Evaluating Conditions for Strong Coupling ... - ACS Publications

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Evaluating Conditions for Strong Coupling between Nanoparticle Plasmons and Organic Dyes Using Scattering and Absorption Spectroscopy Gülis Zengin,† Tina Gschneidtner,‡ Ruggero Verre,† Lei Shao,† Tomasz J. Antosiewicz,†,§ Kasper Moth-Poulsen,‡ Mikael Kal̈ l,† and Timur Shegai*,† †

Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden § Centre of New Technologies, University of Warsaw, Banacha 2c, 02-097 Warsaw, Poland ‡

S Supporting Information *

ABSTRACT: Interactions between surface plasmons in metal nanoparticles and electronic excitations in organic chromophores have resulted in many notable findings, including single-molecule Raman scattering, nanoscale lasing, and enhanced fluorescence. Recently, plasmon−exciton interactions have been shown to reach the strong coupling limit, a nonperturbative regime in which a coupled plasmon− exciton system should be treated as a unified hybrid. Strong coupling effects could open up exciting possibilities for manipulating nanoparticle plasmons via molecular degrees of freedom, or vice versa. Optical properties of such hybrid systems can differ drastically from those of noninteracting components. Specifically, optical spectra of a strongly coupled system are expected to exhibit mode splitting due to Rabi oscillations of excitation energy between the system components. However, the interpretation of optical spectra in terms of strong coupling is not a straightforward matter. Here we clarify the nature of plasmon−exciton coupling for the case of rhodamine-6G (R6G) interacting with localized surface plasmons in silver nanodisks using scattering and absorption spectroscopy. We show that this system is only marginally able to reach the strong coupling limit, even for very high molecular concentrations and despite the appearance of obvious mode splitting in scattering. For lower molecular concentrations, the mode splitting we observe should be interpreted as being due to surface-enhanced absorption rather than strong coupling. These results allow us to evaluate the critical concentration necessary for reaching the strong coupling limit and propose conditions for observing strong coupling between single-particle plasmons and organic dyes, such as R6G.

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INTRODUCTION Noble metal nanostructures are well-known for exhibiting pronounced surface plasmon resonances (SPRs), collective oscillations of conduction electrons at the metal surface, when excited by visible light. Nanoparticle plasmons are characterized by deep subwavelength optical mode volumes and strong electromagnetic field enhancement, characteristics which make them suitable as optical antennas for coupling light to molecules, as is the case in surface-enhanced Raman scattering,1−3 fluorescence,4 infrared absorption,5,6 refractometric SPR sensing,7 molecular imaging,8 plasmon-enhanced lightharvesting in dye-sensitized solar cells,9 and plasmon-enhanced dye-lasers.10 In most of the above examples, it is reasonable to think about the metal nanostructures as passive antenna devices that may drastically amplify the molecular optical cross sections but which are themselves not strongly influenced by the interaction; in other words, the plasmon and the molecule(s) are weakly coupled, and the back-action of the molecule on the plasmon is negligible in terms of inducing new coupled resonances. © 2016 American Chemical Society

The strength of the plasmon−molecule coupling, however, can be increased through adjustment of several parameters, including the plasmon and exciton spectral overlap,11,12 mode volume,13,14 damping rates, oscillator strength of the molecular resonance,15−18 number of interacting molecules,19 and position and orientation of the molecule(s) relative to the enhancing antenna structure.20,21 In particular, under special circumstances, that is, when the plasmon and the molecular exciton are degenerate in energy and the plasmonic mode is so compressed that the coupling strength surpasses both the molecular and the plasmonic damping rates, the interacting system can enter the strong coupling regime in which an excitation oscillates back and forth between the metal and the molecule(s) at a characteristic Rabi frequency.22,23 Strong Special Issue: Richard P. Van Duyne Festschrift Received: January 8, 2016 Revised: February 19, 2016 Published: February 23, 2016 20588

DOI: 10.1021/acs.jpcc.6b00219 J. Phys. Chem. C 2016, 120, 20588−20596

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The Journal of Physical Chemistry C

Figure 1. (a) Schematic illustration of the attachment of R6G via a thiol bond on the surface of silver nanodisks. (b) AFM image of Ag nanodisks made by colloidal lithography on a glass substrate. (c) Four different ways to adsorb R6G on Ag nanodisk: ordinary and thiolated R6G dissolved in methanol and water. (d) Left: photograph of Ag nanodisk samples covered with R6G in four different ways, samples 1−4. Right: dark field micrographs of the same samples.

