Size-Modulation of Plasmonic Nanorings Obtained ... - ACS Publications

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Size-Modulation of Plasmonic Nanorings Obtained by the SelfAssembly of Gold Nanoparticles and Block Copolymers Jean-François Lemineur,† Silvere Schuermans,‡ Joseph Marae-Djouda,‡ Thomas Maurer,‡ and Anna M. Ritcey*,† †

Département de chimie and CERMA, Université Laval, Pavillon Alexandre.-Vachon 1220, 1045 avenue de la Médecine, G1 V 0A6 Québec, Canada ‡ Laboratoire de Nanotechnologie et d’Instrumentation Optique, ICD CNRS UMR n°6281, Université de Technologie de Troyes, CS 42060, 10004 Troyes, France S Supporting Information *

ABSTRACT: Metal nanoparticles exhibit interesting optical properties due to the collective excitation of conduction electrons called the plasmon. Within appropriate metal nanostructures, cooperative plasmon modes appear and the resonance plasmon frequency is modified. This article reports a simple method for the formation of such structures, in the form of self-assembled nanorings. Rings of alkanethiol-capped gold nanoparticles are obtained by the Langmuir− Blodgett technique and a block copolymer (PS-b-P2VP) template. With this approach, organized nanoparticle arrangements covering a large surface area are obtained. Furthermore, geometric parameters such as ring diameter, ring-to-ring separation, and ring width can be systematically varied by the addition of homopolymer or in situ nanoparticle regrowth. Optical extinction spectra recorded for the nanoparticle rings depend both on ring diameter and particle size. In particular, after in situ particle regrowth, the plasmon extinction spectrum exhibits a red-shift that increases with ring diameter. Theoretical spectra generated with the discrete dipole approximation indicate that this spectral shift can be attributed to plasmon coupling that extends over an increasing number of particles as the ring is enlarged.

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INTRODUCTION Plasmonic nanorings (NRs) are of increasing scientific interest because of their remarkable characteristics. For example, they present a high degree of symmetry1 and can simultaneously exhibit both magnetic and electric coupling modes.2,3 For this reason, NRs currently represent one of the best options for the development of meta-materials with negative refractive indices at visible wavelengths.4 In addition, research conducted by Larsson et al.5 has shown that the localized surface plasmon resonance (LSPR) for NRs is more sensitive to variations in local refractive index than for conventional nanodisks. Consequently, NRs are good candidates for designing ultrasensitive LSPR sensors,6 including potential combinations with other sensing technologies such as surface enhanced Raman scattering (SERS),7 metal enhanced fluorescence (MEF),8 and surface enhanced infrared absorption (SEIRA).9 Furthermore, since the LSPR frequency is directly dependent on the NR diameter,5 these nanostructures seem to be an attractive way to manipulate, confine, and guide light more effectively and below the diffraction limit.10 To date, very few studies have reported the synthesis of plasmonic NRs, illustrating the difficulty of fabricating such structures over large surface areas while maintaining control of © 2016 American Chemical Society

ring geometry. The majority of existing methods rely on complex and expensive procedures like lithography or other top-down approaches.11 Among bottom-up processes, the evaporation-induced assembly of gold nanoparticles (NPs) into NRs12 or solution phase techniques13 show promise, but maintaining order over large surface areas remains problematic. Herein, we employ a very simple method14,15 for the formation of two-dimensional NRs composed of metallic NPs capped by octanethiol ligands. We previously demonstrated15 that block copolymer monolayers spread at the air−water interface can serve as templates for the directed assembly of metal NPs. Furthermore, the resulting composite films can be easily transferred onto macroscopic solid substrates via the Langmuir−Blodgett technique. Depending on the exact choice of block copolymer, NP size, and capping ligand, a variety of organized structures can be obtained by this approach, including NP clusters, lines, and rings. Of these, rings are of particular interest because of the unique plasmonic properties described above. In the present article, we show that block copolymer templating is amenable to the systematic modReceived: February 18, 2016 Revised: April 1, 2016 Published: April 5, 2016 8883

