Far-Field and Near-Field Investigation of Longitudinal Plasmons of

Jun 21, 2016 - We studied the localized surface plasmons of AgAuAg-nanorods (NRs) using far-field dark-field spectro-microscopy and scattering-type ...
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Far-Field and Near-Field Investigation of Longitudinal Plasmons of AgAuAg Nanorods Deok-Soo Kim,†,‡ Sung-Hyun Ahn,§,∥ Jinwook Kim,§ Daeha Seo,⊥,# Hyunjoon Song,⊥ and Zee Hwan Kim*,† †

Department of Chemistry, Seoul National University, Seoul 08826, Korea Department of Chemistry, Korea University, Seoul 02841, Korea ⊥ Department of Chemistry, Korea Advanced Institute of Science and Technology, Deajeon 34141, Korea §

S Supporting Information *

ABSTRACT: We studied the localized surface plasmons of AgAuAg-nanorods (NRs) using far-field dark-field spectromicroscopy and scattering-type scanning near-field optical microscopy (sSNOM) techniques. We observe that the far-field scattering spectra of individual AgAuAg-NRs exhibit longitudinal dipolar and octupolar resonances that are mainly determined by the overall lengths of the NRs and are fairly insensitive to the relative compositions of Ag and Au. Corresponding near-field distributions measured by sSNOM further reveal plasmonic local field patterns that closely resemble those of monolithic Au-NRs. These show that the longitudinal plasmons of AgAuAg-NRs oscillate along the entire length of the NRs, although resonance widths indicate appreciable damping by the Ag−Au interfaces. The result constitutes an interesting counter-example of tunability of plasmons by composition variation and, thus, provides an important design rule for composition-tunable bimetallic plasmonic structures. AgAuAg nanorods (NRs).16 The Ag and Au subunits in AgAuAg-NRs have epitaxial (i.e., without defect) interfaces, offering an excellent test ground to access the influence of configuration of subunits and intermetallic interfaces on plasmon resonances. Recent reports by Ahn et al.1 and Mayer et al.17 on similar AgAuAg-NRs suggest that the longitudinal plasmons may be uninfluenced by the relative compositions of Ag and Au. To explore in more detail the roles of Ag−Au interfaces and their compositions in plasmon resonances, we have carried out the dark-field scattering spectroscopy and the scattering-type scanning near-field optical microscopy (sSNOM)18−24 measurements to directly study the far-field scattering resonances and the plasmonic local field distributions of NRs. Direct plasmonic local field mapping is possible with electron microscopy17,25 and fiber-based near-field microscopy,26 as well as with sSNOM. However, the sSNOM offers the polarization-specific local field maps with phase-contrasts, offering fairly detailed information on plasmonic local field distributions. We find that far-field longitudinal dipolar and octupolar resonance wavelengths are nearly insensitive to the

1. INTRODUCTION The bimetallic nanostructures, the nano-objects made of two different kinds of metallic subunits,1−9 promise compositiontunable plasmonic resonances, adding a new parameter for manipulating the plasmonic resonances and field enhancements for plasmon-based sensing and surface-enhanced spectroscopy.8,10,11 A particularly interesting bimetallic structure is the one with two different metallic subunits making a direct contact with each other. When two metallic units with different dielectric functions are making direct contact to each other, new plasmon modes emerge as a result of conductive plasmonic coupling.10 The new plasmon resonances may be influenced by the physical states (such as crystalline disorder4 or interfacial alloying6,8,12) of the intermetallic interfaces, as well as by the dielectric functions of the subunits. Currently, a majority of studies are focused on the core−shell structures with polycrystalline intermetallic interfaces. In addition to the composition-tuning, a significant resonance broadening was observed, which is attributed to the electron scattering at Ag− Au interfaces.7,12−15 For more general classes of bimetallic nanostructures, however, detailed structural (interfaces, geometries, and dielectric functions) influences on plasmon oscillations and their damping remain largely unexplored. Here we study the dipolar and octupolar plasmon resonances and their local field distributions of bar-coded bimetallic © 2016 American Chemical Society