dimer excitonic states.28 Similarly, spectral dips and mode splitting at the absorption maxima of R6G interacting with SPRs at Ag films were interpreted as a vacuum Rabi splitting by Hakala et al.29 More recent studies have focused on R6G coupled to nanoparticle lattices.30−32 Although dispersion curves showed anticrossing behavior, which is often considered characteristic of strongly coupled systems, the extracted coupling energies were found to be smaller than both the R6G and the plasmon line widths and thus outside the strong coupling regime. This brings up the question of how to interpret spectral modifications, in particular “spectral dips”, observed in scattering and transmission and extinction spectra in terms of coupling.25 It was pointed out in several works on cavity−exciton coupling that absorption and emission are reliable quantities for drawing conclusions about the coupling strength.33,34 However, for plasmonic cavities, especially in the case of single-nanoparticle studies, elastic scattering is often the primary experimental choice. This complicates the interpretation of optical properties of plasmon−exciton hybrids and may even lead to erroneous conclusions.17 This contribution focuses on clarifying the mechanism(s) leading to spectral dips in scattering and transmission spectra of plasmon−R6G systems through experiments with improved control over the molecular adsorption process, as illustrated in Figure 1a. In most previous studies, R6G was unspecifically adsorbed on the metal surface by drop casting or spin coating or by simply mixing the dye and plasmonic nanoparticles in solution.30,32,35 Here, we instead use a newly synthesized thiolated form of R6G with an ability to form a self-assembled monolayer (SAM) on a silver surface. In our experiments, R6G is coupled to disordered arrays of nanodisks produced by colloidal lithography36 (Figure 1b). We observe significant

coupling effects are highly interesting from both fundamental and applied points of view, in particular in the context of possible quantum optics applications.24,25 The specific molecule that we investigate here, rhodamine6G (R6G), is a popular organic fluorophore that has a strong and distinct color, high photostability, and a fluorescence quantum yield close to unity. Its absorption bands overlap with some of the most common laser lines: 514.5 nm Ar+ line and the Nd:YAG second harmonic at 532 nm. These features make R6G extremely useful in applications ranging from dye lasers to biotechnology. In one of the first reports on interactions between R6G and plasmonic nanoparticles, a spectral “dip” in the extinction spectra of Ag island films at a wavelength coinciding with the R6G absorption band was observed and interpreted as a surface-enhanced absorption effect.26 However, since the first experimental observation of intense surface-enhanced Raman scattering (SERS) originating from R6G adsorbed on colloidal Ag nanoparticles by Hildebrandt and Stockburger,27 most studies, including some of the first single-molecule studies,3 have concentrated on SERS. Nevertheless, over the past few years, several studies focusing on resonant interactions between R6G and plasmonic structures have been performed.12,28−32 Van Duyne’s group studied resonant interactions between silver nanotriangles covered with R6G using extinction spectroscopy and found complicated dependence of plasmon resonance peak position on the dimensions of nanoparticles, but no strong coupling was reported in this case.12 Cade et al. studied nanostructured Ag films with adsorbed R6G and found complex three-peaked extinction spectra and anticrossing dispersion behavior that was interpreted as strong coupling and hybridization of the plasmon and R6G in monomer and H20589


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Table 1. Calculation of Effective Permittivity of the Molecular Layers around the Ag Nanodisks in FDTD Calculationsa ordinary R6G

molecule densityb number of molecules per nanodisk reduced oscillator strength, f 2/f1c

thiolated R6G

sample 1

sample 2

sample 3

sample 4

methanol ∼0.16 nm−2 ∼3.5 × 103 0.016/0.005

water/methanol (4:1)

methanol

water/methanol (4:1)

∼1.0 nm−2 ∼2.4 × 104 0.024/0.008

∼4.2 nm−2 ∼1.0 × 105 0.0098/0.0033

NA ∼1.8 × 106 0.15/0.05d

a

Measured molecular density and number of molecules per nanodisk as well as oscillator strengths of the main and the secondary peaks in each considered case. The molecule density is calculated from thiol exchange experiments. bCalculated assuming the same thickness of the dye layer as in FDTD calculations (1.5, 6, 4, and 24 nm for samples 1−4, respectively). cNote nonoptimal f in the case of sample 3 (see text for details). dThe inner 4 nm of the coating is assumed to have the same optical properties as the SAM of sample 3.

