PlasmonâExciton Coupling between Metallic Nanoparticles and Dye

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Plasmon-Exciton Coupling Between Metallic Nanoparticles and Fluorescent Monomers Andrea L Rodarte, and Andrea R. Tao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08905 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry

Plasmon-Exciton Coupling Between Metallic Nanoparticles and Dye Monomers Andrea L. Rodarte and Andrea R. Tao* Department of NanoEngineering, University of California, San Diego, 9500 Gilman Dr. MC 0448, La Jolla, California 92039-0448, United States

KEYWORDS: localized surface plasmon, exciton, strong coupling, Rabi splitting, dark field spectroscopy

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ABSTRACT: We investigate electromagnetic coupling between localized surface plasmons and excitons in a Ag nanoparticle (NP) and monomer dye conjugate. Strong coupling is observed as Rabi splitting in the optical scattering response of the conjugate. We measure the strength of coupling as a function of NP-dye separation distance using bifunctional molecules that link the dye and NP with nanoscale control. Coupling strength follows a single exponential decay with a decay length of 13.7 ± 5 nm, indicating that the conjugates can be used as a plasmon ruler. In addition, we find that at separation distances below 2 nm, the coupling strength is strongly decreased by quantum effects such as electron tunneling which interfere with plasmon-exciton hybridization. Using single NP spectroscopy to investigate conjugate coupling energy, we are able to tune separation distance to achieve coupling energies as high as 600 meV, among the highest energies reported for Ag NPs.


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INTRODUCTION Energy transfer based on optical near-field interactions has been instrumental for the detection, measurement, and observation of small separation distances in molecules and systems of interest. Förster resonance energy transfer (FRET) is a mechanism in which the energy of an excited molecule can be non-radiatively transferred to a molecule with lower transition energy through dipole-dipole coupling. FRET pairs are commonly used to probe small separation distances, including cell-material interactions at the molecular level1, use as a molecular sensor2, to study the length and flexibility of polymers and biomolecules3, to investigate molecular interactions and conformal changes of proteins4, as well as a way to investigate biomolecular dynamics such as RNA folding5 and small DNA bending angles6. In FRET, energy transfer is characterized by the Förster radius Ro which is defined as the separation distance at which the efficiency of energy transfer is 50%. Ro is given by = 0.211 ()

(1)

where ϕD is the quantum yield of the donor (D) in the absence of the acceptor (A), κ2 is the dipole orientation factor, n is the refractive index of the medium, and J(λ) is the spectral overlap integral of the donor and acceptor emission and absorption spectra7. Small changes in separation distance lead to large changes in the observed fluorescence spectrum, as shown in Figure 1A, and it is for this reason that FRET is commonly used as a spectroscopic ruler. Because of the dipole-dipole interaction the efficiency of coupling is proportional to the inverse sixth power of the distance between the molecules. This effectively limits FRET to function in the range below 10 nm.


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Figure 1. Schematics of various energy transfer mechanisms. (a) FRET energy transfer between two fluorophores where donor molecule transfers energy to the acceptor, increasing the acceptor fluorescence. (b) Plasmon coupling for a plasmon ruler consisting of a NP pair, where close-coupling results in a red-shift of the LSPR peak wavelength. In the vicinity of an Ag NP a fluorophore can exhibit (c) weak coupling resulting in enhanced fluorescence (FL) or (d) strong coupling, resulting in Rabi Splitting depending on the position and shape of the exciton transition with respect to the LSPR of the Ag NP. More recent work has focused on the use of plasmon rulers composed of noble metal nanoparticles (NPs). Optical excitation can induce a collective oscillation of conduction electrons with a resonant frequency that is dependent on NP size, shape and composition. These localized surface plasmon resonances (LSPRs) are responsible for generating extinction cross-sections that are up to 100 times the physical cross section of the NP8. The strong scattering cross-section allows nanoparticles to be easily visualized with a dark field microscope. In addition, metallic nanoparticles do not suffer from blinking or bleaching, making them prime candidates for sensing applications and are often used for sensing even a small change in the index of refraction of the surrounding media9, evidenced by a shift of the peak LSPR wavelength. In a plasmon


