Plexcitons: The Role of Oscillator Strengths and Spectral Widths in

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Plexcitons: The Role of Oscillator Strengths and Spectral Widths in Determining Strong Coupling Reshmi Thomas, Anoop Thomas, Saranya Pullanchery, Linta Joseph, Sanoop Mambully Somasundaran, Rotti Srinivasamurthy Swathi, Stephen K. Gray, and K George Thomas ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06589 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Plexcitons: The Role of Oscillator Strengths and Spectral Widths in Determining Strong Coupling Reshmi Thomas,1 Anoop Thomas,1 Saranya Pullanchery,1 Linta Joseph,1 Sanoop Mambully Somasundaran,1 Rotti Srinivasamurthy Swathi,1,* Stephen K. Gray,2,* and K. George Thomas1,*

1

School of Chemistry, Indian Institute of Science Education and Research

Thiruvananthapuram (IISER-TVM), Vithura, Thiruvananthapuram 695551, India

2

Center for Nanoscale Materials, Argonne National Laboratory Argonne, Illinois 60439, USA

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ABSTRACT

Strong coupling interactions between plasmon and exciton-based excitations have been proposed to be useful in the design of optoelectronic systems. However, the role of various optical parameters dictating the plasmon-exciton (plexciton) interactions is less understood. Herein, we propose an inequality for achieving strong coupling between plasmons and excitons through appropriate variation of their oscillator strengths and spectral widths. These aspects are found to be consistent with experiments on two sets of free-standing plexcitonic systems obtained by (i) linking fluorescein isothiocyanate on Ag nanoparticles of varying sizes through silane coupling and (ii) electrostatic binding of cyanine dyes on polystyrenesulfonate-coated Au nanorods of varying aspect ratios. Being covalently linked on Ag nanoparticles, fluorescein isothiocyanate remains in monomeric state and its high oscillator strength and narrow spectral width enable us to approach the strong coupling limit. In contrast, in the presence of polystyrenesulfonate, monomeric forms of cyanine dyes exist in equilibrium with their aggregates: coupling is not observed for monomers and Haggregates whose optical parameters are unfavorable. The large aggregation number, narrow spectral width and extremely high oscillator strength of J-aggregates of cyanines permit effective delocalization of excitons along the linear assembly of chromophores, which in turn leads to efficient coupling with the plasmons. Further, the results obtained from experiments and theoretical models are jointly employed to describe the plexcitonic states, estimate the coupling strengths and rationalize the dispersion curves. The experimental results and the theoretical analysis presented here portray a way forward to the rational design of plexcitonic systems attaining the strong coupling limits.

KEYWORDS: Plexcitons, oscillator strength, spectral width, plasmons, excitons, Jaggregates, FDTD method 2 ACS Paragon Plus Environment

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The large optical near-fields that arise in the vicinity of noble metal nanostructures due to surface plasmon resonances have led to surface-enhanced Raman scattering,1-4 surfaceenhanced fluorescence5,6 and other spectroscopies7,8 of relevance to chemical and biological sensing. Surface plasmon excitations can also induce physical phenomena such as hotelectron injection which are relevant for solar energy harvesting and catalysis.9,10 The interaction of surface plasmon excitations with molecular excitations lies at the heart of most of these phenomena. On one extreme, the weak-coupling regime is observed, wherein the exciton-plasmon interactions result in various surface-enhanced phenomena. On the other extreme, hybrid “plexcitonic” states (i.e., plasmon-exciton states) occur in the strong coupling regime when the energy transfer process occurs faster (~10-12 s) than the electronic relaxation of the excitons. The latter systems have incredible potential in sensing, switching, digital data storage and as metamaterials.11-14 Initial efforts towards strong coupling were focused on the interaction between various Fabry-Pérot microcavity modes and atomic, organic as well as semiconductor systems.15-17 Recent advances in the strong coupling studies created opportunities such as modulation of the landscape, specificity and dynamics of various chemical reactions.18-21 However, their practical utility has not hitherto been explored due to complexities involved in the design of cavity-based systems. Alternatively, such studies can conveniently be carried out in solution phase by designing free-standing plexcitonic systems with spectral tunability in a wide range from ultraviolet to near-infrared region. Approaches in this direction include the coupling of plasmonic nanostructures such as nanoparticles, nanorods, nanodisks and nanoshells with various excitonic systems like organic chromophores, semiconductor quantum dots and transition metal dichalcogenides.22-33 In all these cases, a dip or splitting of the plasmon spectral feature occurs when the exctionic and the plasmonic resonances match. Some of the


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reported systems with such behavior include (i) Au nanostructure-cyanine dye hybrids,23, 34 (ii) hybrid metallic nanodisk dimers and chromophoric J-aggregates26 and (iii) planar silver thin films with CdSe nanocrystals.35 Efficient coupling between excitons and plasmons has been experimentally established by varying parameters such as the concentration of the molecular layer,36,37 absorbance of the molecules,38 distance between the dye molecules and the plasmonic surfaces34,

39

and conformations of the molecules.12,40 The dynamics of the

hybrid states of Au nanorods and J-aggregates of a chromophoric dye were recently investigated in the strong coupling regime using ultrafast spectroscopy demonstrating the coherent coupling between the excitons and the plasmons.41-43 The strongly coupled systems also display interesting spectroscopic properties: notable ones are the Rabi splitting in photoluminescence44,45 and Raman signal enhancement based on polariton states.46 The Rabi splitting is defined as the spatial overlap of the transition dipole moment of the chromophoric system and the electric field of the plasmonic system.26 Indeed, strong light–matter interactions have been achieved to a large extent by fine-tuning of plasmons and excitons - so strong to cross even the threshold of ultra-strong coupling regime where the Rabi splitting energy is larger than ∼15 % of the excitonic transition energy.47-49 Various theoretical approaches have been effectively used to explain the plexcitonic interactions in strongly coupled systems. Methods ranging from classical coupled oscillator models, finite-difference time-domain (FDTD) simulations and quantum two-state models to full quantum mechanical models, wherein both the field and the system are described quantum mechanically, have been used to explain the origin of the peaks and estimate the coupling strengths.50-57 The plasmonic and the excitonic features that dictate the plexcitonic interactions in hybrid systems are (i) oscillator strengths (OS; also denoted as f - see the Supporting Information for details on their calculation) of the plasmons and the excitons,58,59 4 ACS Paragon Plus Environment

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(ii) spectral widths (denoted as γ) of the plasmons and the excitons and50, 60 (iii) the strengths of coupling between the plasmons and the excitons (denoted as U).61,62 Tuning the coupling strengths in plexcitonic systems has been investigated in detail by several research groups.26, 63

In this manuscript, the roles of f and γ of the excitons and the plasmons on strong coupling

are investigated in detail. Chromophoric dyes are usually used as excitonic systems and they have a distinctive feature of existing in different forms, having varying values of f and γ, depending on the environment.64,65 Herein, we have (i) defined the role of these optical parameters on strong coupling using ideas based on both a classical coupled oscillator model and a quantum two-state model, (ii) employed the insights from the models to realize plexcitons by a judicious combination of plasmonic and excitonic components and (iii) further provided a comprehensive interpretation of the plexcitonic hybrid states within the framework of the theoretical models48 and the FDTD simulations.26, 36 By choosing spherical Ag nanoparticles (Ag NPs) of varying diameters and Au nanorods (Au NRs) of varying aspect ratios as the plasmonic elements, this manuscript describes a joint experimental and theoretical investigation on the strong coupling interactions of these systems with the various forms (monomer/aggregate) of fluorescein isothiocyanate (FITC) and cyanines. Employing the monomeric, J- and H-aggregates of these chromophoric dyes, the role of f and γ of excitons on strong coupling is probed. By a careful choice of the composition and the geometry of the metallic systems, plasmonic features (f and γ of plasmons) are tuned in the spectral region of 400 nm to 800 nm to establish a perfect resonance condition with the excitonic counterparts, resulting in clear peak separations indicative of an approach to the strong coupling regime.


