Surface Plasmon Coupling in Dimers of Gold Nanoparticles

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Surface Plasmon Coupling in Dimers of Gold Nanoparticles: Experiment and Theory for Ideal (Spherical) and Non-ideal (Faceted) Building Blocks Jun Hee Yoon, Florian Selbach, Ludmilla Schumacher, Jesil Jose, and Sebastian Schlücker ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01424 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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Surface Plasmon Coupling in Dimers of Gold Nanoparticles: Experiment and Theory for Ideal (Spherical) and Non-ideal (Faceted) Building Blocks Jun Hee Yoon,* Florian Selbach, Ludmilla Schumacher, Jesil Jose, and Sebastian Schlücker* Chair of Physical Chemistry I, Department of Chemistry, and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 45141 Essen, Germany KEYWORDS Dimer, gold nanosphere, faceted gold nanoparticle, surface plasmon coupling, gap morphology.

ABSTRACT The surface plasmon (SP) coupling in dimers of spherical and faceted gold nanoparticles is investigated experimentally and computationally at the single-particle level. Single ideal dimers of two spherical gold nanoparticles with a constant gap, filled by a self-assembled monolayer of 1,8-octanedithiol linker molecules, exhibit highly uniform dark-field scattering spectra. In contrast, single non-ideal dimers of two faceted gold nanoparticles with the same constant gap exhibit a high degree of spectral non-uniformity. We attribute this significant spectral heterogeneity to the many possible gap morphologies, i.e., the many possible configurations of the crystal facet orientations of the two faceted particles in non-ideal dimers. This configurational complexity is reduced in so-called hybrid dimers, which comprise an ideal spherical particle and a non-ideal

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faceted particle. Hybrid dimers have therefore been investigated theoretically and experimentally for revealing the influence of crystal facet orientation in the gap. Computed scattering spectra of ideal dimers (two spheres) and three different configurations of hybrid dimers (sphere and icosahedron) with face, edge, or point contact, respectively, are obtained using the finite-difference time-domain (FDTD) method. Theory predicts a blue-shift of the longitudinal bonding dipolar plasmon coupling mode for hybrid dimers with point and edge contact, but a red-shift for face contact due to the stronger SP coupling. Experimental dark-field (DF) scattering spectra of 62 individual hybrid dimers are measured for comparison with the predictions from theory. All singleparticle DF scattering spectra exhibit either a blue-shift (two distinct classes) or a red-shift relative to the corresponding ideal dimers, in agreement with the predictions from computer simulations.


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Surface plasmon (SP) coupling is the hybridization of discrete surface plasmon resonance (SPR) modes.1 This intriguing light-matter interaction is achievable by placing plasmonic nanostructures sufficiently close to each other and is controllable by changing the inter-particle gap distance.2 In theory, a dimer of two nanospheres separated by a constant gap is the simplest model to investigate SP coupling and it has been extensively investigated by computer simulations.3-7 In contrast, experimental research has long been lacking far behind due to the difficulties in preparing such ideal dimers of spheres and their subsequent characterization at the single-particle level.8-15 Due to the high structural uniformity of the ideal dimers, their experimental dark-field (DF) scattering spectra are highly uniform.16 Since the building blocks in ideal dimers are spherical, there is just a single geometrical configuration for the two spheres at a constant gap distance. This is fundamentally different for dimers of non-spherical, faceted particles due to their corners, edges, and faces. The relative orientation of corners, edges, and faces in the inter-particle junction of nonideal dimers creates various different gap morphologies. Few groups have reported on the role of gap morphology on the plasmonic properties of dimers12 including the nanoparticle-on-mirror (NPoM) system.17-19 Interestingly, for faceted gold nanoparticles (AuNP) on a gold film, a blueshift of the plasmon coupling band at longer wavelengths compared to that of gold nanospheres (AuNS) has been reported.19 The present work was therefore undertaken in order to systematically investigate the differences in plasmonic properties of ideal (two spheres), hybrid (sphere & faceted particle), and non-ideal dimers (two faceted particles) at the single-particle level.