Special Issue: Richard P. Van Duyne Festschrift Received: April 14, 2016 Revised: June 17, 2016 Published: June 21, 2016 21082

DOI: 10.1021/acs.jpcc.6b03813 J. Phys. Chem. C 2016, 120, 21082−21090

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Figure 1. (a) Electron microscopy images of monolithic Au-NRs. (b, c) Electron microscopy images of bimetallic AgAuAg-NRs with short (total length, Ltot ∼ 200 nm, (b)) and long (Ltot ∼ 500 nm, (c)) total lengths. The brighter and darker regions of NRs correspond to the Au and Ag subunits, respectively. The scale bars in (a−c) correspond to 200 nm. (d) Schematic of dark-field spectro-microscopy setup: L = objective lens; Pol = polarizer; S = spectrometer. (e) Schematic of scattering-type scanning near-field optical microscope (sSNOM): L = objective lens; BS = beam splitter; PD = Si-photodiode; M = mirror. The inset of (e) illustrates how the dielectric tip with an effective polarizability (αeff) records the plasmonic local field (Eloc) excited by the incident light (E0), through the detection of scattered field (Escat).

scattering spectroscopy, the NRs are dispersed on top of indium tin oxide (ITO)/glass substrates with position markers. For the sSNOM measurement, the NRs are dispersed on top of SiO2/Si-substrates. The far-field scattering spectroscopy of individual NRs was carried out using a standard dark-field spectro-microscopy setup1 (see Figure 1d) with white-light sources (xenon and halogen lamps), an oil-immersion dark-field condenser, an objective lens (NA = 0.9), and a spectrometer. A polarizer is placed in front of the spectrometer to select the longitudinal and transverse components of the scattering from an NR. The spectrometer has a spectral resolution better than 2 nm around λ ∼ 600 nm. The scattering spectra are background-corrected and normalized by the spectra of the light sources. The plasmonic local field distribution is directly recorded with a side-illuminated, scattering-type scanning near-field optical microscope (sSNOM),18,19,27 which is composed of a tapping-mode atomic force microscope (AFM), a continuouswave laser (HeNe, λ = 633 nm), a Michelson interferometer, and a silicon-photodiode. The tip is dithered near the resonance frequency of the cantilever (∼300 kHz) with a full cantilever oscillation amplitude of 20−50 nm above the sample surface. A linearly polarized light (E0) from a laser is focused onto the tip−sample junction with an angle of 30° with respect

relative compositions of the Ag and Au, and that the corresponding local field distributions (both intensity and phase) are remarkably similar to those of monolithic Ag-NRs and Au-NRs with comparable dimensions. At the same time, however, the far-field octupolar resonances of AgAuAg-NR display broader line widths than those of monolithic NRs. These suggest that the longitudinal plasmons in AgAuAg-NR oscillate along the entire length of the NRs, with non-negligible damping by epitaxial Ag−Au interfaces.

2. EXPERIMENTAL SECTION A single crystalline AgAuAg-NR sample (see Figure 1b,c for the scanning electron microscopy (SEM) images) is synthesized by the method of Seo et al.16 The Ag and Au subunits are composed of five single crystalline domains, and form epitaxial Ag−Au interfaces. We have prepared two batches of AgAuAgNRs with two different average total lengths (Ltot ∼ 200 and 500 nm; hereafter we call them short and long AgAuAg-NRs, respectively), by changing the growth conditions. As can be seen in the SEM images, the sample has noticeable dispersion in the relative composition (length of Au subunit (LAu)/total length (Ltot)), yet the overall diameter (D) is fairly uniform. As a control sample, monolithic Au-NRs with Ltot ∼ 200 nm and D ∼ 100 nm are also prepared (see Figure 1a). For the dark-field 21083