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ruler, two metal NPs are used to measure separation distance. Capacitive coupling between nearby NPs is known to result in a strong redshift of the LSPR wavelength10 as shown in Figure 1B. NP coupling strength depends strongly on separation distance with a R-3 (as opposed to R-6 for FRET)11. Pairs of covalently linked NPs have been demonstrated as plasmonic rulers that can sense distances far beyond the 10 nm limit of FRET12. These plasmon rulers have been demonstrated for observing the cleavage of DNA13, testing the efficacy of pro-apoptopic cancer drugs14, and as colorimetric DNA switches15. For in vitro or in vivo applications, it would be advantageous to combine the high signalto-noise of plasmon NP rulers with the utility of fluorescence tags that are commonly used in FRET pairs to label biomolecules of interest. One strategy is to carry out energy transfer using a plasmonic NP and a molecule that possesses a strong electronic transition as the donor/acceptor pair. When this molecule is a fluorophore, energy transfer can be observed by quenching or enhancement of fluorescence, depending on the separation distance from the NP surface16,17. This NP-fluorophore interaction is typically the result of weak coupling18 as shown in Figure 1C. Plasmon-mediated energy transfer has also been reported for NP-chromophore pairs where the optical scattering intensity of the plasmonic NP is quenched due to the overlap of LSPR band of the NP and the absorption band of the chromophore molecule19. Weak coupling is characterized by a modification of the LSPR band of the plasmonic NP or the fluorescence peak of the chromophore molecule while the less common strong coupling is typically the result of strong coupling between plasmons and excitons, resulting in a hybridization of the electronic states. Previous studies have identified some of the parameters necessary to achieve strong coupling in a coupled plasmon-exciton system. In order to achieve strong coupling, the oscillator strength must be sufficiently strong, as indicated by a narrow optical absorption peak linewidth20-


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. Plasmon-exciton coupling of the system can manifest as a Fano resonance which is observed

by an asymmetric dip in absorbance at the exciton transition frequency, or as Rabi splitting in which the degenerate states split to form lower and higher energy states as shown in Figure 1D. In a hybridized system, strong coupling is determined by the separation of energy between the low and high energy states. Gomez et al. determined that Ag NPs achieve stronger coupling than Au NPs, and that coupling strength is highest when the exciton transition frequency is slightly higher than that of the uncoupled LSPR frequency23. Plasmonic NPs coupled to J-aggregates  where several dye molecules organize into supramolecular structures on the surface of the NP are well-studied systems for understanding plasmon-mediated energy transfer mechanisms. J-aggregates exhibit a strongly red-shifted and narrower absorption peak than the monomeric dye molecule, and thus possess high oscillator strengths and narrow linewidths that enable strong coupling. Fofang et al. observe that this strong coupling could be observed for colloidal NP suspensions. Au-coated silica shells with Jaggregates adsorbed on the surface exhibited strong Rabi splitting. They measured a coupling energy of 120 meV to the dipole LSPR and a slightly lower coupling energy of approximately 100 meV to the quadrupole LSPR resonance24. Lebedev et al. predicted that large nanoparticles with additional multipole plasmon resonances would have additional peaks as a result of the exciton coupling to higher order resonances25,26. The coupling energy was shown to increase for dye molecules in the presence of a strong electric field, such as those generated by the electromagnetic hot spots within NP dimers. Schlather et al. investigated J-aggregates situated in between Au NP dimers with a 15 nm gap and observed coupling energies ranging from 230 meV to 400 meV, dependent on the size of the dimers27. Faucheaux et al. predicted that the coupling energy is determined by the relative oscillator strength of the system as well as a coupling


