is attributed to an in-plane or out-of-plane mode) shows a simple (single) MCD peak that corresponds to its 3 ACS Paragon Plus Environment


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absorption maximum. Simultaneous deconvolution analysis of electronic absorption and MCD spectra should be done using Gaussian functions, indicating the presence of inhomogeneous spectral broadening caused probably by the prism shape distribution. Most importantly, the MCD spectroscopy has great advantage in effectively determining the surface plasmonic origin for the anisotropic metal nanostructures.

• EXPERIMENTAL The plasmonic system we investigated is three kinds of triangular Ag nanoprisms with different aspect ratios (AR), which can be typically defined as congruent edge-length (L) divided by thickness (T). The samples were prepared via a thermal method developed by Mirkin and co-workers.23 Typically, an aqueous solution (25 mL) containing AgNO3 (0.1 mM), trisodium citrate (1.5 mM), polyvinylpyrrolidone (Mw = 40000, 0.7 mM), H2O2 (5.0–20 mM) were combined with various amounts of NaBH4 solution (0.4–1.0 mM). Depending on the concentration of NaBH4 and H2O2 used,23,24 the size (or AR) of the nanoprisms was changed; we hereafter call the prepared Ag nanoprism samples Pr-1, Pr-2, and Pr-3, which were different in edge-length and thickness. Note that the position of main (in-plane dipolar) LSPR peak can be a good indicator for general nanoprism architectures since it strongly depends on their aspect ratio.14,25 That is, the in-plane dipole LSPR peak experiences a red shift as the AR value increases, which is due to electron accumulation in the corners of the nanoprism and the large distance between the negative/positive polarization supporting lower energy oscillations.13,14 MCD spectroscopic measurements were carried out with a JASCO J-820 spectropolarimeter equipped with a JASCO permanent magnet (PM-491LB) of 1.6 T (tesla) with parallel and antiparallel fields. Rectangular 5-mm cuvettes made of quartz were used for the measurements. UV-vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. In addition, they


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were simultaneously obtained together with the MCD spectra under the applied magnetic field. Field emission scanning transmission electron microscopy (FE-STEM) was conducted with a Hitachi S-4800 electron microscope (30 kV).

• RESULTS AND DISCUSSION Absorption Spectroscopy and Electron Microscopy. Figure 1a shows UV-vis absorption spectra of aqueous Pr-1, Pr-2, and Pr-3 samples together with their dispersion photos, in which the main in-plane dipolar LSPR bands appeared at 654 nm, 729 nm, and 780 nm with the width of ~5100 cm–1, 4600 cm–1, and 3900 cm–1 (FWHM), respectively. Other weak LSPR peaks could be also detected at 450–500 nm and ~338 nm in all samples. On the basis of theoretical calculations using DDA (discrete dipole approximation), the sharp peak observed at ~338 nm can be reasonably assigned to the out-of-plane quadrupolar LSPR maximum, whereas the broad peak at 450–500 nm is due to the in-plane mode of quadrupolar LSPR.12–14 Figure 1b shows scanning transmission electron microscopy (STEM) images of the samples, exhibiting the formation Ag nanoprisms with high yield and considerable uniformity in all samples. In certain observation areas (a typical example is shown in inset in Figure 1b), the Ag nanoprisms arranged in a stacked formation with their flat faces perpendicular to the grid could be found (N.B. apparently, they seem as if they were rods), making us convenient to estimate both the prism thickness and aspect ratio. Distributions of edge-length (L), thickness (T), and aspect ratio (AR) of the samples are shown in Figure 2. We found that the mean edge-length increased (from 25.5 to 40.8 nm) with a slight increase in the thickness of the nanoprism (from 3.9 to 4.7 nm) upon changing the sample from Pr-1 to Pr-3, but the edge-length increase was more drastic so the mean aspect ratio resultantly increased from 6.3 to 8.8. It is then clear that Ag nanoprisms have higher average aspect ratio as the spectral position of the main in-plane dipolar LSPR shifted to red. Note that the tip morphology (that is, snipping or truncation) may introduce a



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blue shift in the main in-plane dipole LSPR due to a decreased electron density around the corners of the triangular nanoprism.13,14 Indeed, in our samples, Ag nanoprisms with slightly rounded corners would also exist, but from Figure 1b, their triangular morphology was quite similar to each other, suggesting the similarity in the snip effect. This is supported by the observation that the linewidth of the in-plane dipole LSPR linearly increased as the energy of the resonance increases, since the scattering spectra of individual Ag nanoprisms with similar morphologies are found to have similar spectroscopic trend due to an increased radiation damping and nonradiative decay in the prisms.26

Figure 1. (a) UV-vis absorption spectra of Ag nanoprism samples (Pr-1, Pr-2, and Pr-3) examined in this study. Photo of the dispersion samples is also shown. (b) STEM images of the Ag nanoprism samples showing the triangular morphology. The inset shows a typical example of the STEM image in which Ag nanoprisms stand vertically on a STEM grid surface.


