Comparative Study of Plasmonic Resonances between the Roundest

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Comparative Study of Plasmonic Resonances between the Roundest and Randomly Faceted Au Nanoparticles-on-Mirror Cavities Ji-Hyeok Huh, Jaewon Lee, and Seungwoo Lee ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00856 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Comparative Study of Plasmonic Resonances between the Roundest and Randomly Faceted Au Nanoparticles-on-Mirror Cavities Ji-Hyeok Huh1,+, Jaewon Lee1,+, and Seungwoo Lee1,2* 1

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

2

Department of Nano Engineering & School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea *Email: [email protected]

Keywords: Nanoparticle-on-mirror cavity, Plasmonic nanogap, Antenna mode, Waveguide mode, Dark-field spectral reliability +

Equally contributed to this work

Abstract: Over the last decade, the synthesis of a relatively large-sized (> 50 nm), spherical gold nanoparticles (Au NPs) has undergone significant progress, from the initial demonstration of the hydroquinone-mediated synthesis of randomly faceted Au nanospheres (NSs) to iterative growth and dissolution of highly spherical and ultrasmooth Au NSs. The iterative growth and dissolution method can synthesize the roundest Au NSs. However, the roundest Au NSs have not been used in nanoparticle-on-mirror (NPoM) cavities; thus, the effects of Au NS roundness and facets on the plasmonic resonance of an NPoM cavity needs to be understood. In this work, we synthesized the Au NSs of the same size but with different facets and used them in NPoM cavities. Using these plasmonic models, we systematically compared round and randomly faceted Au NSs in terms of plasmonic resonance and spectral reliability. On the basis of these experimental results, we theoretically defined the accessible plasmonic mode volume with an NPoM cavity.

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In 1982, a simple but highly efficient nanophotonic system, this is, metallic nanoparticles (NPs) on a flat metallic mirror (simply referred to as a nanoparticle-on-mirror (NPoM) cavity), was suggested to robustly reach the limits of plasmonic enhancement.1,2 For example, spherical metallic NPs on a flat metallic mirror can form a point-like space (< 1 nm3), and the plasmonic field can be resonantly squeezed and enhanced within this ultrasmall volume via antenna mode (i.e., the electric dipole within the NPs oscillates vertically with respect to the mirror, referred to as the longitudinal dipole plasmon).1–6 Using colloidal metallic nanocubes (NCs) rather than nanospheres (NSs) can result in additional plasmonic resonance by which the dipoles or multipoles oscillate laterally within the interfacial gap between NCs and the mirror (i.e., transverse gap plasmon via waveguide mode).6,7 Since this system was first described, the ease and robustness of forming an ultrasmall mode volume by placing metallic colloids on a flat metal film have made NPoM an important tool to enable various extreme nano-optics, such as enhancing spontaneous emission, coupling singlemolecular excitons to plasmons, surface-enhanced Raman scattering (SERS), plasmonic color pixels, and single-molecule optomechanics.8–18 Nevertheless, the practical applications of NPoM for nano-optics were compromised by heterogeneity of the metallic colloids. For example, the relatively large gold (Au) and silver (Ag) NSs (> 50 nm), used in NPoM research so far, were randomly faceted rather than truly spherical.4,9,13–16,19–22 Imperfect control over the nanogap resulted in nondeterministic plasmonic modes and hindered reliable studies on the ultimate limit of plasmonic enhancement. Furthermore, common methods to visualize the nanogap morphology of the NPoM cavities, such as scanning electron microscopy (SEM), cannot access the region beneath NP. Thus, reliably and systematically investigating the associated plasmonic modes is difficult. Recently, iterative growth and dissolution synthesis has been suggested to dramatically improve the roundness of Au NSs.23–28 Thus, the question arises whether the roundest Au NSs can indeed address the plasmonic nanogap challenges of the NPoM cavities and enhance spectral reliability. We here comparatively studied NPoM resonant properties between the roundest and randomly faceted Au NSs of the same size (60 nm). Both the roundest and randomly faceted Au NSs were chemically synthesized with a high yield and then placed on an atomically flat Au mirror to form an NSoM cavity. We precisely controlled the plasmonic nanogap morphology (i.e., nanogap facet) of the NSoM geometry and systematically compared (i) the spectral reliability and (ii) plasmonic resonance between the roundest and randomly faceted Au NSoM cavities. The structural integrity of the NSoM, mainly defined by the Au NS facets and uniformity/smoothness, correlated well with theoretically predicted resonant behaviors. Finally, we experimentally defined the achievable nanogap facet with the roundest and randomly faceted Au NSs NPoM cavities; on the basis of the experimental results, we theoretically outlined the corresponding plasmonic mode volume.