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constant that is proportional to 1/d3, where d is the distance between the center of mass of the dipoles21. This distance-dependence suggests the potential of NP-fluorophore pairs as optical rulers to measure nanoscale separation distances. Here, we demonstrate the dependence of separation distance on coupling strength between plasmonic Ag NPs and common fluorophores used to label cells and tissue in fluorescence microscopy and cell biology. In our design, the main readout is optical scattering from the Ag NP. While previous work has been carried out for coupling between metal thinfilms and monomer dyes28,29, very little work has explored interactions within conjugates of monomer dye and metal NPs30. We conjugate a monomeric dye, Alexa Fluor 488 (AF488), to the surface of spherical Ag NPs and observe strong plasmon-exciton coupling resulting in Rabi splitting. This coupling is strong enough that the scattering spectra of the NPs is significantly modified even when the LSPR peak is detuned 30 nm from the AF488 absorption peak. We control the separation distance between the dye and the NP using two linker molecules to probe two distance regimes. The first regime represents > 3 nm in separation distance, where we expect to observe a decrease in coupling energy with increasing distance due to the evanescent field of the plasmonic NP. The second regime occurs for separation distances < 2 nm, where strong coupling is expected21,31-33. In this regime, we observe the effects of quantum electron tunneling that competes with the hybridization of states. We utilize single-NP spectroscopy to quantify the NP-fluorophore coupling energy for each of the linker molecules used. This work is the first step to creating a novel hybrid plasmonic ruler. RESULTS AND DISCUSSION


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Ag nanospheres were selected as the plasmonic NP component in the hybrid dye-NP system due to their well-known size-dependent and tunable LSPRs that are excited in the visible range. Ag NPs are also known to exhibit a stronger plasmon-exciton coupling than Au NPs21,23. Ag NPs were synthesized as according to previously published protocols34. Briefly, we used the polyol precursor injection method to reduce 1 mL of 1.77 M aqueous silver nitrate (AgNO3 Sigma Aldrich) into 50 mL of hot 0.468 mM polyvinylpyrrolidone (PVP, Sigma Aldrich) in ethylene glycol (HOCH2CH2OH, Sigma Aldrich) mixture, resulting in nanospheres capped with a PVP layer. By adjusting the reaction temperature, NP diameter can readily be tuned. In this manner, we were able to tune the scattering peak corresponding to dipolar LSPR excitation between 400 to 500 nm. Initial experiments were performed using Ag NPs with a peak LSPR wavelength at 465 nm, as shown in Figure 2A (black line). A SEM image of the 75 nm NPs is shown in Figure 2B. Alexa Fluor 488 Tetrafluorophenyl (AF-TFP) Ester (Life Technologies) was selected as the monomer dye because it exhibits an absorption peak at 495 nm (Figure 2A, green line) which matches well with Ag NP LSPR (Figure 2A, black line) excitation and a narrow linewidth (223 meV) which is expected to enable strong coupling20. In addition, AF-TFP is an amine reactive dye that forms a very stable amide bond between the dye and amine terminated biomolecules. In order to promote covalent conjugation of the dye to the Ag NP surface, we reacted AF-TFP with 3-amino-1-propanethiol (APT) to functionalize the dye with a thiol group. Figure 2B shows an SEM image of the Ag NPs and the conjugation scheme is shown in Figure 2C. The APT molecule serves as a linker to the Ag NP and also serves to control the separation distance between the dye and Ag NP. Briefly, 100 µg (0.155 µmol) of dry AF-TFP is dissolved in dimethyl sulfoxide (DMSO, VWR International) at a concentration of 0.2 mM. We added 0.155


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µmol of various linker molecules dispersed in ethanol to the AF-TFP solution. The reaction was stirred in the dark for 2 hours and stored at 2⁰ C prior to use.