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Figure 2. Distributions of the edge-length (L), thickness (T), and aspect ratio (AR) of the Ag nanoprism samples.

MCD Spectroscopy. Figure 3 shows MCD spectra of the aqueous Pr-1, Pr-2, and Pr-3 samples. The MCD spectra were recorded with a spectropolarimeter equipped with a permanent magnet of 1.6 T (tesla) with parallel (denoted as +1.6 T) and antiparallel (denoted as –1.6 T) fields.19,27 The MCD signals appeared at the whole spectral region including the absorption peaks, suggesting strong MO effects in all LSPR modes. Here, the sign of the MCD signal was completely reversed when the field was switched (that is, mirror symmetry), confirming that signatures are not from an experimental artifact but originated from real MO responses. With a close inspection of the MCD spectra, we found that (i) in a region of in-plane dipolar LSPR (600~800 nm), the signal lineshape was (+/–) derivative-like, but it was significantly asymmetric and the position



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(wavelength) crossing zero was not in accordance with that of the LSPR absorption maximum; (ii) in a region of in-plane or out-of-plane quadrupolar LSPR (300~600 nm), the MCD signal exhibited a simple peak whose energy was equal to that of the absorption of quadrupolar LSPR, meaning that the mode of LSPR (dipolar or quadrupolar resonance) in Ag nanoprisms can be easily distinguishable with MCD spectroscopy; in other words, MCD spectroscopy can be used for the rapid assignment of the plasmonic modes in such metal nanoarchitectures.

Figure 3. MCD spectra of Ag nanoprism samples in aqueous solution. Applied magnetic fields are +1.6 T (blue curve) and –1.6 T (light-brown curve).

The bisignated MCD response originated from the dipolar LSPR mode has been explained in terms of two (circular) modes of “surface magnetoplasmon”.17–21 At zero magnetic field, the two 8 ACS Paragon Plus Environment

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(left- and right-handed) circular magnetoplasmonic modes are degenerate. When an external magnetic field is applied along the light propagation direction which is perpendicular to the flat surface of the prism, the degeneration should be removed by the Lorentz forces acting on the collective electrons that oscillate in circular orbits. The circular charge motion will be confined according to the left-handed or right-handed helicity of the plasmonic mode with frequency ω+ or

ω–, respectively, and a shift in the resonance (ω+ – ω–) can be observed. It is of note here that, if the architectures display local optical chirality, which may occur in plasmonic planar nanostructures with no spatial chirality or imperfect symmetry at the nanoscale,28–30 the left- and right-handed circular modes may not strictly be degenerate; however, the present samples are composed of an ensemble of nanoprisms with various morphologies of triangles, so the difference between the two circular modes would be averaged over the structures. Quantitative Assignment. To more quantitatively evaluate the characteristic features of the surface magnetoplasmonic resonances in these triangular Ag nanoprisms, spectral deconvolution analysis of both absorption and MCD data was simultaneously conducted since the MCD signal essentially arises from the same transitions as those seen in the electronic absorption spectrum.31–33 The fitting was conducted for the MCD spectrum obtained at –1.6 T as well as for the electronic absorption obtained simultaneously with MCD under the applied magnetic field. In the present system, to obtain a numerically adequate description, deconvolution using seven Gaussian bands was successful (bands 1–6 plus one additional component that would be attributed to a contribution from the interband transitions of silver).22 The spectral fits of MCD as well as UV-vis absorption spectra for the Ag nanoprism samples (Pr-1, Pr-2, and Pr-3) are shown in Figure 4. Fitting parameters (peak energy, bandwidth, and relative intensity for each component) obtained are listed in Table 1. Importantly, the deconvolution analysis is constrained by the requirement that a “single set” of Gaussian components with identical band parameters such as peak energy and linewidth be used for the fitting of both the absorption and MCD spectra.19,27 9 ACS Paragon Plus Environment


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In the analysis, the spectra of Ag nanoprisms could not be fit by the Lorentzian lineshape but well by the Gaussian function,26 indicating the substantial presence of inhomogeneous broadening. This can be due to a distribution of average size and shape (including the snip effect) of nanoprisms that have strong sensitivity of the optical response; for example, small triangular nanoprisms with sharp tips typically show a main dipole LSPR peak in a low-energy region whereas large triangular prisms with rounded (snipped) tips have a peak relatively in a high-energy region,34 inducing spectral inhomogeneities.