solubility in water because of the hydrophobic linker; therefore, the water solution was diluted by methanol in a 4:1 water/ methanol volume ratio to fully dissolve the dye. The HCL samples were immersed in four R6G solutions: ordinary R6G in methanol, ordinary R6G in a 4:1 water/methanol solution, thiolated R6G in methanol, and thiolated R6G in a 4:1 water/ methanol solution for at least 18 h to saturate the surface. After that, all substrates were rinsed twice with the corresponding solvents to remove excess dye. The samples were then dried with compressed nitrogen. Quantification of the Number of R6G Molecules. A thiol-exchange reaction was used to quantify the number of R6G molecules absorbed on Ag nanoparticles for both thiolated and ordinary R6G. Each sample was dipped in 20 mM 1dodecanethiol in DMF. 1-Dodecamethiol is a short thiol molecule that is able to replace dye on the Ag nanoparticle surface. The replaced R6G molecules were then collected and quantified using optical absorption. This data is summarized in Table 1. The details of the thiol exchange experiment are given in Figure S9. Optical Measurements. Dark-field measurements were done using an inverted microscope (Nikon TE-2000E) equipped with a variable NA oil-immersion objective (100×; NA = 0.5−1.3, Nikon) and a dark-field condenser (Nikon, NA = 0.8−0.95). To average the scattering signal from many nanoparticles (∼50 μm area), we use a Bertrand lens to perform back focal plane spectroscopy. We measured different parts of the sample to check the reproducibility and uniformity of the spectra. For photobleaching, we used a 532 nm laser (irradiance 20 W/cm2) in epi-illumination mode and the NA of the objective was set to 1.3. Spectroscopic measurements were done by a fiber-coupled spectrometer (Andor SR-303i-B). Color images were taken by a Nikon D300s DSLR camera. FDTD Simulations. Numerical electrodynamics calculations were carried out using a commercial FDTD solver (FDTD Solutions 8.7.1, Lumerical Inc.). An 80 nm diameter Ag disk with a thickness of 27 nm was placed on top of a substrate with a refractive index of 1.45. Between the gold and glass we placed a 1 nm Cr adhesion layer. Such structures quantitatively matched the peak position and width of the experimentally measured samples. The permittivity of an R6G molecule was modeled by two Lorentzians with resonances positioned at 499 and 530 nm with reduced oscillator strengths of f1 = 0.23 and f 2 = 0.69, respectively;40 line widths of 165 meV; and background permittivity of 2.10 in accordance with

modifications of scattering and absorption spectra of silver nanodisks upon interaction with R6G. Nevertheless, numerical calculations using the finite-difference time domain (FDTD) method, as well as morphological characterization using wide field optical microscopy and atomic force microscopy (AFM), clearly show that surface-enhanced absorption rather than strong coupling is the main mechanism behind these spectral modifications. Only in the case of the densest molecular coverage here considered does the system just reach the strong coupling regime, although proof for this is obtained only from FDTD calculations. Furthermore, we discuss conditions for observation of strong coupling between nanoparticle plasmons and R6G molecules based on general principles. We conclude that true strong coupling between a nanoparticle plasmon and R6G requires immense amount of dye, in line with experiments. Our findings are in agreement with a number of previous studies on strong coupling between Fabry−Pérot cavities and organic dyes.37−39

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MATERIALS AND METHODS Synthesis of Thiolated R6G. The synthesis of the thiolated form of R6G is summarized in the Supporting Information. A detailed description of the synthesis and characterization of the intermediate compounds can be found in Figures S1−S6. The absorption spectrum of the dye is characterized by a peak with the maximum at wavelength of λ = 529 nm accompanied by a secondary peak at wavelength of λ = 493 nm in methanol as shown in Figure S7. Comparison of the thiolated R6G with ordinary R6G is given in Figure S8. Preparation of R6G-Coated Silver Nanodisk Arrays. Silver nanodisks on glass substrates were made using so-called hole−mask colloidal lithography (HCL).36 In short, 80 nm polystyrene beads served as a mask to produce Ag disks with a diameter of 80 nm by evaporating 25 nm of Ag on top of 2 nm of Cr used as an adhesion layer on top of the glass substrate. The samples were cleaned by ethanol and deionized water and then dried with compressed nitrogen. We note that Ag nanodisks used in this study are polycrystalline, which reduces their quality factors in comparison to single crystalline nanoparticles because of additional electron scattering at grain boundaries. However, in this study we were not concerned with minimizing material losses, because inhomogeneous broadening plays an equally important part in these samples. Both thiolated and ordinary R6G were prepared in 1 mM concentration in two different solvents (methanol and water) in order to study the effect of solubility. We thus obtain four different samples (samples 1−4) as sketched in Figure 1c,d. Both ordinary and thiolated R6G exhibit better solubility in methanol than in water. However, thiolated R6G has very poor