Control of the structural integrity of NSoM cavity In general, relatively large Au NPs (> 50 nm) have been synthesized by controlled reduction of Au in the presence of a seed (i.e., seed-growth method). Unfortunately, the reduced Au atoms tend to crystallize into polygonal NPs (e.g., Au cubes, octahedras, and rhombic dodecahedras) rather than spherical NPs due to the thermodynamics.23–25 In the presence of a specific organic ligand such as


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hydroquinone (HQ), Au seeds grow into relatively spherical Au NPs of 50–200 nm, but the randomly faceted surfaces are still problematic.29 Iterative growth and dissolution of polygonal Au nanocrystals has been recently suggested to achieve highly uniform and smooth Au NSs.23–25 Since polygonal Au nanocrystals can be readily synthesized with high uniformity, selective dissolution of their vertices can result in nearly perfect and uniform Au NSs. To the best of our knowledge, this method can produce the roundest Au NSs. Herein, we prepared both randomly faceted and highly spherical 60 nm Au NSs (see Figure 1) and used them to develop NSoM cavities (see Methods part). Top panels of Figures 1a-b highlight the synthetic pathways for each Au NS. In synthesizing randomly faceted Au NSs, 18 nm Au seed NPs, which were already polydisperse, were used as seeds. HQ-mediated growth resulted in randomly faceted Au NSs with an average size of 60 nm (see bottom panels of Figure 1a). Au nanorods and nanoplates were simultaneously obtained (see Figure S1 of Supporting Information). Thus, we tried to refine Au NP seeds to be highly uniform by iterative reductive growth and oxidative dissolution (Figure 1b).25 In particular, the ends of Au nanorods can be selectively etched into highly uniform Au NSs by oxidative dissolution. These Au NSs were further grown into uniformly distributed Au concaved rhombic dodecahedra (CRD) with 8 sharp vertices. The vertices were selectively etched and transformed into highly uniform and smooth Au NSs (see bottom panels of Figure 1b and Figure S2 of Supporting Information). The faceting of Au NSs can be quantified by algorithmic analysis of the rotationally distributed center-to-surface distance. Figure 1c shows representative transmission electron microscope (TEM) images of Au NSs used to quantify the facets. For the roundest Au NSs, the center-to-surface distances generally ranged from 29 nm to 31 nm according to the rotational angle (blue dots in Figure 1d). Also, fluctuations at a specific rotation range were clearly visible, indicating that even roundest Au NSs had a facet. This analysis showed that the lateral dimension of the facet was about 15 to 20 nm. In our experiments, a facet of 15 to 20 nm was the lower limit with iterative growth and dissolution. Other relevant studies showed a similar faceting of the roundest Au NSs.24,25 In stark contrast, the rotationally distributed center-to-surface distance of randomly faceted Au NSs varied from 27 to 34 nm with a significant fluctuation (pink dots in Figure 1d), that in turn, indicates lateral dimension of the facets from 5 to 45 nm. More analyses on the facets of Au NSs are included in Figure S3-S4, Supporting Information. Additionally, the NS aspect ratio (the ratio of major to minor axes, shown in Figure 1e) indicates size uniformity of the Au NSs.25 The aspect ratio of a perfectly round Au NS would be 1.0. The aspect ratio of the randomly faceted Au NSs was broadly distributed from 1.00 to 1.31 (pink dots in Figure 1e), whereas the roundest Au NSs showed a much narrower distribution of aspect ratios less than 1.10 (blue dots in Figure 1e). Both the randomly faceted and roundest Au NSs were placed onto a bare flat Au mirror to complete the NSoM cavity (Figure S5, Supporting Information). Once the atomically flat Au (200 nm thick), attained with the stripping method,30 is used as a mirror, the nanogap morphology of the NSoM and the resultant plasmonic mode are mainly defined by the faceting of Au NSs. As the bare Au mirror is hydrophobic, drop-casting the aqueous Au colloidal suspension formed a droplet on the mirror with a relatively high contact angle. The Au colloid droplets remained on the mirror during evaporation. Thus, the Au NSs were frequently clustered due to evaporation-induced capillary force ACS Paragon Plus Environment

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(Figure S5, Supporting Information). We carefully chose the Dark-field (DF) scattering spectrum from a single Au NS having an average size of 60 nm. At least 10 NSoMs were investigated (see the numbers in DF images of Figure S5, Supporting Information). We judged whether the measured DF scattering spots indeed correspond to individual 60 nm Au NSs by correlation of them with SEM image (see Figures S6-S9, Supporting Information). Thus, by using randomly faceted and roundest Au NSs, we can systematically verify the effect of Au NS uniformity and roundness on the deterministic construction of an NSoM cavity. Through this comparative study, we defined the accessible nanogap facet and plasmonic enhancement/mode volume with an NPoM cavity.

