Article Cite This: ACS Photonics 2018, 5, 413−421
Comparative Study of Plasmonic Resonances between the Roundest and Randomly Faceted Au Nanoparticles-on-Mirror Cavities Ji-Hyeok Huh,†,‡ Jaewon Lee,†,‡ and Seungwoo Lee*,†,§ †
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea Department of Nano Engineering and School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
§
ACS Photonics 2018.5:413-421. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/15/18. For personal use only.
S Supporting Information *
ABSTRACT: Over the past 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. KEYWORDS: nanoparticle-on-mirror cavity, plasmonic nanogap, antenna mode, waveguide mode, dark-field spectral reliability
I
istic 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
n 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 (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 nondetermin© 2017 American Chemical Society
Received: July 30, 2017 Published: November 22, 2017 413
DOI: 10.1021/acsphotonics.7b00856 ACS Photonics 2018, 5, 413−421
Article
ACS Photonics
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 a function of the rotational angle. (d) The rotational distribution of the center-to-surface distance of randomly faceted and roundest Au NSs. (e) Distribution of aspect ratios (the ratio of major to minor axes) of randomly faceted and roundest 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). Top panels of Figure 1a,b highlight the synthetic pathways for each Au NS. In synthesizing randomly faceted Au NSs, 18 nm Au seed NPs, which were already polydispersed, were used as seeds. HQmediated 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 fourteen sharp vertices. The vertices were selectively etched and transformed into highly uniform and smooth Au NSs (see bottom panels of Figures 1b and S2 of Supporting Information).
cavities; on the basis of the experimental results, we theoretically outlined the corresponding plasmonic mode volume.
■
CONTROL OF THE STRUCTURAL INTEGRITY OF THE 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 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 have 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. 414
DOI: 10.1021/acsphotonics.7b00856 ACS Photonics 2018, 5, 413−421
Article
ACS Photonics 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 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 15−20 nm. In our experiments, a facet of 15−20 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 Figures S3 and 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, dropcasting 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 evaporationinduced capillary force (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.
Figure 2. (a) 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.
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 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 64°, 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 quasi-quadrupolar 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
■
EFFECT OF FACETS ON PLASMONIC RESONANCES OF NSOM CAVITIES Figures 2 and 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 415
DOI: 10.1021/acsphotonics.7b00856 ACS Photonics 2018, 5, 413−421
Article
ACS Photonics
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) nonspherical 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 (Figure 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 Figure 4e,f, the main peak positions of l1 mode were found to be ranged from 650 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 is 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 (Figure 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 Figure 4a,b, the DF scattering spectra with a significantly hybridized antenna and waveguide modes (two distinct peaks at
Figure 3. (a) 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) 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).
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 (Figure 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., 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 is 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 Figure 4a−d. Herein, a total of 160 randomly faceted NSoM cavities were investigated by DF scattering spectra. Not 416
DOI: 10.1021/acsphotonics.7b00856 ACS Photonics 2018, 5, 413−421
Article
ACS Photonics
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 (
