A Numerical Spectroscopic Investigation on the ... - ACS Publications

J. Phys. Chem. C 2010, 114, 13825–13831

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A Numerical Spectroscopic Investigation on the Functionality of Molecular Excitons in Tuning the Plasmonic Splitting Observed in Core/Shell Hybrid Nanostructures Demet Gu¨len* Department of Physics, Middle East Technical UniVersity, Ankara 06531 Turkey ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: June 30, 2010

Coupling between localized surface plasmon resonance (LSPR) of metal nanoparticles and molecular excitons in core/shell hybrid nanostructures attracts increasing attention. Emphasis has been on fine-tuning exciton-plasmon coupling by controlling the LSPR of metal nanoparticles, while functionality of the intact excitonic resonance in this fine-tuning has received less attention. Our assertion has been that as a localized polarized excitation, just like the LSPR, the excitonic resonance should also sense the effective dielectric medium provided by the metal core and embedding medium. We have thereby provided a series of numerical experiments to investigate effective medium effects on the absorption of the core/shell hybrid complexes. Excitonic resonance is shown to experience an effective dielectric medium induced band splitting. Despite existence of two excitonic resonances, plasmonic splitting is shown to be due to coupling between blueshifted exciton and plasmon resonances. Further investigation of dependencies of excitonic band splitting and exciton-plasmon coupling on the shell thickness and excitonic oscillator strength yields important insight into the mechanism of exciton-plasmon coupling. Introduction Core/shell complexes composed of metallic nanoparticle cores coated with molecular layers, such as J-aggregates, in resonance with the localized surface plasmon resonance (LSPR) provide very flexible inorganic-organic hybrids for controllable design of optical response at nanoscale. Understanding of their optical response has great potential in the development of nanophotonic devices with functionalities such as molecular imaging, biological sensing, optical signal amplification, and plasmonic resonance energy transfer.1-7 The motivation for understanding the optical response of such core/shell complexes has increased greatly with the recent observations of strong coupling between excitonic resonance(s) provided by the J-aggregate shell and LSPR of the metal core.5-22 The evidence for strong coupling between the excitonic and the LSPR resonances is typically provided by measuring optical response of the hybrid complex. Strong coupling is characterized by the formation of a pair of peaks split around the nearly degenerate exciton and plasmon resonances in the optical response.7-20 This coupling is attributed to strong coupling between excitons of the J-aggregate shell and electronic polarizations of the metal core. Potential applications mentioned above would greatly benefit from the controllable tuning of the exciton-plasmon coupling strength. The current emphasis has been on tuning of LSPR resonance to achieve strongest possible exciton-plasmon coupling and fine-tuning is achieved by exploiting the wealth of information existing on the control of LSPR of metal nanoparticles.1,5,23,24 On the other hand the involvement of the intact optical response of the excitonic partner in fine-tuning has received less attention.20,22 Our assertion is that as a localized polarized excitation, just like the localized surface plasmon excitation, the intact excitonic response should also sense both the effective dielectric medium * To whom correspondence should be addressed. Phone: +90 (312) 2105060. Fax: +90 (312) 210-5099. E-mail: [email protected].

provided by the metal core and the embedding medium, in addition to the well-known electromagnetic field enhancement inside the shell provided by the plasmonic response of the core.5 In this study we will provide an investigation on the effects of effective dielectric medium provided by the metal core and the embedding medium on the optical response of the Jaggregate shell. The effects predicted will then be used to understand the optical response of a specific core/shell complex. The investigation has been carried out through simulations of the optical response of the intact J-aggregate shell and the core/shell hybrid complex in classical electrodynamics,25 and by a subsequent analysis of the hybrid optical response in terms of the coherent coupling between the excitonic states of the intact J-aggregate shell and the intact surface plasmon state in the framework of quantum mechanics.31 Materials and Methods Definition of the Absorption Cross-Section in Classical Electrodynamics. The model geometry used is a prolate spheroid of core spherical radius a, and of aspect ratio r, which has been uniformly coated by a J-aggregate shell of thickness tJ. The spheroid is embedded in a homogeneous nonabsorbing medium of dielectric constant εD. εM(ω) and εJ(ω) are the dielectric functions of the metal core and the J-aggregate shell, respectively. It is widely acknowledged that classical electrodynamics provides a quantitative description of the optical response of noble metal nanoparticles and their hybrids.1-25 A recent review article6 can be referred to for an overview of analytical and numerical methods that can be used to describe the optical response of such systems. A more detailed account of the method we have used can be found in ref 25. If the size of the core/shell spheroid is much smaller than the wavelength of the incident light it is sufficient to consider only the dipolar response to incident light and the quasi-static approximation is appropriate. In brief the electric potential of the system described above is evaluated by