2

ε(ω) = ε∞ + ∑ j = 1 f j ω 2

0, j

ω0,2j − ω2 + iγ0, jω

. These parameters yield the

absorption cross-section of a “numerical” R6G molecule of the order of 3 × 10−16 cm2, which is in good agreement with the 20590


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Figure 2. Scattering of thiolated and ordinary R6G on silver disks prepared in two different solvents: sample 1, ordinary R6G in methanol; sample 2, ordinary R6G in a 4:1 water/methanol mixture; sample 3, thiolated R6G in methanol; and sample 4, thiolated R6G in 4:1 water/methanol. (a) Experimental scattering spectra for these four cases. The scattering spectra of Ag particles covered with R6G before and after photobleaching of the dye are shown in magenta and blue, respectively. (b) Simulated scattering spectra for these four cases. Magenta curves show Ag nanodisks coated with R6G, and blue curves show the uncoupled case.

lack of morphological characterization, in analysis of sample 4 we rely on FDTD calculations. Scattering and Absorption of R6G Coupled to Nanoparticle Plasmons. The scattering spectra of Ag nanoparticles coated by ordinary and thiolated R6G molecules in two different solutions are shown in Figure 2. The experimental and calculated spectra are shown in the top and bottom columns, respectively. The magenta curves represent the spectra of coupled dye−Ag nanoparticles, while blue ones represent the photobleached spectra. We observe a disappearance of molecular absorption features and a concomitant recovery of the plasmon resonance peak after photobleaching in all studied samples, which proves that the spectral modifications of the optical response of these nanoparticles are caused by R6G. We observe that both the thiol linker and solvent effects influence the molecular surface coverage. As expected, the thiolated version of R6G can be adsorbed on the metal surface in higher density because of chemical affinity of thiols to noble metals. Additionally, both ordinary and thiolated R6G exhibit low solubility in water; therefore, higher surface densities can be expected in comparison to pure methanol, because of formation of insolvable molecular aggregates which subsequently bind to the metal surface. These expectations are consistent with the thiol exchange reactions (see Table 1 for data and extracted molecular coverages). We now turn to a detailed description of spectral modifications in the plasmonic optical response caused by R6G. For the case of ordinary R6G in methanol (see Figure 2a, sample 1) we observe no significant spectral differences between scattering spectra of coupled and uncoupled systems. Thiol exchange experiments indicate that the number of adsorbed molecules in this case is ∼3.5 × 103. Assuming all these molecules are adsorbed on the disk (with dimensions of D = 80 nm and H = 25 nm), the resulting surface coverage is ∼0.16 nm−2, which is about ∼1.6 times lower than a dense monolayer of R6G molecules lying flat on the metal surface.41

experiment. To calculate the permittivity of each layer, which we assumed is made up only of R6G molecules, the initial values of reduced oscillator strengths, f1 and f 2, were modified in accordance with the volume fraction that the R6G occupies in each effective molecular layer in the four studied cases. We did this by measuring the number of molecules per Ag disk and relating their volume to the volume of the dielectric layer they form. Here, we assumed that an ordinary R6G layer is 1.5 nm thick (approximate molecule length), while a thiolated one is 4 nm thick (thiol + molecule length). In a simple volumetric analysis, an ordinary R6G molecule occupies approximately 0.24 nm3, while the thiol linker 0.12 nm3. The calculations are summarized in Table 1.