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RESULTS AND DISCUSSION Dimers were prepared by a substrate-based sequential dimer assembly: the deposition of the first particle on a glass substrate, the addition of 1,8-octanedithiol (C8) for the formation of a selfassembled monolayer (SAM) on the surface of the first particle, and the conjugation of the second particle to the first particle. The characterization of the monomeric building blocks, i.e., AuNS and faceted AuNP, is shown in Figure S1 of Supporting Information. The assembly scheme of the dimers is shown in Figure S2 of Supporting Information.16 Dimers were characterized by singleparticle DF spectroscopy and transmission electron microscopy (TEM). Figure 1 shows the direct comparison of ideal dimers comprising AuNS (50.0 ± 2.5 nm) and non-ideal dimers comprising faceted AuNP (50.9 ± 6.1 nm). Ideal dimers exhibit a highly uniform geometrical structure (Figure 1a), which produces highly uniform DF scattering spectra at the single-particle level (Figure 1c). In particular, the position of the longitudinal bonding dipolar plasmon coupling (LBDP) band at longer wavelength is extremely regular. Although non-ideal dimers have the same C8 linker molecule and therefore the same gap distance (Figure 1b), the corresponding single-particle DF scattering spectra are very non-uniform (Figure 1d). The observed spectral irregularity in non-ideal dimers is attributed to the inhomogeneity of the gap morphology resulting from many different configurations of the opposing two faceted AuNP.12 Figure 1 shows representative DF scattering spectra from seven individual dimers of each class. For more reliable statistics, we analyzed DF scattering spectra from a larger number of ideal (N=19) and non-ideal dimers (N=46). Specifically, we determined the spectral position (λmax) of the LBDP band in each scattering spectrum and therefore obtained the corresponding histogram showing the distribution of λmax. The result is shown in Figure 2a. For ideal dimers (Figure 2a top), a narrow distribution (735 ± 6.7 nm) is observed. In contrast, for non-ideal dimers (Figure 2a bottom), a ca.


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three-times broader distribution (696 ± 20.8 nm) is obtained. This broadening is assigned to the spectral irregularity (Figure 1d), which is attributable to poor monodispersity of the faceted AuNP compared to the AuNS (see Figure S1c and d of Supporting Information). Most importantly, the mean of λmax is blue-shifted by 39 nm (Figure 2a). The blue-shift of the LBDP band position is clearly observable in the statistical analysis of the entire scattering spectrum recorded from all individual dimers (Figure 2b). In the mean scattering spectrum of the non-ideal dimers, the LBDP band is blue-shifted by 34 nm. Overall, this significant blue-shift of the LBDP band in the singleparticle DF scattering spectra of non-ideal dimers was rather surprising for us since the gap distance is the same as in the ideal dimers (ca. 1.3 nm20 assuming an all-trans conformation of the C8 linker16,21,22) and the average size of the building blocks is very similar (only 0.9 nm difference between the mean values). This blue-shift for non-ideal dimers may have different physical origins. Since the ~1.3-nm gap of both ideal and non-ideal dimers is filled with a non-conductive C8 SAM, we excluded quantum effects and aimed at an explanation using classical physics.23 Nonlocality, which is critical for small-sized systems like small particles24,25 or small gaps (< 1 nm),26,27 was also not considered. We hypothesized that the characteristic shift in the LBDP band is indicative of the specific gap morphology, which strongly influences the degree of SP coupling.12,28 For understanding the underlying physics in terms of SP coupling, we decided to perform finite-difference time-domain (FDTD) simulations on dimer model systems. An icosahedron with rounded corners and edges (Figure S4a of Supporting Information) was employed as a model for faceted AuNP since it has the maximum number of faces among the Platonic bodies and has been experimentally found in faceted AuNP grown by the citrate reduction method.29,30 We restricted our computational analysis to an ideal dimer model (two spheres) and the hybrid dimer model (one sphere and one icosahedron)


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for one major reason: The isotropic sphere in the hybrid dimer model eliminates the variations in both the relative orientation of the sphere with respect to the icosahedron and the dihedral angle between the sphere and the icosahedron. In contrast, a pair of identical icosahedra as a model for a non-ideal dimer has many different possible configurations, depending on both orientation and dihedral angle of the two opposing icosahedra. Overall, the hybrid system bridges the ideal case (two AuNS) and the non-ideal case (two faceted AuNP). We calculated the scattering spectra of four different classes of dimer models: the ideal dimer model with its zero-dimensional point contact (only one configuration possible) and three different configurations for the hybrid dimer model. Please note that in the following we employ the term contact for describing the gap morphology for the non-touching particles, and the gap distance is kept at 1 nm (Figure 3 and Figure 4) or at 1.25 nm (Figure 4), respectively. Figure 3a shows a top view on the three geometrical configurations of the hybrid dimer models (left) together with a view along the dimers axis from the sphere (right) in order to highlight the gap morphology; point contact (zero-dimensional), edge contact (one-dimensional), and face contact (two-dimensional) are color-coded in blue, green, and red, respectively. Figure 3b top left shows the simulated scattering spectra of the two individual building blocks (sphere versus icosahedron): the SPR band for the sphere is centered at 553 nm, while it is red-shifted by ca. 13 nm in the case of the icosahedron. This computational result is consistent with the experimental observation for DF scattering spectra from single AuNS and AuNP (Figure S1e of Supporting Information). Figure 3b bottom left shows the simulated scattering spectra of the ideal dimer model (two spheres, zerodimensional point contact) and the three different hybrid dimer models (one sphere and one icosahedron with zero-dimensional point, one-dimensional edge, or two-dimensional face contact, respectively). The ideal dimer model exhibits a LBDP band at 732 nm. In contrast, all hybrid dimer