DOI: 10.1021/acs.jpcc.6b01689 J. Phys. Chem. C 2016, 120, 8883−8890

Article

The Journal of Physical Chemistry C

the reduction of Au3+ to Au+, the substrate-supported monolayer was immersed in the solution for 10 min at room temperature. Characterization Techniques. Transmission electron microscopy (TEM) images were obtained with a Jeol 1230 microscope at an acceleration voltage of 80 kV. Monolayer samples were prepared by attaching carbon-coated microscope grids to the glass slides used for Langmuir−Blodgett deposition. Atomic force microscopy (AFM) images were recorded with a Nanoscope III Multimode microscope (Digital Instruments) in tapping mode. Height, amplitude, and phase images were collected simultaneously with high resolution tips. Extinction spectra of NP assemblies were measured with a transmission optical microscope coupled to a microspectrometer using a multimode optical fiber as confocal filtering (Figure 1). A ×50 objective lens (NA = 0.15) allowed for a detection area of ∼50 μm2. For each sample, spectra were recorded at several different spots.

ification of NR dimensions, through either the addition of homopolymer or in situ NP growth. Optical extinction spectra of self-assembled NRs of gold NPs are also reported, as a function of both ring diameter and particle size.

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METHODS Synthesis of Gold NPs. Gold NPs were synthesized via a modified Brust method.16 Chloroauric acid (HAuCl4) was dissolved in ultrapure water and transferred to chloroform by a phase-transfer agent, tetraoctylammonium bromide (TOAB). Gold was then reduced by addition of an aqueous solution of sodium borohydride (NaBH4). The resulting organic phase colloidal suspension was washed with dilute sulfuric acid and ultrapure water several times. After displacement of TOAB by the addition of 0.1 mL of octanethiol (C8-SH), NPs (denoted C8-NPs) were purified by three centrifugations in a chloroform/methanol mixture at 15000 rpm, dried and conserved under vacuum until use. Ultrapure water (18.2 mΩ·cm) was obtained using a Nanopure II filtration system. Spreading Solutions. Spreading solutions were prepared by dissolving polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP), in chloroform at a fixed concentration (1.8 mg/mL). C8-NPs were added to yield a concentration of about 2 mg/mL and the solution was treated in an ultrasonic bath for 5 min. Modification of ring diameter was achieved by the addition of polystyrene homopolymer (PS) at different concentrations (0.00, 0.25, 0.50, 0.75, and 1.00 mg/mL) and the resulting solution was stirred overnight at room temperature. Poly(2vinylpyridine) homopolymer (P2VP) was added to the PS-bP2VP spreading solutions for modification of micelle spacing. All polymers were commercially obtained from Polymer Source, and their molecular weight characteristics are provided in Table 1.

Figure 1. Schematic of the optical setup dedicated to extinction spectroscopy.

Theoretical Calculations. The theoretical extinction spectra were calculated using the DDSCAT 7.3 code developed by Draine and Flatau.17 It is based on the discrete dipole approximation (DDA) in which the target is replaced by a series of polarizable points. The targets used for simulations were composed of 6 or 10 nm NPs arranged in rings of various diameters. The interparticle distance was kept constant at 1.8 nm. The Johnson and Christy gold optical constants were used for the NP dielectric function and the refractive index of the ambient medium was set to 1.59. Rings were oriented in the YZ plane while the incident beam propagated along the X-axis with two different polarization directions (along the Y and the Z axis). The code was also employed to calculate electric field strength within and near the targets.

Table 1. Molecular Weight Characteristics of Polymers Used in This Study PS-b-P2VP PS P2VP

mean molecular weight (g/mol)

polydispersity index

54900-b-50100 52000 3700

1.07 1.07 1.44

Langmuir−Blodgett Film Formation. Monolayers were formed by spreading chloroform solutions described above on a KSV 3000 Langmuir trough filled with ultrapure water. Compression isotherms were recorded for monolayers spread from 25 μL. Surface pressure was recorded during symmetric compression (10 mm/min) by a platinum Wilhelmy plate microbalance. Langmuir−Blodgett films were prepared from monolayers spread from 50 to 100 μL of chloroform solution. Films were compressed to a surface pressure of 15 mN/m and held at this pressure for 30 min before transfer. Langmuir−Blodgett films were obtained by transfer to glass microscope slides at a dipping speed of 5 mm/min. In Situ NP Regrowth. A solution of regrowth precursor was prepared by dissolving 0.0197 g of HAuCl4 (2.5 × 10−4 M) and 6 g of cetyltrimethylammonium bromide (CTAB) in 200 mL of ultrapure water with vigorous stirring. A solution of ascorbic acid (0.1 M) was prepared in ultrapure water, and 0.05 mL of this solution was added to a 10 mL aliquot of the growth solution. Once the resulting mixture became white, indicating