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RESULTS AND DISCUSSION

Distribution of R6G Molecules around Nanodisks. Plasmon−exciton interactions are very sensitive to the number of involved molecules as well as their distribution and orientation around nanostructures. To evaluate molecular distribution around the nanostructures, we have performed morphological studies using atomic force microscopy (AFM) (see Figure S13). We observe that for samples 1−3, the nanostructures look similar to bare nanoparticles, implying that the core−shell model shown in Figure 1a is a valid approximation. The thickness of dye layer in these cases can be accessed from a combination of AFM measurements, total number of molecules deduced from thiol exchange reactions, and FDTD calculations. The results are summarized in Table 1. The case for thiolated R6G in water was quite different though. In this case, the quality of AFM scans was very poor (data not shown), likely because the interface was too soft to allow acquisition of better images. We attribute this to a formation of soft insoluble fragments of dye at the interface, as confirmed by dark field microscopy images of sample 4 (Figure 1d). These kinds of large insoluble fractions of dye are likely formed because of the hydrophobic nature of R6G. Because of 20591


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Figure 3. Absorption of thiolated and ordinary R6G on silver disks for four different cases. Sample 1, ordinary R6G in methanol; sample 2, ordinary R6G in a 4:1 water/methanol mixture; sample 3 thiolated R6G in methanol; sample 4 thiolated R6G in 4:1 water/methanol. (a) Experimental absorption spectra of R6G on silver disks are plotted with green, and silver disks without dye are plotted with dashed brown lines. (b) Simulated absorption cross sections: green curves show the total absorption cross-section of the coupled R6G-Ag nanoparticles system, while magenta and orange curves show separate contributions of metal and dye, respectively. Dashed curves represent absorption of the same amount of R6G with no plasmonic particles present multiplied by the enhancement factor.

sample 3). This is a surprising result, which could arise for several possible reasons. Unfavorable molecular orientation with respect to the local electromagnetic field;20 strong intermolecular interactions in dense molecular layers; and increased sensitivity of the thiolated R6G to its chemical environment, in particular pH (Figures S6 and S7), are possible scenarios. FDTD simulations shown in Figure 2b, sample 3 are quantitatively similar to the experiments, but only because the oscillator strength of the model dye was decreased 10-fold with respect to the value resulting from the filling fraction. In other words, in such a dense SAM, the absorption efficiency of each individual molecule is greatly reduced. This reduction cannot be explained by differences in absorption cross sections of ordinary and thiolated dyes, because they are very similar (see Figure S8), but could instead originate from the abovementioned factors or their combination. Finally, a thiolated R6G in water/methanol mixture is shown in Figure 2a, sample 4. Spectral measurements in this case reveal very significant spectral modifications of the scattering spectra at the absorption lines of R6G. Through thiol exchange experiments we estimate the coverage to be ∼1.8 × 106 molecules per nanodisk, corresponding to very thick molecular overlayers. AFM data in this case is not available; therefore, we have to rely on FDTD in estimating the parameters of this system. By using a 20 nm thick outer dye layer with the reduced oscillator strength of 0.15 (which accounts for ∼30% of the total number of molecules per nanodisk) and an inner 4 nm thick layer with the same characteristics as in sample 3, we find good agreement with the experiment. The other 70% of the molecules are concentrated in large insoluble fragments (see Figure 1d, sample 4), which on one hand do not interact with nanoparticle plasmons and on the other do not significantly contribute to the scattering response. We therefore neglect these large clusters in FDTD calculations, which yet seem to reproduce the experiment well (see Figure 2, sample 4). We note, however, that in the case of sample 4 we only assumed a decrease of the oscillator strength of the inner 4 nm (as for

AFM data suggests that the difference between the bare sample and sample 1 was negligible, which is in line with the low molecular coverage. The calculated scattering cross-section is in good agreement with the experimental observations, assuming the adsorbed molecules are evenly distributed within a 1.5 nm layer. As the dye density is increased up to ∼1.0 nm−2, the spectral dip becomes more prominent (see Figure 2a, sample 2). Such molecular surface density is nearly 4 times denser than a single monolayer of flat oriented molecules and 6.25 times denser than in sample 1. We interpret this as being due to formation of multiple R6G overlayers on the metal surface, likely because of poor solubility of R6G in water. This is confirmed by AFM data (Figure S13), which shows that particle heights are increased with respect to the bare sample. In calculations (Figure 2b, sample 2) we observe a good agreement with experiment, assuming the molecules are distributed over a thicker, 6 nm, layer due to formation of these overlayers. In the case of thiolated R6G, the surface coverage is further increased because of the ability of thiols to form SAMs on gold or silver surfaces (see Figure 2a, sample 3). The thiol exchange experiments in this case yielded an even higher amount of adsorbed molecules, ∼1.0 × 105 per nanodisk, which corresponds to a dense SAM coverage of 4.2 nm−2. This is in good agreement with earlier reports on alkanethiols HS(CH2)x-CH3 SAMs on Ag(111), which suggested a molecular surface density of ∼5.45 nm−2.42 We thus conclude that in this case we form a SAM of thiolated R6G on the silver surface. This is further confirmed by AFM measurements, which did not show significant changes in particle profiles in comparison to bare nanodisks in this case, in line with just a singlemolecular layer present at the surface. Note that the molecular density within these thiolated R6G SAMs is much denser than in samples 1−2, which implies differences in molecular packing and orientation in comparison to those samples. However, despite a significant increase in molecular coverage (26-fold), we observe only a weak scattering dip in this case (Figure 2a, 20592