Figure 4. Gaussian band fits of the electronic absorption (upper panels) and MCD spectra (lower panels) of the Ag nanoprism samples. Red curves display the reconstructed absorption and MCD spectra. Black dots indicate the experimental spectra. Deconvoluted components (1–6) obtained are also shown.

The deconvoluted bands 1 and 2 in the lowest-energy region are ascribed to two modes of surface magnetoplasmon originated from in-plane dipole LSPR in the Ag nanoprisms because of their opposite signs (positive and negative for bands 1 and 2, respectively).19 Component 1 (corresponding to a clockwise helicity of conduction electrons with frequency ω–) has a stronger MCD amplitude as compared to component 2 (with ω+), resulting in the non-equivalent


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magnetoplasmonic splitting. In addition, their peak positions and widths are strongly AR-dependent. As the AR becomes small, the bandwidth broadening is significant; for example, according to Table 1, the widths of bands 1 and 2 for sample Pr-1 are ~2000 cm–1 and ~2600 cm–1, whereas those for sample Pr-3 are ~1500 cm–1 and ~2200 cm–1, respectively. The observed broadening in the main dipolar LSPR is in good agreement with the previous report on the bandwidth measurements for individual Ag nanoprisms.26 On the other hand, the reason for the inconsistency between the energy of the dipolar LSPR absorption peak and that crossing zero in the MCD response is not clear at present (see dashed blue lines in Figure 4), but may be that each dipolar magnetoplasmonic band will be composed of various, counteracting sub-components with non-equal MCD amplitudes and widths, which is probably caused by the anisotropic circular movement of collective charges in some kinds of Ag nanoprisms. The peak position of component 3 is also strongly AR-dependent, and it can be attributed to the in-plane quadrupolar surface magnetoplasmon according to the theoretical DDA simulations.12–14 Importantly, even after the deconvolution procedure, the quadrupolar magnetoplasmon resonance has the same energy with the LSPR peak in the electronic absorption (see dashed green lines in Figure 4). This is also the case for the narrow band 6, which is ascribed to the out-of-plane quadrupolar LSPR.12–14 Therefore, MCD spectroscopy can effectively identify whether an observable plasmonic peak is originated from dipolar or quadrupolar oscillation. Bands 4 and 5 have similar (and appreciable) absorption intensity with each other, but the MCD intensity of band 4 is much weaker than that of band 5. Here, the peak position (350–360 nm) and negative sign of the MCD response (at –1.6 T) of component 5 are reminiscent of those detected in spherical Ag nanoparticles,19 and thus, it (= band 5) can be assigned to a small deviation from the smooth curve of the imaginary part of the dielectric function of silver.35,36 Finally, for component 4, the possible assignment includes in-plane hexapolar or out-of-plane dipolar LSPR in the Ag nanoprisms according also to theoretical DDA calculations.12–14 It has been revealed that the in-plane 11 ACS Paragon Plus Environment


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hexapolar resonance, which appears at the energy slightly (1000~2000 cm–1) higher than that of the quadrupolar excitation (band 3), begins to resolve when the aspect ratio is larger than ~10. In our samples, the obtained AR values are smaller than this criterion so this mode seems unlikely. In the out-of-plane polarization, on the other hand, a dipolar resonance with a relatively low absorption intensity is possible at ~400 nm.12 This mode is known to exhibit a weak dependence on the nanoprism thickness (T) with a small shift toward lower energy with an increase in T (at a constant edge-length), while a very small blue shift is found with an increase in the nanoprism edge-length (L) at a fixed thickness. Additionally, the theory indicates that this out-of-plane excitation should be a broad resonance, so in general, sole absorption measurements do not give sufficient resolution.12 The band 4 well matches the above spectroscopic behaviors, so we believe this can be due to the out-of-plane dipolar LSPR. Very weak MCD amplitudes for component 4 would result from the ensemble average of various derivative-like signals canceling out with each other. Consequently, two different spectral patterns (electronic absorption and MCD) gave clear information on the multipolar surface magnetoplasmonic resonances in the anisotropic Ag nanoarchitectures. Characteristic Features of In-Plane Quadrupolar Surface Magnetoplasmon. One of the most remarkable results on the MCD responses of triangular Ag nanoprisms is that quadrupolar excitations exhibit a simple (single) peak whose energy (position) is identical with its absorption maximum, whereas dipolar modes have a derivative-like spectrum. Since the sole measurements of electronic absorption cannot determine the mode of LSPR whether it is dipolar or quadrupolar, the observed MCD character should be an important indicator for efficient LSPR identification. In consideration with the fact that two (that is, left-handed and right-handed) circular modes of magnetoplasmon are created by the additional Lorentz force imparted on the collective charge motion, the above features can be interpreted from a viewpoint of polarizations in the prisms.13,14 Note here that the Lorentz force (F) can then be induced by in oscillating dipoles (