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configurations exhibit different LBDP band positions due to the difference in the degree of the capacitive coupling strongly influenced by gap area or curvature.31,32 Specifically, both hybrid dimer models with point contact (blue spectrum) and edge contact (green spectrum) exhibit a blueshifted LBDP band. In contrast, the hybrid dimer model with face contact (red spectrum) exhibits a red-shifted LBDP band. Finally, Figure 3b right shows the corresponding plasmon hybridization diagram together with the monomeric building blocks (top) and gap morphology in the dimer model (bottom). Overall, the computer simulations predict both a blue-shift (two cases: point and edge contact) as well as a red-shift (one case: face contact) for the LBDP mode of hybrid dimer models (Figure 3b left bottom and right). Previous reports on assemblies with C8 linker molecules have assumed a gap distance of ca. 1.3 nm.20 Therefore, additional FDTD simulations with a gap distance of 1.25 nm were performed since this is close to assumed gap distance of 1.3 nm. The comparison of the results for the gap distances of 1 nm and 1.25 nm are shown in Figure 4. As expected, the longer gap distance resulted in a constant blue-shift of the LBDP band for all three hybrid dimer configurations, indicating a weaker SP coupling. Both series of computed scattering spectra show the same trend for the position of the LBDP band relative to that of the ideal dimers: for the face contact a red-shift in contrast to a smaller and larger blue-shift for edge and point contact, respectively. Individual hybrid dimers (Figure 5a) were characterized experimentally by single-particle DF scattering spectroscopy for testing predictions from the FDTD computer simulations. We recorded 62 DF scattering spectra of single hybrid dimers. All single-particle scattering spectra were found to be a member of one out of three different categories due to their similar LBDP peak positions as shown in Figure 5b and c. Based on the predictions of the FDTD simulations, the class of hybrid dimers which all exhibit a red-shifted LBDP band (771.5 ± 13.2 nm) in the corresponding single-


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particle scattering spectrum was assigned to the face contact case. Similarly, the other two classes of hybrid dimers which exhibit a blue-shifted LBDP band at (726.9 ± 5.6) nm and (707.7 ± 3.1) nm, respectively, were assigned to the edge contact case (smaller blue-shift) and point contact case (larger blue-shift), respectively. Please note that the experimentally observed differences in the mean LBDP band positions for the three classes of hybrid dimers are significantly larger than the corresponding standard deviations for each class. In other words, the inter-class variation is significantly larger than the intra-class variation. Again, we emphasize that the assignment of the three classes of hybrid dimers is only possible due to the predictions from theory. Overall, our findings support the idea that the gap morphology is indeed the most decisive factor in the observed trends for the LBDP band shifts.12,19,32 In addition to the effect of gap morphology discussed in the context of Figure 3, also other factors could, in principle, be responsible for the experimentally observed blue-shift in the DF scattering spectra of non-ideal dimers: Differences in monodispersity with respect to both i) size and ii) sphericity of the building blocks (highly monodisperse spherical AuNS versus faceted AuNP with broader size and sphericity distributions), the difference in iii) the stabilizing agent on the surface of the dimers ((11-mercaptoundecyl)-N,N,N-trimethylammonium bromide (MUTAB) and 11mercaptoundecanoic acid (MUA)). We therefore performed control simulations and experiments to test the relevance of the above mentioned factors. For probing i) the influence of the size of the building blocks, we performed FDTD computer simulations for comparing the ideal dimer model (two 50 nm spheres) with a size-asymmetric dimer model comprising a 50 nm and a 47.5 nm sphere as well as another size-asymmetric dimer model comprising a 50 nm and a 57.3 nm sphere (Figure S4i and j of Supporting Information). These diameters were chosen because of the different distances in a rounded icosahedron: 57.3 nm