DOI: 10.1021/acs.jpcc.6b03813 J. Phys. Chem. C 2016, 120, 21082−21090

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Figure 2. (a) Far-field scattering spectra of a short AgAuAg-NR (Ltot = 230 nm; LAu = 130 nm, D = 92 nm). (b) Far-field scattering spectra of a short monolithic Au-NR (Ltot = 240 nm; D = 115 nm). In (a) and (b), the spectra in red and blue are the scattering polarized along the longitudinal and transverse directions of the NR, respectively. The inset images are the electron microscopy images of the corresponding NRs (scale bar = 100 nm). (c) Longitudinal scattering spectra of long AgAuAg NRs with various total lengths (Ltot). Also shown in gray trace is the transverse mode spectrum of a long AgAuAg-NR with Ltot = 520 nm; LAu = 211 nm. (d−f) Corresponding electrodynamics simulation of the far-field scattering spectra of NRs based on bulk dielectric constants of Au and Ag. (g) Upper panel: the correlation of resonance wavelength (λmax) and the total length (Ltot) of long AgAuAg-NRs (red circles) and the linear fit (solid line). Middle panel: correlation of λmax and the composition of Au (LAu/Ltot). Bottom panel: correlation of λmax and the aspect ratio (Ltot/D) of the NRs. (h) Comparison of lineshapes of experimental spectrum (red, Ltot = 520 nm, LAu = 211 nm), simulation based on bulk dielectric constants of Johnson and Christy28 (black dashed line), and simulation based on modified dielectric constants (black solid line, see main text). The spectra are shifted in x-axis match the λmax positions. Part of the data shown in (c) is reproduced with permission from ref 1. Copyright 2013 PCCP Owner Societies.

For sSNOM measurement of plasmonic field, the tip serves as a polarizable nanoparticle with an effective polarizability (αeff), sampling the plasmonic local field (Eloc) around the NR. Under the condition that tip−sample coupling is sufficiently weak,19

to the sample surface via an objective lens (NA of 0.42). Backscattered light (Escat) from the tip−sample junction is collected by the same lens and homodyne-amplified by a Michelson interferometer and is detected by a Si-photodiode. 21084

DOI: 10.1021/acs.jpcc.6b03813 J. Phys. Chem. C 2016, 120, 21082−21090

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The Journal of Physical Chemistry C the Escat is proportional to the Eloc. To avoid overly strong tip− sample coupling (which may lead to higher order scattering), we use the tips made of silicon (Nanosensors, PointProbe Plus; the radii of curvature are ∼10 nm). Conically shaped tip is dominantly polarizable along the tip-axis (z-axis), sampling mostly the Ez-component (perpendicular to the sample plane) of the Eloc. The polarization direction of the excitation beam is controlled by a half-wave plate. A quarter-wave plate placed in the reference arm of the interferometer controls the linear polarization of the reference field. The polarization of the reference field is set such that only z-component of the Escat is selectively amplified and detected by the detector. The far-field background is rejected by the third harmonic demodulation via a lock-in amplifier to give separate intensity (|s3|2) and phase (ϕ3) information on the scattered field. The sSNOM intensity (|s3|2) and phase (ϕ3) images, which, for our purpose, is the same as the map of the |Eloc,z (x, y)|2 (where x and y are the lateral position of sample) and the ϕz, the phase-shift of Eloc,z(x, y), with respect to the excitation field, E0, respectively. The images are acquired by raster-scanning the sample and recording the optical (sSNOM) and topographic (AFM) signal simultaneously. The simulated spectra and local electric field distributions are calculated by the finite-difference time-domain (FDTD) method using the FDTD-Solutions software package by Lumerical Inc. with the bulk dielectric constants of gold and silver by Johnson and Christy.28 For the analysis of octupolar resonances of AgAuAg-NR, we make a correction to the bulk dielectric constants of Ag and Au with an additional ad hoc damping term (see below). The presence of substrates (ITO/ glass and SiO2/Si) is explicitly taken into account in the simulation.