Effect of facets on plasmonic resonances of NSoM cavities Figures 2-3 summarize the theoretically predicted plasmonic resonant characteristics of ideally spherical (Figure 2) and faceted (Figure 3) 60 nm NSoM cavities (finite element method-based numerical simulation). DF scattering spectra (Figures 2a and 3a-b) together with spatial maps of both |E|/|E0| and surface charge at signature peaks, shoulders, and dips (Figures 2b and 3c) were calculated to determine the resonance behaviors of NSoM cavities. For these simulations, the gap between Au NS and a bare mirror was set at 1.5 nm, comparable to the organic ligand thickness of our Au NSs. It turned out that a 1.5 nm thick organic ligand was enough to avoid conductive contact. With this gap, classical electrodynamics, excluding nonlocal correction, allowed for an accurate prediction of NSoM plasmonic resonance.3,5,6 The faceting area was designed as a simple circle. To elucidate the role of facets in NSoM resonance, the diameter (w) of the circular facet was gradually varied from 0 nm to 45 nm, while the vertical Au NS dimension remained unchanged (60 nm) because regardless of the facet, the average size of the randomly faceted Au NSs was 60 nm in our experiment. This constraint is in contrast with previous work from J. Aizpurua and colleagues, where the vertical dimension of the Au NSs decreased as w increased.5 Figure S10 of Supporting Information details our simulation models for Au NSs having the same vertical dimension but different w. The incident polarization and angle were p-pol and 64º, respectively. Three important features are noticeable. First, three antenna modes, highlighted by peaks at 650 nm (l1 corresponding to longitudinal dipolar radiative mode), 550 nm (l2 corresponding to quasiquadrupolar radiative mode (intermediate state between longitudinal and transverse modes)), and 520 nm (l3 corresponding to transverse quadrupolar radiative mode), were present with an ideally spherical NSoM (w is 0 nm). However, in this case, l2 and l3 were not significantly differentiable. Second, as w increased slightly to 15 nm (matching with our roundest Au NSs), l2 mode became stronger (compare green line with purple line and see the spectral evolution in Figure 2a). Moreover, the increase in w leads to a red-shift of l1 mode (to 710 nm) due to enhanced capacitive coupling between Au NSs and the mirror. A modal analysis of l1, l2, and l3 resonances for a w of 15 nm (i.e., |E|/|E0| at the middle of the gap (top and middle panels) together with corresponding surface charge (bottom panels)) is summarized in Figure 2b. Finally, gradual increases in w make the lower-energy transverse waveguide modes (i.e., S11, S02, S12, S03, S13 and so on) become elusive (Figures 3a-b).5 More importantly, these waveguide modes become hybridized with antenna modes especially beyond w of 40 nm. As J. Aizpurua and colleagues reported, antenna modes interact with even order waveguide modes (i.e., S02, S03, and so on), whereas odd order waveguide modes (i.e., ACS Paragon Plus Environment

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S11, S12, S13 and so on) remain intact.5 Figure 3b presents DF scattering spectra (i.e., far-field scattering intensity) for a w of 45 nm together with near-field variation in plasmonic nanogap as a function of wavelength (taken at 0.75 nm height with respect to mirror surface). Two peaks of DF scattering at 660 nm (j1a mode) and 725 nm (j1b mode) are visible resulting from the hybridization between l1 and S02 modes. Particularly, it’s noteworthy that such two scattering peaks of j1a and j1b hybrid modes are not Lorentzian. Also, the maxima position of near-field intensity corresponding to S02 mode matches well with the dip between j1a and j1b modes. These results confirm that j1a and j1b modes indeed originate from the hybridization between l1 and S02 modes. The resonance natures of each mode for a w of 45 nm (i.e., j1a, j1b, and S02 modes) are detailed in Figure 3c. The DF scattering spectra and optical microscope images of assembled, randomly faceted NSoM cavities are shown in Figures 4a-d. Herein, total 160 randomly faceted NSoM cavities were investigated by DF scattering spectra. Not surprisingly, randomly faceted NSoM cavities showed highly polydisperse DF scattering spectra, which can be categorized into two main parts (Figure 4b): (i) the relatively spherical (Au NSs with relatively symmetric spheres (83.12 %), highlighted by purple and blue outlines in Figure 4b) and (ii) non-spherical Au NPs (asymmetric Au nanorods (12.34 %) and nanoplates (4.54 %), respectively highlighted by green and orange outlines). We selectively separated the DF scattering spectra of relatively spherical Au NSs and further divided them into two groups according to their spectral similarity compared with theoretical predictions in Figures 2a and 3a-b: (i) w less than 35 nm, showing distinct l1, l2, and l3 modes (purple outline of Figure 4b, 59.74 %) and (ii) w larger than 35 nm, showing distinct j1a and j1b modes (blue outline of Figure 4b, 23.38 %). The representative DF optical microscope and SEM images of the Au NSs are included as well (Figures 4c-d). Interestingly, both groups of the DF scattering spectra were obtained from the almost same-sized, randomly faceted Au NSoM cavities (~ 60 nm); but, showing a significantly different resonant feature. To quantify the practical limit to the smallest plasmonic nanogap with randomly faceted Au NSoM cavities, we further analyzed DF scattering spectra of w less than 35 nm (Figure 4e). The direct comparison between the experimentally measured and theoretically predicted peak positions of l1 mode was used to spectrally figure out w of relatively spherical, but randomly faceted Au NSoM cavities. This is because l1 mode is highly sensitive to w and the resultant capacitive coupling. As shown in Figures 4e-f, the main peak positions of l1 mode were found to be ranged from 650 nm to 720 nm; corresponding to w of 5 ~ 30 nm. Particularly, a relatively narrow nanogap (e.g., w less than 15 nm) was formed frequently, even with the randomly faceted Au NSs (see Figure 4f). Also, the spectral hallmarks corresponding to l2 and l3 modes were not obviously detached across several cavities, so further confirmed the formation of nanogap with w less than 15 nm. More importantly, it turned out that a few NSoM cavities (3.75 %) can reach to much lower w of 5 to 6 nm. This was the empirical lower limit of plasmonic nanogap, accessible with randomly faceted Au NSoM cavities. A comparison between these DF scattering spectra and theoretical predictions shows good agreement. Meanwhile, it’s noteworthy that the main peak positions of l1 mode were also relatively uniform. This finding implies that the longitudinal antenna mode of the NSoM cavity is less sensitive to the Au NS shape and roughness. On the basis of the above experimental results, we theoretically defined the corresponding mode volume of Au NSoM cavities (Figure 5). To extract plasmonic mode volume at each resonance, both Purcell factor and Q-factor were calculated by using finite-difference, time-domain method ACS Paragon Plus Environment