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RESULTS AND DISCUSSION To achieve the conditions of strong coupling, it is indeed important to choose excitonic and plasmonic components with matching optical properties. In addition to the spectral features, spatial overlap between the plasmonic and the excitonic components is found to play a crucial role in determining the extent of coupling, as discussed in the literature.26 The most convenient and well-established methods to describe the plexcitonic states in terms of the properties of the isolated components are the classical coupled oscillator model and the quantum mechanical two-state model. Employing these, we first define the role of oscillator strengths and spectral widths in governing strong coupling. This analysis provides guiding principles towards the selection of the plasmonic and the excitonic components. Conditions for Strong Coupling: We begin our analysis with a simple quantum two-state model. Let pl denote the plasmon resonance energy in the absence of the exciton, and ex denote the corresponding excitonic resonance energy. The eigenvalues of the Hamiltonian matrix H=

(1)

represent the expected extinction peaks as a function of the photon energy = ℏ in a system such as a plasmonic nanostructure coated by the excitonic counterpart. In the above equation, is the plasmon-exciton coupling energy. The eigenvalues of the above matrix correspond to the plexcitonic levels and are given by: 1

1

± = 2 + ± 2 ( − )2 + 42 .

(2)

When the excitonic absorption maximum is in resonance with the extinction maximum of the plasmonic nanostructure, = and Equation (2) implies 2 = − . Thus, from the experimental spectral peak separation of the plexcitonic resonances under the condition that


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the plasmonic peak is in resonance with the excitonic peak, it is possible to compute the coupling energy. The role of losses in Equations (1) and (2) cannot be completely neglected and this aspect has been discussed in detail in Supporting Information. Actually, the coupling energy can be related to the dimensionless effective oscillator strengths of the plasmonic and the excitonic resonances (

and

)

by employing

the formalism of the classical coupled oscillator model (Supporting Information). On resonance, i.e., for Epl = Eex = E0, one then has ℏ

$

≈ # ' " % &

(

)*

+, +

.

(3)

In the above equation, M is a dipolar coupling factor, ω0 is the resonance frequency given by 0 = 0/ℏ and e and me are the charge and the mass of the electron, respectively. Nex is the number of excitonic units (either monomers or aggregates as the case may be) about each plasmonic system. For spherical nanoparticles of radius a, assuming that the excitonic species are close to the metal surface, M is approximately 2/a3 in Gaussian units. In the strong coupling limit, one has two distinct spectral peaks at energies (E0 – U) and (E0 + U), each with full-width at half-maximum of

ℏ "

0 + 0 (Supporting Information). To be distinct, ℏ

the energy separation of the two peaks, 2U > 0 + 0 or, using Equation (3): " 1

2

# ' 2 +, 0

1

> 2 #0 + 0 '.

(4)

Equation (4), particularly owing to the approximation of Equation (3), should be viewed only as a rough indicator of the trends. The OS values in question are effective ones such that, for example, fpl must be much greater than unity reflecting the collective nature of a plasmonic excitation.66 While we developed an expression for the interaction energy (U) in terms of the effective oscillator strengths of the excitons and the plasmons (fex and fpl), related expressions in terms of the exciton dipole moment (4) and the plasmon mode volume (Vm) have been


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frequently used in the literature.67,68 In particular, in relation to such treatments, we note that 4 5 +

and 6) 5

7

89:

. Also, one should note that we are drawing a distinction between OS

values (fpl and fex) and the corresponding decay constants or spectral widths (ℏγpl and ℏγex). This is valid if various inhomogeneous broadening effects (e.g., dephasing) are the dominant contributions to the spectral widths, which is the case in most experiments on colloidal and solution-phase molecular systems and, indeed, many other systems. However, if a spectral width is dominated by just homogeneous spontaneous emission, as with atoms or quantum dots at ultra-cold temperatures, then the emission rate and the corresponding OS values are functions of the relevant transition dipole moment and thus are linearly dependent. Although rather approximate, Equation (4) nonetheless permits us to develop strategies for designing plexcitonic systems by meaningfully varying the values of spectral widths and oscillator strengths. Chromophoric dyes can exist in monomeric as well as J- and H-aggregate forms, which enables us to play with γex and fex values. On the other hand, the size- and shape-dependent plasmonic properties of metal nanoparticles permit us to match the frequencies of the plasmons with those of the excitons and to vary γpl and fpl. Based on these guiding principles, we have selected a set of plasmonic nanostructures and monomeric as well as aggregate forms of chromophoric dyes as given below. Selection of Excitonic and Plasmonic Systems: The selection of excitonic and plasmonic systems is based on the rationale of tuning the plexcitonic interactions by varying the plasmon resonance wavelengths to either side of the excitonic resonances. In order to probe the strong coupling interactions in the entire visible region of 400-800 nm, we have chosen Ag NPs of varying diameters and Au NRs of varying aspect ratios as the plasmonic elements. Ag NPs and Au NRs are labeled with a numerical extension denoting their peak plasmon resonance wavelength. Ag NPs possessing high quality factor and tunable surface plasmon resonance frequency in the range of 400 nm to 550 nm are chosen for the design of hybrid 8 ACS Paragon Plus Environment

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systems in the shorter spectral range. Plexcitonic interactions of these nanoparticles are investigated using FITC, a chromophoric dye (Figure 1) with sharp absorption maximum at 494 nm (ℏγ ~ 180 meV) and large extinction coefficient (ε = 8.54 x 104 M-1cm-1 at 494 nm). The OS of the monomeric form of FITC is estimated from the integrated absorption coefficient as 0.64 (Table 1 and Supporting Information), which is similar to that reported in the literature.69 FITC is covalently linked on to the Ag NPs of various diameters (vide infra). By tuning the size of the Ag NPs, considerable overlap of the plasmon resonance with the excitonic absorption of FITC, an essential condition for strong coupling is achieved. In order

Figure 1: Representative TEM images of (a) Ag NPs of varying sizes and (b) Au NRs of varying aspect ratios used for the strong coupling studies. (c) Structures of the chromophores under study: (i) fluorescein isothiocyanate (FITC), (ii) 1,1′-diethyl-2,2′-cyanine iodide (Cy+), (iii) 1,1′-diethyl-2,2′-carbocyanine iodide (CCy+), (d) Normalized absorption spectra of the monomeric forms of FITC, Cy+ and CCy+ (in methanol). Normalized extinction spectra of (e) Ag NPs of varying sizes (f) Au NRs of varying aspect ratios used in the present study.