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RESULTS Modification of Surface Micelle Diameter. When PS-bP2VP block copolymers are spread at the surface of water, they self-assemble and spontaneously form a hexagonal network of circular surface micelles that can serve as templates for the formation of NRs of NPs.14 The micelle diameter depends on the size and nature of the constituent block copolymer and, for a given sample, is invariant with surface pressure. In order to swell the diameter of the hydrophobic micelle cores, PS homopolymer was added to the spreading solutions. The homopolymer molecular weight was selected to be close to that of the PS block of the copolymer to ensure miscibility. Five different concentrations (0.00, 0.25, 0.50, 0.75, and 1.00 mg/ mL) were tested and the compression isotherms for the 8884


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Figure 2. Compression isotherms obtained after spreading 25 μL of chloroform solutions of PS-b-P2VP (without NPs) mixed with PS homopolymer (a) or P2VP homopolymer (b) in the specified quantities.

Figure 3. (a) AFM images of monolayers obtained by spreading 50−100 μL of chloroform solutions containing PS-b-P2VP copolymer (1.8 mg/mL) and PS homopolymer at concentration indicated on the images. All films were transferred to glass at 15 mN/m and do not contain gold NPs. (b) Schema depicting the molecular organization of an amphiphilic block copolymer surface micelle and the addition of homopolymer to the hydrophobic domains.

corresponding composite PS-b-P2VP/PS monolayers are presented in Figure 2a. The results presented in Figure 2a show that the compression isotherm of PS-b-P2VP is relatively insensitive to the addition of PS homopolymer. This observation is in agreement with the accepted model18 for surface micelles of amphiphilic block copolymers. As illustrated in Figure 3b, this model includes circular domains of aggregated hydrophobic segments (PS in the present case) riding on top of a monolayer of the hydrophilic component (P2VP in the present case), that spreads to cover the entire water surface. The depicted structure applies to surface pressures below the pseudoplateau. This plateau corresponds to the reorganization of the hydrophilic blocks (P2VP), including possible solubilization and is followed by a steep rise in surface pressure as the rigid PS domains come into contact. As predicted by this model, at compression beyond the plateau, the isotherms of Figure 2a shift progressively to larger molecular areas with the increasing concentration of PS homopolymer, consistent with an increase in the diameter of the hydrophobic domains. The increase in the diameter of the hydrophobic domains upon the addition of PS homopolymer is confirmed by the atomic force microscopy (AFM) images presented in Figure 3a. The dimensions of the PS domains, as evaluated from the AFM images, are reported in Table 2.

Table 2. Evolution of Surface Micelle Dimensions, as Determined from AFM Images, of PS-b-P2VP/PS Composite Films as a Function of PS Homopolymer Content polystyrene concentration (mg/mL)

average area of PS domains (nm2)

average diameter of PS domains (nm)

average height of PS domains (nm)

0.00 0.25 0.50 0.75 1.00

1900 3300 5200 8400 10200

50 64 82 104 114

8 9 12 14 15

These results clearly demonstrate that the addition of homopolymer is a convenient way to modulate the dimensions of the block copolymer template that does not require access to samples of variable molecular weight. With this approach, the average diameter of the micelles can be more than doubled, increasing from 50 to 114 nm upon the addition of 1 mg/mL of PS. There is, however, a limit. At concentrations above 1 mg/ mL of homopolymer, the periodic surface micellar structure is deformed, as illustrated by Figure S1 of the Supporting Information. At higher PS content, the hydrophobic domains appear to merge to form oblongs that coexist with circular surface micelles. 8885


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Figure 4. AFM images of monolayers, at three different scales, obtained by spreading 50−100 μL of a chloroform solution containing PS-b-P2VP copolymer (1.8 mg/mL) and P2VP homopolymer (about 0.8 mg/mL). Films were transferred to glass at a surface pressure of 15 mN/m.