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(Figures 5a-b).15 Mode volumes were generally increased, as w is increased (Figure 5c); l1 resonance showed the smallest mode volume. In our experiments, 5 nm was the smallest w, obtainable with the randomly faceted Au NSoM cavities; corresponding plasmonic mode volume for w of 5 nm was 138 nm3. This was the lower limits of the mode volume in our experiments. Thus, both w and modal characteristic limited the available mode volume in NPoM cavities. As expected, formation of a plasmonic nanogap with randomly faceted Au NSs was not so deterministic. As shown in Figures 4a-b, the DF scattering spectra with a significantly hybridized antenna and waveguide modes (two distinct peaks at 625 to 725 nm), evidencing the formation of relatively large facets (w larger than 40 nm), were also common with randomly faceted Au NSoM cavities (23.38 %). Furthermore, the peaks corresponding to the hybrid mode were not relatively uniform (see blue outlines in Figure 4b). Nevertheless, our results indicate that the vertices or sharp edges of randomly faceted Au NSs can have advantageous over rounder Au NSs, particularly for reducing facet (w of ~ 5 nm) and plasmonic mode volume (e.g., 138 nm3 at l1 mode), because the round Au NSs had uniformly distributed facets with a w of 15 to 20 nm. Figures 6a-b present the DF scattering spectra of the roundest Au NSoM cavities together with the corresponding DF optical microscope images. Indeed, the hallmarks of the l2 and l3 modes, scattering peaks at 520 nm and 570 nm, were clear and reliable; also, the l1 mode was found to be red-shifted at 705 nm. These experimental DF spectra matched well with the theoretical spectra. Thus, the smallest plasmonic nanogap of the roundest Au NSoM (w of 15 to 20 nm) was larger than that achievable with a randomly faceted Au NSoM (w of ~ 5 nm). Thus, plasmonic mode volume of the roundest Au NSoM particularly at l1 mode can be ranged from 259 nm3 and 360 nm3 (see Figure 5), much higher than that of the randomly faceted counterpart. Although a rounder Au NSoM guarantees much more reliable plasmonic resonance when compared with a randomly faceted Au NSoM (see the detailed uniformity comparison of DF scattering spectra, shown in Figure 6c), the ultimate limit to the plasmonic enhancement was not comparable to that of a randomly faceted Au NSoM. From an experimental point of view, stopping the etching process when an Au NS facet approaches 0 nm is extremely difficult,23–25 whereas HQ-based synthesis inherently results in molecularly sharp edges and vertices.29 Thus, the randomly faceted Au NSs can be readily used to practically develop an NSoM cavity with a truly point-like plasmonic nanogap.

Conclusions Herein, we systematically compared the plasmonic resonance of the roundest and randomly faceted Au NSoM cavities, which can be practically accessible with recent advances in chemical synthesis. From an experimental point of view, randomly faceted Au NSs, despite having non-uniform DF scattering spectra, were advantageous over the roundest Au NSs in terms of reducing plasmonic mode volume. Of course, rounded Au NSs with a w less than 15 nm may be synthesized in the future; but, using randomly faceted Au NSs, which already have sharp vertices and edges, can still provide a practical and effective platform to reach the ultimate limit of plasmonic enhancement.