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to obtain strongly coupled exciton-plasmon interactions in the longer wavelength region, we employ Au NRs, a well-known plasmonic system with tunability in the 500-800 nm region by varying their aspect ratio. The high stability of the Au NRs is an added advantage, making them an appropriate choice for the present study. To investigate plexcitonic interactions with Au NRs, two cyanine dyes namely, 1,1′-diethyl-2,2′-cyanine iodide (Cy+) and 1,1′-diethyl2,2′-carbocyanine iodide (CCy+) are chosen (Figure 1). The cyanine dyes used in the present study exist in monomeric as well as aggregated forms with distinct spectral features (vide infra). This provides additional opportunity of matching the monomeric as well as the aggregate absorption with the plasmonic frequencies of the Au NRs. The monomeric forms of Cy+ and CCy+ possess absorption maxima at 524 nm (ℏγ ~ 248 meV) and 600 nm (ℏγ ~ 288 meV), respectively with a shoulder in the short wavelength region. The f values of the monomeric forms of Cy+ and CCy+ are estimated as 0.38 and 0.41, respectively from their integrated absorption coefficients (Table 1 and Supporting Information). These values are in agreement with the reported values.70 Au NRs possess a positive zeta potential, therefore, the

0.6

0.6 0.3 0.0 400

450 500 550 Wavelength (nm)

600

J-aggregate

0.2 0.0


700

0.4

Absorbance (norm.)

0.4

b J-aggregate monomer

0.9

Absorbance

0.6

Absorbance (norm.)

a Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H-aggregate Monomer

0.9 0.6 0.3 0.0

450


H-aggregate * Jaggregate

0.2 0.0

400

500 600 Wavelength (nm)

700

Figure 2: Absorption spectral changes of (a) Cy+ and (b) CCy+ with the addition of PSS. Deconvoluted absorption spectra of monomeric and the aggregated forms of Cy+ and CCy+ are presented in the inset (details are presented in Supporting Information; *CCy+ forms small amounts of J-aggregates along with predominant H-aggregate formation).


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chromophoric dyes Cy+/CCy+ are bound on to the nanorod surface after overcoating with a negatively charged polymer, polystyrenesulfonate (PSS) (Methods). It is reported that the cyanines form J- and H-aggregates in microheterogeneous environments as well as in polymeric matrices depending upon the nature of the dye.65, 71,72 To understand the spectral properties of the dyes in the presence of the polymer, varying quantities of PSS are added to Cy+/CCy+ and the results are presented in Figures 2a,b. Interestingly, distinctly different spectral features are observed on interaction of Cy+/CCy+ with PSS. The monomeric peak of Cy+ at 524 nm underwent a decrease in the intensity with the concomitant formation of a sharp J-aggregate band at 570 nm, with a ℏγ value of ~33 meV. On further increasing the concentration of PSS, formation of a small amount of H-aggregates is also observed along with the J-aggregates. In contrast, CCy+ forms H-aggregates (ℏγ ~389 meV) in the presence of PSS and the spectral changes are presented in Figure 2b. The characterization of these bands is based on the previous investigations on the aggregation properties of cyanine dyes.72,73 The lowering of ℏγ of the J-aggregate band relative to the monomeric band is referred to as exchange narrowing and has earlier been demonstrated within the framework of the exciton model.74 For J-aggregates, the absorption to the lowest state of the excitonic band contributes to the narrow spectral widths. In contrast, in case of H-aggregates, absorption to the higher excitonic states resulted in broader spectral widths due to additional decay channels. Chromophoric dyes (Cy+ and CCy+) are labeled with alphabetic extensions M, J and H denoting monomeric, J- and H-aggregates, respectively. The OS of J-aggregates predominantly depends on the aggregation number. It is reported that aggregation number of the J-aggregates of Cy+ (Cy+(J)) varies from 4 to 50,000 depending on the experimental conditions.75-77 Increase in aggregation number results in very high integrated absorption coefficients, leading to large f values. One of the convenient ways to theoretically estimate the aggregation number in linear aggregates is from the linewidths, as described by Knapp.78 11 ACS Paragon Plus Environment

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Employing this analysis, we find the aggregation number of Cy+ to be 55 (using the ℏγ values provided in Table 1) in the presence of PSS, which is in agreement with the reported value. Wiersma and coworkers have shown that the average OS for the Cy+(J) in ethylene glycol/water glass is 49 times that of the monomeric form, which is attributed to the delocalization of the excitonic states over a large number of molecules in the aggregate.79 In contrast, H-aggregates are often referred to as dimers in view of their low aggregation number.80 These features of the Cy+(J) make it an ideal candidate for plexcitonic investigations compared to monomeric as well as H-aggregates. The experimental extinction coefficients and the OS values of Ag NPs and Au NRs are estimated in conjunction with TEM and inductively coupled plasma (ICP) analysis (Table 1 and Supporting Information). Table 1: The effective oscillator strengths (ƒ) and spectral widths (ℏγ) of the excitonic and the plasmonic systems

System under study FITC Cy+

CCy+

Monomer (in methanol; RIa =1.33) Monomer (in methanol; RIa =1.33) J-aggregatec (in water; RIa =1.33) Monomer (in methanol; RIa =1.33) H-aggregate

Ag NPe Au NRg

Ext. coefficient (ε), (M-1cm-1) at λmax

Spectral width, ℏγ

8.54 x 104 at 494 nm 3.46 x 104 at 524 nm 1.8 x 106 at 570 nm 4.4 x 104 at 600 nm

1454 cm-1 (180 meV) 2000 cm-1 (248 meV) 270 cm-1 (33 meV) 2323 cm-1 (288 meV) 3140 cm-1 (389 meV) 6720 cm-1 (833 meV) 3459 cm-1 (429 meV)

-

LSPRf (in water; RIa =1.33) Longitudinal LSPRf (in water; RIa =1.33)

2.1 x 1010 at 436 nm 2.29 x 109 at 582 nm

Effective oscillator strength, ƒ 0.64 0.38 (0.39)b 2.88

0.41 (0.56) b > 1, which may surprise some readers. It is important to distinguish these effective oscillator strengths from those of atomic spectroscopy that have one as an upper bound and satisfy the sum rules. In the present case, the oscillator strength is a measure of the actual extinction cross-section maximum relative to that which would arise from a single, harmonically bound electron. In the case of the metal nanostructures, f then is a measure of the number of active conduction electrons.66 Design of Plexcitonic Nanohybrids: In this study, we focus on the design of a series of hybrid systems wherein the plexcitonic interactions can be tuned by varying the plasmon frequencies of the metal nanostructures. To observe the plexcitonic interactions in the strong coupling regime, chromophoric units should be tightly bound onto plasmonic nanostructures. It has been reported in the literature that the distance between the plasmonic and the excitonic systems plays a crucial role in determining the extent of coupling.23 In the present manuscript, excitonic and plasmonic systems are coupled by adopting two different strategies: (i) silane coupling of FITC to Ag NPs and (ii) electrostatic binding of cyanine dyes onto Au NRs. Schematic representation of the design of plexcitonic nanohybrids is presented in Scheme 1. For a given combination of excitonic and plasmonic systems, the distances are kept constant (vide infra).


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Scheme 1: Schematic illustration for the (a) FITC functionalization of Ag NPs, (b) Cy+/CCy+ functionalization of Au NRs. FITC Functionalization of Ag NPs: FITC is covalently linked onto Ag NPs having varying diameters (in the range of 25 nm to 120 nm) and details on the synthesis of these nanohybrids are presented in Methods and in Supporting Information. In general, isothiocyanate group of FITC is first functionalized with 3-aminopropyltrimethoxysilane (APS) through a thiourea linkage (Figures S15 and S16). The obtained adduct (FITC-APS) is coupled onto the surface of polyvinyl pyrrolidone (PVP)-coated Ag NPs (Ag@PVP) and characterized using TEM analysis. Representative TEM images of Ag450-FITC are presented as Figures 3a,b. Ag450@PVP possesses a layer of organic shell having a thickness of ~2 nm (Figure S14). Interestingly, we observed dense dots embedded in the thin organic shell after coupling with FITC-APS, featuring the formation of small islands of silica. The signature of Si Kα line in the energy-dispersive X-ray (EDX) spectrum of the Ag450-FITC nanohybrids (Figure 3c) gave conclusive evidence for the binding of FITC-APS onto the surface of Ag@PVP. From the TEM images, it is clear that the Ag-FITC hybrids remain isolated, further ruling out the possibility of aggregation of nanoparticles.