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from corner to corner, 50.0 nm from edge to edge, and 47.5 nm from face to face (see Figure S4a of Supporting Information). As expected, the simulated scattering spectrum of the dimer model with the smaller 47.5 nm sphere exhibits a minor blue-shift of the LBDP band, while the dimer model with the larger 57.3 nm sphere exhibits a major red-shift. The average of both computed scattering spectra exhibits a red-shifted LBDP band (Figure S5a of Supporting Information), indicating that the experimentally observed blue-shift for non-ideal dimers is not due to differences in the size monodispersity of the building blocks. For probing ii) the influence of the sphericity of the building blocks, we compared the ideal dimer model (two 50 nm spheres) with a dimer model comprising a 50 nm sphere and an ellipsoid. The ellipsoid is tri-axial (57.3 nm longer axis, 50 nm middle axis, and 47.5 nm shorter axis) and can be arranged in different orientations relative to the sphere (Figure S4f-h of Supporting Information). None of the computed sphere-ellipsoid configurations exhibits a blue-shifted LBDP band (Figure S5b of Supporting Information), indicating that the experimentally observed blueshift is not due to differences in the shape of the building blocks. For experimentally testing iii) the influence of the stabilizing agent, we prepared ideal dimers stabilized by either MUTAB or MUA. Their corresponding UV-vis extinction spectra at the ensemble level almost look identical (Figure S3 of Supporting Information), indicating that the stabilizing agent does not influence the position or width of the LBDP band. Overall, neither the stabilizing agent nor the size or minor shape variations of the building blocks can account for the experimentally observed blue-shift of the LBDP mode in single-particle DF spectra for non-ideal dimers. Also a decrease or increase in the gap distance would only result in an overall constant red/blue-shift in the DF spectra.


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Similar experimental results have been recently reported for the NPoM geometry with AuNS and faceted AuNP on a gold film.19 In that work, faceted AuNP on a gold film produce highly nonuniform DF scattering spectra and the SP coupling band at longer wavelengths is blue-shifted compared to AuNS on a gold film. Their result based on a dimer model system is consistent with our experimental observations on non-ideal dimers. Our theoretical predictions and single-particle DF experiments on hybrid dimers provide unique insights into the SP coupling in real particlebased dimers, which is similar yet distinct from the NPoM geometry where image dipoles in the metal film are involved. Furthermore, in contrast to our real particle-based dimers, the NPoM geometry also does not allow experimentalists to directly observe the gap by electron microscopy. We employed scanning transmission electron microscopy (STEM) on hybrid dimers to directly observe the dimer gap (see Figure S6 of Supporting Information). STEM images of four individual hybrid dimers unambiguously show a small gap between the two particles (Figure S6a of Supporting Information). In an initial attempt to at least qualitatively investigate the gap morphology, we recorded STEM images at different tilting angles around the dimer axis (Figure S6b of Supporting Information). We observed minor differences in the two-dimensional cross sections: an initially narrow gap region, which can be potentially assigned to a point contact (Figure S6b bottom of Supporting Information), turns into a relatively broader gap region, which could be assigned to an edge or face contact (Figure S6b top of Supporting Information). At this stage, however, more conclusive interpretations are unfortunately not possible. Nevertheless, all five representative STEM images in Figure S6 of Supporting Information at least exclude the presence of a broad gap, which has previously been used as an argument for explaining the experimentally observed blue-shift in the NPoM geometry.19


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CONCLUSIONS In this work, we have shown that non-ideal dimers of faceted AuNP produce polydisperse LBDP positions which are blue-shifted compared with ideal dimers, although the average size of their building blocks (Δ < 1 nm) and the gap distance (ca. 1.3 nm) are the same. We propose that the gap morphology is the dominant factor influencing the degree of SP coupling in the classical regime where we exclude quantum effects. Negative control experiments and computer simulations investigating the role of stabilizing agents as well as the size, sphericity of the building blocks and gap distance support this conclusion. Computer simulations predict both blue- and redshifted LBDP bands for certain configuration in hybrid dimer models. These results are also supported by our single DF scattering experiments on hybrid dimers. Compared to the NPoM system with a particle and its image dipole, the presented dimers comprise two particles and allow the direct observation of the gap by electron microscopy. Our investigations emphasize that ideal and non-ideal dimers are not directly comparable and that an ensemble of non-ideal dimers does not reflect the properties of an ideal dimer. We believe that ideal dimers with their uniform structure and properties are ideally suited for precision plasmonics, e.g., for quantum plasmonics and surface-enhanced Raman scattering (SERS) which are extremely sensitive to gap properties such as gap distance and gap morphology. Last but not least, experimentally prepared ideal dimers provide unique access to the direct comparison with predictions from theory typically using idealized three-dimensional structures.