Figure 2c shows a set of longitudinal scattering spectra of long AgAuAg-NRs with a range of total lengths (Ltot = 416− 520 nm), displaying a major resonance (λmax) that systematically red-shifts with increasing Ltot (see Figure 2g, upper panel). The spectral shift could be fitted to a linear equation: Ltot = 1.1λmax − 315 (units in nm). At the same time, the λmax is found to be nearly uncorrelated to the relative compositions of the NRs (middle panel of Figure 2g, LAu/Ltot). Linear Ltot − λmax relation above is similar to the ones for octupolar modes of monolithic Ag-NRs29 (Ltot = 0.95λmax − 229) and is also consistent with the Fabry-Pérot type standing wave condition for octupolar charge oscillation (Ltot = mλmax/2, m = 3), again suggesting that the octupolar plasmons of AgAuAg-NR oscillate along the entire length of NR, uninfluenced by the Ag−Au boundaries. The dark-field spectra of short AgAuAg-NRs show only the longitudinal dipole resonances, whereas those of long AgAuAg-NRs show only the longitudinal octupolar resonances. For the long AgAuAg-NR (Ltot ∼ 500 nm), longitudinal dipole resonance is expected to be found λmax ∼ 1.8 μm (see Supporting Information, Figure S1, for the simulated spectra), which is out of spectral window of our measurement. For short AgAuAg-NR (Ltot ∼ 200 nm), octupole resonance, which is expected to occur at λmax < 400 nm, according to Fabry-Pérot resonance condition, is strongly damped by the interband transition of gold. For these reasons, we were able to observe only the octupole mode for long NR, and dipole mode for short NR, respectively. It is important to note that two different regimes of plasmon resonance exist for NRs in general:30,31 for very small NRs with the length (L) and diameter (D) much smaller than the wavelengths (L, D < 100 nm), electrostatic Mie-Gans theory applies, and thus, the λmax is believed to be governed by the aspect ratio (L/D). On the other hand, for relatively large NRs (with L ∼ wavelength and D > skin depth of silver or gold), Fabry-Pérot standing-wave condition primarily governs the resonance, and hence, the λmax is governed by L, not by L/D. Marginal dependence of material’s dielectric function arises from the fact that the end-facets of the NRs are not perfect reflectors (as in macroscopic laser cavities), causing minor material-specific variations in phase-shifts and effective cavity lengths. The longitudinal resonances of our AgAuAg-NRs belong to the latter case, which is further corroborated by clear linear λmax ∼ Ltot correlation (Figure 2g, upper panel), and poor (or no) λmax ∼ Ltot/D correlation (Figure 2g, bottom panel). We do not have a sufficient experimental data to fully discuss the transverse modes of AgAuAg-NR in detail. Nevertheless, we have carried out electrodynamics simulation (see Supporting Information, Figure S2) to examine the transverse coupling between the Ag and Au units in AgAuAg-NRs. Unlike longitudinal resonance spectra, in which entirely new resonances appear due to strong conductive coupling between the Ag and Au units, transverse resonance spectra largely retain spectral characteristics of separate Ag and Au units because of weak dipolar coupling. In transverse mode, three dipoles (two Ag and one Au units) are placed side-by-side, and they oscillate in-phase, leading to π*-type plasmon hybridization (which is similar to the plasmon mode of a nanoparticle dimer that is transversely excited). Such π*-type plasmon hybridization results in a major resonance peak that is slightly blue-shifted (∼10 nm) with respect to that of the transverse dipolar peak of Ag subunits. The degrees of blue-shift, as well as the relative intensities of Ag and Au peaks, change with compositional variation.

3. RESULTS AND DISCUSSION 3.1. Far-Field Scattering Spectra of AgAuAg Nanorods. Figure 2a,b compare the longitudinal and transverse scattering spectra obtained from a short AgAuAg-NR (Ltot = 230 nm; LAu = 130 nm; D = 92 nm) and a monolithic Au-NR with a comparable overall dimension (Ltot = 240 nm; D = 115 nm). Most strikingly, the longitudinal spectra (polarized along the NR axes) of the two NRs are fairly similar in terms of resonance wavelengths (λmax ∼ 800 nm) and the resonance widths (Δλmax ∼ 160 nm). The transverse spectra (polarized perpendicular to NR axes) of the two NRs are also similar, but in this case the AgAuAg-NR shows slightly broader resonance width than that of Au-NR, possibly due to the overlapping transverse dipolar resonances of weakly coupled Ag and Au subunits (see below for more detailed discussion). The electrodynamics simulation (Figure 2d,e), which is based on bulk dielectric constants satisfactorily reproduce the spectral features. The simulation somewhat overestimates the spectral difference between the transverse modes of Au-NR and AgAuAg-NR. This may arise from the NR-to-NR variation in the shape of end-facets of NRs, which has not been fully taken into account in the simulation (in the simulation, the same decahedral shapes are assumed for AgAuAg-NR and Au-NR). The spectral features, together with the sSNOM measurement (see below), allows us to assign the longitudinal peaks of AgAuAg-NR and Au-NR as the dipolar plasmon modes, and the spectral similarity strongly suggests that the dipolar plasmon oscillates along the entire length of NR, nearly unhindered by the Ag−Au boundaries. 21085