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Figure 3: (a,b) TEM images of Ag450-FITC at different magnifications. (c) EDX spectrum of Ag450-FITC (note: peaks at 0.9, 8.1, and 8.9 keV in the EDX spectrum correspond to the Lα, Kα, and Kβ energy lines of Cu and peak at 0.21 keV corresponds to the Lα energy line of carbon originating from the carbon-coated Cu TEM grid). (d) Extinction/absorption spectral features in the control experiment carried out by mixing Ag450@PVP and FITC in methanol: (i) Ag450@PVP, (ii) FITC and (iii) mixture of Ag450@PVP and FITC. (e) Absorption spectra of FITC-APS in the absence and presence of PVP in methanol. (f) Extinction/absorption spectra of Ag450-FITC in methanol before and after etching the silver core. The extinction spectrum of Ag450-FITC nanohybrids (Figure 3f) is distinctly different from the resultant spectrum obtained when Ag450@PVP is mixed with FITC-APS conjugate in methanol (Figure 3d). Interestingly, a splitting in the extinction spectrum is observed in the former case whereas the latter one is found to be an additive spectrum of the individual components. These experiments substantiate that the close proximity of FITC to Ag NPs helps in the effective coupling between the plasmons and the excitons. Two control 15 ACS Paragon Plus Environment

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experiments are carried out to ensure that the observed splitting in the extinction spectrum is due to the plexcitonic interactions in the nanohybrids. The absorption spectrum of FITC-APS remained unaffected on addition of PVP (Figure 3e). The spectral features of the FITC are recovered on etching the metallic core of the Ag450-FITC hybrid on treatment with sodium cyanide (Figure 3f and Supporting Information). Thus, the possibility of degradation of the FITC in the presence of PVP or Ag NPs is ruled out. These results further confirm that the splitting observed in the extinction spectrum of Ag450-FITC is due to plexcitonic interactions with the monomeric form of the chromophoric dye rather than with the aggregates (vide infra). Cyanine Functionalization of Au NRs: Au NRs possess a high positive zeta potential (ζ) owing to the presence of cetyltrimethylammonium bromide (CTAB) as the capping agent. For example, ζ of Au680 is found to be +29 meV. Therefore, electrostatic interaction is a convenient strategy to bind charged chromophoric dyes onto Au NRs for the design of nanohybrids with potential plexcitonic interactions. However, in the present case, both the components (Au NRs and Cy+/CCy+) are positively charged. Hence, to facilitate electrostatic binding with Cy+/CCy+, Au NRs are coated with a negatively charged polymer, namely polystyrenesulfonate (PSS) as illustrated in Scheme 1. On coating with PSS, Au NRs possess negative ζ, for example, -30 meV for Au680 (Figure 4a). The PSS-coated Au NRs are monodisperse and the formation of a uniform layer of organic shell with a thickness of ~1 nm is evident and their extinction features remain unaffected (Figure 4b and Figure S18). By adopting the above strategy, PSS-coated Au NRs having varying aspect ratio (1.0 to 3.0) are synthesized, enabling the tuning of the longitudinal plasmon resonance frequency in the spectral region of 520-800 nm. Design of hybrid nanostructures is achieved through successive addition of Cy+/CCy+ to PSS-coated Au NRs and the binding of the chromophores is probed by following their Raman signals. Cy+ molecules in methanol showed negligible


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a

0.9

b

NR with PSS NR

-30 meV

29 meV

0.6 0.3 0.0 -140

600

20 nm

-70 0 70 140 Zeta Potential (mV)

c

9 d

NR + Cy+ Cy+

400

Intensity (a.u.)

Counts (norm.)

1.2

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 0

NR NR + Cy+

6 3 0

600

900

1200

1500

Raman Shift (cm-1)

10

100 1000 Size (nm)

10000

Figure 4: Characterization of PSS-coated Au NRs: (a) Zeta potential measurements of Au NR before and after PSS coating. (b) TEM image of the PSS-coated Au NR. Characterization of Au-Cy+: (c) Raman spectra of Cy+ molecules in the presence and absence of Au NRs. (d) DLS of Au NRs in the presence and absence of Cy+. Raman scattering (the signal intensities are weak). It is well established that the electric field prevailing on the surface of plasmonic materials enhances the Raman signal intensities. The enhanced Raman peaks observed in the present case clearly indicate that the Cy+ molecules are bound to the surface of Au NRs (Figure 4c). The hybrid nanostructures are further characterized using TEM and dynamic light scattering (DLS) studies. It is observed that the nanorods remain isolated in TEM images and the DLS profiles are unaffected upon binding (Figures 4b,d). These independent investigations rule out the possibility of the aggregation of Au NRs on addition of Cy+. Tuning the Plexcitonic Coupling: In the Ag-FITC and Au-Cy+/Au-CCy+ hybrids, we could tune the frequency of the plasmonic systems from blue to red in the visible spectral region (400 nm to 750 nm). The chromophoric systems used in the present study exist in monomeric 17 ACS Paragon Plus Environment

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as well as aggregated forms, enabling us to probe the role of excitonic features in governing strong coupling. FITC has a monomeric band at 494 nm (in methanol; ℏγ = 180 meV) and two aggregation bands at 468 nm and 508 nm, respectively.82 Interestingly, the dip observed in the Ag-FITC hybrid system at 494 nm in methanol matches with the monomeric absorption band rather than the aggregated bands. In contrast, Cy+ exists as monomeric, Hand J-aggregated forms in the presence of PSS (Figure 2a), whereas CCy+ forms predominantly H-aggregates (Figure 2b; note: J-aggregates are also observed at higher concentration of CCy+). The tunability of the longitudinal band of Au NRs, by varying the aspect ratio indeed permits the matching of the plasmon frequencies with the excitonic frequencies of the (i) Cy+(M) and CCy+(M), (ii) Cy+(J) and (iii) CCy+(H). It is found that the Cy+(J) band couples with Au587 and hence investigations are first focused on optimizing the chromophoric concentration required for obtaining an efficient coupling (Figure 5). The extinction spectra of the hybrid systems are distinctly different from the additive spectra of the individual components (Au587 and Cy+(J)). With more Cy+ adsorbed onto PSS-capped Au NRs, a dip is observed at 589 nm with two new peaks on either side. The position of the dip matches well with the longitudinal plasmon resonance of Au587 and the absorption

0.8

Extinction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

Cy+ (0-5.8 µM)

0.4 0.2 0.0


Figure 5: Extinction spectra of PSS-coated Au587 with increasing concentration of Cy+ (05.8 µM) in water. 18 ACS Paragon Plus Environment