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METHODS Synthesis of AuNS and faceted AuNP. CTAB-capped AuNS (50.0 ± 2.5 mn) prepared via chemical etching of polyhedral particles and characterized in our previous report were used.16 Citrate-capped AuNP (50.9 ± 6.1 nm) were synthesized by the seeded growth method.33 In detail, a HAuCl4 solution (200 mM, 0.08 mL) was added into D.I. water (61 mL) at 80 °C, followed by the addition of a citrate solution (55.8 mM, 1.92 mL) at 100 °C. The reaction was continued until the color turned red which indicates the formation of seeds (15.1 nm in diameter). A portion of the prepared seed solution (61 mL) was diluted in D.I. water (10 mL) and heated. When the temperature reached 90 °C, the seeds were grown by the consecutive addition of D.I. water, citrate solution (60 mM), and HAuCl4 solution (25 mM) under vigorous stirring. The volumes of such additives and the reaction times are listed in the Supporting Information (Table S1). Assembly of Dimers. The overall dimer assembly scheme is illustrated in the Supporting Information (Figure S2 of Supporting Information). AuNS and AuNP adsorbed on glass substrate were employed as a starting material. Glass slides (25 mm × 12 mm) as substrate were cleaned for the adsorption of AuNS16,34 and were amine-functionalized for the adsorption of AuNP20,35 by using reported protocols, respectively. The cleaned glass slide (HO-glass) was immersed in 5 mL of a CTAB solution (5 μM) containing CTAB-capped AuNS (5 pM) for 17 h at 30 °C. This optimal concentration of the additional CTAB enhances the stability of CTAB-capped AuNS but not interrupt the interaction between CTAB-capped AuNS and HO-glass during the adsorption.34 The prepared amine-functionalized glass slide (NH2-glass) was immersed in 5 mL of a HCl solution (1 mM) containing citrate-capped AuNP (10 pM) for 15 h at room temperature. The protonation of NH2-glass by HCl ensures the interaction between citrate-capped AuNP and NH2-galss.22 Following assembly steps were carried out at 30 °C. In the assembly process, the glass slides after


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washing with the solvent used in the prior step and to be used in the next step were immediately transferred to the next step. Step 1: For forming a well-ordered linker SAM on the surface of particles, the glass slides where AuNS and AuNP are electrostatically adsorbed were immersed in the mixture of an ethanolic C8 solution (1 mM, 5 mL) and an aqueous NaBr solution (250 mM, 20 μL) for 1 h. The combination of EtOH and NaBr effectively destroys the robust CTAB layer on AuNS which critically hinders the formation of Au-S bond.16 Step 2: For the synthesis of the ideal dimers, the glass slide coated with AuNS was soaked into 5 mL of acetonitrile containing CTAB-capped AuNS (20 pM) and NaBr (200 μM) for 5 h. For the stability of suspended CTABcapped AuNS during dimerization, EtOH was exchanged with acetonitrile and NaBr concentration was lowered.16 For the synthesis of the hybrid dimers, the glass slide coated with AuNS was immersed into 5 mL of water containing citrate-capped AuNP (20 pM) for 5 h. For the synthesis of the non-ideal dimers, the glass slide coated with AuNP was dipped into 5 mL of ethanol containing citrate-capped AuNP (150 pM) for 3.5 h. The added particles for the dimer formation do not interact with those glass slides (HO-glass and NH2-glass) under such types of solvents. Step 3: For stabilizing dimers, the glass slides were immersed in the mixture of an ethanolic MUTAB solution (1 mM, 5 mL) and an aqueous NaBr solution (250 mM, 20 μL) for 1 h. The use of negatively charged MUA instead of positively charged MUTAB also stabilizes dimers but produces negatively charged dimers. Step 4: Dimers were dispersed in solvent medium by sonication of the glass slides for 30 s. For further stability of dimers, the area newly exposed after the detachment from glass slide should be stabilized with capping agents used in step 3. Hence, ethanol containing MUTAB or MUA (10 μM, 5 mL) was employed as a dispersion medium. Characterization. In order to characterize the dimers and their monomeric constituents on the glass substrate and in the solutions, we employed TEM (Zeiss, EM 910), STEM (JEOL, JEM-