Modification of Surface Micelle Spacing. An analogous approach can be employed to control micelle separation within the monolayer by the addition of P2VP homopolymer to the spreading solution. In this case, the compression isotherms show important variations, particularly at surface pressures below the pseudoplateau (Figure 2b). This observation is again in agreement with the model in which the hydrophilic P2VP forms a monolayer at the air−water interface. Since the added homopolymer occupies part of the surface, the monolayer is expanded, resulting in a larger mean molecular area per block copolymer P2VP repeat unit. The influence of the addition of P2VP homopolymer on film morphology is illustrated in the AFM images of Figure 4. These images demonstrate that, while surface micelle diameter remains invariant with the addition of P2VP, the separation between the hydrophobic domains is greatly increased. The periodicity of the surface micelle arrangement is, however, lost. The ability to modulate intermicelle separation is vital for the eventual spectroscopic characterization of individual nanostructures since such measurements require separation distances that exceed optical resolution. Modification of Plasmonic Ring Diameter. As previously reported for PS-b-PMMA,15 the spatial distribution of small gold NPs in block copolymer surface micelles is controlled by the size of particles and the chemisorbed ligands. With a fixed size of 6 nm and octanethiol ligands at their surfaces, NPs assemble at the PS−PMMA interface to form NRs. As illustrated in Figure 5, similar NP assembly can be achieved with a PS-b-P2VP template. All of the PS domains are surrounded by NPs and the resulting plasmonic NRs have almost the same diameter as the micelle core (56 nm in average). The size of the NP ring was systematically modified by addition of PS homopolymer as described above. The corresponding AFM images and numerical results are reported in Figure 6 and Table 3, respectively. It can be noted that, while the addition of C8-NPs does not significantly affect the film morphology, it does result in a slight increase (on the order of 10%) in micelle size. The location of the gold NPs within the composite films was determined by TEM. As shown by the images of Figure 7, in the presence of PS homopolymer, the NPs continue to assemble at the periphery of the hydrophobic domains. The diameters of the NRs, measured from the TEM images, are as indicated. In is relevant to note that at higher PS homopolymer concentrations (0.75−1.00 mg/mL) in the presence of NPs, the morphology of the composite films is sensitive to experimental conditions. Typically, LB films transferred to solid substrates immediately after monolayer compression

Figure 5. TEM image of a composite LB film of PS-b-P2VP and C8NPs formed by the spreading of 100 μL of chloroform solution at the air−water interface and transfer to a glass microscopic slide at a surface pressure of 15 mN/m.

exhibit a lamellar morphology. In order to obtain circular micelles surrounded by NP rings, the monolayers must be allowed to stabilize for 30 min at 15 mN/m prior to transfer to solid substrates. This phenomenon is illustrated by the TEM images provided in Figure S2 of the Supporting Information. Modification of the Thickness of the Rings. As previously reported by Lamarre et al.,15 NRs are only obtained for a specific NP size. Since particle−particle attraction increases with particle size, larger NPs are poorly dispersed within the block copolymer template and NRs are not obtained. Therefore, in order to obtain thicker rings, in situ NP regrowth was employed to increase the size of preincorporated NPs. The regrowth method, described in the experimental section, is based on a combination of the seed-mediated solution growth process developed by Jana et al.19 and the substratesupported diffusion rate-controlled growth reported by Muller et al.20 Once the 6 nm gold NPs are organized into rings within the block copolymer template, the supported monolayers are immersed in a growth solution containing a metal precursor and a very weak reducing agent (ascorbic acid). Gold ions diffuse from the growth solution to hydrophobic surface domains with the help of surfactant micelles (CTAB) and are reduced only at the surface of seed particles.19 After only a few minutes, the red color of the substrate-supported film intensifies while the growth solution remains transparent. As illustrated by the TEM images of Figure 8, this color change can be attributed to an increase in particle size within the NRs without significant nucleation of new particles elsewhere in the 8886


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Figure 6. AFM images of monolayers obtained by spreading 50−100 μL of chloroform solutions containing PS-b-P2VP copolymer (1.8 mg/mL) and PS homopolymer at the concentration indicated on the images. All films were transferred to glass at 15 mN/m and contain gold NPs (1 to 2 mg/mL).