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maximum of Cy+(J). Moreover, with the increasing concentration of Cy+, the dip becomes more prominent, indicating coherent coupling of the surface plasmons of Au587 with the Cy+(J) excitons in the hybrid system (Figure 5). Aggregation being an equilibrium process, Cy+ exists in monomeric as well as J-aggregate forms in the presence of PSS (Figure 2a). However, no interaction is observed between the monomeric form of Cy+ and the longitudinal plasmon peak of the corresponding Au NR (Au525), when they are in resonance (Figure S19). Further, we have experimentally and theoretically investigated the evolution of the plasmon-exciton coupling in Ag-FITC and Au-Cy+/CCy+ systems by tuning the plasmon frequency to either side of the exciton peaks (Figure 6). The (higher energy) and the (lower energy) hybrid states are generated in the case of Ag-FITC systems by tuning the plasmon resonance of Ag NPs in the range of 400-500 nm, with respect to the FITC absorption (Figures 6a, c, Figure S17). Though we could tune the Ag NP plasmons to longer wavelength regions (> 500 nm) by choosing particles of larger diameters (~100-130 nm), the occurrence of multipolar transitions in such nanoparticles complicates the analysis. Hence, in order to avoid the coupling of the quadrupolar resonances to FITC, we limit our discussions to Ag NPs of diameters less than 120 nm (Ag420-FITC, Ag440-FITC, Ag450-FITC, Ag460FITC, Ag480-FITC and Ag500-FITC systems). Multipolar transitions in Ag NPs (Figure 1e) limit the exploration of plexcitonic interactions in Ag-FITC systems beyond 500 nm. In contrast, the tunability of the longitudinal band of Au NRs in the spectral region of 525-725 nm permits the exploration of their plasmonic interaction with various forms of Cy+ and CCy+ (Figures 1d,f and Figure 2). Contrary to the Ag-FITC system, strong coupling interaction is observed when the plasmonic bands of Au NRs are in resonance with the Cy+(J), rather than with Cy+(M). As we move to either side of the J-aggregate band, the extent of coupling reduces leading to an asymmetric


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3.0 b 2.7 2.4

2.4

Au630 Au606 Au587

Energy (eV)

Au660

Ag480 Ag460 Ag450 Ag440

2.1

d Au682

Ag500

Extinction

Ag 460 Ag4 50 Ag44 0 Ag420

c

Ag420

e

f Au623

Extinction

Extinction

Ag 50 Ag 0 480

Energy (eV)

a

Extinction

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2.1

Au606 Au587 Au573

1.8

Au573 Au557


Au557

1.8

2.1 2.4 2.7 3.0 Plasmon Energy (eV)

400

600

800

Wavelength (nm)

Figure 6: Experimental and computed extinction spectra and dispersion curves of Ag-FITC and Au-Cy+(J) systems. Experimental extinction spectra of (a) Ag-FITC and (d) Au-Cy+(J) showing variation in the intensities of the higher energy ( ) and the lower energy ( ) bands. Theoretical extinction spectra of (c) Ag-FITC and (f) Au-Cy+(J) calculated using the classical coupled oscillator model. Dispersion curves corresponding to (b) Ag-FITC and (e) Au-Cy+(J), illustrating the chromophoric states (purple traces), the Ag NP/Au NR (green squares) states, and the anti-crossing of the (red dots) and the (blue triangles) states. The solid black traces denote the plexcitonic energy levels computed using the quantum twostate model. Methanol and water are used as solvents for Ag-FITC and Au-Cy+ systems, respectively. lineshape and the extent of transparency weakens (Figures 6d,f, Figure S19). As in the case of the Ag-FITC system, the coupling between the plasmon and the exciton leads to two new hybrid modes, the upper branch mode ( band) and the lower branch mode ( band), as illustrated in Figure 6e. The new hybrid states formed have both excitonic and plasmonic character when the two unhybridized components are close to resonance. Far away from resonance, the hybrid states are principally equivalent to the additive spectra of uncoupled


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plasmons and excitons. The variation in the intensities of the and the bands is more apparent in the present system (Figures 6d, f). Both the bands show comparable intensities when the plasmon and the chromophore are in resonance (Au587-Cy+(J)). Further, as we move to the shorter wavelength (Au557-Cy+(J) and Au573-Cy+(J)), band showed higher intensity while the band is predominant in the longer wavelength region (from Au606Cy+(J) to Au682-Cy+(J)). When the plasmon wavelength is maximum detuned with respect to the exciton wavelength, for e.g., in case of Au726, wherein the longitudinal plasmon wavelength is 137 nm away from the exciton wavelength, the coupling is found to be minimum (Figure S19A). We could observe a shift in the longitudinal peak and formation of a new peak at 579 nm, substantiating weak coupling between Au726 and Cy+(J). Although coupling is observed for the longitudinal plasmon mode, no interaction with the transverse mode is observed, ruling out the possibility of a transverse plasmon-exciton coupling. We could observe selective coupling of the longitudinal plasmon resonances of Au NRs with the J-aggregates of the excitons. Thus, the calculations are performed under longitudinal polarization of the incident light and therefore the theoretical spectra do not possess the peaks corresponding to the transverse mode (Figure 6f). The aggregation number of Cy+ in the presence of PSS is estimated as ~55 (Table 1) and the strong coupling observed herein is attributed to the effective delocalization of excitons in these linear aggregates, leading to rather large effective oscillator strength of ~2.88 (Table 1). Based on our investigations on Ag-FITC and Au-Cy+(J) hybrids and the theoretical models employed herein, we have observed that the spectral widths and the effective oscillator strengths of the chromophoric systems play a significant role in governing plexcitonic interactions. To further establish these features, we considered the interactions with another chromophoric dye, namely CCy+ with similar structural characteristics. Interestingly, CCy+ exhibits equilibrium between various forms in the presence of PSS: monomeric form, H- and


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J-aggregates (Figure 2b). In the presence of PSS, CCy+ forms small amounts of J-aggregates, along with predominant H-aggregate formation and these observations are in agreement with earlier reports.71,

73

Hybrid nanostructures are synthesized through electrostatic binding of

CCy+ onto the surface of PSS-coated Au NRs by adopting a similar methodology as is used for the design of Au-Cy+ systems. The possibility of coupling of various forms of CCy+ with the longitudinal plasmon oscillation of Au NRs is further investigated. This is achieved by tuning the plasmon frequency in the spectral range of 525-725 nm by varying the aspect ratio of Au NRs (Figure 1f). We obtained a spectral dip in the extinction profile of Au-CCy+(J) in the case where the plasmon oscillation of Au NR (Au630) effectively overlaps with the CCy+(J) band (Figure 2b and Figure S20). Though CCy+ exists predominantly in H-aggregate and monomeric forms, plexcitonic interaction is observed only with the J-aggregates. Since the amount of J-aggregate formed is small, construction of the dispersion curve for the AuCCy+ hybrids is challenging and hence not pursued. Classical Coupled Oscillator Model for Plexcitons: Further, the experimentally obtained extinction spectra of the Ag-FITC and Au-Cy+(J) hybrid systems are compared to the theoretical extinction spectra obtained from the classical coupled oscillator model. In this model, we consider a system consisting of two dipoles interacting with an applied electromagnetic field, which are further coupled to one another via the resulting local electromagnetic fields that each feels as a result of the other (Supporting Information). Such models have a long history in describing metal nanoparticle-molecule interactions dating back to the seminal works of Gersten, Nitzan and Brus.83-85 Following Shah et al., we take the equations of motion for the two dipoles86 to be ; ( ) + 0

;) + 14 (>)E

(5)

= ? GA cos(>) + 14 (>)H.