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absorption of coupled R6G molecules is enhanced 22 times; however, this is because the reference calculation was also done using nonoptimal 10-fold reduced oscillator strength. On the basis of this discussion above, we conclude that samples 1−3 correspond to realization of the surface-enhanced absorption scenario, despite observation of pronounced scattering dips.17 This implies an inherent difficulty of studying strong coupling phenomena based exclusively on scattering optical signal, a situation that is rather widely spread in the literature.25 Our results for samples 1−3 are in line with the earlier data of van Duyne et al., which showed no splitting but rather plasmonic shifts in the extinction measurements of silver nanotriangles covered with dense R6G layers.12 Interpretation of sample 4, the one containing the largest amount of R6G, is however more complicated. The main difficulty here is that the experiment seems to show no clear splitting, but rather a strongly red-shifted plasmon band on top of which absorption peaks of R6G are visible, while FDTD calculations indicate entering the onset of strong coupling (see Figure 3, sample 4). The situation gets further complicated because of presence of uncoupled molecules, which partially conceal the dip both in experiments and calculations. To overcome this complexity, we rely on FDTD calculations in further analysis. In particular, to unequivocally answer the question of whether this system is able to reach strong coupling, we performed a numerical calculation of mode anticrossing. Because of lack of natural variable parameters that would preserve the metal-to-dye ratio, we vary the plasma frequency, ωp, of the metal in a range from 7 to 12.5 eV (natural ωp is about 9 eV, as accessed from the Drude model fitted to experimental measurements43). In Figure 4 we show

sample 3), the thickness of a thiolated SAM, while the rest of the dye had normal parameters. We interpret this as indicating molecular orientation, intermolecular coupling, and chemical environment of the majority of R6G in this case were different from those of sample 3. Interestingly, the concentration of R6G molecules in the dye layers of samples 3 and 4 is so high, ∼1 nm−3, that it corresponds to densities within closely packed molecular crystals. We now turn to the interpretation of the spectral modification in the scattering response due to the presence of R6G molecules as shown in Figure 2. In all scattering spectra we observe scattering dips due to plasmons interacting with excitons. It is especially evident in the case of water/methanol mixtures: samples 2 and 4. Naively we could assign such spectra to strong coupling interaction with giant Rabi splittings reaching ∼100 meV for sample 2 and ∼500 meV for sample 4. However, in the following part of the article we will argue that it is not enough to observe spectral dips, even as pronounced as we see in sample 4, to draw accurate conclusions about the interaction regime. Our arguments are based on absorption spectra of the coupled system, FDTD simulations, and morphological study of dye molecule distribution. So far we have discussed scattering of disordered silver nanodisk samples interacting with R6G molecules. Although scattering is an important quantity, especially for single-particle measurements, the absorption cross-section is more appropriate for studying strong coupling, as has been suggested earlier.17,22 Here, we evaluate absorption via performing transmission and reflection measurements at near normal incidence by A = 1 − T − R (a detailed transmission and reflection data used for evaluation of absorption spectra is shown in Figure S11). Note that we neglect diffuse scattering when evaluating the absorption spectra. Importantly, the absorption data is very different from the scattering. In particular, we observe no spectral dips in the absorption spectra (Figure 3), which is in sharp contrast with the scattering discussed above (Figure 2). FDTD calculations further corroborate absence of splitting in the absorption data for all cases except sample 4 (see Figure 3, bottom row). In addition, our calculations allow evaluating separate contributions of metal and R6G into the total absorption. We plot total absorption in the whole metal−dye hybrid (green curves), as well as absorption contributions in metal (magenta) and dye layers (orange). This last value is compared with the absorption in the same amount of dye without the plasmonic disk (dashed blue) multiplied by the effective enhancement factor given in the inset. We observe that the absorption enhancement factor decreases for the denser layers R6G, presumably because of the fast decay of electromagnetic field outside of silver nanoparticle, resulting in molecules being exposed to a diminished enhancement. Accordingly, for ordinary R6G dissolved in methanol, calculations show that absorption in the submonolayer of R6G on Ag nanoparticles is enhanced 30-fold (Figure 3b, sample 1), while in the densest case (Figure 3b, sample 4), the presence of R6G aggregates implies that most of the molecules do not interact with the enhanced plasmonic near-fields. Hence, when comparing absorption of R6G with and without the Ag disk, we arrive at an enhancement of only ∼2. This is because in this case the spatial extent of R6G (layer thickness of 24 nm) goes beyond the decay length of the electric field (d1/e ≈ 20 nm) resulting in effectively lower enhancement values. We should also note that in the SAM case (Figure 3b, sample 3),