10.1021/jp101658b  2010 American Chemical Society Published on Web 07/27/2010

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solving Laplace equation with the corresponding boundary conditions when subjected to a uniform external electric field along a specified direction. The polarizability, R(ω), of the system is found by dividing the induced dipole moment by the external field strength. The replacement of Maxwell’s equations by electrostatic Laplace equation is known as the quasistatic approximation. In this limit the polarizability acquires its wavelength/frequencydependence through frequency-dependence of the dielectric functions of core and shell. The absorption cross-section σ(ω) is expressed as

σ(ω) )

ω Im[R(ω)] πcR2

(1)

where ω is the angular frequency of the incident electromagnetic radiation, c is the speed of light, R ) a + tJ is the radius of the core/shell spheroid. In this limit, the exact solution for the polarizability along the long axis of the core/shell prolate spheroid embedded in a homogeneous dielectric medium is25 R(ω) ) [εJ(ω) - εD][εJ(ω) + (εM(ω) - εJ(ω))(L(M)-qL(J))]+ V

qεJ(ω)[εM(ω) - εJ(ω)] [εJ(ω) + (εM(ω) - εJ(ω))(L

-qL )][εD + (εJ(ω) - εD)L(J)] +

(M)

(J)

qL(J)εJ(ω)[εM(ω) - εJ(ω)]

(2) In eq 2, epsilons are defined above, V is total volume of the core/shell spheroid, q is the fraction of the total volume occupied by the core. L(M) and L(J) are geometrical factors for the metal core spheroid and J-aggregate spheroidal shell. For a prolate spheroid of aspect ratio r, L factor along the long axis is defined as

L)

[

]

1 r-2 1 + (1 - r-2)1/2 ln -1 (1 - r-2) 2(1 - r-2)1/2 1 - (1 - r-2)1/2

(3)

Dielectric Functions. The dielectric function of metal core is described by a Drude model

εM(ω) ) ε∞,M

ω2p ω(ω + iγp)

(4)

where ε∞,M is the contribution from the bound electrons in the metal, ωp is the bulk plasmon frequency of metal and γp is the plasmon relaxation constant. To interpret the real-life observations of the optical response of core/shell complexes it would be better to employ the experimental dielectric function data of the metal core.26 However, eq 4 with the Drude parameters given below is not only a satisfactory approximation commonly used to model the optical response of the gold and silver nanoparticles1,4-6 but also serves as a useful analytical model for the purposes of this study. Further discussion on the origin of parameters can be found in other studies.1,5,6,25 The dielectric function of the J-aggregate shell is described using a homogeneously broadened one-oscillator Lorentzian model as

εJ(ω) ) ε∞,J +

2 fωJ,0 2 ωJ,0 - ω2 - iγJω

(5)