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2200FS), UV-vis spectroscopy (Jasco, V-630), and single-particle DF spectroscopy with a homebuilt DF microscope.16 Raw DF scattering spectra were smoothed by a Savitzky-Golay filter. Computer Simulations. Simulated scattering spectra of monomers and dimers thereof having 1 nm gap distance were carried out using the program package FDTD Solutions developed by Lumerical Solution, Inc. A simulation model in the override region (0.5 or 0.25 nm mesh) was illuminated by a linearly polarized total-field scattered-field plane wave source (400–900 nm). For the dielectric constant of gold, a polynomial fitting of the experimental data obtained by Johnson and Christy was used. The effective refractive index of the surrounding medium was set to 1.45. The FDTD simulation started with the development of building block models (Figure S4a of Supporting Information). We developed an icosahedron whose corners and edges are rounded (curvature diameter = 8 nm) to represent faceted (but not that sharp) AuNP. In order to define the size of the icosahedron which is comparable to a 50 nm sphere (representing AuNS), the length of edge-to-edge (from an edge to the opposite edge) was set at 50 nm. Then, we built the aforementioned three configurations of the hybrid dimer model comprising the icosahedron and the sphere (Figure 3a and Figure S4c-e of Supporting Information). In order to test the effect by anisotropy of the icosahedron, the scattering spectra were simulated by applying the linearly polarized plane wave source towards either the top or the front of the override region where a simulation model is centered (Figure S4b of Supporting Information). We checked that simulated scattering spectra with two differently directed wave source are almost identical. It indicates that the effect from the anisotropy of the rounded icosahedron is negligible. In this manuscript, scattering spectra simulated with the wave source towards the top of the override region are shown. An ideal dimer model of the 50 nm spheres was simulated for comparison. Monomer models


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remained after removing the sphere in dimer models were also simulated for plasmon hybridization diagram.


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ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. More detailed information (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation; project number YO 237/1-1 and project number 278162697 - SFB 1242 Nonequilibrium dynamics of condensed matter in the time domain, project A04, and INST20876/2018-1 FUGG) is acknowledged.