Figure 4. Anticrossing in a numerical experiment in which the plasma frequency of metal changes from ca. 9 eV (appropriate for silver) in a range from 7 to 12.5 eV to sweep the plasmon across the R6G absorption lines. (a, c) Scattering and (b, d) absorption for (a, b) ordinary R6G and (c, d) thiolated R6G. The curved dashed lines trace approximately the peaks of the observed resonances. The vertical lines in absorption spectra show the main transitions in R6G. 20593


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R6G was adsorbed on top of ordered arrays of plasmonic nanoparticles by spin-casting of extremely concentrated dye solutions.30,32,35 For the case of disordered arrays studied here, we calculate the mode volume of an individual plasmonic disk as ∼V = 105 nm3, which then results in NR6G on the order of 105 per nanoparticle necessary to reach the strong coupling limit. By inspecting Table 1, we observe that these high numbers are achievable only for samples 3 and 4. However, in the case of sample 3, the scattering data indicates effective lowering of the oscillator strength. Thus, it is only sample 4 that satisfies eq 1. Importantly, both simple estimate from eq 1 and numerical anticrossing results shown in Figure 4 indicate that the necessary concentration of R6G molecules around plasmonic nanostructures is close to 1 nm−3, which is at the physical limit of densely packed molecular crystals. It is thus interesting to compare these findings to results reported in refs 30 and 32 where very concentrated dye solutions (30 wt % and 200 mM in ethanol, respectively) were used. In Figure S14, we report similar measurements using high concentrations of spin-casted dye. We also perform a morphological study on these samples, which shows that large insoluble fragments of R6G that do not couple to plasmonic nanoparticles are found (Figure S15). These results again emphasize that molecular distribution around nanoparticles is very important, and care should be taken before drawing conclusions on whether the hybrid system can be found in the strong coupling regime, especially based on scattering experiments.

results of these calculations for sample 2 (low packing density of ∼0.1 nm−3) and sample 4 (high packing density ∼1 nm−3) cases of ordinary and thiolated R6G in water. In the low-density case (sample 2, Figure 4a), scattering shows an avoided crossing and splitting of ∼200 meV. However, in absorption (Figure 4b), the resonances cross, indicative of enhanced absorption.17 For thiolated R6G in water, the scattering (Figure 4c) shows wider splitting (∼500 meV) and simultaneously absorption also shows anticrossing behavior (Figure 4d). Hence, this interaction can be classified as strong coupling. Using absorption to estimate the splitting, we obtain ∼400 meV, a value smaller than that estimated from scattering. Comparing this value to the widths of the plasmon (300 meV at ωp = 9 eV) and R6G (165 meV), we note that the splitting is greater than both of these values. It is noteworthy that Figure 4d also shows crossing-like behavior, which is indicative of a considerable amount of dye not coupled to the plasmon.17 This superfluous dye disturbs not only the absorption but also the scattering spectrum by absorbing part of the scattered light originating from the coupled metal−dye hybrid and inflating the splitting. This highlights the danger of using large amounts of dye to obtain dense layers around metal nanoparticles. While such dense layers undoubtedly increase the coupling efficiency, dye not coupled to particle plasmons may distort the scattered signal, which complicates the analysis. Conditions for Observation of Strong Coupling between Nanoparticle Plasmons and R6G. We now turn to more general conditions that must be fulfilled for R6G to reach strong coupling with the nanoparticle plasmons. Let us perform an order-of-magnitude estimate of the number of molecules, N, that are necessary to reach strong coupling. In accordance with refs 23 and 39, ℏΩR = 2 N μe|Evac|, where ℏΩR is the Rabi splitting, |Evac| =