where ε∞,J is the high-frequency component of the dielectric function, ωJ,0 is the frequency of excitonic transition, γJ is the line width of the excitonic transition, and f is the reduced oscillator strength. Experimentally observed spectral signature of the lowest energy excitonic state(s) of J aggregates (J-band) is a narrow absorption band with a pronounced asymmetry on the higher energy side, i.e., more like an asymmetric Lorentzian. Previous works27-30 may be consulted for more information on the physical origin of the peak position, width, and shape of the J band. Equation 5 with the parameters given below serves as a sufficient model for the purposes of this study. Numerical Values of the Parameters for the Electrodynamics Calculations. A core spheroid of spherical radius, a, of 25 nm, and an aspect ratio, r, of π is considered. The metal core is assumed to be gold. The dielectric constant of embedding medium, εD, is set equal to 1.768 (aqueous). The gold core and J-aggregate parameters are assigned by following the experimental results. The parameters for the gold core are assigned as ε∞,M ) 9.84, pωp ) 9 eV, and pγp ) 67 meV.11,16 The excitonic transition of the J-aggregate is taken almost resonant with the surface plasmon resonance wavelength of the gold spheroid (∼693 nm), and pγJ ) 52 meV.11,16 When necessary for discussion the relaxation rates are assigned differently (see below). There is no consensus on the value of ε∞,J in the literature. ε∞,J values ranging between 1 and 1.8 have been used.9,11,16,17,20 Here we take ε∞,J ) εD ) 1.768 for the ease of discussion (see below). The thickness, tJ, and the oscillator strength, f, are varied around the values reported in the literature.11,16,17 The ratio of (tJ/a) is varied as 0.05, 0.1, 0.15, and 0.20. The oscillator strength values are varied between f ) 0.02 and f ) 0.1. The coupling strength is self-adjusted once the exciton and plasmon parameters are set. Quantum Mechanics of Two-State System.31 For convenience the quantum mechanical notation and sign conventions are shown in Figure 1. The states |1〉 and |2〉 represent “localized” excitations corresponding to the plasmonic resonance in the presence of the J-aggregate shell and to the excitonic resonance of the J-aggregate shell in the presence of metal core, both evaluated in the presence the embedding medium. Their respective energies are E1 and E2. The states |1〉 and |2〉 are coupled by an interaction V into states | +〉 and | -〉

|-〉 ) cos ξ|1〉_ sin ξ|2〉

(6a)

|+〉 ) sin ξ|1〉 + cos ξ|2〉

(6b)

These states have respective energies

E( )

(E1 + E2) 1 ( (4V2 + (E2 - E1)2)1/2 2 2

(7)

The mixing parameter ξ is defined by

tan(2ξ) )

2V E2 - E1

(8)

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Figure 1. Conventions for the eigenvalues and eigenfunctions for the standard two-state problem. In our usage E1 e E2 and E- e E+ for all values of the coupling strength V. The mixing parameter ξ has the sign of V and ranges from 0 to π/4. Scales are arbitrary except that the units are the same for both axes. The triangle provides a ready connection among the eigenvalues, the unperturbed energy splitting and ξ.

The absorption cross-section in the presence of mixing/ coupling can be calculated as 2 σ(ω) ) [σ+ 1 (ω) + σ2 (ω)] cos ξ + [σ1 (ω) + 2 σ+ 2 (ω)] sin ξ (9)

where σiφ(ω) (i ) 1, 2 and φ ) +, -) is the “unperturbed” absorption cross-section for excitation i whose peak is shifted to Eφ. Results and Discussion For the numerical values given above the uncoupled core spheroid exhibits a (longitudinal) plasmon resonance peaking at 692.815 nm, which is slightly “blue-shifted” with respect to the resonance peak of the uncoupled J-aggregate shell at 693.125 nm (shown in Figure 2a). Here the term “uncoupled” refers to free-standing structure in the embedding medium. For the core/shell spheroid, optical response is first evaluated using classical electrodynamics (eq 1). The dependencies of absorption spectra on the shell thickness, tJ, and the oscillator strength of the excitonic transition, f, are simulated. The results at two different shell thicknesses are shown in Figure 2b. Parts c and d of Figure 2 show the position of the peaks observed as plotted against (tJ/a) for different f. We note that arbitrary units (au) related to absorption, σ, are given on the same scale throughout the manuscript. The results presented in Figure 2 demonstrate that optical response of core/shell complexes consisting of metal nanoparticles coated with J-aggregates significantly differs from a simple superposition of the responses of uncoupled components as already discussed in a number of theoretical and experimental accounts.7-21 The optical response mainly consists of a pair of peaks split around the nearly degenerate uncoupled exciton and plasmon resonances. As discussed recently,17 the amount of splitting increases as the oscillator strength, f, and/or the thickness, tJ, of the J-aggregate shell gets larger. What is not been noted in earlier numerical experiments is the development of a shoulder in the gap between the two peaks, which is pronounced at large oscillator strengths. The shoulder is actually present at all shell thicknesses. At small thicknesses it is manifested as an asymmetry in the line shape, which is especially notable on the short wavelength side of the red-shifted