(6)


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(

The Supporting Information (Section 20) shows that ? = )

*

and ? = ,

(

)*

where Nex is the number of excitons interacting with the plasmon and that the analysis of the energy differences between the peaks in the classical cross-sections leads to the approximate relation for the coupling energy, Equation (3). The classical model, in addition to allowing us to have a different means of estimating the plasmon-exciton coupling energy U, allowed us to develop equations that relate U to the quantities of interest i.e., effective oscillator strengths and decay constants. The theoretical extinction spectra for each of the plexcitonic systems are obtained as the sum of the absorption and the scattering cross-sections and the parameters in the model can be varied to fit the experimental spectra. The experimental and the so-fitted theoretical extinction spectra for the Ag-FITC and Au-Cy+(J) systems are in fairly good agreement (Figure 6). FDTD Simulations for Plexcitons: To obtain a clearer insight on how the features of the excitonic and the plasmonic systems influence the strong coupling, FDTD simulations are also carried out to model the Ag NP/Au NR-chromophoric hybrid systems (Supporting Information). Computational methods like discrete dipole approximation and FDTD method have recently been employed to model the extinction and the scattering features of hybrid plexcitonic systems.36,

50, 54

In the present study, we use the FDTD method to model the

optical properties of the Ag NPs and Au NRs in the presence and absence of the excitonic molecular layers. Firstly, the extinction properties of the bare Ag NPs are calculated. Subsequently, the effect of the plexcitonic interactions between the Ag NP plasmons and the FITC molecular excitons on these features is investigated by introducing a molecular layer on the surface of the Ag NPs of varying sizes. Further, in the case of Au NR-chromophore systems, the extinction features of the PSS-capped nanorods (refractive index of PSS = 1.40) are calculated in the absence and presence of excitonic molecular layer and the extent of coupling in the case of NRs of varying aspect ratio is investigated. 23 ACS Paragon Plus Environment

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The plasmonic nanostructures are modeled considering the average dimensions obtained from the TEM analysis. Ag NPs and Au NRs are characterized using a frequency-dependent dielectric function. Palik’s bulk dielectric data is used in the case of Ag, whereas Johnson and Christy dielectric data is used for modeling the frequency dependence of the dielectric constant of Au. The permittivity of the molecular layers is represented by a Lorentzian IJ (K) = IJL +

KJN M (KJ KJ OPK) N

,

(7)

where fL is the dimensionless Lorentzian oscillator strength, γ is the spectral width, A is the absorption maximum and Q"L is the background permittivity (Figure S21).54,

59

The

Lorentzian oscillator strength, fL in Equation (7) should not be confused with the effective oscillator strength, fex for the individual dye molecules or for the J- or H-aggregates that appear in Table 1. The Lorentzian oscillator strength, fL is a parameter describing the dielectric constant for an absorbing dye layer around a metal nanoparticle and this layer contains a number of excitonic units within it. Suppose that it is known from experiment or other estimates that the dye layer has a certain thickness and also that it contains a certain number Nex of active excitonic units within it. The overall effective oscillator strength of the layer would be approximately Nexfex, where fex is the oscillator strength for one excitonic unit. One way of determining a suitable fL would be to carry out Mie theory or FDTD absorption cross-section calculations for just the dye layer with dielectric constant of Equation (7) and to consider results for a range of possible fL values. The absorption maxima and the spectral widths of the chromophoric systems obtained from the experimental absorption spectra are used in the simulations (Table 1). The FITC molecular layer is modeled using a layer thickness of 2 nm with fL = 0.1. The numerical value of Q"L used in the calculations is 1.77. The strong coupling between the Ag NPs and FITC is well evident from the simulated extinction spectra (Figure 7), in accordance with the experimentally obtained spectra.


4

4x10

4

2x10

4

0

a

Ag450-FITC


4 6x10 b

4x10

4

2x10

4

0

Ag480-FITC


Cross-section (nm2)

6x10

Cross-section (nm2)

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Cross-section (nm2)

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6x104

c

Ag500-FITC

4

4x10

4

2x10

0


Figure 7: Simulated extinction spectra of the bare Ag NPs (black traces) as well as the AgFITC hybrid systems (red traces): (a) Ag450-FITC (b) Ag480-FITC and (c) Ag500-FITC. Further, using the same approach, Au NR and Cy+/CCy+ hybrid systems are simulated. The thickness of chromophoric layers is assumed to be 0.5 nm for both the systems. The Cy+ and CCy+ monomer layers are modeled with Lorentzian oscillator strengths of fL= 0.1 and spectral widths of ℏγ = 248 meV and 288 meV, respectively (Table 1). In order to probe systems with strong plexcitonic interactions (λmax = 524 nm for the monomer of Cy+), simulations are carried for the Au530, Au587 and Au606 systems (Figure S22). Whereas, in case of monomer of CCy+ (λmax = 600 nm), the Au587, Au606 and Au660 systems are considered (Figure S22). In parallel with the experimental results, no significant coupling is observed for these monomeric dye systems. The Au-Cy+(J) system is further simulated with the Cy+ layer having fL = 0.8 and spectral width ℏγ = 33 meV. The simulated extinction spectra computed for Au NRs of varying aspect ratio in presence of Cy+(J) (Figure 8a) are found to be in reasonable agreement with the experimental spectra. Specifically, the calculated spectra exhibited a strong dip at the excitonic resonance frequency and two distinct plexcitonic resonances, namely, a higher energy mode on the blue side of the exciton and a lower energy mode on the red side are obtained. A major limitation in comparing the computed spectra with the experimental spectra is that the latter are for an ensemble of Ag NPs/Au NRs and their size distribution is not taken into account in the simulations. However,


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the trends in the variation of the plexcitonic features are in very good agreement with the experimental results.

a

2.4 Au684

Energy (eV)

Cross-section

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Au660 Au606

2.2 2.0 1.8

Au587 Au530

400

b

600

Wavelength (nm)

800

1.8

2.0 2.2 Plasmon Energy (eV)

2.4

Figure 8: (a) Simulated extinction spectra of the Au-Cy+(J) plexcitonic hybrids formed from Cy+(J) and Au NRs of varying aspect ratio, showing variation in the intensities of the higher energy and the lower energy bands. (b) Dispersion curve for the hybrids illustrating the chromophoric (purple trace) and the plasmonic (green squares) energy states and the anticrossing of the lower energy (red dots) and the higher energy (blue triangles) hybrid states. Anticrossing Behavior of Plexcitonic States: The anticrossing of the hybrid plexcitonic states at the resonance wavelength of the plasmon and the exciton is another characteristic feature of strong coupling. From the experimental extinction spectral features, energies corresponding to the and the states are calculated and plotted as a function of the plasmon energy for the Ag-FITC system (Figure 6b). The green trace corresponds to the energies of the plasmon bands of Ag NPs of varying size and purple trace corresponds to the FITC absorption. It is evident from Figure 6b that the dispersion curve of Ag-FITC hybrid systems shows excellent anticrossing behavior (both the traces do not cross at the resonant wavelength). The Rabi splitting energy (2U) for the Ag-FITC hybrid system is calculated from the energy differences of the and states and is found to be 383 meV. The splitting energy for the Ag-FITC hybrid system is found to be considerably higher than the reported


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values for inorganic semiconductors and other dyes like cyanine or Zn-porphyrins, which are in the range of 50 to 150 meV.16, 23, 24 As in the case of Ag NPs, the dispersion curve for the Au-Cy+(J) system is constructed from the energies of the and the states as obtained from the experimental spectra. The anticrossing behavior of the strong coupling interaction is clearly noticeable from the dispersion curve (Figure 6e). The Rabi splitting energy calculated from the dispersion curve is found to be 229 meV, lower than the value obtained for the Ag NP system. Subsequently, the dispersion curve for the Au-Cy+(J) system is constructed using the results from the FDTD simulations and is shown in Figure 8b. Thus, we have validated that the size of the spherical nanoparticles and the aspect ratio of the nanorods offer a means of controlling the plasmonexciton coupling, as also demonstrated earlier in the case of Au nanorods, Ag nanoprisms etc.23, 24, 47 Furthermore, regardless of the state (monomer, J-aggregate or H-aggregate) of the molecular system, if the effective oscillator strengths and spectral widths of the excitonic systems are suitably chosen, they would have the prospect of coupling strongly to the plasmonic systems. The two-state model energies, Equation (2) are then fitted to the experimental data (Supporting Information) and the solid lines in Figures 6b,e are the results, which agree well with the experimental data. These results suggest that 2U = 380 meV for the Ag-FITC system and 2U = 220 meV for the Au-Cy+(J) system. Both these cases are approaching closely the strong coupling limit, although not yet fully there. While 2U is larger for the Ag-FITC system, when one considers the decay widths, the Au-Cy+(J) system is a ℏ

little closer to the strong coupling limit of 2U > 0 + 0 . For Ag-FITC, one thus has 2U "