According to the plasmon ruler equation established by Jain et al.,21 the relative LSPR shift induced by particle−particle coupling decays exponentially with interparticle distance, with a characteristic decay length that increases with the particle size. Normalization with respect to particle size leads to a universal decay length of about 0.2 times the NP diameter. For a given gap, coupling will therefore be stronger for larger NPs, and if the capping ligands limit NP approach to distances greater than the decay length, coupling will not be observed. DDA calculations carried out for the present system yield decay lengths of 0.9 and 1.5 nm for 6 and 10 nm NPs, respectively (see Figure S3 in the Supporting Information). These values are somewhat smaller than those predicted by the universal normalized decay length reported by Jain et al.,21 possibly because of the much smaller particle size in the present case. The classical approach of DDA may also not be sufficient for modeling coupling at such narrow interparticle gaps where electron tunnelling effects may be operative.22 In any event, the TEM images of Figure 8 clearly show separation distances of several nanometers for the majority of the NPs in the NRs. Some closely packed particle pairs are, however, observed. For such pairs, the octanethiol capping ligands can be estimated to limit particle−particle approach to about 2 nm. This distance is significantly larger than the decay length calculated for the 6 nm NPs, thus explaining the absence of coupling in this case. Experimental extinction spectra of the NP rings after in situ particle growth are presented in Figure 9b. In this case, there is a strong LSPR red-shift when the ring diameter is increased by the addition of PS homopolymer. This strong red-shift cannot be explained by the increase in NP size. Indeed, as shown in Figure S4 of the Supporting Information, both Mie theory and the DDA calculations predict a negligible difference in the LSPR frequency between 6 and 10 nm gold colloids with a surrounding medium refractive index of 1.59. The observed strong red-shift can rather be attributed to the appearance of a coupled plasmonic mode.23 During particle regrowth, both the

Table 3. Surface Micelle Dimensions in the Presence of C8NPs, Evaluated from AFM Images, as a Function of PS Homopolymer Concentration polystyrene concentration (mg/mL)

average area of PS domains (nm2)

average diameter of PS domains (nm)

average height of PS domains (nm)

0.00 0.25 0.50 0.75 1.00

2200 3800 7800 10000 11300

52 70 100 112 120

9 10 12 14 16

film. The particle size histograms show that, after 10 min of regrowth, the average particle diameter increases by about 4 nm. The interest of this regrowth is 2-fold. First, larger NPs provide higher extinction. Second, when the particles grow without changing their positions, the interparticle gap decreases and the coupling between neighboring gold NPs increases. Optical Properties of Gold NP Rings. Extinction spectra of the NR arrays transferred to glass substrates were recorded with a transmission optical microscope coupled to a microspectrometer as described above. As illustrated in Figure 9, spectra were recorded for two particles sizes (6 and 10 nm) and five ring diameters. In the case of small (6 nm) particles, the NR extinction spectra all exhibit a maximum near 530−540 nm and show no dependence on ring diameter. Although the NR spectra are significantly red-shifted with respect to that of isolated particles in suspension (maximum at 520 nm), this shift cannot be attributed to particle coupling. The observed LSPR spectral shift is rather the result of the different refractive indices of the surrounding medium; 1.44 in chloroform suspension versus 1.59 in the PS block copolymer domains. The spectra of Figure 9a therefore indicate that plasmon coupling is negligible in the case of small particles, presumably because the interparticle gap is relative large with respect to NP size.