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Figure 3. sSNOM images of a short AgAuAg-NR and a monolithic Au-NR obtained with the excitation light at λex = 633 nm. (a) Illumination direction (black arrows) and the polarization (blue arrows). (b, f) AFM topography. (c, g) sSNOM intensity (|s3|2) images. (d, h) sSNOM phase (ϕ3) images. (e, i) Line profiles of sSNOM intensity and (red) and phase (blue) sampled along the lines indicated in (c), (d), (g), and (h). The double-sided arrows in (c), (d), (g), and (h) represent the excitation polarization directions. (j−o) Corresponding FDTD simulation of the intensity (j, m) and phase (k, n) of Eloc,z profiles. The simulated Eloc,z is sampled at a curved conformal surface that is 5 nm above the top surfaces of NRs. White dashed lines in (j) and (m) sketches the shapes of the NRs, and red dashed lines show the Ag−Au boundaries. The scale bars in experimental sSNOM images and the simulation correspond to 100 nm.

Upon closer examination, we also find that the resonance widths are unusually broad (see Figure 2h, Δλ = 100−160 nm) and nearly symmetric: normal octupolar resonances of monolithic NRs have narrow line widths of Δλ ∼ 50 nm, which is caused by the reduced radiative damping rates as compared with those of dipolar resonances. Additionally, such octupolar resonances have Fano-type asymmetrical lineshapes,29 which is caused by the interference of dipolar and octupolar eigen-modes. The electrodynamics simulation based on bulk dielectric constants28 (see Figure 2f) does reproduce the trends of resonance wavelength shifts with increasing Ltot, yet it fails to reproduce the observed widths and shapes (see Figure 2h for comparison). This suggests that octupolar longitudinal plasmons of AgAuAg-NRs are significantly damped by the Ag−Au interfaces, possibly by the interfacial electron scattering.7,13−15 As shown in Figure 2h, the observed line widths and lineshapes could be reproduced by the electrodynamics simulation based on modified dielectric functions incorportating an ad hoc damping term in the intraband components of the bulk dielectric constants.7,32

Overall, spectral similarities of AgAuAg-NR and Au-NR with comparable dimensions, and the Ltot-dependence of the resonance wavelengths strongly suggest that the longitudinal plasmons (dipolar and octupolar) of the AgAuAg-NRs are mostly determined by the overall length of the NR. At the same time, the spectral widths indicate non-negligible damping by the Ag−Au interfaces. 3.2. Near-Field Distributions of Plasmon Modes of AgAuAg Nanorods. To further characterize the longitudinal plasmons of AgAuAg-NR, we have directly imaged the plasmonic field distributions around the NRs using sSNOM. Figures 3a-i compare the sSNOM images of a short AgAuAgNR (Ltot = 220 nm; D = 100 nm) and a Au-NR (Ltot = 200 nm, D = 100 nm; see AFM topography images in Figure 3b,f). An spolarized (in-plane) light at λex = 632 nm, which is nearresonant with the longitudinal modes of the NRs, excites the longitudinal plasmons and the Eloc,z vector component is imaged. The sSNOM intensity maps of a short AgAuAg-NR and a Au-NR (Figure 3c,g,e) show enhanced field intensities at the 21086