= 380 meV which is smaller than

ℏ "

0 + 0 = 506 meV; and for Au-Cy+(J), one may

ℏ

compare 2U = 220 meV to 0 + 0 = 231 meV. "

The classical dipole-based approximation for the interaction energy, Equation (3) can be difficult to estimate for the present case owing to uncertainties in the number of excitonic 27 ACS Paragon Plus Environment

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species about each plasmonic particle, Nex, as well as the approximate nature of the electromagnetic coupling factor, M. For the Au-Cy+(J) case, however, we have estimated (Supporting Information) that as many as Nex ~ 100 J-aggregates could be around each nanorod. Using this, and also taking a = 14 nm (the average of the two radial dimensions for the appropriate nanorods), one infers from Equation (3) that U ≈ 54 meV, which is within a factor of two of the more correct result. Thus, we confirm that, for excitonic systems with weak oscillator strengths and large spectral widths, as in case of monomeric Cy+/CCy+, the interaction between the plasmon and the exciton is negligible and the extinction spectrum remains unaffected. Whereas, in the case of monomeric form of FITC, with a smaller ℏγ and a larger OS, we found that the coupling becomes more effective and the system approaches the strong coupling regime, i.e., the coupling overcomes the decoherence channels. Lastly, in the case of Cy+(J) with a very sharp peak and exceptionally high OS, we observed highly efficient plexcitonic coupling. This indicates that, an excitonic system with a narrow spectral width and substantially large OS is indeed a suitable candidate for probing plexcitonic interactions. Our main objective herein is to elucidate the role of the effective oscillator strengths and the spectral widths of the plasmonic as well as the excitonic systems in governing strong coupling. The OS values of conventional chromophoric systems are typically less than unity. However, by considering J-aggregates of chromophores, it is possible to increase the OS values beyond unity. Herein, the Cy+(J) system is shown to possess an OS value of ~2.8, which enables us to achieve its strong coupling with Au NRs. The sharp spectral resonance in J-aggregates also additionally contributes to its role in strong coupling. On the other hand, in case of plasmonic systems, we report the OS of Ag NPs and Au NRs to be ~104 and ~105, respectively. This results in strong coupling of the Ag NP system with the FITC system, although the OS of FITC is less than unity. Thus, by employing Ag NPs and Cy+(J) of high 28 ACS Paragon Plus Environment

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effective oscillator strengths, both the Ag-FITC system and the Au-Cy+(J) system were driven towards achieving the strong coupling limits. It is of course ideal to have both the excitonic and the plasmonic systems with desirable properties of high OS values and sharp spectral features. However, strategies to bring such systems together for attaining strong coupling are limited by experimental protocols. CONCLUSIONS The significant role of effective oscillator strengths and spectral widths on strong coupling in plexcitonic systems has been explored with extensive experimental and theoretical studies using Ag/Au plasmonic systems and various forms of chromophoric dyes. Precisely, the combined or individual increase in the effective oscillator strengths or/and decrease in the spectral widths of the plasmons and the excitons are the key features that drive the coupling. As a proof-of-concept, two strategies are adopted for the design of plexcitonic systems by binding chromophoric units, covalently as well as electrostatically, on plasmonic structures by tuning their resonance wavelengths in the entire UV-Visible range (400 nm-800 nm). Indeed, the experimental studies on Ag-FITC and Au-cyanine nanohybrids conclusively support the significance of spectral widths and effective oscillator strengths in governing plexcitonic coupling. On comparing the interaction of FITC and cyanines in their monomeric form with the plasmonic substrates, it is observed that the favorable optical parameters of the former (ℏγ ~ 180 meV; ƒ = 0.64) permit strong coupling. More pronounced coupling with distinct plexcitonic states is observed in the case of the J-aggregates of cyanine dyes due to significant narrowing of the spectral width (ℏγ ~ 33 meV) and large effective oscillator strength (ƒ = 2.88). The formed hybrid states are attributed to the exceptional delocalization of the excitons in J-aggregates with aggregation numbers as large as 55, leading to increased propensities for obtaining larger Rabi splittings. The joint experimental and theoretical


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studies presented here provide guidelines for choosing suitable plasmonic and excitonic systems that may approach the strong coupling regime. METHODS All reagents are purchased from Sigma-Aldrich and are used as such. Photophysical studies are carried out using spectroscopic grade solvents and used as such unless specified. The UVVis spectra are recorded on a Shimadzu 3600 spectrophotometer and Analytic Jena Diode Array spectrophotometer. TEM measurements are carried out in FEI Tecnai 30 G2 (300 kV) HRTEM. The samples are prepared by dropcasting 100 µL of the solution (same solution is used for spectroscopic investigations) onto a carbon coated copper grid and solvent is allowed to evaporate. Synthesis and Characterization of Ag NPs Ag NPs having diameter in the range of 30 nm are synthesized by reducing silver nitrate in a mixture (1:3) of glycerol and water using sodium citrate as the reducing as well as the capping agent. When the extinction maximum reached ~417 nm, the reaction is arrested by cooling to room temperature. Citrate-stabilized colloidal Ag NPs (diameter in the range of 60-80 nm), having extinction maxima in the range of 440-480 nm, were synthesized by sodium citrate reduction method. By following the extinction spectra, the nanoparticles of desired diameter are synthesized. A seed-mediated method was adopted for the synthesis of Ag NPs having large sizes (> 90 nm). To a solvent mixture (1:6) of glycerol and water containing polyvinylpyrrolidone (PVP), seed nanoparticles as prepared earlier with a size of 30 nm are added under moderate stirring, followed by the addition of silver nitrate and ammonia. Addition of ascorbic acid triggered the growth process and the reaction is arrested upon attaining the desired size. Details of the synthesis of Ag NPs of various diameters are presented in the Supporting Information. We have kept a constant distance between the excitonic and the plasmonic systems by adopting the following strategies. In the Ag-FITC 30 ACS Paragon Plus Environment