Figure 7. TEM images of NRs obtained by spreading 50−100 μL of chloroform solutions containing PS-b-P2VP copolymer (1.8 mg/mL), PS homopolymer at the concentration indicated above the images, and gold NPs (1 to 2 mg/mL). All films were transferred to carbon coated microscope grids at a surface pressure of 15 mN/m. 8887


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Figure 8. TEM images of composite LB films of PS-b-P2VP and C8-NPs (a) before and (b) after immersion in the regrowth solution. Corresponding particle size dispersion histograms are provided in (c) and (d).

Figure 9. Experimental extinction spectra of LB films composed of PS-b-P2VP, PS homopolymer and C8-NPs organized into NRs for (a) 6 nm NPs before particle regrowth and (b) 10 nm NPs obtained by in situ regrowth. Spectra are provided for various NR diameters, as identified in the inset legend. The observed shift in the LSPR extinction maximum, relative to that of the smallest ring, is plotted in (c) as a function of NR diameter for both particles sizes.

reduction of the interparticle separation distances and the increase in the plasmon decay length lead to more efficient particle−particle coupling. Furthermore, the spectra in Figure 9b show a strong dependence on NR size, indicating that plasmon coupling is not limited to isolated particle pairs, but extends over a significant portion of the ring. Similar results were obtained by the DDA simulation of NR extinction spectra. As shown in Figure 10a, no plasmon coupling is predicted for NRs composed of 6 nm particles. This is also illustrated by the near-field maps in the top row of Figure 10c which shows that the plasmon extinction can be attributed exclusively to the dipolar mode of individual particles. Calculated spectra for NRs of larger particles are presented in Figure 10b. In this case, the spectra are significantly red-shifted with increasing NR diameter. The corresponding near-field maps of Figure 10c reveal that this spectral shift can be attributed to plasmon coupling that extends over a number of NPs that increases with ring diameter.

The experimental spectra of Figure 9b are much broader and less-well resolved than the corresponding calculated spectra of Figure 10b. This is to be expected from the heterogeneities of the experimental samples. In addition to NP size polydispersity, the TEM images also reveal considerable variations in particle spacing, as well as the presence of clusters and NP pairs with smaller separation gaps. With the experimental setup described in Figure 1, extinction spectra are recorded from an ensemble of approximately 5000 NRs. Because of variations in interparticle spacing and particle size, the extent of plasmon coupling will differ from one NR to another, leading to significant spectral broadening. Furthermore, many of the NRs are incomplete. It can therefore be anticipated that the experimental extinction spectra will contain a relatively important contribution from individual NPs in addition to the coupled modes. It is for this reason that future work will include the spectroscopic characterization of individual rings. Although the calculated spectra consistently shift with increasing ring diameter, the evolution of the plasmon 8888


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Figure 10. DDA simulated extinction spectra of 6 (a) and 10 nm (b) gold NPs organized into NRs of different diameters. (c) Mapping of the electric field calculated by the DDA approach at the wavelength corresponding to maximum plasmon intensity. The calculations correspond to normal incidence with horizontal linear polarization. Ring diameter (D) and NP diameter (NPs) are as indicated in the images.

diameter, thus explaining the delayed onset of spectral shifts in the experimental data.

resonance frequency is far from linear. As shown in Figure 10b, an important red-shift is observed for ring expansion from 54 to 64 nm, whereas subsequent increases in ring size produce only modest spectral changes. This result is in very good agreement with previously reported observations for linear NP chains.24,25 For example, Barrow et al.24 recorded extinction spectra for 64 nm gold NP chains of variable length (ranging from monomers to hexamers) and found that the magnitude of the red-shift decreases progressively with chain extension, eventually reaching a plateau. In the case of 10 nm NPs, the electric field maps of Figure 10c indicate that plasmonic coupling is restricted to approximately three particles in 54 nm NRs but extends to five or six NPs upon modest ring expansion to 64 nm. Despite the important difference in particle size, this corresponds very closely to NP chain lengths over which Barrow et al.24 observed the largest spectral shifts. Finally, it must be noted that the experimental variation in plasmon frequency with NR diameter differs significantly from the theoretical predictions. Unlike the calculated spectral evolution shown in Figure 10b, the experimental spectra of Figure 9b only exhibit measurable red-shifts at the largest ring diameters. This observation can be tentatively attributed to imperfections in the experimental NRs. The presence of incomplete rings and isolated particles will result in optical extinction near the single particle resonance frequency and can potentially mask red-shifted coupled contributions. According to the calculations, a significant spectral shift will only result from plasmonic coupling that extends over more than three NPs. The probability of forming uninterrupted arched chains exceeding this length can be expected to increase with NR