Figure 2. (a) Plasmonic and excitonic responses of the uncoupled core and J-aggregate shell. The spectra are normalized at their peaks. The excitonic response is simulated by letting the core to be aqueous and for f ) 0.02 and tJ/a ) 0.05. (b) Optical response of the core/shell complex at different f and two different (tJ/a). In each panel f changes between 0.02 (smallest splitting) and 1.0 (largest splitting), the f values in-between are 0.04, 0.06, and 0.08. (c)/(d) λ+/λ-: the wavelength for the higher/lower energy peak vs (tJ/a) at different f. For higher accuracy peak positions are determined using pγp ) pγJ ) 0.1 meV. Color coding, f ) 1 (magenta), f ) 0.08 (green), f ) 0.06 (red), f ) 0.04 (blue), f ) 0.02 (gray). The remaining parameters are assigned as given in Materials and Methods.

hybrid peak. Position of the shoulder, however, remains more or less fixed around the uncoupled exciton/plasmon resonance wavelength. These observations are taken evidence for our assertion that as a localized polarized excitation the intact excitonic response should sense the effective dielectric medium provided by the metal core and the embedding medium. As mentioned above the high-frequency component of the dielectric function of the shell, ε∞,J, is taken equal to the dielectric constant of the embedding medium, εD. With this choice the surface plasmon resonance frequency of the intact core becomes independent of shell thickness. This choice has

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Figure 3. (a) Optical response of the intact J-aggregate shell at pγJ ) 52 meV (solid lines) and pγJ ) 5.2 meV (dashed lines) at tJ/a ) 0.2 for different oscillator strengths, f ) 1 (magenta), f ) 0.08 (green), f ) 0.06 (red), f ) 0.04 (blue), f ) 0.02 (gray). The remaining parameters are defined in Materials and Methods. (b) Peak positions of the red- and the blue-shifted bands of intact J-aggregate spectra as a function of (tJ/a) at different f (on the left) and the spectra (normalized at the blue-shifted peak) for f ) 0.10 at different (tJ/a), 0.05, 0.1, 0.15, and 0.20 (from top to bottom) with pγJ ) 5.2 meV. For a better accuracy the peak positions are determined for pγJ ) 0.1 meV.

been merely made to simplify the discussion of the excitonic effects at a fixed intact plasmon frequency. It should be noted that the predictions of this paper on the spectral response of the intact shell and on the nature of hybridization survive unless ε∞,J ) εD ) ε∞,M. The “intact” response of the shell is simulated by assigning ε∞,M as the dielectric constant of nonabsorbing core while the shell is still in the embedding medium. The intact responses simulated for a specific shell thickness (tJ/a ) 0.20) at different oscillator strengths ranging between 0.02 and 0.10 and for two different exciton relaxation rates, pγJ ) 52 meV and pγJ ) 5.2 meV, are shown in Figure 3a. The latter value, an order of magnitude narrower than the experimental bandwidth, pγJ ) 52 meV,11,16 is arbitrarily assigned to distinguish the underlying spectral components contributing to the intact shell spectra. Figure 3b shows the dependence of peak positions observed in Figure 3a as a function of (tJ/a) at different f. The results presented in Figure 3 clearly demonstrate that the optical response of the J-aggregate shell is modified by the metal core acting as nonabsorbing dielectric medium. The intact response is characterized by two transitions: a red transition peaking at wavelengths around the transition wavelength of the uncoupled J-aggregate shell and a blue-shifted transition whose position determined, to a large extent, by the magnitude of the oscillator strength. The blue-shifted peak reshifts slightly to the red with increasing thickness of the shell and the red peak is very slightly blue-shifted as the shell gets thicker. As can be judged from the narrow bandwidth simulations the band widths of the two transitions are similar. Except in a very recent work