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Methods Chemicals: All chemicals used in this work, except for epoxy precursors, were purchased from Sigma-Aldrich and used as received. Epoxy precursors were obtained from 3M Scotch-Weld™. Synthesis of the randomly faceted Au NSs: Randomly faceted Au NSs were synthesized by the seedgrowth method. First, 18 nm Au NSs were synthesized by reducing gold (III) chloride trihydrate (HAuCl4) with trisodium citrate. A total of 100 mL of 0.125 mM HAuCl4 in water was heated to boiling, and 2.02 mL of 18.7 mM sodium citrate was quickly injected with a syringe under vigorous stirring. The color of the aqueous solution gradually changed to yellow, colorless, dark purple, and light red. The light red color indicated the formation of highly uniform 18 nm Au NSs. To synthesize average 60 nm randomly faceted Au NSs, these 18 nm Au NSs were used as seed. As-synthesized, 18 nm Au NS seed aqueous solutions (170 µL) were mixed with 47.2 µL of HAuCl4 (50 mM), and deionized water was added to bring the total solution volume to 8 mL. Quickly adding 40 µL of trisodium citrate (15m M) and 80 µL of hydroquinone (25 mM) induced randomly faceted Au NSs to grow. Synthesis of the roundest Au NSs: First, Au nanorods were synthesized as follows. A total of 125 µL of 10 mM HAuCl4 was added to 5 mL of 100 mM cetyltrimethylammonium bromide (CTAB) aqueous solution. Ice-cold sodium borohydride (NaBH4) (300 µL of 10mM) was rapidly injected into the aqueous solution with vigorous stirring for 1 minute to nucleate seeds. The seed solution was incubated in a 25 °C water bath for 30 minutes. Then, 200 mL of 100 mM CTAB, 10 mL of 10 mM HAuCl4, 1.8 mL of 10 mM silver nitrate (AgNO3), 1.14 mL of 100 mM L-ascorbic acid, and 240 µL of seed solution were added and stirred for 1 minute to synthesize Au nanorods. The assynthesized Au nanorod solution was then washed by repetitive centrifugation and resuspension in 50 mM CTAB aqueous solution. To selectively etch the ends of the Au nanorods, they were dispersed in 50 mM CTAB aqueous solution and mixed with 80 µM HAuCl4 aqueous solution. Au nanorods were transformed into 20 nm Au NSs. The Au NSs was washed in 100 mM cetylpyridinium chloride (CPC) aqueous solution. To synthesize the roundest 60 nm Au NSs, these 20 nm Au NSs were used as seeds. First, 30 mL of 10 mM CPC, 6.75 mL of 100mM ascorbic acid, 1.5 mL of seeds (at 1 optical density [OD]), and 525 µL of 6.775 mM HAuCl4 were mixed in deionized water. Au NS seeds grew into CRD and then these Au NSs was washed with 50 mM CTAB aqueous solution. Next, HAuCl4 aqueous solution was added to the Au CRD solution and stirred for 4 hours at 40 °C to selectively etch the CRD vertices (i.e., oxidative dissolution). These reductive growth and oxidative dissolution (or etching) steps were repeated until the Au NS aspect ratio was minimized (i.e., iterative growing and etching). Quantification of the Synthesized Au NSs: To do this, we used the custom-built algorithmic analysis code, in which the rotational distribution of the center-to-surface can be both automatically and algorithmically analyzed, as previously reported in Ref. 25. Briefly, the edges of each Au NSs were identified by a Laplacian of Gaussian subroutine-enabled definition of a zero-threshold of image intensity; the areas within such defined edges were filled with offset intensity. Then, the computation algorithm defined the centroid of the Au NSs; the distance from the centroid to the edges of Au NSs was automatically calculated as a function of rotational angle.