ℏω 2εε0V

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CONCLUSIONS In this study we have investigated coupling between nanoparticle plasmons and electronic excitations in one of the most commonly used chromophores, R6G. We further performed an optical comparison study between R6G and newly synthesized thiolated R6G in two different solvents. The solubility of both dyes significantly decreases in water compared to methanol leading to an increase in the number of molecules interacting with Ag nanoparticles. Depending on the solvent, thiolated R6G can be adsorbed on Ag surface either as SAMs or dense molecular overlayers. For higher dye concentration, stronger modifications in the optical response, manifested as dips in the scattering spectra, were observed. FDTD calculations agree well with the experimental observations and suggest that surfaceenhanced absorption is the dominating effect responsible for these spectral modifications; however, the sample with the densest dye layer is just reaching the strong coupling limit. The number of R6G molecules adsorbed on Ag nanodisks was quantified using thiol exchange experiments and ranged from a few thousands to a few millions per nanoparticle. Despite these large numbers, strong coupling between R6G and nanoparticle plasmons was not reached in samples 1−3 and only marginally reached for the densest molecular coverage, sample 4. By using numerical calculations and estimations based on the mode volume of metal disks, we were able to derive a critical concentration of R6G molecules necessary to reach the strong coupling limit, 1 nm−3 or 105 per particle if V = 105 nm3. These values are very high and are comparable to the dye packing in the densely packed molecular crystal. Thus, although reaching strong coupling using organic dyes like R6G is marginally feasible, we warn against the risk of utilizing high dye concentrations without checking molecular distribution around nanostructures. Among possible ways to facilitate observation of strong coupling, we list usage of high transition

the vacuum field, and

V the mode volume. We further assume that N ≈ ncV, representing the case of dye molecules saturating the nearest vicinity of the nanoparticle at some unknown critical concentration, nc, which is just enough to reach the strong coupling condition. For simplicity, we assume optimal molecular orientation and ignore inhomogeneity of the field distribution over the mode volume. We also assume that the saturation of coupling is achieved by filling the volume roughly comparable to the mode volume. The expression for the Rabi splitting thus reduces to ℏΩ R = 2μe

ncℏω 2εε0

. By further setting

the strong coupling condition to be ℏΩR ≈ ℏγpl and substituting Q = ωpl/γpl, we obtain the condition for the critical dye concentration: nc =

ℏγpl εε 0 2Q μe 2

(1)

Note that V does not enter the final expression. Furthermore, by setting reasonable parameter values, such as μe ≈ 1 D, Q ≈ 10, and ℏγpl ≈ 100 meV, we obtain nc ∼ 1.9 nm−3. Such density is rather high even in comparison to closely packed molecular crystal (density of R6G crystal is 1 g/cm3, which corresponds to molecular density of ∼1 nm−3). Despite its order-of-magnitude accuracy, this result illustrates one important point: reaching strong coupling with dye molecules that have a transitional dipole moment of about μe ≈ 1 D requires extreme densities of dye to be packed within the near-field zone of the nanoparticle. Indeed, this agrees well with the recent experiments, in which 20594


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dipole moment materials, such as, e.g., J-aggregates or quantum dots, and compression of the mode volume by using smaller nanoparticles or nanoparticle pairs.23,44 We believe these observations shed new light on strong coupling in plasmonics and will help to design applications based on exciton−plasmon interactions.

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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.6b00219. Additional discussion of experimental procedures, sample preparation, calibration procedures, AFM data, and transmission and reflection spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from Swedish Research Council (VR), Swedish Foundation for Strategic Research, Göran Gustafsson Foundation, and Knut and Alice Wallenberg Foundation. K.M.-P. acknowledges funding from the European Research Council (ERC). T.J.A. also acknowledges support from the Foundation for Polish Science via the project HOMING PLUS/2013-7/1.

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