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Figure 4. (a) Optical response of the core/shell complex at different pγJ for pγp ) 67 meV. pγJ values are 5 meV (dashed-dotted), 25 meV (solid), and 50 meV (dashed). (b) Optical response of the core/ shell complex at different pγp for pγJ ) 52 meV. pγp values are 1 meV (dashed), 10 meV (solid), and 50 meV (dashed-dotted). The remaining parameters are assigned as given in Materials and Methods. The spectra in both panels are normalized at the peak of their red transition.

that appeared at the submission stage of this study,20 in all of the previous studies,7-11,16-19 it has been presumed that the J-aggregate response do not change upon coating on the core. Reference 20 provides experimental evidence for the existence of the two shell transitions, in agreement with the predictions of this study. Our prediction on the existence of two excitonic/J-aggregate transitions requires re-examination of the core/shell optical response7-11,16-19 to decide the modes and strength(s) of coupling between these two transitions and the plasmonic transition. We first show that despite the existence of two J-aggregate transitions, the optical response mainly results from the coupling between the blue-shifted J-aggregate and the plasmon resonances. An explanation of this statement is provided through the simulations shown in Figure 4. The approach of the numerical experiments shown in Figure 4 has been to identify separate spectral components contributing to the optical response of the core/shell hybrid complex and to clarify their hybridization characteristics by arbitrarily varying the values of exciton and plasmon relaxation rates down to very narrow band widths/ very slow relaxation rates. Figure 4a shows the optical response of the core/shell complex at different exciton relaxation rates, pγJ, ranging between 5 and 50 meV, while the plasmon relaxation rate is fixed at its experimental value of 67 meV. Figure 4b shows the optical response at different plasmon relaxation rates, pγp, ranging between 1 and 50 meV, while

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Figure 5. (a) Fit results for different (tJ/a) at a particular f ) 0.06. Peak positions obtained from classical electrodynamics simulations (symbols) and quantum mechanical fits (dashed lines). Also shown are Ep(0) and fit results for V and EJ,B(0). (b) Comparison of the magnitudes of coupling, 2V (dashed lines), and peak splitting, E+ - E- (solid lines), for different (tJ/a) at different f. (c) Comparison between the fit results for EJ,B(0) (solid lines) and the corresponding peak positions of the intact J-aggregate shell (dashed lines) already shown in Figure 3b. (d) Mixing coefficients, sin2 ξ and cos2 ξ vs (tJ/a) at different f. The color coding for different f values are shown in the box.

the exciton relaxation rate is kept at its experimental value of 52 meV. The spectra are drawn on the energy scale to allow an easier comparison of the band widths. This numerical experiment illustrates clearly that the response has in fact three peaks. The middle transition corresponding to the shoulder noted above is around the red transition of the intact J-aggregate shell/plasmon resonance. Its bandwidth follows the assigned exciton relaxation rates. Moreover, its peak energy and bandwidth are not affected by the changes in the plasmon relaxation rates. These observations confirm that it is, and it remains as an uncoupled excitonic transition. On the other hand, even when the exciton/plasmon relaxations are very “slow”, the bands on either side do not relax as slow is a clear confirmation of their hybridized nature. In summary, hybrid optical response is a superposition of an uncoupled excitonic band which is associated with the red band of the intact J-aggregate shell and the two hybrid bands resulting from the coupling between the blue-shifted J-aggregate and the plasmon resonances. Having the red transition of the J-aggregate shell isolated and uncoupled allows one to estimate further plasmon induced modifications on its transition wavelength and intensity. These details will be discussed elsewhere. Here we suffice to remark that the field enhancement does not induce any further splitting and band shape changes. The results presented in relation to Figure 4 are important for two basic reasons. First, having explicitly identified the two states contributing to the hybridization, the coupling strengths can be realistically determined. Second, the characteristics of the hybridized states can be investigated in detail on a solid basis.