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Preparation of the Atomically flat Au mirror: To prepare the atomically flat Au mirror, we used the template stripping method.30 By using electron-beam (e-beam) evaporation, a 200 nm thick Au film was evaporated onto a clean crystalline silicon (Si) wafer. The rate of e-beam evaporation was gradually increased from 0.1 Å/s to 1.5 Å/s until the Au layer was 200 nm thick. Next, the surface of the evaporated Au was covered with liquid epoxy precursors, and a glass substrate was gently put on it. Thermal curing process at 75 °C permanently bound the epoxy glue with the glass substrate and Au layer. Finally, the glass substrate was peeled off from the silicon wafer. NSoM cavity fabrication: To create an NSoM cavity, the Au NS aqueous solution was dropped onto the Au mirror and dried under nitrogen gas. Analysis of dark-field (DF) scattering spectra: The DF scattering spectra were collected by using a custom-built dark field spectroscope, in which both the imaging spectrometer (IsoPlane, Princeton Instruments) and CCD spectrometer (PIXIS-400B, Princeton Instruments) were integrated into an optical microscope (Eclipse Ni-U, Nikon).28 DF scattering spectra from individual Au NSs were isolated by using our custom-built aperture system. A broadband light source was used, and the incident angle was 64°. The source light was polarized to be p-pol. Scattered light was collected with a 100x objective lens with a numerical aperture of 0.9. Numerical simulations: The numerical simulation was performed by both finite-element method and finite-difference, time-domain. The NSoM cavity was designed to be encapsulated by a 3D perfectly matched layer (PML). To rationalize the facet of the Au NSs, a truncated cone was inserted into the bottom of the Au NSs (see Figure S10, Supporting Information). The dielectric constants of Au were taken from Johnson and Christy;31 the real refractive index of the organic ligand of Au NSs was assumed to be 1.4. Incident light was p-pol. According to the numerical aperture of the objective lens used in our experiment, the scattered pointing vector was integrated within a cone of half-angle θ = 64°. To extract plasmonic mode volume at different modes, both Purcell factor and Q factor were numerically calculated.15 In the numerical simulation, we excluded nonlocal effects.

Acknowledgement This work was supported by Samsung Research Funding Center for Samsung Electronics under Project Number SRFC-MA1402-09.

Supporting Information Details about (i) synthesis of the randomly faceted and roundest Au NSs, (ii) analysis of the facets, (iii) measurement of DF scattering spectrum, and (iv) numerical simulation are available in Supporting Information. This material is available free of charge via the internet at http://pub.acs.org.


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References 1. Aravind, P; Metlu, H. Use of a Perfectly Conducting Sphere to Excite the Plasmon of a Flat Surface. 1. Calculation of the Local Field with Applications to Surface-Enhanced Spectroscopy. J. Phys. Chem. 1982, 86, 5076−5084. 2. Aravind, P; Metlu, H. The Effects of the Interaction Between Resonances in the Electromagnetic Response of a Sphere-Plane Structure; Applications to Surface Enhanced Spectroscopy. Surf. Sci. 1983, 124, 506−528. 3. Ciracì, C.; Hill, R. T.; Mock, J. J.; Urzhumov, Y.; Fernández-Domínguez, A. I.; Maier, S. A.; Pendry, J. B.; Chilkoti, A.; Smith, D. R. Probing the Ultimate Limits of Plasmonic Enhancement. Science 2012, 337, 1072−1074. 4. Mertens, J.; Eiden, A. L.; Sigle, D. O.; Huang, F.; Lombardo, A.; Sun, Z.; Sundaram, R. S.; Colli, A.; Tserkezis, C.; Aizpurua, J.; Milana, S.; Ferrari, A. C.; Baumberg, J. J. Controlling Subnanometer Gaps in Plasmonic Dimers Using Graphene. Nano Lett. 2013, 13, 5033−5038. 5. Tserkezis, C.; Esteban, R.; Sigle, D. O.; Mertens, J.; Herrmann, L. O.; Baumberg, J. J.; Aizpurua, J. Hybridization of Plasmonic Antenna and Cavity Modes: Extreme Optics of Nanoparticle-on-Mirror Nanogaps. Phys. Rev. A 2015, 92, 053811. 6. Chikkaraddy, R.; Zheng, X.; Benz, F.; Brooks, L. J.; de Nijs, B.; Carnegie, C.; Kleemann, M.-E.; Mertens, J.; Bowman, R. W.; Vandenbosch, G. A. E.; Moshchalkov, V. V.; Baumberg, J. J. How Ultranarrow Gap Symmetries Control Plasmonic Nanocavity Modes: From Cubes to Spheres in the Nanoparticle-on-Mirror. ACS Photonics 2017, 4, 469−475. 7. Lassiter, J. B.; McGuire, F.; Mock, J. J.; Ciracì, C.; Hill, R. T.; Wiley, B. J.; Chilkoti, A.; Smith, D. R. Plasmonic Waveguide Modes of Film-Coupled Metallic Nanocubes. Nano Lett. 2013, 13, 5866−5872. 8. Yoon, I.; Kang, T.; Choi, W.; Kim, J.; Yoo, Y.; Joo, S.-W.; Park, Q-H.; Ihee, H.; Kim, B. Single Nanowire on a Film as an Efficient SERS-Active Platform. J. Am. Chem. Soc. 2009, 131, 758−762. 9. Park, W.-H.; Kim, Z. H. Charge transfer enhancement in the SERS of a Single Molecule. Nano Lett. 2010, 10, 4040−4048. 10. Moreau, A.; Ciracì, C.; Mock, J. J.; Hill, R. T.; Wang, Q.; Wiley, B. J.; Chilkoti, A.; Smith, D. R. Controlled-Reflectance Surfaces with Film-Coupled Colloidal Nanoantennas. Nature 2012, 492, 86−89. 11. Akselrod, G. M.; Argyropoulos, C.; Hoang, T. B.; Ciracì, C.; Fang, C.; Huang, J.; Smith, D. R.; Mikkelsen, M. H. Probing the Mechanisms of Large Purcell Enhancement in Plasmonic Nanoantennas. Nat. Photonics 2014, 8, 835−840. 12. Hoang, T. B.; Akselrod, G. M.; Argyropoulos, C.; Huang, J.; Smith, D. R.; Mikkelsen, M. H. Ultrafast Spontaneous Emission Source Using Plasmonic Nanoantennas. Nat. Commun. 2015, 6, 7788. ACS Paragon Plus Environment