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Figure 2. (a) Raleigh scattering spectrum for 75 nm diameter Ag nanospheres (black) overlayed with the absorption (green) and emission (red) of Alexa Fluor 488 TFP in ethanol. (b) SEM image of Ag NPs. (c) Conjugation scheme for creating thiolated AF. Dye-NP conjugates were prepared on a glass or silicon substrate for carrying out spectroscopy measurements. Substrates were cleaned in piranha solution prior to being immersed in a 1% vol. solution of (3-aminopropyl)trimethoxysilane (APTMS, Sigma Aldrich) in ethanol for 30 minutes. This chemical modification of the substrate was necessary for immobilizing NPs


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on the substrate and preventing NP diffusion and aggregation during measurements. First, pristine Ag NPs are adsorbed to the substrates by incubating the APTMS functionalized glass in washed Ag NPs redispersed in ethanol for 1 hour. NPs were observed to have adsorbed to the substrate by optical microscopy. The substrate was then immersed in 1 mM NaOH for 2 hours in order to remove any residual APTMS on the surface35. The immobilized NPs were then incubated in a 10 µM solution of thiol-conjugated AF in ethanol at room temperature for 4 hours. After incubation the nanoparticles were rinsed repeatedly in ethanol to remove excess unattached dye and the scattering spectrum of the dye/NP conjugate was collected using a Nikon Eclipse LV 100D-U upright dark field microscope outfitted with a Horiba Syncerity MicroHr spectrometer. Figure 3A shows a typical scattering spectrum collected for AF-conjugated Ag NPs immobilized on a glass substrate (green curve). AF conjugation was carried out using an 3-

Figure 3. (a) Optical scattering spectra collected for pristine nanoparticles (black) and for dye/NP conjugates (green). The conjugate spectrum consists of two peaks (λ1 and λ2) that correspond to two hybridized states created by Rabi splitting. (b) Schematic of Rabi splitting when plasmon and exciton energy is degenerate, two hybrid states are created with higher and lower energies from the initial state. The energy between the branches is considered the Rabi energy, ℏΩR. In the case that the energy states are not degenerate the coupling energy between upper and lower states is ℏΩ..


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Amino-1-propanethiol hydrochloride (APT) linker which has a molecular length of approximately 0.49 nm. Figure 3A also shows the scattering spectrum for immobilized, pristine Ag NPs (black curve) for comparison. The scattering spectrum of the dye-NP conjugates exhibits two scattering peaks that appear at 447 nm and 540 nm. Peak wavelengths were determined by Lorentzian fits (OriginPro 2016). Fluorescence measurements indicate that AF emission at 520 nm is strongly quenched, which is consistent with the short distance between AF and metal surface. We assign the higher intensity peak at 447 nm as the primary peak, λ1, because it appears slightly blue-shifted from the LSPR band of the pristine NPs. We assign the weaker intensity peak at 540 nm as the secondary peak, λ2 because it coincides closely with the absorption band of the unconjugated AF-TFP. The appearance of these two peaks is consistent with Rabi splitting for degenerate plasmon and exciton energy bands36,37. Figure 3B shows a schematic of the energy states that result from Rabi splitting when the LSPR energy and exciton energy are degenerate with one another, i.e. the case where maximum energy overlap is expected. Upon coupling, the plasmon and exciton states hybridize to form two new states. The energy of the upper and lower states can be calculated using a coupled harmonic oscillator model27,38, !" ,!$ (ℏ%& ) =

ℏ!' (ℏ!)*

± ,(ℏΩ. ) + (ℏ%& − ℏ% 1 )

(2)

where ℏωP and ℏωex are the uncoupled energies of the LSPR and exciton bands respectively and ℏΩR is the Rabi energy, or the coupling energy. The coupling energy is dependent on the spectral overlap of the excitonic transition dipole moment and the induced surface plasmon electric field. Strong coupling occurs when ℏΩR is maximized. In the case that the exciton and plasmon energy are detuned, coupling still occurs but the energy between the hybridized states is simply ℏΩ as