To obtain the magnitude of coupling, V, at different values of tJ and f, we have fitted the peak energies of the core/shell complex, shown in parts c and d of Figure 2, using the quantum mechanics of two-state system and have determined the mixing properties using eqs 6-8. Hybridization hampers the deduction of the plasmon induced wavelength shifts/intensity modifications on the blue-shifted excitonic band through simulations. Therefore, unperturbed transition energy of the blue-shifted excitonic state of the intact J-aggregate, EJ,B(0), is let as free fit parameter, while the energy of the intact plasmon state, Ep(0), is kept constant at 692.815 nm. The results are summarized in Figure 5. Figure 5a shows the fit results for different (tJ/a) at a particular f (0.06). Figure 5b compares the magnitudes of coupling, V, and peak splitting, E+ - E-, for different (tJ/a) at different f. In Figure 5c the fit results for EJ,B(0) are compared with the corresponding peak positions of the intact J-aggregate (see also Figure 3b). Figure 5d displays the mixing coefficients, sin2 ξ and cos2 ξ for different (tJ/a) at different f. According to quantum mechanical picture, the states would mix equally if the unperturbed energies are exactly in resonance, or smaller the energy difference between the unperturbed energies of excitonic and plasmonic states, larger is the mixing (Figure 5d). In resonance the amount of energy splitting is equal to twice the coupling strength, or the difference between these two quantities increases as the states get more off resonance (Figure 5b). Fit results for the unperturbed energy of the exciton state, EJ,B(0), do not exactly match the peak positions obtained for the intact J-aggregate shell (Figure 5c), while they are definitely blue-shifted with respect to the original transition energy of the

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uncoupled J-aggregate. The tentative interpretation of the results shown in Figure 5c would then be that the plasmonic modifications act opposite to the dielectric medium related modifications, and plasmon induced red shifts, are not large enough to beat the dielectric induced blue shifts. The magnitude of red shift is dominantly controlled by the strength of the excitonic transition. The excitonic oscillator strength is dominant also in determining the order of magnitude of the coupling strength. Coupling is enhanced further with the increasing shell thickness (see Figure 5b for quantitative values). At this point it is important to remark that coupling strength estimates carried out without taking the modifications on the intact J-aggregate shell transition energies are bound to be overestimates. The trends observed on the dependence of the coupling strength on f and tJ are in qualitative agreement with a coupling between the polarizations. Next we address how well the optical response of the core/ shell complex correlates with the optical responses of the contributing states and with their coupling. Although sometimes mentioned as “the observation of coherent coupling”,16 the current theoretical treatments of the experimental data have merely addressed how strong the coupling is. Degree of coherence could be accessed, for example, by providing a quantitative comparison between the experimental optical response and the theoretical coherent spectral reconstruction. In the absence of information on the intensity of the blue-shifted excitonic band we could only evaluate coherent plasmonic response. As an example, coherent plasmonic response at different shell thicknesses for a specific oscillator strength (f ) 1) are generated using eq 9. The results are given in Figure 6 in comparison with the classical electrodynamics based responses. The magnitudes and relative strengths of the symmetric and antisymmetric hybrid bands are accounted rather well by the coherent plasmonic contributions. This spectral resolution explicitly shows that basic impact of the J-aggregate shell is to mediate the coupling for the plasmonic splitting. It looks promising that an analytical understanding of the mechanism of interaction can be approached by integrating the two effects into coupling between the polarizations of plasmon and intact exciton states. One of which we have demonstrated here through numerical experiments is the dielectric medium induced energetic shifts on the excitonic transitions in the absence of plasmonic response and the other is the plasmon induced enhancement of the excitonic oscillator strength. The challenge is to reach a level of understanding existing, for example, for the case of semiconductor quantum dot metal nanoparticle hybrid systems.32,33

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Figure 6. Coherent plasmonic response for f ) 1 at four different shell thicknesses, (tJ/a), 0.05, 010, 0.15, and 0.20 vs classical electrodynamics based response. Plasmonic contributions from the symmetric exciton-plasmon state (blue dashes) and the antisymmetric excitonplasmon state (red dashes), total coherent plasmonic response (gray), and classical electrodynamics-based response (symbols). The remaining parameters for the electrodynamics based/coherent plasmonic response are assigned as stated in Materials and Methods/in the text. The insert shows the plasmonic response of the uncoupled core as in the same arbitrary units with the rest of the data shown in the main part of this figure.