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REFERENCES (1) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302, 419-422. (2) Nordlander, P.; Oubre, C.; Prodan, E.; Li, K.; Stockman, M. I. Plasmon Hybridization in Nanoparticle Dimers. Nano Lett. 2004, 4, 899-903. (3) Xu, H.; Aizpurua, J.; Käll, M.; Apell, P. Electromagnetic Contributions to Single-Molecule Sensitivity in Surface-Enhanced Raman Scattering. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 62, 4318-4324. (4) Zuloaga, J.; Prodan, E.; Nordlander, P. Quantum Description of the Plasmon Resonances of a Nanoparticle Dimer. Nano Lett. 2009, 9, 887-891. (5) Bachelier, G.; Russier-Antoine, I.; Benichou, E.; Jonin, C.; Del Fatti, N.; Vallée, F.; Brevet, P.-F. Fano Profiles Induced by Near-Field Coupling in Heterogeneous Dimers of Gold and Silver Nanoparticles. Phys. Rev. Lett. 2008, 101, 197401. (6) Dahmen, C.; Schmidt, B.; von Plessen, G. Radiation Damping in Metal Nanoparticle Pairs. Nano Lett. 2007, 7, 318-322. (7) Besteiro, L. V.; Govorov, A. O. Amplified Generation of Hot Electrons and Quantum Surface Effects in Nanoparticle Dimers with Plasmonic Hot Spots. J. Phys. Chem. C 2016, 120, 1932919339. (8) Weller, L.; Thacker, V. V.; Herrmann, L. O.; Hemmig, E. A.; Lombardi, A.; Keyser, U. F.; Baumberg, J. J. Gap-Dependent Coupling of Ag-Au Nanoparticle Heterodimers Using DNA Origami-Based Self-Assembly. ACS Photonics 2016, 3, 1589-1595. (9) Lerch, S.; Reinhard, B. M. Quantum Plasmonics: Optical Monitoring of DNA-Mediated Charge Transfer in Plasmon Rulers. Adv. Mater. 2016, 28, 2030-2036. (10) Vilar-Vidal, N.; Bonhommeau, S.; Talaga, D.; Ravaine, S. One-Pot Synthesis of Gold Nanodimers and Their Use as Surface-Enhanced Raman Scattering Tags. New J. Chem. 2016, 40, 7299-7302. (11) Wustholz, K. L.; Henry, A.-I.; McMahon, J. M.; Freeman, R. G.; Valley, N.; Piotti, M. E.; Natan, M. J.; Schatz, G. C.; Van Duyne, R. P. Structure-Activity Relationships in Gold Nanoparticle Dimers and Trimers for Surface-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2010, 132, 10903-10910. (12) Popp, P. S.; Herrmann, J. F.; Fritz, E.-C.; Ravoo, B. J.; Höppener, C. Impact of the Nanoscale Gap Morphology on the Plasmon Coupling in Asymmetric Nanoparticle Dimer Antennas. Small 2016, 12, 1667-1675. (13) Cha, H.; Yoon, J. H.; Yoon, S. Probing Quantum Plasmon Coupling Using Gold Nanoparticle Dimers with Tunable Interparticle Distances Down to the Subnanometer Range. ACS Nano 2014, 8, 8554-8563. (14) Marhaba, S.; Bachelier, G.; Bonnet, C.; Broyer, M.; Cottancin, E.; Grillet, N.; Lermé, J.; Vialle, J.-L.; Pellarin, M. Surface Plasmon Resonance of Single Gold Nanodimers near the Conductive Contact Limit. J. Phys. Chem. C 2009, 113, 4349-4356. (15) Busson, M. P.; Rolly, B.; Stout, B.; Bonod, N.; Larquet, E.; Polman, A.; Bidault, S. Optical and Topological Characterization of Gold Nanoparticle Dimers Linked by a Single DNA Double Strand. Nano Lett. 2011, 11, 5060-5065. (16) Yoon, J. H.; Selbach, F.; Langolf, L.; Schlücker, S. Ideal Dimers of Gold Nanospheres for Precision Plasmonics: Synthesis and Characterization at the Single-Particle Level for Identification of Higher Order Modes. Small 2018, 14, 1702754.