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13. 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. 14. Benz, F.; Schmidt, M. K.; Dreismann, A.; Chikkaraddy, R.; Zhang, Y.; Demetriadou, A.; Carnegie, C.; Ohadi, H.; de Nijs, B.; Esteban, R.; Aizpurua, J.; Baumberg, J. J. SingleMolecule Optomechanics in “Picocavities”. Science 2016, 354, 726−729. 15. Chikkaraddy, R.; de Nijs, B.; Benz, F.; Barrow, S. J.; Scherman, O. A.; Rosta, E.; Demetriadou, A.; Fox, P.; Hess, O.; Baumberg, J. J. Single-Molecule Strong Coupling at Room Temperature in Plasmonic Nanocavities. Nature 2016, 535, 127−130. 16. Lombardi, A.; Demetriadou, A.; Weller, L.; Andrae, P.; Benz, F.; Chikkaraddy, R.; Aizpurua, J.; Baumberg, J. J. Anomalous Spectral Shift of Near- and Far-Field Plasmonic Resonances in Nanogaps. ACS Photonics 2016, 3, 471−477. 17. Choi, H.-K.; Park, W.-H.; Park, C.-G.; Shin, H.-H.; Lee, K. S.; Kim, Z. H. Metal-Catalyzed Chemical Reaction of Single Molecules Directly Probed by Vibrational Spectroscopy. J. Am. Chem. Soc. 2016, 138, 4673−4684. 18. Stewart, J. W.; Akselrod, G. M.; Smith, D. R.; Mikkelsen, M. H. Toward Multispectral Imaging with Colloidal Metasurface Pixels. Adv. Mater. 2017, 29, 1602971. 19. Sigle, D. O.; Mertens, J.; Herrmann, L. O.; Bowman, R. W.; Ithurria, S.; Dubertret, B.; Shi, Y.; Yang, H. Y.; Tserkezis, C.; Aizpurua, J.; Baumberg, J. J. Monitoring Morphological Changes in 2D Monolayer Semiconductors Using Atom-Thick Plasmonic Nanocavities. ACS Nano. 2015, 9, 825−830. 20. Benz, F.; Tserkezis, C.; Herrmann, L. O.; de Nijs, B.; Sanders, A.; Sigle, D. O.; Pukenas, L.; Evans, S. D.; Aizpurua, J.; Baumberg, J. J. Nanooptics of Molecular-Shunted Plasmonic Nanojuctions. Nano Lett. 2015, 15, 669−674. 21. Huang, S.; Ming, T.; Lin, Y.; Ling, X.; Ruan, Q.; Palacios, T.; Wang, J.; Dresselhaus, M.; Kong, J. Ultrasmall Mode Volumes in Plasmonic Cavities of Nanoparticle-on-Mirror Structures. Small 2016, 12, 5190−5199. 22. 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. 23. Li, C.; Shuford, K. L.; Chen, M.; Lee, E. J.; Cho, S. O. A Facile Polyol Route to Uniform Gold Octahedra with Tailorable Size and Their Optical Properties. ACS Nano 2008, 2, 1760−1769. 24. Lee, Y.-J.; Schade, N. B.; Sun, L.; Fan, J. A.; Bae, D. R.; Mariscal, M. M.; Lee, G.; Capasso, F.; Sacanna, S.; Manoharan, V. N.; Yi, G.-R. Ultrasmooth, Highly Spherical Monocrystalline Gold Particles for Precision Plasmonics. ACS Nano 2013, 7, 11064−11070.