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shown in Figure 3B. The orientation of the transition dipole with respect to the LSPR is known to strongly affect the coupling strength39. Because electric field strength associated with LSPR excitation is known to decay as the cube of the inverse distance from the NP surface, ℏΩR is expected to also become a function of dye-NP separation distance. Previous studies examining the optical response of J-aggregates on the surface of plasmonic NPs24, in the gap of a plasmonic NP dimer27, or adsorbed to a polyelectrolyte layer on the NP surface30 observe strong coupling at separation distances up to 2 nm. To confirm the role of this separation distance on plasmon-exciton coupling strength, we varied the carbon backbone length of the thiol-functionalized linker molecule that is reacted with AF-TFP prior to NP conjugation. First, we examined the optical response of the dye-NP conjugate for separation distances much larger than the molecular size (estimated with and Rg= 1.5 nm) of AF. We used bifunctional amine- and thiol-terminated polyethylene glycol (PEG, Nanocs) chains with varying molecular weights of 1 kDa, 2 kDa, and 5 kDa as the linker molecules. PEG is expected to adopt a brush-like configuration when grafted as a monolayer on NP surfaces. Thus, we estimate that these PEG molecular weights result in separation distances of 3.2, 5.6 and 11.2 nm, respectively40. It should be noted that the use of flexible biological linker molecules precludes the ability to control the orientation of the transition dipole of AF41. The measurements are assumed to be the interaction of the LSPR of the Ag NP with an ensemble of transition dipoles of varying orientations. The radial symmetry of the system and use of unpolarized light minimizes the effect of differing orientations and is assumed to be similar for all linker molecules. Using finite-distance time domain (FDTD simulations of 3D models, we estimated the field enhancement at the wavelength corresponding to the AF absorption peak (495 nm). At distances of 3.2, 5.6 and 11.2 nm from the NP surface the field enhancement is


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calculated to be 9.8, 8.0, and 5.1. Coupling strength is expected to decrease similarly due to its dependence on electric field. Figure 4A shows representative dark-field scattering spectra collected for PEGylated dye/NP conjugates with varying linker length. For the 1 kDa PEG linker (solid black curve), we observe distinctive splitting of the LSPR band associated with strong coupling, giving rise to scattering peaks at λ1=452.9 ± 7.4 nm and λ2=522.1 ± 16 nm. For the 2 kDa and 5 kDa PEG

Figure 4. (a) Representative scattering spectra collected for ensembles of NP/dye conjugates using PEG molecules with increasing molecular weights to construct a spacer layer. The grey bar indicates an absorption peak for free dye molecules in solution. (b) Plot of the change in peak wavelength for peak 1 (blue) and peak 2 (red) as a function of linker length. (c) Plot of the difference in energy of peak 1 and 2 as a function of linker length. Solid curve is a fit to a power law decay y = a/x3 and dotted curve is a fit to the single exponential decay 3 = 4 ∗ 6 1/8 where τ = 13.7 ± 5.4 and a = 0.44 ± 0.07. linkers, peak splitting appears less distinct. We observe that the secondary peak at λ2 decreases in intensity with increasing linker length. We observe peaks at λ1=455.4 ± 2.1 nm and λ2=506.2 ±


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13.4 nm for the 2 kDa linker and peaks located at λ1=465.7 ± 15.8 nm and λ2=506.7 ± 14.5 nm for the 5 kDa linker. It should be noted that using a Lorentizian fit for only two peaks (near the initial LSPR band and absorption band of the dye) resulted in a poor match for the experimental spectrum. In order to obtain reasonable fits, we included a third peak located at the dye absorption band at 495 nm (grey bar). This is to account for light that is scattered off the NP and reabsorbed by the surrounding dye, slightly modifying the scattering spectrum. Figure 4B plots the wavelengths λ1 and λ2 as a function of linker length. Two trends are apparent with increasing linker length. First, λ2 is observed to blueshift as the linker length increases, reaching an asymptote near a separation distance of 11 nm. This asymptotic behavior is consistent with Rabi splitting, since λ2 is expected to occur a lower energy than the exciton absorption peak at 495 nm. Second, λ1 is observed to redshift linearly with increasing linker length. Figure 4C shows a plot of the coupling energy ℏΩ, calculated by taking the energy difference between λ1 and λ2, with respect to linker length. The decrease in ℏΩ with distance follows a power law decay as a function of y = a/r3 (solid line). The data is also fit to a single exponential decay (dashed line) as a comparison to the decay observed in NP dimer coupling, resulting in a decay length of 13.7 ± 5 nm. This is consistent with decay lengths observed for plasmonic NP dimers with respect to NP gap separation distances42. Extrapolating from this decay rate indicates that strong coupling for the dye-NP conjugate should persist beyond the typical FRET distances to approximately 30 nm before ℏΩ reaches zero. For larger spacing, we expect to observe only weak coupling between the NP and monomer dye molecules, which is manifested in the optical scattering response as a single peak that is redshifted from the LSPR wavelength of the pristine Ag NPs.