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system, the NPs are coated with aminopropylsilane (APS) for FITC functionalization. The amine functionalization prevents the system from further growth of the silica shell, thus making the shell thickness fairly uniform. Same procedure is followed for designing all the Ag-FITC systems with varying dimensions of Ag NPs. Preparation of PVP-coated Ag NPs PVP (50 mg, 20 mmol) dissolved in double distilled water (2 mL) is added to the silver colloids in portions of 500 µL in 30 min intervals. After stirring for 24 h, the solution is centrifuged at 4500 rpm for 45 min and the residue is redispersed in double distilled water (5 mL). The PVP-coated Ag NPs thus obtained are stored at 4 0C under dark condition. Functionalizing FITC with 3-aminopropyl(trimethoxy)silane FITC (5 mg, 0.0128 mmol) is mixed with APS (64 µL, 0.366 mmol) and dry methanol (2 mL) in glovebox. The mixture is stirred inside the glove box for 24 h, to yield FITC-APS conjugate. As prepared FITC-APS is bound onto Ag NPs without further purification. The FITC-APS conjugate possesses a primary amine group which is further used to bind onto the surface of Ag NPs coated with PVP. Synthetic scheme adopted is given as Figure S15. Synthesis of Ag-FITC Hybrids Ag@PVP stock solution (50 µL) was added to dry isopropanol (10 mL) and the mixture is stirred at room temperature followed by dropwise addition of ammonia (250 µL, 25 %). FITC-APS solution (20 µL of the stock solution is diluted with 500 µL methanol) is added drop-wise to the reaction mixture. After stirring for 1 h at room temperature, the hybrid nanoparticles are centrifuged at 4500 rpm for 45 min to remove the unbound dye molecules. Purified FITC-tagged nanoparticles are redispersed in water (3 mL) and stored at 4 ºC, under dark condition. Synthesis of Au NRs Gold nanorods are prepared by adopting a photochemical method reported by Yang and coworkers.87 The growth solution was prepared by dissolving 440 mg of CTAB in 15 mL of 31 ACS Paragon Plus Environment

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water and 1.25 mL of 0.024 M HAuCl4 solution was added along with 300 µL of acetone and 200 µL of cyclohexane. Aspect ratio of the Au nanorods is varied by varying the concentration of AgNO3. The solution is irradiated for 1 h using a radiation of 300 nm wavelength and is purified by centrifugation. TEM analysis is used to confirm the formation of Au NRs and to estimate their aspect ratios. Details of the synthesis of Au NRs of various aspect ratio are presented in the Supporting Information. Polymer Overcoating of Au NRs The Au NRs are centrifuged twice to remove the excess of CTAB and are redispersed in water. The PSS (10 mg/mL) is prepared in 1 mM aqueous NaCl solution and sonicated for 30 min. The Au nanorods are then added to the PSS solution under stirring and after 1 h, the solution is centrifuged and the nanorods are separated. Details are provided in the Supporting Information. Au NRs coated with PSS possess shell thickness of ~1 nm in all the cases, which is confirmed through TEM measurements, thus ensuring constant distance between excitonic and plasmonic systems. Binding of Cy+ to Au NRs The PSS-coated nanorods are negatively charged due to the presence of sulfonate group and the surface charge is confirmed from the Zeta potential measurements. The Cy+ molecules possess a positive charge and hence they bind electrostatically onto the nanorod surface. The close interaction of the Cy+ and Au NR is further established through the SERS studies, which show a huge enhancement in the signal of the Cy+ molecules, which otherwise showed negligible Raman signal intensity. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website: Oscillator strength calculations of the excitonic and the plasmonic systems, synthesis, characterization and FITC functionalization of Ag NPs, synthesis of FITC-APS conjugates 32 ACS Paragon Plus Environment

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and their optical features, synthesis and PSS-coating of Au NRs and their characterization, plexcitonic spectra of FITC-functionalized Ag NPs, Cy+(J) bound Au NRs, CCy+(M) bound Au NRs, classical coupled oscillator model, FDTD simulations and quantum two-state model for plexcitons. AUTHOR INFORMATION Corresponding authors *E-mail: [email protected], [email protected], [email protected] Current address of AT: University of Strasbourg, CNRS, ISIS and icFRC, Strasbourg, France; Current address of SP: Department of Chemistry, The Pennsylvania State University, Pennsylvania, United States; Current address of LJ: Department of Physics and Astronomy, Dartmouth College, New Hampshire, USA. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT RSS and KGT thank the Department of Science and Technology (DST Nanomission Project; SR/NM/NS-23/2016), Government of India for financial support. KGT and SKG thank Indo-US Joint R&D Network Joint Center on “Quantum Plasmonics of Hybrid Nanoassemblies” (IUSSTF/3-2014) for travel support. KGT acknowledges the J. C. Bose National Fellowship of DST; RT and AT acknowledge the Council of Scientific and Industrial Research (CSIR) for senior research fellowship. This work was performed, in part, at the Center for Nanoscale Materials, a U. S. Department of Energy Office of Science User Facility, and supported by the U. S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. The authors thank P. P. Rafeeque for graphical support.


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AUTHOR CONTRIBUTIONS RSS, SKG and KGT conceived the project; the experiments were designed by KGT and carried out by RT and AT. The calculations were designed by RSS and SKG and carried out by RT. SP, LJ and SMS were BS-MS students who carried out some preliminary experiments. RT, AT, RSS, SKG and KGT analyzed the results and wrote the manuscript. REFERENCES 1. Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J., Surface-Enhanced Raman Scattering from Individual Au Nanoparticles and Nanoparticle Dimer Substrates. Nano Lett. 2005, 5, 1569-1574. 2. Nikoobakht, B.; El-Sayed, M. A., Surface-Enhanced Raman Scattering Studies on Aggregated Gold Nanorods. J. Phys. Chem. A 2003, 107, 3372-3378. 3. Shanthil, M.; Thomas, R.; Swathi, R. S.; George Thomas, K., Ag@SiO2 Core–Shell Nanostructures: Distance-Dependent Plasmon Coupling and SERS Investigation. J. Phys. Chem. Lett. 2012, 3, 1459-1464. 4. Kumar, J.; Thomas, K. G., Surface-Enhanced Raman Spectroscopy: Investigations at the Nanorod Edges and Dimer Junctions. J. Phys. Chem. Lett. 2011, 2, 610-615. 5. Lakowicz, J.; Geddes, C.; Gryczynski, I.; Malicka, J.; Gryczynski, Z.; Aslan, K.; Lukomska, J.; Matveeva, E.; Zhang, J.; Badugu, R.; Huang, J., Advances in SurfaceEnhanced Fluorescence. J. Fluoresc. 2004, 14, 425-441. 6. Emmanuel, F.; Samuel, G., Surface-Enhanced Fluorescence. J. Phys. D: Appl. Phys. 2008, 41, 013001. 7. Willets, K. A.; Duyne, R. P. V., Localized Surface Plasmon Resonance Spectroscopy and Sensing. Annu. Rev. Phys. Chem. 2007, 58, 267-297. 8. Vo-Dinh, T.; Fales, A. M.; Griffin, G. D.; Khoury, C. G.; Liu, Y.; Ngo, H.; Norton, S. J.; Register, J. K.; Wang, H.-N.; Yuan, H., Plasmonic Nanoprobes: From Chemical Sensing to Medical Diagnostics and Therapy. Nanoscale 2013, 5, 10127-10140. 9. Clavero, C., Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nat. Photonics 2014, 8, 95-103. 10. Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J., Photodetection with Active Optical Antennas. Science 2011, 332, 702-704. 11. Ali, H.; Seyed, M. S.; Étienne, B.; Michel, M., Quantum Dot–Metallic Nanorod Sensors via Exciton–Plasmon Interaction. Nanotechnology 2013, 24, 015502. 12. Schwartz, T.; Hutchison, J. A.; Genet, C.; Ebbesen, T. W., Reversible Switching of Ultrastrong Light-Molecule Coupling. Phys. Rev. Lett. 2011, 106, 196405. 13. Kabashin, A. V.; Evans, P.; Pastkovsky, S.; Hendren, W.; Wurtz, G. A.; Atkinson, R.; Pollard, R.; Podolskiy, V. A.; Zayats, A. V., Plasmonic Nanorod Metamaterials for Biosensing. Nat. Mater. 2009, 8, 867-871. 34 ACS Paragon Plus Environment

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Plexcitons: The Role of Oscillator Strengths and Spectral Widths in

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