Conclusions A quantitative analysis on the optical response of hybrid complexes consisting of a plasmonic metal core and an excitonic molecular shell is given. More explicitly, the effects of effective dielectric medium provided by the metal core and the embedding medium on the optical response of the excitonic molecular shell are investigated. The effects predicted are then used to correlate the optical response of core/shell complexes with the optical responses of intact subsystems and with the coupling between their corresponding localized excitations, exciton, and surface plasmon. It has been shown that the absorption spectrum of molecular/ J-aggregate shell, which is nearly resonant with the LSPR of the metal core in its uncoupled state, can be strongly influenced by the effective dielectric medium offered by the core and the embedding medium. Intact response of the J-aggregate shell has been characterized by two transitions: a red transition peaking

at wavelengths around the transition wavelength of the uncoupled J-aggregate shell and a blue-shifted transition whose position determined, to a large extent, by the magnitude of the oscillator strength of the original excitonic resonance. This new prediction raised two basic questions on the coupling among the two excitonic resonances and the plasmonic resonance: what are the modes of coupling, and what are their strengths? It has been shown that the plasmonic resonance couples only to the excitonic resonance associated with blue-shifted absorption band of the intact J-aggregate leading to a doubly split plasmonic response and the red transition remains as an uncoupled excitonic transition. The association of the hybridized part of the response with two specific states has been discussed to be very functional in determining the coupling strengths and in understanding the contributions of the excitonic and plasmonic partners to the hybridized response. The hybrid optical response

Plasmonic Splitting in Core/Shell Hybrid Nanostructures has been detailed further in terms of its dependencies on the thickness and the oscillator strength of the J-aggregate shell. In addition to contributing to an increased control over tuning the plasmonic response of core/shell complexes for their potential applications, these results can stimulate interest for analytical approaches aiming at a more fundamentally sound understanding of the exciton-plasmon coupling in these hybrid nanosystems. References and Notes (1) Zhao, J.; Sherry, L. J.; Schatz, G. C.; van Duyne, R. P. IEEE J. Sel. Top. Quantum Electron. 2008, 14, 1418. (2) Lim, S. I.; Zhong, C.-J. Acc. Chem. Res. 2009, 42, 798. (3) Liu, G. L.; Long, Y.-T.; Choi, Y.; Kang, T.; Lee, L. P. Nat. Methods 2007, 4, 1015. (4) Anker, J. N.; Hall, W. P.; Lyanders, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442. (5) Schwartzberg, A. M.; Zhang, J. Z. J. Phys. Chem. C 2008, 112, 10323. (6) Zhao, J.; Pinchuk, A. O.; McMahon, J. M.; Li, S.; Ausman, L. K.; Atkinson, A. A.; Schatz, G. C. Acc. Chem. Res. 2008, 41, 1710. (7) Wiederrecht, G. P.; Wurtz, G. A.; Bouhelier, A. Chem. Phys. Lett. 2008, 461, 171. (8) Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Langmuir 2001, 17, 578. (9) Wiederrecht, G.; Wurtz, G.; Hranisavljevic, J. Nano Lett. 2004, 4, 2121. (10) Uwada, T.; Toyota, R.; Masuhara, H.; Asahi, T. J. Phys. Chem. C 2007, 111, 1549. (11) Wurtz, G. A.; Evans, P. R.; Hendren, W.; Atkinson, R.; Dickson, W.; Pollard, R. J.; Zayats, A. V. Nano Lett. 2007, 7, 1297. (12) Evans, P. R.; Wurtz, G. A.; Atkinson, R.; William, H.; O’Connor, D.; Dickson, W.; Pollard, R. J.; Zayats, A. V. J. Phys. Chem. C 2007, 111, 12522.

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