) due to

their associated moving charges;17 12 ACS Paragon Plus Environment

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where

is the applied magnetic field (or magnetic flux density). In dipolar excitation (or circular

oscillation), the effect of additional Lorentz force can split LSPR in the plane perpendicular to the field direction yielding two circular modes of surface magnetoplasmon.17–20 In the quadrupole resonance, on the other hand, the situation is different; half of the electronic cloud moves parallel to the electromagnetic beam polarization while the other half moves toward the opposite direction. Schematic illustration (including polarization vectors of in-plane dipolar and quadrupolar modes) for triangular nanoprisms is shown in Figures 5a and 5b, which are drawn on the basis of the results obtained by Pileni and co-workers.14,37 In the quadrupolar resonance (Figure 5b), Lorentz forces imparted on the two kinds of collective electrons with opposite directions (that is, blue and black round arrows in Figure 5b) would then give two components with frequencies of ω + ωc/2 and ω –

ωc/2, where ω or ωc is the position of the original LSPR peak or cyclotron frequency, respectively, irrespective of the excitation polarization (that is, in both cases of left- and right-circularly polarized light (LCPL or RCPL) excitations). Similarly, a scheme for the out-of-plane quadrupolar resonance is also drawn in Figure 5c. The MCD response is detectable when a sample differentially absorbs LCPL and RCPL in an external magnetic field parallel to the light beam, so in the quadrupolar oscillation, the resultant MCD signal should be small and appear at the almost same energy position with that of the absorption maximum (= ω). In any case, we believe the present work will indicate a potential usefulness for future plasmonic applications based on some metal anisotropic nanostructures such as nanorods and nanocubes.38



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Figure 5. Schematic illustration showing the polarization vectors for the in-plane (a) dipolar or (b) quadrupolar resonance in the Ag nanoprism. Additionally, (c) shows a scheme for the out-of-plane quadrupolar resonance. The symbols + and – indicate the resulting polarity. Round arrows indicate a circular oscillation of collective electrons under LCPL excitation. We here assume that the direction of the magnetic field applied is perpendicular to the page.

• CONCLUSIONS In summary, magnetic circular dichroism (MCD) spectroscopy was demonstrated for the first time in triangular Ag nanoprisms. The Ag nanoprisms examined had a mean aspect ratio ranging from 6.3 to 8.8. Optical absorption exhibited a strong main peak at 650–800 nm that is due to the in-plane dipolar localized surface plasmon resonance (LSPR), along with other two peaks at 450–500 nm and ~330 nm, which can be assigned to the in-plane and out-of-plane quadrupolar LSPR, respectively. The MCD revealed a derivative-like response at the energy of the in-plane dipolar LSPR (that is, dipolar magnetoplasmon), but the position crossing zero did not correspond to its absorption maximum. In contrast, the MCD signal ascribed to a quadrupolar resonance (whether it is from in-plane or out-of-plane mode) showed a simple (single) peak whose position was identical with that of electronic absorption, suggesting that MCD measurements can readily and effectively determine (or identify) the LSPR modes in the nanoprisms. The agreement of the peak energies of quadrupolar LSPR between absorption and MCD could still be explained within the context of a model on the magnetic Lorentz force induced by the collective electrons that 14 ACS Paragon Plus Environment

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circularly move in the nanoprism. We believe that characterizations of remarkable magneto-optical responses or magnetoplasmons as a function of anisotropy in metal nanostructures will bring a wide range of impact to the field of active plasmonics.38

■ AUTHOR INFORMATION Corresponding Author. *E–mail: [email protected] Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The present work was in part supported by Grant-in-Aids for Scientific Research (C: 15K04593 (H. Y.)) from Japan Society for the Promotion of Science (JSPS).

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Table 1. Results of the Gaussian fit analysis of Ag nanoprism samples.


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TOC GRAPHICS


Multipolar Surface Magnetoplasmon Resonances in Triangular Silver

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