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To probe the limits of strong coupling and the emergence of quantum effects, we carried out dye-NP conjugation with separation distances less than 2 nm. We used the following bifunctional linear alkanes as linker molecules for the AF-TFP: APT, 6-Amino-1-hexanethiol hydrochloride (AHT), 11-Amino-1-undecanethiol hydrochloride (AUT) and 16-Amino-1hexadecanethiol hydrochloride (AHDT) (all Sigma Aldrich). By varying the length of the carbon backbones from between 3-16 carbon atoms, we expect to adjust the dye-NP separation distance to 0.49 nm, 0.82 nm, 1.37 nm, and 1.92 nm, respectively43. It should be noted that the index of refraction for a compact monolayer of alkyl chains is significantly lower than that of a

Figure 5. (a) Representative scattering spectra collected for alkyl chains of increasing length. Grey line indicates the peak absorption of dye in solution. (b) Plot of peak 1 (blue) and peak 2 (red) position for hybridized composite as a function of linker length. (c) Percent intensity of peak 1 (blue), peak 2 (red), and absorption peak (green) as a function of linker length. monolayer of brush-like PEG molecules. As a result, the peak wavelengths and coupling energies for the alkyl-modified AF-NP conjugates chains cannot be directly compared with those observed for the PEGylated AF-NP conjugates, though overall optical responses are comparable.


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Figure 5A shows characteristic spectra collected for the alkyl AF-NP conjugates. We observe a clear change in the spectral lineshape for the AF-NP conjugates due to band splitting, as well as a significant change in scattering peak intensity for the secondary scattering peak at λ2 with respect to linker length. Figure 5B shows a plot of the peak scattering wavelengths λ1 and λ2 or the alkyl AF-NP conjugates with respect to linker length. We observe clear Rabi splitting for the shortest APT linker (black curve), whose spectrum can be fit with two Lorentzian peaks separated by ∆E=475 ± 45 meV. Longer linker molecules required the addition of a third peak at the AF absorption band in order to achieve an accurate fit. Increasing the linker length results in a wider separation between λ1and λ2 indicating an increase in coupling strength. For the AHT linker (dashed curve), ∆E=495 ± 57 meV; for the AUT linker (dotted curve), ∆E=505 ± 94 meV; for the longest AHDT linker (dashed-dotted curve), ∆E=597 ± 31 meV. While λ2 experiences a clear redshift of approximately 27.5 nm with increasing linker length, we observe that the position of λ1 stays relatively constant. As a result, the coupling energy ℏΩ increases by 122 meV with increasing separation distance between the dye and the NP surface, which is inconsistent with our expectation of coupling behavior for a decrease in electric field strength. Using FDTD simulations we can estimate the electric field enhancement at 0.49, 0.82, 1.37, and 1.92 nm from the surface of the Ag NP to be 12.5, 12.2, 11.6, and 11.0 respectively. It is true that longer linker molecules provide a larger free volume for the AF molecules to occupy. Chikkaraddy et al. recently showed that the plasmon-exciton coupling strength increases as a function of √n where n is the number of dye molecules in a plasmonic nanocavity39. Based on the increase in number of dye molecules we would expect to see an increase in coupling energy of 0.1, 1.0, and 1.9% for linkers with 6, 11, and 16 carbon chains. Experimentally we observe an increase in coupling


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energy of 4.3, 6.3, and 25.7%, indicating that the change in coupling strength is due to more than the increase in dye molecules. One possibility that would account for this discrepancy is the presence of electron tunneling effects. Electron tunneling effects have observed to be dominant for separation distances

PlasmonâExciton Coupling between Metallic Nanoparticles and Dye

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PlasmonâExciton Coupling between Metallic Nanoparticles and Dye