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(17) Benz, F.; Chikkaraddy, R.; Salmon, A.; Ohadi, H.; de Nijs, B.; Mertens, J.; Carnegie, C.; Bowman, R. W.; Baumberg, J. J. SERS of Individual Nanoparticles on a Mirror: Size Does Matter, but So Does Shape. J. Phys. Chem. Lett. 2016, 7, 2264-2269. (18) Kleemann, M.-E.; Mertens, J.; Zheng, X.; Cormier, S.; Turek, V.; Benz, F.; Chikkaraddy, R.; Deacon, W.; Lombardi, A.; Moshchalkov, V. V.; Vandenbosch, G. A. E.; Baumberg, J. J. Revealing Nanostructures through Plasmon Polarimetry. ACS Nano 2017, 11, 850-855. (19) Huh, J.-H.; Lee, J.; Lee, S. Comparative Study of Plasmonic Resonances between the Roundest and Randomly Faceted Au Nanoparticles-on-Mirror Cavities. ACS Photonics 2018, 5, 413-421. (20) Yoon, J. H.; Lim, J.; Yoon, S. Controlled Assembly and Plasmonic Properties of Asymmetric Core-Satellite Nanoassemblies. ACS Nano 2012, 6, 7199-7208. (21) Wadayama, T.; Oishi, M. Surface-Enhanced Raman Spectral Study of Au NanoParticles/Alkanethiol Self-Assembled Monolayers/Au(1 1 1 ) Heterostructures. Surf. Sci. 2006, 600, 4352-4356. (22) Lee, D.; Yoon, S. Gold Nanocube-Nanosphere Dimers: Preparation, Plasmon Coupling, and Surface-Enhanced Raman Scattering. J. Phys. Chem. C 2015, 119, 7873-7882. (23) Tan, S. F.; Wu, L.; Yang, J. K. W.; Bai, P.; Bosman, M.; Nijhuis, C. A. Quantum Plasmon Resonances Controlled by Molecular Tunnel Junctions. Science 2014, 343, 1496-1499. (24) Scholl, J. A.; Koh, A. L.; Dionne, J. A. Quantum Plasmon Resonances of Individual Metallic Nanoparticles. Nature 2012, 483, 421-427. (25) Piella, J.; Bastús, N. G.; Puntes, V. Size-Controlled Synthesis of Sub-10-Nanometer CitrateStabilized Gold Nanoparticles and Related Optical Properties. Chem. Mater. 2016, 28, 10661075. (26) Mortensen, N. A.; Raza, S.; Wubs, M.; Søndergaard, T.; Bozhevolnyi, S. I. A Generalized Non-Local Optical Response Theory for Plasmonic Nanostructures. Nat. Commun. 2014, 5, 3809. (27) Esteban, R.; Zugarramurdi, A.; Zhang, P.; Nordlander, P.; García-Vidal, F. J.; Borisov, A. G.; Aizpurua, J. A Classical Treatment of Optical Tunneling in Plasmonic Gaps: Extending the Quantum Corrected Model to Practical Situations. Faraday Discuss. 2015, 178, 151-183. (28) Barbry, M.; Koval, P.; Marchesin, F.; Esteban, R.; Borisov, A. G.; Aizpurua, J.; SánchezPortal, D. Atomistic Near-Field Nanoplasmonics: Reaching Atomic-Scale Resolution in Nanooptics. Nano Lett. 2015, 15, 3410-3419. (29) Barnard, A. S.; Young, N. P.; Kirkland, A. I.; van Huis, M. A.; Xu, H. Nanogold: A Quantitative Phase Map. ACS Nano 2009, 3, 1431-1436. (30) Scott, M. C.; Chen, C.-C.; Mecklenburg, M.; Zhu, C.; Xu, R.; Ercius, P.; Dahmen, U.; Regan, B. C.; Miao, J. Electron Tomography at 2.4-Ångström Resolution. Nature 2012, 483, 444-447. (31) Knebl, D.; Hörl, A.; Trügler, A.; Kern, J.; Krenn, J. R.; Puschnig, P.; Hohenester, U. Gap Plasmonics of Silver Nanocube Dimers. Phys. Rev. B 2016, 93, 081405(R). (32) Huang, Y.; Chen, Y.; Wang, L.-L.; Ringe, E. Small Morphology Variations Effects on Plasmonic Nanoparticle Dimer Hotspots. J. Mater. Chem. C 2018, 6, 9607-9614. (33) Bastús, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing Versus Ostwald Ripening. Langmuir 2011, 27, 11098-11105. (34) Guo, L.; Zhou, X.; Kim, D.-H. Facile Fabrication of Distance-Tunable Au-Nanorod Chips for Single-Nanoparticle Plasmonic Biosensors. Biosens. Bioelectron. 2011, 26, 2246-2251.


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(35) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Self-Assembled Metal Colloid Monolayers: An Approach to SERS Substrates. Science 1995, 267, 1629-1632.


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Figure 1. Comparison of ideal (left) and non-ideal (right) dimers linked by a SAM of C8. TEM images (top) and single-particle DF scattering spectra (bottom). Black dash lines are given for eye guidance.


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Figure 2. Statistical analysis showing the blue-shift tendency of the LBDP mode in non-ideal dimers compared to ideal dimers. (a) Distributions of the LBDP band positions. (b) Averaged DF scattering spectra of ideal and non-ideal dimers. The gray lines represent one standard deviation. Black dash lines are drawn at λmax of the LBDP band.


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Figure 3. (a) Three types of hybrid dimer configurations for FDTD simulation. In the view from the sphere, the gap region on the rounded icosahedron is colored to represent the gap morphology. (b) FDTD simulation results of ideal and hybrid dimer models including their building block models (left), and the corresponding plasmon hybridization diagram (right). Color code for hybrid dimer models: blue for point contact; green for edge contact; red for face contact. The ideal dimer model is color-coded in black.


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Figure 4. Computed scattering spectra for hybrid dimers (50 nm Au sphere and 50 nm rounded icosahedron with face, edge, or point contact) with 1.00 nm gap (top) and 1.25 nm gap (bottom), respectively. The data for ideal dimers (two 50 nm Au spheres) is also shown for comparison.


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Figure 5. (a) Representative experimental single-particle DF scattering spectra and the categorized LBDP band positions of hybrid dimers (50 nm AuNS and 50 nm AuNP). Three distinct classes of spectra were identified based on the position of the LBDP mode at (771.5 ± 13.2) nm, (726.9 ± 5.6) nm, and (707.7 ± 3.1) nm, respectively. They are assigned to hybrid dimers with face, edge, and point contact, respectively. (b) TEM image of hybrid dimers.


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Surface Plasmon Coupling in Dimers of Gold Nanoparticles

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