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Figure 4. sSNOM images of a long AgAuAg-NR (Ltot = 530 nm) obtained with the excitation light at λex = 633 nm. (a) AFM topography. (b, c) sSNOM intensity (|s3|2) and phase (ϕ3) images of the longitudinally excited NR. (d) sSNOM intensity (red), phase (blue), and topography (dashed black line) line profiles sampled along the NR-axis (shown in green dashed lines in (a)−(c)). (e, f) sSNOM intensity and phase images of the same NR transversely excited (perpendicular to the sample plane), respectively. (g) Line profiles of sSNOM images (e, f) sampled along the green dashed lines in (e, f). (h−m) Corresponding simulated local field distribution (intensity = |Eloc,z|2 and phase = ϕz) of AgAuAg-NR calculated by FDTD method. (n−s) Simulated local field distributions of a monolithic Au-NR with the same overall dimension. The scale bars (white horizontal lines) correspond to 100 nm length scales.

D = 110 nm) that is longitudinally (Figure 4a−d) and transversely (Figure 4e−g) excited with a light at λex = 633 nm (resonant with features around λmax = 600−700 nm in Figure 2c). The longitudinal sSNOM intensity map (Figure 4b) reveals eight intensity maxima (hotspots). By comparing with the topography, we recognize that four of the hotspots are positioned along the top edge line of pentagonal cross sections, and the other four are positioned at one of the two side-edges of the pentagon. Considering only the pentagonal cross sectional shapes of NRs, we expect to see a total of 12 hotspots (four along the top edge line and eight along the two

ends of the NRs, and the corresponding phase images (see Figure 3d,h,e) show a 180° phase-shift across the centers of the NR (the 180° phase shift in Ez-vector correspond to the electric-field vector pointing up and down with respect to the sample plane, and thus indicates the positive and negative instantaneous charge distributions on either sides), revealing characteristic dipolar longitudinal field distributions for the two NRs. The simulated Eloc,z distributions shown in Figure 3j−o qualitatively reproduce the features observed by sSNOM. Figure 4 displays a set of sSNOM images of a long AgAuAgNR (Ltot = 530 nm, as determined from the AFM topography; 21087

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Figure 5. (a) Real (solid) and imaginary (dotted) parts of bulk dielectric functions28 of Au (yellow) and Ag (blue) plotted as a function of wavelength. (b) Simulated far-field spectrum of a AgAuAg-NR (Ltot = 520 nm, LAu = 160 nm, substrate = glass), showing octupolar resonance (ii, λ = 607 nm) and a higher order resonance (i, λ = 441 nm). The inset images are the local field distribution of AgAuAg-NR at the resonance positions (i) and (ii). Vertical red lines denote the Ag−Au boundaries. Shaded region in (a) and (b) indicates the wavelength regions showing appreciable dielectric mismatch.

substantiated below. By comparing the boundary positions (red vertical lines in Figure 4a, b, and e) with the locations of intensity hotspots in longitudinal sSNOM images (Figure 4b), we find that the longitudinal hotspot positions do not coincide with the positions of Ag−Au boundaries. This further indicates that the longitudinal octupolar mode is largely uncoupled from the Ag−Au boundaries. The electrodynamics simulation (Figure 4h−s) not only reproduces the features of sSNOM images of AgAuAg-NR, but it also displays the close similarities of plasmon modes of Au-NR and AgAuAg-NR. In addition, the field intensity of the transversely excited AgAuAg-NR (Figure 4k,m) reproduces the weaker hotspots at the Ag−Au boundaries, substantiating the interpretation of the features in sSNOM images shown in Figure 4g. Overall, the sSNOM images in Figures 3 and 4 clearly reveal that both the longitudinal dipolar and octupolar plasmon modes of AgAuAgNR are nearly unmodified by the Ag−Au boundaries. The far-field spectra and the local field distributions demonstrate that the longitudinal plasmons of AgAuAg-NRs are fairly insensitive to the relative compositions of Ag and Au, even though there exists some degree of damping at the Ag−Au interfaces. We believe that such insensitivity arises from the similarity of the dielectric functions (especially the imaginary part) of Ag and Au near the longitudinal resonance wavelengths (λ > 600 nm, see Figure 5a) and from the nature of Fabry-Pérot standing-wave condition. In fact, at shorter wavelengths (λ < 500 nm), in which imaginary parts of dielectric functions are significantly mismatched, we predict that the AgAuAg-NRs support plasmon modes that are highly confined to the Ag subunits. As an example, we show in Figure 5b the local field distribution of a longitudinal mode (i) at λ = 441 nm, showing a shape that is distinct from a regular octupolar field distribution (ii). Such plasmon mode will be highly dependent