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CONCLUSIONS The self-assembly of an amphiphilic block copolymer (PS-bP2VP) and alkanethiol-capped gold NPs at the air−water interface is a convenient method for the fabrication of NP rings. Importantly, this approach allows for the systematic variation of NR diameter, in the range of 50 to 120 nm, through the addition of hydrophobic homopolymer. Furthermore, particle size can be increased by in situ growth, without loss of the ring structure. This is a significant result because NRs composed of smaller (6 nm) particles do not show plasmon coupling and larger NPs (10 nm) do not spontaneously self-assemble into rings. Finally, optical extinction spectra show that the extent of plasmon coupling increases with NR diameter. Since red-shifted coupled modes are predicted to be highly sensitive to local refractive index changes, the results reported here are highly relevant to the development of plasmon-based sensing applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01689. AFM image of a mixed PS-b-P2VP/PS Langmuir− Blodgett film at high homopolymer content; TEM images of mixed PS-b-P2VP/PS Langmuir−Blodgett films before and after surface relaxation; DDA simulated 8889


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Liquid-Liquid System. J. Chem. Soc., Chem. Commun. 1994, 0, 801− 802. (17) Draine, B. T.; Flatau, P. J. Discrete-Dipole Approximation for Scattering Calculations. J. Opt. Soc. Am. A 1994, 11, 1491−1499. (18) Choi, M.; Chung, B.; Chun, B.; Chang, T. Surface Micelle Formation of Polystyrene-B-Poly(2-Vinyl Pyridine) Diblock Copolymer at Air-Water Interface. Macromol. Res. 2004, 12, 127−133. (19) Jana, N. R.; Gearheart, L.; Murphy, C. J. Seeding Growth for Size Control of 5−40 Nm Diameter Gold Nanoparticles. Langmuir 2001, 17, 6782−6786. (20) Müller, M. B.; Kuttner, C.; König, T. A. F.; Tsukruk, V. V.; Förster, S.; Karg, M.; Fery, A. A Plasmonic Library Based on SubstrateSupported Gradiential Plasmonic Arrays. ACS Nano 2014, 8, 9410− 9421. (21) Jain, P. K.; Huang, W.; El-Sayed, M. A. On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation. Nano Lett. 2007, 7, 2080−2088. (22) Marinica, D. C.; Kazansky, A. K.; Nordlander, P.; Aizpurua, J.; Borisov, A. G. Quantum Plasmonics: Nonlinear Effects in the Field Enhancement of a Plasmonic Nanoparticle Dimer. Nano Lett. 2012, 12, 1333−1339. (23) Lamarre, S.; Sarrazin, A.; Proust, J.; Yockell-Lelièvre, H.; Plain, J.; Ritcey, A. M.; Maurer, T. Optical Properties of Au Colloids SelfOrganized into Rings via Copolymer Templates. J. Nanopart. Res. 2013, 15, 1−6. (24) Barrow, S. J.; Funston, A. M.; Gomez, D. E.; Davis, T. J.; Mulvaney, P. Surface Plasmon Resonances in Strongly Coupled Gold Nanosphere Chains from Monomer to Hexamer. Nano Lett. 2011, 11, 4180−4187. (25) Teulle, A.; Bosman, M.; Girard, C.; Gurunatha, K. L.; Li, M.; Mann, S.; Dujardin, E. Multimodal Plasmonics in Fused Colloidal Networks. Nat. Mater. 2014, 14, 87−94.

extinction spectra of gold NP pairs for various interparticle distances; DDA and Mie simulated extinction spectra of 6 and 10 nm gold NPs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 418-656-2368. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of le Fonds de recherche du Québec Nature et Technologies (FRQNT) and the National Sciences and Engineering Research Council of Canada (NSERC).

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Size-Modulation of Plasmonic Nanorings Obtained ... - ACS Publications

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