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25. O’Brien, M. N.; Jones, M. R.; Brown, K. A.; Mirkin, C. A. Universal Noble Metal Nanoparticle Seeds Realized Through Iterative Reductive Growth and Oxidative Dissolution Reactions. J. Am. Chem. Soc. 2014, 136, 7603−7606. 26. Kim, D.-K.; Hwang, Y. J.; Yoon, C.; Yoon, H.-O.; Chang, K. S.; Lee, G.; Lee, S.; Yi, G.-R. Experimental Approach to the Fundamental Limit of the Extinction Coefficients of UltraSmooth and Highly Spherical Gold Nanoparticles. Phys. Chem. Chem. Phys. 2015, 17, 20786−20794. 27. Kim, M.; Lee, S.; Lee, J.; Kim, D. K.; Hwang, Y. J.; Lee, G.; Yi, G.-R.; Song, Y. J. Deterministic Assembly of Metamolecules by Atomic Force Microscope-Enabled Manipulation of Ultra-Smooth, Super-Spherical Gold Nanoparticles. Opt. Express 2015, 23, 12766−12776. 28. Park, K. J.; Huh, J.-H.; Jung, D.-W.; Park, J.-S.; Choi, G. H.; Yoo, P. J.; Park, H.-G.; Yi, G.R.; Lee, S. Assembly of “3D” Plasmonic Metamolecules by “2D” AFM Nanomanipulation of Highly Uniform and Smooth Gold Nanospheres. Sci. Rep. 2017, 7, 6045. 29. Perrault, S. D.; Chan, W. C. W. Synthesis and Surface Modification of Highly Monodispersed, Spherical Gold Nanospheres of 50−200 nm. J. Am. Chem. Soc. 2009, 131, 17042−17043. 30. Nagpal, P.; Lindquist, N. C.; Oh, S.-H.; Norris, D. J. Ultrasmooth Patterned Metals for Plasmonics and Metamaterials. Science 2009, 325, 594−597. 31. Johnson, P. B.; Christy, R. W. Optical Constants of Noble Metals. Phys. Rev. B 1972, 6, 4370.


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Figure 1. Schematic for synthesis of (a) randomly faceted gold nanospheres (Au NSs) and (b) roundest Au NSs (top panels). The representative transmission electron microscope (TEM) images of both Au NSs are included (bottom panels). (c) TEM images of both Au NSs, used to quantify the center-to-surface distance as function of the rotational angle. (d) The rotational distribution of the center-to-surface distance of randomly faceted and roundest Au NSs. (e) The distribution of aspect ratios (the ratio of major to minor axes) of randomly faceted and roundest Au NSs.


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Figure 2. (a) The numerically simulated scattering cross section (SCS, nm2) of NSoM cavities with different facet diameters (w less than 35 nm). (b) Plasmonic mode analyses of the NSoM with w of 15 nm. l1, l2, and l3 modes were analyzed.


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Figure 3. (a) The numerically simulated SCS (nm2) of NSoM cavities with different facet diameters (w). Two hybrid modes between antenna and waveguide modes (j1a and j1b) are excited in NSoM cavities with w larger than 40 nm. (b) The numerically simulated far-field and near-field intensities for the NSoM with w of 45 nm. (c) Plasmonic mode analyses of the NSoM with w of 45 nm. S02, j1a, and j1b modes were analyzed (from left to right).


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Figure 4. (a) DF scattering spectra of randomly faceted Au NSoM cavities (i.e., total 160 Au NSoM cavities were measured). (b) Categorization of DF scattering spectra of randomly faceted Au NSoM cavities into (i) nanorods (orange outline), (ii) nanoplates (green outline), and (iii) nanospheres (blue and sky colored outlines). This categorization was carried out by the correlation between DF scattering spectra and scanning electron microscope (SEM). The portions per each motif are included: 12.34 % for nanorods, 4.55 % for nanoplates, and 83.11% for nanospheres. The Au nanospheres can be further divided into two regimes: (i) a relatively large w (> 35 nm) and (ii) a relatively small w (< 35 nm). (c-d) Representative scanning electron microscope (SEM) images (top panels) and DF optical microscope images (bottom panels) of spherical Au NSoM cavities with (c) w smaller than 35 nm and (d) w larger than 35 nm. All these Au NSs showed an average size of 60 nm. (e-f) More detailed DF scattering spectral analysis of the randomly faceted Au NSoM cavities having a relatively small w (< 35 nm). (e) Herein, the DF scattering spectra of total 96 Au NSoM cavities having a relatively small w (< 35 nm) are summarized. The peak position corresponding to l1 mode acted as an indicator of w, as this antenna mode highly depends on the capacitive coupling between Au NS and mirror. (f) The frequency of w is summarized. We found that w of randomly faceted Au NSoM cavities can be reduced to 5 nm. Also, w of 10 nm to 20 nm is most frequently observed.


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Figure 5. (a) Purcell factors, (b) Q-factors, and (c) plasmonic mode volumes for 60 nm Au NSoM cavity with a controlled facet (w).


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Figure 6. (a) DF scattering spectra of the roundest Au NSoM cavities. (b) The corresponding DF optical microscope image. (c) Comparison of DF spectra uniformity between randomly faceted and roundest Au NSoM cavities.


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Comparative Study of Plasmonic Resonances between the Roundest

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