side-edges of pentagon). Missing (or weak) four hotspots and the overall field asymmetry perpendicular to the NR axis (yaxis) arise from the illumination retardation effect33 (i.e., finite excitation phase gradient experienced by an NR along the laser propagation direction, denoted as k⃗ in Figure 4), not from tipartifact or from substrate perturbation. Corresponding phase image (Figure 4c) further shows 180° phase shifts between the neighboring hotspots along the NRaxis. Such features, which are summarized in the line-profile in Figure 4d, correspond to octupolar plasmon oscillation along the entire length of the NR. We also note that the phase spans +180° ∼ −180° range instead of +90° ∼ −90°, as is observed for dipolar field profiles (for example, see Figure 3e,i). This is unrelated to the nature of AgAuAg-NR. Rather, this mainly arises from the contribution of dipolar field component in the local field measured at λ = 633 nm (Fano-type mixing34): For the NR with Ltot ∼ 500 nm, we expect a strong dipolar resonance appearing at λ = 1.8 μm. At the wavelength λ = 633 nm, there is still a residual dipolar field component, resulting in the field pattern that is a coherent superposition of the octupolar and dipolar field patterns, with a nonzero “mixingangle” (i.e., phase-shift between the two modes) between the two modes. This leads to the retarded oscillation of the plasmon modes observed. Figure 4e−g show the sSNOM images of the same AgAuAgNR that is transversely excited (along z-axis), showing a plasmon mode that arise from the weak coupling of Ag and Au subunits. The intensity image (Figure 4e) shows two hotspots at the two top vertexes of the NR, simply reflecting electrostatic lightning-rod effect of sharp edges. Additionally, we also observe two minor hotspots (see asterisk marks in Figure 4g) along the top-edge line of the NR. We attribute the features as the hotspots arising from Ag−Au boundaries, which will be 21088

DOI: 10.1021/acs.jpcc.6b03813 J. Phys. Chem. C 2016, 120, 21082−21090

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The Journal of Physical Chemistry C

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upon the Ag/Au compositions and the states of Ag−Au interfaces. Likewise, we predict that highly damped metals (such as Pt or Ni) can be optically driven with a nanorod configuration, such as AgNiAg-NR, provided that the resonance wavelength is sufficiently long.

4. SUMMARY AND CONCLUSION To sum up, the far-field scattering spectra and the sSNOM images of longitudinally excited AgAuAg-NRs reveal that the longitudinal plasmons oscillate along the entire length of the NR without significant perturbation by the Ag−Au interfaces. The result constitutes an interesting counter-example of tunability of plasmons by composition variation and, thus, provides an important design rule for composition-tunable bimetallic plasmonic structures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03813. Figure S1: simulated longitudinal scattering spectra of AgAuAg-nanorods; Figure S2: simulated transverse scattering spectra of AgAuAg-nanorods (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ‡

Department of Energy Science, Sungkyunkwan University, Gyeonggi-do 440−746, Korea. ∥ Dow Chemical Korea, Seoul 135−728, Korea. # Department of Otolarynology, University of California San Francisco, CA 94115, U.S.A. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by BioNano Health-Guard Research Center funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea as Global Frontier Project (HGUARD_2013M3A6B2078947) and the Research Resettlement Fund for the New Faculty of SNU.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b03813 J. Phys. Chem. C 2016, 120, 21082−21090

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DOI: 10.1021/acs.jpcc.6b03813 J. Phys. Chem. C 2016, 120, 21082−21090